Using the Engineer's Lathe in Clockmaking E-Book

Laurie Penman

• Herausgeber: The Crowood Press
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 212

Veröffentlichungsjahr: 2022

Beschreibung

This book is a practical guide to selecting, setting up and using an engineer's lathe to create parts specifically for clockmaking or clock repairs. It begins as an instructional manual and consequently the first chapter assumes that the reader does not yet have a lathe and needs advice on choosing one. With over 160 diagrams and photographs, topics include: choosing and setting up the lathe; turning - materials and facing; boring and milling techniques; chucks, collets and face-plates; creating a height-centring gauge; pivots, wheels and pinions; removing and mounting gear wheels; fly cutters; making a flat depthing tool and, finally, creating centre marking tools. The intent is to provide a gentle learning curve for the practical use of the lathe.

Leseprobe

First published in 2022 by NAG Press, an imprint of The Crowood Press Ltd, Ramsbury, Marlborough Wiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2022

© Laurie Penman 2022

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 0 7198 3152 2

Cover design by Maggie Mellett

Contents

Acknowledgements

Chapter 1 Introduction

Chapter 2 Turning

Chapter 3 Facing, Boring and Drilling

Chapter 4 Screw Threads

Chapter 5 Additional Machining Functions

Chapter 6 Chucks and Collets

Chapter 7 Height-Centring Gauge

Chapter 8 Pivots

Chapter 9 Removing and Mounting Gear Wheels

Chapter 10 Fly Cutters

Chapter 11 Flat Depthing Tool

Chapter 12 Centre-Marking Tools

Epilogue

Glossary

Index

Acknowledgements

Fig. 1.01 by kind permission of Myford Ltd (www.myford.co.uk)

Figs 1.02 and 2.12 by kind permission of Chester Machine Tools (www.chesterhobbystore.com)

Fig. 1.03 by kind permission of Sherline Products (www.sherline.com)

Figs. 2.20; 3.12 and 4.04 by kind permission of Machine Mart Ltd (www.machinemart.co.uk)

Figs 4.12a, 4.12b, 5.06, 6.15 and 10.01 by kind permission of RDG Tools Ltd (www.machinemart.co.uk)

Chapter 1

Introduction

This book begins as an instructional manual, and consequently the first chapter assumes that the reader does not have a lathe at the moment and needs advice on choosing one. From there the chapters deal with preparing a cutting tool; the possible techniques; and several machining projects that are needed when repairing clock movements or that will prove useful clockmaking (or model-making) tools for your workshop. The intent is to provide a gentle learning curve for the practical use of the lathe.

CHOOSING A LATHE

Enjoy getting to know the engineer’s lathe, as it is almost a universal tool. There are very few machining operations that it is not capable of – it is even possible, with a little cunning, to use it to make a larger lathe.

The small engineer’s or centre lathes that are used for clockmaking and repair perform very differently from the much larger lathes used in general engineering, although their functions are much the same; however, general engineering makes far greater demands upon the machine.

Fig. 1.01 A Myford lathe.

Lathes such as the Myford (Fig. 1.01) have all the characteristics of a larger lathe: lateral adjustment on the tailstock, top slide, complete apron with traversing handle and screw-cutting controls and back gearing; while mini-lathes usually have a swivelling headstock for taper turning, traversing at the end of the bed, no top slide and a fixed centre tailstock. Other differences result from the type and size of the materials that are machined.

Parts for clocks do not generally demand cutting tools with a wide variety of rakes and clearances for working within the limits of the tool material. It is quite often the case that old-fashioned carbon steel tools are perfectly adequate for the job in hand. So long as the speed is kept down, a carbon steel tool is actually harder than most high-speed steels (HSS).

Fig. 1.02 A Chinese 7 × 12 small lathe.

Fig. 1.03 A Sherline mini-lathe with a swivelling headstock.

For convenience (and to avoid undue advertising), I will refer to ‘small lathes’ and ‘mini-lathes’, the first being simply a scaled-down engineering machine such as the Myford or the Chinese 7 × 12 (Fig. 1.02), and the second one with a swivelling headstock like the Sherline (Fig. 1.03).

Fig. 1.04 The parts of a lathe.

So far as the basic clockmaker’s or watchmaker’s lathe is concerned, the only differences in operation fall under the heading of ‘graving’, that is, the use of a hand-held tool called a graver. Very often a top slide is added to the watchmaker’s lathe and then it is really a tiny engineer’s lathe. Some reference will be made to this, but I cannot think of any operation that cannot be performed at least as well on the normal centre lathe (which is the general term for the type of lathe discussed in this book). Precision turning can be carried out with a graver, but it has limited application to modern clockmaking. It must be almost impossible to use a graver to remove an amount smaller than 0.025mm from a turned diameter and leave a high finish, or to modify a taper by a similar amount. Clockmaking uses files and burnishers for these tasks and they can be used just as easily on the centre lathe as on the traditional clockmakers’ machine. The parts of the lathe are shown in Fig. 1.04.

Fig. 1.05 The two types of milling machine: rising head, fixed knee (left) and fixed head, rising knee (right).

Larger work, such as turning a bar larger than 25mm (1in) and longer than 300mm (12in); using a collet on a rod of 12mm (0.5in) diameter that passes right through the collet; the making of clock gears and tools or carrying out shaping and milling operations will all call for the larger machines. Gear-cutting requirements can be met on the Myford with attachments (either purchased, or made on the lathe), for gears up to about 200mm (8in) diameter. Many mini-lathes are sold with the choice of milling attachments and these will enable the cutting of gears up to about 75mm (3in) diameter, but the attachments are not as rigid as a purpose-made milling machine or wheel-cutting engine and will usually only have a small range of tooth counts available for the dividing device.

Vertical milling machines of solid construction, with a rising head rather than a rising ‘knee’ are available and not much more expensive than the mini-lathes. Wheel-cutting engines are even more useful for gear cutting and range in price from less than a vertical miller to just over. However, they will not carry out other milling operations. Milling machines with rising knees lift the work up to a rotating cutter; the rising head version has a static support for the work (with two slides) and the cutter is brought down to it. The support in small machines is cast as one with the frame of the machine and is much sturdier than the rising knee milling machine.

A professional clockmaking shop really needs all three of the machines mentioned (small lathe, mini-lathe, vertical miller or wheel-cutting engine) but the beginner will find that the mini is quite sufficient to begin with, progressing to larger machines if and when the work demands it.

Lathe Qualities

The major requirements of a good lathe are listed below:

Sufficient power A useful rule of thumb for the light lathes used by clockmakers is 50 watts per inch of ‘swing’ (ignore any gap), for speeds up to 2,000rpm. ‘Swing’ refers to the maximum diameter that may be accepted over the lathe bed. An 18cm (7in) lathe should have at least 350 watts (just under 0.5hp) and a 7.5cm (3in) lathe 100 watts. There are two electric motor ‘ratings’ – intermittent and continuous. Intermittent rating will require the motor to be switched off at frequent intervals so that it can cool off; continuous rating is self-explanatory: there is no need to switch off the motor.

A stiff bed Every part of a lathe is related to the bed, and if it shows any tendency to distort under working conditions, the relative positions of headstock, tailstock and tool will change. This can result in ‘chatter’ or dimensional and geometric inaccuracy in the work piece. There is no way that you can test this but, as a rough rule, the width of the bed (the sliding surfaces) should be much the same as the height of the spindle over the bed. The higher the spindle (lathe arbor) in relation to the bed, the less stiff the machine becomes.

Accuracy The importance of this very much depends on the type of work that the machine is required to do. If the lathe is to be used for boring out hydraulic cylinders, it would be expected to maintain a parallel cut over a longer distance than what is needed for clockmaking. In standard machine shop practice, the taper allowance on a 300mm-long turned bar is 0.025mm on the diameter and a concavity of 0.025mm on a 300mm diameter face. Practical requirements for clockmaking would be about 0.001in per inch for turning and 0.001in concavity on a 1in-diameter face (0.025mm per 2.5cm). This is from a new lathe, but experience will enable a turner to produce accurate work on a robust machine with much looser tolerances than this. Accuracy is very much a matter of how the work is tackled, but an accurate lathe (in the terms set out above for clock repairing) is easier to ‘set up’ than an inaccurate one.

Range of speeds Speed changing can be managed either by means of pulleys, or gears from a constant-speed motor, or by using a variable-speed motor. A geared, or pulley-driven headstock, has the advantage of maintaining its speed well regardless of the size of cut (as long as the motor is powerful enough), but it has to work to a limited range of speeds. A variable-speed motor has an infinitely adjustable speed between a stated upper and lower limit, but the speed can alter as the amount of cut varies, or the state of the material being worked alters. An additional advantage not usually stated is that variable-speed motors can often be made to rotate at very low speeds indeed when there is no load on them. The ability to turn the chuck at 5 or 10rpm when setting work up is very convenient.

Fig. 1.06 The scales are engraved with divisions of 0.025mm or 0.001in. The rotating scale is attached to the traversing screw and shows how far the tool has been moved.

Convenient hand-wheels The hand-wheels for moving the slides should be large enough to get one’s fingers on and operate smoothly. An otherwise good lathe can be spoilt by this lack, and it would be a very good idea to increase the size of hand-wheel by making another that attaches to it or replaces it. Much the same applies to the tailstock handle. This is only an issue with mini-lathes in the main. Scaled dials should always be a part of the handles, marked clearly in either 0.025mm or 0.001in divisions. Some machines provide an adjustable scale ring that can be moved to zero and then locked in position, but on a small or mini-lathe the attachment tends to be a little unreliable.

Headstock rigidity It should be possible to turn a steel rod that protrudes from the chuck four times as long as the diameter of the piece without the work riding up and over the tool, or chattering. (The work piece must be stout enough not to bend and chatter on its own account, which is why its length is quoted its length in terms of its diameter.) This is quite a normal requirement in model- and clockmaking.

Sturdy head bearings The arbor may be supported by a variety of bearings – ball, roller and solid. I prefer the last two because they tend to last longer in service. For non-industrial turning (clock repairing, for instance) solid bearings have the great advantage that when, after a decade or two, they are too worn for good work they can be faked up for one last job – turning a new set of head bearings for the same lathe. The sturdiness of the headstock may be tested by mounting the largest diameter of steel or brass bar that will pass through the mandrel (spindle) and gripping it with the chuck before straining it manually up and down – in other words, trying to wiggle it.

Economic Considerations

The choice between a small lathe and a mini will depend on the cash available and the type of work that it is proposed to tackle. Myford lathes are no longer being made but a refurbished and guaranteed model with motor, three-jaw chuck, tailstock chuck, hard centre and a couple of high-speed tool pieces (the minimum ‘ready to use’ specification), would be around £1,500. A mini-lathe with the same accessories costs between £300 and £450 (at 2018 prices) but is much more limited in what it can do. Second-hand small lathes are more likely to be in good condition than second-hand minis, because they are much more robust machines.

If the work is to be limited to repair techniques – polishing, pivoting, bushing and so on – then a good mini-lathe will be quite sufficient.

Pros and Cons of Typical Lathes

Sherline and Similar 90mm (3.5in) Swing Lathes

Pros: High speed; sturdy; suitable for machining bar stock up to about 12mm (0.5in) diameter or short pieces and light turning of possibly 25mm (1in) and a disk held on a mandrel up to about 65mm (2.5in) safely. In particular, the Sherline has a very large range of add-ons, such as dividing head, quick-change tool posts, CNC and so on, making it very suitable for making and repairing many domestic clock parts. It’s also inexpensive.

Cons: This is a small machine and cannot be used to repair spring barrels, for instance.

Myford

Pros: A big machine and capable of producing any part of a domestic clock, even gear wheels for tower clocks. There are very many extras, such as vertical slides, milling attachments, dividing heads, quick-change tool posts and CNC.

Cons: An excellent machine but over the top for simple clock repair tasks; it is more useful for clockmaking but expensive.

7 × 12 Chinese Lathes

Pros: These are inexpensive, simple, basic machines with a good range of speeds (infinitely variable) and sturdy; suitable for clock repair and making. Quick-change tool posts are available.

Cons: Not easily adapted for gear cutting.

LATHE OPERATIONS

The main operations will fall under the headings turning, boring, facing and milling (screw cutting and honing will be dealt with separately).

Turning The action of producing a cylinder by rotating the work and holding a single pointed tool against the outside. Without the use of specialist devices, the effect is always a truly circular cross-section – unless there is a fault in the machining method. Sometimes it is convenient to use a flat file instead of a turning tool; early clockmakers frequently used a file for arbors, posts and pins.

Facing Machining surfaces that are at right angles to the axis of the lathe.

Boring The production of circular holes by rotating the work piece and holding a single pointed tool against the inside circumference.

Drilling Boring cylindrical holes by holding a twist drill (or any form of drill) in either the tailstock chuck or the headstock chuck.

Milling The production of machined surfaces using a rotating tool with one or more cutting faces. The work is usually not rotated and is either advanced in one dimension against the cutting action of the tool, or the tool itself is advanced in one plane to produce the same effect. The lathe may be used for milling operations without modifying it by holding work on the cross slide and mounting a milling cutter (end mill or slot drill) in the chuck. Movement of the cutting tool in more than two planes is unusual in clockmaking or repairing operations.

Chapter 2

Turning

Turning for most clockmaking tasks does not call for the same variety of tool forms that are needed in industry; clock repair machining (where production speed is of less importance than quality and cost of tools) rarely requires any more exotic material for the cutting tool than high-speed steel (HSS). I shall, however, briefly touch on other cutting materials because there are occasions when only a special tool will solve a machining problem.

A tool for plain, or cylindrical, turning has:

• A side rake (or top rake) to clear the metal being removed and provide one plane of a cutting edge

• Side relief to prevent the tool from rubbing on the work and to provide the other plane to the cutting edge

• Front relief, to avoid rubbing the newly exposed surface of the work.

Table 1 Typical rake/clearance angles for turning brass, mild steel and high-carbon steel

Table 1 shows typical angles for these three faces when turning free-cutting (machining) brass, mild steel and high-carbon steel (silver steel or drill rod). Note that these angles are for work that is demanding; however, most small work (such as clock parts) can make use of the same angles, simply modifying the turning speed to prevent the tool from overheating. This makes for a great deal of economy in tool usage. The angle is about 10 degrees for the top clearance and 5–10 degrees for the clearance of front and side. There is rarely any need for rake.

These are guidelines only, as the angles for front and side clearances affect the support to the cutting edge and point. The tougher the work, the smaller this angle should be. The top rake affects the sharpness of the tool, the speed at which waste metal (swarf) is cleared away from the work piece, the mass of metal behind the cutting edge and consequently the rate at which heat is drawn away from it.

Fig. 2.01 The terms used to describe the features of a tool for plain or cylindrical, turning.

METALS

70/30 brass The numbers indicate a metal composed of approximately 70 per cent copper and 30 per cent zinc. This type of brass is used for hammering or ‘drawing’ on a press; it is not good for machining. When turned, the metal produces long, curling swarf (the metal equivalent of wood shavings). In clockmaking it is mainly used sheet as sheet and employed for parts like lifting pieces, rack hooks and so on. It becomes springy if hammered and it is brittle when red hot (hot short). Cast parts are often made from this alloy, and before the mid-eighteenth century all brass parts were 70/30 with a small amount of lead.

Free-cutting brass (machining brass) The proportion of copper to zinc in this alloy is approximately 65:35. It is a brass that machines easily and is used for pipe fittings, clock wheels, collars and mountings. The swarf sprays off the tool in short chippings. This alloy tends to crack when bent cold but is readily deformed when red hot (cold short). It is also used for forgings.

Mild steel This low-carbon steel that does not harden appreciably with heat treatment. It can however be case-hardened where a thin stratum of carbon-rich steel is produced by a heat treatment. The stratum is really thin and, when hard surfaces are required, high-carbon steel is more reliable. Mild steel is used for arbors and semi-hard parts. There is a certain amount of hardening during ‘working’ (work hardening) and burnishing is reputed to provide a thin, hard surface to pivots.

Silver steel or drill rod (high-carbon steel) This is used for arbors that have hard parts (pivots), or parts that are subject to wear such as pinions and escapement pallets. Tool steel has a higher carbon content (with other elements) and can be used for making tools that cut or punch. As its name implies, it is used more for making tools than clock parts.

HSS This steel retains its cutting edge at higher temperatures than silver steel or drill rod and is commonly used for light turning jobs – the main use in clockmaking.

Tungsten carbide This very hard material requires a green-grade grinding wheel or diamond to sharpen it. It is rarely needed for clockmaking tasks.

BASIC TURNING TOOLS

Fig. 2.02 shows a tool for turning the pivots, arbors and spindles up to about 12.5mm in diameter. This is not the upper limit of the metal that can be machined, but above this diameter it will be found that the speed of the metal over the tool will require light cuts in comparison to the diameter. A cut of 0.25mm on a steel bar of 3mm diameter (a pivot, for example) is a moderately heavy cut and there will be a tendency for the bar to bounce unless it is short in length (less than four times the finished diameter) but the same amount removed from a 19mm-diameter bar is a very light cut. As far as the tool is concerned, however, the amount of work being done is the same.

Fig. 2.02 This is a tool for turning the diameter of pivots, arbors and spindles up to about 12.5mm (0.5in) in diameter.

Fig. 2.03 A light cut taken along a machined bar indicates whether it is running ‘true’ or not.

Fig. 2.04 Diagram showing the ideal position of the tool against a carborundum stone to finish the diameter and the shoulder.

This tool can be used for roughing out small work and then, after touching up the cutting edge (if necessary) with a carborundum stone and by swivelling it to the position shown in Fig. 2.04, it may be used for finishing the diameter and the shoulder. It will also provide a slight undercut at the angle of the shoulder. I prefer the tool to be as keen as a graver when finishing diameters like those of a pivot. This will make the use of a fine Arkansas stone to finish the faces imperative. Test the keenness of the point by holding it loosely between thumb and forefinger and dragging it across your other thumbnail. If it slides it is not sharp enough; if it catches then it is just right. Finally, use the Arkansas stone to slightly dull the vertical edge by no more than a hair’s breadth.

Fig. 2.05 Fine Arkansas stone is used to slightly dull the vertical edge.

Fig. 2.06 Standard turning tools: the cutting edge slopes to the right and there is a chamfer or radius at the tip.

A glance at any of the books on commercial turning will show that the forms of turning tools are nearly all as shown in Fig. 2.06: the cutting edge slopes to the right and there is an appreciable chamfer or radius at the tip – this is to prevent the tip from being broken or worn easily. By comparison, the tools that I use mostly have very little work to do and I am more intent upon producing faces and shoulders that make a right angle to the long axis of the work.

When grinding a stick of HSS to make a new turning tool, the top rake is ground without touching the side surface and a ‘witness’ of 0.1mm is left along the left side of this top face. When the side relief is ground, a similar witness is left on the side relief or upright face. The tool is finished with a carborundum or emery stone to remove the witness and produce a sharp edge that as near as possible is the original edge of the HSS stick. When the tool needs sharpening, only the front needs to be ground away to expose fresh cutting edges. I used to have tools in my box that had been used for more than ten years without either of the relief surfaces needing to be refreshed.

The dimensions of the stick of HSS should be chosen so that when held in a square tool post, the top surface lies on the centre line of the lathe. Most lathes will be produced with a particular size of stick in mind. If this is not the case for your lathe, make a packing piece for the size of HSS that you intend to use. This is simply a piece of metal that is machined or filed to a thickness that keeps the top edge of the HSS level with the lathe’s centre line.

Fig. 2.07 The top rake is ground without touching the side surface, leaving a ‘witness’ of 0.1mm along the left side of the top face.

Fig. 2.08 Ensure that the top surface of the stick of HSS lies on the centre line of the lathe.

Fig. 2.09 A parting tool is used to either cut the work from the metal bar or to make a channel.

Fig. 2.09 shows a parting tool, which is used either to cut the work from the metal bar or make a channel. The front of this is angled so that when cutting off the finished work, there is a very small ‘pip’ left at the break-off. If the tool is to be used for cutting grooves for circlips, keyhole washers and the like (or even as a help in left-hand turning), the front edge is best left square.

Turning the right-hand side of a bar, with the tool moving from right to left, is termed right-hand turning, and all tools used on this surface are also ‘right-hand’. Turning in the opposite direction, with a larger diameter on the right, is left-hand turning. This all seems fairly obvious, but it is not an international understanding; for example, pivot files made in Switzerland for the right hand are for some reason called left-hand. If a tool is to be purchased in the ground condition, play safe and enclose a sketch of the form expected.

The size of the metal used for tools is not