Sharpening Common Workshop Tools E-Book

Marcus Bowman

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2020

Beschreibung

Using sharp tools is one of life's great joys. A sharp tool feels right; the job seems that much easier; and the quality of the cut usually means there is less work to do to produce a finished surface on the object. This book provides an understanding of what is involved in sharpening common workshop tools. With over 580 colour photographs and illustrations it covers sharpening techniques for the most commonly used tools - scissors, shears, knives, lathe tools and drills, screwdrivers, chisels and punches, along with the more specialist gravers and scrapers. Advice on types of abrasives, grades, grits and mesh sizes is given. It describes what happens during the sharpening process and provides practical guidance on using sharpening stones and grinding wheels on a bench grinder. Provides essential maintenance instructions such as how to restore the surface of a flat stone and the face of the wheel on a bench grinder. Finally, it gives advice on bench grinders, and includes examples of the use of readily available tools and accessories such as vee blocks, toolposts, collet holders, protractors and gauges.

Leseprobe

CROWOOD METALWORKING GUIDES

SHARPENING

COMMON WORKSHOP TOOLS

DR MARCUS BOWMAN

CROWOOD METALWORKING GUIDES

SHARPENING

COMMON WORKSHOP TOOLS

DR MARCUS BOWMAN

First published in 2020 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2020

© The Crowood Press 2020

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 668 5

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.

Contents

Acknowledgements

Introduction

1 Sharpening Media

2 Basic Movements

3 Bench Grinders

4 Sharpening in Practice

Tool type 1: Scissors, shears and knives

Tool type 2: Screwdrivers

Tool type 3: Chisels

Tool type 4: Gravers

Tool type 5: Punches

Tool type 6: Scrapers

Tool type 7: Turning tools

Tool type 8: Drills

Further Information

Index

A DMT Diamond Wave Sharpener featuring convex and concave abrasive surfaces.Photo: DMT.

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 EuroTrade Ltd

John Baguley

R. D. Barrett Small Tools Ltd

David Beattie

Bowland Trading Ltd

Celtic INC

Chronos Ltd

DMT

Eccentric Engineering

W. E. Falck

John Florence

Glendo LLC

Glenn Evans of Glennview.com

Gränsfors BRUK AB

Haskpro LLC

Nigel Hill

Alan Hooper

David Haythornthwaite

Horotec SA

Jack Sealey Ltd

Leon Kirk

Lansky Sharpeners

Lee Valley Tools Ltd

Donald Mitchell

John Moran at gadgetbuilder.com

Morse Flight Systems

Howard at MYFORD-LATHES.COM

Spyderco LLC

Steve Lindsay Engraving and Tools

T D Fitchett 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

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 which 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 support made the writing task so much easier.

Every effort has been made by the author and publisher 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

The underlying purpose of this book is to give you an understanding of what is involved in sharpening common workshop tools, from scissors, shears and knives, to lathe tools and drills. The range includes screwdrivers, chisels, and punches, along with the more specialist gravers and scrapers. The aim is to equip you with the knowledge of what happens during the sharpening process, and provide practical examples of how to sharpen a range of common tools by using sharpening stones and grinding wheels on a bench grinder.

This is not a book about tool and cutter grinding; and it does not deal with sharpening milling cutters, reamers, taps or dies. Nor is it a book about designing and making your own cutting tools.

Using sharp tools is one of life’s great joys. A sharp tool feels right; the job seems that much easier; and the quality of the cut usually means there is less work to do to produce a finished surface on the object. This book is all about sharpening tools; giving them precise cutting edges; and ensuring the tool angles are suited to the way the tool is to be used and the materials to be cut.

The means of sharpening range from using an abrasive stone to sharpen a knife or restore the shape of a screwdriver blade, to using a bench grinder to give more life to a chisel, or put a keen edge on a range of lathe tools.

The tools range from the everyday knives and scissors to punches, clockmakers’ gravers and lathe tools, with a substantial section on devices for sharpening drills.

Abrasives are essential, and there is information on types of abrasives, grades, grits and mesh sizes, as well as forms of abrasives like stones, wheels and papers.

The equipment is centred around abrasive stones used on the bench, and bench grinders fitted with a range of abrasive wheels, as these are widely available and easy to use. However; buying a stone or a bench grinder is one thing, but looking after abrasive surfaces is another, and many a shed or workshop has a flat stone and a bench grinder which seem ineffective because their abrasive surface has been clogged or damaged and is no longer capable of creating a keen edge on any tool. Restoring abrasive surfaces is a common requirement, so there is information on how to maintain the surface of a flat stone and the face of the wheel on a bench grinder.

Fig. 1: In this book, sharpening is based around a bench grinder, bench stones, and accessories such as templates and setting gauges. This grinder has a replacement wheel on the right, together with a home-made wheel guard.

Much of the ancillary equipment shown in this book is commercially available, but, to get the best out of any flat abrasive stone or a bench grinder, it helps if you have good metalworking skills and are able to make some simple accessories. These might include simple devices to hold a tool against a flat stone securely and at an accurate angle, or alternative rests (or at least larger platforms for rests), sliding plates and adaptors, and, optionally, some small devices for holding work so that it can be positioned accurately and safely as it is presented to a grinding wheel. There are lots of photos of self-made equipment, but because these are relatively simple items, there are no dimensioned drawings. Instead, the photos give ideas and inspiration, allowing you to make accessories to suit your own grinder and your own ways of working.

There is general advice on bench grinders, and there are examples of the use of readily available tools such as vee blocks, toolposts, collet holders, protractors and gauges.

There is a little maths and a few handy formulae, but there are also tables which can be used instead.

The specific examples included in the book are designed to demonstrate a range of techniques which allow a variety of surfaces and edges to be restored using appropriate forms of abrasive. Although it is easy to dip into the contents to find techniques suited to particular tools, reading through all the examples from beginning to end will provide an ordered sequence designed to help demonstrate a full range of methods; and although you may not envisage sharpening every type of tool, it is worth reading all the examples, because there are usually other applications for the techniques you will learn.

Approached in the right frame of mind, sharpening is a satisfying task, and its end result is to produce a tool well suited for use in the workshop. If taken at an unhurried pace, it is an almost Zen-like way in which to occupy time as you restore the capabilities of the equipment in the workshop, knowing that the tools will be ready for whatever challenge they face. Their owner, too, will be prepared for the next job.

STAY SHARP; STAY SAFE

The workshop is a place of industry but it should also be a place of tranquillity, focus, and the pleasure which 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 which slips, a saw blade which wanders, or a machine with unguarded cutting edges which 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 for a blunt blade 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 which 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 materials 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 practice 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 set ups for sharpening tools, many of the photographs show grinders which 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

Any part of a tool may be shaped by rubbing it with an abrasive material. The abrasive rubs and removes material, changing the shape of the tool. In particular, a cutting edge may be rubbed with an abrasive so that some of the material is removed, leaving the edge sharp. Often, the area immediately behind or adjacent to the cutting edge is rubbed into a particular shape with the abrasive, to give the edge particular properties such as a long shape tapering to a thin cutting edge (like a razor blade or a scalpel) which can penetrate material deeply using little force, or a short steep slope to give the edge sufficient strength (Fig. 2) to withstand heavy pressure (like a chisel).

Abrasives were originally naturally occurring materials such as emery, Arkansas stone, corundum, cubic boron nitride (cBN) and diamond, used in their natural state, or refined to give them a relatively uniform structure. Modern abrasives are often manmade synthetic materials which benefit from a consistent particle size and a precise composition designed to remove material at a particular rate or in a particular way. Synthetic materials complement rather than replace the natural abrasives, providing a wider range of choice of materials. Table 1, in the Further Information section at the end of this book, lists commonly available abrasive materials, their relative hardness, and some applications.

Fig. 2: Cutting edges may have different thicknesses and angles, to suit their purpose and the materials they will cut.

An abrasive material contains particles which act like small cutting tools and can remove some of the material being rubbed by the abrasive. As the particles rub and cut, they become blunt, so an important part of the action of an abrasive is to allow the blunted particles to be removed. In some materials this happens naturally as the abrasive rubs away. When sharpening a knife blade on a soft stone, for example, the stone may appear to wear and be reduced to dust as it rubs the knife blade. That dust contains particles which have rubbed the blade and been blunted in the process, and the bond between those particles and the others remaining in the stone is sufficiently weak that the blunted particles separate from the stone. This reveals new, sharp, particles which can then act on the knife blade. In this way, although the stone wears away, it provides a continuously refreshed face to rub the blade, removing material from the blade, changing its shape and sharpening it in the process. Some stones do not degrade as quickly, and retain their particles for longer. This can result in blunt particles rubbing on the work, and the steel of a knife blade will tend to smear on the stone, clogging it and tending to prevent the release of blunt particles. That, in turn, leads to a less efficient cutting action. So there is a balance between the rate at which the stone releases blunt particles, the speed of the cutting action, and the rate at which the stone wears.

Synthetic abrasives work in the same way as naturally-occurring abrasive materials, but the shape and size of the grains can be accurately controlled, making the cutting action more predictable and consistent. The abrasive particles are mixed with a bonding material, and the mixture is moulded to a useful shape such as a wheel or a flat stone.

The choice of bonding material affects the way in which blunt particles are released from the material, and the strength of the bond can be arranged to suit the requirements of the material being abraded, as well as controlling the economics of the operation. However, just as with the natural stone, a lower release rate may result in contamination of the grinding medium by the material being sharpened because blunt particles are not being released sufficiently quickly. This can lead to clogging of the abrasive material, reducing the effectiveness and the efficiency of the sharpening process.

Abrasives may be classified as ‘soft’ or ‘hard’. Soft abrasives release abrasive particles before they are fully blunt, while hard abrasives hold onto their particles even after they have been made blunt. The most desirable condition would be somewhere in between, with the abrasive neither too soft nor too hard, but just right compared with the material being sharpened. In more recent times, abrasives based on diamonds or cBN have been termed 'superabrasives', because they are effective against hard materials, and their particles stay sharp for a relatively long time.

FRIABILITY

A material which 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.

The degree of friability in an abrasive is an important factor in its ability to remove material from a workpiece.

BONDED ABRASIVES

Abrasive particles can be mixed with a bonding agent, then moulded and/or pressed into shape. This process is used to form flat surfaces (Fig. 3), cylindrical or conical shapes such as wheels (Fig. 4), or other shapes of abrasive (e.g. triangular, tapered or curved) for specific purposes (Figs 5 and 6). Commonly-used bonding materials are listed in Table 2.

Fig. 3: A range of flat abrasive surfaces.

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

Fig. 5: Abrasives may be moulded in various shapes.

Fig. 6: An Arkansas stone of triangular cross-section.

Grinding wheel bond types

Bond type

Description

Bond reference letter

Resinoid

Originally obtained from trees, most resins are now synthetic and reinforced by other materials. Resinoid wheels are often suitable for heavy grinding.

B

Shellac

A secretion of the female lac insect. Although the bond is of low strength, shellac wheels can providuce a good final finish on work.

E

Metal

Powdered metal and diamond or cBN particles are sintered (pressed together under high pressure and heat) to form a metal-bonded abrasive for wet grinding, ceramics and glass-based materials.

M

Oxychloride

Magnesia oxychloride cement, made from magnesium oxide and magnesium chloride, produces a relatively soft bonding compound. Oxychloride wheels can create very sharp edges on cutting tools.

O

Rubber

Natural and artificial rubbers create an elastic wheel often used for centreless grinding. Rubber-bonded wheels can provide a very fine finish.

R

Silicate

Sand and sodium carbonate (soda ash) are combined at high temperature to provide a bonding material. A wheel made with silicate bonds generates relatively little heat during grinding.

S

Vitrified

The most widely used bonding agent, made from types of clay. Wheels with a vitrified bond are rigid and strong, and are capable of both high rates of material removal and of precision grinding.

V

Table 2: Common types of wheel bond.

COATED ABRASIVES

A flat surface can be coated with a bonding agent and abrasive particles. The surface may be flexible or rigid, and typical types of coated abrasives include emery paper (Fig. 7), soft-cored abrasive pads, abrasive belts, tape and string (Fig. 8).

Fig. 7: Emery paper made using corundum powder.

Fig. 8: Abrasive belt, tape (abrasive strip), soft-cored pad, and abrasive-coated string (cord).

For some materials such as diamond, the coating process may be technically complex and include chemical vapour deposition or plating (often involving depositing nickel to hold the diamond particles in place). Coatings are normally applied to metal plates which are either stand-alone or may be embedded in plastic or resin to form a composite plate. The metal plate in a composite plate may be perforated (Fig. 3), and the holes allow the diamond slurry developed during a sharpening session to become trapped but remain active there, or, where the resin substrate is at the same level as the abrasive surface, the particles may become embedded in the resin, enhancing the abrasive action of the plate.

ABRASIVES IN OTHER FORMS

Abrasives are available in paste or liquid form, including products such as grinding paste (often used to grind valves and valve seats), rubbing compounds (often used to burnish paint), polishing pastes, and polishing liquids (Fig. 9).

Fig. 9: Abrasive pastes and liquids.

Fig. 10: Abrasive bars consisting of paste and abrasives, for applying to a hone.

Fig. 11: Impregnated cloth and wadding, and soft impregnated block.

Another approach is to impregnate a soft material such as leather or canvas with abrasive particles, to make a hone or a strop. The abrasive particles may be applied by spraying, but they are often held in suspension in a paste, frequently moulded into the shape of a bar, rather like a bar of soap (Fig. 10), and the bar is rubbed against the hone to transfer abrasive particles. The surrounding paste in the bar is also transferred, and acts as a lubricant. The colour of the bar denotes the grade or grit size of the abrasive particles. Pre-impregnated cloths, wadding, and soft impregnated blocks are also available (Fig. 11) and these are useful for surfaces which feature corners and curves.

GRINDING STONES AND PLATES

A rectangular abrasive block with flat faces (Fig. 3) can be used to sharpen a wide variety of tools. These abrasive blocks are often called stones, whether they are made of naturally occurring or synthetic materials. Flat-faced stones (rectangular blocks) are normally used manually. The stone is most commonly laid on a bench and the tool is held by hand or in a simple guide, and moved over the abrasive surface manually (Fig. 12). In some cases, the size or shape of the tool makes it easier to hold the stone and move it across the edge being sharpened. Sheet metal shears, for example, are too awkward to sharpen if the stone is on a bench, but can easily be sharpened by holding the stone in one hand and rubbing it across the cutting edge of the shears (Fig. 13). Stones are commonly available in a range of sizes and grits, and man-made stones may have a uniform grit throughout, or may have different grit sizes on two sides, for convenience, like the combination carborundum stones shown in Figs 14 and 15 which have a coarse grit on one side and a finer grit on the other.

Fig. 12: A tool can be shaped or sharpened by moving it across the surface of an abrasive stone or plate, while pressing fírmly.

Fig. 13: A tool can also be sharpened by moving a stone across the cutting edge of a stationary tool.

Fig. 14: Combination stones have a different grit size on each major face, giving both coarse and fine faces on a single stone.

Fig. 15: A combination stone has two stones of different grades stuck together.

Fig. 16: Shaped stones are useful for sharpening shaped tools.

Fig. 17: A triangular stone is useful for sharpening internal corners with the same angle as the corners of the stone, or larger.

Fig. 18: A Diamond Honing Cone with a convex curve. The diameter changes along the cone.

Photo: DMT.

Fig. 19: A DMT Diamond Wave Sharpener with both convex and concave abrasive curved surfaces.

Photo: DMT.

Fig. 20: A circular abrasive stone with flat upper and lower faces. Photo: GRÄNSFORS BRUK, SWEDEN.

Non-flat shapes of abrasive stones have their uses too, and stones required to sharpen tools which have flat, externally and/or internally curved faces may have a combination of flat and rounded faces (Figs 5, 6 and 16–20). The flat faces of stones with a triangular cross-section may be used in internal corners such as on the vee tool shown in Fig. 18. The flat upper and lower faces of a circular abrasive stone (Fig. 19) provide equal access from all angles.

GRINDING WHEELS

Grinding wheels have a central hole and are designed to be mounted on a rotating spindle. One convenient shape of grinding stone is the cylindrical flat-sided stone found on most basic bench grinders (Fig. 21). This is a useful shape because if the wheel is rotated at a uniform rate (typically 1400 or 2800 revolutions per minute), all points on the curved face of the periphery 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. That helps provide a uniform rate of grinding. Although the circular flat-sided stone is useful, that basic shape does not suit all tools, so wheels suitable for tool and cutter sharpening are available in a range of shapes depending on the intended purpose of the wheel. As with abrasive stones, the shape of the wheel makes some tasks easier or more difficult than others. A convex surface (curving outwards) allows the creation of narrow angled faces by raising a horizontal tool above the centre height of the wheel (Fig. 22), while a tapered wheel can reach more easily into a flute (Fig. 23).

Fig. 21: Bench or offhand grinders are normally fitted with cylindrical flat-sided abrasive wheels.

Fig. 22: Raising a horizontal tool above wheel centre height creates an angled face on a tool. The greater the height, the greater the angle A degrees.

Fig. 23: A tapered wheel has clearance at the back, and can reach into a confined area such as a flute.

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. 24 and Table 3 contain details of typical wheel shapes for tool and cutter grinders.

Fig. 24: 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

7

Wheel recessed on both sides

3

Wheel tapered on one side

12

Dish wheel

11

Taper cup wheel

6

Straight cup wheel

Table 3: Grinding wheel shapes in common use.Reference numbers refer to Fig. 24.

Fig. 25: On a rotating wheel, the linear speed of a particle depends on its radius.

The linear speed of points on the side of the wheel depends on their radial distance from the centre of the wheel (Fig. 25). 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 x π x R x RPM metres per minute, and all points on the blue circle travel at 2 x π x r x RPM metres per minute. Because r is smaller than R, points on the blue circle travel more slowly than points on the red circle.

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 RPM) of the wheel (Fig. 26).

Fig. 26: By law, all grinding wheels must carry a label showing the 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.

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. 27). 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. 28). For wheels of this type the grinding face is usually relatively narrow (6 to 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.

Fig. 27: The effective diameter of a particle depends on its distance from the centre of the wheel.

Fig. 28: On this type 6 cup wheel, the chisel experiences different cutting speeds at different points.

rotational speed (RPM) x π x

wheel diameter(mm) / 1000

surface speed (m/min) x 1000 / (π x wheel diameter (mm))

or

rotational speed (RPM) x π x wheel

diameter(inches) / 12

For example:

Wheel diameter 200mm

Rotational speed 1420RPM

Wheel diameter 6 inches

Rotational speed 2840 RPM

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.

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 nonmetallic material (e.g. stone).

cBN (Borazon)

Hard steels.

Table 4: Abrasives used to grind common materials.

Table 4 lists abrasives and the workpiece materials for which they are best suited.

Grinding wheel shapes 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 coded by manufacturers using their own proprietary codes.

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. 29: Interpreting British Standard wheel markings.

Fig. 29 shows how to interpret a “specification mark” according to BS ISO 525:2013. Note that the meanings of Optional codes (termed 'specification marks') 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.

Mounting a wheel

To mount a grinding wheel on an arbor (shaft) 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. 30). Sleeves are often made of plastic and are readily obtained at very low cost. In many cases, the reducing bushes 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 Table 5 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 which 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 (Fig. 31). Loose flanges have the advantage that the small amount of friction drive may allow the wheel to slip more readily if it jams, but does limit the torque which can be applied to the wheel and the work. Blotters must be fitted between wheel and flange, on each side of the wheel (Fig. 31). 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. 30: Wheel centre hole sizes may be reduced by using adaptor bushes.

Fig. 31: Wheels are mounted between paper blotters and steel flanges.

Table 5: 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.

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 ready for cutting work. This is particularly important if the wheel has become clogged with soft material (Fig. 32).

Fig. 32: A grinding wheel clogged by grinding soft mild steel, resulting in particles of steel sticking to the surface, reducing the effectiveness of the wheel.

Although the grit sizes in a wheel are determined during manufacture, different dressing techniques may result in a relatively fast-cutting wheel which does not heat the work much, or a wheel which produces a much finer finish but more heat in the tool.

Using a star wheel dresser

A tool with a group of hardened disc wheels and star-shaped wheels (Fig. 33) can be manually pressed against the surface of a wheel and will knock grains from the wheel. This is a messy operation compared to other methods, but it is quick and leaves a relatively rough finish to the wheel surface. Typically, star wheel dressers are used on bench grinders not used for fine tool grinding.

Fig. 33: A star wheel dresser contains hardened disc wheels and star-shaped wheels.

Most star dressers have two protruding legs which can be placed on the top surface of the tool rest (Fig. 34), or hooked over the front edge of the grinding rest and used to guide the dresser as it is traversed across the surface of the wheel (Fig. 35). Set the dresser and rest so that the star wheel is close to, but not touching, the grinding wheel. Tilt the dresser by lifting the handle, to make contact. Then traverse the dresser across the surface of the wheel.

Fig. 34: The legs of the dresser can be used to steady the dresser on the rest, before sliding it into contact with the wheel.

Fig. 35: The star wheel dresser can be held on the rest with its legs hooked over the front edge of the rest, and tilted against the face of the wheel while traversing from one side to the other.

Fig. 36: Hooking the dresser over a bar clamped to the rest allows smoother control of the depth of cut by tilting the dresser.

Some people recommend that this kind of dressing should take place after the grinder has been switched off and the wheel is coasting to a stop, but small grinders tend to have less momentum and the wheel is comparatively easily stopped, so that the grinder has to be repeatedly started and stopped. As a result, dressers are most commonly used while the grinder is under power. The star wheel dresser can be used to take a heavier cut than some other methods of dressing, although deep ridges or scores already in the wheel need to be dressed out in several stages. Ideally, the tool rest should be broad enough (side-to-side) that the dresser can sit securely clear of the wheel on either side, to give a secure start and finish during the whole of the traversing movement. A round bar can be clamped across the rest, to locate the legs of the dresser (Fig. 36), which is useful if the tool rest does not have a straight, uninterrupted, front edge. A round bar allows a smoother action and finer control of depth of cut by tilting the dresser, raising the rear handle to pivot the head downwards, to increase the cut. The bar shown in Fig. 36 is mounted clear of the rest, to allow room for the dresser feet to swing downwards to bring the star wheel into contact with the grinding wheel. The face of the wheel will be dressed parallel to the bar.

Using a dressing stick

A dressing stick is a rectangular stick of abrasive material which is pressed against the surface of the wheel in much the same way as a star wheel dresser (Fig. 37). 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.

Fig. 37: A dressing stick is a rectangular stick of abrasive material capable of removing grains from the surface of the wheel.

Using a 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 Fig. 38.

Fig. 38: A single-point dresser consists of a diamond mounted in a steel bar.

Multi-point diamond dressers contain many diamonds embedded in the end face of a short length of bar (Fig. 39). The multipoint dresser is used in much the same way as a star wheel or stick dresser, traversing the dresser across the face of the wheel.

Fig. 39: A multi-point dresser contains many diamonds mounted in the end face of a small block.

Fig. 40: A single-point dresser in an adjustable holder which allows the depth of cut to be varied.

Fig. 41: 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 (mm)

Distance below centre height (mm)

100

9 - 13

125

11 - 16

150

13 - 19

180

16 - 23

200

17 - 26

Table 6: Setting the dresser horizontal and at the given distance below centre produces an effective angle of 10 to 15 degrees.

The single-point dresser is traversed across the face of the wheel, but needs to be held so that the diamond trails the grains of the 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 which sets the angle (Fig. 40). On a bench grinder, this angle can be achieved by setting the rest below centre height (Fig. 41) at a height listed in Table 6.

Recommended depths and feeds for dressing

When using a dresser which has a single diamond or a cluster of same-size diamonds, 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 do the job perfectly well on wheels of diameter 150 or 200mm (6 or 8 inches).

The effect produced by diamond dressers, and to some extent by dressing sticks, depends on the depth of feed and the speed at which the dresser is traversed across the face of the wheel. Fine infeeds towards the wheel of 0.01 (fine grit) to 0.025mm (coarse grit) per pass should be enough, and that can be controlled by clamping the holder onto the table or rest then applying feed using the table feedscrew or by using a screw feed within the holder itself. The recommended depth of cut when dressing a wheel is the same for all types of dresser.

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 which 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. For most applications, let the grade of the wheel determine the roughness, and use a medium rate of traverse, as suggested in Table 7, which gives an indication of the time to traverse across the faces of wheels of different thicknesses, on grinders with different spindle speeds. 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.

Table 7: Traversing time across a selection of wheel widths, at a medium rate of traverse.

Flattening stones

Natural and synthetic stones wear as tools are rubbed along their surface (Fig. 42), in much the same way that grinding wheels wear. It is important that stones have uniformly flat surfaces, otherwise it becomes difficult to maintain a consistent angle between tool and stone. Just as a wheel is dressed to reform its working surface, so a stone should be flattened once it has worn. Flattening a stone also reveals fresh grains, restoring the cutting action of the stone. Two stones can be flattened by rubbing them together using a lubricant such as water or oil (Fig. 43), and a circular or figure-of-eight pattern is more effective than a simple back-and-forward motion. The high spots on each stone will be in contact and will wear, bringing lower spots into contact, until the whole of each surface is in contact, eventually. This process is most effective if the two stones are of the same material, so that they wear at the same rate, or if one stone is very flat and has a much harder more coarse-grained abrasive (Table 8). Two India stones of the same grade in contact with one another will flatten each other at a uniform rate, for example. Using two similar stones with different grades, such as waterstones of 1000 and 6000 grits will result in more rapid wear of the finer stone (6000 grit) but will eventually produce two flat stones. To save premature wear on the finer stones, a stone may be flattened by rubbing against an abrasive paper with a coating which is harder than the stone, such as aluminium oxide or silicon carbide 'wet or dry' paper. Place the paper on a flat surface, and rub the stone, working face down, against the paper (Fig. 44). Use a paper with a coarser grit than the stone, for faster results, and to expose fresh grains in the stone rather than simply blunting those grains and slowing the flattening process. At the other extreme, a DMT Dia-Flat lapping plate, which is very flat and covered in hard diamond grains, will flatten any other stone, grinding it to conform to the flat shape of the Dia-Flat plate (Fig. 45). Diamond is sufficiently hard to be able to cut any other abrasive material.

Fig. 42: A soft abrasive stone, worn concave by years of use sharpening pen knives.