Screwcutting E-Book

Marcus Bowman

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

Veröffentlichungsjahr: 2015

Beschreibung

Screwcutting is a guide to the theory and practice of threads and thread-making, whether that is threading a hole using hand tools or cutting a thread using a lathe. The book covers details of the major threadforms, such as metric, Whitworth and Unified threads, as well as the British Association (BA) and Model Engineering (ME and MME) series, the smaller metric and Unified threads, pipe threads, and specialist threads such as ACME, trapezoidal and RMS microscope threads. Techniques for making threads manually, as well as screwcutting in the lathe are also covered. As well as covering the basics of screwcutting, this book examines higher-level and advanced techniques, using case studies to demonstrate what can be achieved - fine, accurate and well-finished work. Illustrated throughout.

Leseprobe

CROWOOD METALWORKING GUIDES

SCREWCUTTING

DR MARCUS BOWMAN

THE CROWOOD PRESS

First published in 2015 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2015

© The Crowood Press 2015

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 000 3

Frontispiece: thread mill

Contents

Acknowledgements

Introduction

  1 From Beginning to End

  2 Threadforms

  3 Thread Calculations

  4 Measuring Threads

  5 Using Taps and Dies

  6 Preparing for Screwcutting in a Lathe

  7 Screwcutting Tools and Cutting Speeds

  8 Cutting Threads

  9 Varying Pitch

10 Thread Milling, Grinding and Rolling

Projects:

1 Female Mandrels and Ring Gauges

2 Threaded Male Mandrels

3 Small Screw Chuck

4 Reducing a Bolt

5 Water Heater Bleed Valve

6 Large Screw Chuck

7 Machinist’s Top

Reference Tables

Further Information

Index

Threading a hole in a steel plate, using a tap held in an auto-reversing tapping head mounted in a drilling machine.

Acknowledgements

The author would like to thank those individuals and companies who expended time and effort to contribute photographs and information for this book:

Fred Anderson

Axminster Tool Centre Ltd

Kirk Burwell of Hemingway Kits

Theodore Clarke

John Dammeyer of Auto Artisans

Andy Franks

Gagemaker, LP

Jean-Claude Gerber

Glen McKechnie

Jacques Maurel

Mitutoyo (UK) Ltd

Brian Mock

Manuel Marti Moreno

Kate Nicholson

Paul Norman of Racing Vincent

George Nutt

Dennis Nutt

Steffen Pahlow

Joe Pogan

Jim Schroeder

Trend Machinery & Cutting Tools Ltd

James Villegas

Rebecca Webb of Just Riding Along

RMS Engineering Ltd

Erik and Chris Schmidt of

Whizcut of Sweden AB

Roger Williams

Wiseman Threading Tools Ltd.

But most of all, a very special thanks to my long-suffering personal support team, Hazel and Rachael, whose support made the writing task so much easier.

The author also wishes to commend those Standards organisations which have laboured long and hard to produce accurate specifications of the various threadforms, including:

British Standards Institution

American National Standards Institute

American Society of Mechanical Engineers

ISO: International Organisation for Standardization

Every effort has been made by the author and publisher to contact the copyright holders of the works illustrated in this book. Should any omissions have been made and/ or the correct source not been acknowledged, the publishers will rectify this at the earliest opportunity upon reprint.

Introduction

This book is about the theory and practice of threads and threadmaking, whether that be threading a hole using hand tools, or cutting a thread using a lathe.

The contents include details of the major threadforms, such as metric, Whitworth and Unified threads, as well as the British Association (BA) and Model Engineering (ME and MME) series, the smaller metric and Unified threads, pipe threads, and specialist threads such as ACME, trapezoidal and RMS microscope threads. Specialist reference books provide details of every size of every thread you could imagine, but this book provides a comprehensive reference for the types and sizes of threads you are most likely to meet in the workshop.

This is a practical book, with instructions on how to thread manually, and screwcutting in the lathe, either using a conventional lathe tool or an accessory such as a thread mill, with enough information to allow you to cut accurate, well finished threads of all sorts using a variety of machining techniques. Measuring methods are included so that you can be sure you have created male and female threads of the right size and shape, to an appropriate tolerance to fit mating threaded holes or fasteners.

The basics of screwcutting are not difficult to master, but this book aims to help you produce fine, accurate and well finished work, which is repeatable and quite beautiful to behold. This is the kind of work you will be proud to display, as it demonstrates your mastery of the considerable skills involved in work at this level.

PRE-REQUISITE SKILLS

This book assumes you have a lathe and know how to do basic turning to a good standard, so there are no explanations about how to do things such as turn a plain diameter, turn a shoulder or a groove, drill from the tailstock, or centre work accurately. Nor are there explanations about common lathe accessories for plain turning, such as centres, faceplate, cross-slides, top-slides, toolposts or any of the information you will find in a basic lathework book. The focus is on screwcutting, rather than on turning.

Manual screwcutting assumes you have a place to work – normally a bench with a vice. Some operations assume you have a means of drilling a hole accurately, and that usually means a drilling machine. As with basic lathe operation, there is no explanation of the basic use of a pillar or bench-mounted drilling machine. As with the lathe, this book assumes that if you have a drilling machine you know how to use it.

Producing threads is an everyday activity in the workshop or on site. It is an activity from which one can take pleasure, as the thread appears as if by magic, a screw fits a special nut just as it should, or the quality of finish on a thread flank brings tears of joy. A well finished, accurate thread is a thing of beauty and much to be admired.

However, as with all mechanical processes, there are dangers, so we need to take care as we go about our work. A little forward planning and good working practices help assure safety.

BE SAFE

Have a safe place to work: in the workshop, manually threading a shaft or a hole demands that you have sufficient clear space in which to work. Slips and trips are largely avoidable if there is room to work, and there are appropriate tools for the job. Greasy hands make for an insecure grip on tools, particularly where the gripping surface is polished steel, as it is with many tap wrenches and die holders, and clean hands make for safer work, as well as a more digestible lunch.

Take particular care when using powered machines: powered machines such as a drilling machine or a lathe bring their own dangers, but one of the most basic precautions is to make sure the machines are in good working condition. Whether in a factory or a home workshop, all machines should be subject to a detailed safety inspection at regular intervals, as well as a more cursory check before beginning any job. Electrical safety is essential, but some of the simpler checks are life critical, such as making sure the chuck key is not still in the chuck before switching the machine on. These are elementary checks, but they must never be omitted, as the consequences can be injurious and potentially fatal.

When screwcutting in the lathe, there are particular dangers arising from the operation itself. At any speed, co-ordinating the movements of eyes, hands and arms with the relentless spinning of the lathe headstock and the movements of a powered carriage demands absolute concentration, and any potential distractions must be designed out of the workshop environment before beginning these tasks. Take steps to deal with likely interruptions such as unexpected loud noises, callers ringing the doorbell, phones ringing, emails beeping, the house alarm going off, dogs or cats entering the workshop, and the less likely outbreak of fire, air-raid sirens or fog horns. You should have definite ways of dealing with all these interruptions to your concentration, and while you may have dealt with any of these on an ad hoc basis up until now, screwcutting demands practice in the seamless management of all of them. At the very least be sure to do the following:

Turn off phones and computers

Designate someone else to deal with callers at the door

Train potential visitors to avoid sudden noises or loud speech designed to catch your attention, perhaps by arranging that they flash the room light instead of making a noise

Close the workshop door to dogs, cats and unintended waifs and strays

Train yourself to hear, but not react to, loud noises; while you may need to react to a fire alarm, do that in a calm and deliberate manner, having practised the procedure beforehand

BE PREPARED

In all of this, arrange your workspace and machinery in such a way that you can always reach the emergency trip switch to power off the machine quickly and safely. Also, make sure you have a means of summoning immediate assistance if you need help: this is especially true if you are alone in the workshop. It’s a bit like being alone on a hill climb in winter, in a gale and driving rain, in sub-zero temperatures: someone needs to know you are out there, and you need to be able to call out the rescue team quickly, if necessary.

Seconds count, if you are bleeding profusely – sharp tools, rotating shafts which catch and tear, chuck key impaled in the chest – or knocked unconscious – hit by a flying object thrown from the drilling machine, struck on the temple by a chuck key, knocked out as you slip and hit the corner of the bench or the concrete floor. These are serious matters, and you will want to be assured that when you have been revived or repaired by whoever assists and treats you, you are able to return to the tranquillity of the workshop once again.

No-volt and overload release switches are essential on all powered machinery. They may add to the cost, but where a fault develops they not only shut off the power but, importantly, prevent the machine from restarting when the power is reconnected.

Foot-level safety switches are useful. Some operate when pressed or kicked, and allow you to kick the power off even if your hands are occupied. Others demand that your foot press the switch to allow power to flow to the machine, and act as a 'dead man's switch', cutting the power when released by removing your foot. If the lathe grabs your sleeve, and you cannot reach the off switch with your other hand, you will be grateful that you can move your foot and save yourself from further injury.

Knee-level mushroom-headed 'OFF' switches are also useful. In a commercial workshop, overhead trip wires and drop strings with prominent red balls allow anyone to reach up and pull to trip the power off, even if they are not near the machine. They are just as useful in a home workshop.

Modern no-volt and overload switches are designed so that if they are tripped off you must take positive action to reset the switch and restore power. There is usually a recessed green 'ON' button and a protruding mushroom-headed 'OFF' button. When the unit trips and power is cut off, the red button pops out and must be manually pressed and turned to reset the switch, before further use. That's a reminder to think about what caused the trip to operate.

You will not, of course, want to wear loose clothing, unsecured belts or, worst of all, neck ties or scarves, in a workshop: not ever.

Screws have been used since early times, and a screw can create enough pressure to squeeze oil from olives. © KATE NICHOLSON.

1 From Beginning to End

The helical groove and ridge form the basis of the screw thread, and, like the wheel or the lever, the origins of this device have been lost in the mists of time. Some historians say wooden screws were in use as early as 400bc, but metal screws did not appear until the 1600s, echoing developments in materials and especially cutting tools capable of machining metals.

The lathe has an older history, dating back some 3,000 years to early applications shaping rotating pieces of wood with a hand-held chisel. Threads lent themselves to being cut on a lathe, using a chisel guided by hand.

Although wood is soft, a great deal of skill is required to cut a thread with sufficient accuracy that the screw will turn in a mating nut. Despite that, some woodworkers still cut threads by hand in the present day, although more as an exercise of skill than as a commercial proposition.

Early applications made use of the fact that a screw rotating in a fixed nut causes the screw to move through the nut, converting rotary motion to linear motion and providing some mechanical advantage. Whereas it may be difficult to push a rod forwards against a load, rotating a screw is easier, even if it does demand a greater rotary movement in many cases. Applications include wine pressing (from earliest times) through the printing press (fifteenth century) to present-day fasteners and precision movements of machine parts such as tables and slides.

Despite being a simple device, the screw thread has been of critical importance in the development of industrial processes and applications. As the industrial revolution dawned and techniques of mass production were developed, thread production not only moved towards the use of standards and the production of interchangeable parts, but, many would argue, drove key developments in manufacturing technology.

In 1568, the French mathematician Jacques Besson invented the first screw-cutting lathe by using pulleys to co-ordinate the longitudinal movement of the tool with the rotation of the spindle.

In the mid-1700s, Antoine Thiout improved on Besson’s method by adding a screw drive instead of pulleys to control longitudinal movement of the toolpost and carriage on a lathe. This was a significant development, laying the groundwork for the kind of accurate tool movement imparted by the modern-day lead screw. To extend the range of movement available with a single pitch screw, Thiout used a system of levers to vary the effective pitch of the main screw. An example of Thiout’s machine can be seen in Paris, in the Musée des Arts et Métiers.

In the 1770s, Jesse Ramsden (1735–1800), the English mathematician and renowned instrument maker, improved the lathe as a screwcutting machine, and that led ultimately to Henry Maudsley’s screwcutting lathes incorporating increasingly accurate leadscrews as well as change gears. That led to his ability to cut accurate micrometer screw threads, creating a simple means of making accurate measurements.

Once accurate screwcutting lathes became available, manufacturers began producing threads to their own specifications, with little interchangeability between systems.

To address this problem, Joseph Whitworth proposed a system of standardized threads based on a 55-degree flank angle and a standard number of threads per inch for each diameter of thread. This led to the British Standard Whitworth system of threads being adopted in Britain in 1841, which proved to be a critical moment in the development of standards for the interchangeability of parts in mass production.

In the USA, William Sellers proposed a similar system based on a threadform with 60-degree flank angle, and that led to the Unified Thread Series standard on which the popular Unified series of threads is based.

In more recent times the almost universal adoption of ISO thread standards, and of ISO Metric threads has given the world a range of universally interchangeable threads.

Thread cutting by a variety of methods is an important industrial process responsible for the creation of increasing numbers and types of fasteners that not only secure component parts of an assembly, but, crucially, allow disassembly without damaging the individual parts. From complex machines, to simple flat-packed furniture, the humble screw has certainly proven its use.

A versatile, compact and powerful car jack, based on concentric square threads.

2 Threadforms

Fig. 2.1: A plain shaft can slide along the common axis inside a plain hole.

A plain hole constrains a shaft of similar size so that the shaft can slide in and out of the hole, and can rotate (Fig. 2.1), but the axis of the shaft and the axis of the hole must remain co-incident.

Fig. 2.2: Cross-section of a helically-ridged shaft inside a hole carrying a matching helical groove.

Fig. 2.3: A shaft carrying a helical ridge can only move into or out of a hole carrying a matching helical groove if the shaft rotates.

Creating a uniform helical groove in the wall of the hole, and a matching helical ridge on the shaft (Fig. 2.2), means that if the nut remains stationary, the only way to slide the screw in and out of the hole will be to rotate the shaft (Fig. 2.3). This feature also means that rotating the shaft will force it to move in or out of the nut. Several useful consequences follow from this arrangement.

Fig. 2.4: A micrometer makes use of the fact that rotating a shaft with a helical ridge inside a hole with a matching helical groove causes precise movement of the shaft.

Rotating a shaft with a uniform helical ridge in a hole with a mating groove will provide a uniform and predictable linear movement of the shaft for a given rotation, allowing precise control over the movement of the shaft (Fig. 2.4).

Fig. 2.5: In this boring head, the shaft carries a helical ridge and can rotate but is prevented from moving along its axis. The hole in the slide contains a matching helical groove, so turning the shaft forces the slide to move a distance related to the rotation of the shaft.