Basic Benchwork for Home Machinists E-Book

Les Oldridge

• Herausgeber: Fox Chapel Publishing
• Kategorie: Lebensstil
• Sprache: Englisch
• Veröffentlichungsjahr: 2020

Seitenzahl: 180

Beschreibung

For apprentices and amateur metalworkers, this book is a practical, hands-on guide to engineering benchwork that teaches all the valuable hand tool skills and procedures for files, punches, hand filers, and more. Well-illustrated with concise technical diagrams, tables, and black and white photos, you'll learn all the tricks and gain a solid foundation in the basics of engineering benchwork that will become second nature over the course of your career as a metalworker. Not only are these proper practices essential knowledge to get started in the industry, but they will also save you spoiled work and tools.

Leseprobe

BASIC BENCHWORK FOR HOME MACHINISTS

BASIC BENCHWORK FOR HOME MACHINISTS

Les Oldridge

© Les Oldridge 1988

Published in the UK by Special Interest Model Books 2013

This edition published in 2020 by Fox Chapel Publishing Company, Inc., 903 Square Street, Mount Joy, PA 17552.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the copyright holders. Basic Benchwork for Home Machinists includes edits by George Bulliss to update the material and adapt the text for American readers.

Fox Chapel Publishing Edition

Technical Editor: George Bulliss

Editor: Anthony Regolino

Layout: Christopher Morrison

Print ISBN 978-1-4971-0057-2

eISBN 978-1-6076-5727-9

Library of Congress Control Number: 2019950252

To learn more about the other great books from Fox Chapel Publishing, or to find a retailer near you, call toll-free 1-800-457-9112 or visit us at www.FoxChapelPublishing.com.

We are always looking for talented authors. To submit an idea, please send a brief inquiry to [email protected].

Contents

Chapter 1   Introduction

Chapter 2   Materials

Chapter 3   Reading Engineering Drawings

Chapter 4   Hacksaws

Chapter 5   Files and Filing

Chapter 6   Hammers, Chisels, and Punches

Chapter 7   Scrapers and Scraping

Chapter 8   Measuring

Chapter 9   Marking Out

Chapter 10   Drills and Reamers

Chapter 11   Screwed Fastenings, Spanners, Screwdrivers, and Pliers

Chapter 12   Taps and Dies

Chapter 13   Riveting

Chapter 14   Soft Soldering

Chapter 15   Silver Soldering, Brazing, Bronze Welding, and Engineering Adhesives

Chapter 16   Welding

Chapter 17   Hardening and Tempering Tools

Chapter 18   Keys, Keyways, Splines, Collars, and Shafts

Chapter 19   Sheet Metalwork

Appendix

Acknowledgments

Chapter 1

Introduction

Modern engineering workshops are equipped with machine tools capable of producing components to such accurate limits that hand fitting at the bench is no longer necessary. Mass production methods render the skills possessed by old-time fitters in danger of being forgotten forever. This is a pity, as in many situations, the ability to complete a job using only hand tools is a great asset. In any case, it is often quicker to bring a component to the correct dimensions using a hand method than it is to spend time setting up the job in a milling machine or shaper, even if one is available.

Machine tools owned by the model engineer are often limited to a lathe and, perhaps, a bench drilling machine, so he has to become skilled in the use of hand tools. The purpose of this book is to describe the basic skills he must acquire. It takes a great deal of practice to reach the standard required, and although disappointment may be experienced at first, with the slow progress made, the satisfaction when the job is concluded is well worth the work involved.

It must be emphasized that no textbook, however comprehensive, can take the place of actual experience at the bench. The best advice is to “have a go,” perhaps on a bit of scrap material, to gain the necessary skill and confidence before working on a valuable casting.

Throughout this book, special emphasis will be made on safe working. In industry, this is looked after by the Health and Safety at Work Executive, but in the amateur’s workshop, there is no legislation to ensure the worker has a safe working environment. It is up to the individual to look after himself and to help to this end the various hazards likely to be encountered will be outlined from time to time.

A simple first aid kit, and the knowledge of how to use it, is desirable. A fire extinguisher is also a good investment and the Fire Prevention Officer from the local fire department will be pleased to give free advice as to the best type to purchase to suit your particular needs.

If good work is to be produced, a sturdy, rigid bench, fitted with a good quality vise, is essential. Most model engineers are not wealthy and setting up a workshop is a costly business. I hope to suggest, where possible, ways of cutting costs without sacrificing quality. For example, good secondhand timber is often available at low prices; when buildings are being demolished, a tactful word with the site foreman may provide just what is needed to build a bench at low cost.

Fig. 1.1Vise clamps bent up in aluminum, copper, or lead, dimensions to suit vise.

Legs made from at least 3 in. square timber and the top from 2 in. planks should be aimed at. The space between the legs can be used to house a useful cupboard.

The size of the vise will depend on the type of work to be undertaken, but it is better to have one a little larger than is thought to be necessary, to allow for “future expansion,” that is, for the bigger jobs which may come along later.

The height of the bench should be such that the top of the vise jaws are in line with the point of the user’s elbow. This makes filing accurately much easier, but more about that later.

The vise jaws are serrated to prevent the work slipping when roughing down. Clamps, sometimes called clams, to fit over the jaws are needed to prevent these serrations from damaging finished surfaces. They may be made of lead, copper, aluminum, or fiber or any other soft material (see Fig. 1.1).

Various special clamps can be made to hold round, or odd, shaped work securely in the vise. Fig. 1.2. shows an easily made and useful device for holding round bar or pipe. Holes of a size to suit the bars in common use are drilled, as shown, in a piece of mild steel 25mm × 12mm (½ in. × 1 in.) and of a length to accommodate the number of holes required. A saw cut is made through the center of all the holes, except the end one. A similar gadget for holding threaded material can be made in a like fashion, except that the holes are threaded before the gadget is split with the type of thread generally used. These two projects form a useful exercise after reading the chapters on hacksawing, drilling, and cutting screw threads!

Fig. 1.2Vise clamp for holding round bars.

Chapter 2

Materials

Before looking at the various tasks which are performed at the bench, the materials on which we shall be working and their properties must be discussed. It is important that the most suitable material for the job in hand is chosen. Often this will be specified in the drawing from which we are working, but sometimes we have to decide what to use.

The following properties then have to be considered:

STRENGTH. The strength of a material is its ability to withstand stress without breaking. The load, or stress, may tend to stretch, compress, twist, or cut the material. These are termed tensile, compressive, torsional, or shear forces. See Fig. 2.1. The strength of a material varies with the type of stress to which it is subject. For example, cast iron has good compressive strength but relatively poor tensile strength; it is about four times stronger when it is squeezed than when it is stretched.

ELASTICITY is the ability of a stressed material to return to its original shape when the load is removed. Spring steel has a high elasticity factor. Plasticine has practically no elasticity. Most materials are elastic below a certain limit, known as their elastic limit. If the stress applied exceeds this limit, the material is permanently deformed.

PLASTICITY is the reverse of elasticity and is the property of a material to retain any deformation produced by loads after the load has been removed. Steel is plastic at red heat and can be forged to shape.

DUCTILITY is the ability in a material to be drawn out by tensile forces beyond its elastic limit without breaking. This property is important in the production of wire, the wire being produced by drawing metal through dies that get progressively smaller.

MALLEABILITY is a similar property to ductility, except that the material is deformed beyond the elastic limit by compressive forces, such as rolling or hammering, instead of by a tensile force. Lead is a malleable material but lacks ductility because of low tensile strength.

BRITTLENESS. A material is brittle where fractures occur with little or no deformation. Glass is a classic example of a material with this property.

TOUGHNESS is the ability to withstand shock loads.

HARDNESS is the ability of a material to resist penetration, scratching, abrasion, indentation, and wear. In the laboratory, it is measured by applying a load to a small area of material by a hard steel ball or pointed diamond, and measuring the depression made into the material under a given load. Chisels, lathe tools, and center punches, for example, must have this quality to do the job for which they are intended. Unfortunately, the harder carbon steel tools are made the more brittle they become, so some hardness must be sacrificed for toughness in the tempering process. This will be discussed more fully in the chapter on hardening and tempering.

Fig. 2.1Compressive, tensile, shear, and torsional stresses.

SOFTNESS, obviously, is the opposite property to hardness. Soft materials may be easily shaped by filing, drilling, or machining in a lathe, milling machine, or shaper. In many cases the component is hardened by one means or another, to be discussed later, after the shaping process is completed.

MATERIALS

Materials can be divided into a number of groups, such as:

1. Metals, which can be subdivided into ferrous and non-ferrous metals. This is the group with which we are most concerned but the others will be met from time to time.

2. Plastics, are now widely used in industry and which the model engineer will occasionally use them.

3. Timber.

4. Ceramics—the name originally given to materials made from clay but now used to cover a wide range of materials.

FERROUS METALS

These are the metals containing iron. Metals are rarely used in their pure state but are combined with other metals to form an ALLOY. In the case of iron, carbon is the most important addition. Although it is only present in small amounts, it causes big changes in the property of the metal.

CAST IRON this form the iron has been melted and poured into a mold, usually made of sand, in which it is allowed to solidify. This is a simple, convenient, and relatively cheap process to manufacture components of a complicated shape. Cast iron is an alloy of iron and carbon with small amounts of manganese, silicon, sulfur, and phosphorus. It contains about 3% of carbon.

There are two types, gray and white. Both get their names from the appearance of the metal when fractured. In white cast iron, all the carbon present is cementite; in gray cast iron, most of the carbon is present as flakes of graphite, and there is usually a remainder which is in the form of pearlite. Because cementite is intensely hard, white cast iron is hard and durable, though very brittle. Graphite is soft and is a good lubricant, so gray cast iron is readily machinable, less brittle, and suitable for sliding surfaces. Being hard and brittle white cast iron is rarely used alone but it is the material used for the production of malleable iron.

GRAY CAST IRON, then, is the type in common use; it is cheap and easy to cast and machine. As a typical example, a motor car cylinder block contains 93.32% iron, 3.3% carbon, 1.9% silicon, 0.8% manganese, 0.14% sulfur, and 0.18% each of phosphorus, molybdenum, and chromium. The carbon content of approximately 3.3% consists of about 0.7% of combined carbon and about 2.6% of free carbon.

Because of the free carbon content, cast iron is easy to machine and file; the carbon flakes act as a lubricant, enabling the cast iron to be machined dry. Drilling or tapping of cast iron components is fairly easy, no lubricant being required. There is, however, a hard skin in which some of the molding sand may still be present. This is particularly hard on lathe tools, and when it has to be filed, an old file should be used; a new one would probably be ruined.

Cast iron is used for model engine flywheels, internal combustion engine cylinders, model locomotive wheels, and a host of other parts. Because of its self-lubricating properties, it is an ideal material for plummer block bearings. The spindle of the Model Engineer sensitive drilling machine runs directly in cast iron bearings and shows little signs of wear after years of use.

Cast iron has low tensile strength and poor shock resistance.

THE STEELS

There are standard specifications for steels contained in BS 970, which dates back to 1942, but since that date, there have been several revisions. In 1970, the specifications underwent a radical change and in 1983 the Standards were again restructured. Originally an EN code was used but this is now replaced by a six-digit system. It will be some time before the EN numbers disappear altogether and, in fact, some manufacturers show both the EN numbers and the current specifications where the two are closely aligned with only a point or two variation in analysis. For example, the free-cutting steel 212M36 corresponds to the old EN8M.

The North American naming convention uses a four digit number that helps to identify the steel type and its carbon content. Common steel types used by hobbyists include 1018, a common, low carbon steel, and 12L14, a low carbon steel with lead added to improve its machinability.

PLAIN CARBON STEELS. The main difference between cast iron and steel is the carbon content. Plain carbon steel has never more than 1.5% carbon, whereas cast iron, as has been stated above, has about 3%.

MILD STEEL, containing about 0.15% to 0.3% carbon combined with the iron, is ductile and malleable. It is easy to weld, machine, forge, or press into a new shape. It may be worked hot or cold. Because of its low carbon content, it cannot be hardened by heating and quenching, but can be case-hardened, a process which will be described later. It is supplied in bar form with hexagon, round, square, or flat sections in a “black” or “bright” form, and in sheets of varying thicknesses.

MEDIUM CARBON STEEL, with a carbon content of 0.35% to 0.5%, is much stronger than mild steel. Its hardness and strength can be increased by quenching the metal from a red heat. It can be tempered, rendering it suitable for many general engineering purposes where the stresses imposed are greater than could be withstood by mild steel.

HIGH CARBON STEEL, with a carbon content of 0.55% to 1.5%, is used for most tools after being hardened and tempered. Chisels, files, drills, and reamers are made from this steel.

ALLOY STEELS. In order to improve the properties of steel and to suit the metal to special applications, other substances beside carbon are added to the steel. NICKEL improves the ductility and toughness of the metal. CHROMIUM and MOLYBDENUM increase its hardness, while VANADIUM improves the elasticity, strength, and fatigue resistance of the steel. All steel contains MANGANESE but sometimes more is added to improve the steel’s mechanical properties.

STAINLESS STEEL is principally an alloy of iron, nickel, and chromium. It has a high resistance to corrosion, but in some forms, it is difficult to machine. However, by introducing a free machining agent into the alloy, this drawback can be overcome. DRILL ROD, a common tool steel used by model engineers, is a carbon steel with 1.1% to 1.2% carbon, 0.35% manganese, 0.45% chromium, and 0.1% to 0.25% of silicon.

TINPLATE. Sheets of mild steel are coated with tin to provide the metal used for the familiar food containers and for many other purposes. It is a useful material for the model engineer, being easily worked and soldered and can be obtained without cost from discarded cookie tins, etc.

NON FERROUS METALS

ALUMINUM is the lightest of the commonly used metals. It is too soft to use in its pure state, but when alloyed with copper, magnesium, and manganese, it is widely used for many components. It is a good conductor of electricity but is impossible to solder by the usual methods.

COPPER is soft, ductile, and of low tensile strength. It is an excellent conductor of electricity and is easy to solder or braze. It is the base of the brass and bronze alloys. Copper hardens with age and also work-hardens, that is, it becomes hard when it is bent or stretched. It can easily be returned to its soft, ductile state by annealing. This is done by heating to a red color and then allowing it to cool.

LEAD is soft, ductile and of very low tensile strength. It is often added to other metals to make them free cutting. It is typically used for lead acid battery plates and in soft solder.

TIN is corrosion resistant and is used to coat mild steel plate to make “tin plate.” It is used in soft solder and is an alloying agent in bronze, and is the basis of “white metal” bearings.

BRASS AND BRONZE. When copper is alloyed with zinc, brass is formed. Bronze is an alloy of copper and tin, and usually about 10% tin is used. Sometimes about 0.5% of phosphorus is added, the alloy then being called phosphorous bronze. There are various classes of bronze made especially for particular applications. It is, for example, an excellent bearing material.

IDENTIFICATION OF FERROUS METALS

Several metals have a similar appearance and new bar materials are often color coded by painting the end with a distinctive color paint. Often, off-cuts are used up and it is essential that these are identified. Most model engineers have a scrap box where all sorts of odds and ends are stored. Trouble will be experienced if, for example, a piece of high carbon steel is selected when a free cutting mild steel is what is required. There are several ways in which metals can be identified and some tests appear below.

APPEARANCE. Cast iron has a dark rough finish; the mold joint line is probably visible. A section of iron away from the skin has a gray appearance and a fracture appears crystallized.

Mild steel comes in two forms, black and bright. The former, “hot-rolled steel,” has a smooth scale with a blue/ black sheen. Cold-rolled steel (CRS) has a bright, silver-gray surface. Medium carbon steel has a smooth scale and a black sheen, while high carbon steel has a rougher scale.

GRINDING. A popular test is to grind the metal and note the color, quantity, and type of sparks given off. This is a difficult procedure to describe: a video film is really necessary, and it is equally difficult for the beginner to recognize the different types of sparks. It is suggested that an experiment is carried out using steels of known types and comparing the differences.

Cast iron gives off a short stream of red sparks, which at some distance from the grinding wheel, burst into a yellow spark formation. Plain carbon steel produces a lighter and brighter spark in a greater profusion than cast iron. As the carbon content increases, the sparks become lighter, are in greater quantities, and occur nearer the wheel. The high carbon steels produce secondary bursts, bunching out from the primary sparks.

If materials are drilled, it is very noticeable that the cuttings from cast iron are granular in form, while those from steel come off in long spirals. The cuttings (swarf) from medium carbon steel may turn brown or blue, but still be in spiral form. Swarf is very sharp and can cause nasty cuts if handled, so special care is needed when clearing the cuttings away from a drill.

PLASTICS

These materials become plastic above certain temperatures and, while plastic, they can be squeezed into dies or molds to give them the required shape that they retain on cooling. There are two main types, THERMOSETTING and THERMOPLASTIC. The former group do not become plastic on re-heating. They are hard, rigid, and rather brittle. They are used particularly for electrical equipment as they are good insulators. Bakelite comes within this category.

Thermoplastics may be softened by heat so they cannot be used at temperatures much above 212°F (100°C). Some of them, celluloid and plexiglass (Perspex) for example, are transparent and most can be colored by adding a suitable pigment.

POLYVINYLCHLORIDE (PVC) comes in this class, and is the flexible and rubberlike substance commonly used for insulating electric cables.

POLYTETRAFLUORETHYLENE (PTFE) is similar to PVC but has a very low co-efficient of friction, which makes it particularly suitable for making bushes which need not be lubricated. NYLON, one of the earliest plastics, is used for a variety of purposes, including small gearwheels.

REINFORCED PLASTIC. Laminated plastic such as TUFNOL consists of a fibrous material like paper or woven cloth impregnated with phenolic resin. The sheets of fabric are then laid up in a hydraulic press and squeezed and heated so that they become solid sheets, rods, or tubes.

GLASS FIBERS can be bonded together by polyester or epoxy resins to form large and complex moldings. Crash helmets and boat hulls are examples of things made in this way. The customary term is “glass-reinforced plastic” or GRP.

Chapter 3

Reading Engineering Drawings