Gearing of Lathes for Screwcutting E-Book

Brian Wood

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

Veröffentlichungsjahr: 2017

Beschreibung

Gearing of Lathes for Screwcutting is aimed specifically at the engineer for use in the workshop. It is intended to take away as much as possible of the mathematics and mystique from calculating gear ratios, so that the more enjoyable work of using your lathe to make things becomes as easy as possible. Topics covered in this latest addition to the Crowood Metalworking Guides include Myford and other types of lathes; approximations and alternatives; errors and their significance and the non-gearbox mini-lathe. Fully illustrated with 102 colour photographs.

Leseprobe

CROWOOD METALWORKING GUIDES

GEARING OF LATHES FOR SCREWCUTTING

BRIAN WOOD

THE CROWOOD PRESS

First published in 2016 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2017

© The Crowood Press 2016

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data

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

ISBN 978 1 78500 251 9

Dedication

In memory of a dear friend, Seffie

Contents

Introduction

PART 1 THE GEARBOX LATHES

1 MYFORD ML7 LATHES AND VARIANTS

2 THE MYFORD ML7 LATHES WITH THE DAVID MACHIN GEARBOX

3 THE SOUTHBEND LATHE AND ITS MANY CLONES

Includes the English Boxford, Australian Hercus, Smart and Brown Sabel and others.)

4 THE SMART AND BROWN TOOL ROOM LATHES

5 THE RAGLAN 5- AND 5½-INCH LATHES

6 THE HOLBROOK MODEL B No. 8 TOOL ROOM LATHE

7 THE COVMAC 13- AND 17-INCH LATHES

8 ALTERNATIVE APPROXIMATION GEARING

9 THE SIGNIFICANCE OF ERRORS

PART 2 THE NON-GEARBOX MINI LATHE

10 THE MINI LATHE

11 FUTURE TRENDS

References

Acknowledgements

Index

Introduction

This book is not a guide to making things, nor is it a guide on ‘how to do things’ such as the screwcutting operation itself; there is already plenty of information available elsewhere on those topics. It is principally a work of reference aimed specifically at the engineer for use in their workshop. It is intended to save them the trouble and time involved in working out the intermediate gear ratios needed for cutting screw threads – ratios that are not in the screwcutting language their machine was built for, or that become necessary because the pitch is unusual in some way.

The inclusion of a screwcutting gearbox on a lathe gives a convenient and compact way of quickly selecting from a range of the different choices of gearing within the gearbox to give alternative screw pitches without having to make any alterations to the change wheels outside the gearbox. In lathes equipped with a separate feed shaft for surfacing and cross cutting, gearbox changes also affect those, as they are usually linked together by gearing. However, this book is aimed purely at the screw thread pitches, as the other feeds are usually defined on the machine itself by some simple fixed ratio to screw thread pitch.

Any pitch that falls outside the selections available from the gearbox itself requires changes to the external gearing down from the lathe spindle, just as would be the case on a non-gearbox lathe. Manufacturers usually fix plates to the change wheel gear cover for English or metric conversions (depending on the machine), although other conversions for, say BA pitches, might only be listed in a handbook.

Shown in this book are detailed set-up tables of gearing for the most frequently owned lathes in model engineers’ and other light engineering workshops, with one notable exception. A wide range of pitches are covered, and for the first time many are presented by including pitches outside the more usual conversions, such as metric from imperial. Some obscure pitches are included for those engaged in restoration projects.

In the case of the Myford lathe fitted with the company gearbox, a radically different approach to conversion gearing is described which preserves the greatly valued fine-feed setting for all pitches. When the conversion is conducted in the manufacturer’s published manner, the fine-feed setting is no longer accessible, as an inconvenient consequence of converting the lathe to cut non-imperial pitches. To remedy this, some minor and discreet equipment modifications to Myford lathes need to be made. These are simple and are fully described with the necessary drawings; all of them are well within the ability of any practical engineer. I make no apology for showing a bias towards Myford lathes, having been an owner for over fifty years, but fair treatment of the other, often better built and larger capacity machines is presented in the same uncomplicated way.

The whole purpose of this book has been to avoid as much as possible the mathematics and mystique of calculating gear ratios – which for some may be a dreaded task – so that the more enjoyable work of using your lathe to make things becomes as easy as possible.

One of the most versatile uses of the lathe as a machine tool is to be able to cut screw threads in widely varying pitches that are not limited by the diameter of the work. This freedom is simply not possible with fixed size threading dies, and with the lathe itself usually having been used to size the work in the first place any such threading as a further operation will also be guaranteed to be co-axial with the work.

Gear wheels (more correctly known as change wheels in this context, as from now on they will be called) can be arranged at will to alter the gearing ratio between the lathe spindle, and ultimately the carriage leadscrew, to vary the pitch of the resulting screw threads that can be produced.

Fig. 0.1 A typical gap bed lathe showing relevant detail (drawing by Gary Wooding).

Fig. 0.1 is a simplified drawing of a typical screw-cutting lathe where these essential features are identified. Since the tailstock takes no part in screw-cutting calculations, the drawing does not include a tailstock. The position of a screwcutting gearbox is shown; it is the last item in the gearing chain between the spindle in the headstock and the leadscrew that moves the tool carriage, for the very good reason that it falls nicely to hand for the lathe operator to use as required.

DEVELOPMENT OF SCREWCUTTING LATHES

We learn from the pages of Wikipedia that Leonardo da Vinci drew sketches for screwcutting lathes; one of these is recorded as being fitted with twin leadscrews and change wheels to vary pitches.

In the late eighteenth century Jesse Ramsden constructed the first recognizably modern screwcutting lathe; it was equipped with a leadscrew, tooling slide rest and change wheel mechanism. Henry Maudslay is credited with bringing this successful design to the world in 1800 as a machine tool that could be reliably and mechanically guided to cut metal screw pitches. From then on, large scale industrial production of screwed parts became possible.

It was Joseph Whitworth who brought some very necessary order in 1841, along with a system to classify and consolidate the plethora of screw threads that were by then being produced by multiple companies. Before Whitworth, there was no hope of going down to a hardware store for a replacement screw or nut; all these companies had their own non-interchangeable standards. You will find pitch tables in this book for threads whose origins go back to those early days. Some were used at the small arms factory in Enfield, England, and others from the late eighteenth century by the London-based lathe makers Holtzapffell in the production of their ornamental turning lathes.

Industrial demands

Change wheels methods allow the very widest range of pitches to be possible from a supply of change wheels. However, the use of change wheel methods to vary the pitch of lathe-cut screws is not well suited to the requirements of industrial production, being far too slow in operation.

Initially of course there was no alternative to altering change wheel arrangements manually, after which the proper meshing of the selected wheels needs to take place. To cope with the slow pace of production brought about by these limitations, it is quite probable that different lathes would have been set up to do different threading tasks, along with an operator to each machine in an effort to get some production speed. Industrial pressure at that time would have grown steadily for something to speed up the whole process, and the selective screwcutting gearbox was eventually born.

Today that kind of operation would be conducted by a cam operated automatic, churning out tens of thousands of individual screws at a time. Even the bar feed from a storage hopper into the machine is automatic, so it will operate day and night on the task.

Historical patents

In the course of researching the development leading up to a successful screwcutting gearbox, I was directed to a small book by Oscar E. Perrigo which was published in New York in 1903.1 In his book, Perrigo cast an engineer’s eye over the list of patents related to the screwcutting gearbox; there were 164 then held in the US patent office, having been issued over a time scale of just under half a century from 1854 to 1903.

From his exhaustive study of this intensely fruitful period, for patents at least, he condensed the work to a shortlist of twenty-nine patents. These he considered contained the germ of ideas and ingenuity, many of which were taken forward by others, and these therefore had the greatest influence on the outcome. In making his selection he discarded the ‘also-rans’ from the point of view of practicality or durability. Many of these he considered must have taxed the patent office in establishing what the new patent application had about it in the way of merit when compared to something very similar. We have to bear in mind here that reading and understanding a patent application is not easy, and on a topic new to start with it becomes doubly difficult.

To summarize his work as succinctly as possible, the concept of a cone of gears, from which to select individuals for gearing the leadscrew, originated from John Humphreys of Chicopee, Massachusetts, when it appeared in patent 83,774 issued on 3 November 1868.

There then followed many variations on this theme with some quite extraordinarily dangerous inventions, all of open gears, in a profusion of different forms. Just the mess alone of oil being flung off these devices, and the noise generated by them in the course of use, would have made them extremely unpleasant machines to use. Some were also remarkably complex with multiple cones of gearing to be meshed correctly – a task surely above the ordinary turner who would have been expected to grasp the complexity and use it to produce results.

The first gearbox: Norton

Out of all this emerged the first device to contain all the necessary mechanism, suitably compacted, to fit inside a separate box that could be installed in the drive to the lathe leadscrew. It contained a twelve-step cone of gears directly connected to the leadscrew and simple robust means of accessing the individual gears in the cone.

Fig. 0.2 Extract from the patent granted to Wendall P. Norton (with kind permission of the US Patent and Trademark Office).

That design went on to become the clear front runner and US patent 470,591 was issued on 8th March 1892 to Wendall P. Norton, then of Mount Vernon, New York, and later at Torrington, Connecticut, when he added a reversing refinement in a further patent. His name is now firmly associated worldwide with screwcutting gearboxes. Fig. 0.2 is a copy of the drawing from that patent, presented just as it would be to an operator of the lathe. The lathe headstock is at the top of the figure.

Norton’s design has since taken over completely in this regard, and industrial lathes have been fitted with gearboxes generally conforming to it for well over a century. Norton was clearly fortunate in finding a backer in the Hendey Company of Torrington, Connecticut at the right time for the industrial application of his design.

Hendey fitted a gearbox of his design to a lathe of theirs in the same year as the patent was issued. The combination propelled Norton forward to prominence, and other machine tool companies sat up and took serious notice. The burgeoning US automobile industry alone had a vast appetite for machine tools of all sorts, and Hendey was just one of the many companies that supplied into it. In under thirty years, Hendey were turning out lathes all equipped with a Norton screwcutting gearbox on a full scale production line basis.

So, commercially at least, the prize goes to Wendell P. Norton for producing a gearbox of a compact form based on John Humphrey’s original idea of a cone of gears, the general design of which is still in use today.

On some modern tool room lathes, these gearboxes can become quite complex with lever selection, or at most, one easily reversible cluster gear to provide access to other pitches. After that, fast selection from a further range of other pitches can then be provided via the gearbox.

Fig. 0.3 Image of a modern screwcutting gearbox fitted to a German tool room lathe (with kind permission of www.lathes.co.uk).

An example from a German manufacturer is shown in Fig. 0.3 which reveals something of the level of complexity that can be built into a sophisticated screwcutting gearbox. The basic gear cone is clearly visible in the right-hand half, a twelve-step version in this case. However, the way the ratios are mixed in the left-hand side of the gearbox housing would be frankly bewildering without appropriate guidance in tabular form to list all the choices it can provide. It is quite likely to contain gearing for other languages, simply accessed by lever position.

For what might be termed the amateur market in the UK, the cost and size of even simple gearboxes prohibited inclusion on the smaller lathes produced for that market. For those machines, the continued use of change wheels for screwcutting was all that was available until well after the end of the Second World War.

Fig. 0.4 An early Drummond treadle lathe, acquired as a rusty heap of parts; carefully restored and rebuilt by the club members (by kind permission of Peter Goring, Epping Forest Horological Club).

Fig. 0.5 A close-up photo of the Drummond lathe. It is now a working machine and is regularly exhibited at various events (by kind permission of Peter Goring, Epping Forest Horological Club).

The photograph in Fig. 0.4 was taken of an early Drummond treadle lathe, with a close-up picture of the headstock shown in Fig. 0.5. This picture shows very clearly that it is still a working machine which has not been cleaned of recent brass swarf from an exhibition demonstration. It also shows quite graphically that no operational room existed for a gearbox on the leadscrew drive on a lathe of that design. Any useable space has been compromised by having the drive belt to the headstock running through it.

Fig. 0.6 Image of an early treadle lathe made by Johann Weisser in Germany, complete with a Norton style gearbox (with kind permission of www.lathes.co.uk).

To overcome this problem the German manufacturers, ever inventive as engineers, created a treadle lathe from the early twentieth century, shown here in Fig. 0.6, which included a ten-stage Norton pattern gearbox coupled directly to the leadscrew. The gearbox was built on as an outrigger from the headstock end of the lathe, thus placing it well away from the belt drive. It would surely have been a very tiring machine to use with only foot treadle power available, and having to overcome the added frictional losses from within the gearbox, quite apart from the effort involved in machining metal. Later similar designs by the same company were powered from a motorized counter shaft.

In the US, every scope existed for innovation and invention in machine tool design on many fronts. These included the arms manufacturing industry (Remington, Winchester and Colt to name a few) and the fast expanding railroad network, later joined by the growing automobile field, along with general mass production manufacturing of all kinds.

Alternatives to Norton

Arising directly from this fertile period, Perrigo also noted other gearbox designs which were granted patents; two of these were of radically different design to the Norton patent.

Fig. 0.7 Detail image of the Flather patent circular gearbox fitted to a Hamilton lathe (with kind permission of www.lathes.co.uk).

Fig. 0.8 An image taken in 2011 of a Hamilton lathe with the Flather gearbox. Now owned by the father and son standing behind it, the lathe is fully restored and working (with kind permission of www.lathes.co.uk).

The first of these was granted a patent on 2 April 1895 to Edward Flather of Bridgeport, Connecticut. The concept was further developed by others. His design was for a most ingenious circular gearbox from which individual gears could be selected on eccentrically mounted shafts on which they all ran. They were fitted to lathes made by the Hamilton Machine Tool Company of Hamilton, Ohio. Figs 0.7 and 0.8 are two images of this very unusual gearbox. Pitches were selected by pushing and pulling knobs attached to short shafts in the housing to position the wheels that then engaged with the internally cut teeth on a drum-shaped housing, from which the drive was taken on to the leadscrew. However, unfortunately for Flather, the Hamilton Company did not appear to recognize the value of the patent by claiming the gearbox to be a universal unit of their own design. Sadly therefore, without the kind of backing that Norton had received, it failed to gain recognition. Given the right opportunity, it might perhaps have been capable of extensive development.

The other completely different design that Perrigo drew attention to was based on nested concentric discs with gear teeth formed on the rim that could be selected by pushing forward the disc concerned out of the stack of discs. It then meshed with a follower gear to take the drive onwards (the analogy to a juke box selector mechanism seems apt) and patent number 541,385 was issued on 18 June 1895 to Carl Johann Paulson, a Swedish engineer of Brooklyn, New York. This seems to be as far as the concept went, there being no surviving hardware to show that it was ever made or fitted to a working lathe.

COMMERCIAL DEVELOPMENT

The US market

The whole American machine tool industry was based on a very much larger footing to serve a big country, well in excess of that in the UK, and it was easily able to cater with all these supply requirements. Neither had the US industry been hamstrung by the overriding demands of war related production which had so hobbled forward progress, in the British small lathe field in particular, through the two world wars. Indeed, American industry thrived on the supply of machine tools and equipment to assist the UK and later on for their own defence when they joined the conflict after Pearl Harbour in December 1941.

Because of this unfettered freedom, development and innovation in the smaller lathe market in the US took place more or less hand in hand with design and development in the huge industrial field and it flourished accordingly. By 1944, for example, the long-established Southbend Lathe Company of Indiana were producing 9- and 10-inch centre lathes all equipped with multistage Norton screwcutting gearboxes which gave forty directly accessible imperial pitch selections.

North American lathe size designation is by swing over the bed, so those sizes would equate to 4½ and 5-inch centre height lathes in the UK. A further selection of eight pitches could be had by making one outside wheel change to halve the ratio of the gearing. This in turn doubled the potential number of pitches available from the gearbox, but it was of course only of real use in the coarsest range already provided by the gearbox. The altered gearing thus brought the full capability of non-duplicated selectable pitches up to forty-eight different values.

Fig. 0.9 A publicity photograph of the lathe assembly shop at the Hendey Machine Tool Company (with kind permission of www.lathes.co.uk).

There are publicity photographs in the historical records from the Southbend factory and other machine tool companies. This one in Fig. 0.9 was taken in 1943 as a factory tour for the Hendey Machine Company, and it is typical for the time in showing a large factory with serried ranks of lathes stretching away into the distance, all in varying stages of assembly. Incidental points of interest in the picture show that women were also employed on the shop floor, the chap in shirt and tie is clearly a supervisor, and the picture must have been taken in the summer, judging by the open banks of windows to capture any breeze worth having. There would have been no air-conditioned working in those days. The rather inadequate looking stove whose flue was piped out of the same window would surely have been a cosy magnet in the winter months. The lathes shown here all have Norton screwcutting gearboxes and the view is of the final assembly stage of production.

Rather unusually for an American manufacturer, Southbend lathes could also be specified to order fitted with the alternative metric gearbox and its matching 3mm pitch leadscrew. A total of fifty-six metric screwcutting pitches then became available instead from four ranges. with screw pitch threads that were halved by outside wheel change on three of these ranges. Special translation change wheels, made as cluster wheels, were also provided to change imperial pitch lathes to generate metric pitches or vice versa. These were part of the standard supply to cater for such needs that the customer might be called upon to meet.

These well-made and well-equipped lathes were mass produced for a huge market which numbered in the tens of thousands. As well as being supplied industrially, they were also available for sale to smaller and amateur workshops worldwide.

With such a successful design, it became widely copied, and the many clones include the English-made Boxford and Smart and Brown lathes, and the Australian-made Hercus, to name a few of the better known makers. Other clones were produced for local sale in Brazil and Sweden.

English lathes

The Raglan Company in Nottingham was noted for producing high quality lathes right from its foundation date in 1942, and it began selling a top of the range 5-inch centre height Little John lathe, equipped with a screwcutting gearbox, from 1966. These machines were better built and specified and therefore much more expensive than those produced by their nearest rival at Myford Machine Tools, who were also based in Nottingham. Myford eventually bought the company out and finally closed the factory in 1971 when further operation became uneconomic.

The Covmac lathes were big and rugged machines of 13- and 17-inch centre height, weighing about one and a half tonnes. They were designed and produced by the Coventry Machine tool company, who along with a number of other manufacturers, operating under licence from the parent company, supplied lathes for war time demands to a design which dated back to the middle 1930s. They have a rather limited two-stage imperial pitch screwcutting gearbox but it is equipped with an interesting and novel range change method compared to those fitted to the other lathes mentioned. There are now only six intact examples known to be in existence worldwide.

In contrast, the Holbrook tool room lathe was a beautifully produced top quality 4-inch centre height English made machine, produced for a limited precision market in the 1950s. Rather surprisingly, for a tool room lathe and the application it was intended for, the screwcutting gearbox built for this lathe was even more limited than that of the Covmac. In fact it was much more like the Norton patent original and depended completely on the rearrangements of change wheels in the outside gear train to be able to provide the multiples of the central nine-step gear cone.

While all these gearboxes conform to the general Norton design they have differing internal gearing ratios to suit the lathe to which they are fitted. What was different about the gearbox package made by Myford for their ML7 lathe was in the outside gearing arrangement which has not been apparent elsewhere.

THE MYFORD MACHINE TOOL COMPANY

Being a Myford owner, I have a much closer knowledge of the lathe than of the others and it is hoped the reader will accept the rather more detailed analysis given to these lathes as a result in the chapter describing Myford gearboxes and their operation.

When the Myford Machine Tool Company launched their 3½-inch centre height ML7 screwcutting lathe in 1945, it was to an entirely new design to replace their ageing ML4 model. The lathe ushered in a new era in the UK home market for smaller sized, modestly priced good quality machinery aimed at both the domestic market and light industrial use.

Fig. 0.10 Photograph from 1948 literature for the Myford ML7 lathe (by kind permission of Myford Ltd).

This new and robust machine, with a built-in mains powered countershaft as standard equipment, came as a complete revelation for the model engineers’ market, after years of putting up with older pre-war designs – some, like the Drummond shown earlier, dated back to the days of treadle lathes. Fig. 0.10 has been reproduced from a 1948 Myford publicity brochure for the lathe. The price list for the accessories which were available for the lathe at that time makes interesting reading.

Before long schools, training establishments, laboratories and research organizations from all over the UK had examples of the lathe in their workshops. The Myford ML7 or the later Super 7 (one of the successor designs) have also appeared in hospital maintenance workshops, and even in the restricted space available on Royal Naval nuclear submarines. Sales of all the series were brisk, and individuals were quick to propose useful modifications which have added to the wide appeal these machines have built up over the years.

Soon after the Myford launch, Lawrence Sparey produced a screwcutting gearbox for the lathe. It was designed for screwcutting imperial thread pitches and was fabricated in sheet metal. It used standard Myford gears internally, and many owners were keen to build their own copy of his design from the kits he also produced. The necessary ‘outside’ gearing continued to use the standard Myford quadrant and wheels selected from the standard set of change wheels that were supplied with the basic lathe on new supply.

In the mid 1950s Myford introduced their cast iron cased gearbox; it was a miniaturized Norton style gearbox based on by now very well established industrial experience. Like Sparey’s gearbox, this too was designed with the primary aim of generating twenty-four imperial pitch threads. It was sold either factory fitted to new lathes or as an accessory that owners could buy and fit themselves. The fitting instructions were clear and concise, but the task assumed a good deal of engineering knowledge and a reasonably well equipped workshop; it was not a job for a beginner.

The outside gearing construction that was supplied with it was very different from the normally supplied quadrant and it appears to be unique for the lathe. The quadrant is made of cast iron and is fitted with two hardened fixed position pins on which run two hardened gear clusters each of 57 teeth mated to one of 19 teeth. One of the clusters is reversible on its pin to give a fine feed geared reduction ratio of 1 to 9. The equipment supply was completed by a single 24-tooth case hardened steel gear, used as the mandrel wheel, and a double width 72-tooth gear fitted to the gearbox input shaft. No other change wheels were offered nor were they catered for.

Fig. 0.11 This is very similar to the standard supply from Myford, when a gearbox has been fitted. The door that normally covers the wheels has been removed. In this photo, a wheel of 34T is in place of the usual mandrel wheel of 24T. The reverser, shown here with the yellow knob, is a nonstandard modification which I made, based on the design by Graham Meek.

Fig. 0.11 shows the quadrant set-up, with a 34-tooth mandrel wheel in place of the 24-tooth wheel. The lathe had recently been used to cut a 1.00mm pitch thread and not returned to imperial working. To set up for screwcutting the cluster gear nearest to the gearbox has been reversed on its pin to give a direct one-to-one drive through the whole drive chain from the mandrel to the gearbox. The same gearbox package is still sold today, sixty years later, and the design has changed very little in the intervening years. One early improvement was made to fit hardened components throughout the gearbox after failures were traced back to the use of unhardened gears and dog clutch parts.

Later still David Machin introduced a kit build gearbox. It was a clone of the Sparey design with identical gearing but instead of a sheet metal casing it used rigid aluminium castings. Kits are available for these through Hemingway Kits but as supplied they do not include any of the necessary twenty DP Myford gears that are needed internally. This gearbox will be examined in more detail further on as its design and operation is subtly different to that of the Myford product.

The metric conversion set

Fig. 0.12 Myford screwcutting gearbox as it was supplied to the English market.

Fig. 0.13 This is a copy of the data plate for the export gearbox (by kind permission Myford Ltd).

At some stage, the date of which I do not know, Myford also produced an export gearbox aimed at the North American market in direct competition for the Southbend business. It differed from the English gearbox by incorporating gearing to generate 23 tpi threads to suit North American standards for pipe threading pitches. The change was made possible by substituting a 23-tooth gear in the gear cone in place of the 19-tooth gear used to generate 19 tpi pipe thread pitches to suit English standards. The other gears in the cone were shuffled up accordingly to allow it to be fitted. In every other respect the gearbox is identical in both size and method of use. The pitch and feed data plates for both of these gearboxes are shown in Figs 0.12 and 0.13.