Using the Engineer's Lathe in Clockmaking

Laurie Penman

• Publisher: The Crowood Press
• Year: 2022
• Print ISBN: 9780719831515
• eBook ISBN: 9780719831522
• Topic: Technology & Engineering
• Subtopic: Engineering General
• Index: Technology & Engineering

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.

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-industria...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• 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