CROWOOD METALWORKING GUIDES

TOOL AND CUTTER
GRINDING

DR MARCUS BOWMAN

Quorn tool and cutter grinder

CROWOOD METALWORKING GUIDES

TOOL AND CUTTER
GRINDING

DR MARCUS BOWMAN

First published in 2021 by
The Crowood Press Ltd
Ramsbury, Marlborough
Wiltshire SN8 2HR

enquiries@crowood.com
www.crowood.com

This e-book first published in 2021

© The Crowood Press 2021

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.

ISBN 978 1 78500 861 0

Disclaimer
Safety is of the utmost importance in every aspect of metalworking. The practical workshop procedures and the tools and equipment used in metalworking are potentially dangerous. Tools should be used in strict accordance with the manufacturer's recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

Cover design by Maggie Mellett

Acknowledgements

The author would like to thank those individuals and companies who have expended time and effort to contribute photographs and information for this book:

Arc Euro Trade Ltd

John Baguley

R. D. Barrett Small Tools Ltd

David Beattie

Bowland Trading Ltd

Celtic INC

Chronos Ltd

Didier Descouens

DMT

Eccentric Engineering

W. E. Falck

T.D. Fitchett Ltd

John Florence

Glendo LLC

Glenn Evans of Glennview.com

Gränsfors BRUK AB

Nigel Hill

Alan Hooper

David Haythornthwaite

Steve Lindsay Engraving and Tools

Machinery Plans

Donald Mitchell

John Moran at gadgetbuilder.com

Morse Flight Systems

Howard at MYFORD-LATHES.COM

Jack Sealey Ltd

The Society of Model and Experimental Engineers

Tormek AB

Toulouse Museum

Versalab INC

Niels Vrijlandt of Nielsmachines.com

Warren Machine Tools Ltd

Weald Enterprises

A Jones & Shipman model 520 grinder, useful for many tool grinding tasks.
Photo: HOWARD at MYFORD-LATHES.COM.

The author wishes to commend those members of bulletin boards, discussion groups and bloggers who have selflessly shared their work, provided solutions to problems and stimulated ideas.

Thanks, too, to the sterling work done by those Standards organisations that have laboured long and hard to produce usable and practical standards for tool features, geometry and terminology.

British Standards Institution

American National Standards Institute

ISO: International Organisation for Standardization

But most of all, a very special thanks to my long-suffering personal support team, Hazel and Rachael, whose encouragement made the writing task so much easier.

Every effort has been made by the author and publishers to contact the copyright holders of the works illustrated in this book. Should any omissions have been made and/or the correct source not been acknowledged, the publishers will rectify this at the earliest opportunity upon reprint.

Introduction

Historians and archaeologists tell us that humans began making and using tools a very long time ago, and museums are well-stocked with examples of early hammers, axes and arrowheads (Fig. 1). What led to that first development is not known with any certainty.

Fig. 1: Pointe de flèche. An early stone arrowhead shaped and sharpened by chipping pieces from the edges. The arrowhead shown dates from 3000 to 2400BCE. Museum of Toulouse (France). Photo: DIDIER DESCOUENS.

Using a stone as a hammer seems relatively intuitive, but deliberately making the edges of a flat stone sharp to create a large hand-axe capable of cutting wood and softer materials is much more of a puzzle, because that tool serves a very different purpose. The archaeological record suggests that cutting and piercing tools developed using 'flake' technology, in which a stone was shaped and given a sharp edge or point either by chipping flakes off the stone to shape the edge, or by shaping the stone so that ready-to-use flakes could then be removed by a single blow. Flakes resulted in the production of shaped spear and arrow points, and scrapers used to clean and prepare an animal hide or shape wood.

Core tool-making technologies led to some cultural and geographic diversity in the shape and purpose of tools, so that some communities began to produce tools shaped in particular ways for specific purposes. Communities with ready access to sources of fish, for example, developed the basic spear shape by adding a hook, making it rather like the head of a harpoon.

This is an example of the specialization of tools and the development of their shapes; a trend that continues to the present day.

Tools changed as humans discovered other materials and began to be able to manipulate metals. Copper is believed to be the first metal that humans could extract from ore by using fire. Although copper is a soft metal, the later development of the alloy (mixture) of copper and tin we know as bronze ushered in a new age, because bronze is capable of being hardened to a much greater degree than pure copper, and could be used to make better tools than stone. This was a sufficiently significant advance that it is recognized as a specific stage in human development, as humans moved from the Stone Age to the Bronze Age. Bronze tools included many for working with wood or stone. Axes, various shapes of adze (essentially an axe with its edge set at right angles to the shaft), saws and drills were all used to enable relatively sophisticated woodworking, and there is evidence from sites such as the Pyramids that bronze chisels were sufficiently hard to be used for working stone.

Iron followed bronze, as smelting techniques improved. Some bronze tools were made by casting, pouring the metal into cavities in moulds to create shaped tools. Iron tools may be cast, but Iron Age tools were also produced by forging, using hammers to strike the iron and beat it into shape. That beating action also changes the structure of the iron, improving the performance of the finished tool. So a blacksmith hammering a tool not only forges it into an appropriate shape for its intended use, but improves the metallurgy so that the tool is stronger and, in some cases, holds a better edge.

Steel followed iron, and the development of the Bessemer process that led to the production of high speed steel allowed the manufacture of tools that would hold their shape and their cutting edge even when red hot. This allowed modern-day machining processes for shaping iron, steel, and other softer metals, and began the huge strides forward in tool-making we have seen in our own lifetimes.

A key element in all of this has been the ability of humans to create a cutting edge on a tool. A hammer may change the shape of an object, or force one object into another, but a cutting tool divides material into separate parts. A sharp cutting edge allows precise, clean, division of material and enables accurate work in a wide range of substances, from the sharp blade of a sheet metal guillotine to the fine edge on a surgeon's scalpel. Keeping an edge sharp is important, because as the edge loses its sharpness it changes shape, altering the whole cutting action and often preventing the tool from cutting cleanly or cutting at all.

This book is about creating and maintaining a sharp edge on a range of engineers’ cutting tools, because without that sharp edge tools may not only fail to perform as intended but may actually become dangerous. With a keen, sharp edge, any tool will be ready for use whenever necessary, and may allow fine work to be executed effectively and with minimum effort, in safety.

STAY SHARP; STAY SAFE

The workshop is a place of industry but it should also be a place of tranquillity, focus, and the pleasure that comes from creating things. The sharp cutting edge plays an important part in most workshops, but it also represents a source of danger.

Workshops can be dangerous places, for all sorts of reasons, but there are specific dangers associated with sharp edges. A sharp edge can easily cut flesh, and careless or imprecise use of a sharp tool brings the danger that the cutting edge may penetrate skin, sever sinew and nerve tissue, or slice through bone. A knife that slips, a saw blade that wanders, or a machine with unguarded cutting edges that may allow contact between blade and body parts are all potential sites of serious injury to humans or animals. Take every care to avoid all of these.

A blunt cutting tool is almost as bad, and while it may not cut quite so quickly or effectively, there is the real added danger that a blunt tool must be forced to cut. That forcing action may increase the danger that the tool slips or the work moves under the force, and the unguided cutting edge, blunt though it may be, causes serious injury to nearby parts of the body. Gently stroking a sharp hacksaw over a sheet of thick steel will part the sheet cleanly and accurately, while a blunt blade must be pressed much more firmly into the surface to have any effect. With increased pressure comes loss of accurate control, and the danger that if the blade breaks, hands, head and chest may come down hard on the sharp edge of the work. Whereas a gentle movement can easily be arrested as a sharp blade finishes cutting and drops off the material being cut, the kind of high pressure needed to make a blunt blade cut often means a loss of control as a knife is pulled heavily off the material. The nearest targets then become an adjacent hand, arm, stomach, ribs or leg.

Material being cut may move unless firmly secured. Cutting steel with a chisel creates sufficient force for lightly clamped work to be pushed out of a vice, at which point it becomes a projectile capable of damaging whatever lies in its path. A blunt chisel requires much more force to make it cut, and even tightly clamped work may move under the pressure. Failing to attend to the condition of parts of a tool that are intended to be struck by a hammer, like the end of a chisel for example, may result in the danger that fragments fly off and cause injury. A misshapen cutting edge on a chisel or a punch may shatter, producing flying projectiles akin to the fragments of a hand grenade.

Sharp cutting edges and well-maintained tools are a basic requirement and the first precaution in the workshop.

Impact-proof goggles are an essential, and good ear defenders are a wise addition where tools make noise. The noise of a grinder sharpening a tool is loud and contains many high frequencies, which are quite capable of causing serious hearing impairment, so you should take precautions to protect your ears. The dust created by grinding gets everywhere, and can cause serious lung problems, so wear a good quality dust mask.

Gloves may be a useful form of protection, but a sharp knife can easily cut cloth or leather, and modern material such as Kevlar or woven stainless steel wire may be more appropriate, depending on the task.

Abide by appropriate Health and Safety recommendations when using tools, even in the home workshop or craft studio, because those regulations represent good practice, designed for your own protection. When using grinding wheels and grinding machines, obtain a copy of the relevant Health and Safety regulations, particularly in respect of mounting and using wheels safely. These regulations are mandatory in an industrial situation, but are essential for safety even in a home workshop. Skill is an important aspect of safety, and you should take the time to practise using tools.

Many of the photographs shown in this book are of grinders. For clarity, so that you can see as much detail as possible of the various setups for sharpening tools, many of the photographs show grinders that have had all or part of their protective wheel guards removed. Under no circumstances should you operate a grinder without those protective guards in place.

As with everything else, the best safety equipment is only effective when it is worn; and the most appropriate safe working methods are only useful when they are practised. Take your time; think about what you are doing; and enjoy working safely.

1 Sharpening Media

Abrasion is the process of wearing away a material by rubbing it with a second, rougher material. An abrasive is a material used to wear away, or change the shape of, another material. Using an appropriate abrasive, the shape of a tool may be changed to produce a cutting edge. So, a soft stone may be rubbed with a hard stone to shape the softer stone to a sharp edge to make it into a knife or an arrowhead. More to the point, a hard metal may be ground (rubbed) with a harder material such as diamond, to produce a cutting edge on a tool designed to cut material slightly softer than itself. So a high speed steel tool can be ground to produce a sharp cutting edge capable of cutting mild steel. The specific shape of the cutting edge will depend on the job for which the tool is designed, and will affect the durability of the edge. A razor-thin edge may cut some materials beautifully, but will crumble under pressure, while a chisel-shaped edge has more material immediately behind the cutting edge and although it will not be able to cut soft material particularly well, it will be much more able to withstand the heavy pressure required when cutting harder materials such as hardened steel. However; although it seems obvious that a hard abrasive can grind a softer steel, it is also true that a softer abrasive material can be made to grind a harder steel.

Early abrasives were originally naturally occurring materials, but modern abrasives are usually either refined natural materials or man-made synthetic materials, both of which benefit from a consistent particle size and a precise composition designed to remove material at a particular rate or in a particular way. There is a place for both, in tool and cutter grinding. Further Information: Table 1 lists commonly available abrasive materials, their relative hardness, and some applications.

Fig. 2: An early hand-operated sandstone grinding wheel with a water trough.
Photo: (c) Flominator/ Wiki Commons/GFDL.

Fig. 3: A range of flat abrasive surfaces.

Fig. 4: Abrasives may be moulded to make wheels.

Although natural materials may be shaped then used as abrasive stones or wheels (Fig. 2), modern composite abrasives consist of consistently sized particles of natural or synthetic materials that are normally embedded in a bonding material that holds the particles and allows the abrasive to be moulded into a specific shape such as a block (Fig. 3) or a wheel (Fig. 4). The particles act like small cutting tools and remove material being rubbed against the abrasive. Those particles rub and cut, and like any cutting tool, they become blunt, so an important part of the action of an abrasive is to allow the blunted particles to be removed. Natural materials require sufficient hardness to be able to grind material, but they also require sufficient friability (the ability to break down and shed particles) so that as particles become blunt they are shed, exposing fresh particles to the work, to continue grinding. That means the choice of abrasive must be based on a careful match between the hardness of the material being ground, and the hardness and friability of the grinding material. Particle size is also important, and although larger particles tend to remove material more quickly, smaller particles work more slowly but tend to leave a finer finish on the ground surface. There are limits to how well these factors can be controlled when using natural materials, so synthetic materials have been developed to allow a better match to suit specific materials and grinding requirements.

Fig. 5: A grinding wheel clogged by grinding soft mild steel, resulting in particles of steel sticking to the surface, reducing the effectiveness of the wheel.

Bonding materials play an important role in controlling the rate at which blunt particles are released from the abrasive, and the strength of the bond can be arranged to suit the requirements of the material being ground. Although releasing particles at a high rate means an abrasive wheel wears more quickly, a lower release rate may result in contamination of the grinding medium by the material being sharpened, leading to clogging of the abrasive material (Fig. 5), reducing the effectiveness and the efficiency of the sharpening process. Control of the strength of the bonding agent produces a range of abrasives that either release abrasive particles before they are fully blunt ('soft' abrasives), or hold onto their particles even after they have been made blunt ('hard' abrasives). The ideal situation is when the hardness of the abrasive is suitable for the material being sharpened. Commonly used bonding materials are listed in Further Information: Table 2. In more recent times, hard abrasive surfaces based on diamonds or cBN, termed 'superabrasives' have been produced by embedding the abrasive particles in a nickel matrix deposited electrically, in a plating process.

Abrasive
Workpiece material

Arkansas stone, water stone, India stone, Coticules, Belgian Blue whetstone, ceramic stone, sandstone, oilstone, emery, rotten stone (Tripoli)

Mild steel, carbon steel, high speed steel. Usually used in the form of flat stones, for honing, or sharpening, rather than initial shaping. The harder the steel, the slower the process.

Aluminium oxide (corundum)

Mild steel, carbon steel, high speed steel. Softer steels have a tendency to clog the wheel more quickly than harder steels.

Silicon carbide (carborundum, Crystolon)

Tungsten carbide

Diamond

Tungsten carbide, non-ferrous metals, hard non-metallic material (e.g. stone)

cBN (Borazon)

Hard steels

Table 3: Abrasives used to grind common materials.

Ideally, abrasives should be matched to the material being abraded, so Table 3 lists abrasives and the materials for which they are best suited. Further Information: Table 4 lists reference grades for a range of types of abrasive, to allow comparison between commonly available forms and types.

FRIABILITY

A material that can easily be worn away by being reduced to smaller particles (like a soft stone wearing to dust) is termed ‘friable’. All materials have a degree of ‘friability’, but some are more friable or less friable than others, just as all materials have the property of ‘hardness’ but some are harder or less hard than others. A hard material is termed ‘indurate’, although, as with friability, some materials are more indurate or less indurate than others.

GRINDING WHEELS

The speed at which a surface can be shaped by an abrasive depends largely on the rate at which the abrasive can be rubbed against the surface. As a result, although rubbing a tool along an abrasive stone by hand is an ancient method still used today for some tools, powered wheels of abrasive material are more commonly used to grind the majority of tools. The abrasive wheels may be made of a natural abrasive (Fig. 2), but it is easier to control the shape of the wheel and the consistency of the performance of the abrasive if the abrasive particles are held in a bonding agent such as resin or vitrified clay. Close control of the manufacturing process allows precisely shaped wheels to be formed, each designed to be useful for particular grinding operations. In addition, using precisely made wheels in a grinder that allows accurate control of movement of the wheel and the workpiece helps ensure accuracy of grinding operations.

Fig. 6: Cross-sections of common wheel shapes for tool and cutter grinding. Arrows indicate working faces. Reference numbers correspond to wheel types listed in Table 3.

Grinding Wheel Shapes

Wheel type reference number

Description

1 (General)

Straight wheel for general applications

1 (Saw)

Thin straight wheel for saw sharpening

5

Wheel recessed on one side only

7

Wheel recessed on both sides

3

Wheel tapered on one side

12

Dish wheel

11

Taper cup wheel

6

Straight cup wheel

Table 5: Grinding wheel shapes in common use. Reference numbers refer to Fig. 6.

The basic shapes of grinding wheels are defined in national and international standards documents, available from national standards bodies such as ANSI (American National Standards Institute), BSI (British Standards Institution) and ISO (International Organisation for Standards), and these shapes may be augmented by individual manufacturers, to produce a wide range of useful shapes for general or specific tasks. Fig. 6 and Table 5 contain details of typical wheel shapes for tool and cutter grinders.

Fig. 7: Bench grinders are often fitted with a type 1 wheel. The operating surface is the outer curved cylindrical face of the wheel. The face of the wheel on the right has embedded diamond particles.

Fig. 8: On a rotating wheel, the linear speed of a particle depends on its radius.

The type 1 grinding wheel is the cylindrical flat-sided stone found on most basic bench grinders (Fig. 7). The working face of a type 1 wheel is the curved periphery, and not the flat sides of the wheel. The abrasive particles on the curved face of the periphery are all at the same distance from the centre of the wheel, so they all travel at the same linear speed (the speed measured as if they were travelling in a straight line on-the-flat), known as the surface speed. For a tool in contact with the curved working face of the wheel, each part of the tool being ground will experience the same grinding rate. Grinding a chisel against the flat working face of a type 6 cup wheel means using abrasive particles travelling at different linear speeds, because the particles nearer the centre of the wheel travel at a lower linear speed than those nearer the outer edge of the face (Fig. 8). The linear speed of points on the side of the wheel depends on their radial distance from the centre of the wheel (Fig. 8). Unrolling the red and blue paths and laying them flat shows that the red path is longer, so particles on the red and blue paths travel different distances in the same time (the time taken for one revolution). All points on the red circle travel at a speed of 2 × π × R × rpm metres per minute, and all points on the blue circle travel at 2 × π × r × rpm metres per minute. Because r is smaller than R, points on the blue circle travel more slowly than points on the red circle.

Fig. 9: A type 1 wheel is useful for grinding a chisel because raising the chisel above the centre line allows a match between the angle of the face and the curve of the wheel.

Fig. 10: A type 3 wheel is a better choice of shape when grinding the front faces of the teeth of a circular saw, as it avoids damage to adjacent teeth.

In practice, this is seldom a problem, because most grinders use a sideways motion ('traverse') of the tool across the whole face of the wheel, and although each part of the face of the tool experiences particles travelling at different rates, they experience those in the same way, and the face is finally ground by particles travelling at the same speed, either fast (if traversing to the right) or slow (if traversing to the left). There is more on the traversing movement in Chapter 3. The range of wheel shapes simply allows more convenient access to some faces or edges of specific tools than others. So, although a type 1 wheel is useful for grinding a chisel (Fig. 9), it cannot easily be used to grind the front face of a saw tooth without damaging adjacent teeth, and a type 3 wheel is likely to be more convenient for that task (Fig. 10). Grinding wheels with convex or flat working faces have a range of uses, and these are explored in Chapter 3.

GRINDING WHEEL SPEED

By law, every grinding wheel must carry a label stating its recommended Maximum Operating Speed (MOS) which is the maximum allowable safe rotational speed (in revolutions per minute or rpm) of the wheel (Fig. 11).

Fig. 11: By law, all grinding wheels must carry a label showing their maximum permissible rotational speed, in rpm. Many also show the maximum permissible linear speed at the periphery, often in metres per second (m/s).

That allows a match between motor speed, usually expressed in rpm, and the grinding wheel rotational speed. However, the important speed is the linear speed of the periphery of the wheel, in metres per minute (m/min) or feet per minute (ft/min). To distinguish between linear speed and rotational speed (revs per minute), the term 'surface speed' is often used, and the units then become surface metres per minute (smpm) or surface feet per minute (sfpm). The significance of rotational speed is that the faster an abrasive particle is travelling, especially at the periphery of a wheel, the greater the force attempting to throw the particle off the wheel, and the stronger the bond must be to hold it in place. As rotational speed increases, these forces increase, but the strength of the bond remains constant. When the forces produced by the wheel’s rotational speed exceed the strength of the bond, the wheel will shed particles and break apart, endangering everything nearby.

Fig. 12: On this type 6 cup wheel, the chisel experiences different cutting speeds at different points located nearer or further from the centre of the wheel.

The significance of surface speed is that each abrasive particle is essentially a cutting edge, and there is a range of effective cutting speeds for each material being ground. There is also a balance to be struck between effective cutting and reasonable (and cost effective) wear on the wheel. Ideally, the surface speed should lie in an appropriate range for the material being ground, and that means taking the effective diameter of the wheel into account. Where the face of the wheel is the curved peripheral surface, the effective diameter is the diameter of the wheel, but where type 6, 11 or 12 wheels are used, the effective diameter is the diameter of the point at which the work contacts the wheel (Fig. 12). If the grinding face is wide there will be a range of diameters in contact with different points on the work simultaneously, producing a range of speeds across the work (Fig. 12). For wheels of this type the grinding face is usually relatively narrow (6–10mm being common), and it is usually possible to find a method of presenting the work to the wheel so that this variation is minimized. That normally involves the cutting edge of the work being traversed across the face of the wheel, so that only a portion of the cutting edge is in contact with the face of the wheel at any moment.
Rotational speed, surface speed and wheel diameter are related by the following formulae (where π = 3.14):

Surface speed (m/min) = rotational speed (rpm) × π × wheel diameter (mm) / 1,000

Rotational speed (rpm) = surface speed (m/min) × 1000 / (π × wheel diameter (mm))

or

Surface speed (feet/min) = rotational speed (rpm) × π × wheel diameter (in) / 12

Rotational speed (rpm) = surface speed (feet/min) × 12 /( π × wheel diameter (in))

For example:

Wheel diameter 200mm

Rotational speed 1,420rpm

Surface speed(m/min) = 1,420 × 3.14 × 200 / 1,000 = 89,1760 / 1,000 = 891.760 = 892m/min

Surface speed (m/sec) = surface speed (m/ min) / 60 = 892 / 60 = 14.87m/s

Wheel diameter 6in

Rotational speed 2,840rpm

Surface speed (feet/min) = 2,840 × 3.14 × 6 / 12 = 53,505.6 / 12 = 4,458.8 = 4,459 ft/min (or 4,459 / 60 = 74.3ft/sec)

GRIT SIZE

Abrasive particles sizes are usually stated as having particular ‘grit’ sizes, which are traditionally determined by passing the particles through a series of fine meshes. The finer the mesh, the greater the number of holes in a given area, and the smaller the size of the particles that will pass through. Low grit numbers correspond to a low number of larger holes in a standard-sized area of the mesh, so the particles are larger and the abrasive is coarser. High grit numbers correspond to a large number of smaller holes in the same standard-sized area of mesh, through which smaller particles may pass. Lower-numbered grits are coarser than higher-numbered grits, so a 20 grit abrasive is much coarser than a 1,000 grit abrasive. Abrasives may be roughly classified into Macrogrits (coarse; with grit numbers of less than 240) and Microgrits (fine; with grit numbers of 240 or more). For a given grit size, the particles are not necessarily all of identical size, but although there is some variation, the range of sizes is small, so they can be considered more or less uniform, for each grit size.

Grinding wheel shape and grit codes

The range of wheel shapes, types, and grit sizes is wide, so international standards define several aspects of the codes used to categorize wheels. Other aspects are indicated by manufacturers using their own proprietary codes.

Fig. 13: Interpreting British Standard wheel markings.

International standards define the dimensions e. g. (BSI: BS ISO 603-6:1999 Bonded abrasive products- Dimensions- Part 6: Grinding wheels for tool and toolroom grinding), (BS ISO 525:2013 Bonded Abrasive Products. General Requirements), (ANSI: B74.13-1990:Markings for Identifying Grinding Wheels and Other Bonded Abrasives). Fig. 13 shows how to interpret a 'specification mark' according to BS ISO 525:2013. Note that the meanings of optional codes in positions highlighted in green must be found by consulting a manufacturer's literature, as these vary from one manufacturer to another. The example code 32A46-JVBE translates to aluminium oxide, grain size 46 (coarse), hardness J (soft), vitreous bond, with manufacturer’s optional codes 32 (mixture of abrasive types) and BE (special code).

In the USA, ANSI B74.13 - 1990 (Revised 2007) codes are almost identical to those of British Standard ISO 525:2013, although the 'Mixture of abrasive types' is termed 'Prefix', and the 'Manufacturer's special code' is termed the 'Manufacturer's Record' in the ANSI code.

MATCHING ABRASIVES TO WORKPIECE MATERIALS

The hardness of the material being ground is a key factor in selecting an abrasive, because grinding works best when used on hardened material. Mild steel, in its unhardened state, tends to clog a grinding wheel, so a soft grade of wheel is required to avoid this. Hardened steel, on the other hand, can be ground using a harder grade of abrasive. Not only will the finish tend to be better, but the abrasive wheel will not have the same tendency to clog as when grinding softer steel. The grit size is important too, with a coarser grit (smaller grit number) cutting faster but leaving deeper scores on the surface than a finer grit size (higher grit number). However, the grit size should be chosen bearing in mind the tendency for fine wheels to heat work more than coarse wheels, because a fine wheel will cut more slowly and the work will be in contact with the wheel for longer. Some jobs may require initial grinding using a relatively coarse wheel, followed by a finish grind using a finer wheel. On a tool and cutter grinder, grade 38 (or a smaller number) would be relatively coarse, while grade 60 or 100 would be a fine wheel. Finer grades would produce a finer and more polished finish on the work.

TESTING THE INTEGRITY OF A WHEEL

One of the dangers of using a grinder is that of a wheel bursting, so it makes good sense to perform a simple test before mounting a wheel onto a grinder. Suspend the wheel on a string, and tap it gently with a metal rod (Fig. 14). A cracked wheel will make a dull sound, while a good wheel will make more of a ringing noise. It will not ring like a bell, but the clearer sound is a good indication of the integrity of the wheel. Cracked wheels cannot be repaired and should be disposed of immediately.

Fig. 14: Test the integrity of a wheel by tapping the suspended wheel with a steel bar.

MOUNTING A WHEEL

To mount a grinding wheel on a shaft (sometimes referred to as an arbor,although that term is more commonly used for a shaft held between centres on the table) the hole through the centre of the wheel should match the diameter of the shaft. It is not unusual to find that the hole is much larger than the shaft, but that can be remedied by fitting a reducing sleeve to the hole in the wheel (Fig. 15). Sleeves are often made of plastic and are readily obtained at very low cost. In many cases, the reducing sleeves come free with the wheels if the shaft diameter is specified at the time of purchase. The wheel should be held between steel flanges, and Further Information: Table 6 gives guidance on flange sizes, based on recommendations in BS 4581-2: 1984, which also specifies flange shapes and minimum flange thicknesses. Smaller low-cost offhand grinders often use two thin loose flanges, but toolroom grinders normally have one flange keyed to the shaft, to provide positive drive. A keyed flange may also have a hole in its periphery that allows a rod to be inserted so that the flange can be held stationary. This holds the shaft stationary, and helps when tightening the nuts at each end of the grinder shaft. Flanges should be relieved in the centre, so that contact between wheel and flange occurs only on a band near the outside diameter. Flanges that are not keyed have the advantage that the small amount of friction drive may allow the wheel to slip more readily if it jams, but that does limit the torque that can be applied to the wheel and the work before the wheel slips. Blotters must be fitted between wheel and flange, on each side of the wheel (Fig. 16). These should be made of soft paper or card, and are intended to accommodate the difference in surface textures between the wheel and the flanges, preventing undue stress being created as a smooth, hard, flange crushes the points of the grains.

Fig. 15: Reducing sleeves enable a wheel with a large centre hole to fit a shaft with a smaller diameter.

Fig. 16: Flanges should be relieved in the centre, and thin card blotters should be used between flange and wheel.

DRESSING A WHEEL

Before use, a grinding wheel needs to be dressed so that it rotates concentrically with the spindle, and its surface is in a fit state to grind work, with freshly exposed grains on its working face. After mounting a wheel in the grinder or grinding spindle, it is wise to dress it; even if the wheel is mounted permanently on its own arbor or boss.

The purpose of dressing is:

• to shape the working face of the wheel by making it concentric with, or at right angles to, the axis of the spindle; or to give the operating face of the wheel a particular shape (such as giving the corner of a wheel a specific radius).

• to expose fresh grains. This is particularly important if the wheel has become clogged with soft material (Fig. 5).

Although grit sizes are determined during manufacture, different dressing techniques may result in small differences in abrasive performance, producing a slightly faster-cutting wheel that does not heat the work much, or a wheel that produces a finer finish but creates more heat in the tool.

The star wheel dresser

Fig. 17 shows a star wheel dresser that consists of alternating plain and star-shaped hardened steel discs. This type of dresser may be suitable for dressing the type 1 wheel on a bench grinder, but it is not suited to the kind of precise dressing required on a tool and cutter grinder.

Fig. 17: A well-used star wheel dresser consisting of hardened plain and star-shaped discs.

Using a single- or multi-point diamond dresser

Single-point diamond dressers consist of a single large industrial diamond mounted in a holder (usually a short length of cylindrical steel bar), as shown in Figs 18 and 19. This is the most versatile, and probably the most commonly used type, particularly because a single diamond in a rod can be mounted in a workhead (Fig. 20) and traversed accurately and at a controllable rate.

Fig. 18: A single point dresser consists of a diamond mounted in a steel bar.

Fig. 19: A single-point dresser in an adjustable holder which allows the depth of cut to be varied.

Fig. 20: A single-point dresser in a holder mounted in a workhead ready to be traversed across the face of a wheel.

Fig. 21: A multi-point dresser contains many diamonds mounted in the end face of a small block.

Multi-point diamond dressers contain many diamonds embedded in the end face of a short length of bar (Fig. 21). The multipoint dresser is used in much the same way as a star wheel or dressing stick (Fig. 23), guiding the dresser by hand while traversing across the face of the wheel. However; it is not as easy to hold a multi-point dresser in a workhead, as a single-point dresser.

When a single-point dresser is traversed across the face of a wheel, it needs to be held so that the diamond trails the grains of the wheel, pointing away from the direction of travel both vertically, by 10 to 15 degrees, and horizontally by 15 to 10 degrees.

Traversing across a flat-faced wheel

If the wheel is rotating downwards at the front, set the diamond at right angles to the wheel (or radial to the wheel, for the curved surface of a cylindrical wheel) then tilt it downwards by 10 to 15 degrees. The angle is best achieved by mounting the dresser in a holder that sets the angle (Figs 19 and 20).

Traversing across a curved-face wheel

Because the wheel is curved, Fig. 22 shows that the trailing angle can be created by setting the dresser horizontal and at centre height, then lowering the dresser (or raising the wheel) as shown in Table 7. In addition, trail left to right by 5–10 degrees.

Fig. 22: The angle between dresser and wheel can be set by holding the dresser 'horizontal' a specific distance below centre height.

Height below centre for ‘horizontal’ single-point dresser

Diameter of wheel

Distance below centre height (mm)

100

9–13

125

11–16

150

13–19

180

16–23

200

17–26

Table 7: Setting the dresser horizontal and at the given distance below centre produces an effective angle of 10 to 15 degrees.

Using a dressing stick

A dressing stick is a rectangular stick of hard abrasive material that is pressed against the surface of the wheel and removes material from the wheel (Fig. 23). The stick is made of a material similar to the wheel, or harder, and the bonding material is usually stronger than the wheel bond, so that the stick removes grains from the wheel. Sticks such as Norbide, made by Norton Saint-Gobain, are made from boron carbide which is one of the hardest man-made materials and is well suited to dressing most abrasive wheels except diamond. Although diamond dressers are the tool of choice for dressing a wheel to produce a flat face or a precise shape or angle, the dressing stick has its place as a freehand tool, and its broad face allows the corner of a wheel to be dressed to a curve more easily than a diamond dresser.

Fig. 23: A dressing stick is a rectangular stick of abrasive material capable of removing grains from the surface of the wheel.

Recommended depths and feed rates for dressing

When using a diamond dresser the diameter of the grinding wheel and the width of the face influence the optimum size of diamond needed to prevent excessive heat build-up. In practice, a diamond of 0.25kt, 0.33kt or 0.5kt will work well on wheels of diameters up to 200mm (8in).

The effect produced by diamond dressers depends on the depth of feed and the speed at which the dresser is traversed across the face of the wheel. Fine feeds towards the wheel of 0.01 (fine grit) to 0.025mm (coarse grit) per pass should be enough.

Relatively coarse feeds and a rapid traverse produce a more open finish to the grains on the wheel, resulting in a coarser finish on the work, but a relatively cool cutting process. Relatively fine feeds and a slow traverse will produce a smoother more closed finish to the wheel face, resulting in a finer finish on the work, but one that requires finer feeds and will cut more slowly, thereby generating more heat in the work. Although this is affected by the grade of the wheel, and cannot be used to make a very coarse-grained wheel behave like a very fine-grained wheel, it is a noticeable effect caused by the diamond cutting grains rather than ripping them from the surface of the wheel. Cut grains are less able to abrade the work and behave more like blunt grains, so this effect should be applied with care.

Table 8: Traversing time across a selection of wheel widths, at a medium rate of traverse.

For most applications, let the grade of the wheel determine the roughness, and use a medium rate of traverse, as suggested in Table 8. Traversing times are calculated based on linear movements in mm/rev to produce coarse (0.3mm/rev), medium (0.2mm/rev) and fine (0.08mm/rev) finishes on the wheel face.

T = W/(LS × rpm)

where T is the Traversing time in seconds

W = wheel width (mm)

LS = linear speed (mm/min)

rpm = spindle speed in revs per minute

Table 8 shows speeds for a range of typical wheel sizes. Absolute precision is not required, especially when traversing manually, and it is usually possible to get reasonably close with a little practice. If in doubt, err on the side of greater traversing speed.

2 Tool and Cutter Grinders

Tool and cutter grinders (T&C grinders) are specifically designed to sharpen a range of tools, and, even in their basic forms, are provided with toolholding attachments, work tables and precision spindles, making them versatile machines (Fig. 24).

Fig. 24: Component parts of a typical tool and cutter grinder: Spindle (the shaft carrying the wheel), wheelhead (the assembly consisting of the spindle and its housing), height adjustment handlwheel, grinding wheel, table, crossfeed and traverse lever (or the table may be pushed by hand).
Photo: HOWARD at MYFORD-LATHES.COM.

T&C grinders use a range of wheels with shapes designed for particular tool types. Most wheels have a narrow face and the tool is normally traversed across the face of the wheel, so the narrow face is not a disadvantage. In many cases, the face of the wheel is dressed so that the area of contact is even smaller, to prevent heat build-up. Small taper and dish wheels are useful where the wheel needs clearance in a restricted area such as the gullet between the teeth of a saw. White aluminium oxide wheels are common, but silicon carbide ('green grit') and diamond-impregnated wheels are often used for sharpening carbide tools.

BASIC WHEEL AND WORK POSITIONS

Tool and cutter grinders normally have a comprehensive range of movements and adjustments for the wheelhead, table or rest, and workholding arrangements for the tool. On many grinders the head can swivel to present the wheel at an angle to the work, or a table that can be set at an angle to the wheel, and most can accommodate a range of wheels of varying diameters and shapes. Figs 25 and 26 show two common arrangements, with the axis of the wheel and the axis of traverse at right angles (Fig. 25) or parallel to each other (Fig. 26).

Fig. 25: The axis of the spindle (i.e. the shaft carrying the wheel) can be set at right angles to the direction of traverse of the table.

Fig. 26: The axis of the spindle (i.e. the shaft carrying the wheel) can be set parallel to the direction of traverse of the table.

Most grinders allow the height of the wheelhead to be altered via an accurate screw, so that the tool can always contact the wheel at a convenient spot, even if the tool is tilted up or down. As a result, a wide range of operations is normally possible on a T&C grinder, to suit a range of tool types. Where table or workhead movements are controlled by leadscrews, the results are usually highly accurate.

Swing, tilt and rotate

On a grinder, tools are normally held in a workhead provided with a toolholder to suit the shape and type of tool. Workheads can rotate about one or more axes, providing at least one, and often all three, of the basic movements of swing, tilt and rotate (Fig. 27) as explained in more detail in Chapter 3. Compound workheads capable of all three movements make work-setting straightforward, provided care is taken with the inter-relationships between the axes. (see the important note on workhead geometry, on page 31). Some workheads incorporate linear movements to adjust the tool position, or to apply feed towards the wheel.

Fig. 27: Workheads may provide rotation around three axes, producing swing (yellow), tilt (blue), and rotate (green) movements to position cutting edges.

WORK TABLES AND WORKHEADS

Grinders such as the Clarkson (Figs 28 and 33), Stent (Fig. 24), Brookes (also known as the Brookes-Stent) (Fig. 106, page 36), Kennet (Fig. 29),K.O. Lee (Fig. 30) and Worden (Fig. 31) have a table that can move side-to-side, past the wheel in a traversing movement, or, in most cases, towards or away from the wheel. Tools are gripped in a workholder or held between centres. Movements towards or away from the wheel are used to provide fine control of depth of cut, while traversing past the wheel causes the wheel to grind the tool. Depth of cut movements are normally under the control of a feedscrew, while traversing movements need to be more rapid and are more commonly controlled by hand or by rack and pinion gearing (Fig. 32), belt, wire or mechanical linkage. Multi-start coarse-pitch leadscrews also provide rapid movement, allowing compound X-Y tables to be used. On some grinders, such as the Worden, the table is traversed by hand, while guided by circular bars underneath.

Fig. 28: The Clarkson allows the table assembly to be swung to the side. Traverse takes place parallel to the axis of the base of the table. Photo: DEBI CARTER.

Fig. 29: Kennet grinder. Photo: JOHN BAGULEY.

Fig. 30: A K.O. Lee tool and cutter grinder. The large central knob provides crossfeed, and the two wheels at the rear elevate the head. The lever on the right rotates to traverse the table.
Photo: GLENN EVANS at GLENNVIEW.COM.

Fig. 31: Worden grinder built by Donald Mitchell. The table on this grinder tilts up and down, and is traversed by hand. The wheelhead is fixed and has no height adjustment. The toolholder swings from side-to-side, guided by the semi-circular slot in the table. Depth of cut is controlled by the knob at the back of the toolholder (nearest the operator).

Fig. 32: On some grinders, traverse is provided using rack and pinion gears.
Photo: JOHN FLORENCE of THE SOCIETY OF MODEL AND EXPERIMENTAL ENGINEERS.

Fig. 33: A sub-table (or the top surface of the table itself) may be swivelled to allow tapered cutters to be sharpened. Photo: T.D. FITCHETT.

The Clarkson, Stent, Brookes, Kennet, and most larger industrial grinders have a table with one or more tee slots. The Worden has a table that features a single curved slot to allow a workholder to pivot (swing) to present a cutter at an angle to the wheel. The Kennet has a similar angle-setting facility. The Clarkson table can be swivelled to allow the cutter to be presented at an angle or set parallel to either the front or side faces of the wheel (Figs 28 and 33). Some grinder tables carry sub-tables that can be set at an angle to the main table without changing the direction of traverse of the main table (Fig. 33) to allow tapered cutters to be sharpened. If the axis of the cutter is parallel to the axis of the sub-table, the main table ways continue to guide the table parallel to the face of the wheel, but if the cutter is held at an angle to the direction of travel it will be ground tapered (see Chapter 3).

Fig. 34: Single-point 'universal' grinders do not have a table, but use a workhead to hold tools. The workhead of this Deckel SO single point 'universal' grinder slides on a horizontal bar. Photo: NIELS VRIJLANDT of NIELSMACHINES.COM.

Fig. 35: The workhead traverses in an arc, across the face of the grinding wheel.
Photo: NIELS VRIJLANDT of NIELSMACHINES.COM.

Fig. 36: The Deckel SO workhead allows rotation around three axes, providing the equivalent of swing, tilt and rotate movements. Pivoting around the front bar creates a traversing motion across the wheel face. Sliding along the horizontal bar allows the work head to be positioned.
Photo: NIELS VRIJLANDT of NIELSMACHINES.COM.

Fig. 37: Workhead slides provide forward-and-back and side-to-side adjustment of the cutter position in relation to the axes of swing and tilt.
Photo: NIELS VRIJLANDT of NIELSMACHINES.COM.

Fig. 38: Sharpening lathe tools on a Deckel SO or Universal grinder requires a collet or sleeve with a hole shaped to grip the tool, or a modified tool holder in place of the collet holder.

Some grinders have tables, and others simply have a workhead. The Alexander/ Deckel universal grinder is designed to sharpen single-point tools, engraving cutters and some drills. The workhead swivels to present the tool at an appropriate angle to the wheel. Sliding the workhead along the bar varies the distance between the tool and the wheel (Fig. 34). Clamping the workhead to the bar and using a graduated knob to move the bar backwards or forwards provides fine control of movement to create depth of cut. Pivoting the workhead around the bar allows the cutter to be moved in an arc past the face of the grinding wheel that lies at right angles to the axis of the bar (Fig. 35). This kind of grinder is usually intended to be used with a type 6 cup wheel. The workhead allows rotation in at least three planes (Fig. 36), providing the equivalent of swing, tilt and rotate movements. The position of the tool can be adjusted using the forward-and-back and side-to-side movements of the slides built into the top of the workhead (Fig. 37). The rear of the workhead also allows the tool to be rotated, and the graduated collar allows angles to be set, while the plunger at the front is part of a dividing arrangement that uses holes drilled in the rear collar to rotate the tool to predetermined positions. This configuration means the grinder is well-suited to grinding tools like engraving cutters, single-point fly cutters and end mills. Using the grinder to sharpen lathe tools requires a collet shaped to grip the tool (such as a collet with a square hole), or a modified toolholding arrangement in place of the collet workholder, to hold lathe tools (Fig. 38).

Fig. 39: The Quorn is a 'universal' grinder, capable of a wide range of grinding operations. The workhead can be fitted with a tailstock to allow cutters such as taps or reamers to be mounted between centres. The versatility of the Quorn does come at a price, which, for many tools, means setup times can be lengthy.

Fig. 40: The ColyerCaseley grinder retains the useful features of the Quorn for sharpening commonly used tools such as lathe tools and end milling cutters. Plans available from Birmingham Society of Model Engineers Ltd. Photo: ALAN HOOPER.

The Quorn takes the table-less workhead-only design a stage further by extending the range of possibilities for the workhead with facilities for adding a tailstock (Fig. 39) and providing a greater range of movements and adjustments between cutter and wheel. This allows the Quorn and its derivatives to sharpen multi-tooth cutters (including milling cutters, reamers, taps and dies). The Colyer-Caseley (Fig. 40) removes a lot of the complexity of the Quorn, for a range of everyday sharpening tasks, by omitting many of the seldom-used features. That also simplifies the process of setting up the grinder.

Fig. 41: A versatile standalone wheelhead with elevation via a handle, and fine adjustment of height via a knob. Design, construction and photograph by DAVID HAYTHORNTHWAITE.

A different approach is shown in Fig. 41 where the wheelhead has been constructed as a stand-alone unit, capable of being deployed with a range of different baseplates, tables and workheads.

3 Basic Movements

All tool and cutter grinders are designed to sharpen tools by bringing one or more faces of the tool into contact with the wheel, so that material is removed from each face, leaving the cutting edge sharp. That basic concept presents the challenge of positioning each face accurately in relation to the wheel (Figs 42 and 43), then precisely controlling the movement of the tool past the wheel.

Fig. 42: Faces of a lathe tool are positioned in relation to planes representing the bottom of the tool, and the face of the grinding wheel or abrasive stone.

Fig. 43: An axis and two planes are used to define the initial position of the faces and cutting edge of the chisel, in relation to the surface of the wheel, and the rest or table of the grinder.

In part, positioning then moving a tool depends on both the physical arrangement of the wheelhead and the traversing mechanism, as well as the geometry of the workhead.

TOOL REFERENCE PLANES AND AXES

Every tool has surfaces that are related to the faces or flanks that form the cutting edges, or act as reference planes (Fig. 43). Some of these planes can be used to locate the tool in a toolholder. Reference planes might be chosen as the base and side of a lathe tool, as shown in Fig. 42, and the tool can be located accurately by sitting it on its base in a toolholder, with the side sitting against the inner face of a slot in the toolholder. The toolholder can then be swung or tilted to locate the faces, flanks and cutting edges of the tool in relation to the wheel.

Similarly, most tools have at least one axis around which they can rotate. If a tool is held on an arbor, or gripped in a sleeve or collet it may be rotated to orientate the cutting edge at a specific angle, or index from one tooth to another.

There are established methods and setups for sharpening most tools, and those suggest planes and axes convenient for those tasks. The movements of wheelheads, workholders and tables are designed to enable positioning of the tool ready for sharpening.

AXES AND PLANES

Axes and planes are imaginary geometric features that can act as useful references.

An axis is a line that acts as a reference. Every axis is infinitely long, but has no thickness, and exists in just one dimension.

An axis of rotation is a stationary line about which an object can rotate.

A plane is an imaginary flat surface that acts as a reference. A plane exists in two dimensions and is infinitely large.

Real objects exist in three dimensional space, but a two dimensional plane is useful when using a flat surface as a reference. Horizontal and vertical reference planes are also useful, as is the plane of the grinding surface of a flat-faced wheel, or the plane that is tangent to the line where a tool contacts a curved wheel.

The undersides of many tools are flat, so distances and angles can be measured from the plane of the underside. Grinder rests and tables are usually flat, and provide a useful plane of reference common to tool and rest.

SETTING UP A GRINDER

Because tool and cutter grinders take various physical forms, it is important to be able to establish a reference position for work, not least because constantly having to repeat a range of setup instructions will make the descriptions of the various grinding operations in this book very lengthy. Instead, this chapter contains the basic setup instructions for a range of grinders that have different physical characteristics, including grinders with an axis at right angles to a moving table, such as the Stent, Clarkson or Worden; or grinders with a universal workhead that moves in an arc across the face of the grinding wheel, such as the Deckel, U3 Universal or Quorn.

Basic relationships

On any grinder, the basic setup is the starting position where the relationship between the toolholder, the face of the wheel and the various axes of the movable parts of the grinder are established.

Many of the basic relationships between parts of a grinder are created during manufacture, and, depending on the design of the grinder, that may include the orientation of the spindle, the path of a moving slide, or graduation marks that indicate the angle between spindle and workholder. Individual work setups often require angles or motion paths to be set up quickly, with or without reference to graduated scales or markings on the grinder.

Traverse

The work traverses past the face of the wheel, and will be ground parallel to the axis of traverse. This is true even if the face of the wheel or the axis of the spindle is not quite parallel to the axis of traverse (Figs 44, 45 and 46). Tapers can be ground by deliberately setting the axis of the work at an angle to the axis of traverse (Fig. 47).

Fig. 44: Work will be ground parallel to the axis of traverse.

Fig. 45: Although the tool is not at right angles to the traverse, the front cutting edge will be ground parallel to the axis of traverse.

Fig. 46: Work will be ground parallel to the axis of traverse, even if the face of the wheel is not parallel to the traverse.

Fig. 47: A taper can be ground by setting the tool at an angle. Rotating the tool about its long axis while traversing grinds the tip conical.

Setting the axes of traverse and spindle

If the axes of the spindle and the axis of traverse are both fixed, as on the Worden or the Kennet, no adjustment is necessary or possible. Where the spindle can be swung, the two most useful setups are with the axis of the spindle at right angles to the axis of traverse (Fig. 48), or the spindle axis parallel to the traverse (Fig. 49). In either case, the face of the wheel is dressed parallel to the axis of traverse.

Fig. 48: The axis of the spindle set parallel to the axis of traverse.

Fig. 49: The axis of the spindle set parallel to the axis of traverse.

If the spindle can swing, as on the Boxford and Quorn, or the axis of traverse can be altered, as on the Clarkson, the choice is between setting the axes of the spindle and the table using the graduations on the machine, or adjusting as below:

Fig. 50: Mount a DTI on the table or in a stationary workhead with its shaft horizontal and its foot against the setting bar.

• Mount a plain cylindrical bar in the spindle, by gripping it in a collet or by using a bar with a taper to fit the spindle (Fig. 50). Swing and/or tilt the spindle and set it approximately horizontal, and parallel to the axis of traverse.

Fig. 51: Use a round-ended foot on a DTI, against a flat face, and a large, flat, elephant’s foot against a cylindrical bar.

Fig. 52: Mount a DTI on the table or in a stationary workhead with its shaft vertical and its foot against the setting bar.

• Choose a plunger-style Dial Test Indicator (DTI) equipped with an elephant's foot (a large diameter, flat-ended foot) for use against a cylindrical face (Fig. 51), and mount it on the table or in a stationary workhead. Position the DTI so that its plunger is horizontal, with its foot resting against the side of the spindle bar, then lock the workhead (Fig. 52).

• Traverse back and forward and note the readings on the DTI. Adjust the spindle swing until the reading remains unchanged during the whole length of traverse. That is a zero position. Adjust the spindle swing scale reading to zero at this position, or to 90 degrees if the work is to point towards the wheel.

• Reposition the supporting arm of the DTI so that its plunger is vertical and its foot rests against the top of the bar (Fig. 52).

Fig. 53: A digital angle gauge can be used to set the spindle tilt angle to zero. If possible, use a gauge with a vee-shaped underside.

• Traverse back and forward, adjusting the tilt until the reading remains unchanged. Set the tilt scale reading to zero. Alternatively, use a digital angle gauge. Zero the gauge on the table surface; sit the gauge on the bar and adjust the tilt until the gauge reads zero (Fig. 53).

Some digital angle gauges have magnets and a vee on the bottom to sit securely on a cylindrical bar. Accuracy is typically +/− 0.2 degrees.

Aligning a workhead

A workhead can be aligned using the same process as for a spindle, but the DTI must be mounted on a fixed part of the grinder:

Fig. 54: Mount a DTI on a fixed part of the grinder, with its shaft horizontal and its ball-ended foot against the flat face of the setting bar held in the workhead.

• Use the workhead to grip a parallel bar and set it approximately horizontal and parallel to the axis of traverse (Fig. 54).

• Mount a DTI horizontally on a fixed, stationary part of the grinder, and not on the table. Attach a suitable shape of tip to the DTI, using a round-ended tip against a flat tool side, or a large, flat, elephant's foot against a round bar (Fig. 51).

• With the ball-ended foot of the DTI against the flat face of the bar or tool (Fig. 54), traverse, noting the reading on the DTI, and adjust the swing of the workhead until the reading stays constant throughout the traverse. That sets the zero position for the swing.

Fig. 55: Some work holders can be aligned using an engineer's square held against the reference surfaces for the tool.

• Alternatively, it is often possible to grip an engineer's square in the workholder, then align the blade of the square against a setting bar in the spindle (Fig. 55).

Fig. 56: With the DTI standing vertically and attached to a fixed part of the grinder, traverse, keeping the foot of the DTI in contact with the tool or bar.

• Mount the DTI vertically, on a fixed part of the grinder, and set the foot against the top surface of the tool (Fig. 56). Traverse, adjusting the workhead tilt until the reading stays constant throughout the traverse. That sets the zero position for the tilt.

Setting a zero position for rotation or for indexing

Preparation for establishing a zero position for rotation usually begins by setting some feature, like a face or a cutting edge, horizontal or vertical. The reference will normally be either the surface of the table or the face of the wheel. The surface of the table can serve as a horizontal reference, and an engineer's square may be used to set an edge or a tooth vertical, by visual comparison (Fig. 57) or by contact (Fig. 58). Alternatively, use a height gauge fitted with a transparent plastic screen that has been scribed with horizontal or vertical lines (Fig. 59) aligning a tooth or an edge vertically or horizontally by eye. A different method is to set a tooth or the tip of a tool on the horizontal centre line by rotating the cutter down onto a pre-adjusted centre-height gauge (Fig. 60). Once aligned, the angular scale on the axis of rotation can be set to zero. There are further details for specific tools in the Sharpening section.

Fig. 57: A cutting edge can be set vertical by visually aligning it with the edge of an engineer's square.

Fig. 58: A cutting edge can be set vertical by bringing it into contact with the edge of the blade of a square.

Fig. 59: A cutting edge can be set vertical or horizontal by comparison with the lines engraved on a clear plastic panel held on the arm of a height gauge.

Fig. 60: A tooth can be set against a centre-height gauge.

Once set to a vertical or horizontal position, the scale on the axis of rotation can be used to turn the tool to any other angle. An alternative method is to set a tooth or cutting edge against a preset centre-height gauge, then calculate the vertical distance the tooth must rise or fall in order to create a clearance angle. Set an adjustable height gauge to the resulting new height, and set the tooth or cutting edge against that gauge. The section entitled ‘Wheel and Cutter Heights’ discusses how to set the vertical distance.

Setting the work axis on centre

For some sharpening operations, the axis of the work needs to be set at the wheel centre height, initially. This is particularly important when using a type 1 wheel to grind the cutting edges of fluted cutters like reamers or many types of milling cutter. For other toothed cutters, like circular saws, it is important to be able to set individual teeth at wheel centre height, initially.

Fig. 61: Set the spindle at workholder centre height using two half-diameter setting bars.

Fig. 62: Lower the spindle until the flats on the two setting bars sit against each other.

Fig. 63: A centre-height gauge can be set with the flat top of its finger uppermost. The half-diameter setting bar should be free to rotate.

Fig. 64: A centre-height gauge set with its finger flat facing down is used against upward-facing teeth.

The axis of a workhead, particularly a collet holder, is usually at a fixed height when the tilt is zero. The grinding wheel can be set to the same centre height by using two half-diameter flats on circular bars mounted in the workhead and the wheel spindle (Fig. 61). The diameter of the bars is not important, although larger-diameter bars provide larger flats for ease of use. Set the flats approximately horizontal, but free to rotate, then bring them into full contact with one another by adjusting the height of the spindle (Fig. 62). It is also useful to set two reference centre-height gauges, which can be used later for setting features of the work at centre height. General-purpose height gauges are useful, but a pre-set centre-height gauge with a finger set to the centre height of the spindle allows a tooth or an edge to be set at centre height quickly (Figs 63 and 64). An additional adjustable-height gauge can be set to a specific height using a conventional height gauge, or by comparison with an existing centre-height gauge. The adjustable gauge can be set to any height above or below centre height, as required, and used to set a tooth or edge to that specific height without losing the setting of the fixed centre-height gauge.

WHEEL AND CUTTER HEIGHTS

When grinding against the flat face of a wheel, the height at which the tool contacts the wheel is not particularly important, but grinding against the curved face of a type 1 wheel is another matter, and the relative heights of the axis of the work and the axis of the spindle are critical in most cases.

Fig. 65: The top of a rectangular tool at wheel centre height lies on a radial line.

Fig. 66: Radial lines need not be horizontal, but each can act as an 'artificial horizontal'.

Fig. 65 shows that when a rectangular-section tool, like a typical lathe tool, is aligned with the centre of a curved wheel, the top of the tool lies along a radius (a line drawn from the centre of the wheel to the point of contact with the tool). For convenience, the diagram shows that as a horizontal line, but Fig. 66 shows that radial line does not have to be truly horizontal, but can act as an ‘artificial horizontal’. It is convenient to simply call this the ‘horizontal’. On any circle (such as the outer circumference of the wheel) the tangent line is at 90 degrees to the radius, and touches the circle at just one point as it passes by. In this initial position the top and tip of the tool are aligned with the centre of the wheel and lie on a radial line. The front face of the tool is aligned vertically with the tangent to the wheel, and unless the tool is pressed into the wheel, the wheel face will not touch the front face below the horizontal, because the wheel curves from above towards the tangent point, then curves away below that point.

Fig. 67: Raising the tool vertically causes the wheel to grind the area below the tip, creating clearance.

Fig. 68: Lowering the tool will cause material to be ground from the area at the top of the vertical face.

Raising the tool vertically, keeping the top front edge of the tool against the wheel, raises the point of contact between the top edge of the tool and the wheel (Fig. 67). The ‘horizontal’ is the radial line between the centre of the wheel and the top surface of the tool. Raising the tool vertically (parallel to the tangent), then moving the tool horizontally until the tip touches the wheel will cause the wheel to grind a portion off the lower section of the tool face (Fig. 67). That can be useful, as it creates front clearance without having to tilt the tool. The more the tool is raised vertically, the greater the effective clearance angle. Similarly, lowering the tool parallel to the tangent will cause the wheel to grind a portion off the upper section of the tool face (Fig. 68).

Fig. 69: On most tools, a secondary clearance angle prevents the face from rubbing against the work. The wheel grinds a narrow primary clearance.

It is true that grinding in this way produces a hollow front face (Figs 67 and 68), but most tools are designed to contact the revolving work for only a short distance below the cutting edge, as the tool penetrates the work. The remainder of the front face is usually ground back at a secondary clearance angle, to take it clear of the work (Fig. 69). That is the case with most cutting edges and most tools, and is not restricted to lathe tools. There is more on the shape of cutting edges, and the associated clearances, in the Sharpening sections, where there are further details of how to sharpen the cutting edges on different types of tools.

Fig. 70: The clearance angle is related to the radius of the wheel (r) and the height the cutting edge of the tool is raised above the horizontal centre line (h).

Fig. 71: A negative clearance angle is related to the radius of the wheel (r) and the height the cutting edge of the tool is lowered below the horizontal centre line (h).

Figs 70 and 71 show the relationship between the height above the radial line of contact (the ‘horizontal’) and the angle created on the front face of the tool is:

h = r × SinA (where A is in degrees)

or

h = D / 2 × SinA (where A is in degrees)

For example:

on a 200mm diameter wheel, grinding an angle of 5 degrees would require:

h = 100 × Sin5 = 100 × 0.087 = 8.7mm

Set the tool horizontal and at wheel centre height, then raise the tool 8.7mm (or lower the wheel) vertically.

Table 9: Heights above centre to create clearance angles at the tip of a tool, when using different diameters of wheel.

Table 9 lists the angles produced by raising a tool vertically (or lowering the wheel) to create a front clearance angle at the tip, for a range of different wheel diameters. See also Further Information: Table 9A.

STRAIGHT TEETH ON CIRCULAR CUTTERS

Some circular cutters, such as slitting saws and some milling cutters, have individual teeth set on the periphery, with the cutting edges parallel to the central axis of the cutter (Fig. 72). The challenge with these cutters is in positioning the teeth against a grinding wheel to produce the correct rake and clearance angles. There is more on this in the sections dealing with specific types of cutters, but there are some useful general principles and techniques that apply to all cutters of this type. These cutters may be sharpened against either a flat or curved wheel face.

Fig. 72: A cutter with peripheral teeth and cutting edges parallel to the axis of the cutter.

Setting up against a flat wheel

Fig. 73 shows the relationship between a tooth on a circular cutter and the flat face of a wheel, with the cutting edge of the tooth set at the same height as the centre of the cutter. The wheel face is on the cutter tangent line, and the tooth face will be ground tangent to the cutting edge, so there will be no clearance behind the cutting edge of the tooth. The clearance may be created by rotating the cutter through the clearance angle C degrees (Fig. 74). However, not all workheads are capable of easily setting a small angle accurately, so an alternative is to calculate the distance the tip of the tooth should drop (move downwards) as the wheel is rotated, and measure the drop instead of the angle. The centre of the wheel remains at the same height. The relationship between the clearance angle (C degrees), the radius of the cutter (r), and the amount by which to lower the tip of the cutter (h) is:

h = r × SinC

Using the diameter (d) of the cutter instead of the radius:

h = ½ × d × SinC

For example:

when the cutter diameter is 75mm and the clearance angle is to be 7 degrees:

h = 0.5 × 75 × Sin7

h = 0.5 × 75 × 0.121869

h = 4.57mm

Fig. 73: A tooth positioned against a flat-faced wheel, at wheel centre height.

Fig. 74: Clearance can be created by rotating the cutter through the clearance angle.

Table 10: Tooth drop height below centre when sharpening a toothed cutter against a flat-faced wheel.

Table 10 lists angles of rotation and drop distances for common cutter diameters when grinding a toothed cutter against a flat-faced wheel. See also Further Information: Table 10A.

Setting the drop

Before applying any drop, the cutting edge should be set to contact the wheel when the cutting edge is on a horizontal plane at right angles to the face of the wheel and passing through the centre line of the cutter (Figs 73 and 75). Identify that point with the help of a centre-height gauge. To create the drop, an adjustable height gauge may be set away from the grinder to centre height minus the drop (Fig. 76), then placed on the grinder table. Rotate the cutter until the cutting edge rests on the finger.

Fig. 75: The clearance angle can be set by measuring the distance the tip of the wheel drops as it rotates through the clearance angle.

Fig. 76: Set an adjustable height gauge (seen on the left) to the cutter centre height minus the drop required to create the clearance angle at the tip of the tooth. The blue outline shows the initial on-centre setup, for comparison.

Setting up against a curved wheel

To avoid fouling an adjacent tooth when using a flat-faced wheel, clearance can be created by using a curved wheel (Fig. 77) and raising the centre of the wheel above the common line of centres, without rotating the cutter. The centre of the grinding wheel remains unchanged. The relationship between the clearance angle (C degrees), the radius of the abrasive wheel (rg) and the amount by which to raise the centre of the wheel (h) is calculated in much same way as when setting up against a flat wheel, except that the radius and diameter are of the grinding wheel and not the cutter:

h = rg × SinC

where h is the height, rg is the radius of the grinding wheel and C is the clearance angle. Using the wheel diameter (Dg) instead of the radius:

h = ½ × Dg × SinC

For example:

when the wheel diameter is 100mm and the clearance angle is to be 5 degrees:

h = 0.5 × 100 × Sin5

h = 0.5 × 100 × 0.08716 = 4.36mm

Fig. 77: A clearance angle can be created by setting a toothed cutter at centre height against a curved wheel, then raising the wheel vertically. If the cutter is inverted, lower the wheel by the same distance.

Table 11: Wheel height above centre when sharpening a toothed cutter against a curved wheel. If the cutter is inverted, lower the wheel instead.

Table 11 lists clearance angles and the associated increase in wheel height for a range of grinding wheel diameters when grinding a toothed cutter against a curved wheel. See also Further Information: Table 11A.

WORKHEADS

Workheads are adjustable so that the faces of the tool can be presented at the correct angle for grinding. The main movements in a workhead are:swing, tilt and rotate (abbreviated to S, T and R, for convenience). Fig. 81 shows a tool in an initial reference position, with the long axis of the tool at right angles to the axis of traverse.

Fig. 81: A typical initial reference position has the long axis of the tool (dashed red line) at right angles to the axis of traverse (solid red line).

Swing

Swing is a rotation around an axis normal to the reference plane of the tool (Figs 82–86). Usually, swing is used to bring the cutting edge parallel to the axis of traverse and the face of the wheel (Fig. 82).

Fig. 82: 'Swing' is a rotation around an axis normal to the reference plane of the tool, and is often used to bring the tool cutting edge parallel to the traverse.

Tilt

Tilt is a rotation around an axis that is parallel to the axis of traverse (Fig. 83). In practice, once a tool has been swung to bring the cutting edge parallel to the traverse, tilt is often used to create the clearance on the adjacent flank or face.

Fig. 83: 'Tilt' is a rotation about an axis parallel to the axis of traverse (blue arrow). Tilt is often used after a swing, to create clearance on a face.

Rotate

A rotation may be around any axis, but it is very useful when the axis of rotation is the long axis of the tool (Fig. 84). ‘Rotate’ allows the reference face of a tool to be set horizontal or vertical. It is also useful when indexing from one tooth to another, or when rotating to generate a shape like the conical point on a punch.

Fig. 84: 'Rotate' is often used to rotate the tool about its own axis and is useful in setting the reference face of a tool, or when indexing.

NORMAL

Right angles

A right angle is 90 degrees, and is measured in 2 dimensions. So a line could be at right angles to a plane when measured in one position (Fig. 78), but at a different angle when measured in another position (Fig. 79). On a drawing, right angles are often denoted by a small square.

Fig. 78: These two arms appear to be at right angles to the base, in this view.

Fig. 79: Seen from the side, only the arm of the engineer's square is at right angles to the base.

Normal

A line that is normal to a plane is at right angles to the plane, no matter where the angle is measured (Fig. 80).

Fig. 80: A line is normal to a plane if it is at right angles to the plane, in all directions.

Defining the angles between faces and edges

There is more than one system for defining the angles between the various parts of a tool, but this book uses the Tool-in-Hand system from British Standard BS1296 - 2: 1972. For most practical purposes, that defines the angles of faces as measured at right angles to the cutting edge between the faces. So, for example, on the lathe tool shown in Fig. 85, taking the reference as the horizontal plane of the base of the tool, the clearance angle of the Major (side) flank is measured as shown between the green line in the vertical plane and the red line that is on the flank at right angles to the Major cutting edge. The top rake is measured between the dark blue line in the horizontal plane and the bronze line that is on the top face and at right angles to the Major cutting edge. On the tool shown in Fig. 86 there are two cutting edges, each with an associated flank. The top face of the tool is common to both cutting edges. The clearance angle of each flank is measured in the same way as for the tool shown in Fig. 85. Each cutting edge produces a top rake angle on the top face, causing the top face to lie at a compound angle.

Fig. 85: Clearance and rake angles are measured from a vertical or horizontal plane to a line lying along a flank or face at right angles to the cutting edge.

Fig. 86: On a tool which has two cutting edges, the top face will lie at a compound angle affected by the top rake angle of each cutting edge.

S-T-R workheads

An S-T-R workhead has the axis of swing above the axis of tilt (Fig. 87), and the axis of tilt is usually set parallel to the axis of traverse, so it is easy to swing until the cutting edge is parallel to the axis of traverse independently of the axis of tilt. The workhead can then be tilted to set the angle of the face or flank (Fig. 87).

Fig. 87: The basic S-T-R workhead has the axis of swing (yellow line) above the axis of tilt (blue line). The toolholder may be rotated independently (green axis) to initially set the tool reference surface horizontal or vertical.

Rotation within a workhead is usually about the tool's own axis, and is often used before other movements, to set the tool reference surface horizontal or vertical or at another angle to suit the tool geometry.

S-T-R workheads are found on the Worden and Deckel/Universal/Quorn grinders.

The S-T-R workhead is used predominantly throughout this book, because of the relative simplicity of the setups, and because, for most jobs, it does not require that the grinder has a swinging table or a sub-table. S-T-R and T-S-R workheads (see below) can often be used interchangeably for sharpening the same tools by using different tool reference positions, such as holding the tool base horizontal (S-T-R) or vertical (T-S-R).

T-S-R workheads

A T-S-R workhead has the axis of tilt above the axis of swing (Fig. 88), so tools are tilted before swinging. That can be useful for some setups such as grinding a face at a compound angle that is not at right angles to a cutting edge. The compound rake angle on a lathe tool, for example, can be set up more easily on a T-S-R than on an S-T-R workhead, despite the setup for the flanks and cutting edges being simpler on the S-T-R. T-S-R workheads are commonly found on industrial grinders, where the swing is provided by the table. In this book, the T-S-R workhead is shown where it is the most appropriate choice for the task on any particular tool.

Fig. 88: The basic T-S-R workhead has the axis of tilt (blue line) above the axis of swing (yellow line). The toolholder may also be rotated independently (green axis) to initially set the tool reference surface horizontal or vertical.

Grinders with a tilting table

Grinders such as the Worden and Kennet incorporate a tilting table. The axis of tilt is parallel to the axis of traverse, so if the workholder provides swing, this combination provides movements equivalent to an S-T-R workhead (Fig. 89).

Fig. 89: On a Worden grinder, the workhead swings on the surface of the table (yellow axis), but tilt (blue arrow and angle) is provided by the table (blue axis).

Grinders with a swinging table or sub-table

Grinders such as the Clarkson and many industrial models have a swinging table that pivots around an axis normal to the surface of the table. Many of these grinders have a tilting workhead that cannot swing, so the combination of workhead and table is equivalent to a T-S-R workhead (Fig. 90).

Fig. 90: The table on a Clarkson Mk1 grinder can swing around its central bolt. When coupled with a workhead which can tilt, and allows independent rotation of the tool, the combination is equivalent to a T-S-R workhead.
Photo: DEBI CARTER.

Composite moves

Swing, tilt and rotate movements of a point or a face take place in 3-dimensional space, and the sequences of moves may be important. In some cases, changing the order of the movements results in the point or face ending up at a different place or at a different angle. It is important to either follow the sequence of moves suggested in the examples in this book, or, at the very least, to think through the outcome of a sequence.

Fig. 91: On an S-T-R workhead, after a swing to bring the Major cutting edge of the tool parallel to the axis of traverse, the orange line shows that the Major cutting edge is parallel to the axis of tilt (blue line). The distances from the axis of tilt to the outer corners of the cutting edge (the grey and brown lines) are equal.

The main difference between the S-T-R and T-S-R workheads lies in the effect of the second movement. Fig. 91 shows an S-T-R workhead holding a lathe tool. The workhead has been swung to bring the Major cutting edge of the tool parallel to the traverse (which is the same as the front of the base plate, in this photo). After the swing, the traverse, Major cutting edge and tilt axis are all parallel. The distances from the axis of tilt to the corners of the cutting edge are equal, at this stage, as shown by the grey and brown lines.

Fig. 92: On a T-S-R workhead, after a swing to bring the Major cutting edge of the tool parallel to the axis of traverse, the orange line shows that the Major cutting edge is not parallel to the axis of tilt (blue line). The distances from the axis of tilt to the outer corners of the cutting edge (the grey and brown lines) are not equal.

The same swing on a T-S-R workhead is shown in Fig. 92, where the Major cutting edge has been brought parallel to the axis of traverse (the front of the baseplate). However, because the axis of tilt moves with the swing, it is no longer parallel to either the traverse or the Major cutting edge.

Fig. 93: Using an S-T-R workhead, an initial swing sets the tool cutting edge parallel to the traverse (orange line). Because the axis of tilt is parallel to the traverse, tilting the tool places the cutting edge along a cylindrical path, where the cutting edge remains parallel to the traverse (blue and orange lines) and the face of the wheel (shown here as parallel rectangular planes).

Fig. 94: Using a T-S-R workhead, an initial swing sets the tool cutting edge parallel to the traverse (orange line). Because the axis of tilt is not parallel to the traverse, tilting the tool places the cutting edge along a conical path, where the tip of the cutting edge travels further than the rear corner (red line). The edge-on view of the parallel planes shows that the rear corner will be ground away as a result.

The result is that a subsequent tilt will now produce a different effect on each workhead. Because the S-T-R axis of tilt is parallel to the cutting edge, tilting causes the cutting edge to lie on the surface of a cylinder (Fig. 93). Drawing parallel vertical planes that represent the face of a grinding wheel in contact with the cutting edge shows that the cutting edge always lies against that face. On the T-S-R workhead, on the other hand, the axis of tilt is not parallel to the cutting edge, so tilting causes the cutting edge to lie on the face of a cone (Fig. 94). Drawing parallel vertical planes shows that as the cutting edge swings upwards, the rear corner moves further away from the plane than the front corner. Grinding to sharpen the whole cutting edge grinds the rear corner back as the wheel moves far enough towards the tool to touch the front corner. That means the angle at the tip of the tool changes depending on the amount of tilt. The effect on a lathe tool may be relatively small, but, if the tip angle is important (as it will be on a threading or vee-shaped tool), the angles will not be as intended.

Fig. 95: Turning a lathe tool through 90 degrees allows a T-S-R workhead to behave like an S-T-R workhead. The tilt axis can be used to bring the (orange) cutting edge vertical; swing then brings the flank parallel to the traverse and the face of the wheel.

The S-T-R arrangement produces true angles, but T-S-R workheads and swing-table grinders require some forward planning so that the final ‘tilt’ takes place around an axis parallel to the axis of traverse (and the face of the wheel). In the case of a lathe tool, mount the tool on its side with its reference face (the base) vertical, as shown in Fig. 95. Swing and tilt are then effectively swapped, so tilting the workhead brings the cutting edge parallel to the face of the wheel, but does not change the position of the swing axis. The next movement is a swing, either of the workhead or the table, which creates the clearance angle. The Major flank will then be ground at the correct angle to all three axes, and the cutting edge approach angle will not be changed by the ‘tilt’ movement. Throughout this book, differences in setup and in the use of the axes of swing, tilt and rotate are highlighted, so that tools may be successfully ground on either an S-T-R or T-S-R workhead, or a swing-table grinder.

GRINDING A RADIUS

In mathematics, a radius is a line from the centre of a circle to any point on its circumference. In tool grinding, the term ‘grinding a radius’ refers to grinding part of the circumference of a circle on a tool by swinging the tool around a centre point while the face of the grinding wheel is one radius away from the centre point (Fig. 98).

Fig. 96: Swinging around the centre of rotation creates an arc with radius the distance to the wheel.

Fig. 97: Swinging the tool about an axis of rotation in the centre of the rotary table grinds a circular arc.

The basic setup uses a flat-faced grinding wheel, and a tool whose reference surface lies on a plane at right angles to the wheel. Fig. 97 shows a tool with a curved end (a scraper) held on a rotary table. The surface of the table is at right angles to the face of the wheel, and the radius is measured from the centre of rotation that lies on an axis at right angles to the surface of the rotary table and parallel to the face of the grinding wheel.

Fig. 98: A view from above shows the tool (blue), a reference edge (red), and a centre of rotation halfway across the tool. Swinging through a half-turn grinds a semi-circular curve.

Fig. 99: A view from above shows that if the centre of rotation is positioned one radius away from the face of the wheel, a quarter-turn rotation will grind a quarter-circle on the corner of the tool.

Fig. 100: Begin with the centre of rotation further away from the wheel than its final position. Repeatedly feed forward, and grind, until the centre is in its final position, one radius from the wheel.

Fig. 101: A view from above shows the tool (blue), the centre of rotation, and the curve which will be ground (yellow) as the tool rotates past the wheel. Half a turn of the tool grinds a circular arc on the nose of the tool.

Figs 98–101 are views from above, showing the general arrangement for a rectangular lathe tool, with the yellow dashed line representing the circumference of the circle, and the red line being a visual reference for one edge of the tool. Swinging the tool about the centre of rotation takes the end past the wheel and the curve that is ground varies depending on the distance to the wheel, and the position of the centre of rotation within the work. Grind in stages to avoid excessive depths of cut (Fig. 100).

Tilting the axis

When the axis of rotation is parallel to the face of the wheel, rotating the tool creates a vertical cylindrical surface. If the axis of rotation is tilted backwards or forwards as shown in Fig. 102, the ground surface will be conical, providing a constant clearance angle, equal to the angle of tilt, around the nose of the tool. However, if the tool is tilted but the axis of rotation remains parallel to the face of the wheel, the ground surface will be cylindrical in the tilted position, when viewed from above, but elliptical when the tool is laid back onto its base (Fig. 103). The work is horizontal at diagram 1, and inclined at diagram 2. Diagram 2 also shows the work in two positions, half a turn apart, so each point on the edge moves up and down as the work rotates. At two points, the edge will be vertical against the face of the wheel (between A and B at 3) and parallel to the axis of rotation. Viewed from above, at diagram 4, the work appears circular, but when laid flat, at diagrams 5 and 6, the curve is elliptical, and the upper and lower surfaces are staggered, as these correspond to the upper and lower surfaces shown at diagram 3. Diagram 5 shows that this curve can be used to provide an angle on the Major flank that reduces as the curve runs around the edge, blending into flat sides a quarter of a turn around in either direction. The blend line is tilted at the angle at which the work is tilted from the horizontal, at diagrams 2 and 3.

Fig. 102: Tilting the axis of the work creates a conical edge. The yellow section is the imaginary cone top.

Fig. 103: Tilting the work but rotating it about a vertical axis creates a changing angle around the edge. The tilted work is cylindrical when viewed from above, but elliptical when laid horizontally.

Internal curves

Grinding a consistent external radius down the thickness of the work requires a vertical spindle and a cylindrical wheel or, for smaller curves, a ‘pin’ that spans the vertical face of the work. Figs 104 and 105 show that if the centre of rotation is located on the far side of the axis of the wheel, swinging will produce an internal radius. This requires a wheel with a smaller radius than the radius of the curve. As with an external curve, changing the position of the centre of rotation in relation to the grinding wheel, changing the radius, and altering the initial position of the tool, all results in a range of different internal curves (Figs 104 and 105).

Fig. 104: Using a cylindrical pin, mounted vertically, and swinging the work produces an internal curve with a radius equal to the distance from the centre of rotation to the outer surface of the pin.

Fig. 105: Different internal curves can be produced by varying the initial position of the work in relation to the centre of rotation and the distance from the centre of rotation to the furthest point on the face of the pin.

As with external curves, a vertical axis creates a curve with a cylindrical face, as on an external curve with a vertical axis. When the axis of rotation is tilted, the face of the internal curve will lie on the surface of an imaginary cone. That is useful when creating clearance on the Major flank of an internally curved tool.

4 Sharpening in Practice

In this section, you will find practical details of how to sharpen eight types of precision metalworking tools typically found in an engineering workshop. Keeping tools sharp will help improve the surface finish on work that is machined in the lathe or the milling machine, and may well lead to more accurate work. Where necessary, tools can be modified by grinding, to alter the cutting edges to create special-purpose tools, or tools that perform better under specific conditions.

The first three chapters of this book provide information about abrasives, tool and cutter grinding machines, and the basic movements required to position cutting edges to recreate cutting angles and clearances. The tool types in this current chapter illustrate a range of sharpening techniques. Although it is possible to simply follow the instructions for each tool, there is more to be learned by treating each set of procedures as a useful addition to the range of sharpening techniques that can be applied to all sorts of similar tools.

It is possible to dip into the tool types that are of immediate interest, but there is progression in the techniques used, from type one to type eight, and a recommended plan would be to read the first three chapters, then skim read all eight tool types, returning to read in more depth the tool types of immediate interest.

Fig. 106: A modified, home-constructed Brookes-Stent grinder made by Jim Schroeder. Photo: JIM SCHROEDER.

There are two principal workhead types, as discussed in Chapter 3, and both are in common use. Each has its own strengths, so it is important to understand the differences in their geometry and the resulting ways they move a tool to position the cutting edge. Where possible, examples of typical setups have been given for both types of workhead, for each tool. However, rather than presenting complex geometric theory, the methods recommended for positioning a tool ready for sharpening are based on some simple procedures that can be applied to all the tools in this section, as well as more generally to most other tool types.

There are many illustrations of accessories in use, throughout this section. These are used to hold tools in particular ways and to enable movements of the workhead to allow cutting edges to be positioned easily, to recreate the cutting edge geometry. Many of these accessories can be bought off the shelf, but others are not commercially available, so must be self-made. There are no specific drawings or instructions for their manufacture, but the photographs and drawings show their purpose and their major features. If you feel inclined, and have a decent workshop, you may find pleasure in making these items. If not, simply use something that performs a similar function; or modify the procedures to suit the equipment you have at hand.

The ability to sharpen precision metalworking tools is a considerable achievement, and there is joy in exercising the skills involved, as well as reward from the results.

ARROW COLOUR KEYS

Throughout this book, coloured arrows are used to indicate the fundamental movements of swing, tilt and rotate.

Yellow arrow – swing
Blue arrow – tilt
Green arrow – rotate

An arrow with a dashed line indicates an optional movement. The arrowheads indicate the direction, but if a line has two arrowheads, that means the direction is uncertain. In those cases, there is guidance in the accompanying instructions for that operation.

Lines with no arrow indicate axes, and the dashed sections indicate the axis continuing inside the workhead.

Fig. 107: On this S-T-R workhead, the yellow line indicates the swing axis, and the yellow arrow indicates a swing movement in the direction of the arrow. The blue line indicates the tilt axis, and the blue arrow indicates a tilt in the direction of the arrow. The green line indicates the rotate axis, and the green arrow with the dashed line indicates an optional rotate movement. The two green arrowheads indicate that the rotate movement could be in either direction (but there will be guidance in the accompanying instructions).

Tool type 1:
Simple flat faces

Tools such as chisels, gravers, many screwdrivers, and some lathe tools and simple milling cutters have cutting edges formed between flat faces (Fig. 108),and are relatively easy to sharpen using basic movements. Tools that have more complex geometric properties are found in other chapters.

Fig. 108: Many tools have cutting edges formed between flat faces.

Flat faces can be ground by mounting a flat-faced grinding wheel, then setting each tool face parallel to the axis of traverse, and at right angles to the axis of the grinding spindle. For each tool, establish an initial reference position by setting an axis or a face of the tool in a known position in relation to the spindle of the grinder.

CHISELS

A chisel usually has two faces, with a cutting edge where the faces intersect. Grinding the two faces produces a fresh cutting edge. There is the odd exception, such as a diamond-faced chisel, but the principle remains the same, with each cutting edge formed by the intersection of two faces.

Fig. 109: The included angle of a cold chisel is the angle between the faces behind the cutting edge.

The included cutting angle of a cold chisel (Fig. 109) is typically 60 degrees for cutting relatively soft material such as aluminium,70 degrees for cutting soft steel, or 90 degrees for cutting hard steel. A smaller angle allows the chisel to penetrate soft work relatively easily, but a larger angle makes the tip of the chisel stronger and better able to withstand the higher pressure required for harder materials. Chisels may also have a secondary relief angle to provide clearance for cut metal passing over the tip of the chisel.

Holding a chisel

The shank of a chisel is not always precisely formed, and on a chisel with a hexagonal shank there is no guarantee that any of the flat sides will be aligned with the cutting edge of the chisel. Put the chisel in a circular sleeve and clamp it using grubscrews (Fig. 110). Small-diameter sleeves may be gripped in a collet, but larger sleeves may need a purpose-made clamp (Fig. 111). Before finally tightening, set the cutting edge vertical by eye, comparing the cutting edge with the vertical blade of an engineer's square (Fig. 112), or a line scribed on a plastic plate held in a height gauge.

Fig. 110: Chisels may be gripped in a circular sleeve.

Fig. 111: Larger sleeves may need a different holder.

Fig. 112: Set the cutting edge vertical by comparison with the blade of a square.

Sharpening a chisel against a flat-faced wheel

• Mount, then dress, a type 6 cup wheel of 60 or 80 grit.

Fig. 113: Set the chisel in a reference position.

• Set the chisel in a reference position by aligning its long axis horizontal (usually parallel to the surface of the table) and parallel to the axis of traverse (Fig. 113).

Fig. 114: Swing through half the cutting angle, to set one primary face parallel to the axis of traverse.

• Swing through half the tip cutting angle, to set one primary face parallel to the axis of traverse (Fig. 114).

• Apply a cut towards the wheel, then traverse to grind.

• Rotate the chisel about its long axis, through 180 degrees, then traverse to grind the other primary face.

• Repeat until the cutting edge is sharp.

SCREWDRIVERS

A classic screwdriver has two angled faces and a flat end between the faces. The head may be the same width as the shaft, termed ‘cabinet-style’, or the tip may be flared (Fig. 115). A hollow-ground tip (or head) has parallel faces with blending curves where the faces meet the shaft (Fig. 116) which require a different grinding technique.

Fig. 115: The tip of a screwdriver may have the same width as the diameter of the shaft, but an alternative style of head has wider flared shoulders.

Fig. 116: A hollow-ground tip has parallel faces.

Holding a screwdriver

A screwdriver needs to be held securely, but with provision for indexing through half a turn. A vee block and clamp provide a secure way of gripping most screwdriver shafts. Circular-section shafts may not have a uniform diameter along the their length, so use two clamps, for safety. Circular-section cabinet-style screwdrivers may be held in a sleeve (Fig. 117), but screwdrivers with a flared tip cannot. For taper-ground cabinet-style screwdrivers, indexing can be arranged by gripping them in a square-section sleeve (Fig. 118), or by mounting a square-section collar on the shaft, between the handle and the sleeve (Fig. 119). Flared-tip screwdrivers will need to be set in the reference position twice; once for each face. That is no great hardship, given there are only two faces.

Fig. 117: A cabinet screwdriver gripped in a sleeve and held in a vee block. A square-section collar mounted on the shaft makes indexing easy.

Fig. 118: A square-section sleeve facilitates indexing.

Fig. 119: Set the shaft in a reference position.

Setting a reference position

• Set the wheel spindle axis at right angles to the axis of traverse. Mount, then dress, a type 6 cup; type 11 taper cup; or type 12 saucer wheel of 60 or 80 grit.

• Set the screwdriver in a reference position, with the shaft horizontal and parallel the to axis of traverse (Fig. 119).

Restoring the end face

The flat end at the tip of a screwdriver is best ground with its broad edge horizontal. That way, the end face will be ground parallel to the traverse, and any vertical tilt errors in the alignment of the blade will have minimal effect because they occur across the narrow dimension of the face.

Fig. 120: Set the blade horizontal against the arm of a height gauge.

• Set the blade horizontal, perhaps by rotating the screwdriver about its own axis and nestling one face of the blade against the horizontal arm of a height gauge (Fig. 120).

Fig. 121: Swing to set the shaft at right angles to the axis of traverse.

• Swing to set the screwdriver shaft at right angles to the direction of traverse and facing the flat face of the wheel (Fig. 121).

• Apply a cut towards the wheel, then traverse to grind. Repeat as necessary.

For a hollow ground blade this is normally sufficient, unless the length of the straight part of the blade is now too short. However, grinding the end face of a taper-ground screwdriver makes it thicker, because of the tapering sides, so the two flat side faces must also be ground, so that the blade still fits the range of slots for which it was designed.

Grinding flat faces

• Create a reference position with the blade sides vertical and the axis of the shaft parallel to the traverse. Then swing the screwdriver through half the tip angle to bring one face parallel to the traverse and the face of the wheel (Fig. 122). If the tip angle is not known, simply set the face against the wheel. Lock the swing.

Fig. 122: Swing through the tip half-angle to set one face parallel to the axis of traverse.

• With the tip clear of the wheel, apply a cut towards the wheel. Traverse to grind.

• Rotate the screwdriver through a half-turn, and grind the other face.

Fig. 123: Position affects the direction of striations.

There is a continuing discussion about whether the striations (scratches) on the ground face of the screwdriver should run parallel to the axis of the shaft, or parallel to the end face (Fig. 123). Grinding along the horizontal centre line of the wheel will produce striations parallel to the end face. Grinding across the top or bottom of the flat face of the wheel will produce striations parallel to the axis of the shaft. Choose whichever suits your own theory of the direction that provides the best grip against the inside of a slot.

Grinding hollow-ground faces

Hollow-ground faces can be ground in two different ways, producing different results.

Thinning the blade without blending

Grinding both faces of the blade will result in a thinner blade, to suit a narrower slot in a screwhead. This is easy to do using a wheel with a flat face, but results in a step at the inner end of each face.

• Set the spindle at right angles to the axis of traverse, then mount and dress a flat-faced wheel.

Fig. 124: Grinding the blend curve produces a step.

• Set the screwdriver in a reference position, with the shaft horizontal and parallel to the traverse, and the blade vertical. Use a collar and spacer to set the projection from the holder (Fig. 124). Remove the spacer before grinding.

• Apply a cut and grind the face of the blade (Fig. 124). This will produce a curved step that end of the blend curve.

• Set a traverse stop at this point.

• Release, then rotate the screwdriver through a half-turn about its long axis, and remount, using the collar and distance piece to set the projection.

• Grind the other face, up to the traverse stop. That should result in a similar step on this side of the blade. This is not essential, but is visually more balanced.

Thinning or extending with blending

Grinding the faces using the curved edge of a small-diameter wheel makes it possible to produce a blending curve at the inner end of each face. Careful control of the depth of grind allows a minimal reduction of the thickness of the blade, although it is difficult to avoid some thinning.

• Mount a small-diameter wheel designed for surface grinding. This looks like a small type 6 cup wheel, but its operating face is the curved periphery. Typically, a 50mm diameter wheel will produce a pleasing blend curve, but any wheel of the same diameter, or smaller than, the original curve will do. The blend curve is simply a pleasing way of making the transition to the full diameter of the shaft.

Fig. 125: Set wheel and traverse axes at 90 degrees.

• Set the axis of the wheel at right angles to the axis of traverse (Fig. 125).

• Set the screwdriver with its blade horizontal and its long axis horizontal and parallel to the axis of traverse.

Fig. 126: Initially, the wheel should just touch the underside of the lower face of the screwdriver.

• Alter the height of the wheel so that it just touches the underside of the lower face of the screwdriver (Fig. 126).

• Retract using the traverse, then apply a cut by raising the wheel.

• Traverse to grind, stopping when the blend curve has been ground. When extending the length of the faces, grind far enough into the blend curve to allow for the end face of the screwdriver to be ground to remove any damage. Set a traverse stop at that point, then use the traverse to retract.

• Rotate the screwdriver through a half-turn about its long axis, and grind the other face.

Fig. 127: Grind the end of the tip against the flat face.

If necessary, grind the end face of the tip, to remove any damage. Use the flat face of the wheel (Fig. 127).

• Swing the workholder so that the long axis of the screwdriver is at right angles to the traverse.

• Alter the wheel height so that the tip is approximately at the wheel centre height.

• With the screwdriver clear of the wheel, apply a cut towards the wheel, then traverse to grind. Repeat as necessary, to remove any damage from the end face.

GRAVERS FOR TURNING

A graver used for turning has one main face and the cutting edges are formed between the front face, which acts as the Major flank, and one of the sides of the graver, which acts as the Major face (Fig. 128). The cross-section of the shank of a graver is often square or lozenge-shaped, but could be circular. The most common method for sharpening a graver is not grinding, but honing. Grinding would only be used where the graver faces were being formed for the first time, or where there is significant damage to the faces or edges, or the sharp point of the graver.

Fig. 128: The cutting edges are formed between the front face and one of the sides of the graver.

Holding a graver for grinding

A vee groove is useful for gripping square- or circular section shafts, and Fig. 129 shows a square-section graver held in a small vee block attached to a spindle in a workhead. The vee block incorporates threaded holes for clamp screws, allowing the graver to be gripped directly in the groove. However, it is often more convenient to grip the graver in a sleeve before clamping it in the block. That way, the graver can be rotated to set one side in a convenient starting position. Fig. 130 shows that one way of setting the angle of the graver front face is to use a template against one side of a square-section graver shaft. A graver with a lozenge-section shank can also be gripped in a square hole in a sleeve, pressed against one side of the hole by grubscrews, then rotated in the same way. The axis of the graver need not be concentric with the axis of the sleeve, but those axes must be parallel, so one side of the graver does need to be pressed firmly against one of the sides of the hole in the sleeve. A graver with a circular-section shank can be gripped in a collet (Fig. 131) or mounted directly in the vee block (Fig. 132) or held in a split sleeve in a lathe-style tool-post (Fig. 133).

Fig. 129: A square-section graver may be held in a small vee block attached to a workhead spindle.

Fig. 130: Holding a graver in a sleeve allows rotation to set the initial orientation of the sides.

Fig. 131: A circular-section graver can be held in a collet.

Fig. 132: If it has a sufficiently large diameter, a circular-section graver can be held directly in a vee block attached to a workhead spindle.

Fig. 133: A circular-section graver can be clamped in a split sleeve in a lathe-style toolpost.

Grinding a graver face

• Mount and dress a type 6 cup, type 11 taper cup, or type 12 saucer wheel, with the face of the wheel set parallel to the axis of traverse. Choose a fine grit appropriate to the state of the faces and cutting edges.

• Establish a reference position by setting the long axis of the shank horizontal and parallel to the axis of traverse. If the grinder crossfeed is sufficiently long, the graver shaft can be nestled against the face of the newly dressed wheel, before retracting enough to allow for a later swinging movement to place the tip against the wheel. Alternatively, a vee block groove, or the rear of the slot of a lathe-style toolpost workholder can be set parallel to the traverse by using an engraved scale or by using a DTI against a test bar in place of the graver.

Fig. 134: Set the point-to-heel axis horizontal.

• Rotate the graver so that the axis from the point to the heel of the front face lies horizontal (Fig. 134). On a square- or lozenge-section shank, the axis runs between two opposite ridges on the shank. On a circular-section shank, the face will be an ellipse and the major axis will run across the widest part of the ellipse. The axis can be aligned by eye, by comparison against the arm of a height gauge or a horizontal line on a clear plastic screen. Alternatively, if the graver has flat sides, rotate the sleeve so that the angles between two adjacent sides and the table surface are equal (45 degrees for a square-sided shank).

Fig. 135: Swing through the tip angle to set the graver face parallel to the traverse and the wheel face.

• Swing the workholder through the tip angle of the graver, to bring the front face parallel to the axis of traverse (Fig. 135).

Fig. 136: On a Quorn/Universal grinder, set the point-to-tip axis vertical, and tilt to align the front face with the traverse and the face of the wheel.

Fig. 136 shows an alternative method on a Quorn or Universal grinder, where the graver point-to-tip axis is set vertical, with the workhead tilted to make the front face parallel to the face of the wheel. Traverse is a sweeping action.

• Use the crossfeed to bring the face close to the wheel. Traverse clear to one side; apply a cut by feeding towards the wheel; then traverse past the wheel to grind the face. Repeat as necessary, until the face is flat and the cutting edges around the sides of the face are sharp.

• Repeat the process, using a fine cBN wheel (for carbide gravers) or a diamond wheel (for high speed steel) to give the faces a good finish. Progress from 260 mesh to 600, 1000 and, optionally, 2000 mesh. In the case of a graver, any marks on the cutting edges and faces will be reproduced on the workpiece, so the faces need to be polished by honing.

Honing the faces

A powered hone (Fig. 137) allows graver faces to be finished quickly, using either fine diamond-coated discs (Fig. 138) or a plain ceramic wheel that can be coated with successively finer grades of diamond paste or spray as honing proceeds.

Fig. 137: GRS Power Hone. Photo: GLENDO LLC.

Fig. 138: Diamond-coated discs with grades from 300 to 1200, and diamond paste in a syringe.

Fig. 139: An adjustable workhead. Photo: GLENDO LLC.

• Mount the graver in an adjustable workhead (Fig. 139), and recreate the angles used to grind the main graver face, so that it lies flat against the wheel.

• Using successively finer grades of wheel, paste or spray, hone the face.
When using a ceramic wheel, spread a dab of paste, or spray a mixture of water and diamond particles onto the wheel. When changing to a finer mesh size, take care to remove all traces of the previous paste or spray.

• The honing process can stop when the main graver face has a mirror finish.

• Hone the sides to produce a similar finish near the cutting edges.

Tool type 2:
Lathe tools

Sharp tools are especially important in a lathe, to allow the tool to cut relatively hard materials such as metals, or soft materials like plastics. Each material is likely to require different, and quite specific, angles on the faces of the tool, and the shape of the tool is often influenced by the way in which it is required to cut the material (Fig. 140). So, for example, a tool required to reduce the cylindrical outer surface of a bar needs to have a different shape to a tool designed to cut inside a cylindrical hole in the end of a bar. Although sharpening is normally undertaken to restore existing tool faces, and to preserve or restore the angles of those faces, there are occasions when grinding is used to form the shape of a tool from a basic rectangular or cylindrical tool blank. To make it easier to discuss and refer to particular faces and edges of tools of different shapes, this book uses terms outlined in British Standard BS1296 - 2: 1972 which defines systems for naming faces, edges and angles of single-point lathe tools (Figs 140 and 141). Most are based on the ‘tool-in-hand’ reference system. Fig. 141 shows the names of the main faces and edges of a general -purpose ‘knife’ tool often used for turning, but the same features are found in most lathe tools. The next page contains definitions of those terms, all of which will be used in descriptions of sharpening procedures.

Fig. 140: Reference planes are used to refer to features of a tool which are used to position the tool for grinding.

Fig. 141: Names of the main features of a right-hand 'knife' tool. These features are found in most lathe tools. The names are chosen from the 'tool-in-hand' reference system employed by British Standard BS1296 - 2: 1972. Although alternative names are often used when referring to some of these features, this book follows the convention used in the diagram. Page 45 contains definitions for each of these features.

LATHE TOOL TERMS

A range of standard terms can be used to refer to parts of a lathe tool. These are illustrated in Figs 141 to 146.

Cutting edge
The cutting edge is forced under the surface of the rotating workpiece, which helps part material from the work. The Major cutting edge does most work, but there may be a Minor cutting edge that allows the tool to cut when travelling in another direction. A knife tool designed mainly to cut when travelling to the side using its Major cutting edge may also be capable of cutting when travelling forwards, taking a facing cut, using the Minor cutting edge.

Face
Cut work (chips or swarf ) flows over a face. The face is often the top surface of the tool.

Flank
A flank is adjacent to the surface of the work. The new work surface passes by the flank, but the cut material passes over an adjacent face. The flank is often the surface running downwards from the cutting edge (Fig. 142).

Fig. 142: Cut material passes over a face and passes by a flank, rubbing just under the tool tip.

Corner (or Nose)
The corner (or nose) is the junction between Major and Minor cutting edges. The corner may be sharp, or it may be curved.

Heel
The heel is the edge between flank and base.

Base
On a tool with a square-section shank, the base is the bottom surface.

Reference plane
The reference plane is an imaginary flat surface that extends infinitely in all directions and is used when positioning the tool ready for sharpening (Fig. 140).

Working plane
The front of the cutting part of the tool, which usually coincides with the workpiece, is the working plane.

Tool reference plane
The base of the tool (or a plane parallel to it) is the tool reference plane.

Tool back plane
The tool back plane is at right angles to both the tool reference plane and the working plane. Note that it is not at the back of the tool.

Cutting edge angle
The cutting edge angle is measured from the working plane to the cutting edge (Fig. 143).

Fig. 143: Viewed from above, each cutting edge lies at an angle to the working plane (usually aligned with the work surface).

Clearance angle
The clearance angle lies between the flank and a ‘vertical’ reference plane containing the cutting edge and is normal to the tool reference plane (Fig. 144).

Fig. 144: The clearance angle is measured in relation to a vertical plane which includes the cutting edge.

Rake angle
The rake angle of the top face of the tool is measured at right angles to the cutting edge from a line parallel to the tool reference plane (Fig. 145).

Fig. 145: The rake angle is measured at right angles to the cutting edge.

‘True’ top rake angle
Where a tool has two cutting edges sharing the same top face, and each cutting edge has an associated top face rake angle, there will be one angle that produces a flat fop face containing both cutting edges. The angle of that face lies between the top face angles, and is called the ‘true’ top rake angle. It is measured from a plane parallel to the tool reference plane, touching the tool tip (Fig. 146).

Fig. 146: The ‘true’ rake angle lies between the two cutting edge angles, on the plane of the top face.

Swing, tilt and rotate
These movements take place around axes as shown in Fig. 147.

Fig. 147: Workhead axes.

HOLDING LATHE TOOLS

Lathe tools with a square-section shank can be held in a slot in a toolholder, a sleeve with a square hole or a vee block (Fig. 148). Cylindrical shanks can be gripped in a sleeve or collet held in a workhead (Fig. 149).

Fig. 148: Tools with square-section shanks may be held in lathe-style toolholders, a sleeve with a square hole, or a vee block.

Fig. 149: Tools with cylindrical shanks may be gripped in a collet or a sleeve in a workhead, a split square sleeve in a toolpost, or a vee block mounted on a workhead.

However the tool is held, it must be possible to set the appropriate reference planes, the faces and the flanks in known positions.

SETTING REFERENCE POSITIONS

Any reference position must take account of the way the workhead axes are set in relation to one another and to the axis of traverse. A good way to begin is by setting the tool in a known reference position in a workhead that has its axes in a known position.

Fixed reference surfaces and axes

The table surface acts as a ‘horizontal’ reference plane, and the face of the spinning wheel acts as a ‘vertical’ reference plane (Fig. 150). On a type 1 wheel, the vertical plane is tangent to the face of the wheel.

Fig. 150: The face of the table acts as a 'horizontal' reference surface. Unless the grinding spindle is tilted, the furthest forward spot on the face of the spinning wheel acts as a 'vertical' reference plane.

The axis of traverse and the axis of the wheelhead spindle are usually set either at right angles to one another (Figs 152 and 154), or parallel (Figs 151 and 153).

Workhead reference position

On a workhead used to sharpen lathe tools, there are typically three axes. Some have more; some have fewer. The orientation of the axes depends on the type of workhead; S-T-R or T-S-R.

S-T-R workhead

Swing sits above tilt, and the axis of tilt is usually set parallel to the traverse. The axis of swing is set normal to the table (Figs 151, 152 and 147). The axis of rotation is usually at right angles to the axis of swing, and is either at right angles to the traverse (Figs 152 and 147) or parallel to the traverse (Fig. 151). Occasionally, it is useful to have the axis of tilt at right angles to the traverse.

Fig. 151: On this S-T-R workhead, the axis of traverse is set at right angles to the spindle axis, but parallel to the face of the wheel. The axis of rotation (the axis of the workholder, in this photo) is often set parallel to the axis of traverse.

Fig. 152: On this S-T-R workhead, the axis of the spindle is set at right angles to the axis of traverse. In this photo, the axis of rotation is set parallel to the spindle axis, and at right angles to the traverse.

T-S-R workhead

Tilt sits above swing, and the axis of swing is set normal to the table and at right angles to the axis of traverse (Figs 153 and 154). The tilt axis is at right angles to the axis of swing, and a useful initial reference position is with tilt parallel to, or at right angles to, the traverse (Figs 153 and 154). The axis of rotation of the tool is at right angles to the tilt.

Fig. 153: On this T-S-R workhead, the axis of swing is set normal to the table and the axis of traverse. The axis of tilt is set at right angles to the swing and the traverse. The axis of rotation is parallel to the traverse and at right angles to the spindle axis.

Fig. 154: On this T-S-R workhead, the axis of swing is set normal to the table and the axis of traverse. The axis of tilt is set parallel to the traverse. The axis of rotation is set at right angles to the traverse and parallel to the spindle axis.

T-S-R workheads sometimes have no swing axis, but are mounted on a sub-table that provides the swing. On a T-S-R workhead, swing affects the axis of tilt, which is undesirable and leads to complex setups involving equally complex geometry. As a result, it is not unusual for a tool to be mounted on its side, so that the actions of the tilt and swing axes are effectively swapped. The T-S-R then acts as an S-T-R workhead.

Tool reference position

Typical reference positions for the tool are with the base (the tool reference plane) either parallel to the plane (the surface) of the table (‘horizontal’) (Fig. 155) or at right angles to the plane of the table (‘vertical’) (Fig. 156). Individual faces or flanks on the cutting part of the tool may initially be set approximately at right angles to, or parallel to, the axis of traverse, depending on the position of the face of the wheel, although their final position will be established later, by using swing, tilt and/or rotate movements.

Fig. 155: A lathe tool gripped in a lathe-style toolholder, with its flat reference surface set horizontal and, initially, parallel to the table. The tool is upright, with its Major face upwards.

Fig. 156: A lathe tool gripped in a lathe-style toolholder, with its flat reference surface set vertical and, initially, parallel to the traverse. The Major face is opposite the wheel.

Tools with flat bottom faces

For many lathe tools, the flat bottom surface is a natural choice for the tool reference plane. The long axis through the centre of the shank usually runs parallel to the flat bottom surface. That makes a lathe-style toolholder a good first choice to hold the tool (Figs 155–157), because the bottom face of the slot in which the tool is clamped can be set in a known position, normally ‘horizontal’, and the inner side of the slot can be set ‘vertical’ and parallel to the grinder's axis of traverse. The bottom (reference) face of the tool may sit horizontal or vertical.

Inverting the tool

In some cases, mounting a tool upright, as it would be mounted in a lathe toolpost, is inconvenient, because when the toolholder tilts backwards to create a sizeable clearance angle the bottom of the toolholder or workhead may tilt forwards and foul the wheel. The solution may be to mount the tool upside down in the holder, and grind it with the top face and the cutting edge nearest the table (Fig. 157). In that position, some movements may need to be reversed. The traversing movement is likely to be the same, although the positions of traverse stops may need to be different.

Fig. 157: Inverting a tool may improve access to some faces while avoiding fouling the spindle body.

Cranked tools

Some tools have the cutting part set at an angle to the shank (Fig. 158). One consequence may be that it is physically difficult to position the wheel and cutting edge in the required relationship without fouling the wheel or the grinder guards. That may determine the best type of toolholder to use and the best orientation for mounting the tool (Figs 159 and 160). The position of the faces on the wheel can also help, and choosing a wheel that has its face at the side (type 1 or 3) rather than at the front (type 6 or 11) may simplify some setups.

Fig. 158: A cranked tool has its cutting part set at an angle to the shank.

Fig. 159: A cranked tool inverted to allow easy access to its Minor face.

Fig. 160: A cranked tool held on its side to simplify setting the angle of the Major face.

Tools with circular-section shanks

Tools that have circular-section shanks can easily be gripped in a collet or sleeve (Fig. 149), but because they do not have a convenient flat base, they must be aligned by setting a face or flank at a known angle to an imaginary base acting as a reference plane (Fig. 161). Use a workhead in which the sleeve or collet holder can rotate so that the tool can be aligned after the sleeve or collet has been tightened onto the tool. The set position then becomes the zero reference position from which all subsequent movements are measured.

Fig. 161: A circular section tool can be aligned by setting a face or flank at a known angle.

Getting a good finish

In general, the finer the grit, the finer the finish. However, there is a balance to be struck between speed, heat and finish. Using an aluminium oxide wheel, choose 36–60 grit to restore damaged flanks and faces, to avoid heat building up. Then use a finer wheel of 80–120 grit to provide a better finish. Use smaller depths of cut, but maintain traverse speeds. A 120 grit wheel will produce a good finish, acceptable for many lathe tools. Tools requiring a better finish must be ground using an even finer wheel, or honed using diamond paste. A fine cBN wheel of grain 91 to grain 46 (finest) will provide a very good finish, but should be used with very shallow depths of cut (0.02mm or 0.001in, if possible).

RIGHT-HAND KNIFE TOOL

The right-hand knife tool is a common lathe tool that, when viewed from the front, has its Major (main) cutting edge on the right. Viewed from the rear, a right-hand tool cuts as it moves to the left (Figs 162 and 163). There may be an additional Minor (subsidiary) cutting edge at the front of the tool. Each cutting edge has an associated face and flank, and angles are measured from tool reference planes (Figs 144, 163 and 164). Viewed from above, there may be Major and Minor cutting edge angles, often termed ‘front rake’ and ‘side rake’ angles (Fig. 144), and a clearance angle on each flank (Figs 163 and 164).

Fig. 162: A typical knife tool with Major and Minor cutting edges and top rake. The Major cutting edge is not parallel to the side of the shank.

Fig. 163: Rake and clearance angles for a knife tool which has a single rake angle on the top face, running back at right angles to the Major cutting edge.

Fig. 164: Some knife tools have both Major and Minor top rake, and both cutting edges slope downwards.

When viewed from the front, the Minor cutting edge will slope downwards because of the Major face rake angle on the top face (Fig. 163, top right). If (and only if ) there is also Minor face rake on the top face, the Major cutting edge will slope downwards from the point of the tool towards the shank (Fig. 164, left-hand diagram). The cutting edges always lie in the same plane as their associated flank, so, when aligning either cutting edge with the traverse and the face of the wheel, it is only necessary to align it with the traverse when viewed from above. The next move will align the whole flank with the plane of the face of the wheel.

Using the flat face of a cup, plain or tapered wheel

Minor (front) flank: S-T-R workhead

Mount a flat-faced wheel and dress the face parallel to the traverse. Dish the wheel slightly towards the centre, to leave a narrow flat outer ring where the tool will first contact the wheel. This helps prevent heat building up and protects the tool.

Reference position:

Fig. 165: S-T-R workhead: reference position for a knife tool Minor flank.

Fig. 166: Quorn/Universal workhead: reference position for a knife tool Minor flank.

Axis of the cutting part of the tool at right angles to the traverse, tip facing the wheel. Tool base horizontal, and top surface uppermost (Figs 165 and 166).

Fig. 167: S-T-R workhead: swing to bring the Minor cutting edge (orange) parallel to the traverse.

• Swing the tool through the Minor cutting edge angle, to set the Minor cutting edge parallel to the traverse (Figs 167 and 172). When viewed from above, the cutting edge should now be parallel to the axis of tilt. The tool reference face (the base) should remain horizontal.

Fig. 168: S-T-R workhead: tilt upwards through the clearance angle to set the flank vertical.

• Using an axis of tilt parallel to the traverse (and to the Minor cutting edge), tilt the tool upwards at the front, through the Minor (front) flank clearance angle (Figs 168 and 173).

• Set a traverse stop. Use the crossfeed to apply a cut, then traverse to grind. Repeat as necessary. On the Quorn/Universal, traverse is a sweeping movement across the face of the wheel.

Minor (front) flank: T-S-R workhead

Reference position:

Fig. 169: T-S-R workhead: reference position for a knife tool Minor flank.

Axis of the cutting part of the tool at right angles to the axis of traverse, tip facing the wheel. Tool base vertical and at the back of the slot or sleeve (Fig. 169).

Fig. 170: T-S-R workhead: tilt, to set the Minor cutting edge vertical.

• Tilt the tool downwards at the front, through the Minor cutting edge angle to set the Minor cutting edge vertical, and parallel to the axis of swing, when viewed from the left-hand side (Fig. 170).

Fig. 171: T-S-R workhead: swing, to bring the Minor cutting edge and the flank parallel to the traverse.

• Swing through the Minor flank clearance angle, to bring the Minor flank parallel to the traverse (Fig. 171).

• Use the crossfeed to apply a cut, then traverse to grind. Repeat as necessary.

Fig. 172: Quorn/Universal workhead: swing to bring the Minor cutting edge parallel to the traverse.

Fig. 173: Quorn/Universal workhead: tilt upwards through the clearance angle to set the flank vertical.

Major (side) flank: S-T-R workhead

Reference position:

Fig. 174: S-T-R workhead: set the cutting part of the tool (or the shank of a straight tool) parallel to the traverse and the face of the wheel.

Tool base horizontal; top face up; tip facing the wheel (Fig. 174).

Fig. 175: S-T-R workhead: swing to bring the Major cutting edge parallel to the axis of traverse, then tilt to set the Major flank vertical.

• Swing the tool to position the Major cutting edge parallel to the axis of traverse when viewed from above (Fig. 175).

• Tilt the tool upwards at the front, through the Major flank (side) clearance angle (Fig. 175), to set the flank vertical.

• Apply a cut, then traverse to grind. Repeat as necessary.

Major (side) flank: T-S-R workhead

Reference position:

Fig. 176: T-S-R workhead: 'tip up' reference position for a knife tool Major flank.

Axis of the cutting part vertical; tool tip at the top; tool base vertical; Major face nearest the wheel (Fig. 176).

Fig. 177: T-S-R workhead: tilt to bring the Major cutting edge vertical and parallel to the axis of swing, then swing the Major flank parallel to the traverse.

• Tilt through the cutting angle, to bring the Major cutting edge vertical and parallel to the axis of swing when viewed from the side (Fig. 177).

• Swing through the clearance angle to bring the Major flank parallel to the face of the wheel (Fig. 177).

• If necessary, adjust the height of the wheel to avoid grinding the shank below the end of the Major flank.

• Use the crossfeed to apply a cut, then traverse to grind. Repeat as necessary, until the Major cutting edge is sharp.

Top face with Major face rake only

The Major (side) cutting edge of a right-hand knife tool that cuts mainly as it travels to the left (Figs 178 and 162) does all the work, and chip flow is across the top face of the tool, away from the Major cutting edge. The top rake angle depends on the material the tool is designed to cut. Brass, for example, requires no top rake, while steel machines best with 10–20 degrees, and cast iron works well with 7–10 degrees. However, unless there is a good reason to change, sharpening should preserve existing angles.

Fig. 178: A knife tool may have Major face rake only, on the top face., with no Minor (top) face rake.

Major (top) face rake only: S-T-R and T-S-R workheads; rotate and tilt method

Reference position:

Fig. 179: S-T-R workhead: reference position for a knife tool with a single Major (top) face rake angle.

Grinder spindle horizontal and at right angles to the traverse. Axis of the cutting part of the tool parallel to the traverse (Fig. 179). Tool base vertical, and furthest away from the face of the wheel, with the tool lying on its side. Major face opposite the face of the wheel. Major cutting edge facing upwards but not necessarily horizontal. Workhead tilt axis parallel to the traverse. Axis of rotation at right angles to the axis of tilt.

Fig. 180: S-T-R workhead: rotate to set the Major cutting edge horizontal; then tilt to the rake angle.

• If necessary, rotate the toolholder through 90 degrees minus the Major cutting angle, to bring the Major cutting edge parallel to the axis of tilt (Fig. 180).

• Tilt through the Major face rake angle, taking the Major cutting edge away from the face of the wheel (Fig. 180).

• Set a traverse stop, apply a cut, then grind the face.

Top face with a compound rake angle

Some knife tools have both Major and Minor cutting edges and are used to cut in two directions. Surfacing takes place using the Major cutting edge (at the side of the tool). Facing takes place across the end face of the work, using the Minor (front) cutting edge. Each cutting edge requires top rake, so the single top face on the tool must lie at a compound angle to both cutting edges – the ‘true’ rake angle of the top face (Fig. 181). One consequence is that the Major cutting edge is tilted down from the tip to the shank as a result of the Minor face angle, and the Minor cutting edge is tilted from the tip to the opposite side of the shank as a result of the Major face angle (Fig. 146). Sharpening always removes material from the top face, no matter which flank or face is ground, and that lowers the height of the tip above the base. Although the two rake angles should be measured running back from each edge at 90 degrees, it is really only feasible to grind one plane surface on the top face. The correct end result should be a plane surface incorporating the Major and Minor cutting edges and tilted at the ‘true’ rake angle (Fig. 181).

Fig.181: The top surface of the tool lies at a compound 'true' rake angle tilted at both the Major and Minor rake angles simultaneously.

Calculating an accurate top face angle

Fig. 182: When the Tool Reference Plane (the base) is horizontal and the Major cutting edge is parallel to the traverse, the 'true' rake angle can be set using a swing from the Major cutting edge, and a tilt upwards from the Tool Reference Plane.

When the included angle between the Major and Minor cutting edges is at (or close to) 90 degrees, the compound ‘true‘ rake (TR) angle will have a value between the Major and Minor rake angles. A top face at that angle is a plane that lies at an angle to the base of the tool and includes both cutting edges (Fig. 182). To set this up on a grinder we need to know where the ‘true’ rake angle lies in relation to the cutting edges, and the orientation of the axis to use for the tilt that will bring the top face flat against the face of the wheel. That axis is always at right angles to the ‘true’ rake angle. Fig. 182 shows this as a swing (S) from the Major cutting edge, and a tilt (T) about an axis at right angles to the ‘true’ rake. S and T can be calculated using the formulae:

S = arcTan(Tan(Major face rake)/Tan(Minor face rake))

T = arcTan(Tan(Minor face rake)/CosS)

For example:

If the Major rake angle is 10 degrees, and the Minor rake angle is 5 degrees:

S = arcTan(Tan10/Tan5)

= arcTan(0.176/0.087) = 63.7 degrees

T = arcTan(Tan5/Cos63.7)

= arcTan(0.087/0.443) = 11.1 degrees

Fig. 183: When the tool is on its side, rotate through S and swing through T to position the top face.

Fig. 183 shows how these angles work with the tool on its side, rotating through S then swinging through T, to bring the ‘true’ rake top face horizontal and parallel to the traverse and the face of the wheel.

Fig. 184: If the tool is held with its axis vertical (position 1), rotate to set the Major cuttng edge vertical (position 2), then rotate through angle 'S' to set the 'true' rake angle line vertical. Seen from the side, the tool will now be in position 4. Tilt through angle T to bring the 'true' top face vertical, ready to be ground at the composite angle.

Fig. 184 shows the movements required when the tool is initially held vertical.

Further Information:Table 12 contains values of S and T for some typical rake angles.

Top face ‘true’ rake angle: S-T-R workhead

An S-T-R workhead should be configured as shown in Fig. 185. The workhead should have an axis of tilt parallel to the traverse, and a rotate axis parallel to the spindle and to the surface of the table. Follow the steps in Fig. 184, as listed below.

Fig. 185: S-T-R workhead: reference position, with the 'rotate' axis horizontal and acting as a tilt axis.

Reference position:

Grinder spindle at 90 degrees to the traverse; tool axis vertical, with the tool reference plane (base) vertical and parallel to the axis of traverse. The Major face is then opposite the face of the wheel, and the cutting part of the tool is at the top (Fig. 185).

Because the tool is vertical compared to Fig. 182, the angle S is a rotate angle, which will bring the brown ‘tilt’ axis horizontal (Fig. 184 (3)). The angle T is then a tilt around that horizontal axis.

Fig. 186: S-T-R workhead: (1) rotate to set the Major cutting edge horizontal, then (2) rotate down through S degrees to set the (red) ‘true’ rake direction vertical, and normal to the surface of the table.

• Using the ‘rotate’ axis, rotate the tool through 90 degrees minus the Major cutting edge angle, to set the Major cutting edge horizontal (Fig. 186 (1)).

• Rotate the tip of the tool anticlockwise, through S degrees (Fig. 186 (2)) so that the ‘true’ rake direction (blue arrow) lies vertical and at right angles to the (brown) tilt axis.

• Tilt the tool tip away from the wheel, through T degrees so that the ‘true’ rake plane is parallel to the traverse and the wheel face (Fig. 184 (4) and (5)).

• Apply a cut and traverse to grind.

Fig. 187: S-T-R workhead: with the 'true' rake direction set vertical, the brown axis (Figs 182 and 184) is now parallel to the axis of tilt, so tilt through the angle T to position the true Major face plane.

Top face ‘true’ rake angle: T-S-R workhead

A T-S-R workhead should be configured as shown in Fig. 188, with an axis of rotation (the tilt axis) parallel to the spindle axis and the cutting part of the tool, and at right angles to the traverse. Follow the steps shown in Fig. 183, and as listed below.

Fig. 188: T-S-R workhead: reference position to grind the Major face at the ‘true’ rake angle.

Reference position:

Mount the tool on its side, with the tool reference plane (normally the base, when the tool is in the normal cutting position) vertical and parallel to the traverse. The long axis of the tool should be ‘horizontal’ (parallel to the table). The ’top’ face is then opposite the face of the wheel, and the cutting part of the tool protrudes to the left (Fig. 188).

Fig. 189: T-S-R workhead: (1) tilt up to set the Major cutting edge horizontal, then (2) tilt down through S degrees to set the (red) ‘true’ rake direction horizontal, and at right angles to the axis of swing.

• Tilt the toolholder upwards through 90 degrees minus the Major cutting edge angle, to bring the Major cutting edge horizontal (Figs 183 (1) and (2), and 189 (1)).

• Tilt the tip of the tool down, through S degrees (Fig. 189(2)) so that the ‘true’ rake direction lies horizontal and at right angles to the axis of swing (Fig. 183(2) and blue arrow, and Fig. 189(2) and red arrow).

Fig. 190: T-S-Rworkhead: swing through the ‘true’ rake angle T, to position the true Major face plane parallel to the traverse.

• Swing the tool tip away from the wheel, through T degrees so that the ‘true’ rake plane is parallel to the wheel face (Figs 190 and 183 (4) and (5)).

• Apply a cut towards the wheel and traverse to grind the top face.

Top face composite rake shortcuts

Where the Major and Minor cutting edges are at 90 degrees to each other, one common shortcut involves setting the top face at the Major face rake angle, then tilting around an axis at right angles to the Major cutting edge to set the top face at the Minor face rake angle. Larger or smaller angles between the cutting edges cause increasing errors in the Minor face rake angle, but small differences may not significantly affect the performance of most knife tools.

Use a toolholder that has an axis of rotation at right angles to the shank of the tool (Fig. 191). That allows an initial alignment of the Major cutting edge, leaving the next axis for creating the Major rake, and the third axis for creating the Minor rake.

Shortcut – top face: S-T-R workhead

Reference position:

Fig. 191: S-T-R workhead: orientation of axes in the reference position when setting up to grind the composite Major (top) face rake angle of a lathe tool. The axis of rotation is at right angles to the traverse.

Mount the tool vertically, with the tool reference face (the base) vertical, the long axis of the shank vertical, and the top face opposite the face of the wheel. The Minor (front) flank should point upwards (Fig. 191).

Fig. 192: S-T-R workhead: rotate to set the cutting edge vertical; swing to the Major face rake angle; then tilt to the Minor (top) face rake angle.

• Rotate the toolholder to bring the Major cutting edge vertical and parallel to the axis of swing (Fig. 192).

• Swing to create the Major (top) face rake angle, taking the Major cutting edge away from the wheel (Fig. 193).

• Tilt the tool through the Minor (top) face rake angle, taking the tip away from the face of the wheel (Fig. 194).

• Adjust the height of the wheel to avoid fouling the shank; apply a cut, then traverse to grind.

Shortcut – top face: T-S-R workhead

Reference position:

Fig. 193: T-S-R workhead: reference position to grind the Major (top) face at the 'true' rake angle.

Set the tool reference face (the base) vertical with the top face nearest the face of the wheel. The Minor flank should point to the left; with the long axis of the shank horizontal and parallel to the axis of traverse (Fig. 193).

Fig. 194: T-S-R workhead: rotate, swing and tilt to set the Major face at the ‘true’ rake angle.

• Rotate the toolholder to bring the Major cutting edge horizontal (Fig. 194).

• Tilt the Major cutting edge up, to create the Major face rake angle (Fig. 194). Because the Minor cutting edge is at right angles to the Major cutting edge, the Minor cutting edge is now parallel to the axis of swing, in preparation for the next positioning movement.

• Swing the tool through the Minor (front) rake angle, taking the tail nearer the face of the wheel (Fig. 194).

• Use the crossfeed to apply a cut, then traverse to grind. Repeat as required.

Using the curved face of a wheel

The curved face of a type 1 wheel can be used to create some of the angles on the flanks of a tool. This can eliminate the need to tilt a face to create a clearance angle, and reduce the number of axes required on a workhead. However, that leaves the face curved rather than flat. In practice, that is not a disadvantage, because only the area immediately below the cutting edge touches and affects the surface finish on the work. The basic principle is explained in detail in Chapter 3, but it involves raising or lowering the tool above or below the wheel centre line to allow the curve of the wheel face to create an angle on the tool.

When using this method, the reference positions for S-T-R and T-S-R workheads are essentially the same for the Minor and Major flanks. Top face setups differ only because the initial move to bring the Major cutting edge horizontal is a tilt on a T-S-R workhead, but a rotation on an S-T-R workhead.

Minor (front) and Major (side) flanks

Both S-T-R and T-S-R workheads:

Reference position:

Fig. 195: S-T-R workhead: reference position when using the curve of a wheel to create the Minor (front) or Major (side) flank clearance angles.

Fig. 196: T-S-R workhead: reference position when using the curve of a wheel to create the Minor (front) or Major (side) flank clearance angles.

Axis of the spindle parallel to the axis of traverse. Swing axis normal to the table. Axis of the cutting part of the tool at right angles to the axis of traverse. Tool base horizontal, and top surface uppermost. Tip facing the front. Figs 195 and 196 show the setups.

Fig. 197: T-S-R workhead: swing, to bring the Minor cutting edge parallel to the traverse, then lower the wheel to create the Minor flank clearance.

• Swing the tool through the Minor (or Major) cutting edge angle, to set that cutting edge parallel to the axis of traverse when viewed from above (Fig. 197).

• Lower the wheel below centre height, to create an angle on the tool face.

For example:

For a clearance angle of 5 degrees, and a 100mm diameter wheel, find the required height of the tip of the tool above centre by consulting Table 9 (page 27) or by calculation (Chapter 3).

Height = r × TanC

where r = 50mm and C = 5 degrees

Height = 50 × Tan5 = 4.35mm Lower the wheel by 4.35mm (Fig. 197).

• Apply a cut and traverse to grind.

Top face

S-T-R workheads require a toolholder to hold the tool with its axis at 90 degrees to the axis of rotation (Fig. 198). In effect, this makes the top two axes of the S-T-R workhead behave in the same way as the tilt and swing axes of the T-S-R workhead.

In this example, the tool is inverted, to prevent the toolholder from fouling the body of the spindle. The method remains the same, but the spindle will be raised to create the Major cutting edge rake angle.

Reference position:

Axis of the cutting part of the tool parallel to the axis of traverse. Tool base vertical, and parallel to the traverse and the face of the wheel. Top face opposite the curved face of the wheel, with the Major cutting edge at the bottom edge, in this example, because the tool is inverted (Figs 198 and 199).

Fig. 198: S-T-R workhead: reference position for the curve of a wheel to create the Major face rake angle. The tool is inverted in this photo.

Fig. 199: T-S-R workhead: reference position for the curve of a wheel to create the Major face rake angle. The tool is inverted in this photo.

Fig. 200: S-T-R workhead: rotate the tool holder down to set the Major cutting edge horizontal, then swing to the Minor top rake angle. Lower the wheel to create the Major face rake angle.

Fig. 201: T-S-R workhead: tilt the tool holder down, to set the Major cutting edge horizontal, then swing to the Minor top rake angle. Lower the wheel to create the Major face rake angle.

• Set the Major cutting edge horizontal by S-T-R: Rotating the toolholder (Fig. 200 green arrow).
T-S-R: Tilting the toolholder (Fig. 201 blue arrow).
Both:The angle will be 90 degrees minus the Major cutting edge angle. If the answer is positive, tilt down; but if it is negative, tilt upwards.

• Swing the tip of the tool away from the wheel, through the Minor top rake angle (Figs 200 and 201, yellow arrows).

• Set the centre of the wheel at the height of the tip of the tool. Find the distance required to raise the wheel to create the Major face rake angle, by consulting Table 9 (page 27) or by calculation (see earlier section), then raise the wheel.

• Set traverse stops. Apply a cut and grind.

Finishing the nose

A knife tool can be ground until it has a sharp corner between the Major and Minor flanks, forming a sharply defined point. To give that point some additional strength, and to improve the finish the tool will leave on the work, it is often preferable to create a small flat, or a curve with a small radius, to form a larger cutting surface at the nose and blend from the tip to the Minor cutting edge. Where a specific radius, or a large blending radius, is required, refer to the later section entitled ‘Radiused Tools’.

Using a flat-faced wheel to create a flat

A small flat, at a small angle to the work, can improve the finish produced by the tool (Figs 202 and 203). Note that if a blending radius is required, it is not advisable to begin by grinding a flat. Coating the cutting part of the tool with layout blue or dark ink helps highlight the flat as the grind takes place.

Fig. 202: A left-hand knife tool for brass, with a small flat ground at the nose.

Fig. 203: Swing the tool through a small angle to form a narrow flat at the nose.

Reference position:

Set the tool with its axis at right angles to the surface of the wheel (the normal cutting position when the tool is mounted in the lathe).

• Swing through a small angle of 0.5 to 2 degrees, to bring the Minor cutting edge closer to the wheel (Fig. 203).

• Tilt until the edge between the Major and Minor flanks is parallel to the face of the wheel. It is not essential that this edge is precisely aligned, because only a short distance below the cutting edge is in contact with the work.

• Apply a small cut, and traverse to grind a flat of perhaps 1–1.5mm wide. Inspect the width frequently during the grind.

Using an abrasive stone to create a curve

An alternative to a flat is a small curve between the flanks, but the combination of a curve and a short flat works well too. To form a small curve, shape the tool nose on an abrasive stone.

Fig. 204: Pull the tool while raising the tail, to create a radius at the nose.

• Hold the tool vertically, with its nose in contact with the stone, then tilt it so that the whole of the sharp corner between the flanks is in contact with the stone (Fig. 204, left-hand position).

• Swing the tail of the tool to the side, until the Major flank is in contact with the abrasive stone.

• Pull the tool along the stone, at right angles to the corner, keeping the corner in contact with the stone, but swinging the tool tail-upwards with a swiping action, to create a small radius (Fig. 204). Stop when the small nose flat is against the stone.

PARTING TOOL (LATHE TOOL STYLE)

A lathe parting tool (Fig. 205) has a single Major cutting edge at the front, and is fed forwards at 90 degrees to the axis of the lathe spindle, cutting the work into two pieces. The Minor cutting edges have side clearance and, aside from a small scraping action near the tip, normally play no part in the cutting operation (Fig. 206). The Major face typically has a rake angle, but seldom has side rake from one or other of the Minor cutting edges. Some parting tools have a few degrees of side rake on the Major flank, angling the flank backwards from left to right, or from right to left. The purpose of that side rake is to leave a clean face on the work remaining in the chuck, or on the end of the work that is parted off, with any centre ‘pip’ left on the opposite face (the end of the material left in the chuck, or the face of the work that has been parted off).

Fig. 205: A lathe parting tool with a narrow centrally located blade.

Fig. 206: A parting tool has its Major cutting edge at the front. A 'neutral' cutting edge is set at 90 degrees to the shank, but the cutting edge may instead be raked to one side or the other, to ensure a clean cut on the work, next to the furthest protruding tip.

Sharpening a parting tool

Note that where the blade tapers from front to back, sharpening reduces the width of the blade. Use a flat-faced wheel.

Major (front) flank: S-T-R workhead

Reference position:

Fig. 207: S-T-R workhead: reference position for a flat-faced wheel to grind the Major flank.

Axis of the tool at 90 degrees to the traverse. Tool base horizontal, Major face up, and tool tip facing the wheel (Fig. 207). Tools with a cylindrical shank can be positioned by using an angle gauge to set one Minor flank at the clearance angle (Fig. 161).

Fig. 208: S-T-R workhead: if necessary, swing to create Major face side rake. Then tilt to create the Major flank clearance angle.

• Option: Swing the tool tip to create the Major cutting edge side rake (Fig. 208), which will bring the Major cutting edge parallel to the axis of tilt.

• Tilt upwards to create the Major flank clearance angle (Fig. 208).

• Apply a cut and traverse to grind.

Major (front) flank: T-S-R workhead

Reference position:

Fig. 209: T-S-R workhead: reference position for a flat-faced wheel to create the Major face rake angle.

Axis of the cutting part of the tool at 90 degrees to the axis of traverse. Tool base vertical, Major flank facing the wheel (Fig. 209).

Fig. 210: T-S-R workhead: if necessary, tilt to create Major cutting edge side rake, and bring the Major cutting edge parallel to the swing axis. Then swing to create the Major flank clearance angle.

• Option: Tilt the tool tip to create the Major cutting edge side rake (Fig. 210), which will bring the Major cutting edge parallel to the axis of swing.

• Swing the tool to create the Major flank clearance angle (Fig. 210).

• Apply a cut, then traverse to grind.

Major (top) face: S-T-R workhead

Use a toolholder with an axis of rotation at 90 degrees to the axis of tilt.

Reference position:

Fig. 211: S-T-R workhead: reference position for a flat-faced wheel to grind the Major (top) face.

Axis of the cutting part of the tool parallel to the axis of traverse. Tool base vertical; Major (top) face towards the wheel (Fig. 211).

• Option: If the Major cutting edge has a right-hand or left-hand rake (Fig. 206), rotate the tool through the Major cutting edge angle, to bring the Major cutting edge vertical and parallel to the axis of swing (Figs 213 and 216).

Fig. 212: S-T-R workhead: if necessary, rotate to set the Major cutting edge vertical and parallel to the axis of swing. Then swing through the Major face rake angle.

• Swing the tool tip away from the wheel to create the Major face rake angle (which runs back towards the shank) (Figs 206 and 212).

• Set a traverse stop, apply a cut and traverse to grind.

Major (top) face: T-S-R workhead

Reference position:

Fig. 213: T-S-R workhead: reference position for a flat-faced wheel to grind the Major (top) face.

Axis of the cutting part of the tool parallel to the axis of traverse. Tool base vertical, Major (top) face towards the wheel (Fig. 213).

Fig. 214: T-S-R workhead: if necessary, tilt to set the Major cutting edge vertical and parallel to the axis of swing. Then swing through the Major face rake angle.

• Option: If the Major cutting edge has a right- or left-hand rake (Fig. 206), tilt the tool through the Major cutting edge angle, to bring the Major cutting edge vertical and parallel to the axis of swing when viewed from the side (Fig. 214).

• Swing the tool tip away from the wheel to create the Major face rake angle (which runs from the Major cutting edge back towards the shank) (Figs 206 and 214).

• Set a traverse stop, apply a cut and grind.

Minor (side) flank: S-T-R workhead

The Minor cutting edge does little work, so the simplified method outlined below defines the Minor flank clearance as though it was measured from the horizontal, rather than from the downward-sloping Minor cutting edge.
Reference position:

Axis of tilt at right angles to the traverse. Axis of swing normal to the table. Axis of the tool parallel to the traverse. Tool base horizontal, and Minor flank facing the wheel (Fig. 215).

Fig. 215: S-T-R workhead: optionally, rotate to set the Major cutting edge vertical and parallel to the axis of swing. Then swing through the Major face rake angle.

Fig. 216: S-T-R workhead: if necessary, rotate to set the Major cutting edge vertical and parallel to the axis of swing. Then swing through the Major face rake angle.

• Swing to bring the Minor cutting edge parallel to the axis of tilt (Fig. 216).

• Tilt upwards to create the Minor flank clearance angle (Fig. 216).

• Set a traverse stop, apply a cut and traverse to grind.

Minor (side) flank: T-S-R workhead

Use a toolholder with an axis of rotation at 90 degrees to the axis of tilt.

Reference position:

Fig. 217: T-S-R workhead: reference position for a flat-faced wheel to grind the Minor (side) flank.

Axis of tilt at right angles to the traverse. Axis of swing normal to the table. Axis of the cutting part of the tool horizontal, Minor flank facing the wheel (Fig. 217).

Fig. 218: S-T-R workhead: rotate to set the Minor flank vertical. Optionally, tilt to set the Minor cutting edge horizontal. Then swing through the Minor cutting edge rake angle.

• Rotate the tool through the Minor flank clearance angle, to set the flank vertical (Fig. 218).

• Option: Tilt through the Major (top) face rake angle to set the Minor cutting edge horizontal (Fig. 218).

• Swing the tool to create the Minor cutting edge rake angle (Figs 218 and 206).

• Apply a cut, then traverse to grind.

PARTING TOOL (BLADE STYLE)

Parting tool blades sit in a holder at an angle (tilt), effectively creating (or adding to) a top rake angle, but requiring an increase in the Major (front) flank clearance angle by the same amount (Fig. 219). That tilt angle must be taken into account when grinding.

Fig. 219: Most parting tool blade holders incorporate a tilt, to set the top rake and minimize the amount of grinding required when sharpening the blade.

PARTING TOOL BLADE FIXTURE

• The fixture is based on a plate with holes for pins and clamp screws (Figs 220 and 221). The blade is aligned against two pins, and is held in place by two strap clamps (Fig. 221).

• Pins of equal diameter align the blade with the side of the fixture. Pins of different diameter hold the blade at an angle to the edge of the fixture, and are most useful when the angle of the blade is the same as the top rake angle, so that the top edge of the blade is set parallel to the top edge of the fixture.

• Varying the pin diameters creates different rake angles. Keep the small pin the same diameter, but set the rake angle by varying the diameter of the large pin, which can be found using the formula:
D = d + 2 × C × SinR
where:
d = diameter of the small pin
C = distance between the pin centres
R = rake angle, in degrees

For example:
If d = 4mm, C = 30mm and R = 5 degrees
D = 4 + 2 × 30 × Sin5
D = 4 + 2 × 30 × 0.087
D = 4 + 5.22 D = 9.22mm

Fig. 220: Main features of a fixture for use when sharpening parting tool blades.

Fig. 220 shows a simple fixture designed to hold rectangular-section blades flat. Interchangeable pins on the right-hand side allow quick setting of the clearance angle, from 0 degrees to 3, 5 or 7 degrees, and eliminate the need to swing the fixture. A plain plate without pins may be used instead, by clamping the blade parallel to the side of the fixture. A clearance angle is then created by swinging or tilting the fixture.

Fig. 221: The parting tool blade fixture mounted on a workhead.

Fig. 221 shows the fixture mounted in a workhead. Insert the appropriate pins, then clamp the blade with its top face against the pins and a short length protruding from the front of the fixture.

Fig. 222: Parting tool blade sections. From the left: rectangular section; tapered; and hollow-ground.

Fig. 223: A fixture designed to hold both rectangular-section and taper-ground blades. Adjusting screws at the back counter the tilt in the blade.

Rectangular-section blade blanks have parallel sides (Fig. 222), and would benefit from having a taper ground on each side to create Minor flank clearance. Tapered or hollow-ground blades already have Minor flank clearance, but that makes them difficult to hold vertically. Fig. 223 shows a second fixture that can hold all blades securely, butted against an adjustable fence aligned along the top edge of the fixture. Grubscrews accessible from the rear can be extended out from the front of the plate to support a taper-ground blade so that, when viewed from the front, a line drawn down the centre of the blade will lie parallel to the flat plate. The top and bottom face of the blade will then be at right angles to the flat plate. Check using the stock of an engineer's square. More than one set of screws may be required, to suit blades of different height. An alternative is to omit the screws and allow for the tilt of the blade when setting up for grinding.

Major (front) flank: S-T-R workhead

Reference position:

Fig. 224: S-T-R workhead: reference position for grinding the Major (front) flank of a parting tool blade.

Set the blade on its side (vertical), with the narrow base of the blade horizontal, and the long axis of the blade at right angles to the traverse. The tool tip should face the wheel (Fig. 224). If the blade is tapered and has one tapered side vertical, make adjustments as noted below.

Fig. 225: S-T-R workhead: if necessary, tilt; then swing to set the Major flank clearance.

• If necessary, swing to set the Major cutting edge parallel to the traverse, when viewed from above (Fig. 225).

• Tilt to set the Major flank vertical, taking account of any existing angles from a toolholder or a fixture (Fig. 225).

• Apply a cut, and traverse to grind.

Major (front) flank: T-S-R workhead

Reference position:

If the blade is in a fixture, or a lathe parting toolholder, hold it there. Set the blade horizontal, with the long axis of the blade at right angles to the traverse, and the tool tip facing the wheel (Fig. 226).

Fig. 226: T-S-R workhead: reference position for grinding the Major (front) flank.

Fig. 227: T-S-R workhead: if necessary, tilt through the Major cutting edge angle; then, if necessary, swing to create the Major flank clearance.

• If necessary, tilt through the Major cutting edge angle to set the Major cutting edge vertical when viewed from the side (Figs 226 and 227).

• If necessary, swing to create the Major flank clearance angle, taking account of any angle incorporated into the fixture or holder (Figs 219 and 227).

• Apply a cut, and traverse to grind.

Minor (side) flank: rectangular blades only

Note: Taper-ground blades do not require the sides to be ground. Normally, there is no Minor cutting edge side rake on a parting tool blade, as that not only weakens the blade, but makes it difficult to regrind further back along the blade.

S-T-R workhead: rectangular blade:

Reference position:

Fig. 228: S-T-R workhead: reference position for grinding the Minor flank.

Set the tilt axis parallel to the axis of traverse. If the blade is in a fixture, or a lathe parting toolholder, hold it there. Set the blade upright (vertical), with the long axis horizontal and parallel to the axis of traverse and the face of the wheel (Fig. 228).

Fig. 229: S-T-R workhead: if necessary, swing to create the Minor (side) rake, then tilt to create the Minor flank clearance.

• Option: swing to create the Minor cutting edge (side) rake and bring the Minor cutting edge parallel to the traverse (Fig. 229).

• Tilt the fixture through the Minor face clearance angle, taking the Major cutting edge further away from the wheel (Fig. 229).

• Apply a cut and traverse to grind.

• Before grinding the other Minor flank, the blade must be removed from the fixture and replaced upside down.

T-S-R workhead: rectangular blade:

Reference position:

Fig. 230: T-S-R workhead: reference position for grinding the Minor flank.

Set the tilt axis at right angles to the traverse. If the blade is in a fixture, or a lathe parting toolholder, hold it there. Set the blade vertical, with the tip uppermost and the side of the blade parallel to the traverse and the face of the wheel (Fig. 230).

Fig. 231: S-T-R workhead: tilt through the Minor face clearance angle.

• Tilt to bring the Minor cutting edge vertical and parallel to the axis of swing (Fig. 231).

• Swing through the Minor flank clearance angle, to set the Minor face parallel to the traverse (Fig. 231).

Both types of workhead:

• Traverse to grind the blade, using a stop to avoid grinding the clamps or the fixture. The fence is a sacrificial item and may be ground along with the blade.

• Remove the blade and turn it end-for-end before clamping it at the other end of the fixture. Leave the tilt angle unchanged, so that each side of the blade is ground to the same taper angle.

• Move the workhead to the other side of the wheel; set a stop, and grind the second side.

Top face

If the parting tool blade is held at the Major face rake angle in the lathe toolholder, the top face should not be ground. Instead, sharpening of the Major cutting edge should be confined to grinding the Major flank only. The exception is where the Major face has a chip clearance groove which must be extended (see Adding a chip clearance groove). The following instructions are only for blades held horizontal in the lathe.

Both types of workhead:

Reference position:

Fig. 232: S-T-R workhead: reference position for grinding a parting tool Major (top) face.

Fig. 233: S-T-R workhead: swing to set the Major (top) rake angle, then, if necessary, swing through the Major cutting edge rake angle.

Fig. 234: T-S-R workhead: reference position for grinding a parting tool top face.

If the blade is in a fixture, or a lathe parting toolholder, hold it there. The blade should lie horizontal, on its side, with the Major flank central axis horizontal thereby compensating for any Minor flank clearance angle on a taper-ground blade. The long axis should be parallel to the traverse, with the Major face nearest the wheel (Figs 232 and 234). Set the axis of tilt parallel to the traverse (Figs 232 and 234).

S-T-R workhead

• Swing to set the top rake angle, taking into account any top rake angle built into the blade holder (Fig. 233).

• Option: Tilt through the Major cutting edge rake angle to set the Major cutting edge parallel to the traverse and the face of the wheel (Fig. 233).

• Set a traverse stop to limit the length of the top face. Feed towards the wheel and traverse to grind the blade.

T-S-R workhead:

Fig. 235: T-S-R workhead: if necessary, tilt through the Major cutting edge angle, then swing to create the top rake angle.

• Option: Tilt through the Major cutting edge angle (Fig. 235) to set the Major cutting edge vertical.

• Swing to set the top rake angle, taking into account any top rake angle built into the blade holder (Fig. 235).

• Apply a cut, and traverse to grind.

Adding a chip clearance groove

A chip clearance groove may be ground using a wheel with its curved face dressed to a 90 degree vee. A small-diameter wheel works best, as that produces a smaller curve at the inner end of the groove.

Both workheads:

Reference position:

Fig. 236: Both workheads: set the blade horizontal, and position the wheel directly above the centreline of the top face of the blade.

Set the axis of the spindle at right angles to the traverse; the long axis of the blade horizontal and parallel to the traverse, with the blade vertical and the top face uppermost. Set the wheel above and directly over the centre line of the blade (Fig. 236).

• Lower the wheel to create feed, and traverse to grind the groove. Repeat until the groove is deep enough, but leaving a small land on either side of the groove.

VEE-POINT TOOLS

A vee-shaped chamfering tool is designed to cut a chamfer using a cutting edge facing to the left or to the right, and a threading tool cuts a vee groove of a specific angle for a thread. These are, arguably, tools with a single Major cutting edge that bends around the point of the tool, and no Minor cutting edge. However, because two distinct setups will be needed when sharpening the Major cutting edge, it is more useful to think of the tool as having two faces, one for each side of the cutting edge. There are, effectively, two Major cutting edges and two Major flanks, usually with the same clearance angles. On a chamfering tool, each cutting edge is used independently.

The effect of the thread helix angle

All vee-point tools need Major flank clearance. For a chamfering tool this is the same as for a knife tool, but the groove cut by a threading tool is a helix that travels around the work at an angle (Fig. 237). The flank clearances must lie at the helix angle, or at least be sufficiently large to avoid fouling the sides of the helix (although over-large side clearances will reduce support for the cutting edges, and weaken the tool). Note that cutting a left-hand thread normally requires a tool ground differently from a right-hand thread, because the helix angles tilt in opposite directions. When creating the clearances, one flank has its clearance angle increased by the helix angle, and the other flank has its clearance reduced by the same helix angle.

Fig. 237: Every thread has a helix angle. The front view of the lathe tool shows that the effect is to change the clearance angle on each flank. Note: when cutting, the tool will face the thread and the helix angles will match.

Finding the helix angle

Find the helix angle by consulting thread tables or by using the formula:

Helix angle = arcTan(Lead/pitch circumference)

where
Pitch circumference = Pitch diameter × π

Note that pitch diameter is not the external diameter of the thread, but can be read from thread tables. Further Information: Table 13 lists helix angles for many common thread pitches.

Add the helix angle to the flank clearance angle of one side.

Subtract the helix angle from the flank clearance angle of the other side.

For example:
an M8 × 1.25 thread has a pitch of 1.25mm. Further Information: Table 13 shows that its helix angle is 3.1683 degrees (or 3 degrees 10 minutes), or approximately 3.2 degrees.

Assuming side clearance of 7 degrees, adding the helix angle to one side results in a clearance angle of 7 + 3.2 degrees = 10.2 degrees;

subtracting the helix angle from the other side results in a clearance angle of 7 – 3.2 degrees = 3.8 degrees.

On most grinders, it will be difficult to set these angles accurately, but a little extra clearance will not go wrong. The angles may then be 10.5 and 4 degrees.

Use a flat-faced wheel.

Major flanks: S-T-R workhead

Reference position:

Fig. 238: S-T-R workhead: reference position for grinding a vee tool.

Hold the tool with its base horizontal, Major face uppermost, and the axis of the cutting part parallel to the traverse (Fig. 238).

Fig. 239: S-T-R workhead: swing one cutting edge parallel to the traverse, then tilt away from the wheel to create the Major flank clearance.

Threadform
Included angle of vee

Metric

60

Unified

60

Whitworth

55

British Association

47.5

Table 14: Included angle of the vee point on a lathe thread cutting tool.

• Swing, to bring one cutting edge parallel to the traverse when viewed from above (Fig. 239). For a threading tool this is half the included thread angle shown in Table 14.

• Tilt the tool tip backwards through the Major flank clearance angle (Fig. 239).

• Apply a cut using the crossfeed, then traverse to grind the face.

• Repeat the swing/tilt/grind sequence for the other face.

Major flanks: T-S-R workhead:

Reference position:

Fig. 240: T-S-R workhead: reference position for grinding the Major flank of a chamfering tool.

Hold the tool with its long axis vertical, with its base vertical and normal to the traverse, and the tip with the Major flanks pointing upwards (Fig. 240).

Fig. 241: T-S-R workhead: tilt to set one cutting edge parallel to the traverse, then swing to create the Major flank clearance.

• Tilt, to bring one cutting edge parallel to the traverse when viewed from the side (Fig. 241).

• Swing through the Major flank clearance angle to set the flank parallel to the traverse and the face of the wheel.

• Apply a cut using the crossfeed, then traverse to grind the face (Fig. 241).

• Repeat the tilt/swing/grind sequence for the other face.

The effect of top rake

Fig. 242 shows the outline of a tool with a theoretically correct vee shape. However, threading and some chamfering tools typically have no top rake. Adding top rake may improve the cutting action, but will produce a vee point that is too narrow. The sides of the tool are angled inwards as they slope downwards, to provide side clearance, so adding top rake will cause the top face to intersect the tapering sides of the tool at a pair of increasingly lower points further back, making the distance between those points narrower, and reducing the angle of the vee point. So the vee ground on the tool must have a larger included angle to compensate. This can be dealt with visually, by comparing the tip of the tool with a lathe thread setting gauge (Fig. 246), or a homemade gauge for chamfering tools. Hold the setting gauge parallel to the base of the tool, and sight down from directly above, preferably using high magnification. Any discrepancy in the angle should be remedied by altering the angle at which each flank is ground. Make a similar adjustment in the angle setting for each flank, to keep the nose central and the axis of the vee aligned with the axis of the shank. Many threads involve a shallow depth of cut, so, in most cases, a small error in the angle will not prevent the thread from mating with a typical commercially produced nut, or cutting a chamfer that looks good.

Fig. 242: Plan view of a theoretical profile for thread cutting. The outer black lines indicate the edge of the vee. The green indicates the bottom face. Red indicates the sides and angles when there is top rake.

Major face

Reference position:

Fig. 243: Both workheads: reference position for grinding the Major (top) face of a chamfering tool.

Swing axis vertical; tool reference face vertical; tool axis horizontal and parallel to the traverse (Fig. 243).

Fig. 244: Both workheads: swing to the Major face rake angle.

• Swing the tool tip away from the wheel, to the Major face rake angle (Fig. 244).

• Apply a cut and traverse to grind.

Dealing with the nose

A chamfering tool will benefit from a small curve at the nose, but the tip of a threading tool has a specific shape to match the thread being cut, and the shape of each threadform is defined by national and international standards. There is a basic theoretical shape for each threadform, and a set of tolerances for the features of each thread. ISO metric and UTS threads have a 60 degree vee shape with a sharp point. In practice, the external thread is given a flat crest and the root is rounded. The internal thread has its root rounded to avoid having to cut a perfectly sharp vee inside a nut. These changes provide for clearance and tolerance, and allow a male and female thread to mate in the presence of dirt (Fig. 245). That rounded profile varies according to the pitch (P) of the thread, with a minimum of 0.125 × P and a maximum of 0.14434 × P for a male thread, and a value of 0.072168 × P for the nut. Whitform threads (Whitworth, British Standard Fine, British Standard Pipe, Royal Microscopic Society microscope threads, and Model Engineering series threads) have a 55 degree angle and use rounded crests and roots, although it is common to give the rounded crests a flat top. This simplifies manufacture. It is important to know the dimensions of a thread, and its profile, before grinding a screwcutting tool to suit.

Fig. 245: Threads have additional clearance at the roots, to accommodate dirt and provide clearance for the crests of the mating thread.

Fig. 246: Compare the shape of the tool to a profile gauge.

Rounded profiles on screwcutting tools are tricky to form accurately, but a close approximation is normally sufficient. For most roots, a smaller radius than on the theoretical profile is normally good enough, as that may provide a little extra clearance. It is important to match the shape of the cutting tool to the ideal profile of the threadform, so the tool should be compared to an accurate profile gauge (Fig. 246) or a profile etched on an optical inspection device such as an optical comparator, preferably under high magnification. Although it is usual to create a radius running all the way down the front of a screwcutting tool, that is not essential, as long as the profile at the top is correct and no part of the tool below the top is in contact with the work. In practice, manually creating the radius is most easily done by creating a radius all the way down the front of the tool. This will not be truly radial to the top of the tool, but the error will not affect the thread, as the radius simply creates clearance at the root of the thread.

Produce a nose radius by honing the sharp point of the tool against a flat stone, comparing the nose with an accurate thread gauge as honing proceeds. Hold the tool with one flank trailing against the stone and turn the tool smoothly over its nose while dragging the tool along the stone (Fig. 204). Repeat as necessary to round the nose and give it a fine finish.

Flat nose vee tools

Instead of a sharp point or a radius, a small flat can be ground on the nose of chamfering tools or some thread cutting tools (depending on the thread profile being cut).

Reference position:

Fig. 247: S-T-R workhead: reference position for creating a flat nose on a vee tool.

Fig. 248: T-S-R workhead: reference position for creating a flat nose on a vee tool.

Reference surface (base) of the tool horizontal; long axis of the tool at right angles to the axis of traverse. Axis of tilt parallel to the axis of traverse. For threadcutting tools, there should be an axis of rotation at right angles to the traverse (Figs 247 and 248).

Fig. 249: View from the rear end of the shank: rotate a left-hand threading tool anti-clockwise and a right-hand tool clockwise.

Fig. 250: Both workheads: rotate a thread cutting tool through the helix angle, to set the nose vertical when viewed from the front of the tool.

• Thread cutting tools: Rotate through the helix angle to set the nose vertical (Figs 249 and 250). Viewed from the rear, rotate a left-hand helix anticlockwise, and a right-hand helix clockwise.

Fig. 251: Both workheads: tilt, to bring the front ridge between the flanks approximately vertical.

• Tilt, to bring the front ridge between the flanks approximately vertical (Fig. 251). It is not essential that the ridge is perfectly vertical, because only the short distance below the cutting edge is in contact with the work. When cutting a thread, the rest of the ridge is clear of the work.

• Colour the nose, to make it easier to judge the width of the flat.

• Apply a cut and traverse to grind.

• Option: Hone by hand, to a fine finish.

RADIUSED TOOLS

An external radius can be created swinging the work against the flat face of a wheel, as explained in Chapter 3. Fig. 252 shows a simple radius grinding attachment that can be used to control the distance from the tip of the tool to the swing axis, and the sideways position of the tool. That allows the centre point of the radius on the tool to be positioned accurately in relation to the swing axis. The grinder crossfeed is then used to advance the grinder workhead, with the attachment, to bring the tool tip into contact with the wheel, while swinging the attachment and tool from side-to-side. The size of the radius depends on the distance the nose of the tool projects beyond the swing axis that acts as the centre of a circle, and the Major cutting edge sits on the circumference of a circle whose radius is the distance from the swing axis to the nose. At that point, the wheel should also be one radius away from the swing axis.

Fig. 252: A radius grinding attachment designed to be mounted in the workhead swing axis. The toolholder can be moved backwards and forwards as well as side-to-side. to position the tool in relation to the swing axis. The setting micrometer pivots around the swing axis and reads zero when the end face of its spindle is on the swing axis.

For example:

Fig. 253: Plan to create a blending curve of 3mm radius between the Major and Minor cutting edges of a left-hand knife tool.

To create an external radius of 3mm on the nose of a left-hand knife tool, between the Major and Minor cutting edges (Fig. 253):

Fig. 254: Set the micrometer to 3mm.

• Using a setting micrometer, set the end face of the micrometer spindle to 3mm (Fig. 254). The setting micrometer should read zero when the end face of the spindle is aligned with the swing axis. Setting the micrometer to 3mm retracts the spindle, placing the end face 3mm beyond the pivot (nearer the wheel). Some radius-grinding attachments use a vertical setting bar, and the radial distances are set using accurate spacers or gauge blocks. Other arrangements include a half-diameter setting rod coincident with the swing axis. That establishes the zero position, but requires some means of accurately moving the tool forwards and to the side, to set the nose at the required distance (3mm in this example) beyond the swing axis.

Fig. 255: Swing the attachment and adjust the protrusion of the tool to align the Major cutting edge against the end face of the setting micrometer.

• Swing the setting micrometer and adjust the protrusion of the tool so that the Major cutting edge is aligned against the end face of the micrometer spindle (Fig. 255).

Fig. 256: Swing the micrometer and adjust the sideways position of the tool carriage to align the Minor cutting edge against the end face of the micrometer.

• Swing the setting micrometer (removing, swinging, then replacing it if necessary) and adjust the sideways position of the tool to align the Minor cutting edge against the end face of the micrometer spindle (Fig. 256).

• Where the cutting edges lies at an angle to the forward/backward and the side-to-side axes, so each movement affects the other. If necessary, repeat the two previous steps until no further adjustments are required. Note that the setting micrometer may have to be removed to allow it to swing past. It is then replaced.

Fig. 257: Tilt, to set the pivot axis at the clearance angle.

• Tilt the whole attachment to the clearance angle (Fig. 257).

• Colour the nose of the tool, using a marker pen or layout blue.

• Repeatedly apply a cut and swing the whole radius grinding attachment, until the wheel just touches the two cutting edges (as shown by the disappearing marker colour). The radius on the tool face will then be 3mm (Fig. 253). Continuing to advance will progressively reduce the radius of the nose.

Fig. 258: When the Major and Minor cutting edges just touch the wheel, the nose will have been ground to the required radius and any colour coat on the flanks will just have disappeared.

It is possible to use a typical type 6 cup wheel when grinding a radius, but there is often a real danger that the wheel will foul the radiusing attachment as it swings. A small tapered type 11 grinding wheel is usually a more practical choice. Careful adjustment of the wheel height may also help.

BORING TOOLS

Boring tools are designed to work inside a hole. Fig. 259 shows the tips of a typical solid boring tool and a boring bar with an inserted tip. The Major cutting edge does most of the cutting (Fig. 260), while the nose often carries a small radius to improve the finish. The Minor cutting edge does little cutting and the side rake takes it clear quite quickly. The smaller the hole, the greater the need for additional clearance at the heel of the tool.which can be created by grinding extra clearance facets at the heel and the rear of the head of the tool (Fig. 261).

Fig. 259: Two boring tools. On the left is a solid tool, and on the right is a bar holding an inserted bit.

Fig. 260: The Major cutting edge does most of the cutting, and the side rake takes the Minor cutting edge clear relatively quickly.

Fig. 261: Boring tools need sufficient clearance while inside a hole. That can be provided by grinding additional clearance facets at point 1 and, for safety, point 2.

Major and Minor cutting edges can be sharpened in the same way as for a lathe tool, although the orientation of the shank is likely to be different.

Holding boring tools

Like lathe tools, boring tools with a square-section shank can be gripped in a lathe-style toolpost, a vee block or a toolholder with a slot and grubscrews to clamp the tool. Boring tools with a cylindrical shank can be gripped in a collet or a sleeve, both of which allow initial orientation by rotation.

Boring bars carry an inserted bit that can be removed and sharpened separately. Alternatively, it is often possible to leave the inserted bit in the bar and sharpen it in situ, although one challenge may be in the setup for the Minor cutting edge, when the bit may have to be rotated in the bar and/or held inverted. A second challenge would be in setting up to grind the Major (top) face, and the bit may have to be rotated through 90 degrees from its normal cutting position. A taper cup wheel is likely to be required, to provide clearance for the toolholder.

Sharpening boring tools

Boring tools are held in a similar manner to lathe tools, although the cutting edges may face in different directions. The examples that follow show a shanked tool, but the same basic principles apply for inserted bits held in a collet or a bar.

Major (front) flank: S-T-R workhead

Reference position:

Fig. 262: S-T-R workhead: reference position for a square-shanked boring tool in preparation for grinding the Major (front) flank.

The base of a square-section shank should be horizontal, with the long axis at right angles to the traverse, and the cutting part facing the wheel (Fig. 262).

Fig. 263: Swing to bring the Major cutting edge parallel to the traverse and the face of the wheel, when viewed from above. Then tilt to the Major flank clearance angle.

• Swing to bring the Major cutting edge parallel to the face of the wheel, when viewed from above (Fig. 263). That sets the cutting edge in a plane parallel to the tilt axis in preparation for the next movement.

• Tilt to create the Major flank clearance (Fig. 263).

• Apply a cut and traverse to grind.

Major (front) flank: T-S-R workhead

Reference position:

Fig. 264: T-S-R workhead: reference position for a square-shanked boring tool in preparation for grinding the Major (front) flank.

The base of a square-section shank should be vertical, with the long axis at right angles to the traverse, the cutting part facing the wheel and the tip uppermost (Fig. 264).

Fig. 265: T-S-R workhead: tilt to set the Major cutting edge vertical when viewed from the side. Then swing through the Major flank clearance angle, to set the flank parallel to the plane of the wheel face.

• Tilt to set the Major cutting edge vertical, when viewed from the side (Fig. 265). That places the cutting edge in a plane parallel to the face of the wheel in preparation for the next move.

• Swing through the Major flank clearance angle to set the Major flank vertical and parallel to the traverse (Fig. 265).

• Apply a cut and traverse to grind.

Minor (side) flank: S-T-R workhead

Reference position:

Fig. 266: S-T-R workhead: reference position in preparation for grinding the Minor (side) flank of a square-shanked boring tool.

Fig. 267: S-T-R workhead: swing to bring the Minor cutting edge parallel to the traverse. Then tilt to the Minor flank clearance angle. Parts of the tool or workhead may project behind the wheel face.

Mount the tool with its shank horizontal, base down and Major face up. Set the axis of the shank parallel to the traverse. The Minor flank should face the wheel (Fig. 266). Swing to bring the Minor cutting edge parallel to the traverse and the face of the wheel. Be prepared for parts of the shank and workhead to be sited behind the plane of the wheel face (Fig. 267).

• Tilt to the Minor flank clearance angle (Fig. 267) to set the flank parallel to the plane of the front of the wheel.

• Apply a cut and traverse to grind.

Minor (side) flank: T-S-R workhead

The position of the Minor cutting edge may require part of the workhead to be sited behind the plane of the face of the wheel.

Reference position:

Fig. 268: T-S-R workhead: reference position in preparation for grinding the Minor (side) flank of a square-shanked boring tool.

Mount the tool with its shank vertical; base on the right and at right angles to the traverse. The Minor flank should face the wheel (Fig. 268).

Fig. 269: Tilt, to set the Minor cutting edge vertical and parallel to the axis of swing when viewed from the side, then swing to the Minor flank clearance angle.

• Tilt, to set the Minor cutting edge vertical when viewed from the side (Fig. 269).

• Swing to the Minor flank clearance angle (Fig. 269).

• Apply a cut and traverse to grind.

Major (top) face: S-T-R workhead

As with the lathe tool composite rake shortcuts (page 54), the following method may result in a small error. If the Major and Minor cutting edges are not 90 degrees apart.

Reference position:

Fig. 270: S-T-R workhead: reference position in preparation for grinding the Major (top) face of a square-shanked boring tool.

Tool axis parallel to the axis of traverse. Tool base vertical, tool tip uppermost, and the Major face nearest the wheel. Normally, for a right-hand tool, the tip of the tool will be on the left (Fig. 270).

Fig. 271: S-T-R workhead: swing through the Major face rake angle, then tilt through the Minor face rake angle.

• Swing through the Major face rake (Fig. 271).

• Tilt through the Minor face rake (Fig. 271).

• Apply a cut and traverse to grind.

Major (top) face: T-S-R workhead

As with the lathe tool composite rake shortcuts (page 55), the following method may result in a small error if the Major and Minor cutting edges are not 90 degrees apart.

Reference position:

Fig. 272: T-S-R workhead: reference position in preparation for grinding the Major (top) face of a square-shanked boring tool.

Tool axis parallel to the axis of traverse; tool base vertical, tip uppermost, Major face nearest the wheel. Normally, for a right-hand tool, the tool tip will be on the left (Fig. 272).

Fig. 273: T-S-R workhead: tilt through the Minor face rake angle, then swing through the Major face rake angle.

• Tilt through the Minor face rake, until the Major cutting edge is vertical when viewed from the end nearest that edge (the left end) (Fig. 273).

• Swing through the Major cutting edge rake angle (Fig. 273).

• Apply a cut and traverse to grind.

Creating clearance at the heel

Boring tools are often expected to begin boring inside a relatively small pilot hole where there is little clearance between the back of the bar and the wall of the bore (Fig. 261). In this situation, the lower limit on the bore size is set by the amount of clearance at the front of the tool, at positions 1 and 2 in Fig. 261. To ensure adequate clearance, grind enough material from the front and rear of the heel to avoid fouling against the walls of the bore.

Creating a solid boring tool

On a commercial small-diameter boring tool the cutting part projects further than the shank that follows (Fig. 274). Grinding a boring tool (as opposed to a bit for a boring bar) from a cylindrical high speed steel blank means the neck must be ground to a smaller diameter than the head (Fig. 275). The neck could be ground by eye, but it is much more satisfactory to grind the neck as a cylinder, with its axis offset from the axis of the bar (Fig 275). The clearance is twice the offset, and grinding eccentrically gives strength, creating clearance at the front, behind the Major cutting edge, while leaving the maximum amount of material towards the rear of the shaft. In the absence of a purpose-made tool, an adjustable boring head can be used to create the offset between the axes. Mount the boring head in a workholder on a shaft that can rotate about its axis, and adjust the boring head so that the bar turns concentrically. Concentricity can be measured using a DTI fitted with an elephant's foot (Fig. 276). Then raise or lower the boring head slide by the required offset distance (Fig. 277). Set the bar parallel to the face of the wheel, and rotate the head and bar assembly so that the bar is brought closest to the wheel. Feed at right angles to the face of the wheel, while rotating the head and bar (Fig. 278). The bar will touch the wheel during the eccentric part of the rotation, and the neck will gradually be formed. Depending on the width of the face of the wheel and the required length of the neck, it may be necessary to traverse slowly as the assembly is rotated. Stop when the neck has been ground back sufficiently to create the required clearance behind the head. Grinding can stop just as the wheel kisses the rear of the shaft. If necessary, a raising or blending curve may be created between the neck and the head and shank by setting the tool at an angle to the wheel after the main clearance has been ground. Alternatively, dress each corner of the wheel to a radius that will create a blend curve; before carrying out the whole grinding operation.

Fig. 274: A small boring tool with a tip which projects beyond the shaft diameter.

Fig. 275: Offsetting the centre of rotation allows the creation of a shaft with a reduced diameter.

Fig. 276: Use a DTI with an elephant's foot to centre a tool blank in a boring head.

Fig. 277: Offset the shank by raising or lowering the boring head slide.

Fig. 278: Rotate the workhead while traversing between stops to grind the eccentric neck.

Fig. 279: A completed small diameter boring tool, showing the eccentric shaft behind the cutting head. The head was created in a separate operation, after the neck had been ground.

Tool type 3:
Fly cutters

Fly cutters are single-point tools (Fig. 280) that can be swept across a workpiece, creating a flat surface (Figs 281 and 282). They are often used in milling machines, where they can provide a fine finish to a surface. Fly cutters often use inserted toolbits with a square or circular cross-section. In this book, the term ‘fly cutter’ refers to the toolbit.

Fig. 280: Fly cutters can be ground to many different shapes to suit the job. Some may be designed to sweep across a surface, while others may act as form cutters to recreate their shapes in the workpiece.

Fig. 281: A fly cutter mounted in the spindle of a vertical milling machine.

Fig. 282: A fly cutter gripped in a holder mounted underneath a faceplate can be used to produce a flat surface on work secured to a milling machine table. This arrangement allows the fly cutter to sweep over a large area.

The best way to hold a fly cutter for sharpening depends on its shape. Some may be held in a toolpost, like a lathe tool, while others are best held in a collet, a sleeve or a chuck.

SHARPENING A SURFACING FLY CUTTER

Fly cutters used for surfacing are designed to sweep across a surface, making it flat. They usually have a Major cutting edge lying along the work surface, and a Minor cutting edge rising up away from the surface (Figs 283 and 284). The Major cutting edge does most of the cutting, but the nose and the Minor cutting edge may also contribute. Often, these cutting edges are approximately horizontal and vertical, but this need not necessarily be so. Surfacing fly cutters are often mounted with their shanks at an angle to increase their radius of sweep (Fig. 283) but they may also be mounted vertically on the end of a long bar, to sweep a much larger radius (Fig. 284). The positions of the cutting edges in relation to the work remain the same, for all angles of the shank. The Major cutting edge is usually at the end of the cutter and sweeps over the work in a plane at right angles to the axis of the spindle. The Minor cutting edge is at the side.

Fig. 283: A view looking at the Major face of a typical surfacing fly cutter. The nose may be straight, curved, or fully radiused.

Fig. 284: Fly cutters may operate with their shanks held vertical, but the cutting edges remain in the same position relative to the workpiece, as shown in Fig. 283, and the cutting action is the same.

Fig. 285: Faces, cutting edges, rake and clearance angles on a typical facing fly cutter. The cutter shown is designed to be held vertically, so that the Major cutting edge sweeps across the work in the horizontal plane.

Surfacing fly cutters (Figs 283–285) may be sharpened in a similar way to a lathe boring tool, and an inserted toolbit may remain in its holder, provided there is enough clearance around the tip, or it may be removed and held in a collet, chuck or sleeve. The instructions that follow assume the tool-bit will be held in a chuck or collet, unless stated otherwise. If the toolbit is held in its holder, orientating the toolbit may require additional movements during the setup process, to take account of the angle at which the toolbit is mounted in its holder and ensure the toolbit is in the appropriate reference position.

Cutting edges are sharpened by grinding adjacent flanks and faces, and the general method when setting up to grind a face or flank is to swing to position the cutting edge parallel to the direction of traverse, then tilt to bring the face or flank parallel to the traverse (and the face of the wheel), creating a clearance angle or rake for the cutting edge.

Reference position:

Fig. 286: The reference plane for a rectangular section tool is best set parallel to the base.

Fig. 287: The reference plane for a circular-section tool must be positioned so that the angles of the cutting edges and faces are correct when measured from the plane, which effectively acts as the base.

For tools ground from a square- or rectangular-section blank, a suitable reference plane is the base of the tool when the tool is sitting horizontally, so that it can be held in the same way as a lathe tool (Fig. 286). For tools with a circular cross-section, the reference plane is parallel to the long axis of the shank (Fig. 287). Hold circular-section tools in a collet or sleeve, and set the reference plane horizontal by using one of the following techniques:

Fig. 288: Set the Major face vertical, then rotate through 90 degrees to bring the face horizontal.

• Where the Major face has no side rake, set the Major face vertical (Fig. 288) then rotate through 90 degrees to bring the Major face horizontal.

Fig. 289: Set the Minor flank vertical, then rotate through the Minor flank clearance angle.

• Where the Major face has side rake, set the Minor flank against a vertical edge, then rotate clockwise through the Minor flank clearance angle (Fig. 289). The Minor flank is used to avoid complications where the Major face has a compound rake angle.

• Set the long axis of the cutter parallel to the axis of traverse.

S-T-R AND T-S-R WORKHEADS

The examples in this chapter are for S-T-R workheads. If you are using a T-S-R workhead, or a tilting workhead on a swing-table grinder, either:

• rotate the fly cutter through 90 degrees then use tilt instead of swing, to set the cutting edge vertical instead of horizontal; and swing instead of tilt, to create clearance angles.

or

• follow the instructions in the section entitled ‘Using a curved wheel’. Swing is created by swinging the table, but the clearance angles are created using the curve of the wheel face rather than by tilting.

There are many examples showing the use of T-S-R workheads, in Chapter 3: ‘Lathe tools’.

Leaving a land

An optional technique to improve the surface finish left by a surfacing fly cutter is to create a small flat land on the flank immediately behind each cutting edge. This has the effect of rubbing against the cut work surface. Where the cutter has a land, recreate it; where there is no land, it may be omitted, or the flank(s) may be modified to create it. If there is a radiused nose, both flanks should either have, or omit, the land. A land should typically be 0.1–0.2mm (4–8 thou) wide, and should have zero clearance angle.

Sharpening the Major cutting edge

A Major cutting edge is sharpened by grinding the Major flank, with the top face being ground only if it is damaged – in which case the top face should be ground first.

Fig. 290: A fly cutter with long axis at 45 degrees, and with 5 degrees Major cutting edge clearance angle.

Fig. 291: Viewed from above: swing through the approach angle (40 degrees) to bring the Major cutting edge parallel to the axis of traverse.

Rake and clearance angles are measured from the reference plane, at right angles to the cutting edges, in the same way as for a lathe tool. Fig. 290 shows cutting edge angles on a working tool, viewed from the front. Fig. 291 is the grinding setup, viewed from above, showing the angles when positioned for grinding.

Reference position:

Cutter horizontal, with the axis of the cutter parallel to the traverse.

Fig. 292: Swing through the (red) approach angle to produce the same (green) angle between the axis of the tool and the face of the wheel.

• Swing to bring the Major cutting edge parallel to the axis of traverse (Fig. 292). Note that the angle of swing (red) always produces the same angle (green) between the axis of the tool and the face of the wheel (when that is parallel to the axis of traverse).

Using a flat-faced wheel:

• Optionally: Create or re-form a land by grinding the flank while the tool is ‘horizontal’. The land should be (approximately) between 0.2 and 1mm (8 and 20 thou) wide, at this stage.

Fig. 293: Tilt the workhead upwards at the front to create the Major flank clearance angle.

• Tilt the workhead, taking the Major cutting edge upwards, to set the Major flank clearance angle (Fig. 293).

• Coat the Major flank with layout blue, or rub with a marker pen. Apply a cut, and traverse to grind, controlling the final depth of cut to leave the land approximately 0.1–0.2mm (4–8 thou) wide.

Using a curved wheel:

Do not attempt to create a land.

Fig. 294: Set the Major cutting edge at the wheel centre height.

• Set the highest point of the Major cutting edge at centre height (Fig. 294).

Fig. 295: Raise the cutter by 'H' above the wheel centre height, to create the clearance angle.

• Raise the cutter (or lower the wheel) to create the clearance angle (Fig. 295, and see Chapter 3).

• Apply a cut, and traverse to grind.

Sharpening the Minor cutting edge

Fly cutters used for surfacing swing around an axis, so they need enough clearance on the Minor flank to avoid the lower trailing (rear) edge of the Minor flank from fouling the work at the smallest radius of swing (shown as R in Fig. 281. See also Fig. 296). This may mean creating the initial Minor flank clearance, then adding facets further down the Minor flank as required to create additional clearance towards the lower edge of the flank.

Fig. 296: Viewed from above, in the cutting position, the trailing heel may need additional clearance.

Reference position:

Fig. 297: Set the axis of the tool at right angles to the traverse, then bring the Minor cutting edge parallel to the traverse by swinging through 35 degrees.

Cutter horizontal, with the axis of the cutter at right angles to the axis of traverse (Fig. 297). Compared to Fig. 290, the approach angle of 80 degrees is reduced by 45 degrees to leave 35 degrees between the cutting edge and the face of the wheel/axis of traverse.

Fig. 298: Set the axis of the tool horizontal and at 90 degrees to the axis of traverse, with the tool top face uppermost, then swing through 35 degrees to bring the Minor cutting edge parallel to the axis of traverse.

• Set the Minor cutting edge parallel to the axis of traverse by swinging through 35 degrees (Fig. 298).

Using a flat-faced wheel:

• Optionally: Create or re-form a land by grinding the flank while the tool is ‘horizontal’. The land should be (approximately) between 0.2 and 0.5mm (8 and 20 thou) wide, at this stage.

Fig. 299: Tilt the workhead tail-down to create the Minor flank clearance angle.

• Tilt the workhead, taking the Minor cutting edge upwards, to set the Minor flank clearance angle (Fig. 299).

• Coat the Minor flank with layout blue, or rub with a marker pen. Apply a cut, and traverse to grind, controlling the final depth of cut to leave the land approximately 0.1–0.2mm (5–10 thou) wide.

• If required, increase the angle of tilt, and grind a secondary clearance facet to take the trailing edge of the flank clear of the work (Fig. 299).

Using a curved wheel:

Do not attempt to create a land.

• With the cutter horizontal, either raise the cutter or lower the wheel to create the clearance angle (similar to Fig. 295, and see Chapter 3).

• Apply a cut, and traverse to grind.

• Raise the cutter (or lower the wheel) further, to create greater clearance angles on any secondary facets at the heel.

Grinding the top face

Most fly cutters do not need a compound rake angle on the Major face, as any Minor cutting edge rake can be set by rotating the cutter within its holder on the lathe or milling machine. If necessary, calculate the angle and rotation required to create combined Major and Minor face rake in the same way as for a lathe tool (see page 52). The following instructions assume there is no compound rake angle on the top face.

Reference position:

Use a small type 1 wheel. The workhead only requires a tilt axis; other axes are unused. Grip the cutter bit in a sleeve with a tapered nose to provide clearance.

Lay the cutter on its side, with the Major face vertical (parallel to the wheel face) and the long axis of the tool parallel to the axis of traverse.

Fig. 300: Fly cutter set with its axis parallel to the traverse, and its Major cutting edge horizontal.

Fig. 301: Lower the wheel to create the Major face rake angle.

• Tilt to set the Major cutting edge horizontal (Fig. 300). Set the wheel centre at the same height as the Major cutting edge. Calculate the wheel drop (H) to create the Major face rake angle (see Chapter 3); then lower the wheel through the drop distance (Fig. 301).

• Set stops, then traverse to grind.

Rounding the nose

To round the nose manually, hold the cutter vertical on a bench stone, tilt it backwards, to the flank clearance angle, then swipe it from left to right, while following the shape of the nose by rolling the tool from left-tilt to right-tilt (Fig. 204). A few strokes should be enough. Finish on a fine stone. For a more precise radius, use a radius grinding attachment (see following).

To create the land around the nose

Reference position:

Use a flat-faced wheel. On the radius grinding attachment, set the axis of swing vertical. Set the tool horizontal.

• Set the setting micrometer to the radius required for the nose of the toolbit. Option: Add a small grinding allowance of around 0.2mm (8 thou).

Fig. 302: Swing the micrometer and adjust the protrusion and sideways position of the tool to align both cutting edges against the end face of the micrometer.

• Adjust the protrusion and sideways location of the toolbit so that the cutting edges both align against the end face of the micrometer spindle (Fig. 302).

• Feed the workhead forward while swinging the radius attachment from side-to-side. Stop when the cutting edges are one radius from the wheel.

To blend the clearance flanks

Reference position:

Set up the radius grinding attachment as for the land.

Fig. 303: Tilt to the flank clearance angle.

• Tilt the axis of swing backwards, to the flank clearance angle (Fig. 303).

• Colour the nose of the toolbit using a marker pen or some layout blue.

• Advance the workhead using the crossfeed, while swinging the radius grinding attachment from side-to-side. Watch the colour of the tip, and stop before the land disappears.

Shaping and sharpening a gashing fly cutter

Fly cutters are sometimes used to gash gaps between teeth when making a simple gear, or to create a groove when fed along a workpiece (Fig. 304). In both cases, the cutter normally rotates about an axis at right angles to the axis of the cutter. A gashing fly cutter may be followed by a precisely shaped gear cutter, or another fly cutter shaped to create the sides of the teeth.

Fig. 304: Side and top views of a typical gashing fly cutter for gashing clock gears.

The initial sequence for shaping or sharpening is the same as for a lathe parting tool, and the toolbit is best held in a collet or sleeve.

Creating continuous Major flank clearance

The Major flank of a gashing fly cutter can be curved to provide clearance for the heel, by creating an eccentric motion using a boring head. Mount the cutter in a holder, set at its normal radius of operation, and hold it in a boring head (Fig. 305). Set the Major cutting edge horizontal, and the Major face upwards when the cutter is at right angles to the slide of the boring head. A zero-clearance arc will be created on the Major flank (see above), if the centres of rotation of the workhead and of the cutter are the same (Fig. 306). If the centre of rotation of the cutter is lowered, using the feedscrew on the boring head, without altering the axis of rotation of the workhead, rotating the workhead about its axis will produce a continuously increasing curved clearance behind the cutting edge. In Figs 306 and 307, the black circle shows the path of the cutter tip when the axes of the holder and the workhead are the same. The circumference of the green circle shows the path of the tool tip when the axis of the holder is below the axis of the workhead. The solid green area shows the additional material removed during grinding, producing additional clearance. The clearance is a maximum after approximately a quarterturn, but by then the curve will have cleared the tip of the cutter, so the fact that the clearance reduces after that point has no practical consequence.

Fig. 305: When the axis of the cutter holder and the axis of the workhead coincide, the tip of the cutter travels around the circumference of a circle.

Fig. 306: Offsetting the axis of the holder from the axis of the workhead will create increasing clearance (shown in green), at least for the first quarter turn.

Fig. 307: When the boring head is adjusted to lower the axis of the cutter holder below the axis of the workhead, an increasing amount is ground from the underside of the tip of the cutter as the workhead rotates through the first 80 degrees about its axis.

• Coat the tip of the cutter in layout blue, or smear it using a felt-tipped marker.

• Set the workhead so that the axis of adjustment for the offset is vertical, then lower the cutter holder approximately 0.1mm. The exact distance can be judged after each grind.

• Rotate the workhead and use the crossfeed to bring the heel of the cutter to touch the wheel. Rotate the workhead back towards the horizontal position, then inspect the blued tip to check the extent of the ground area.

• Increase the cut and rotate the workhead about its axis, to create the clearance, in stages.

Tool type 4:
Saw blades

SHARPENING SLITTING SAWS

A slitting saw is a flat circular multi-tooth cutter (Fig. 308) used to saw work (Fig. 309) or create a groove (Fig. 310).

Fig. 308: Slitting saws are flat circular multi-tooth cutters.

Fig. 309: Cutting through a motorcycle top yoke.

Fig. 310: Sawing a slot in a screw head.

Side clearance prevents the teeth from jamming in the cut., and that is provided by:

Fig. 311: Thin lines of light between ruler and saw show that hollow grinding provides side clearance.

• Hollow grinding (‘dishing’) both sides of the saw during initial manufacture (Fig. 311), making the centre slightly thinner than the periphery; or

Fig. 312: Wider teeth create side clearance.

• Making the teeth wider than the blade, and creating side clearance on the teeth (Fig. 312); or

Fig. 313: Radial cutting edges running from the periphery to near the centre will cut clearance.

• Providing cutting edges that run radially from the periphery almost to the centre of the wheel (Fig. 313).

Saws with teeth wider than the blade, or with radial side-cutting edges will be considered as ‘side and face’ milling cutters (see Tool type 6: Milling cutters).

Fig. 314: Most saws have cutting edges at right angles to the sides of the blade and parallel to the axis of the saw.

Fig. 315: Some saws have teeth with a single clearance angle.

Fig. 316: Some saws have teeth with both a primary and a secondary clearance angle.

Fig. 317: Most saws have cutting edges which are at right angles to the flat side, and parallel to the axis of the saw.

Fig. 318: Some saws have teeth with a side rake angle. Alternate teeth are angled in opposite directions.

On each tooth, the Major cutting edge is at the tip, and the Major face runs from the cutting edge towards the centre of the cutter, although it is not truly radial, because of the rake angle (Fig. 314). On many slitting saw blades, especially those with fine-pitched teeth, the individual teeth have a single Major face with a rake angle, and a single Major flank with one clearance angle, as shown in Fig. 315. However, some saws have a single Major face, and a Major flank that has both a primary and a secondary clearance angle, as shown in Fig. 316. On most small slitting saws, the Major cutting edges all lie at right angles to the side of the saw, parallel to the central axis (Fig. 317), although some saws incorporate a side rake angle created by arranging for the cutting edges to be at an angle to the axis of the saw (Fig. 318). On those saws, the side rake on adjacent teeth lies in opposite directions, so the teeth must be sharpened in two distinct groups, changing the sideways tilt of the cutter between grinds. Saws with side rake will be treated as milling cutters (see Tool type 6: Milling cutters).

Holding saws

For sharpening, saws may be mounted on their own arbor, or a separate purpose-made arbor, but thin slitting saws will require additional side support to prevent flexing during grinding (Fig. 319). Fig. 320 shows an adjustable fixture designed to hold a range of sizes of slitting saws. It consists of a plate with a slot that guides a post. A bush on the post locates in the bore of the saw, and the clamp knob pushes a large cup washer against the saw, trapping it and giving the saw support during grinding. The fixture is designed to attach to a grinding workhead so that it can be swung or tilted, if necessary, and advanced towards the wheel. Fig. 321 shows the same fixture fitted with an alternative holder designed to cater for small saws. Fig. 322 shows the component parts of the fixture, together with two simple setting gauges to align the saw teeth.

Fig. 319: Saws may be held on a purpose-made arbor to suit the bore of the saw, but thin saws will require substantial support for the sides.

Fig. 320: Shown here mounted on a swivelling workhead, an adjustable fixture makes holding saws easy.

Fig. 321: Small saws may be mounted on a little arbor and gripped in a suitably-sized holder. The riser block allows clearance for the grinding wheel.

Fig. 322: Component parts of a saw sharpening fixture. The large and small setting gauges are used to position a saw tooth on centre. The small saw holder mounts on the main support plate and holds saws of less than 20mm diameter.

Choosing a wheel

A suitable grinding wheel must be able to traverse down the Major flank or the Major face of each tooth without colliding with any other part of the saw. Depending on the tooth shape, grinding wheels might be a taper cup, saucer cup, dish or thin type 1. Thin type 1 wheels, 1–6mm thick, are available specifically for sharpening saws, and if the edge is dressed to an angle to avoid touching adjacent faces in a gullet, they make a very effective choice (Fig. 323). Dress the wheel true, then use a diamond dresser to create the angle, and a dressing stick, used freehand, to form a curve on the other edge.

Fig. 323: A thin type 1 wheel, dressed to provide an angled face and a rounded front corner.

Indexing

The basic sharpening method involves grinding one face of a tooth, then repeatedly moving the next tooth into position and sharpening the same face of that tooth, until all the teeth have been ground identically. There are several methods for indexing from one tooth to the next, but some give more consistent results than others, particularly over repeated sharpening sessions.

Positioning teeth by feel and sight

Often used to grind the Major face of each peripheral tooth on a circular saw blade for woodworking, this method involves simply bringing the face of each tooth against the flat face of a grinding wheel. Continuing to turn applies a little pressure to create a feed, grinding the tooth. The action is repeated for every tooth. Although this technique is used on some low-cost commercially available sharpening machines for circular saw blades, it cannot give consistent results, and will inevitably result in teeth with faces and Major cutting edges at irregular intervals, especially after repeated sharpening sessions. In addition, this method changes the geometry of the teeth each time it is used, progressively reducing the rake each time grinding takes place. Consistent and accurate results really need a reliable mechanical method of setting each tooth in position.

Using a detent

Once the first tooth has been positioned ready for grinding, a ‘finger’ or detent is located against that tooth to establish the position (Fig. 324). The first tooth is sharpened, then the next is rotated into position, against the detent. Typically, this means rotating the cutter until the detent clicks into position against the next tooth, and rotating slightly backwards, to ensure the detent is located firmly. The detent should locate against a part of the tooth that will not be ground during the sharpening session. If, for example, the primary clearance on each tooth is to be ground, the detent should be located against the Major face of the tooth being ground (Fig. 324). The challenge is in locating the detent where it will not itself be ground. Clearance may be very small (Fig. 325), and one shape of detent is unlikely to be suitable for all situations and shapes of teeth (Fig. 326).

Fig. 324: A springy detent 'finger' is located against the tooth to be ground.

Fig. 325: Despite the hooked end, the adjacent tooth prevents this detent from sitting far enough back to avoid being ground.

Fig. 326: Creating a dropped end to the detent allows the main finger to sit further back, avoiding the grinding wheel.

If the Major face of each tooth is to be ground, locating the detent against the Major face of another tooth may seem to be convenient. However, as sharpening progresses, the detent will eventually locate against at least one tooth that has been ground, and that will move the reference position slightly. Over successive sharpening sessions, the effect will be for tooth spacing to become increasingly irregular. Instead, locate the detent against the Major face of the tooth, lock the tooth in position, then swing the detent aside. Using a block with a pin as a stop allows the detent to be able to return to its original position for the next tooth (Fig. 344).

A different approach is to locate the detent against another tooth, further round the periphery; but against a face which will not be reground in that session. One tactic is to prepare for this when the blade is new, and before it does any cutting. Use the front of each tooth as the reference for the detent, and grind the backs of the teeth accurately. To sharpen the fronts, locate the detent against the backs of the teeth. If the backs ever need regrinding, locate the detent against the fronts of the teeth.

Using a dividing device

A collar with accurately spaced holes, a dividing disc or a gear wheel may be used to provide an independent reference to ensure uniform positioning of tooth faces (see Chapter 2). Set the dividing device in its zero position, leaving the saw free to rotate. Position the first tooth (see later), then tighten the saw onto the dividing device. Indexing from one tooth to the next is by means of the dividing device. To sharpen a different part of the teeth, loosen the saw, set the first tooth in position, then tighten onto the dividing device, and proceed as before.

Setting angles

Compared to an initial reference or ‘zero’ position, the angles of the faces on the saw are most accurately set using a finely graduated scale, by measurement or by comparison with a gauge.

Setting a reference position

The basic ‘zero’ position of the wheel and the saw will depend on which tooth face is to be ground, and on how the saw is mounted.

Fig. 327: On a saw, the basic reference is the radial line from the axis of the arbor (effectively the centre of the saw) to the tip of a tooth.

On the saw, the basic reference is the radial line from the axis of the arbor to the tip of a tooth (Fig. 327).

Fig. 328: For a saw mounted horizontally on an arbor, the axis of swing, the pivot post, and the tip of a tooth should all lie on the same straight line.

When the saw is mounted vertically, the most useful ‘zero’ position is with the tip of a tooth facing upwards and lying on a horizontal radial line (Figs 327 and 328). For a saw mounted on an arbor, the radial line begins at the central axis of the arbor. For a saw mounted on a fixture plate, there should be a straight line from the axis of swing, through the centre of the saw pivot post, to the tip of a tooth (Fig. 328).

Fig. 329: Set the swing/pivot axis (or the reference face) at right angles to the direction of traverse, by using the swing axis scale.

Fig. 330: Set the reference face on the side of the fixture at right angles to the direction of traverse, by using the swing axis scale.

For the workhead, one ‘zero’ position is with the swing/pivot/tip axis parallel to the direction of traverse (Fig. 329),and another is with the swing/pivot/tip axis at right angles to the direction of traverse (Fig. 330). When using a horizontal fixture plate, it is useful if there is a reference face on the side of the plate that is parallel to the swing/pivot/tip axis. That allows the swing/pivot/tip axis to be aligned by aligning the side of the fixture plate instead.

Fig. 331: Hold the setting gauge against the side of the fixture, and rotate the saw to align a tooth tip with the end of the gauge.

Figs 331 and 339 show a setting gauge (the ‘large saw setting gauge’) that is held against the reference side of the sharpening fixture. The edge of the protruding blade has been aligned with the centre of the pivot on which the saw will be mounted. Holding the gauge in position, use high magnification to align the tip of a tooth with the edge of the blade.

Fig. 332: A fixture plate mounted so that its working face is vertical.

Fig. 333: Set a height gauge against the face of a freely-rotating centre-setting pin, and tighten the locking collar under the height gauge block.

If the saw is held on a fixture plate whose face is vertical (Fig. 332), remove the fixture and its shaft, then put a freely revolving centring pin in the tilt axis; set a centre-height gauge to the pin (Fig. 333), and place a locking collar under the finger of the centre-height gauge. Then replace the fixture and shaft and use the gauge against the saw pivot post to align that with the centre of the tilt axis. This requires that the height gauge be above its set height by half the diameter of the pivot post, so adjust the gauge finger using one of the following methods:

Fig. 334: Place a washer of half the thickness of the pin, between the locked collar and the centre height gauge finger block.

• Add a washer of half the post thickness, between the locked collar and the underside of the finger block (Fig. 334); or

Fig. 335: Place a drill with a diameter half the thickness of the pin, between the locked collar and the finger block.

• Place a drill of half the post thickness between the locked collar and the underside of the finger block (Fig. 335); or

Fig. 336: Under the foot of the height gauge, place spacers of half the thickness of the pin, as indicated by the red arrows.

• Under the foot of the gauge, add spacers of thickness half the diameter of the post (Fig. 336).

Fig. 337: Remove any washers, drills or spacers, then rotate the saw to set the tip of an upward-facing tooth against the underside of the height gauge.

Fig. 338: On a vertical saw, set the tip of an upward-facing tooth against the underside of the gauge.

For a saw held vertically and with upward-facing teeth, whether mounted on an arbor or held on a fixture plate, rotate the saw until the tip of a tooth lies against the underside of the height gauge finger (Figs 337 and 338).

Sharpening the cutting edges

Choose the sharpening method according to the amount of damage or wear on the cutting edges. Most sharpening avoids grinding the Major faces of the teeth, except when there is damage to those faces, when they should be ground before the flanks.

When the teeth are slightly blunt, sharpen by grinding the Major flank at the primary clearance angle. This will reduce the diameter of the saw slightly. After successive sharpening sessions makes the primary clearance flank more than, say, 1.5–2mm long, grind the secondary clearance angle. However, it is not unusual for the whole back of the tooth to form a single clearance angle, especially on small saws.

The procedure involves rotating, swinging (a horizontal saw) or tilting (a vertical saw) to bring the clearance face parallel to the direction of traverse. The same procedure is used for both primary and secondary clearances, although for secondary clearances, the angle of swing or tilt will be larger.

Grinding the faces of a saw held horizontally

• Mount a thin type 1 wheel (Fig. 323) with its face parallel to the axis of traverse, then dress and shape the face and edge. Swing the trailing edge of the wheel 0.5 to 1 degree away from parallel, to provide clearance. The centre of the wheel should be at approximately the same height as the saw. Using a thin type 1 wheel allows all three faces of a saw to be ground using the same wheel, for convenience. An alternative, applicable only to grinding the primary face, is to mount a type 11 taper cup or type 6 cup wheel with the face parallel to the axis of traverse.

Fig. 339: With the teeth facing anti-clockwise, use a setting gauge to set the tip of a tooth on centre.

• Mount the fixture plate with the swing/ pivot/tip axis at right angles to the axis of traverse. Mount the saw on the fixture with the teeth facing anticlockwise, and use a centre-setting gauge to set the tip of a tooth on the fixture plate axis (Fig. 339).

The next stage is to swing the fixture plate to set the clearance face parallel to the axis of traverse. The best method depends on whether there is a single clearance face, or a primary and secondary clearance face on each tooth. Where there is a single clearance face, treat that as if it was a secondary clearance face.

To grind the primary clearance (where the tooth has both primary and secondary clearances):

Fig. 340: Swing the fixture plate through the primary clearance angle. The detent block rests against a stop pin to allow the detent to be swung or lifted clear then replaced.

• Where the fixture plate can swing, and has a graduated scale, swing it through the primary clearance angle (Fig. 340). This is the easiest method for all clearance faces, and the best method for teeth that have a single clearance face.

Fig. 341: Calculate the linear horizontal distance (H) the tip of the tooth must move to rotate the saw to the primary clearance angle.

• Where each tooth has both a primary and secondary clearance, and there is no swing scale, calculate the linear horizontal distance (H) the tip of the tooth needs to move, to rotate the Major flank (the primary clearance face), as shown in Fig. 341, using the formula:
distance = radius × SIN(clearance angle)

For example:
If a saw has diameter 75mm (radius 37.5) and a clearance angle of 7 degrees,
distance = 37.5 × SIN(7) = 37.5 × 0.122
= 4.575mm (4.6mm will be near enough).

Fig. 342: Place a slip of material between the fixture plate reference face and the centre-setting gauge.

• Rotate the saw on its pivot post until the tip of the tooth has moved 4.6mm, in a direction that brings the clearance face parallel to the axis of traverse. Accurate rotation may be difficult to judge, so use a slip of material of the required thickness (4.6mm in this example) between the side of the centre-setting gauge and the reference side of the fixture. Then set the tooth to the gauge (Fig. 342).

To grind the secondary clearance face (if there is one), or a single clearance face:

• Mount a thin type 1 wheel. Dress and shape the face and the edge so that the wheel will not foul the adjacent Major face as it reaches the bottom of the gullet (Fig. 323).

Fig. 343: Swing the fixture to bring the secondary clearance face parallel to the axis of traverse.

• Where the fixture plate can swing, and has a graduated scale, swing it to the secondary (or single) clearance angle (Fig. 343).

• Where there is no scale, align the face against the side of the wheel, by eye. The method of calculating a distance instead of a rotation, as used for the primary clearance, usually results in an inconveniently large thickness for a distance piece to be placed between the setting gauge and the reference side of the fixture. As the secondary clearance angle is nominal, visual alignment is sufficient.

Fig. 344: Create a depth of cut using the crossfeed, then set a detent against the Major face of the tooth.

• Position a detent against the Major face (Fig. 344), and set a traverse stop to limit the travel to the bottom of the gullet (unless the gullet needs to be increased in depth due to repeated sharpening). This helps blend the clearance face into the gullet.

• Set the depth of cut using the crossfeed.
Traverse to grind, then index and repeat for the other teeth. Grinding takes place at the front left corner of the wheel, and not the side of the wheel.

To grind the Major faces:

When grinding clearances is not enough to sharpen the Major cutting edge, usually because of heavy wear or damage, grind the Major face, ideally before grinding the clearances. The challenge in grinding the Major face is to maintain the geometry of the face in relation to the centre of the saw. Once ground, the Major cutting angle should be unchanged. Several intermediate stages are required during the setup, to allow the wheel and the axis of traverse to stay in that existing relationship. The spindle is usually at right angles to the axis of traverse.

• Place the saw on the fixture pivot so that the teeth face clockwise. This allows sharpening using the same shaped type 1 wheel as used for the clearances.

Fig. 345: Six stages in setting up a saw tooth ready for grinding the Major face.

• To establish a ‘zero’ reference position, set the reference side of the fixture plate parallel to the axis of traverse (Fig. 329), then set the tip of a tooth on the saw pivot/swing pivot centre line, using a centre-setting gauge (Figs 339 and 342). Use the crossfeed to position the tip of the tooth against the wheel (Fig.345 (A)).

The next step is to bring the Major face parallel to the axis of traverse. This involves rotating the saw through the rake angle, as shown in Fig. 345 (B), turning anticlockwise if the saw has positive rake and clockwise if it has negative rake, using one of the following methods:

• Where the fixture plate can swing, and has a graduated scale, swing it through the rake angle (Fig. 345 (B)) to bring the Major face parallel to the axis of traverse.

• Where there is no swing scale and no separate rotary scale for the saw, calculate the distance the tip of the tooth needs to move, to rotate the Major face backwards into position, bringing the Major face parallel to the axis of traverse (distance H in Figs 345 (B) and 340). Use the same method of calculation as for the primary clearance face.

• Use the crossfeed to move the saw the same distance, H, to sit against the Major face, as shown in Fig. 345 (C).

• Use the crossfeed to take the saw further away from the wheel face, to create a depth of cut (shown as f in Fig. 345 (D)). A typical depth of cut would be approximately 0.02mm (0.0005in).

• Rotate the saw so that the tip of the tooth rests against the face of the wheel as shown in Fig. 345 (E). Note: This is a rotation and not a swing. Lock the saw in that position, then traverse to take the saw clear of the wheel.

Fig. 346: Set a swing-clear detent against the Major face of the tooth to be ground.

• Use the crossfeed to advance the saw the same distance, f, as the depth of cut chosen previously, to create the cut (Fig. 345 (F)). Set a swing-clear detent with an auxiliary position pin (Fig. 346) against the Major face of that tooth.

• Swing or lift the detent away. Traverse to grind, setting a stop as the wheel reaches the bottom of the gullet.

• Retract; rotate the saw and replace the detent against its position pin so that the Major face of the next tooth lies against the detent. Lock the saw. The next tooth is now ready for grinding (and the feed has been applied automatically). Repeat the grinding sequence in this last paragraph for each tooth.

Grinding the faces of a saw held vertically

To sharpen a vertically held saw, use a vertically mounted, shaped and dressed narrow type 1 wheel (Fig. 323) with the tapered edge to the rear, or a type 12 wheel if there is space between adjacent teeth (Fig. 347).

Fig. 347: When choosing the wheel, check that when traversing to grind the clearances the wheel avoids grinding the next tooth.

To grind the primary clearance faces (the Major flanks):

Fig. 348: Set the saw arbor parallel to the axis of traverse, and the wheel spindle at right angles to the traverse. Position one tooth tip on centre, with its Major face upwards.

• To establish a ‘zero’ reference position, set the saw arbor, or the fixture plate swing/pivot axis horizontal and parallel to the axis of traverse. Mount the saw vertically, with one tooth tip (the cutting edge) on centre and the Major face upwards (Fig. 338). Mount the wheel vertically, with its spindle at right angles to the axis of traverse (Fig. 348).

• Rotate the saw clockwise (viewed from the saw clamp side), through the primary clearance angle, to bring the primary clearance face vertical, and set a detent against the Major face of that tooth (Fig. 349). On an arbor, use a rear-mounted rotation scale. On a fixture plate with a tilt scale, that can be done by tilting the plate upwards referring to the tilt scale markings. On a fixture plate or an arbor with no tilt or rotation scale, begin by calculating the vertical distance the tip of the tooth must move during rotation, using the same method as for the primary clearance on a horizontally held saw (see page 84). Raise the centre-height gauge by the calculated amount, using one of the techniques shown in Figs 334, 335 and 336; rotate the saw to touch the detent; then lock the saw in position.

Fig. 349: Traverse, taking the saw past the wheel, grinding only the primary clearance and avoiding the next tooth and the detent.

• Set the height of the spindle so that the wheel will grind the whole of the primary clearance face without touching the Major face of the adjacent tooth (Fig. 349). Then traverse the saw clear, positioning it so that the wheel will be rotating upwards when it contacts the tooth.

• Create a cut using the cross-slide.

• Traverse, grinding only the primary clearance face.

• Repeatedly index to the next tooth, and repeat the grind, for all the other primary clearance faces.

To grind the secondary clearance faces:

• Rotate the saw clockwise, through the secondary clearance angle, to bring the secondary clearance face vertical. On a fixture plate with a tilt scale, tilt the plate upwards, referring to the tilt scale markings. Otherwise, it is sufficiently accurate to set the secondary clearance face by sight or feel, against the face of the wheel. Lock the saw in position and set a detent against the Major face of the tooth.

• Set the height of the spindle so that the wheel will grind the whole of the secondary clearance face without touching the Major face of the adjacent tooth (Fig. 347). Then traverse the wheel clear of the saw, positioning it so that the wheel will be rotating upwards when it contacts the tooth.

• Create a cut using the cross-slide.

• Traverse, taking the wheel through the gap between the saw teeth, grinding only the secondary clearance face.

• Repeatedly index to the next tooth, and traverse until all secondary clearance faces have been ground similarly.

To grind the Major faces:

The geometry of the Major face of each tooth is set up in the same way as for a horizontally held saw (Fig. 345).

Fig. 350: With the finger at the top of the finger block of the centre-height gauge, and facing upwards, set the gauge against a downward-facing half-centre.

• Prepare by setting the finger of the centre-height gauge to face upwards, at the top of the finger block, and set the height against a downward-facing half-centre setting arbor in the workhead (Fig. 350).

Fig. 351: Set the tip of one tooth on the horizontal centre line., For this operation, the finger of the centresetting gauge is inverted and faces upwards.

Fig. 352: Rotate through 90 degrees to set the tip of the tooth on the vertical centre line.

• Reverse the saw on the arbor or the pivot post, so that the Major faces of the teeth face the wheel. Set the tip of one tooth on the horizontal centre line (Fig. 351). Rotate the saw, or tilt the fixture plate through 90 degrees, clockwise, to bring the tip of the first tooth onto the vertical centre line (Fig. 352).

Fig. 353: Rotate the saw arbor, or tilt the fixture plate, through the rake angle to set the Major face of the top tooth vertical.

• Rotate the saw arbor, or tilt the fixture plate, through the rake angle (Fig. 353); clockwise for positive rake, but anticlockwise for negative rake.

Fig. 354: Bring the wheel against the Major face of the tooth.

Fig. 355: Move the wheel away, to create a feed (f).

• Use the crossfeed, and adjust the wheel height to bring the side of the stationary wheel against the Major face of the tooth (Fig. 354). Create a depth of cut by backing off the crossfeed to leave a gap (f ) between wheel and tooth (Fig. 355).

Fig. 356: Rotate the saw to bring the tip of the tooth against the face of the wheel.

• Rotate the saw so that the tip of the tooth rests against the face of the wheel (Fig. 356). Lock the saw.

Fig. 357: Use the crossfeed to move the saw to apply the feed (f).

• Traverse the saw clear, then apply the crossfeed (f) to move the saw blade to create the cut (Fig. 357). Set a swing-clear detent against the tooth face.

• Swing the detent clear, then traverse to grind the tooth face. Repeatedly replace the detent; index to the next tooth; lock the saw; swing the detent clear; and repeat the grind, until all the Major faces have been ground similarly. The same cut is automatically applied each time a tooth is indexed against the detent, because that cut was created when the detent was set against the first tooth.

• Additional, deeper cuts are applied by using the sequence shown in Figs 355–357.

Deepening the gullet

The gullet will not require regrinding unless the cutter has been sharpened many times, or has required deep grinding to eliminate damage to the teeth. The precise shape of the bottom of the gullet is not important, provided it does not contain any sharp corners that would act as stress risers. It is possible to deepen the gullet as part of the processes of grinding the Major face and the clearance faces, avoiding having to do this as a separate operation. Dress the edge of a thin type 1 or type 12 wheel to a suitable rounded shape (Fig. 323) before sharpening any face. Grind the Major face first, deepening the gullet. When grinding the clearance face on the back of the tooth, adjust the cuts and the travel to create a neatly blended curve at the bottom of the gullet, while avoiding the Major face of the adjacent tooth. Index to the next tooth, and repeat, until all gullets have been ground in the same way.

Sharpening machines

Saw blade sharpening is such a common requirement that purpose-designed saw blade sharpening machines are available commercially. Fig. 358 shows a sharpening machine designed to cater for blades of diameter 90–400mm. The blade remains stationary during grinding, and the motor and grinding wheel assembly is moved inwards, across the face of a tooth, then back out. The blade may lie horizontal, or may be tilted at up to 25 degrees, to sharpen teeth that have side rake. The sharpener uses a 100mm diameter diamond wheel for sharpening carbide teeth, but a ceramic wheel can be purchased for sharpening HSS teeth. Indexing takes place using a ‘finger’ stop against a tooth further round the periphery of the saw. This design of sharpener incorporates a travel stop so that the bottom of the gullet is not ground, and one corner of the wheel is rounded to blend into the gullet. The narrow face of the wheel is also dressed at an angle to allow the backs of the teeth (the primary clearance faces) to be sharpened by moving the saw blade sideways to align the back of a tooth with the face of the wheel. The Major face rake angle is set by snuggling the first tooth against the side of the wheel and creating a depth of cut by rotating the saw slightly, then setting the ‘finger’ stop against a tooth. Individual teeth are sharpened by pulling the motor unit and wheel forward, then back, and the ‘finger’ stop (detent) controls indexing to the next tooth. Teeth that have side rake can be sharpened by tilting the motor/blade unit using a built-in scale to control the angle.

Fig. 358: A typical commercial machine for sharpening saw blades. Photo: JACK SEALEY LTD.

SHARPENING HOLE SAWS

The working end of a hole saw is like a tube with forward-facing teeth around the edge (Fig. 359). Hole saw teeth have a similar geometry to the teeth on slitting saws, with a Major cutting edge and an associated rake angle on the Major face, as well as clearance behind the cutting edge. Some smaller saws may only have a single, primary clearance angle (Fig. 360), while larger saws are more likely to have both a primary and a secondary clearance angle (Fig. 361), making the teeth stronger and better suited to cutting metal. The rake angle, primary and secondary clearance angles of a saw depend on the material it is designed to cut. If possible, replicate the angles originally created for the saw being sharpened, but suggested approximate values for cutting steel are: Major face rake angle 5–7 degrees, single clearance angle 15 degrees; or primary clearance angle 10 degrees and secondary clearance 25 degrees. Some modern saws have laser-cut teeth that have a continuously curved back that begins with a primary clearance angle, then smoothly changes to a secondary clearance angle (Fig. 362). When sharpening, these are best dealt with by grinding separate flat primary and (if necessary) secondary clearances. The transition curve will remain between the clearance facets, but will progressively reduce with each sharpening.

Fig. 359: A hole saw has forward-pointing teeth around the periphery. From left to right: small hex-shafted arbor; small diameter saw; threaded sharpening arbor with cap; large diameter saw; threaded sharpening arbor with cap; large hex-shafted arbor.

Fig. 360: The teeth of small diameter saws may only have one clearance face.

Fig. 361: Larger diameter saws usually have separate primary and secondary clearance faces.

Fig. 362: Laser cut teeth may have a continuous curve between primary and secondary clearance faces.

As with slitting saws, sharpen hole saws by grinding the primary clearance faces. If that is not enough to sharpen the cutting edges, grind the Major faces. If wear is heavy, or there is visible damage to the cutting edges, grind the Major faces first, if possible, as that speeds the sharpening process.

Fig. 363: Hole saw teeth often have a 'set', created by bending alternate teeth outwards or inwards.

To create clearance as a hole saw cuts into the work, the teeth are often given a ‘set’, which is a sideways tilt. Every second tooth is tilted outwards, and the teeth in between are tilted inwards (Fig. 363). That makes the saw cut a groove wider than its tubular body, giving some room for waste material. Major cutting edges and faces may have a side rake, to help the cutting action, but on most saws this is simply a consequence of the set on the teeth, and can be ignored during sharpening.

The main challenge is in choosing the type and size of wheel, and creating setups that will sharpen one tooth without inadvertently grinding adjacent teeth. Once a tooth has been sharpened, the saw is indexed to bring the next tooth into position for sharpening.

Fig. 364: Tooth spacing may be irregular, and partiallyformed teeth are not unusual.

The spacing of the teeth on a typical hole saw tends to be irregular in places, especially where the saw is made by bending a flat toothed strip and welding to make a tube, but that often leaves a partially formed tooth at the join (Fig. 364). Some hole saws are deliberately made with ‘progressive’ tooth spacing that increases around the saw, making the cutting action more efficient and reducing harmonic vibration. If the saw is placed teeth-down against a flat surface all the tips should touch the surface, but irregular spacing means the depths of the gullets will differ, because teeth closer together will have a shallower gullet.

Holding hole saws

Hole saws are designed to be screwed onto an arbor that usually has a hexagonal shaft and a central hole for a drill. Except for small-diameter hole saws, the drive to the saw is via two pegs, rather than by the threaded portion, so the saw can move on the arbor and would not be sufficiently tightly held to be able to use the arbor for sharpening. A purpose-made arbor provides a more secure arrangement, and Fig. 359 shows two such arbors. Each is threaded to suit the hole saw thread, and each has an additional cup washer that can be held against the inside of the hole saw by a cap head screw. The cup washer adds security, preventing the hole saw from unscrewing during sharpening. The two most common threads in hole saws are 1/2inch × 20tpi UNF, for the smaller diameter saws, and 5/8inch × 18tpi UNF for larger saws. These apply for both metric and imperial diameter hole saws.

Choosing a wheel

The grinding setups depend on the type of wheel chosen. There is one approach for a thin type 1 wheel that caters for primary and secondary clearances as well as the Major faces, and another approach that uses a type 6, 11 or 12 wheel for the primary clearances but is difficult to use for the secondary clearance or the Major face of fine-pitched (narrowly spaced) teeth. The recommended wheels are a small type 11 taper cup, to sharpen the primary clearance faces, and a narrow-faced type 1, dressed, swept back on the right-hand edge to clear adjacent teeth and rounded at the left corner to blend into the gullet (Fig. 323).

Grinding the primary clearance faces

Where a hole saw has teeth with both primary and secondary clearances, the flat face of a type 11 wheel can be used to grind the primary clearances.

Fig. 365: Set the axes of the spindle and the workhead parallel to each other and at right angles to the traverse. Those axes are shown as red lines. Then swing the workhead clockwise, through the primary clearance angle (shown in green). The final position of the workhead axis is shown as a blue line.

• Mount a small type 11 taper cup wheel with its axis at right angles to the axis of traverse (Fig. 365), which will set its face parallel to the axis of traverse. Hold the saw on an arbor in the workhead, and set the axis of the workhead at right angles to the axis of traverse, then swing the workhead clockwise, through the primary clearance angle. Arrange the centre of the wheel and the top tooth on the saw to be at approximately the same height, but be prepared to adjust the vertical height of the wheel, if necessary, to avoid fouling lower teeth.

Fig. 366: A centre gauge fitted with a vertical side extension to allow it to be used with large diameter saws which protrude above the body of the workholder.

Fig. 367: Set the tip of the topmost tooth on centre.

• Use a centre gauge to set the tip of the topmost tooth on centre (Figs 366 and 367). An alternative method is to use a centre-height gauge to set the tip of one tooth on the horizontal centre line through the arbor then rotate the saw through 90 degrees to place that tooth at the top. Set a detent against the Major face of the tooth.

• Set a traverse stop so that the wheel will not grind far enough to touch an adjacent tooth.

Fig. 368: Traverse to grind the primary clearance, but use a stop to avoid grinding the adjacent tooth. The detent is located against the Major face of the tooth being ground.

• With the saw clear of the rear of the wheel, set a cut using the crossfeed. Traverse to grind the clearance face (Fig. 368). Retract, index to the next tooth, and grind that tooth. Repeat until all the teeth have been ground.

Grinding the secondary clearance faces, or single-angle clearance faces

Hole saws have either a primary and secondary clearance face behind each tooth, or a single clearance face. The secondary, or single, clearance angles are too steep to allow the teeth to be ground using the same setup as for the smaller primary clearance angle. However, secondary faces and single-angle faces may be ground using a common setup. Choose a wheel that has a swept back side, to avoid grinding the Major face of the adjacent tooth when the wheel reaches the bottom of the gullet (or the end of the straight section forming the secondary clearance face). This may be a type 11 taper cup or type 12 dish wheel, but a dressed and shaped thin type 1 wheel will avoid snagging even closely spaced adjacent teeth.

Fig. 369: Set the wheel spindle at right angles to the axis of traverse, and the saw arbor or wheelhead axis at right angles to the traverse, with the saw teeth facing the side of the wheel.

• Set the wheel spindle at right angles to the axis of traverse (Fig. 369), and arrange the height of the wheel so that it will grind the bottom tooth but will not touch the teeth on the top edge of the cutter. Then swing the wheel approximately 1 degree to take its trailing edge away from the axis of traverse.

• Set the axis of the saw arbor at right angles to the axis of traverse, so that the teeth face the end of the wheel spindle.

• Align the tip of a top tooth with the axis of the saw arbor, using a setting gauge designed to hold a blade above the centre line of the axis (Fig. 367). Then rotate the cutter through a half-turn about the axis of the arbor, and lock the cutter in that position. That should position the first tooth at the bottom of the saw, at approximately the same height as the centre of the wheel arbor.

Fig. 370: Swing the workhead through the secondary (or single) clearance angle, to bring the clearance face of the bottom tooth parallel to the axis of traverse. The outer edge of the thin front face of the wheel grinds along the tooth clearance face.

Fig. 371: After swinging through the secondary clearance angle, use the traverse and crossfeed to bring the side of the wheel against the secondary clearance face. Note that it is the corner and front face of the wheel that do the cutting.

• Swing the workhead anticlockwise through the secondary clearance angle (Figs 370 and 371), then set a detent against the Major face of the first (bottom) tooth. Set a traverse stop to avoid grinding too deeply at the bottom of the gullet.

• With the wheel stationary, nestle the secondary clearance face against the side of the wheel. Traverse the saw clear of the wheel, create a depth of cut using the crossfeed, then grind along the clearance face.

• Index to the next tooth, using the detent, and grind. Repeat for all the teeth. Treat each tooth individually, and be prepared to monitor and, if necessary, limit the traverse to avoid grinding the Major face of each adjacent tooth, especially in a shallow gullet.

Grinding the Major faces

To grind the Major face of each tooth:

Fig. 372: Set the axis of the spindle at right angles to the axis of traverse, and the axis of the saw arbor parallel to the axis of traverse, so that the saw teeth point towards the face of the wheel.

• Use a thin type 1 wheel dressed to provide clearance on the edge nearest the body of the grinder (Fig. 323), or a small-diameter type 11 wheel, chosen so that it can traverse along the Major face without grinding the back of the adjacent tooth. In either case, dress the outer edge (nearest the axis of traverse) to an approximate match for the bottom of the gullet (Fig. 323). Set the axis of the spindle at right angles to the axis of traverse, then swing the wheel 0.5 to 1 degree so that the rear edge swings away from the axis of traverse. Arrange the height of the wheel so that the centre of the spindle and wheel are at approximately the same height as the top tooth of the hole cutter (Fig. 372).

• Set the axis of the saw arbor parallel to the axis of traverse (Fig. 372) so that the teeth are facing the thin face of the type 1 wheel, or the outer corner of the type 11 wheel.

• Align the tip of a tooth with a setting gauge designed to hold a blade above the centre line of the cutter (Fig. 367) and lock the cutter in that position.

Fig. 373: Swing the saw anti-clockwise, through the Major face rake angle.

• Swing the saw anticlockwise, through the Major face rake angle (Fig. 373), to bring the Major face parallel to the axis of traverse.

Fig. 374: After swinging the workhead anti-clockwise, through the Major rake angle, the traverse and cross-feed can be used to bring the side of the wheel against the Major face. It is the corner and front face of the wheel which do the cutting.

• Move the stationary wheel so that its side (type 1) or face (type 11) rests against the Major face of the top tooth (Fig. 374), then retract the saw.

• Create a cut by rotating the saw so that the top tooth moves towards the wheel. Rotation can be controlled using a scale on the workhead indexing collar, or by placing a slip of material, such as a feeler gauge, against the Major face of the tooth before setting the detent against the material. Remove the material and rotate the saw to position the Major face of the tooth against the detent, then lock the saw arbor. Use a swing-clear detent that sits against a stop pin and can be lifted and swung out of the way to allow the tooth to be ground, but swung back into position to allow indexing to the next tooth. Set a traverse stop to avoid grinding beyond the bottom of the gullet.

• Swing the detent away, then traverse to grind the Major face. Replace the detent (which will not quite touch the Major face now that it has been ground).

• Repeatedly index to the next tooth, and repeat the grind, until all the Major faces have been ground similarly. Treat each tooth individually, but monitor and, if necessary, limit the traverse to avoid grinding the gullet too deeply, or fouling the adjacent tooth, especially when using a type 11 wheel.

Keeping the cutting edge radial

On a slitting saw, the rake angle of the Major face is measured in relation to a radial line (Fig. 375). On a hole saw, the Major rake angle is measured in relation to an axial line parallel to the axis of the saw (Fig. 376), but the Major cutting edges each lie along a radial line from the centre of the saw (Fig. 377). The centre-setting gauge aligns the tip of a tooth on the radial line from the centre of the saw, and if the grinding wheel is vertical, and touches the cutting edge, rotating the hole saw to create a cut ensures that the cutting edge remains radial. Applying crossfeed would create an angle in relation to the original radial position (Fig. 378).

Fig. 375: On a slitting saw, the Major rake angle is measured from the Major face of a tooth to a radial line from the centre of the saw.

Fig. 376: On a hole saw, the Major rake angle is measured from a line parallel to the axis of the saw.

Fig. 377: On a hole saw, the Major cutting edges lie along a radial line from the centre of the saw.

Fig. 378: Crossfeeding the saw offsets the axis, altering the angle of the Major cutting edges. Additional material removed is shown in orange.

Tool type 5:
Faceted drills

The traditional ‘jobber’ twist drill has two cutting edges and a pair of helical flutes running along the drill (Fig. 379). Behind the cutting edges, material is ground away to provide continuously increasing clearance so that the point of the drill does not rub against the bottom of the hole being cut. Nowadays, there are many good sharpening jigs, machines and procedures readily available to sharpen the cutting edge and create the clearances, as shown in Fig. 380.

Fig. 379: Features of a typical twist drill.

Fig. 380: A self-contained machine for drill sharpening. Photo: JACK SEALEY LTD.

However, while retaining the basic concept of cutting edges and flutes, alternative forms of the drill point cut more effectively than a standard jobber drill. Depending on the geometry, ‘four-facet’ and ‘split-point’ drill points make a twist drill self-centring, and improve the cutting action. Web thinning, corner ‘dubbing’ and creating a consistent front clearance angle all help improve the cutting action, without being difficult to carry out.

FOUR-FACET SHARPENING

Figs 381–383 show a drill on which each cutting edge is followed by a primary flat surface at a clearance angle, and a secondary surface at a greater clearance angle, giving four flat faces or ‘facets’ in total. One advantage is that the facets meet at a point in the centre of the drill, and the length of the noncutting section at the centre is likely to be smaller than on a conical-point drill, reducing the force required when drilling without a pilot hole. If the tip is ground accurately, a four-facet drill point is less likely to wander if it is used without first drilling a pilot hole. The flat primary and secondary surfaces can be ground by traversing across a flat wheel. That simplifies the grinding setups, compared to creating the continuously increasing clearance of a typical ‘conically pointed’ twist drill.

Fig. 381: Behind each cutting edge of a 'four-facet' drill are a primary and a secondary facet. The junction between each primary and secondary facet lies on the centre line of the drill tip.

Fig. 382: Behind each cutting edge there is a relatively narrow primary facet ground at the primary clearance angle. That is followed by a secondary facet ground at a steeper secondary clearance angle.

Fig. 383: On a four-facet drill, the primary and secondary facets meet at the tip. Viewed end-on, they form a straight line through the centre point.

Setting up to sharpen (regrind) a four-facet drill involves grinding the primary facets to sharpen the cutting edges, then carefully grinding the secondary facets to position the junctions between the primary and secondary facets on the same straight line (Fig. 383) that passes through the centre point of the drill. The most effective ways to hold a drill in a workhead are to use a drill chuck or a collet. The drill chuck must grip on the plain drill shank, whereas a multi-fingered collet such as the ER series can grip reliably on the shank or on the fluted section, and offers the benefit of a smaller projection of the drill point.

Drills can be sharpened against a flat-faced wheel, with the cutting edges held horizontally or vertically, by creating the point half-angle and clearances using swing and tilt, or tilt and swing movements. In either case, prepare by mounting and dressing a flat-faced grinding wheel.

Cutting edge horizontal

• Set the drill with the long axis of its shank horizontal and parallel to the traverse. Rotate the drill about its own axis to set the cutting edge horizontal by comparison with the scribed line on a clear plastic setting gauge (Fig. 59). Alternatively, set the cutting edge vertical by comparison with the vertical blade of an engineer's square, or the vertical scribed line on a setting gauge, and rotate the drill through 90 degrees to bring the cutting edges horizontal.

Fig. 384: Swing to create the point half-angle; then tilt to create the primary clearance angle.

• Swing to create the point half-angle for the right-hand cutting edge – as viewed from above while standing at the rear (Fig. 384). Typically the half-angle will be 59 degrees for drilling mild steel or softer materials.

• Tilt downwards to create the primary clearance angle (Fig. 384).

• Feed and traverse to grind that facet.

• Rotate the drill through 180 degrees, to bring the other cutting edge into position.

• Traverse to grind the primary clearance angle behind that lip, feeding to the same depth as for the previous facet.

Fig. 385: Use a pointer to compare the tips.

• Check that the outer tips of the cutting edges are at the same relative height. This is made simpler if the workhead can be removed and replaced accurately. Hold it vertical and check the outer corners are at the same height, and that the lengths of the cutting edges are equal and meet in the centre of the drill. Now is the time to remedy any differences by grinding a little from one side. An alternative is to use a pointer mounted on the workhead, setting it level with the outer corner of one cutting edge (Fig. 385). Rotate the drill through 180 degrees and compare the other tip with the position of the pointer.

• Tilt to the secondary clearance angle.

• Feed and traverse to grind that secondary facet, leaving a narrow primary facet.

• Rotate the drill half a turn, to bring the other cutting edge into position.

• Lightly grind the secondary clearance angle behind that lip, using a series of passes to feed to the same depth as for the previous facet, and checking that the two primary facets are of the same width, with the mating edges of all four facets lying in a straight line (Fig. 383).

Cutting edge vertical

• With the drill axis horizontal, set the cutting edge vertical. This can be done by comparing the position of the cutting edge with the blade of an engineer's square or a scribed vertical line on a screen attached to a height gauge. Then set the axis of the drill at right angles to the direction of traverse.

Fig. 386: Tilt to create the point half-angle.

• Tilt down through 90 degrees minus the half-angle, to create the half-angle of the point between the axis of the drill and the face of the wheel (Fig. 386).

Fig. 387: Swing to create the primary clearance.

• Swing to create the primary clearance angle of, typically, 5 degrees (Fig. 387).

• Feed and traverse to grind that facet.

• Rotate the drill through 180 degrees to bring the other lip into position.

• Traverse to grind the primary clearance angle, feeding to the same depth as for the previous facet.

• Check the outer corners are at the same height, and that the lengths of the cutting edges are equal (so that they meet in the centre of the drill). Now is the time to remedy any differences.

• Swing to create the secondary clearance angle; of, typically, 12 degrees.

• Lightly grind that facet. The ultimate aim is for the line between primary and secondary facets on each side of the drill to be aligned with each other, appearing as a straight line. There is just one depth of cut that will produce this.

• Rotate through 180 degrees to bring the other cutting edge into position.

• Traverse to grind the secondary clearance angle behind the primary facet, feeding to the same depth as for the previous facet, finishing with a light grind to ensure the rear edges of the primary facets form a straight line.

Using a jig

All of this can be done conveniently using a jig, and Fig. 388 shows a four-facet grinding jig created by Donald Mitchell for use on the Worden grinder. The drill is held in an ER collet, with the cutting edges vertical. The body consists of a platform set at an angle of 59 degrees to create the corresponding half-angle between the axis of the drill and the face of the wheel (Fig. 389). The body sits on the Worden grinder base, which incorporates an accurate feedscrew. Fig. 390 shows a collet holder that has a square body with steps machined from each corner. The collet and holder body share the same axis, so the drill will be held centrally between the sides of all four steps. This is the key to grinding the drill symmetrically. The rear face of the holder has four tapped holes, one in the centre of each of the protruding sections of the sides (Fig. 391).

Fig. 388: Four-facet drill grinding jig for the Worden grinder, made by Donald Mitchell.

Fig. 389: If the drill cutting edges are orientated vertically, the jig creates a fixed half-angle of 59 degrees for each cutting edge.

Fig. 390: The jig uses a collet holder with a square body which has a step machined at each corner.

Fig. 391: A single securing screw is inserted into one of the tapped holes in the rear face, to prevent the holder slipping forwards.

In use:

• Mount the body on the slide and secure by the screw at the rear. Lock the holder against the right-hand side of the slot by tightening the small screw on the left side, near the front of the jig (Fig. 389) – note the locating dimple near the front of the corner step in Fig. 390.

• Swing the jig to the primary clearance angle.

• Use the feedscrew to control the cut then traverse to grind the primary facet. Note the final feedscrew reading.

Fig. 392: After a half-turn the holder is replaced in the body.

• Back off, remove the collet holder; turn through 180 degrees, and replace (Fig. 392). Then use the feedscrew to grind this facet to the same depth.

• Inspect the drill, checking the corners of the primary facets are at the same height, and the chisel edge is central, so that the cutting edges are of equal lengths.

• Swing the assembly to the secondary clearance angle and use the feedscrew to lightly grind the secondary facet. Repeat the process for the other lip, then take time to adjust the depth of cut and regrind as necessary until the edges between the primary and secondary facets form a straight line through the centre of the drill tip.

• Inspect the drill tip carefully once again, using high magnification. Check that the cutting edges are the same shape. Ideally, these should be straight, and neither convex nor concave.

Fig. 393: The angles of contact on a convex edge (red) vary along the curve and differ from those on a straight edge (green).

The shape of the cutting edges is affected by the point angle and the helix angle of the flutes. For a particular helix angle there is a point angle that should produce a straight cutting edge. A steeper tip (smaller tip angle) will produce a convex cutting edge, while a flatter tip (greater tip angle) will produce a concave cutting edge. Both shapes of edges will cut, but Fig. 393 shows that points on a curved edge contact the work at different angles. The most consistent cutting action occurs when the cutting edge is straight. However, if the point angle is chosen to suit the material and does not provide a perfectly straight line, a reasonable compromise is that the edges are of the same shape even if they are slightly curved because of the fixed helix angle.

Fig. 394: A four-facet sharpener designed and constructed by John Moran, which uses a trunnion table to tilt the drill about the centre line of its tip. The large white plastic block is used when grinding a split point on a drill. Photo: JOHN MORAN at WWW.GADGETBUILDER.COM.

Fig. 394 shows a dedicated four-facet drill sharpener designed and built by John Moran (aka Gadgetbuilder) that uses a trunnion table to simplify and speed up the sharpening process.

Fig. 395: The common axis of the trunnion table and the drill lies along the face of the vertical wheel. Photo: JOHN MORAN at WWW.GADGETBUILDER.COM.

Fig. 383 shows that on the tip of a four-facet drill the centre line is the dividing line between the primary and secondary facets. If the drill could be tilted about that line, the primary and secondary facets would automatically meet on that line, simplifying and speeding up the grinding process. This can be arranged if the drill holder sits horizontally on a rest that pivots on the centre line of the drill tip, by using a trunnion formed using two protruding arms attached to the rest (Fig. 395). The face of a vertical grinding wheel is fixed on the trunnion line.

Fig. 396: The trunnion table features quick adjustments for primary and secondary angles. The chuck block locates on pins fixed in the table, and has in-built controls for feed and indexing. Photo: JOHN MORAN at WWW.GADGETBUILDER.COM.

Fig. 396 shows the trunnion table with angle adjustment, and the collet block that controls forward feed and indexing of the drill. Fig. 397 shows the essential features of the collet block and the arrangements for quickly setting the angle of the table (which sets the primary or secondary angle of the facets). The table is fitted with pins positioned so that the block and drill are held at the point half-angle.

To use the device, grip a drill in the collet with the cutting edges horizontal, and set the protrusion at the front, using a simple gauge. Put the collet block onto the fixed pins on the trunnion table. Tilt the table to the secondary clearance angle, apply feed using a screw on the collet block, then grind one secondary facet. Index to the other secondary facet using an index ring on the collet block, then grind that facet using the same depth of cut. Remove the collet block and check the secondary facets, regrinding with a deeper cut if necessary. Tilt the table to the primary clearance angle, then, without altering the feed, grind the primary facets, indexing from one to the other using the index ring. Remove the collet block and inspect the result. The lines between the primary and secondary facets should appear as a single straight line.

SPLIT-POINT DRILLS

On a conventional conically ground drill the angle of the chisel edge (Fig. 379) is determined by the point angle and the primary relief angle (clearance angle behind the cutting edge). A point angle of 118 degrees and a relief angle of 10–12 degrees will create a chisel edge angle of approximately 125–135 degrees, often stated simply as 130 degrees. No cutting takes place along the chisel edge, which has to scrape its way into the work, requiring relatively high force.

Fig. 397: A split-point drill has two additional facets at the chisel edge, and two 55 degree clearance faces. Each facet provides an additional cutting edge.

A split point has a 55 degree secondary clearance, which is taken forward sufficiently far to create an additional facet with a vertical edge on approximately one third of the chisel edge, on each side (Fig. 397). This creates two small auxiliary cutting edges, each up to one third of the original length of the chisel edge. That leaves approximately one third of the chisel edge, reducing the force required to penetrate the work.

Initial shaping

To initially shape the secondary clearance on a standard jobber drill use a cup or taper cup wheel, but stay at least 0.5mm clear of the chisel edge.

• Begin with the drill horizontal and at right angles to the traverse.

• Rotate the drill about its long axis to set the chisel edge vertical (that's the chisel edge, not the cutting edge). Use an optical device such as a vertical line engraved on clear plastic to assist (Fig. 59). Use high magnification if possible, because the chisel edge is short and precise alignment is a challenge. Misalignment will cause the chisel edge to protrude further than the auxiliary cutting edge, preventing it from cutting into the work efficiently.

Fig. 398: Tilt downwards 55 degrees to bring the secondary clearance face vertical.

Fig. 399: Traverse controls the position of the auxiliary cutting edge.

• Tilt the drill downwards by 55 degrees (Figs 398 and 399). In this position, the crossfeed will control the depth of cut on the secondary clearance face.

• Sideways traverse controls the position of the auxiliary cutting edge in relation to the chisel edge, so set a traverse stop to ensure that the edge of the wheel will not come closer than approximately 0.5mm to the side of the chisel edge.

• With the drill clear of the left edge of the wheel, apply a cut and traverse to the right. Repeat as necessary, until the new clearance face is just level with the end of the chisel edge (Fig. 397).

• Rotate the drill through 180 degrees, and repeat the grind to create the other secondary clearance face.

• Repeat the previous two steps, increasing the forward feed each time, to increase the length of each secondary cutting edge to a little less than a third of the length of the chisel edge. Stop when the two cutting edges and the remaining chisel edge lie parallel to one another and almost on the same straight line as the remaining chisel. The traverse stop ensures the additional facets are offset by approximately 0.5mm on each side of the line through the remaining chisel. Final finishing is done at the sharpening stage.

Inspecting the face being ground is not easy because the drill tip is tilted downwards and faces the wheel and can only be inspected with the wheel stationary by using a mirror and good lighting or a webcam. A better arrangement is to make the workhead removable, as on John Moran's grinder (Figs 394 and 396) and Donald Mitchell's Worden (Figs 388 and 390).

Sharpening the auxiliary cutting edges:

Sharpening is best carried out using a cBN taper cup wheel or a diamond wheel with the edge dressed square. Prepare by carrying out the initial shaping. Then:

• Set a traverse stop so that the edge of the wheel will touch the line running through the chisel edge. Setting the stop will require fine tuning as the grind proceeds. It also demands a fine sideways stop adjustment.

• With the drill clear of the left edge of the wheel, apply a small cut and traverse to the right. Repeat as necessary, until the new clearance face extends to approximately one third of the length of the chisel edge and forms a straight line with the chisel edge. Take care with the depth of feed, as small depth changes make a significant difference to the distance advanced along the chisel edge. Err on the side of caution.

• Rotate the drill through 180 degrees, and repeat the grind to create the other auxiliary cutting edge.

• If the grinder has vertical travel of the grinding spindle, traverse up and down the face of each additional facet, to ensure the face is flat and the edges are straight. If the grinding head cannot traverse easily, accept that the facets and their cutting edges will be slightly curved but will still cut.

WEB THINNING

The chisel edge represents the top edge of the web, so thinning the web reduces the length of the chisel edge. On a jobber drill, the thickness of the web increases gradually towards the shank of the drill. As the drill is sharpened and the flutes become shorter, the web becomes thicker, the chisel edge becomes longer, and the force required to penetrate the work becomes higher. Web thinning is not required with small drills or where the pilot hole has a larger diameter than the thickness of the web of the drill that will be used to open up the hole. On large diameter drills, the web may have substantial thickness, and web thinning is often useful, especially where a small pilot hole, or no pilot at all, is used. There are many ways to thin the web, but each involves removing material from the web and the trailing edge of the clearance near the centre of the drill.

Type X thinning creates a split point, by turning the web into two additional cutting edges. The method is explained in the previous section on split-point drills. This is the most effective way to thin the web, but takes a little more care in setting up than for other methods.

Fig. 400: Type H thinning creates two notches on each side of the chisel edge, at the chisel angle.

Type H thinning involves using a thin rounded wheel to cut a notch parallel to the chisel angle, on each side of the drill (Fig. 400). The objective is to thin the web, rather than to create a split point.

• Use a taper cup wheel or a thin wheel of 1–3mm thickness. Ideally, use a grinder that can position the wheel above the drill, and feed vertically downwards.

• Beginning with the axis of the drill horizontal and parallel to the face of the wheel, rotate the drill to set the chisel edge vertical.

Fig. 401: Move the drill sideways, to align the chisel edge with the face of the wheel. Then grind into the web at the drill tip.

• Use the crossfeed to move the drill sideways, to align the chisel edge with the face of the wheel, but keep the wheel face just clear of the cutting edge, so that when it advances the wheel will grind into the trailing edge of the adjacent secondary clearance, but not the cutting edge (Fig. 401). Adjust the height of the spindle to give a shallow depth of cut.

• Traverse carefully, or feed the wheel down parallel to the chisel edge, to create a notch that extends towards the centre for a maximum of a third of the length of the chisel edge, as for a split-point drill. Set traverse or feed stops.

• Retract, then turn the drill through 180 degrees and repeat for the other side. Adjust the position of the drill, if necessary, to bring it close to, but not touching, the chisel edge.

Fig. 402: For type S, use a thicker wheel to create a broader relief.

Type S thinning involves a wheel sufficiently thick to grind across most of the trailing edge of the secondary clearance (Fig. 402). Align as for type H thinning, and grind using the same motion, to remove a wider band of material without creating a split point. The chisel point remains intact, and it is only the web that is reduced in thickness.

CREATING CONSISTENT RAKE ANGLES

The cutting edge of a jobber drill curves as it approaches the chisel point, and the shape of the flute produces a considerable continuous variation in the rake angle on the flute side of the cutting edge, from the outer tip to the inner chisel edge (Fig. 393). As a consequence, the cutting geometry experienced by the work varies along the cutting edge. One way of making the rake angle more consistent is to grind the inside of the flute, at the rake angle, parallel to the cutting edge, towards the centre of the drill (Fig. 403). Taking this too far in, or too far down the flute, weakens the central core (the web) between the flutes, but larger drills with thick cores may benefit. This operation is sometimes termed ‘dubbing’ and may be combined with web thinning.

Fig. 403: Move the drill sideways, as if traversing, to align the chisel edge with the face of the wheel.

• Mount a thin type 1 wheel, or a type 3 wheel with the flat side facing to the left.

Fig. 404: The flat face of the wheel should face the inside of the flute, at the tip.

• Hold the drill horizontal, pointing towards the thin edge of the wheel, and with the cutting edges vertical (Fig. 404).

• Swing the drill anticlockwise to the desired rake angle.

• Traverse the drill to the right so that the wheel just touches the cutting edge.

• Apply a cut and feed the drill forward, or lower the wheel, just far enough to create a narrow face inside the flute. This will vary in thickness down the face of the cutting edge, being deeper at the web. Adjust the feed or the height of the wheel to grind the face a little below the end of the chisel edge.

• Retract the drill, index through half a turn to bring the other cutting edge into position and repeat for the second cutting edge, using the same depth of cut.

It is useful to set traverse stops for this operation. The left stop should be adjustable, to provide fine adjustment of the cut against the inside of the flute, and the right stop should prevent the back side of the wheel hitting the right-hand side of the flute. An alternative motion is to align the wheel above the drill, then feed down into the flute to create the rake. Grinding takes place as the wheel travels into the flute, either horizontally or vertically. The wheel is not fed sideways while in contact with the drill, as that runs the real risk of breaking the wheel.

DEALING WITH CORNERS

Corners tend to be a weak point on drills, especially when the drill is run a little too fast. Corners can be turned into a short cutting edge by grinding a Secondary Point Angle (SPA), effectively chamfering each corner and giving them much greater strength as well as a short additional cutting edge (Fig. 405). This may be a disadvantage on blind holes as it alters the shape of the bottom of the hole, but will not matter when drilling a through hole. As a rule of thumb, grind the corners at half the half-angle of the tip of the drill (Fig. 405), or 29.5 degrees (half of 59 degrees) for a standard 118 degree point. Thirty degrees will be perfectly satisfactory. The clearance angle should be the same as for the primary cutting edge. For a drill with a clearance angle of 12 degrees, using a type 6, 11 or 12 wheel:

Fig. 405: Creating a corner reduces the chance of damage at the tip.

• Hold the drill with its long axis parallel to the face of the wheel, and the cutting edge nearest the wheel horizontal.

• Swing the drill through half the half-angle of the tip (30 degrees for a standard 118 degree drill), taking the tip towards the wheel.

Fig. 406: With the axis of tilt parallel to the traverse, swing to create the angle of the corner, then tilt to create the clearance angle.

• Tilt the drill downwards, through the clearance angle (12 degrees, in this example), as shown in Fig. 406. Take care that the axis of tilt is parallel to the traverse (and the face of the wheel), so that the angle of the corner cutting edge will not change.

Tool type 6:
Milling cutters

Lathe tools usually remain stationary in a toolpost while the work rotates, but milling cutters (Fig. 407) are mounted in a rotating spindle, while the work is held on a moving table or in a vice. The teeth on a cutter remove material from the work as the work is traversed past the cutter (Fig. 408). In most cases this is a scooping action and, on a multi-tooth cutter, individual teeth cut in sequence as the cutter brings each tooth into contact with the work.

Fig. 407: Milling cutters have teeth arranged to suit particular types of cutting tasks. Rear row, left to right: side and face cutter with straight teeth, shell mill, side and face cutter with angled teeth. Middle row, left to right: thin side and face cutter or saw, involute gear cutter, slitting saw. Front row, left to right: concave form cutter, vee cutter, dovetail cutter, end mill, high-spiral end mill, large dovetail cutter, convex form cutter.

Fig. 408: On a multi-tooth milling cutter, individual teeth remove material using a 'scooping' action.

Fig. 409: Each tooth has a Major face rake angle and a Major flank clearance angle.

Individual teeth normally have a similar geometry, with a major cutting face and associated rake and clearance angles (Fig. 409). Some teeth may also have a minor cutting face with associated rake and clearance angles. Cutting edges may be straight, but on many milling cutters the cutting edges lie along a spiral, with flutes that provide space for chips, rather like a twist drill, and other cutters have shaped faces for specific tasks such as cutting gears (Fig. 407).

Milling cutters take many different forms, ranging from single-tooth fly cutters for facing flat surfaces to multi-tooth cylindrical cutters, gear cutters, dovetail cutters or circular saws that cut on the periphery but which may also have broad teeth running radially from periphery to centre on the side of the cutter (Fig. 407). Types of cutter in common use include end mills and slot drills, as well as fly cutters, slitting saws, slab mills and side-and-face cutters. For sharpening, milling cutters are often held in a chuck or on an arbor, and multi-tooth cutters require indexing facilities to rotate the cutter accurately to bring the next tooth into position for sharpening. The basic actions of swing, tilt and rotate are used to position the cutting edges ready for sharpening, just as for any other tool.

CIRCULAR LANDS AND MARGINS

On a basic milling cutter or fly cutter, the area immediately behind the Major face is termed the ‘land’. That is where clearance is ground, to prevent the rotating cutter from rubbing or scraping the work (Figs 410 and 411). There are two different ways of forming clearances on the peripheral teeth of typical milling cutters, and these produce differently shaped lands and cutter profiles. Some manufacturers refer to these as Archimedes' profile and the double-ended profile.

Fig. 410: Offsetting the clearance curve for a tooth produces continuously increasing clearance towards the gullet at the back of the tooth.

Archimedes' profile has a curved surface behind the cutting edge. For each tooth, the centre of the clearance curve is offset from the centre of the cutter, resulting in continuously increasing clearance (Fig. 410).

Fig. 411: Primary and secondary facets may be ground behind the cutting edge to provide clearance.

The double-ended profile has two flat facets behind the cutting edge, creating a primary clearance and a secondary clearance (Fig. 411). Primary clearance is ground behind the cutting edge, and secondary clearance is ground behind that. Both primary and secondary clearances are ground within the land.

Fig. 412: Some cutters have a 'circular margin' – a small section of the land ground as part of the circumference of the cutter. The margin allows a small amount of sharpening without altering the radius of the cutter.

In addition, some milling cutters, fly cutters and engraving cutters may have a tiny ‘circular land’, or ‘margin’ immediately behind the major cutting edge, before the primary clearance begins. This margin forms part of the circumference of the cutter (Fig. 412) so it has zero clearance, and rubs against the work. However, its purpose is to allow a small amount of sharpening to take place without altering the radius of the cutting edge. The width of the margin should be no more than 0.01mm or 0.0005in.

The double-ended profile is the easiest to recreate, because the challenge with the Archimedes’ profile is in finding the centre and radius of the curve used by the manufacturer. For that reason, some manufacturers recommend that the Archimedes' profile is converted to a double-ended profile the first time the cutter needs to be sharpened.

HOLDING MILLING CUTTERS

The most convenient way to hold a milling cutter for sharpening depends on whether the Major face is flat or helical, and whether the cutter has two centres, a shank or a hole for an arbor. It also depends on which cutting edges are to be sharpened.

Flat Major faces

A straight cutting edge lies on one flat plane, so the Major face is a single flat surface (Fig. 413). A tooth on a side and face cutter, for example, has three straight cutting edges (Fig. 414), with all three sitting on the same plane (the Major face), and short, straight flutes between the teeth. The flanks, however, each sit on a separate plane at the side of the cutter, so the Minor cutting edges lie at the intersection of two planes: the Major face and a Minor flank. The cutting edge of a gear cutter is curved in just one plane, so each tooth has a flat Major face (Fig. 415) and short, straight flutes. Cutters with flat Major faces typically have a centre hole and may be held on an arbor between centres (Fig. 416), or on a stub arbor in a workhead (Fig. 417). A straight arbor between centres may be a plain bar of uniform diameter, with a flange, a threaded section and a nut. The cutter is slid onto the bar and the nut tightened to push and hold the cutter against the flange. Some plain arbors may have a fine taper that will grip the parallel centre hole of a cutter pushed firmly onto the arbor (Fig. 418). A flange or nut is not required on a tapered arbor, which grips using friction alone.

Fig. 413: A straight cutting edge (red) lying on one plane (an infinitely large flat surface), part of which is shown in green.

Fig. 414: A tooth on a side and face cutter has three straight cutting edges (red) all lying on the same plane (the Major face, shown in green). The Minor cutting edges each lie at the intersection of the planes of the Minor flank and the Major face (brown/green and yellow/green).

Fig. 415: Despite the curves, the cutting edge of a gear cutter lies on one plane.

Fig. 416: An arbor may have a flange and a threaded section for a nut.

Fig. 417: Cutters with a central hole may be held on a stub arbor, in a workhead. The top arbor shows the cup washer used to accommodate different thicknesses of cutter.

Fig. 418: Cutters with a central hole may be held on a finely tapered arbor mounted between centres.

Helical Major faces

Slab milling cutters have both helical cutting edges and helical flutes (Fig. 419). The Major faces are inside the flutes, in front of each cutting edge, and they follow the helical contours of the flutes. The shape of the flute immediately adjacent to the cutting edge creates the Major face rake angle. Because of their length, these cutters are normally held on an arbor between centres. End mills have helical cutting edges and helical flutes along their length (Fig. 420). Sharpening helical cutting edges involves rotating the cutter as it is traversed past the grinding wheel, and requires either holding the cutter between centres or gripping its shank in a collet that is free to rotate in a workhead (Figs 421 and 422). Freedom of rotation without shake is essential when sharpening helical cutting edges. While a cutter held between centres is free to rotate, some care is required in arranging the same freedom when the cutter is held in a collet mounted on the end of a shaft and held in a workhead. Jerkiness leads to uneven grinding. Because of this, there are collet holders designed specifically for sharpening end mills, slot drills and other cutters with helical flutes and a shank that can be gripped in a collet. Some of those holders feature air bearings to provide low friction, ease of rotation, and minimal side shake.

Fig. 419: Slab milling cutters have helical flutes which create helical cutting edges.

Fig. 420: The teeth around the sides of an end mill are helical, as are the flutes.

Fig. 421: An end mill held in a collet which is free to rotate in the workhead of an ACTO grinder. Photo: WWW.MACHINERYPLANS.COM.

Fig. 422: An air bearing supporting the shaft of the collet holder of this Marvin Turbo Grind flute grinder ensures the shaft is free to slide and rotate smoothly and with minimum friction. Photo: MORSE FLIGHT SYSTEMS.

End cutting edges

The end cutting edges of end mills and slot drills are straight, and the associated Major faces lie inside the helical flutes (Fig. 423). Although some of these cutters can be mounted between centres, access to the end teeth is restricted. Centre-cutting end teeth have at least one tooth that projects over the centre of the cutter, preventing mounting between centres (Fig. 424). All of these cutters can be held by gripping their shanks in a collet or chuck. Cutters with shaped end teeth, like ball-ended cutters or rounding-over cutters may have cutting edges that are curved in one plane, but their Major faces are flat, so they can be held in the same way as other cutters with end teeth.

Fig. 423: The end cutting teeth of end mills are usually straight except where they meet the flutes.

Fig. 424: On this centre-cutting slot drill one tooth is extended over the centre of the end face.

SHARPENING SIDE AND FACE CUTTERS

A side and face cutter not only has teeth around the periphery, like a slitting saw, but also has cutting edges running down both sides of each tooth (Figs 407 and 425). Peripheral teeth are often staggered, with alternate teeth cutting on alternate sides, and the Major faces and cutting edges of the teeth may lie at an angle to the axis of the cutter (Fig. 426). Each tooth will normally have side clearance, and Fig. 427 shows typical angles for the cutting edges.

Fig. 425: Most side and face cutter teeth have cutting edges along the top and down each side.

Fig. 426: Looking down from above, the Major cutting edges of the teeth on a side and face cutter may lie at an angle to the axis of the cutter. The cutter shown only needs two cutting edges on each tooth; one on the top and the other on the protruding edge.

Fig. 427: Typical clearance angles on a side and face cutter intended for cutting steel.

In general, the methods used for sharpening peripheral teeth, Major and Minor cutting edges, are similar to those used with slitting saws, but the methods of workholding are likely to be different. Although side and face cutters may be held on an arbor mounted between centres (Fig. 416), the most convenient way to hold these cutters for some operations on a grinder with a small table or a single workhead is on a stub arbor (Fig. 417). Some arbors incorporate a standard key to suit the keyway in the cutter and provide a positive lock to prevent rotation of the cutter on the arbor, but the grinding forces are small, so a top hat washer with a central clamping screw is usually sufficient.

Straight peripheral teeth

Straight peripheral teeth have flat Major faces, and straight Major and Minor cutting edges. Each tooth has a Major face, with a primary clearance angle on the Major flank (Figs 427 and 428), and may also have side rake (Fig. 426). There is at least one Minor cutting edge, an associated Minor flank with a clearance angle and a Minor face (as on a lathe tool, this is the same as the Major face) as shown in Fig. 430. Many side and face cutters have teeth with a Major cutting edge rake angle of zero degrees, but have two Minor cutting edges, one on each side, and each has its own Minor flank (Figs 425, 428 and 430) with associated side-clearance (Fig. 429). The cutting edges may be sharpened by grinding the flanks using a flat-faced wheel, such as a type 6 cup or a type 11 taper cup wheel, or the curved periphery of a type 1 wheel. That leaves the Major face unground, preserving its rake. Note that because of the clearance angles on the flanks, grinding any of the flanks or the Major faces of the teeth will reduce the width of the cutter. To reduce this effect, the side teeth may be ground with a clearance angle as small as 0.25 degrees. A reduction in diameter is not such a problem, so clearances on the Major flank may be much greater, and more like those of a lathe tool.

Fig. 428: Each tooth has a Major face, with a primary clearance angle on the Major flank.

Fig. 429: Each Minor cutting edge may have a clearance angle on the Minor flank.

Fig. 430: There is at least one Minor cutting edge, and a Minor flank with clearance angle.

Choices affecting sharpening setups for Major cutting edges

There are inter-related choices to make when setting up to sharpen peripheral teeth: grinding wheel face (curved or flat), detent shape (flat or rounded) and mounting method (moves with the cutter or remains stationary), arbor rotation (fixed during grinding or free to rotate) and the method of dealing with Major cutting edge rake (swing the cutter during setup or auto-rotate while grinding a tooth).

Taking these factors into account, there are two main setups:

One setup caters for all cutters with peripheral teeth, whether they have rake or not. This setup also caters for end mills and slab milling cutters with helical teeth.

The other setup is restricted to teeth that have zero side rake on the Major cutting edge. This is a simpler setup to prepare. Preparatory setup: straight teeth – any rake angle

Set the spindle and workhead or tailstock to a common centre height:

• See Chapter 3. Mount a half-diameter centre-setting bar in the workhead or one arbor centre-holder (tailstock), and set the flat face vertical. Rotate through 90 degrees so that the flat faces downwards, perhaps by using a square-sided collar as an indexing guide, then set a centre-height gauge against that face. Alternatively, allow the centre-setting bar to rotate freely in the workhead, and snuggle the finger of the centre-height gauge up against the flat face of the bar. Lock the finger at that height.

• Set the wheel spindle to the same height as the workhead axis or arbor centre holder, perhaps by mounting a half-diameter setting bar in the spindle, and using the centre-height gauge to set it on centre.

Position the axes:

• Set the axis of the cutter arbor parallel to the axis of traverse (see Chapter 3).

Fig. 431: . When using a flat-faced wheel, set the wheel arbor at right angles to the axis of traverse, and mount the cutter so that the Major face of the tooth nearest the wheel faces downwards. The wheel should rotate downwards at the front.

• Flat-faced wheel: Set the wheel arbor at right angles to the axis of traverse. Curved wheel: Set the wheel arbor parallel to the axis of traverse (Fig. 431).

Fig. 432: When using a type 1 wheel, set the wheel arbor parallel to the axis of traverse, and mount the cutter so that the Major face of the tooth nearest the wheel faces downwards. The wheel should rotate downwards at the front.

• Mount the cutter with the Major face of the tooth nearest the wheel facing the table (Fig. 432).

Sharpening Major cutting edges

Any cutter may be mounted on a freely rotating stub arbor in a workhead that has an axis of rotation coincident with the axis of the arbor (Fig. 433). ‘Freely rotating’ means the arbor must sit in low-friction bearings that allow the cutter to spin freely and with little resistance when flicked.

Fig. 433: A clock gear cutter mounted on a freely rotating stub arbor in a workhead which has an axis of rotation coincident with the axis of the arbor. The extended tapered nose allows access to the small diameter cutter, while providing support.

Fig. 434: A side and face cutter mounted on a freely rotating arbor held between centres aligned with the axis of traverse.

Any cutter may be mounted on an arbor free to rotate between centres aligned with the axis of traverse (Figs 434 and 418).

Dress the wheel so that contact will occur along a narrow band at one edge. For grinding primary clearances, the best choice is a narrow small-diameter type 1 wheel. For large secondary clearances, using the upper section of the flat face of a 50mm diameter taper cup wheel allows easier access, and avoids fouling adjacent teeth.

Reference position:

Follow the preparatory setup instructions (see previous section). Use the centre-height gauge to set the outer tip of the Major cutting edge of the tooth nearest the wheel at cutter centre height. At that stage, the Major cutting edge of that tooth will only be horizontal if the cutter has zero rake (Fig. 425). The direction of rotation of the wheel should tend to make the grinding action push the tooth down towards the table.

• Flat-faced wheel: Create the primary clearance behind the Major cutting edge by rotating the tooth downwards through the clearance angle, or by tilting the workhead downwards through the clearance angle. Because the axes of rotation and tilt are parallel to the cutting edge, either method will ensure the cutting edge remains horizontal. Alternatively, the required rotation can be carried out by rotating or tilting to move the cutting edge through a vertical distance, known as the ‘drop’ without relying on reading an angular scale. Measure the diameter of the cutter, then calculate the drop using the formula: Drop = radius of cutter × SinC where C is the clearance angle.

For example:

If C = 5 degrees, and the diameter of the cutter is 75mm, the radius will be 38.5mm and Drop = 38.5 × Sin5

Fig. 435: Set the lowest point of a tooth against the finger of the height gauge.

Drop = 38.5 × 0.087 = 3.36mm
Using a height gauge, set the head of the centre-height finger to be 3.36mm below centre and reset the cutting edge against the finger. Where the Major cutting edge has side rake, set the lowest point of one tooth against the centre-height finger (Fig. 435).

• Curve-faced wheel: Raise the wheelhead to allow the curve on the face of the wheel to create the primary clearance angle. The distance can be calculated using the formula: Distance = r × SinC where r is the radius of the grinding wheel and C is the clearance angle, in degrees.
For example:
If the radius of the wheel is 50mm and the clearance angle is 5 degrees, Distance = 50 × Sin5 = 50 × 0.087
= 4.35mm
Using a height gauge, set the head of the centre-height finger to be 4.35mm below centre and reset the cutting edge against the finger. Where the Major cutting edge has side rake, set the lowest point of one tooth against the centre-height finger (Fig. 435).

Fig. 436: On a cutter with zero rake, mount a squareended detent to the table or workhead, and seat it against the Major face of the tooth.

• Option, for zero-rake cutters only: Select a square-ended detent, and attach its mount to the workhead or table surface, so that it traverses with the cutter and remains fixed against the Major face of the tooth (Fig. 436).

Fig. 437: Mount a detent with a radiused end to the table or workhead, and seat it against the Major face, just behind the lowest point of the cutting edge.

Fig. 438: Align the highest point on the detent with the side of the wheel, and 1mm or less away from the wheel face. Set it approximately to height, but leave it a sliding friction fit in its holder.

Fig. 439: View looking through the wheel. Rest the tooth in its initial position against the height gauge, and adjust the detent until it contacts the Major face of the tooth, just behind the Major cutting edge.

All cutters (including zero rake where the previous option has not been chosen): Select a detent with a radiused end and attach its mount to a stationary part of the grinder, so that it does not move when the table is traversed. The Major face should then slide over the top of the detent while rotating the cutter as required to keep the cutting edge in contact with the detent (Fig. 437). The smaller the detent end radius, the better. The centre line of the detent radius should be clearly marked on the detent. Align the detent centre line with the outer edge of the tooth and with the edge of the grinding wheel (Fig. 438) so that all three are aligned as the wheel contacts the cutter. The detent should support the tooth just behind the cutting edge, and should be set to support the tooth at the correct height (see previous calculations). This is made easier if the detent is positioned no more than 1mm away from the wheel, set approximately to height using the height gauge, then visually aligned with the edge of the wheel (Fig. 438). Leave the blade a sliding friction fit in its holder. Feed forwards so that the front of a tooth will just barely graze the stationary wheel when the cutter is traversed. Set the height gauge to the required height of the tooth tip, and lower the tooth onto the gauge finger, then slide the detent up to touch the Major face (Fig. 439). Tighten the blade holder fully.

• Apply a cut, using the crossfeed, then traverse to grind. Where the cutter is supported on a detent with a rounded end, take the cutter to the point of first contact, but keep it just above the detent. Rotate the cutter carefully to lower it into contact, then continue traversing, ensuring the tooth stays in contact with the detent by applying a little torque to the end of the cutter arbor.

• Index, by rotating so that the next tooth pushes the detent aside as it passes. Rotate a little more than necessary, so that the detent clicks back into position, then rotate back to seat the Major face against the end of the detent. Grind, then repeat for all the other teeth.

Sharpening side teeth

Side teeth have straight cutting edges, with clearance behind each cutting edge. In addition, side teeth may be angled inwards from the tooth tip towards the centre of the cutter by 0.25–3 degrees, to give additional clearance towards the centre of the cutter (sometimes termed ‘heel relief’). Mount the cutter on a stub arbor and use a small-diameter taper cup wheel.

Reference position:

Fig. 440: Reference position for sharpening side teeth on a side and face milling cutter.

Set the axes of the spindle and the cutter arbor parallel and at right angles to the axis of traverse (Fig. 440). Set the spindle and the arbor axes at approximately the same height.

• Rotate the cutter to position one Minor cutting edge horizontal.

• Choose a detent with a broad flat tip to sit against the Major face of the tooth, mount it on the table, facing upwards, or on the workhead, facing downwards, so that it traverses with the cutter and remains stationary against the tooth.

Fig. 441: If necessary, swing to create heel relief. Tilt to create clearance behind the cutting edge.

• Adjust the detent to support the Minor cutting edge of the tooth. If the detent is mounted on the table and points upwards, choose a tooth whose Major face faces downwards. If the detent is mounted on the workhead and points downwards, choose a tooth with an upward-facing Major face (Fig. 441).

Fig. 442: Support the tooth using a detent mounted on the table or workhead.

• Tilt the workhead, taking the unsupported edge of the Minor flank nearer the wheel, to create the clearance behind the cutting edge (Fig. 442). A typical amount is 0.25 – 3 degrees. Keep the detent at the same height, and keep the Minor cutting edge in contact with the flat tip of the detent as before.

• To complete the setup, lock the cutter, then swing the axis of the arbor, moving the outer end of the tooth away from the wheel, to create any heel relief, if required. Adjust the detent to support the Minor cutting edge in its new position (Fig. 442).

• With the wheel clear of the cutter, apply a cut. Traverse takes place at right angles to the axis of the spindle, stopping before the wheel touches the arbor or any of the teeth on the opposite side of the cutter.

• Index and repeat, for all the teeth.

SHARPENING END MILLS AND SLOT DRILLS

End mills and slot drills (Fig. 443) are essentially similar, and although the terms once implied different characteristics, the lines have been blurred in more recent times, with both terms tending to be used interchangeably by manufacturers and retailers. None of that matters much, but in this section a slot drill is defined as having two cutting edges at the end, one of which overlaps the centre of the cutter, and two flutes (Fig. 443). End mills may have two, three or four cutting edges and flutes. Some end mills also have one cutting edge that overlaps the centre, and these are termed ‘centre-cutting’ end mills (Fig. 443). Other end mills have a gap at the centre, or a small recessed female centre (Fig. 443). In this section, the term end mill is used to cover both end mills and slot drills and all their variants.

Fig. 443: End mills and slot drills. Although the geometry is the same, a slot drill has only two teeth.

The teeth on a square-ended cutter are straight, but the cutting edge along the side of each flute is continuously curved, like the flutes of a twist drill (Fig. 420).

Fig. 444: The main features of the two teeth on the end of a slot drill.

Fig. 445: The main features of the teeth on the end of a slot drill.

Fig. 446: The teeth of a slot drill are separated and defined by a gullet or gash between the teeth.

Fig. 447: Each tooth on an end mill normally has both a primary and a secondary clearance.

Fig. 448: End teeth normally have a small end relief angle so that the outer ends do the cutting first.

Figs 444–447 show the features and angles of typical straight end mill teeth. Each tooth has a primary and a secondary facet, and these are sharpened separately in a similar way to four-facet drills. Because the teeth are close together near the centre of the cutter, the smaller the grinding wheel the easier it is to avoid accidentally grinding an adjacent tooth. However, provided the end mill is not centre-cutting, it is perfectly possible to sharpen small end mills using a 100mm diameter wheel. Although the teeth are straight, they typically have a small end relief angle (Fig. 448) of 1 or 2 degrees, which ensures the outer corner of the end face touches the work, and the inner end of each tooth has just enough clearance to prevent it from dragging on the work. This means the end is effectively dished inwards towards the centre. End teeth can be sharpened using either a flat-faced or a curved-face wheel.

Setting the cutting edge horizontal

On a two-flute cutter (slot drill) there may be a gash between the teeth, but the cutting edges lie on the same straight line (Fig. 449) and are radial. Cutters with more than two teeth require more than one gash between the teeth, so the cutting edges do not lie on the same straight line (Fig. 450) and are not radial. On a four-tooth end mill, for example, pairs of cutting edges are parallel but are spaced above and below a gash that defines the cutting edges, so they are not radial. In addition, no matter the number of teeth, the flutes intersect the cutting edges of end mills and slot drills, creating a curved scallop on the outer end of each cutting edge (Fig. 449). That means it is not possible to set a cutting edge on centre using a centre-setting gauge with a finger. Instead, the best way to orientate the teeth before sharpening is optically, comparing a vertical tooth with the vertical edge of an engineer's square, then rotating the cutter through 90 degrees; or using a centre-setting gauge with a horizontal scribed line.

Fig. 449: The two cutting edges on a slot drill lie in a straight line on a diameter.

Fig. 450: The cutting edges on this end mill are parallel but do not lie on the same straight line.

Sharpening straight end teeth

Flat-faced wheel: Choose a type 11 (taper cup), 12 (saucer) or 6 (cup) wheel. A 50mm diameter type 11 is a good choice for both S-T-R and T-S-R workheads.

Curved wheel: Choose a type 1 wheel of 50mm diameter or larger. Do not use a curved wheel with a T-S-R workhead.

Dress the face of the wheel a few degrees off-parallel compared to the axis of traverse, so that the front corner that will contact the work first protrudes slightly. A flat-faced wheel can be slightly dished. Note that this angle has no effect on the end clearance of the end mill teeth; it helps ensure that the innermost edge of each tooth will be fully ground when the wheel reaches the end of the cutting edge. It also ensures a small area of contact between wheel and work, to minimize heat transfer.

Reference position:

Set the workhead horizontal, and at right angles to the traverse, with zero tilt.

S-T-R workhead: Set the workhead horizontal, mount an end mill, then rotate to set the Major cutting edge of one tooth horizontal when viewed from the front. The Major face (the inside of the flute) should look upwards. Swing the workhead to set the long axis of the cutter at right angles to the traverse.

Flat-faced wheel:

Fig. 451: Reference position when using an S-T-R workhead. The tooth to be sharpened is on the left, in this view. The indexing mechanism is not used.

Set the spindle axis at right angles to the traverse (Fig. 451).

Curved wheel: Set the spindle axis parallel to the traverse.

Fig. 452: Reference position when using a T-S-R workhead.

T-S-R workhead: Set the workhead horizontal, mount an end mill, then set the Major cutting edge of one tooth vertical and in the 12 o'clock position when viewed from the front. The Major face (the inside of the flute) should look to the side (Fig. 452).

Grinding the primary clearance facet

• S-T-R workhead: Create the end relief angle by swinging the workhead 1 or 2 degrees to take the tip of the tooth away from the wheel, bringing the cutting edge parallel to the axis of traverse (Fig. 453).

Fig. 453: Flat-faced wheel: create end relief by swinging (yellow) 1 or 2 degrees. Create primary clearance by tilting upwards (blue) 5 to 7 degrees. Add an indexing detent which rests inside the flute of the tooth being sharpened.

S-T-R workhead: Create the primary clearance angle.

Flat-faced wheel: Tilt the workhead upwards 5–7 degrees (Fig. 453).

Fig. 454: Lower a curved wheel the 'drop' distance to create the primary clearance angle.

Curved wheel: Set the wheel and the tooth cutting edge on a common centre line then lower the cutter below centre to create the clearance angle (see Chapter 3) as shown in Fig. 454.

Fig. 455: T-S-R workhead: create end relief by tilting 1 or 2 degrees upwards. Create primary clearance by swinging 5 to 7 degrees. Add an indexing detent which rests inside the flute of the tooth being sharpened.

• T-S-R workhead: Create the end relief angle of 1 or 2 degrees on the top tooth by tilting the tip of the tooth away from the wheel, bringing the cutting edge vertical (Fig. 455).

• T-S-R workhead: Create the primary clearance angle by swinging the workhead through 5 to 7 degrees (Fig. 455).

Fig. 456: S-T-R workhead: a view through the wheel. Avoid the adjacent tooth tips at A, and, to a lesser extent, at B.

Fig. 457: T-S-R workhead: a view through the wheel. Stay above the tooth tip at A, and take care crossing onto the receding secondary clearance at B.

• S-T-R workhead: Arrange a traverse stop so that the wheel reaches just beyond the inner end of the tooth (Fig. 456). T-S-R workhead: Set the wheel height so that the bottom reaches just beyond the inner end of the tooth (Fig. 457).
Both: On a centre-cutting end mill the stops (S-T-R) or wheel height (T-S-R) must be set differently for the over-centre tooth. A flat-faced wheel may be raised so that the curve on the lower section assists with clearance, avoiding adjacent teeth. This is where a small wheel is an advantage. The height of a wheel with a curved face cannot be altered, as that would alter the tooth clearance angle.

• In preparation for indexing, mount a detent with a rounded end on the workhead or the table, so that it does not move in relation to the cutter as traverse takes place. Set the detent inside the flute, with its flat face approximately parallel to the helix angle, and its tip near the helical edge, a short distance behind the cutting edge (Figs 453–455).

• Use the crossfeed to apply a cut and traverse to sharpen the tooth. Examine the result and, if the tooth has been completely sharpened, note the infeed or set an infeed stop.

• Index and repeat for the other teeth, using the same total infeed. Take care with the traverse, if only one tooth reaches over-centre.

Grinding the secondary clearance face

The secondary clearance on each tooth is ground using the same setup as for the primary clearance, but the angle of tilt is much greater (typically 15 degrees or more). Although the secondary clearance does not affect the end relief, setting the end relief angle back to 0 instead of 1 or 2 degrees then grinding the secondary clearance will result in the primary clearance face tapering, reducing in width towards the centre of the cutter. That, in turn, will provide reducing support behind the Major cutting edge, towards the inner end of the tooth. That may not matter, as the inner end of each tooth will carry least cutting load because of the end relief. The choice is yours.

Gashing

The gashes (gullets) between the end teeth of an end mill separate one tooth from another, and define the cutting edges. Ideally, gashes should follow the helix angle of the flutes, so that the Major face of each tooth (the inside edge of the gash, nearest the helical cutting edge) lies at the Major face rake angle. Use a type 3 tapered wheel or a type 11 dish (saucer cup) wheel.

Reference position:

Fig. 458: Reference position for gashing an end mill.

On most grinders, it is unlikely that the travel can cater for the optimum arrangement where the workhead is parallel to the traverse. Instead, set the workhead horizontal, with its axis at 90 degrees to the traverse, with the spindle parallel to the traverse (Fig. 458).

• Set the cutting edge of one tooth vertical.

Fig. 459: Swing to the flute helix angle, and align the side of the wheel with the cutting edge.

• Swing to the cutting angle by aligning the side of the flute immediately behind the cutting edge with the traverse. This can usually be done by eye, using the face of the wheel as a guide, or by swinging through the cutting angle, where that is known (Fig. 459).

• Use the crossfeed to position the cutting edge so that the wheel will just kiss the cutting edge when grinding takes place. Err on the side of caution, at this stage, and be prepared to adjust the position after the first pass.

• Mount a detent on the workhead or the table, and locate the finger inside the flute, clear of the wheel.

• Raise the wheel so that the gash will end before it grinds the opposite tooth. Where the cutting edges of two opposite teeth form a single unbroken ridge, take care to preserve that feature by using the cross-slide to offset the end mill to create each cutting edge on those two teeth.

Using an end mill sharpening machine

Sharpening end mills is such a common requirement that machines such as the Arc Euro Trade EMG-20 End Mill Resharpening Module (Fig. 460) have been designed to provide a quick way of sharpening end mill end teeth (Fig. 461). Operating like a drill sharpener, end mills are held in sleeves that carry collars incorporating cams chosen to suit the diameter of the cutter and the number of teeth (Fig. 462). The projection and the orientation of the teeth are set using a jig built onto the side of the machine (Figs 460, 463 and 464). The sleeves are inserted into three ‘ports’ or holes in the machine, in sequence (Fig. 465). Each port sharpens one tooth feature. The first port sharpens one cutting edge by grinding the primary clearance. The second port grinds the secondary clearance angle behind that tooth. The third port re-forms the gash between adjacent teeth. This machine can be fitted with either a cBN wheel for sharpening HSS cutters, or a diamond wheel for carbide cutters.

Fig. 460: Arc Euro Trade EMG-20 End Mill Resharpening Module. Photo: ARC EURO TRADE LTD.

Fig. 461: End mill teeth showing square-bottomed gullets (gashes). Photo: ARC EURO TRADE LTD.

Fig. 462: End mills are gripped in collars fitted with integral cams. Photo: ARC EURO TRADE LTD.

Fig. 463: End mill projection is set using a setting device. Photo: ARC EURO TRADE LTD.

Fig. 464: Cutting edges are oriented using jaws in the setting device. Photo: ARC EURO TRADE LTD.

Fig. 465: End mill projection is set using a setting device. Photo: ARC EURO TRADE LTD.

PARALLEL HELICAL CUTTING EDGES

Helical cutting edges are usually found on the sides of specific types of cutters, such as end mills, and some types of reamers and milling cutters. Slab milling cutters, for example, have broad faces with helical cutting edges but no end teeth (Fig. 407). Some reamers have helical cutting edges, but reamers have additional features, and are dealt with in a separate section.

Holding slab milling cutters

Slab milling cutters can be held on a mandrel, mounted between centres (Fig. 466). This may be a plain arbor with a fine taper to hold the cutter securely using a friction fit, or, because these cutters have no end teeth, it may be an arbor with one fixed flange and a clamping nut at the other end.

Fig. 466: Slab milling cutters may be held by friction on a mandrel with a fine taper, and mounted between centres.

Holding end mills

Some end mills have centres in the ends and could be mounted between centres, but centre-cutting end mills, and many others, do not have two centres, so the most effective way to hold end mills is to grip the cutter in a collet on an arbor, in a workhead that will allow the arbor to turn and slide freely (Figs 467 and 421). Commercial attachments for this purpose often use air bearings to reduce friction (Figs 468 and 422).

Fig. 467: End mill held in a collet on a freely rotating arbor, guided by a stationary detent during traverse. Photo, design and construction: JIM SCHROEDER.

Fig. 468: It is important that an end mill holder is free to rotate freely and smoothly, so commercial designs often use air bearings. Photo: WEALD ENTERPRISES.

SHARPENING PARALLEL HELICAL CUTTING EDGES

Sharpening helical cutting edges poses the problem of maintaining contact at the appropriate angle between the wheel and a point on the cutting edge that spirals around the cutter. If the cutter is free to turn and the cutting edge being sharpened is supported by a detent that does not move as the cutter is traversed, the cutter will be forced to turn as it traverses past the wheel. As a result, the same relative point on the cutting edge will remain in contact with the wheel as the cutter traverses and turns, despite the helical shape of the cutting edge and the flute. This is essentially the same method as outlined in the section entitled ‘Sharpening Major cutting edges’ on page 107.

Sharpening slab milling cutters

Reference position:

Cutter fixed to an arbor suspended between centres. The cutting edges should face downwards, and the wheel should rotate downwards at the front, so that the downward force helps to keep the cutter in contact with a supporting finger (see below).

Fig. 469: View from above: dress a flat-faced wheel inwards towards the centre. Then swing the grinding spindle by up to 5 degrees. Dress the wheel edge parallel to the traverse, leaving a flat edge of up to 2mm wide, to avoid heat build up during grinding.

• Choose a wheel type: If the wheel spindle can be swung to an angle of up to 5 degrees (Fig. 469), choose either a small flat-faced taper cup wheel or a type 1 wheel. Check there is enough space between the end of the cutter and the tailstock to accommodate the wheel. If the spindle cannot be swung, choose a type 1 wheel.

• Flat and curved wheels: Set the axis of the spindle to the same height as the axis of the arbor, by first setting a height gauge against a freely rotating half-diameter bar in place of the table-mounted centre. Then place a half-diameter bar in the spindle and set the spindle height against the height gauge (see Chapter 3). Replace the cutter and arbor, and the wheel.

• Flat-faced wheel: Dress the wheel inwards towards the centre, to leave a narrow outer edge. If possible, swing the grinding wheel spindle by up to 5 degrees so that the trailing edge of the wheel will not touch the cutter during traverse (Fig. 469). Dress the outer corner of the wheel parallel to the traverse, to create a face up to 2mm wide.

Fig. 470: View from above: dress a narrow type 1 wheel at an angle to the traverse, to narrow the contact area.

• Curved-face wheel: Swing the wheelhead by up to 5 degrees, or dress the wheel at an angle to the axis of traverse, so that the leading edge (the edge that will first contact the cutter) is 1–2mm wide (Fig. 470).

• Both wheel types: Align the leading edge of the wheel (the edge that will first contact the cutter) with the start of the cutting edge. Use the crossfeed to position the cutter to barely kiss the stationary wheel. A cigarette paper is useful to judge the clearance when crossfeeding up to the cutter. Then traverse the wheel clear of the end of the cutter.

Fig. 471: View from the wheel side: set the lowest end of one cutting edge at centre height by resting the tip of the cutting edge on the arm of a centre-height gauge.

• Set the end of one cutting edge at centre height, by using a centre-height gauge (Fig. 471).

• Flat-faced wheel: Choose the most convenient method of creating the primary clearance angle behind the cutting edge. Either use a Clearance Angle Indicator (Fig. 472) or calculate the drop (see Chapter 3), or read the drop distance from Table 10, page 29.

Fig. 472: The Clearance Angle Indicator attaches to a tailstock, and the clamp with its pointer is slipped over the arbor and tightened in position. The clamp is open at the rear, so that it can be attached and removed without disturbing the arbor.

Clearance Angle Indicator: Fig. 472 shows a Clearance Angle Indicator plate attached to the tailstock, and a removable clamp carrying a pointer arm. The clamp is slipped onto the arbor and tightened with the pointer reading zero (because the tip of the cutting edge is on centre height). Remove the centre-height gauge, rotate the cutter downwards at the rear until the scale indicates the desired primary clearance angle, then support the cutter on a detent (see below). The clamp can then be removed. Use the drop distance: Set a height gauge to centre height minus the drop, and rotate the cutter so that the tip of the cutting edge sits at that height (Fig. 473).

Fig. 473: View from the wheel side: align the highest point on the detent with the leading edge of the wheel and the start of the cutting edge.

• Curved-face wheel: Calculate the distance to raise the wheel to create the primary clearance (see Chapter 3) or read the value from Table 11, page 29, then raise the wheelhead by that amount.

Fig. 474: View from the wheel side: mount a detent (finger) on a fixed part of the grinder, and place it just inside the flute, with the highest point of the detent aligned with the edge of the cutter.

Fig. 475: View from the front: as traverse begins, the leading edge of the wheel, the highest point on the detent (indicated by the scribed line) and the beginning of the cutting edge should all align.

• Both wheel types: Select a detent with a rounded end and prepare to mount it on a fixed part of the grinder that does not move with the traverse. Align the leading edge of the wheel with the start of the cutting edge. Then set the detent against the inside of the flute of the cutting edge to be sharpened. The highest point of the detent should align with the start of the cutting edge, and with the leading edge of the wheel (Figs 474 and 475).

• Rotate the cutter a few degrees to lift it off the detent. With the wheel rotating, use the crossfeed to apply a cut. Rotate the cutter down onto the detent once again, and begin traversing. As traverse continues, make sure the detent remains in contact with the flute.

• Repeat the traverse, in the reverse direction, keeping the detent in contact with the flute, as before.
Option 1: Retract the crossfeed and traverse the cutter back to the start without grinding the cutting edge on this pass. Then return the crossfeed to its former position. This risks introducing a variation in grinding depth from one cutting edge to the next.
Option 2: At the end of the pass, rotate the cutter a few degrees to lift it off the detent. If there is sufficient space, use the clamp for the Clearance Angle Indicator to control the rotation. This should be mounted on the end nearest the start of the traverse, to keep your hands as far from the rotating wheel as possible. Stop the wheel rotating, then traverse back to the start, with the cutting edge clear of the wheel.

• Index to the next tooth, using the detent to control the position of the next cutting edge.

Sharpening end mill helical cutting edges

End mills have helical cutting edges with a similar geometry to slab milling cutters (Figs 444 and 445), and may be sharpened in the same way. Some end mills have two centres and may be sharpened with the cutter held between centres. In theory, they may be sharpened using either a flat-faced taper cup wheel, or a type 1 wheel, but, in practice, there is likely to be insufficient space at the ends of the setup to accommodate a taper cup wheel. A much more convenient method, which applies to all end mills, is to hold the cutter in a collet, on the end of a shaft that is free to turn in a workhead. Whether that is a conventional workhead (Fig. 476) or a purpose-designed attachment with air bearings (Figs 468 and 422) is not important, as long as the shaft will spin for several revolutions when given a sharp flick. If a fixed detent is positioned in a flute, it will keep the helical edge at the same height at the point where it passes the grinding wheel, ensuring a consistent grind along the whole cutting edge.

Fig. 476: An end mill held in a collet attached to a long shaft which is free to slide and rotate smoothly and with low friction in a conventional workhead.

Traversing

There are two approaches to traversing. The first is to use the existing table traverse. In that case, the shaft holding the collet and end mill need only be free to turn. The second is to lock the table traverse and use an extended shaft that not only turns, but extends out from the workhead or sharpening device. One of the benefits of an extended shaft is that a little torque can be applied by hand, to keep the helical cutting edge in position against a detent in the flute. Locking the table traverse also means that a separate sharpening device need not rely on an additional workhead or on the alignment of the table. Where a long shaft is used to provide traverse, precise alignment of the workhead or accessory on the table is not critical, as shown in Figs 44–46. This also simplifies the setup process.

Reference position:

Fig. 477: A narrow type 1 wheel contacts a helical cutting edge at one spot, but a wider wheel may contact spots on two adjacent turns of the helix.

Either mount a narrow type 1 wheel on a spindle parallel to the traverse, and set the axis of the cutter and the spindle at the same height; or mount a small-diameter taper cup wheel on a spindle set at up to 5 degrees away from right angles to the traverse (Fig. 469), and set the spindle at approximately the same height as the axis of the cutter. Set the axis of the cutter parallel to the traverse. If a long-shafted collet holder is used to provide traverse, lock the table traverse. Otherwise, use a DTI to ensure precise alignment between a test bar in the workhead, and the axis of traverse (see Chapter 3). Position both the cutter and the collet to the right of the wheel, so that the cutting edge nearest the wheel faces down (Fig. 477). The rotation of the wheel should be downwards at the right-hand edge, nearest the helical cutting edge. The downward drag of the wheel against the cutting edge will help keep the cutter flute against the supporting detent (see later).

• Choose a wheel type: If the wheel spindle can be swung to an angle of up to 5 degrees (Fig. 469), choose either a small flat-faced taper cup wheel or a type 1 wheel. If the spindle cannot be swung, choose a type 1 wheel.

• Flat-faced wheel: Dress the wheel inwards towards the centre, to leave a narrow outer edge. If possible, swing the grinding wheel spindle by up to 5 degrees so that the trailing edge of the wheel will not touch the cutter during traverse (Fig. 469). Dress the outer corner of the wheel parallel to the traverse, to create a face up to 2mm wide.

• Type 1 wheel: Optionally, swing the wheelhead by up to 5 degrees, or dress the wheel at an angle to the traverse, so that the leading edge (the edge that will first contact the cutter) is 1–2mm wide (Fig. 470). Fig. 477 shows that a narrow wheel will contact a helical cutting edge at one place, but a wider wheel may be able to straddle two spots on adjacent parts of the helix, so the wider wheel must have its trailing edge dressed back out of contact.

• Both wheel types: Align the leading edge of the wheel (the edge that will first contact the cutter) with the start of the cutting edge. Use the crossfeed to position the cutter to almost kiss the stationary wheel. A cigarette paper is useful to judge the clearance. Then traverse the wheel clear of the end of the cutter.

Fig. 478: Set the left-hand end of one cutting edge at centre height, by sitting the tip of the edge nearest the wheel on the arm of a centre-height gauge.

• Set the end of one cutting edge at centre height, by using a centre-height gauge (Fig. 478).

Fig. 479: Attach the collar of a Clearance Angle pointer to the collet holder shaft and set the scale to zero. Then rotate the cutter to the required angle.

• Flat-faced wheel: Calculate the drop required to create the primary clearance angle (see Chapter 3), or read the drop distance from Table 10, page 29. Set a height gauge to centre height minus the drop, and rotate the cutter so that the tip of the cutting edge sits at that height (Fig. 473). An alternative is to use a Clearance Angle Indicator scale that gives a direct reading of angular movement (Fig. 472) – a concept that might be adapted for a range of workheads. The collar of the Clearance Angle Indicator pointer should be attached to the shaft of the collet holder, at the handle end, and positioned so that when the cutter tip is at centre height, the scale indicates zero (Figs 479 and 472). Rotate the cutter to the required angle. Afterwards, remove the collar.

• Type 1 wheel: Calculate the distance to raise the wheel to create the primary clearance (see Chapter 3) or read the value from Table 11, page 29, then raise the wheelhead by that amount.

Fig. 480: Align the highest point on the detent with the start of the cutting edge and the leading edge of the wheel, but avoid the curve at the root of the flute.

• Both wheel types: Select a detent with a rounded end and prepare to mount it on a fixed part of the grinder that does not move as the cutter is traversed. If the table traverse is locked, mount the detent on the table. Some purpose-designed attachments have a detent on their base (Fig. 468). Align the leading edge of the wheel with the start of the cutting edge, and use the crossfeed to bring the cutter to just kiss the wheel, then set the detent against the inside of the flute of the cutting edge to be sharpened (Fig. 480). The highest point of the detent should align with the start of the cutting edge, and with the leading edge of the wheel (Figs 475, 476 and 480). It may help to unlock the table traverse and use that to position the wheel; then lock the traverse. Some purpose-designed holders may provide a detent with a flatter top. In those cases, set the detent at the helix angle inside the flute. It is important that, at this stage in the process, the cutter just barely touches the wheel, and that the detent is just far enough inside the flute to allow for sufficient total depth of cut to remain clear of the wheel. The detent should not be at the back of the flute, because as the crossfeed advances the cutter, the detent remains stationary and must not be pushed forward by the inside of the flute.

Fig. 481: With the cutter engaged with the detent, push the shaft forwards while applying a little turning pressure to ensure that the cutter remains in contact with the detent. The wheel should rotate downwards at the front, so that it helps keep the flute on the detent.

• Gripping the end of the shaft, rotate the cutter a few degrees to lift it off the detent. With the wheel rotating, use the crossfeed to apply a cut. Rotate the cutter down onto the detent once again, and begin traversing (Fig. 481). As traverse continues, make sure the detent remains in contact with the flute.

• Repeat the traverse, in the reverse direction, keeping the detent in contact with the flute, as before.
Option 1: Retract the crossfeed and traverse the cutter back to the start without grinding the cutting edge on this pass. Then return the crossfeed to its former position. This risks introducing a variation in grinding depth from one cutting edge to the next.
Option 2: At the end of the pass, rotate the cutter a few degrees to lift it off the detent. Stop the wheel rotating, then traverse back to the start, with the cutting edge clear of the wheel.

Fig. 482: Some dedicated sharpening devices incorporate an indexing mechanism for end mills with regularly spaced cutting edges. The scale indicates that this device can cater for end mills or reamers with teeth/flutes ranging from 2 to 6. Other similar devices may simply rely on the detent. Photo: WEALD ENTERPRISES.

• Index to the next tooth, using the detent to control the position of the next cutting edge. Some purpose-designed attachments provide built-in dividing arrangements (Fig. 482). These are convenient when the cutting edges are equally spaced around the periphery of the end mill, but the advantage of using the detent is that it automatically replicates the existing spacing, even if that is deliberately non-uniform.

SHARPENING FORM MILLING CUTTERS

Form milling cutters such as gear cutters have teeth with a precisely shaped profile (Figs 483, 410 and 415). Altering the profile normally means the cutter is no longer fit for purpose. The teeth have radial relief, with the back of each tooth being lower than the front, but each tooth has a constant radial profile and can be sharpened by rotating the cutter and grinding each tooth face.

Fig. 483: On this involute gear cutter, the teeth have radial clearance which increases from front to back; but each tooth has a constant radial cross-section.

Fig. 484: Mount the cutter on a horizontal arbor set parallel to the traverse. Mount a wheel on a spindle set parallel to the traverse. If necessary, dress the rear edge of the wheel to allow it to reach the bottom of the tooth gullet.

Mount the cutter on an arbor between centres or on a stub mandrel in a workhead (Fig. 484). Mount and dress a type 12 dish wheel or a thin type 1 wheel. The dish wheel may allow the wheel to reach between adjacent teeth without fouling the rear tooth. The purple type 1 wheel in the photos is 3mm thick and intended for sharpening saws. Dress the rear edge to allow the wheel to reach the bottom of the tooth gullet.

Fig. 485: A swing-clear detent can be used to control indexing, if it is used in conjunction with a repeatable stop. The detent pivot is positioned to the side of the cutter centre line.

The challenge in indexing from one tooth to another is in using the face of the tooth to be ground as the surface against which the detent rests. The technique shown here uses a swing-clear detent (Figs 484 and 485), but alternatives are to use a separate indexing device, such as an indexing disc or collar fixed to the shaft on which the gear is mounted, or to index using the backs of the teeth as reference surfaces. Prepare to use the backs of the teeth when the cutter is new and unground. Set up to index the backs of the teeth, using a detent against the front of the same tooth as a reference. Once the backs have been ground, they should not be reground at any subsequent regrind of the front faces.

Reference position:

Spindle axis parallel to the traverse. Cutter arbor axis parallel to the traverse.

Fig. 486: Set the face of a tooth vertical, by visual comparison with the blade of an engineer's square, or a scribed line on a height gauge or, as shown here, by using a flat vertical plate.

• Rotate the cutter to bring one tooth vertical. This can be done visually, by comparison with the vertical blade of an engineer's square or vertical line scribed on a plastic panel attached to a height gauge (Figs 57 and 59). It can also be done using a flat vertical plate on a vertical post (Fig. 486).

• Bring the stationary wheel against the tooth face (Fig. 484), then traverse clear to create space for the detent.

• Rotate the cutter to create a cut, then set the indexing device to zero. If using a swing-clear detent, set it against the face of the tooth now (Fig. 485). Note that the axis of swing (the vertical post) is set to one side of the tip of the detent, on the swing-clear side, so that the tip will initially swing forward, away from the tooth face, as it swings to the side. The lower block with the pin sets the position for the detent when indexing the next tooth into position (Figs 484 and 485).

• Set the height of the wheel, then traverse to grind the face of the tooth. If necessary, for this first tooth, adjust the height of the wheel to grind the whole depth of the tooth face.

• Repeatedly index and grind the next tooth, until all the teeth have been ground to the same depth of cut.

Note that the crossfeed is not used except for the initial setup. Any subsequent cuts are applied by rotating the cutter.

Tool type 7:
Reamers

A reamer is a cylindrical cutter that has flutes separating a series of cutting edges (Fig. 487).A reamer is inserted into a hole that has been previously drilled or bored a few tenths of a millimetre (a few thousands of an inch) smaller than the required finished diameter. The reamer rotates as it moves into the hole, like a drill bit, and its cutting edges remove any remaining material, leaving the hole an exact, known, diameter and with a fine finish. Figs 487–9 show the main features of a typical hand reamer with helical cutting edges and flutes, a short bevel lead at the tip, and a much longer tapered section before the main parallel section that cuts to a specific diameter. Machine reamers have a short bevel at the tip, where all the cutting takes place, but omit the tapered section. Machine reamers may have straight or helical cutting edges and flutes. On a machine reamer, only the bevel section should be sharpened, while on a hand reamer the bevel should be sharpened first, but the tapered section may also be sharpened, if necessary. There is a narrow circular land (USA: margin) with zero clearance behind each cutting edge on the parallel section, which helps steady the reamer in the hole as it cuts. The challenge is to sharpen a reamer without altering its diameter over the parallel cutting edges, so that the reamer always produces a hole of a consistent, known diameter. It is possible to deliberately alter the diameter of the reamer, but that requires a different approach (see later). Reamers have flutes running down the body, and these may be straight on a machine reamer or helical on a hand reamer. The cutting edges around the circumference may appear to be uniformly spaced, but in practice the spacing is often deliberately non-uniform, with small variations in their positions. This means that it is not easy to replicate the spacing of the cutting edges using conventional indexing methods. Instead, a ‘detent’ (finger) is used to bear on each cutting edge as it is sharpened, thereby using each cutting edge as its own reference, as with milling cutters and saw blades.

Fig. 487: Principal parts of a hand reamer. Machine reamers are similar but omit the starting taper and have tapered shanks.

Fig. 488: End view of a reamer, showing the bevel and the hook angle inside each flute.

Fig. 489: Each tooth has a circular margin which is curved and lies on the circumference of the reamer.

Fig. 490: A small diameter machine reamer which has straight cutting edges and flutes.

The choice of grinding wheel is between a curved (type 1) or flat-faced (type 11 taper cup or type 12 dish) wheel. If the grinder table can swing, the reamer may be held between centres (Fig. 494). Otherwise, hold the reamer in a collet in a workhead (as in the example below).

HOLDING A REAMER

Concentricity is important, and reamers may be held in the grinder in one of two ways: supported between centres (Fig. 491), or gripped in a chuck or collet (Fig. 492). Gripping the reamer in a chuck or collet is very convenient, but any inaccuracies in the chuck or collet will compromise the concentricity of the outer diameter of the reamer in relation to its shank. Reamers are manufactured by grinding between centres, so holding a reamer between centres when sharpening should provide good concentricity, depending on the condition of the centres, but may require an additional clamping arrangement for setup purposes and for some grinding operations. Larger reamers usually have a female centre in each end, but small-diameter reamers may have a male centre on at least the cutting end, requiring a female centre on the grinder tailstock. If the reamer is mounted between centres, use a half-centre at the bevel end, to allow space for the wheel.

Fig. 491: A reamer which has been manufactured with centres in each end may be held between centres. The centres on small diameter reamers may be male, requiring a female centre in the tailstock.

Fig. 492: A hand reamer held in a collet by its parallel shank.

SHARPENING THE BEVEL LEAD

Both machine and hand reamers have a short steep bevel at the tip, which does the initial cutting. This is usually set at 45 degrees to the axis of the reamer, and each tooth has a clearance angle (usually 5 to 10 degrees, for steel). The larger the diameter of the reamer, the smaller the clearance angle. Fig. 493 shows that grinding the bevel at 45 degrees but with no clearance results in the bevels lying on the surface of a cone, as shown by the blue bevel surfaces. Adding clearance by effectively tilting the bevels downwards means the bevel cutting edges lie on the surface of the cone, but the flat bevel lies below the surface, providing room for chips and preventing rubbing (shown by the position of the orange bevel surface in Fig. 493). There is no circular margin behind bevel cutting edges.

Fig. 493: A 45 degree bevel. Blue bevels have no clearance and lie on the curved surface of a cone. The orange bevel is ground flat at a clearance angle, and lies below the surface of the cone.

Reference position:

Fig. 494: If the grinder table can swing, the reamer may be held between centres.

Fig. 495: Reference position for grinding the bevel end of a reamer. In this photo, the reamer is held by its shaft, in a multi-fingered ER collet. A smaller diameter reamer will need additional tailstock support.

Choose a narrow type 1 wheel or a wheel which tapers at the rear (taper cup or dish type). A larger diameter type 1 wheel may help provide clearance around the tailstock. Set the axis of the reamer at 45 degrees to the traverse, with the bevel lead nearest the wheel (Figs 494 and 495).

• Type 1 wheel only: Set the spindle at the same height as the axis of the reamer.

Fig. 496: Rest the outer end of the bevel cutting edge on a centre-height gauge. The tip of the gauge should just reach under the end of the bevel, to avoid the curve of the flute lifting the tip, and to allow room for a detent to be placed behind the cutting edge.

• Position the finger of a centre-height gauge at the end of the reamer, then rotate the reamer to rest the tip at the outer end of one cutting edge of the bevel against the end of the finger (Fig. 496). This is the zero-clearance position.

• Type 1 wheel only: Calculate the distance through which the wheel should be raised to create the clearance angle (see Chapter 3), or consult Table 11, page 29. Raise the wheel through that distance.

Fig. 497: When using a flat-faced wheel, one method of setting the clearance is to use a Clearance Angle Gauge attached to the tailstock. The pointer may be attached to the shank of the collet holder.

• Flat-faced wheels only: If the reamer is held in a collet in a workhead, set the workhead angle reading to zero. Method 1: If the reamer is mounted between centres, attach an angle gauge pointer to the shank, and an angle scale plate to the centre mount at that end, then set the pointer to read zero degrees (Figs 497 and 479). Set the clearance angle by removing the centre-height gauge and rotating the reamer to the clearance angle, by referring to the Clearance Angle gauge. Method 2: Calculate the drop distance for the reamer tip to produce the clearance angle (see Chapter 3), or consult Table 10, page 29. Set a height gauge to the centre-height distance minus the drop, and set the tip of the bevel against that height gauge.

Fig. 498: Set a detent ('finger') against the inside of the flute, just behind the cutting edge of the bevel.

• Set a detent against the inside of the flute, just behind the cutting edge of the bevel (Fig. 498). Alignment with the tip of the bevel is not important, and the detent may be a short distance along the flute, although the nearer it is to the bevel, the better the support. The detent must not move in the flute during traverse, so the detent foot should be mounted on the table, to ensure that it moves with the reamer during traverse.

• Set a traverse stop, apply a cut, and traverse to grind the bevel.

• Repeatedly index to the next tooth, and grind, until the whole bevel is sharp.

SHARPENING THE TAPER LEAD ON A HAND REAMER

A hand reamer has a tapered section, termed the ‘taper lead’ (USA: ‘starting taper’), between the bevel lead and the parallel section of the reamer (Fig. 487). The taper lead does most of the cutting. Sharpening involves grinding material from the primary land on the tapered lead only. This effectively extends that taper, but creates a sharp cutting edge, leaving the full diameter body of the reamer untouched. The taper lead has no circular margin, as that has been ground away by the taper. The taper is usually relatively shallow, at approximately 1 degree half-angle on each side (2 degrees included angle), depending on the manufacturer.

Fig. 499: A workhead which can rotate and move backwards and forwards freely, and with low friction.

A key feature of a workholder used when sharpening the cutting edges on a reamer is that while the base of the workhead remains stationary the shaft assembly must be able to rotate and move backwards and forwards freely, smoothly and with low friction (Fig. 499). When a detent is placed in a flute, moving the shaft assembly out or in causes the reamer to turn, keeping the cutting edge above the detent at a constant height as it passes that point. Typically, a workholder does not require a tilt axis when sharpening reamers, and the workhead shown in this chapter is capable of swing and rotate, but not tilt. A full S-T-R or T-S-R workhead can be used, but tilt will be redundant. The extent of the travel outwards must be at least as great as the length of the section of the reamer being sharpened.

Setting the taper parallel to the traverse

The basic method is to swing the table or workhead and use a DTI with an elephant’s foot resting against a cutting edge. Traverse, and adjust the angle until the reading on the DTI does not vary along the taper (Figs 500 and 501). In either case, use the normal table traverse. However; a helical cutting edge curves out of contact with the DTI during traverse, so an additional step is required. That involves positioning a detent in the flute behind the cutting edge, and setting the DTI against the cutting edge at that point (Fig. 502). If the detent is mounted on a non-moving part of the grinder and remains stationary during traverse, it will force the cutting edge to remain at the same height at that point as it passes the detent. If the DTI is aligned with that point, it will remain in contact with the cutting edge and give a useful reading. Note that this technique may be applied to all reamers, whether they have straight or helical cutting edges, as the detent provides additional support for the cutting edge.

Fig. 500: In this setup, the table is swung; the workhead remains stationary; and table traverse is used. If the taper lead is parallel to the traverse the DTI reading will not vary during traverse. Note that the DTI must be mounted so that it does not move during traverse.

Fig. 501: In this setup, the table is not swung, but remains parallel to the traverse; the workhead is swung; and table traverse is used. If the taper lead is parallel to the traverse, the DTI reading will not vary during traverse. Note that the DTI must be mounted so that it does not move during traverse.

Fig. 502: Align the DTI and the highest point on the detent, before traversing. The DTI and the detent are mounted on a non-traversing part of the grinder, such as the fixed base of the table, the bed of the grinder, or the stationary column supporting the head.

Grinding the taper

Reference position:

Set the cutting edge of the taper parallel to the axis of traverse (Figs 500 or 501).

Flat-faced wheel:

Fig. 503: Initial reference position and setup when using a small flat-faced taper cup wheel to grind the tapered section of the cutting edges on the reamer. The wheel is swung through approximately 5 degrees, taking the leading edge nearer the reamer.

Mount a small-diameter taper cup wheel. Dress the wheel inwards towards the centre, so that only a narrow band on the outside edge will contact the reamer. Swing the wheelhead and spindle up to 5 degrees, so that the trailing edge of the wheel will not touch the reamer during traverse. Fig. 503 shows a setup for a taper cup wheel.

• Set the clearance angle. For the setup shown in Fig. 503, rotating the reamer through the clearance angle will cause the cutting edge to drop below centre height. The amount of that drop can be calculated using the formula:
drop = radius of reamer × Sin(clearance angle)
For a reamer of diameter 12mm, and an angle of 5 degrees, the radius will be 12/2 = 6mm, and the drop will be drop = 6 × Sin5 = 6 × 0.0872 = 0.523mm For convenience, Table 10, page 29, lists useful values for common reamer diameters and typical clearance angles.

• Rest the cutting edge against a centre-height gauge positioned right at the beginning of the taper lead, just after the end of the tip bevel, and lock the reamer so that it cannot rotate. Remove the centre-height gauge, reduce its height by the ‘drop’ distance (0.523mm in this example), and replace it. Then rotate the reamer until the same point at the beginning of the taper lead sits on the gauge. Place a detent with a rounded end inside the flute, just behind the cutting edge, then remove the height gauge.

Type 1 wheel:

Fig. 504: Initial reference position and setup when using a type 1 wheel to grind the tapered section of the cutting edges on the reamer. The reamer is horizontal but the tip is swung towards the wheel, and the cutting edges are set parallel to the traverse.

• Mount a narrow type 1 wheel, and dress the trailing edge a few degrees off-parallel, so that only 1–2mm at the leading edge will contact the reamer (Fig. 504).

• Set the axis of the spindle at the same height as the axis of the reamer.

• Rest the cutting edge against a centre-height gauge positioned right at the beginning of the taper lead, just after the end of the bevel. Remove the centre-height gauge. Place a detent with a rounded end just inside the flute, behind the cutting edge. The detent must be mounted on a part of the grinder that will remain stationary during traverse.

• Set the clearance angle. For the setup shown in Fig. 504, the clearance angle is created by raising the wheel an additional distance above centre-height, calculated using the formula:

distance = radius of wheel × Sin(clearance angle)

For a wheel of diameter 75mm, and a clearance angle of 10 degrees, the height will be height = 75/2 × Sin10 = 37.5 × 0.174 = 6.525mm. Table 11, page 29, lists distances for a range of wheels and angles.

• Raise the wheel by the calculated distance above centre-height (6.5mm in this example). The wheel should rotate downwards as it crosses the cutting edge, to help hold the reamer down on the detent.

• Crossfeed to position the reamer to barely touch the wheel. Align the leading edge of the wheel with the beginning of the taper lead, then unlock the reamer, leaving it free to rotate.

Flat-faced or type 1 wheels:

• With the reamer clear of the wheel, apply a cut.

• With the reamer unlocked, traverse carefully until the reamer can be engaged with the detent. Because the wheel has been raised to create the clearance angle, try to keep the reamer on, or just below the detent as it approaches engagement. Seat the detent as the nose bevel reaches the detent. If the reamer is lowered down onto the detent, there is a danger that the cutting edge may touch the protruding part of the wheel and be over-ground. The height gauge in the flute is fixed in position, so the reamer will rotate as it traverses, presenting the cutting edge at the same vertical position as it passes the finger. The downward rotation of the wheel at the cutting edge will help keep the reamer seated on the detent, but a light torque applied to the collet shaft, at the rear of the tailstock and well away from the wheel, may also help.

SPINNING THE DIAMETER

There may be occasions when the parallel section of the reamer is damaged, or when the reamer must deliberately be reduced in diameter to allow it to produce a smaller diameter hole. In those cases, the outside diameter of the reamer may be ground to a smaller size. Reamers are typically produced slightly oversized, and the common m6 tolerance (for a reamer designed to produce a hole to suit a shaft with tolerance H7) allows reamer diameters to vary from dead-to-size to an oversize allowance dependent on diameter (Table 15). The effect of grinding the reamer depends on the total depth of cut. A small depth may be enough to reduce the diameter of the reamer but leave it still within tolerance for the nominal size of hole for which it was intended. However, the challenge is that the total depth of cut is very small, and the total depth of cut will be half the difference between the maximum and minimum oversize values from Table 15. The bevel lead and any taper lead need not be ground, as those are already smaller in diameter. However, reducing the diameter of the parallel section of the reamer by a significant amount will reduce the length of the lead taper, which may then need to be reground so that it is approximately one and a half times the diameter of the reamer (with a minimum length of 20mm).

Tolerance on reamer diameters

Nominal diameter (mm)

Maximum oversize (mm)

Minimum oversize (mm)

0–3

0.008

0.002

3–6

0.012

0.004

6–10

0.015

0.006

10–18

0.018

0.007

18–30

0.021

0.008

Table 15: Typical oversize allowances for a selection of reamer diameters.

Typical measured reamer margin widths

Reamer diameter (mm)

Margin width (mm)

2.5

0.15

3

0.2

12

0.25

25

0.3

Table 16: Typical measured margin widths for a small selection of reamer diameters.

The cutting edge on the parallel section of a reamer lies on the enclosing circle (Fig. 489), as does the circular lead immediately behind each cutting edge. That is equivalent to a primary clearance angle of zero degrees, and it acts to steady the reamer as it cuts inside a hole. However, the circular land should be relatively narrow, with typical measured values being shown in Table 16. Land widths can be measured using an optical measuring magnifier with a calibrated graticule scale, a microscope or a much larger optical comparator.

In reality, the ‘parallel’ section of most reamers is not parallel along its length, but has a small back taper to ensure that the front of the reamer does the cutting, and to prevent jamming in the hole (Figs 505 and 506). The back taper should be such that it does not take the diameter of the reamer below the minimum tolerance (Table 15). A popular rule of thumb is to create a back taper of one in ten thousand, taking no more than 0.0025mm (1/10 thou) off the diameter per 25mm (1in) of length. That represents a tiny swing of 0.036 degrees, taking the shank nearer the wheel.

Fig. 505: Machine reamers have a short parallel section behind the lead bevel. The remaining length has a back taper of approximately 1 in 10000.

Fig. 506: Machine reamers have a bevel lead, followed by a short length at full diameter. The remaining length has a back taper of approximately 1 in 10000.

Measuring the diameter of a reamer

On a hand reamer, measure the diameter just after the taper lead; on a machine reamer, measure just after the bevel lead. Set the reamer up between centres. Place a DTI with its base on the table and its foot directly above the centre line of the reamer. Position a cutting edge under the foot of the DTI, then turn the reamer back and forth to identify the high point of the cutting edge. Set the DTI to zero. Compare this with a convenient standard of almost the same height, such as the shank or the centre, and note the difference in readings, as well as the diameter of the shank. The diameter of the reamer can be calculated from:

the diameter of the shank plus twice the difference in the readings.

If the shank diameter is 24mm and the difference between the cutting edge and the (smaller) reamer shank is 0.5mm, the diameter of the reamer is:

24mm + 2 × 0.5mm = 25mm

Arrange for a spinning device

Spinning requires some means of continuously rotating the reamer as it is being ground. The safest way to do this is to add a knob or handle to the end of the rear centre, and a catchplate with driving pin and a dog to provide the drive (Figs 507 and 508). The catchplate and handle are fastened onto the rear centre, and prevent that centre from disengaging from the reamer, but allow the centre to turn freely. The dog is fastened to the reamer, and engages with the driving pin. This arrangement should not allow the reamer to turn freely either backwards or forwards, so the dog may have a slot for the catchplate pin. A little clearance is permissible, but a dog with a single arm or pin would need to be held captive to the catchplate pin by wrapping the two together with wire. Spinning while traversing manually requires good co-ordination and some practice and a deluxe arrangement would be to use a motorized attachment. These incorporate the centre and the catchplate with the driving pin, and typically rotate the reamer at 150–300rpm.

Fig. 507: View from the rear, showing the handle, the catchplate with driving pin attached to the centre, and the slotted dog clamped to the reamer.

Fig. 508: Reference position for spinning the circular lands parallel, using a type 1 wheel.

Spin parallel

Reference position:

Mount the reamer between centres which have been set parallel to the traverse. Choose a grinding wheel with a curved working face, such as a narrow type 1 wheel. Set the axis of the wheel to be at the same height as the axis of the reamer. Turn the wheel 1 or 2 degrees from the parallel position, or dress the face at an angle to the traverse, so that contact between wheel and reamer will only be at one edge (Fig. 508).

• Coat the reamer teeth in layout blue or colour them with a marker pen.

• Set the wheel to kiss the highest section of the reamer (the beginning of the ‘parallel’ section). Be guided by the existing run out on the circular lands, and set the wheel where the circular land first appears at full width.

• Rotate the reamer. The reamer must be rotated continuously during grinding, so arrange for a method of hand rotation (Fig. 507) or (preferably) a motor drive unit. The speed of rotation should be approximately 150–300rpm, but considerable variation is possible, provided the traverse is not so slow that the cutting edges overheat.

• Apply a cut, then traverse along the body while continuing to spin the reamer.

• Measure the diameter of the reamer.

• If necessary, increase the cut and repeat, until the target diameter is reached.

Option: spin a back taper

The effect of spinning the circular land of a reamer depends on the total depth of cut. If the depth takes the reamer well below its nominal diameter, the back taper should be reground. But if grinding is light, and the reamer remains within its original tolerance, there is likely to be less need to regrind the back taper. To grind a back taper, set up as for spinning parallel, then swing the table to approximately 0.03–0.04 degrees (or one thirtieth of a degree). This is a tiny angle, and it may be enough to offset the shank end centre tailstock by the thickness of a cigarette paper.

• Allow a short parallel section to remain on the reamer to bring the hole to size (up to one diameter in length). There is no standard for this, but that section needs to be long enough that a subsequent regrind of the bevel lead or the taper lead does not remove all of the parallel section. Once that parallel section has gone, the reamer can no longer cut to its intended size.

• Set the wheel against the reamer at the end of the parallel section, but do not apply an additional cut. Then spin to grind the taper on the remainder of the length.

Option: regrind the clearance

If the circular land ends up too wide (see Table 16, page 125), the clearance behind each circular land should be reground. Treat the whole of the parallel section as truly parallel, as the back taper will not make any significant difference to the secondary clearances. Afterwards, grind the clearance for the flutes on the parallel section. Depending on the diameter of the reamer and the number of cutting edges, a narrow type 1 wheel may be the best choice for grinding the clearances.

Reference position:

Reamer between centres and parallel to the traverse. Spindle axis parallel to the traverse, then swung 2–5 degrees to set the leading edge of the wheel towards the reamer. Spindle and reamer axes at the same height.

Fig. 509: Reference position when regrinding the clearance behind the circular lands of a reamer, using a narrow type 1 wheel.

• Mount a narrow type 1 wheel, and dress to give a small area of contact (Fig. 509).

• Set the tip of one cutting edge at centre height using a centre-height gauge.

• Set a detent inside the flute to support the cutting edge at centre height. The detent must be mounted on a fixed non-traversing part of the grinder.

• Calculate the increase in wheel height required to create the clearance angle, (see Chapter 3), then alter the wheel height.

• Apply a small cut, then traverse to grind. Inspect the width of the circular land, adjusting the depth of cut as necessary.

• Repeatedly index to the next cutting edge and grind.

GRINDING THE MAJOR FACE

Each cutting edge has a circular land running back from the cutting edge. The inside of the flute adjacent to the cutting edge is effectively the Major face, and the hook angle provides the Major face rake (Figs 488 and 510). The circular land is part of the Major flank, but it has zero clearance.

Fig. 510: The Major face must be set vertical before grinding, by taking account of the hook angle.

One way of sharpening the reamer is to lightly grind inside the flute, removing a little material from the circular land, but ensuring that some circular land remains to steady and guide the reamer in the hole. The amount that can be removed is small, but this is a way to restore a dull cutting edge. This method is recommended by some major manufacturers who provide guidance on the process. As with spinning, this technique requires a considerable amount of care, particularly with the depth of cut, so it is not a technique that can remove deep nicks or significant damage.

Setting the Major face vertical

The flute contains the Major face, adjacent to the cutting edge, and regrinding should preserve the Major face rake. On some reamers the inside of the flute is relatively flat just under the cutting edge. Other flutes have a more pronounced curve and a steeper angle. The tighter the curve the more care is needed in setting the Major face vertical (Fig. 510). In practice, it is only the first millimetre or so of the Major face that determines the rake angle.

A height gauge may provide a convenient vertical reference face on a right-angled arm that carries the tip.

Fig. 511: Using the vertical face of the arm of a height gauge as a reference to set the Major face vertical.

Fig. 512: Set the Major face vertical against the edge of the reference surface on the height gauge arm.

• Position the arm of the height gauge inside the top flute (Fig. 511). Using high magnification, set the edge of the vertical reference face against the Major face, bringing the Major face vertical at that point (Fig. 512). Stay away from the curved section of the inside of the flute. Only the first millimetre or so of the Major face needs to be vertical, as that part sets the rake angle.

• Lock the reamer to prevent further rotation; then mark the cutting edge next to that vertical spot with a pen. Remove the vertical finger.

• Arrange for a manual method of applying a light turning force to the reamer during grinding (Fig. 507).

Grinding the Major face rake

Reference position:

Reamer mounted between centres, using a half-centre at the bevel end, with the flat of the half-centre facing upwards. Reamer axis parallel to the traverse, then swung to approximately 2 or 3 degrees less than the helix angle of a flute. In many cases, the reamer axis can remain parallel to the traverse, provided the wheel will not foul the opposite edge of the flute. Set the spindle axis at right angles to the traverse. Mount and dress a small-diameter flat-faced type 12 dish wheel. Round the corner of the wheel to a small curve (to blend into the internal curve of the flute), then create a narrow area of contact with the work (Fig. 513).

Fig. 513: Reference position when grinding the Major face. In this setup, the reamer is parallel to the traverse, as there is enough clearance at the rear of the dish wheel to avoid fouling the adjacent tooth.

• Position the wheel inside the top flute, with the leading edge (the first point of contact) against the Major face at the marked point (see Setting the Major face vertical). Fig. 513 shows the wheel inside the flute, touching only at the leading edge.

• Unlock the reamer. With the wheel stationary, and without altering the height of the wheel, traverse towards the bevel end, clear of the parallel section of the reamer. The wheel may be taken clear of the reamer altogether.

• With the wheel rotating, traverse along the Major face of the flute. The reamer should be guided on entry, and the method recommended by manufacturers is to apply a light torque using finger pressure to keep the cutting edge against the wheel. The object is to create a very small cut, without removing the circular margin. Keep the fingers safe, and well clear of the rotating wheel, by using the handle or knob at the rear of the shank-end centre. The turning motion preserves the rake angle and maintains the correct geometry.

• With the wheel stationary, traverse back to the start. Index visually, aiming simply to have the wheel enter the next flute and rest against the Major face. Then repeat the grind on this flute. Repeat as required until all the Major faces have been reground.

Tool type 8:
Taps and dies

SHARPENING TAPS

Every tap (Fig. 514) has a precise thread-form that conforms to specifications laid down in a Standards document. The cutting action is caused by cutting edges running along the length of the thread that are created by flutes that interrupt the threadform (Fig. 515). To create a female thread, a hole is drilled, of a specific size to suit the thread, and the tap is inserted into the hole. Turning the tap as though it was a bolt, while forcing the tap into the hole causes the cutting edges to remove material from the wall of the hole, recreating the thread of the tap and forming a female thread. Cutting takes place at the tip of the tap, that is normally tapered so that the first few threads begin undersize and gradually increase in height until the thread is fully formed. A full set of conventional hand-operated taps consists of three taps, all with the same threadform but each having a different length of taper at the front of the tap (Fig. 516). Table 17 lists the names of the taps, and the lengths of their respective tapers. Once the taper tap has been used, the thread is fully formed, except at the bottom of a blind hole. To cut the thread closer to the bottom, the taper is followed by the second and the plug (or bottoming) tap, each of which cut an extra turn or two. Flutes on hand taps are straight.

Fig. 514: Taps create female threads in nuts, sleeves or holes, and dies create male threads on fasteners such as bolts, machine screws or studs.

Fig. 515: On a tap, the cutting edge creates a threadform in a nut or hole. The main features of the tap provide cutting edges, Major faces, progressive clearance at the front of the cutting edge, and an entry taper.

Fig. 516: In a set of hand taps, each hand tap has a different length of taper. From the right: tap types are taper, second or intermediate, plug (or taper, plug and bottoming in the USA).

Table 17: The half-angle (the angle on one side) of the tapers on different types of hand taps varies, and the number of threads in the taper depends on the pitch of the thread.

Fig. 517: A spiral point tap has an angled section of flute at the tip.

Spiral-point taps (Fig. 517) are designed for creating threads in through holes, and have an additional angled section of flute at the tip that pushes the chips forwards, leaving the main flutes relatively clear.

Fig. 518: On a spiral fluted tap the helix angle is the angle of the flute.

Spiral-fluted taps (Fig. 518) are designed to cut a thread in one pass, right to the bottom of a blind hole. The spiral flutes are created at a helix angle that helps feed chips back out of the hole.

Geometric or serial taps must be used in sets, as the first tap creates a thread of partial height and only the last tap in the set creates a full-height thread. The flutes and cutting angles are the same as for taps that create full-height threads.

Fig. 519: A cross-section of a typical tap shows some of the geometry. The margin lies on the nominal diameter and has zero clearance. It acts to steady the tap as it cuts the thread inside a hole.

The cutting edge lies where the flute meets the threadform (Figs 515 and 519). At that point, the Major cutting face is inside the flute, the cutting edge is the thread-shaped line where the face meets the thread, and the curved face running around the tap from one cutting edge to the next flute is the Major flank (called the ‘land’). The threadform on a tap has a radial relief, so that only the front of the thread, at the cutting edge, and a small section behind the cutting edge (called the ‘circular margin’ or just the ‘margin’ for short) is fully formed. The radial relief is equivalent to a front flank clearance angle on a lathe tool, but wrapped around the periphery of the tap. The margin guides the tap as it cuts, and sharpening the Major faces of the tap by grinding along the whole length of the flutes will reduce the widths of the margins, producing a sharp tap that has reduced guidance and is less stable when cutting. Grinding heavily or repeatedly will quickly remove the margins and result in a tap that cuts an undersized thread.

Fig. 520: Radial clearance is produced by grinding using a different offset centre for each land. Radial clearance begins behind the margin shown in Fig. 519.

The relief behind the margin is created by grinding using an offset axis (Figs 519 and 520), so that the centre of rotation of the tap must be offset from its own axis. This produces a continuously increasing clearance around the circumference of the tap. It is difficult to find the required offset for grinding each land, but this is not normally required to sharpen a lightly worn tap.

Fig. 521: A drill and tap sharpening attachment that creates the correct relief behind the Major cutting edges. Photo: NIGEL HILL.

Some manufacturers do not recommend sharpening taps smaller than 25mm or 1in diameter. Certainly, the size of the margin is proportional to the diameter of the tap, so small-diameter taps require great care when setting up, and removal of very small amounts at the cutting edges. However, most cutting takes place along the tapered section of the tap, so one strategy for taps that do not have straight flutes is to sharpen the tapered section, leaving the main flutes alone. Where there is substantial damage, reform the taper before sharpening. The best way to sharpen taps is to use a dedicated tap-sharpening machine, as that will preserve the rake angles and the amount of relief, as well as reforming the tapered section on each tap (Fig. 521). Tap grinders are, however, relatively expensive, and, with some adaptation, it is possible to do some sharpening using more general-purpose tool and cutter grinders.

Mounting taps

Taps often have centres at either end, typically a female centre at each end, but sometimes a female at the rear and a male at the front. However, it is not unusual to find that the centres are not in good condition, or have been partially or wholly ground away during manufacture, so it is always worth testing the accuracy of the centres by using a DTI to check run out on the shank (Fig. 522) or at the Major cutting edges (Fig. 523). Top-quality taps usually have well-formed centres with minimal run out, and a consistent reading at each Major cutting edge, but many others do not. As a result, the most convenient way to hold a tap is usually in a collet in a workhead. This greatly simplifies some of the movements required, especially where the flute has a helix angle.

Fig. 522: A DTI can be used to check run out of the shank when a tap is mounted between centres.

Fig. 523: A DTI can be used to check run out of the Major cutting edges (the high points on the threadform) when a tap is mounted between centres.

Fig. 524: A tap mounted between centres, in a Quorn grinder. Note the catchplate and driving dog which prevent unintended rotation.

Where a tap has two well-formed centres and minimal run out, mount the tap between corresponding male or female centres (Fig. 524). That ensures the axis of the tap coincides with the axis of the centres. Use a half-centre at the front (threaded) end of the tap, to allow clearance for the grinding wheel. If the tap is mounted between centres, make arrangements to lock the tap in position, so that it cannot rotate. Use a catchplate (drive plate with a pin) and a dog (small carrier arm) that can be clamped to the shank of the tap and secured against the pin on the catchplate, to lock the tap in position, preventing it from rotating (Fig. 524).

Sharpening the cutting edge on the taper

The general method is to mount a tap with its cutting edge parallel to the axis of traverse, rotate backwards through the hook angle to set the Major flank vertical, then rotate forwards to apply a cut. The following instructions assume the tap is held in a collet, although the same method can be used when the tap is held between centres.

Straight flutes

On a tap with straight flutes, the flutes lie parallel to the axis of the tap.

Fig. 525: The space available for the wheel to run out beyond the end of the tap affects the maximum diameter of the wheel.

• Choose a small-diameter wheel that will have clearance between the rear of the wheel and the back of the flute. The diameter is determined by the distance between the end of the tap and the nearest obstruction such as the body of a centre or a collet holder (Fig. 525).

Fig. 526: Set the face of the wheel on the centre line of the workhead by setting it against the vertical face of a half-diameter setting bar.

• Mount a half-diameter setting bar and use the blade of an engineer's square to set the flat face of the bar vertical. Set the face of the wheel against the flat face of the setting bar (Fig. 526), then raise the wheel clear.

Fig. 527: Set a centre-height gauge against the underside of the half-diameter setting bar.

• Rotate the setting bar through 90 degrees, so that it faces downwards, and set the top face of the finger of a centre-height gauge against the face of the setting bar (Fig. 527).

Fig. 528: Viewed from the wheel side: rotate the tap to bring the fully formed cutting edge against the finger without fouling the curve inside the flute.

• Mount a tap, and rotate to bring one cutting edge against the top face of the finger (Fig. 528). Take care that the top edge or corner of the detent does not foul the inside of the flute.

Fig. 529: Viewed from the workhead end: rotate the tap through 90 degrees to bring one cutting edge vertically above the central axis of the tap.

• Rotate the tap through 90 degrees, to bring the cutting edge vertically above the central axis of the tap (Fig. 529). If the spindle has an angular scale for rotation, set that to zero now.

Fig. 530: Viewed from the workhead end: rotate towards the wheel through the hook angle to set the Major face vertical.

Fig. 531: Viewed from the workhead end: use the crossfeed to bring the cutting edge against the face of the wheel.

• To set the Major flank vertical rotate the tap through the hook angle, to bring the inner face of the flute vertical at the cutting edge (Fig. 530). This can be done by rotating through an angle or by moving a distance.
Rotation through an angle is controlled by an angular scale on the workhead spindle. After rotation, use the crossfeed to move the tap towards the wheel to bring the cutting edge against the face of the wheel. In this position, make sure that the wheel edge is not lower than the cutting edge, and does not foul the curve inside the flute (Fig. 531). Note the angle through which the tap moved (H, in Figs 530 and 531).
Moving a distance involves calculating the horizontal linear distance the cutting edge of the tap should move during a rotation, using the formula:
B = r × SinH
where B is the distance, r is half the diameter of the tap, and H is the hook angle, in degrees
For an M6 tap, with diameter 5.8mm, and a hook angle of 7 degrees, B = 2.9 × Sin7 = 0.35mm
Rotate the tap, taking the top towards the operator, then use the crossfeed to move the tap towards the wheel the calculated distance (0.35mm in this example). Finally, rotate the top of the tap towards the wheel until the cutting edge just meets the lower edge of the wheel. In this position, it is important that the wheel edge is not lower than the cutting edge, and does not foul the curve inside the flute.

Fig. 532: Viewed from the workhead end: use the crossfeed to move the cutting edge away from the face of the wheel by less than the circular margin (f). Then rotate the tap to touch the wheel.

Fig. 533: Viewed from the workhead end: rotate the tap so that the cutting edge touches the wheel.

Fig. 534: Viewed from the workhead end: with the wheel clear, advance the tap by the feed distance (f).

• Creating a cut involves rotating forwards (towards the face of the wheel) by less than the width of the circular margin (Fig. 532). The size of the margin will vary depending on the diameter of the tap, the manufacturer's specification, and whether it has previously been sharpened, so select a value from 0.005 to 0.02mm, erring on the side of caution by beginning with a small value.
Begin with the wheel against the cutting edge (Fig. 531). Using the crossfeed, retract the tap by the feed distance (f, in Fig. 532). Then rotate the tap so that the cutting edge touches the wheel (Fig. 533). Traverse the wheel clear, then use the crossfeed to advance the tap by the feed distance (Fig. 534)

• Traverse to grind. If further cuts are required, use the crossfeed to retract the tap by the feed distance. Rotate the tap so that the cutting edge touches the wheel. Traverse the wheel clear, then use the crossfeed to advance the tap by the feed distance. The feed distances may vary, but the total should be no more than the circular margin.

Fig. 535: Tilting the tap upwards restricts grinding to a short distance beyond the taper.

When a tap is being used, most of the cutting takes place at the front, and is largely complete by the time the first full-height thread enters the work. This means it is not normally necessary to sharpen the whole of the Major cutting edge. Instead, tilt the tip of the tap upwards to bring the tapered section of the cutting edge approximately parallel to the table (Fig. 535). When grinding takes place, only the threads in the taper, and a few beyond, will be ground.

Spiral-point taps

A spiral-point tap has gashes ground at the tip (Fig. 517). Viewed from above, the non-cutting edge of a gash runs parallel, or nearly parallel, to the axis of the tap, but viewed from the side the bottom of the gash lies at an angle (Fig. 536). The sides of a gash are not at 90 degrees to one another, and that creates the hook angle at the cutting edge (Fig. 537). Some spiral-point taps have straight flutes along the remaining threads, but only the tip section needs to be sharpened on a spiral-point tap.

Fig. 536: Viewed from the side, the gash lies at an angle to the axis of the tap (15 degrees on this tap).

Fig. 537: The hook angle (shown in green) is created because the sides of the gash are not at right angles.

Fig. 538: On a small diameter grinding wheel, dress the front face flat, then dress the periphery to create the hook angle (typically 7 degrees, for tapping steel).

• Dress a small-diameter wheel to match the shape of the cross-section of the gash, as shown in Fig. 538.

Fig. 539: Viewed looking towards the front of the tap: rotate the tap until the non-cutting edge of a gash rests on the finger of a centre-height gauge.

• Beginning with the axis of the tap parallel to the axis of traverse and with the tip facing to the right, rotate the tap until one non-cutting edge rests on the upturned finger of a centre-height gauge (Fig. 539). Remove the gauge and rotate the tap through 90 degrees so that one side of the gash is vertical, as shown in Fig. 537. That side is the non-cutting flank.

Fig. 540: Tilt the workhead to bring the bottom corner of the gash horizontal.

• Tilt the tap tip-upwards, to bring the inner corner of the gash horizontal (Fig. 540). The gash on the tap shown in Fig. 536 lies at 15 degrees to the horizontal.

Fig. 541: Crossfeed until the vertical face of the wheel just touches the vertical face of the gash. With a narrow wheel, prioritize grinding the cutting edge.

• Crossfeed until the outer face of the wheel just rests against the vertical face of the gash (Fig. 541). Traverse the wheel clear of the tap. That vertical face does not need to be ground, it is used as a reference face for the wheel. When the wheel is too narrow to span the whole of the bottom of the gash, position it so that the cutting edge will be sharpened.

• Lower the wheel to create a cut along the bottom of the gash. Traverse to grind.

• Rotate the tap through 90 degrees, and grind the second flute.

Spiral -flute taps

On a tap that has spiral flutes (Fig. 518) the cutting edges are continuously curved as they follow the spiralling flutes. This means that it is only the tip of the tap that can be conveniently sharpened, without treating the tap as a milling cutter.

• Begin as for a tap with straight flutes, resting the cutting edge at the tip of the tap (which is often a chamfered tooth) against the finger of a centre-height gauge, then rotating the tap through 90 degrees to bring that tooth vertical.

Fig. 542: Swing to set the axis of the flute parallel to the axis of traverse.

• Swing the tap through the helix angle of the flutes, so that the cutting edge is parallel to the axis of traverse (Fig. 542).

• To set the Major face vertical tilt the workhead through the hook angle, to bring the inner face of the flute vertical at the cutting edge (Fig. 530).

Fig. 543: The Major face can be set vertical by using a tilt instead of a rotation. One effect is to increase the distance the tap moves, because of the greater distance from the axis of tilt to the cutting edge.

On a tap with straight flutes, the axis of the tap and the axis of traverse are parallel, so the cutting edge will remain parallel to the axis of traverse when the tap is rotated. Where the flutes lie at an angle to the axis of the tap, rotating the cutting edge about the axis of the tap effectively rotates that edge about the surface of a cone, relative to the axis of traverse, so the angle of the cutting edge changes. As a result, rotations are best replaced by tilt movements. However, although a tilt is a rotation about the axis of the workhead, that axis is at some distance from the axis of the tap. That magnifies the distance the tap moves towards or away from the wheel at each stage (Fig. 543).

There will now be a gap between the cutting edge of the tap and the face of the wheel. Use the crossfeed to move the workhead horizontally, to close the gap and bring the tap against the wheel (Fig. 531).

If there is no angular scale on the tilt axis, the hook angle can be used to calculate a horizontal distance (Fig. 543), as for a straight-fluted tap, but using the formula:

B = (R+r) × SinH

where B is the horizontal distance, R is the distance between the axis of tilt and the axis of rotation of the tap, r is the radius of the tap, and H is the hook angle, in degrees.

• Creating a cut involves tilting forwards (towards the face of the wheel) by less than the width of the circular margin (Fig. 532), and may be done by retracting the wheel to allow a feeler gauge to be placed between the cutting edge and the face of the wheel. Remove the feeler gauge and tilt to bring the cutting edge against the wheel. Traverse to grind.

Fig. 544: An alternative mounting arrangement is to make the tap face to the right, and to use a type 3 wheel, mounted back-to-front.

Tilting the workhead instead of rotating the tap may take the tap too far away for the crossfeed to be able to bring close the gap between wheel and cutting edge. One alternative is to mount the tap facing to the right, and to mount a type 3 wheel back-to-front (Fig. 544). Dress the flat rear face of the wheel after it has been mounted.

Sharpening dies

Dies are internally threaded and designed to cut external threads on a shaft or bar. Dies are rather like nuts, but the thread is interrupted by clearance holes that create cutting edges (Fig. 545). At each cutting edge, the tangent to the hole, at the intersection with the threadform, provides a major face rake angle (top rake) or hook angle (Fig. 546). The teeth in a die are backed off so that the cutting edge has full-size teeth, but the threadform behind the cutting edge is progressively reduced in height and width towards the trailing edge, to give clearance (Fig. 547).

Fig. 545: A die carries a thread interrupted by clearance holes which create cutting edges.

Fig. 546: Each cutting edge is formed by the intersection between the threadform and a clearance hole. The hook angle is the angle between the tangent to the hole at the point of intersection, and the radial line from the centre of the die to the tip of the cutting edge.

Fig. 547: The teeth of a die are progressively backed off from the cutting edge to the trailing edge, on each threaded section of the die.

Sharpening a die involves grinding a little from the wall of each clearance hole, effectively sharpening each cutting edge. As the holes become larger, the Major face rake angle will decrease slightly, but the small change will not significantly alter the cutting action. A die is a form tool, and removing any significant amount from the cutting edges will soon result in loss of form, so although there may be a small margin of full-form thread immediately behind the cutting edge, any sharpening should involve the smallest cuts consistent with restoring the cutting edge. Grinding beyond that will result in a thread that is larger and tighter than formerly produced by that die.

Fig. 548: Small diameter cylindrical grinding wheels (pins) may be dressed using a diamond dresser.

Fig. 549: Offsetting the dresser to the 'downhill' side ensures the diamond is in the trailing position. Take care with the direction of rotation.

Sharpening can be done using a small-diameter cylindrical grinding wheel of a similar shape to those used in a typical multi-tool. Because of their small diameter, these wheels are usually supplied already mounted on a small-diameter shaft, and are termed ‘mounted’ wheels, or, when the head has a small diameter, ‘pins’. They are available in a range of abrasive types and grit sizes. The ‘pin’ may be dressed in position, using a standard diamond dresser point (Fig. 548) and Fig. 549 shows how this may be offset so that the diamond point is at a small negative ‘trailing’ angle.

Fig. 550: A die may be clamped between two parallel plates. The rear plate is mounted on a hollow shaft, and the shaft is free to rotate in the workhead.

Fig. 551: A die holder consisting of two plates, a hollow shaft, and a centring rod.

Fig. 552: Use the tapered end of the centring rod to centre the die over the hole in the backplate and shaft.

Fig. 553: The taper of the setting rod centres the die by locating against the points at which the cutting edges intersect the entry chamfers (green arrow). The red arrow indicates a gap at each trailing edge.

Fig. 554: Use the crossfeed to apply a cut before entry to the die, then rotate the die holder against the direction of rotation of the wheel while traversing.

In the holder shown in Fig. 550 the die is trapped between two parallel plates (Fig. 551). The holder is mounted on a hollow shaft that provides clearance for the grinding wheel to pass beyond the rear of the die. A centring rod with a tapered point (Fig. 551) can slide snugly inside the shaft hole. The body of the holder appears eccentric, because it can accommodate a range of sizes of die, but individual die holes are always centred on the axis of rotation of the workhead shaft. Align the axes of the wheel spindle and the workhead with the axis of traverse (see Chapter 3). Traverse will be forwards and backwards, parallel to the axis of the wheel spindle, to move the grinding wheel through the die clearance hole. If the whole length of the curved face of the grinding wheel is in contact with the die throughout the traverse that will generate heat, so offset the spindle axis by 0.5 to 1 degree towards the point of contact with the wall of the hole in the die. Ideally, the wheel should contact the front of the wall (nearest the front of the table), as this allows any increase in the crossfeed to cause a corresponding increase in the depth of cut. Mount the die in the holder with its front facing the rear plate, locating the tapered point of the centring rod against the tapered entry ground at the front of the die teeth (Figs 552 and 553), before tightening the front plate and withdrawing the centring rod. Traverse the workholder so that the wheel enters the clearance hole, then move the workholder or the spindle so that the front edge of the wheel touches the wall on the side nearest the front of the table and there is clearance between the curved face of the wheel and the wall, behind the point of contact. Cut is applied by using the crossfeed to move the die holder further towards the point of contact with the wheel. During grinding, simultaneously traverse forwards and backwards to move the wheel along the inside of the hole while rotating the holder against the direction of rotation of the wheel, so that the wheel grinds material uniformly from the wall of the clearance hole (Fig. 554). That sharpens the cutting edges formed at the intersection of the threadform teeth and the wall of the hole. In fact, grinding need only take place on a partial rotation covering a small area on either side of the aperture next to the teeth. However, that will make the hole non-circular, and while that will not affect the cutting action of the die, it will make it difficult to centre the hole for any subsequent sharpening operation. The process is repeated for each clearance hole on the die.

Further Information

BOOKS AND MAGAZINE ARTICLES

Amos, P. (1997) ‘Sharpening End Milling Cutters’, Model Engineers' Workshop, issue 44, August/September, pp. 42–50.

Boxford Machine Tools Ltd. (1975) G200 Tool and Cutter Grinder: Operator's Handbook, Wheatley, Boxford Machine Tools Ltd.

Chaddock, D.H. (1975) Operating Instructions for the Quorn Tool and Cutter Grinder, Sheffield, Model Engineering Services.

Clarkson (Engineers) Ltd. (1962, 3rd edition) Clarkson Cutter Grinder: Operator's Handbook, Nuneaton, Clarkson (Engineers) Ltd.

Clarkson (Engineers) Ltd. (1973) Clarkson MkII Radius Fixture, Nuneaton, Clarkson (Engineers) Ltd.

Dormer Tools Ltd. (2013) Dormer Technical Handbook, Sheffield, Dormer Tools Ltd.

Easterbrook Allcard & Co. Ltd. (1970) Presto Counsellor, Sheffield, Easterbrook Allcard & Co. Ltd.

Oberg, E., Jones, F.D., Horton, H.L. and Ryfell, H.H. (2012, 29th edition) Machinery's Handbook, New York, Industrial Press.

Thomas, W.M. (1975) ‘Grinding staggered-tooth cutters’, Model Engineer, vol. 141, issue 3510, 4 April, pp. 348–351.

CONTRIBUTORS' WEBSITES

Arc Euro Trade Ltd. Suppliers of tools including collet blocks, vee blocks, and toolposts.

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Eccentric Engineering. Manufacturers of innovative tooling.

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W.E. Falck

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T.D. Fitchett Ltd. Suppliers of original Triumph spares.

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John Florence (SMEE)

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Glenn Evans of Glennview.com

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Glendo LLC. GRS and Accu-Finish tools.

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GRS. Power Hone sharpening system.

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Alan Hooper (Colyer-Caseley grinder)

www.bristolmodelengineers.co.uk

David Haythornthwaite

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Steve Lindsay Engraving and Tools. Manufacturer of Lindsay AirGravers, hand engraving tools and resources.

www.lindsayengraving.com
www.airgraver.com

John Moran's extensive site includes useful sections on drill sharpening machines and techniques.

www.gadgetbuilder.com

Morse Flight Systems

www.facebook.com/morseflightsystems

Howard at MYFORD-LATHES.COM

www.myford-lathes.com

Howard buys and sells hobby lathes, mills, cutter grinders, tooling and all live steam items.

Jack Sealey Ltd. Tools and workshop equipment.

www.sealey.co.uk

The Society of Model and Experimental Engineers

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Toulouse Museum (Didier Descouens)

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Niels Vrijlandt of Nielsmachines.com. Preloved machines.

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Weald Enterprises. Machine tool suppliers.

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Table 1: Common abrasive materials.

Table 2: Common types of wheel bond.

Table 6: Guidelines for grinding wheel flange diameters. Summary based on information contained within BS 4581-2: 1984 and relates to wheels with bore diameters less than 76.2mm.

Table 13: Helix angle of common threads.

Table 4: Approximately equivalent particle, mesh and grade values and descriptions for a range of abrasives.

Table 12: Swing and tilt angles to set up for grinding a top face at a 'true' rake angle for a range of Major and Minor face rake angles.

Fig. 70A: The clearance angle is related to the radius of the wheel and the height the cutting edge of the tool is raised above the horizontal centre line.
A negative clearance angle can be created using the same values, but lowering the tool cutting edge below the horizontal centre line.

Table 9A: Heights to which the cutting edge should be raised above centre to create clearance angles on the adjacent flank at the tip of a tool, when using different diameters of grinding wheel. A negative clearance angle can be created using the same values, but lowering the tool cutting edge below the horizontal centre line.

Table 10A: Tooth drop height below centre when sharpening a toothed cutter against a flat-faced wheel.

Fig. 75: Flat-faced wheel: the clearance angle can be set by measuring the distance the tip of the wheel drops as it rotates through the clearance angle.

Fig. 77: Curved wheel: a clearance angle can be created by setting a toothed cutter at centre height against a curved wheel, then raising the wheel vertically. If the cutter is inverted, lower the wheel by the same distance.

Table 11A: Wheel height above centre when sharpening a toothed cutter against a curved wheel. If the cutter is inverted, lower the wheel instead.

Index

Abrasive 10

grit 14

grit codes 14

superabrasive 11

Angle 24

digital 24

face 31

gauge 24

Approach angle 74

Axis 22, 45

Back plane 45

Bond 11

Boring head 71, 77

Boring tool 68

eccentric neck 71

heel clearance 68, 71

Catchplate 125, 130

cBN 11

Centre-height gauge 25, 82, 90, 115

Centre-setting gauge 87, 90

Chisel 38

Clearance 26

angle 26, 45

angle indicator 115, 122

continuous 77

flank 63

heel 71

Collar 38

Collet 42, 121

Corner (see nose) 45

Cutter 28

circular 28

Cutting edge 45

angle 45, 57

Major 45

Minor 45

Neutral 57

radial 93

Detent (see Finger) 81, 108 115

block 84

dropped end 81

stop pin 83

swing-clear 83, 119

Dial Test Indicator (DTI) 24, 71, 123, 130

elephant’s foot 24

Diamond

disc 43

paste 43

Die 134

centring rod 135

clearance hole 134

cutting edge 134

entry chamfer 134

holder 135

trailing edge 134

Dividing 81

Dog 125, 130

Drill 94

angle of contact 97

auxiliary cutting edges 99

chisel edge angle 94

collet holder 97

corner 101

dubbing 101

facet 94, 95

flank 94

flute 94

four-facet 95

heel 94

helix 94

jig 96

point angle 94

point half-angle 95

rake angle 94

secondary point angle 101

sharpening machine 94, 98

split-point 99

web thinning 100

Drop 108, 111, 115, 123

Face 45

hollow-ground 41

Finger 26, 81

Flank 45

radial Minor 79

Fly cutter 72

cutting edges 72

sweep 72

Friability 11

Graver 42

Grinder 18

tilting table 32

trunnion table 98

universal 20

workhead 19

work table 19

Gullet 79, 88, 110

Hardness 11

Height gauge 25, 83, 119, 127

Holder 39

Hole saw 88

Hone 43

Indexing 25

Indurate 10

Knife tool 48–57

Land 74, 103, 120, 129

Milling cutter 102

air bearing 105 114

arbor 104, 107, 114

circular margin 103

clearance angle indicator 115

concave 102

continuous clearance 102

convex 102

dovetail 102

end mill 102, 105, 110, 113

end relief angle 110

form 102, 118

gash 110, 112

gear cutter 102, 104

gullet 110

helical cutting edge 105, 114

helix angle 105, 113, 114

land 103

margin 103

relief 118

sharpening machine 113

shell 102

side and face 102, 109

side teeth 109

slab 114

slot drill 105, 110

vee 102

Normal 30

Nose 45, 57, 65, 76

Parting tool 58—63

blade 60

chip clearance groove 62

fixture 60

hollow-ground 60

tapered 60

Pin 135

Plane 22

horizontal 46

reference 22

tool reference 45

vertical 46

working 45

Radius

creating a radius 34

grinding attachment 67

setting micrometer 67

Rake 45

angle 45, 57

‘true’ angle 45

Reamer

back taper 125

bevel lead 120

circular margin 120

clearance 120

cutting edge 120

flute 120

hand 120

hook angle 120, 126

land 120

machine 120, 125

margin 124

shank 120

spinning 125

taper lead 120, 122

Reference 22, 44

axes 46

tool-in-hand 44

workhead 46

Rotate 19

Saw 78

arbor 79, 89

blade width 78

centre gauge 90

fixture 79, 80

hole 88

hollow grinding 78

side rake 79

set 89

setting gauge 80

sharpening machine 88

slitting 78

tooth spacing 90

tooth width 78

Screwcutting tool 63

Screwdriver 39

Setting bar 25, 131

Setting gauge 82

Sleeve 38

square-section 40

Spindle 18

Swing 19

swinging table 20, 32

Tap

bottoming 128

centre 128

cutting edge 128

flute 128

helix angle 129

hook angle 129, 132

intermediate 128

land 129

margin 129

plug 128

relief 129, 130

second 128

shank 128

spiral-flute 129, 133

spiral-point 129, 132

taper 128

Thread

helix angle 63

included angle 64

nose 65

profile gauge 66

top rake 64

Tilt 19

Toolpost 42

Traverse 18

Vee block 42, 43

Vee-point tool 63

Wheel 12

dressing 16, 17

markings 14

mounting 15

shape 12

speed 13

testing 15

wheelhead 21

Workhead 24, 30

adjustable 43

aligning 24

reference position 46

S-T-R 31, 73

T-S-R 31, 73