Tool and Cutter Grinding E-Book

Marcus Bowman

• Herausgeber: The Crowood Press
• Kategorie: Fachliteratur
• Serie: Crowood Metalworking Guides
• Sprache: Englisch

Seitenzahl: 373

Veröffentlichungsjahr: 2021

Beschreibung

The cutting edges on engineering tools must lie at precise angles to ensure effective cutting, and sharpening must recreate the original geometry of each tool. This book provides an understanding of what is involved in sharpening typical lathe, milling, drilling and threading tools. With over 550 photographs and illustrations this new book covers sharpening techniques for the most commonly used engineering tools, screwdrivers and gravers, lathe, milling, reaming, drilling and threading cutters. It identifies the two principal types of workhead, and discusses the ways in which their geometry affects typical sharpening setups. It teaches how to use the three basic movements of swing, tilt and rotate to position a tool against a grinding wheel to ensure correct tool angles and sharp cutting edges. Contains useful tables for setting cutting and clearance angles and provides general advice on tool and cutter grinders, and includes examples of the use of workholders to suit a range of tools. Includes information on abrasive materials and the types and shapes of grinding wheel suitable for use on a tool and cutter grinder. Finally, it shows photos of accessories that can be made to simplify setups, including workheads, toolholders and fixtures used to hold circular saws, parting tools and dies, as well as an angle gauge to quickly set clearance angles on reamers and milling cutters.

Leseprobe

CROWOOD METALWORKING GUIDES

TOOL AND CUTTERGRINDING

DR MARCUS BOWMAN

Quorn tool and cutter grinder

CROWOOD METALWORKING GUIDES

TOOL AND CUTTERGRINDING

DR MARCUS BOWMAN

First published in 2021 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

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 DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 861 0

DisclaimerSafety 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

Contents

Acknowledgements

Introduction

1 Sharpening Media

2 Tool and Cutter Grinders

3 Basic Movements

4 Sharpening in Practice

Tool type 1: Simple flat faces

Tool type 2: Lathe tools

Tool type 3: Fly cutters

Tool type 4: Saw blades

Tool type 5: Faceted drills

Tool type 6: Milling cutters

Tool type 7: Reamers

Tool type 8: Taps and dies

Further Information

Index

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.

or

For example:

Wheel diameter 200mm

Rotational speed 1,420rpm

Wheel diameter 6in

Rotational speed 2,840rpm

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.

where T is the Traversing time in seconds

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