Engineering workshop I دوسية مشاغل.pdf

Lecture notes of: Engineering Workshop I (10621100) Eng. Luqman Herzallah 2019/2020 Table Contents 1. Safety at the workplace........................................................................................................... 5 1.1. Introduction...................................................................................................................... 5 1.2. Causes of accidents.......................................................................................................... 5 1.3. Safe workplace................................................................................................................. 6 1.4. Responsibilities................................................................................................................ 7 1.5. Control of hazardous substances...................................................................................... 7 1.6. Control of noise at workplace.......................................................................................... 8 1.7. Control of vibration at work............................................................................................. 9 1.8. Fire................................................................................................................................... 9 1.9. Electrical hazards........................................................................................................... 12 1.10. Safety signs and colors.................................................................................... 14 1.11. First aid....................................................................................................................... 16 1.12. The reporting of injuries:............................................................................................ 17 1.13. Some protective equipment........................................................................................ 18 2. Hand processes................................................................................................................ 19 2.1. Engineer’s files............................................................................................................... 19 2.1.1. File identification.................................................................................................... 19 2.1.2. Filing....................................................................................................................... 22 2.1.3. Care of files............................................................................................................. 23 2.2. The hacksaw................................................................................................................... 23 2.3. Cold chisels.................................................................................................................... 25 2.3.1. Cold chisel classification........................................................................................ 25 2.3.2. Using the chisel....................................................................................................... 26 2.4. Engineer’s hammers....................................................................................................... 28 2.5. Screwdrivers................................................................................................................... 28 2.6. Taps................................................................................................................................ 30 2.7. Dies................................................................................................................................. 31 2.8. Power hand tools............................................................................................................ 32 2.8.1. Hand drills............................................................................................................... 33 2.8.2. Screwdriver............................................................................................................. 33 2.8.3. Impact wrench......................................................................................................... 35 1 2.8.4. Grinder.................................................................................................................... 35 2.8.5. Metal shears............................................................................................................ 35 2.8.6. Hammer................................................................................................................... 35 3. Measuring equipment...................................................................................................... 36 3.1. Vernier instruments........................................................................................................ 36 3.1.1. Vernier caliper........................................................................................................ 38 3.2. Micrometers................................................................................................................... 42 3.2.1. Notes on using a micrometer.................................................................................. 46 4. Joining methods............................................................................................................... 47 4.1. Mechanical fasteners...................................................................................................... 47 4.1.1. Machine screws....................................................................................................... 47 4.1.2. Socket screws.......................................................................................................... 48 4.1.3. Self-tapping screws................................................................................................. 48 4.1.4. Bolts........................................................................................................................ 49 4.1.5. Nuts......................................................................................................................... 49 4.1.6. Washers................................................................................................................... 50 4.2. Screw threads................................................................................................................. 50 4.2.1. ISO metric thread.................................................................................................... 50 4.3. Locking devices.............................................................................................................. 51 4.4. Riveting.......................................................................................................................... 51 4.4.1. Solid and tubular rivets........................................................................................... 51 4.4.2. Blind rivets.............................................................................................................. 52 4.5. Welding.......................................................................................................................... 53 4.5.1. Arc welding............................................................................................................. 53 4.5.2. Electron beam welding (EBW)............................................................................... 55 4.5.3. Laser beam welding (LBW).................................................................................... 55 4.5.4. Gas welding............................................................................................................ 56 4.8.5. Flame settings......................................................................................................... 57 4.8.6. Safety in the use of oxy-acetylene welding............................................................ 59 5. Drilling............................................................................................................................ 61 5.1. The sensitive drilling machine....................................................................................... 61 5.2. Tool holding................................................................................................................... 63 2 5.3. Clamping........................................................................................................................ 65 5.4. Cutting tools on drilling machines................................................................................. 66 5.4.1. Twist drill................................................................................................................ 66 5.4.2. Machine reamer...................................................................................................... 66 5.4.3. Countersink............................................................................................................. 67 5.4.4. Counter bore............................................................................................................ 68 5.4.5. Spot face.................................................................................................................. 68 5.5. Drilling operations.......................................................................................................... 69 5.6. Drilling sheet metal........................................................................................................ 72 5.7. Safety in use of drilling machine.................................................................................... 72 6. Material........................................................................................................................... 74 6.1. Physical properties......................................................................................................... 74 6.1.1. Coefficient of linear expansion............................................................................... 74 6.1.2. Specific heat capacity............................................................................................. 75 6.1.3. Density.................................................................................................................... 75 6.1.4. Melting point........................................................................................................... 76 6.1.5. Thermal conductivity.............................................................................................. 76 6.1.6. Electrical resistivity................................................................................................ 76 6.2. Mechanical properties........................................................................................................ 77 6.2.1. Hardness.................................................................................................................. 77 6.2.2. Brittleness............................................................................................................... 77 6.2.3. Strength................................................................................................................... 77 6.2.4. Ductility.................................................................................................................. 77 6.2.5. Malleability............................................................................................................. 77 6.2.6. Elasticity................................................................................................................. 78 6.2.7. Toughness............................................................................................................... 78 6.2. Plain-carbon steel........................................................................................................... 78 6.3. Heat treatment of plain-carbon steel.............................................................................. 79 6.3.1. Annealing................................................................................................................ 80 6.3.2. Normalizing............................................................................................................ 81 6.3.3. Hardening................................................................................................................ 81 6.3.4. Tempering............................................................................................................... 82 3 6.4. Cast iron......................................................................................................................... 84 6.4.1. Grey cast iron.......................................................................................................... 84 6.4.2. Malleable cast iron.................................................................................................. 85 6.4.3. Spheroidal-graphite cast iron.................................................................................. 85 6.5. Aluminum and its alloys................................................................................................ 86 6.5.1. Advantages.............................................................................................................. 86 6.5.2. Forms of supply...................................................................................................... 86 6.5.3. Applications............................................................................................................ 87 6.6. Die-casting alloys........................................................................................................... 87 6.7. Metal protection............................................................................................................. 87 Appendix (A): safety signs................................................................................................ 89 Bibliography................................................................................................................................. 93 4 1. Safety at the workplace 1.1. Introduction Safety in the workplace is a major topic in the workshop technology books, it is because of the hazards that are usually present in the workplace, which if not considered seriously would lead to serious injuries. This chapter clarifies the most common types of hazards that may be present at the workplace, the causes of accidents that may happen, the needed precautions to avoid injuries and the codes that are related to the general safety in the workplace. 1.2. Causes of accidents Whenever accident happen, there should be a faulty behavior or situation led to that accident. And it is worthy to say that the worker and the employer should be aware that any faulty behavior could lead to a serious injury or death to any of the workers in the workplace, and it may defect the tools or the machines in the workplace, so they should consider their duties seriously. Accidents are generally caused when one or more of the following cases are present: - Carelessness, playing at work, practical jokes … - Lack of experience, not sufficient training, poor supervision. - Failure to think ahead. - Exhaustion due to working for long time without sufficient rest, not enough sleeping time. - Illness, medicines that can affect man’s ability to do work safely. - Improper dress, including loose cuffs, trainers, rings, neck ties, chains, hand watches, scarfs, long hair … - Unmaintained machines, unsafe tools. - Unsafe environment in the workplace, bad lighting, poor ventilation, temperature and humidity is out of the comfort region of human, not horizontal or slippery floors, garbage and rubbish are not in rubbish containers, not organized workplace causing objects to fall down … 5 1.3. Safe workplace A safe workplace is associated to heath safety and welfare, where any worker can perform his tasks in a healthy environment, with high safety margins, and where comfort is available. Health in the workplace is related to many aspects that affect the health of the employees in the workplace, which mainly are: - Lighting: where there should be sufficient light, natural or artificial, in order to see things clearly, move safely, and not to experience eye strain. - Temperature: where it should be reasonable, unless if the temperature in the workplace can’t be brought up or down to the comfort zone, such as in the melting furnaces area, then a special cloth should be available in order to keep the worker safe. - Ventilation: where air should be sufficiently displaced in the workplace, especially if the workplace is in a closed area. - Cleanliness: where dust and rubbish are dangerous for health, moreover, the rubbish in the workplace may be toxic, so it should be handled carefully, and not allowed to accumulate in the workplace. Safety in the workplace is related to condition of the workplace itself and the machinery that is used: - Maintenance of the equipment or machinery that are used in the workplace should be at regular intervals - There must be a regular check for the emergency lighting, fire protection system … etc. - All floors should be even and not slippery, and the routes in the workplace should not be narrow. - Any dangerous places, vessels, holes … should be properly fenced. - The workstation should be well arranged in order to perform tasks safely. In every workplace there must be clean toilets, washbasins, clean drinking water, and suitable rest facilities, where employees can eat and rest. 6 1.4. Responsibilities To ensure that work is done safely, employer and employees should consider their responsibilities, which can be illustrated as follows: Employer’s responsibilities: - Provide suitable machinery and tools for the work to be performed. - Provide a safe workplace. - Make a maintenance schedule to ensure that all the machinery and tools work properly. - Provide sufficient training so that the workers can use machines and tools and perform their work safely. Including sufficient knowledge about hazards and safety. - Apply safe practice for storing transporting and handling any special articles, like chemical substances and particular machines. - Ensure suitable conditions in the workplace, where employees can carry out their tasks safely. Employees’ responsibilities: - To understand and apply safe practices at work. - To understand the hazards that could happen if they ignored the safety rules. - To cooperate with the employer on health and safety. - To work seriously, taking responsibility about themselves and the work. 1.5. Control of hazardous substances Hazardous substances are those substances that are dangerous for people’s health, which can bring illness to human related to systems of the human body, such as the diseases of the lungs and the wind pipe, asthma, skin diseases, losing consciousness due to the exposure to toxic fumes, cancer. The employer must make assessment regarding the possible risks to health arising from hazardous substances that are present in the workspace, and then decide suitable precautions needed for the workers to avoid the risk, they must control the storage and transporting and handing with the hazardous substances, and the most important thing is that the workers should be trained well how to deal with the hazardous substances in the workplace. 7 1.6. Control of noise at workplace Noise should be considered in the workplace, where the level of noise may be harmful to hearing, it may cause hearing damage, permanent humming or buzzing in the ears, loss of hearing, that can lead to disturbed sleep. Noise exposure values are related to the level of noise and the duration of exposure. Noise is measured in decibels, and to make it understandable, the levels of noise of known activities are shown in the following table: Table 1-1: noise levels for some known activities Noise source Noise level dB(A) Normal conversation 50-60 Loud radio 65-75 Busy street 78-85 Electric drill 87 Sheet metal shop 93 Circular saw 99 Hand grinding metal 108 Chain saw 115-120 Jet aircraft taking off 25m away 140 The noise exposure limit values are: - Daily or weekly exposure of 87 dB. - Peak sound pressure of 140 dB. In order to keep workers safe, the employer should do the following: 8 - Eliminate the sources of noise if possible. Use quieter equipment if possible. - Reduce the noise exposure that may be risky to the workers. According to the noise exposure limits, which must not be exceeded. - Use sound absorbing materials around the loud machines, to avoid noise transmitting to the workers or the public outside. - Limit the time spent in the noisy areas. - Provide hearing protection, which include earmuffs, earplugs … etc. 1.7. Control of vibration at work Vibration at workplace may be generated by the machinery used, which may be risky if it exceed the limits allowed, it may lead to loss of strength in hands, problems in nerve system, blood vessels, muscles and joints … etc. it may lead to hand-arm vibration syndrome. The daily exposure limit value is set to be of 5 𝑚/𝑠 2. But this may not be clear as it was for the noise levels, because the level of vibration is different from machine to another, and the period of exposure is different from work to another, so it is the duty of the employer to assess the risks, and decide the level of exposure based on the standards, and the manufacturer’s instructions. 1.8. Fire Fire is dangerous to the workers lives, workplace, and environment around. Fire led many times to catastrophic disasters leading to burning forests, death, serious burns and diseases due to the clouds of smoke resulted from the fire, so this this should be considered very seriously. Fire needs 3 things to start; oxygen, fuel and heat (source of ignition). And it may start because of cigarettes and 9 matches, faulty electrical equipment (such as incorrect wiring, damaged cables, overloading … etc.), sparks or heat generated from high speed metal contact in a faulty machine, performing a work that generates sparks and heat next to flammable substances such as welding, storing flammable liquids in contact with hot surfaces, piling rubbish unsafely. To prevent fire from starting or spreading, the following points have to be considered: - Obey safety instructions during work. - Follow the specific precautions for storing flammable substances. - Replace the substances that are used in work with ones that are nonflammable, if possible. Or at least minimize the quantities of flammable substances in workplace. - Use fire resistant floors, doors, roofs … in any place where fire is possible. - Get rid of waste safely, especially if it contain substances that may start fire under certain conditions such as batteries. - Firefighting system must be inserted in the workplace, and it must be checked regularly. and the number of fire extinguishers should be sufficient, and available at different places at the workplace. Fire are classified into six classes, as follows: - Class A fires: freely burning fires fuelled by ordinary combustible materials such as cloth, wood, paper and fabric. - Class B fires: fires fuelled by flammable liquids such as oils and petrol. - Class C fires: fires fuelled by flammable gases such as propane, butane and North Sea gas. - Class D fires: fires involving flammable metals such as Magnesium, Lithium or Aluminum powders or swarf. - Fires involving electrical hazards. - Class F fires: fires fuelled by cooking oils and fats. Types of fire extinguishers. 1. Water: color coded red, suitable for class A fires, rapid cooling 2. Water with additives: it contains water with special additives, that can be more effective in cooling and penetrating the fire, suitable for class A fires. 10 3. Spray foam: color coded red with cream color zone, suitable for class A and B fires, it smothers fire and prevent re-ignition of flammable vapors by sealing the surface of the material. 4. Dry powder: color coded red with blue color zone, suitable for class A,B and C fires, and for electrical hazards because it is non-conductive, it interferes with the combustion process and provides rapid fire knockdown. 5. Carbon dioxide (𝐶𝑂2 gas): color coded red with black color zone, suitable for class B fire and for electrical hazards, it replaces the air with carbon dioxide and extinguishes fire rapidly, it is suitable for computers, because 𝐶𝑂2 is non-toxic, non-corrosive, it is harmless to most delicate equipment. 6. Wet chemicals: color coded red with yellow color zone, especially designed for class F fires, suitable for class A fires, when it is applied to the burning liquid it cools and emulsifies the oil changing it into soap form, extinguishing the flame and sealing the surface to prevent re-ignition. Table 1-2:the suitable fire extinguisher according to the fire type. 11 1.9. Electrical hazards There is no work place that does not use electricity, it is essential for construction works, metal work, carpentry, metal forming works … etc. To use electricity safely many precautions should be followed, otherwise, hazards could happen such as burning, electrical shocks, explosions, fire … etc., those precautions include using well insulated cables, checking the machine to have earth connection, using the right amount of voltage, using the suitable type of wiring in order not to overload... etc. Human body could form a path of electricity to the ground or to any close metal subject. The chances of this increases when the man who operates the machine is wearing any conductive parts in his cloth, such as metal buttons, rings, metal watches … etc. moreover, when the cloth is wet or moisture is present on the surfaces, the chances of electrical chock increase. Anyone who uses electricity to carry out his work, should be aware of the nature of electricity and the basic concepts of electric circuit connections. Electricity is flow of electric charge, and this flow can’t happen generally unless if a closed path is available, this path is called electric circuit. The electric circuit is composed by live line, neutral line and earth line. If any of the live line or the neutral line is disconnected, then the electrical current is zero. The most dangerous line is the live line. If a human touches a line (naked wire) of a closed circuit, then a flow of charge will flow through his body to the ground, causing electric shock or burns depending on the energy transferred through the human body to the ground. (A lot of people say it is the current that kills, and I think it is a combination between current and voltage i.e. without difference in voltage there will be no current, so they work hand to hand in this). 12 To distinguish the lines, a color code was introduced; brown wire refers to live conductor, blue wire refers to neutral conductor and green/yellow wire refers to earth conductor. To ensure a safe usage, the following rules must be followed: - Make sure that the color code is used correctly, i.e. don’t use blue color for live conductor … etc. - Never insert any type of conductor inside the sockets, such as nails, wires… sometimes a match could conduct electricity especially if it was wet. - Make sure that the connections are secure, loose wire may make a short circuit. - Never remove the earth connection. Any external metallic part should be earthed. - Always use the correct cable (the suitable diameter) according to the current rating. - Use the correct fuse as first line of defense. Never replace it with a piece of wire. - Never use old cables. - Never make any adjustment to the tool while it is connected to the power source. - Always ask for a well-trained technician to perform maintenance to your equipment, never do it yourself unless you are well-trained to do so. If you saw a human body came in contact with an electrical conductor resulting in an electric shock, you should do the following: - Switch the power source off. - If switching off is not possible, stand on a dry non-conducting surface, pull or push the victim using any dry non-conductive cloth. And never touch the victim as you will be shocked by forming another path for the current to move through. - Once the casualty is away from the electric source, call the ambulance. Electricity may result in burns, shock, in extreme cases it may stop the heart from beating. 13 1.10. Safety signs and colors A set of standard signs and colors were set to show instructions, precautions, warnings or prohibitions. A prohibition sign is a sign prohibiting behavior likely to increase or cause danger, it is coded by red color. A warning sign is a sign giving a warning of a hazard or danger, it is coded by yellow color. A mandatory sign is a sign prescribing specific behavior, it is coded by blue color. Emergency escape or first-aid sign is a sign giving information on emergency exits, first aid or rescue facilities, it is coded by green color. The signs below are some of the usually used signs in the workplace, and you can find general safety signs in appendix A. Safe condition symbols Hazard warning symbols 14 Mandatory symbols 15 Prohibition symbols Fire equipment symbols 1.11. First aid As long as injuries may happen at the workplace, first aid should take place, especially when the hospital is far away from the workplace. Injuries may be simple and may be serious, for simple injuries a first aid is enough for the casualty, but for serious injuries hospital is essential, but first 16 aid could give the victim a chance of healing before reaching the hospital. Failing to apply first aid led sometimes to unwanted effects and difficulty in healing the injuries as they become more serious. To ensure the first aid is available, the employer should provide a first aid kit or stocked first aid box in the workplace, in addition to appointing one of the employees to take charge of the first aid arrangement, including calling the ambulance, taking care of the first aid kit, and deliver a first aid training course to one of the employees if possible (even though it is not essential, but it would solve a serious problem especially in rural areas). 1.12. The reporting of injuries: Some of the accidents that happen at the workplace should be reported to the related authorities, according to regulations in the state. The most relevant authorities in Palestine are the civil defense department and the health department. But not all accidents should be reported, the accidents that must be reported are those related to the work, and those that result in injuries which are reportable; which are: - Death arising from work relating accident. - Specific injuries including fracture, amputation, loss of sight, crush injuries, serious burns. - Injuries involving members of the public not at work, injured due to accident related to work, and those people were taken to hospital. - Any disease related to work, such as asthma, sever cramp of hand, hand-arm vibration syndrome, dermatitis …etc. 17 1.13. Some protective equipment. 18 2. Hand processes Hand tools are used to remove small amounts of material, usually from small areas of the work piece. This may be done because no machine is available, the work piece is too large to go on a machine, the shape is too intricate or simply that it would be too expensive to set up a machine to do the work. Since the use of hand tools is physically tiring, it is important that the amount of material to be removed by hand is kept to an absolute minimum and that the correct tool is chosen for the task. Wherever possible, use should be made of the available powered hand tools, not only to reduce fatigue but also to increase the speed of the operation and so reduce the cost. 2.1. Engineer’s files Files are used to perform a wide variety of tasks, from simple removal of sharp edges to producing intricate shapes where the use of a machine is impracticable. They can be obtained in a variety of shapes and in lengths from 150 mm to 350 mm. When a file has a single series of teeth cut across its face it is known as single-cut file, and with two sets of teeth cut across its face it is known as double-cut file, Fig. 2.1. The grade of cut of a file refers to the spacing of the teeth and determines the coarseness or smoothness of the file. Three standard grades of cut in common use, from coarsest to smoothest, are bastard, second cut and smooth. In general, the bastard cut is used for rough filing to remove the most material in the shortest time, the second cut to bring the work close to finished size and the smooth cut to give a good finish to the surface while removing the smallest amount of material. Figure 2.1: single-cut and double-cut files 2.1.1. File identification Files are identified either by their general shape – i.e. hand, flat or pillar – or by their cross-section – i.e. square, three-square, round, half-round or knife – Fig. 2.2. 19 Figure 2.2: Types of files 2.1.1.1. Hand file The hand file is for general use, typically on flat surfaces. It is rectangular in cross-section, parallel in width along its length, but tapers slightly in thickness for approximately the last third of its length towards the point. It is double cut on both faces, single-cut on one edge and is plain on the second edge. The plain edge with no teeth is known as the ‘safe’ edge and is designed to file up to the edge of a surface without damaging it. The taper in thickness enables the file to enter a slot slightly less than its full thickness. 2.1.1.2. Pillar file This file has the same section as a hand file but of a thinner section. It is used for narrow slots and keyways. 2.1.1.3. Flat file The flat file is also for general use, typically on flat surfaces. It is rectangular in cross-section and tapers in both width and thickness for approximately the last third of its length towards the point. Both faces are double-cut and both edges single-cut. The tapers in width and thickness enable this file to be used in slots which are narrower than its full width and thickness and which require filing on length and width. 2.1.1.4. Square file The square file is of square cross-section, parallel for approximately two-thirds of its length, then tapering towards the point. It is double-cut on all sides. This file is used for filing keyways, slots and the smaller square or rectangular holes with 90° sides. 20 2.1.1.5. Three-square file The three-square or triangular file has a 60° triangle cross-section, parallel for approximately two- thirds of its length, then tapering towards the point. The three faces are double-cut and the edges sharp. This file is used for surfaces which meet at less than 90°, angular holes and recesses. 2.1.1.6. Round file The round file is of circular cross-section, parallel for approximately two-thirds of its length and then tapering towards the point. Second-cut and smooth files are single-cut, while the bastard is double-cut. This file is used for enlarging round holes, elongating slots and finishing internal round corners. 2.1.1.7. Half-round file The half-round file has one flat and one curved side. It is parallel for approximately two-thirds of its length, then tapers in width and thickness towards the point. The flat side is double-cut and the curved side is single-cut on second-cut and smooth files. This is an extremely useful double- purpose file for flat surfaces and for curved surfaces too large for the round file. 2.1.1.8. Knife file The knife file has a wedge-shaped cross-section, the thin edge being straight while the thick edge tapers to the point in approximately the last third of its length. The sides are double-cut. This file is used in filing acute angles. 2.1.1.9. Dreadnought file When soft material is being filed, the material is more readily removed and the teeth of an engineer’s file quickly become clogged. When this happens, the file no longer cuts but skids over the surface. This results in constant stoppages to clear the file so that it again cuts properly. To overcome the problem of clogging, files have been developed which have deep curved teeth milled on their faces and these are known as dreadnought files, Fig. 2.3. Figure 2.3: Dreadnought file These files are designed to remove material faster and with less effort, since the deep curved teeth produce small spiral filings which clear themselves from the tooth and so prevent clogging. Their principal use is in filing soft materials such as aluminum, lead, white metal, copper, bronze and brass. They can also be used on large areas of steel, as well as on non-metallic materials such as plastics, wood, fiber and slate. 21 This type of file is available as hand, flat, half-round and square, from 150 mm to 400 mm long. The available cuts are broad, medium, standard, fine and extra fine. 2.1.1.10. Needle file Needle files are used for very fine work in tool making and fitting, where very small amounts of material have to be removed in intricate shapes or in a confined space. This type of file is available from 120 mm to 180 mm long, of which approximately half is file-shaped and cut, the remainder forming a slender circular handle, Fig. 2.4. Figure 2.4: Needle file 2.1.2. Filing One of the greatest difficulties facing the beginner is to produce a filed surface which is flat. By carefully observing a few basic principles and carrying out a few exercises, the beginner should be able to produce a flat surface. Filing is a two-handed operation, and the first stage is to grip the file correctly. The handle is gripped in the palm of the right hand with the thumb on top and the palm of the left hand resting at the point of the file. Having gripped the file correctly, the second stage is to stand correctly at the vice. The left foot is placed well forward to take the weight of the body on the forward stroke. The right foot is placed well back to enable the body to be pushed forward. Remember that the file cuts on the forward stroke and therefore the pressure is applied by the left hand during the forward movement and is released coming back. Do not lift the file from the work on the back stroke, as the dragging action helps clear the filings from the teeth and also prevents the ‘see-saw’ action which results in a surface which is curved rather than flat. Above all, take your time – long steady strokes using the length of the file will remove metal faster and produce a flatter surface than short rapid strokes. As already stated, a smooth-cut file is used to give a good finish to the surface while removing small amounts of material. An even finer finish to the surface can be achieved by a method known as draw-filing. With this method, the file, rather than being pushed across, is drawn back and forth along the surface at right angles to its normal cutting direction. An even finer finish can be obtained using abrasive cloth supported by the file to keep the surface flat. Abrasive cloth is available on rolls 25 mm wide, in a variety of grit sizes from coarse to fine. By supporting the cloth strip on the underside of the file and using a traditional filing stroke, extremely fine surface finishes can be obtained while removing very small amounts of material. This process is more of a polishing operation. 22 2.1.3. Care of files A file which cuts well saves you extra work. It is important, therefore, that all the teeth are cutting. Never throw files on top of each other in a drawer, as the teeth may be chipped. Never knock the file on its edge to get rid of filings in the teeth – use a file brush. A file brush should be used regularly to remove filings from the teeth, as failure to do so will cause scratching of the work surface and inefficient removal of metal. Always clean the file on completion of the job before putting it away. Do not exert too much pressure when using a new file, or some of the teeth may break off due to their sharpness – work lightly until the fine tooth points are worn slightly. For the same reason, avoid using a new file on rough surfaces of castings, welds or hard scale. Always use a properly fitted handle of the correct size – on no account should a file be used without a handle or with a handle which is split; remember, one slip and the tang could pierce your hand. 2.2. The hacksaw The hacksaw is used to cut metal. Where large amounts of waste metal have to be removed, this is more easily done by hacksawing away the surplus rather than by filing. If the work piece is left slightly too large, a file can then be used to obtain the final size and surface. The hacksaw blade fits into a hacksaw frame on two holding pins, one of which is adjustable in order to tension the blade. The hacksaw frame should be rigid, hold the blade in correct alignment, tension the blade easily and have a comfortable grip. The blade is fitted to the frame with the teeth pointing away from the handle, Fig. 2.5, and is correctly tensioned by turning the wing nut to take up the slack and then applying a further three turns only. A loose blade will twist or buckle and not cut straight, while an over tightened blade could pull out the ends of the blade. Figure 2.5: Hacksaw The standard hacksaw blade is 300 mm long × 13 mm wide × 0.65 mm thick and is available with 14,18, 24 and 32 teeth per 25 mm, i.e. for every 25 mm length of blade there are 14 teeth, 18 teeth and so on. 23 A hacksaw blade should be chosen to suit the type of material being cut, whether hard or soft, and the nature of the cut, whether thick section or thin. Two important factors in the choice of a blade are the pitch, or distance between each tooth and the material from which the blade is made. When cutting soft metals, more material will be cut on each stroke and this material must have somewhere to go. The only place the material can go is between the teeth, and therefore if the teeth are further apart there is more space for the metal being cut. The largest space is in the blade having the least number of teeth, i.e. 14 teeth per 25 mm. The opposite is true when cutting harder metals. Less material will be removed on each stroke, which will require less space between each tooth. If less space is required, more teeth can be put in the blade, more teeth are cutting and the time and effort in cutting will be less. When cutting thin sections such as plate, at least three consecutive teeth must always be in contact with the metal or the teeth will straddle the thin section. The teeth will therefore have to be closer together, which means more teeth in the blade, i.e. 32 teeth per 25 mm. Like a file, the hacksaw cuts on the forward stroke, which is when pressure should be applied. Pressure should be released on the return stroke. Do not rush but use long steady strokes (around 70 strokes per minute when using high-speed-steel blades). The same balanced stance should be used as for filing. Table 2.1 gives recommendations for the number of teeth per 25 mm on blades used for hard and soft materials of varying thickness. Table 2-1: selection of hacksaw blades Material thickness (mm) No. of teeth per 25 mm Hard material Soft material Up to 3 32 32 3 to 6 24 24 6 to 13 24 18 13 to 25 18 14 Three types of hacksaw blade are available; all-hard, flexible and bimetal: All hard – this type is made from hardened high-speed steel. Due to their all-through hardness, these blades have a long blade life but are also very brittle and are easily broken if twisted during sawing. For this reason they are best suited to the skilled user. Flexible – this type of blade is also made from high-speed steel, but with only the teeth hardened. This results in a flexible blade with hard teeth which is virtually unbreakable and can therefore be used by the less experienced user or when sawing in an awkward position. The blade life is reduced due to the problem of fully hardening the teeth only. 24 Bimetallic – this type of blade consists of a narrow cutting-edge strip of hardened high-speed steel joined to a tough alloy-steel back by electron beam welding. This blade combines the qualities of hardness of the all-hard blade and the unbreakable qualities of the flexible blade, resulting in a shatterproof blade with long life and fast-cutting properties. 2.3. Cold chisels Cold chisels are used for cutting metal. They are made from high-carbon steel, hardened and tempered at the cutting end. The opposite end, which is struck by the hammer, is not hardened but is left to withstand the hammer blows without chipping. 2.3.1. Cold chisel classification Cold chisels are classified as ‘flat’ or ‘cross-cut’, according to the shape of the point. 2.3.1.1. Flat This chisel has a broad flat point and is used to cut thin sheet metal, remove rivet heads or split corroded nuts. The cutting edge is ground to an angle of approximately 60°, Fig. 2.6. Figure 2.6: 'Flat' cold chisel 2.3.1.2. Cross-cut This chisel has a narrower point than the flat chisel and is used to cut keyways, narrow grooves, square corners and holes in sheet metal too small for the flat chisel, Fig. 2.7. Figure 2.7: 'Cross-cut' cold chisel 25 2.3.2. Using the chisel When using a cold chisel on sheet-material, great care must be taken not to distort the metal. To prevent distortion, the sheet must be properly supported. A small sheet is best held in a vice, Fig. 2.8. A large sheet can be supported by using two metal bars securely clamped, Fig. 2.9. Figure 2.8: sheet metal in vice Figure 2.9: sheet metal in support bars To remove a section from the center of a plate, the plate can be supported on soft metal. It is best to mark out the shape required, drill a series of holes in the waste material, and use the chisel to break through between the holes, Fig. 2.10. 26 Figure 2.10: chisel cutting a hole supported on soft metal plate The chisel should be held firmly but not too tight, and the head should be struck with sharp blows from the hammer, keeping your eye on the cutting edge, not the chisel head. Hold the chisel at approximately 40°, Fig. 2.11. Do not hold the chisel at too steep an angle, otherwise it will tend to dig into the metal. Too shallow an angle will cause the chisel to skid and prevent it cutting. Use a hammer large enough to do the job, grasping it well back at the end of the handle, not at the end nearest the head. Never allow a large ‘mushroom’ head to form on the head of a chisel, as a glancing blow from the hammer can dislodge a chip which could fly off and damage your face or hand. Always grind off any sign of a mushroom head as it develops, Fig. 2.12. Figure 2.11: correct angle of chisel Figure 2.12: correct chisel Cold chisels can be sharpened by regrinding the edge on an off-hand grinder. When re-sharpening, do not allow the chisel edge to become too hot, otherwise it will be tempered, lose its hardness and be unable to cut metal. 27 2.4. Engineer’s hammers The engineer’s hammer consists of a hardened and tempered steel head, varying in mass from 0.1 kg to about 1 kg, firmly fixed on a tough wooden handle, usually hickory or ash. The flat striking surface is known as the face, and the opposite end is called the pein. The most commonly used is the ball-pein, Fig. 2.13, which has a hemispherical end and is used for riveting over the ends of pins and rivets. Figure 2.13: Ball-pein hammer For use with soft metal such as aluminum or with finished components where the work piece could be damaged if struck by an engineer’s hammer, a range of hammers is available with soft faces, usually hide, copper or a tough plastic such as nylon. The soft faces are usually in the form of replaceable inserts screwed into the head or into a recess in the face, Fig. 2.14. Figure 2.14: Soft-faced hammers Always use a hammer which is heavy enough to deliver the required force but not too heavy to be tiring in use. The small masses, 0.1 kg to 0.2 kg, are used for center punching, while the 1 kg ones are used with large chisels or when driving large keys or collars on shafts. The length of the handle is designed for the appropriate head mass, and the hammer should be gripped near the end of the handle to deliver the required blow. To be effective, a solid sharp blow should be delivered and this cannot be done if the handle is held too near the hammer head. Always ensure that the hammer handle is sound and that the head is securely fixed. 2.5. Screwdrivers The screwdriver is one of the most common tools, and is also the one most misused. Screwdrivers should be used only to tighten or loosen screws. They should never be used to chisel, open tins, 28 scrape off paint or lever off tight parts such as collars on shafts. Once a screwdriver blade, which is made from toughened alloy steel, has been bent, it is very difficult to keep it in the screw head. There are a number of different head drives. The four most common are slotted, Phillips, Pozidriv and Torx as shown in Fig. 2.15. Always select the screwdriver to suit the size and type of head drive. Use of the incorrect size or type results in damage to both the screwdriver and the screw head, leading to a screw that is very difficult to loosen or tighten. Figure 2.15 : Screw head drivers Phillips and Pozidriv are numbered with 1, 2 and 3 the most common. Torx cover a range numbered T5 to T55 from smallest to the largest. Straight slots in screws are machined with parallel sides. It is essential that any screwdriver used in such a slot has the sides of the blade parallel to slightly tapered up to about 10°, Fig. 2.16(a). A screwdriver sharpened to a point like a chisel will not locate correctly and will require great force to keep it in the slot, Fig. 2.16(b). Various blade lengths are available with corresponding width and thickness to suit the screw size. Figure 2.16: screwdriver point (a) right (b) wrong In the interests of personal safety, never hold the work in your hand while tightening or loosening a screw – the blade may slip and cause a nasty injury. Always hold the work securely in a vice or clamped to a solid surface. 29 2.6. Taps Tapping is the operation of cutting an internal thread by means of a cutting tool known as a tap. When tapping by hand, straight-flute hand taps are used. These are made from hardened high- speed steel and are supplied in sets of three. The three taps differ in the length of chamfer at the point, known as the lead. The one with the longest lead is referred to as the taper or first tap, the next as the second or intermediate tap and the third, which has a very short lead, as the bottoming or plug tap, Fig. 2.17. A square is provided at one end so that the tap can be easily rotated by holding it in a tap wrench, Fig. 2.18. The chuck type of wrench is used for the smaller tap sizes. The first stage in tapping is to drill a hole of the correct size. This is known as the tapping size and is normally slightly larger than the root diameter of the thread. Table 2.2 shows the tapping sizes for ISO metric threads which have replaced most threads previously used in Great Britain. Figure 2.17: set of taps Figure 2.18: Tap wrenches 30 Table 2-2: Tapping sizes for ISO metric threads Thread diameter and pitch (mm) Drill diameter for tapping (mm) 1.6 × 0.35 1.25 2 × 0.4 1.6 2.5 × 0.45 2.05 3 × 0.5 2.5 4 × 0.7 3.3 5 × 0.8 4.2 6 × 1.0 5.0 8 × 1.25 6.8 10 × 1.5 8.5 12 × 1.75 10.2 Tapping is then started using the taper or first tap securely held in a tap wrench. The long lead enables it to follow the drilled hole and keep square. The tap is rotated, applying downward pressure until cutting starts. No further pressure is required, since the tap will then screw itself into the hole. The tap should be turned back quite often, to help clear chips from the flutes. If the hole being tapped passes through the component, it is only necessary to repeat the operation using the second or intermediate tap. Where the hole does not pass through – known as a blind hole – it is necessary to use the plug or bottoming tap. This tap has a short lead and therefore forms threads very close to the bottom of the hole. When tapping a blind hole, great care should be taken not to break the tap. The tap should be occasionally withdrawn completely and any chips be removed before proceeding to the final depth. For easier cutting and the production of good quality threads, a proprietary tapping compound should be used. 2.7. Dies Dies are used to cut external threads and are available in sizes up to approximately 36 mm thread diameter. The common type, for use by hand, is the circular split die, made from high-speed steel hardened and tempered and split at one side to enable small adjustments of size to be made, Fig. 2.19. 31 Figure 2.19: Circular split die The die is held in a holder known as a die stock, which has a central screw for adjusting the size and two side locking screws which lock in dimples in the outside diameter of the die, Fig. 2.20. The die is inserted in the holder with the split lined up with the central screw. The central screw is then tightened so that the die is expanded, and the two side locking screws are tightened to hold the die in position. Figure 2.20: Die holder Dies have a lead on the first two or three threads, to help start cutting, but it is usual also to have a chamfer on the end of the component. The die is placed squarely on the end of the bar and is rotated, applying downward pressure until cutting starts, ensuring that the stock is horizontal. No further pressure is required, since the die then screws itself forward as cutting proceeds. The die should be rotated backwards every two or three turns, to break up and clear the chips. The thread can now be checked with a nut. If it is found to be tight, the central screw is slackened, the side locking screws are tightened equally, and the die is run down the thread again. This can be repeated until the final size is reached. As with tapping, easier cutting and better threads are produced when a proprietary cutting compound is used. Die nuts are also available and are generally used for reclaiming or cleaning up existing threads, not for cutting a thread from solid. 2.8. Power hand tools The main advantages of powered hand tools are the reduction of manual effort and the speeding up of the operation. The operator, being less fatigued, is able to carry out the task more efficiently, and the speeding up of the operation results in lower production costs. Being portable, a powered hand tool can be taken to the work, which can also lead to a reduction in production costs. Accuracy 32 of metal-removal operations is not as good with powered hand tools, since it is difficult to remove metal from small areas selectively. A comparison of hand and powered hand tools is shown in Table 2.3. Table 2-3: comparison of hand and powered hand tools Speed of Cost of tool accuracy Fatigue production Hand tools Low Low High High Powered hand High High Low Low tools Powered hand tools can be electric or air operated. In general, electric tools are heavier than the equivalent air tool, due to their built-in motor, e.g. electric screwdrivers weigh 2 kg while an equivalent air-operated screwdriver weighs 0.9 kg. The cost of powered tools is much greater than the equivalent hand tools and must be taken into account when a choice has to be made. Air-operated tools can be safely used in most work conditions, while electrical tools should not be used in conditions which are wet or damp or where there is a risk of fire or explosion, such as in flammable or dusty atmospheres. A selection of air-operated tools is shown in Fig. 2.21. 2.8.1. Hand drills Electric and air-operated drills are available with a maximum drilling capacity in steel of about 30 mm diameter for electric and about 10 mm diameter for air models. Air-operated tools are more ideally suited to the rapid drilling of the smaller diameter holes, Fig. 2.21(a). 2.8.2. Screwdriver Used for inserting screws of all types, including machine, self-tapping, self-drilling and tapping and wood screws. Some models are reversible and can be used with equal ease to remove screws. The tool bits are interchangeable to suit the different screw-head types, such as slotted, ‘supadriv’, ‘pozidriv ’, hexagon-socket or hexagon-headed. Electric and air-operated screwdrivers are available with a maximum capacity of about 8 mm diameter thread with a variety of torque settings to prevent the screw being over tightened or sheared off, Fig. 2.21(b). 33 Figure 2.21: Air-operated tools (a) hand drill (b) screwdriver (c) impact wrench (d) grinder (e) metal shears (f) hammer 34 2.8.3. Impact wrench Used for tightening and also, with the reversal mechanism, for loosening hexagon-headed nuts and screws. Air-powered models are available with a maximum capacity of 32 mm diameter threads and with torque settings to suit a range of thread sizes. They have the advantage of being able to tighten all nuts or screws to the same predetermined load, Fig. 2.21(c). 2.8.4. Grinder Used to remove metal from the rough surfaces of forgings, castings and welds usually when the metal is too hard or the amount to be removed is too great for a file or a chisel. Electric and air- operated grinders are available with straight grinding wheels up to 230 mm diameter or with small mounted points of various shapes and sizes, Fig. 2.21(d). 2.8.5. Metal shears Used to cut metal, particularly where the sheet cannot be taken to a guillotine or where profiles have to be cut. Electric or air-operated shears are available capable of cutting steel sheet up to 2 mm thick by means of the scissor-like action of a reciprocating blade, Fig. 2.21(e). 2.8.6. Hammer Can be fitted with a wide range of attachments for riveting, shearing off rivet heads, removing scale or panel cutting. Air-operated models are available which deliver between 3000 and 4000 blows per minute, Fig. 2.21(f). 35 3. Measuring equipment Some form of precise measurement is necessary if parts are to fit together as intended no matter whether the parts were made by the same person, in the same factory, or in factories a long way apart. Spare parts can then be obtained with the knowledge that they will fit a part which was perhaps produced years before. To achieve any degree of precision, the measuring equipment used must be precisely manufactured with reference to the same standard of length. That standard is the meter, which is now defined using lasers. Having produced the measuring equipment to a high degree of accuracy, it must be used correctly. You must be able to assess the correctness of size of the work piece by adopting a sensitive touch or ‘feel’ between the instrument and work piece. This ‘feel’ can be developed only from experience of using the instrument, although some instruments do have an aid such as the ratchet stop on some micrometers. Having the correct equipment and having developed a ‘feel’, you must be capable of reading the instrument to determine the work piece size. It is here that the two main standard types of length-measuring instrument differ: the micrometer indicates the linear movement of a rotating precision screw thread, while the vernier instruments compare two scales which have a small difference in length between their respective divisions. 3.1. Vernier instruments All instruments employing a vernier consist of two scales: one moving and one fixed. The fixed scale is graduated in millimeters, every 10 divisions equaling 10 mm, and is numbered 0, 1, 2, 3, 4 up to the capacity of the instrument. The moving or vernier scale is divided into 50 equal parts which occupy the same length as 49 divisions or 49 mm on the fixed scale (see Fig. 3.3). This means that the distance between each graduation on the vernier scale is 40/50 mm = 0.98 mm, or 0.02 mm less than each division on the fixed scale (see Fig. 3.4(a)). Figure 3.1: vernier caliper If the two scales initially have their zeros in line and the vernier scale is then moved so that its first graduation is lined up with a graduation on the fixed scale, the zero on the vernier scale will have moved 0.02 mm (Fig. 3.4(b)). If the second graduation is lined up, the zero on the vernier scale 36 will have moved 0.04 mm (Fig. 3.4(c)) and so on. If graduation 50 is lined up, the zero will have moved 50 × 0.02 = 1 mm. Figure 3.2: Vernier scale Figure 3.3: Vernier scale readings Since each division on the vernier scale represents 0.02 mm, five divisions represent 5 × 0.02 = 0.1 mm. Every fifth division on this scale is marked 1 representing 0.1 mm, 2 representing 0.2 mm and so on (Fig. 3.3). To take a reading, note how many millimeters the zero on the vernier scale is from zero on the fixed scale. Then note the number of divisions on the vernier scale from zero to a line which exactly coincides with a line on the fixed scale. In the reading shown in Fig. 3.5(a) the vernier scale has moved 40 mm to the right. The eleventh line coincides with a line on the fixed scale, therefore 11 × 0.02 = 0.22 mm is added to the reading on the fixed scale, giving a total reading of 40.22 mm. Similarly, in Fig. 3.5(b) the vernier scale has moved 120 mm to the right plus 3 mm and the sixth line coincides, therefore, 6 × 0.02 = 0.12 mm is added to 123 mm, giving a total of 123.12 mm. 37 Figure 3.4: Vernier readings It follows that if one part of a measuring instrument is attached to the fixed scale and another part to the moving scale, we have an instrument capable of measuring to 0.02 mm. 3.1.1. Vernier caliper The most common instrument using the above principle is the vernier caliper (see Fig. 3.6). Because of its capability of external, internal, step and depth measurements (Fig. 3.7), its ease of operation and wide measuring range, the vernier caliper is possibly the best general purpose measuring instrument. They are available in a range of measuring capacities from 0–150 mm to 0–1000 mm. Figure 3.5: Vernier caliper Figure 3.6: External, internal, step and depth 38 To take a measurement, slacken both locking screws A and B (Fig. 3.7). Move the sliding jaw along the beam until it contacts the surface of the work being measured. Tighten locking screw B. Adjust the nut C until the correct ‘feel ’ is obtained, then tighten locking screw A. Re-check ‘ feel ’ to ensure that nothing has moved. When you are satisfied, take the reading on the instrument. Jaws may be carbide tipped for greater wear resistance. Dial calipers (Fig. 3.8) are a form of vernier caliper where readings of 1 mm steps are taken from the vernier beam and subdivisions of this are read direct on a dial graduated in 0.02 mm divisions. Conventional digital calipers (Fig. 3.9) make use of a basic binary system having a series of light and dark bands under the slider and count these as they move along the track. There is no way the system can tell where the slider is, it purely depends on storing the number of bands passed over. Because of this, the jaws must first be closed and the display zeroed to reset the binary system before it starts counting. Digital calipers are now available which can read the slider location at any position without the need to reset zero. This type of caliper makes use of three sensors within the slider and three corresponding precision tracks embedded in the main beam. Figure 3.7: Vernier caliper adjustment Figure 3.8: Dial caliper 39 Figure 3.9: Digital caliper As the slider moves it reads the position of the tracks under the sensors and calculates its current absolute position. This eliminates the need for having to reset the caliper to zero before use. An inch/metric conversion is built into the microprocessor so that a measurement in the desired units can be made. An additional benefit is that the zero point can be set anywhere within the instrument range. With this ability the instrument can be used as a comparator to determine if the work piece is above or below zero and by how much. The resolution of these instruments is 0.01 mm/0.0005”. Although the vernier caliper is extremely versatile in its ability to carry out external, internal, step and depth measurements, calipers with special jaws are available for special purpose applications, a few of which are shown in Fig. 3.10. Figure 3.10: Caliper special jaws a. Point jaw type – for uneven surface measurement. b. Offset jaw type – for stepped feature measurement. c. Neck type – for outside diameter measurement such as behind a shrouded recess. d. Tube thickness type – for tube or pipe wall thickness measurement. 40 3.1.1.1. Notes on using a vernier caliper 1. Before use, clean the measuring surfaces by gripping a piece of clean paper (copier paper is ideal) between the jaws and slowly pull it out. Ensure the instrument reads zero before taking a measurement. Always ensure digital models are zeroed. 2. Wipe sliding surfaces before use. 3. Look straight at the vernier graduations when making a reading. If viewed from an angle, an error of reading can be made due to parallax effect (Fig. 3.11). Figure 3.11: Parallax error 4. Ensure the surface to be measured is clean and free of swarf or grit before taking a measurement. 5. Do not use excessive force when taking a measurement. 6. Do not measure a work piece at a position near the outer end of the jaws. Move the work piece as close to the beam as possible to ensure greatest accuracy of measurement. 7. Keep jaws square/parallel with surface being measured to ensure greatest accuracy. 8. For internal measurement, ensure the inside jaws are inserted as deeply as possible before taking a measurement. 9. Be careful not to drop or bang the caliper such as would cause damage to the instrument. 10. Caliper jaws are very sharp so handle with care to avoid personal injury. 11. Always store the instrument in a clean place when not in use, preferably in a case or box. 12. Do not leave the caliper jaws clamped together when not in use. Always leave a gap between the measuring faces (say 1 mm). 41 3.2. Micrometers The micrometer relies for its measuring accuracy on the accuracy of the spindle screw thread. The spindle is rotated in a fixed nut by means of the thimble, which opens and closes the distance between the ends of the spindle and anvil (see Fig. 3.12). The pitch of the spindle thread, i.e. the distance between two consecutive thread forms, is 0.5 mm. This means that, for one revolution, the spindle and the thimble attached to it will move a longitudinal distance of 0.5 mm. Figure 3.12: External micrometer On a 0–25 mm micrometer, the sleeve around which the thimble rotates has a longitudinal line graduated in mm from 0 to 25 mm on one side of the line and subdivided in 0.5 mm intervals on the other side of the line. The edge of the thimble is graduated in 50 divisions numbered 0, 5, 10, up to 45, then 0. Since one revolution of the thimble advances the spindle 0.5 mm, one graduation on the thimble must equal 0.5 ÷ 50 mm = 0.01 mm. A reading is therefore the number of 1 mm and 0.5 mm divisions on the sleeve uncovered by the thimble plus the hundredths of a millimeter indicated by the line on the thimble coinciding with the longitudinal line on the sleeve. In the reading shown in Fig. 3.13(a), the thimble has uncovered 9 mm on the sleeve. The thimble graduation lined up with the longitudinal line on the sleeve is 44 = 44 × 0.01 = 0.44. The total reading is therefore 9.44 mm. Similarly, in Fig. 3.13(b) the thimble has uncovered 16 mm and 0.5 mm and the thimble is lined up with graduation 27 = 27 × 0.01 = 0.27 mm, giving a total reading of 16.77 mm. Greater accuracy can be obtained with external micrometers by providing a vernier scale on the sleeve. The vernier consists of five divisions on the sleeve, numbered 0, 2, 4, 6, 8, 0, these occupying the same space as nine divisions on the thimble (Fig. 3.14(a)). Each division on the 42 vernier is therefore equal to 0.09 ÷ 5 = 0.018 mm. This is 0.002 mm less than two divisions on the thimble. Figure 3.13: Micrometer readings Figure 3.14: Vernier micrometer readings To take a reading from a vernier micrometer, note the number of 1 mm and 0.5 mm divisions uncovered on the sleeve and the hundredths of a millimeter on the thimble as with an ordinary micrometer. You may find that the graduation on the thimble does not exactly coincide with the longitudinal line on the sleeve, and this difference is obtained from the vernier. Look at the vernier and see which graduation coincides with a graduation on the thimble. If it is the graduation marked 2, then add 0.002 mm to your reading (Fig. 3.14(b)); if it is the graduation marked 4, then add 0.004 mm (Fig. 3.14(c)) and so on. External micrometers with fixed anvils are available with capacities ranging from 0–13 mm to 575–600 mm. External micrometers with interchangeable anvils (Fig. 3.15) provide an extended range from two to six times greater than the fixed-anvil types. The smallest capacity is 0–50 mm and the largest 900–1000 mm. To ensure accurate setting of the interchangeable anvils, setting gauges are supplied with each instrument. Spindle and anvil measuring faces may be carbide tipped for greater wear resistance. 43 Figure 3.15: External micrometer with interchangeable anvils Micrometers are available which give a direct mechanical readout on a counter (Fig. 3.16). The smallest model of 0–25 mm has a resolution of 0.001 mm and the largest of 125–150 mm a resolution of 0.01 mm. Modern digital external micrometers (Fig. 3.17) use sensors and detectors. A tiny disc encoder is connected to and rotates with the spindle along the entire range of the spindle axis, while a detector remains stationary to count the rotational signals and send data to a microprocessor and shows as a reading on an LCD. Figure 3.16: Mechanical counter micrometer Figure 3.17: Digital external micrometer 44 An inch/metric conversion is built into the microprocessor so that a measurement in the desired units can be made. An additional benefit is that the zero point can be set anywhere within the micrometers range. With this ability the instrument can be used as a comparator to determine if the work piece is above or below zero and by how much. The resolution of these instruments is 0.001 mm/0.00005”. Standard micrometers are good for measuring flat and parallel features and outside diameters. However, if the feature to be measured is curved, as in the wall thickness of a tube, an accurate measurement cannot be achieved. In this case a ball or roller can be placed on the inside wall of the tube and a measurement taken (Fig. 3.18). By subtracting the diameter of the ball or roller, the wall thickness can be established. With a digital micrometer, the diameter of the ball or roller can be measured, the micrometer zeroed, and the measurement then made will be the wall thickness shown directly without the need for subtraction. For this type of measurement, an accessory is available called a ‘ball attachment’, which is a ball (usually 5 mm diameter) held in a rubber boot to hold it safely in place on the micrometer anvil while a measurement is taken. Figure 3.18: Measuring tube wall thickness In order to facilitate a wide range of measuring requirements, micrometers are available with a variety of anvils for special purpose applications, a few of which are shown in Fig. 3.19: a. Blade micrometer – for measuring diameter in a narrow groove (the spindle is non- rotational). b. Tube micrometer – for measuring the wall thickness of a tube (anvil has a spherical surface). c. Spline micrometer – for measuring spline-root shaft diameter. d. Disc type micrometer – for measuring spur and helical gear root tangent. 45 Figure 3.19: Micrometer special anvils Micrometers with an adjustable measuring force are available for applications requiring a constant low measuring force such as measuring thin wire, paper, plastics or rubber parts which avoids distortion of the work piece. These are available in a range from 0–10 mm to 20–30 mm. 3.2.1. Notes on using a micrometer i. Before use, clean the measuring surfaces by gripping a piece of clean paper (normal copier paper is ideal) between the anvil and the spindle and slowly pull it out. Ensure the instrument reads zero before taking a measurement. Always ensure digital models are zeroed. ii. Look straight at the index line when making a reading. If viewed from an angle, an error of reading can be made due to parallax effect (Fig. 3.12). iii. Ensure the surface being measured is clean and free of swarf and grit before taking a measurement. iv. Do not rotate the thimble using excessive force. v. Use the ratchet device, if available, to ensure a consistent measuring force. vi. Be careful not to drop or bang the micrometer such as would cause damage to the instrument. vii. Always store the instrument in a clean place when not in use, preferably in a case or box. viii. Never leave the measuring faces clamped together when not in use. Always leave a gap between the measuring faces (say 1 mm). 46 4. Joining methods Some method of joining parts together is used throughout industry, to form either a complete product or an assembly. The method used depends on the application of the finished product and whether the parts have to be dismantled for maintenance or replacement during service. There are five methods by which parts may be joined:  mechanical fasteners – screws, bolts, nuts,  rivets;  soldering;  brazing;  welding;  adhesive bonding. Mechanical fasteners are most widely used in applications where the parts may need to be dismantled for repair or replacement. This type of joint is known as non-permanent. The exception would be the use of rivets, which have to be destroyed to dismantle the parts and so form a permanent joint. Welding and adhesives are used for permanent joints which do not need to be dismantled – any attempt to do so would result in damage to or destruction of the joints and parts. Although soldered and brazed joints are considered permanent, they can be dismantled by heating for repair and replacement. 4.1. Mechanical fasteners Mechanical fasteners can be made from many materials but most bolts, nuts and washers are made from carbon steel, alloy steel, or stainless steel depending upon their industrial use. Carbon steel is the cheapest and most common for general use. To prevent corrosion, mechanical fasteners may be plated or coated in some way, again depending on the application. The most common surface treatments are zinc, nickel and cadmium. Phosphate coatings are also used but have limited corrosion resistance. 4.1.1. Machine screws These are used for assembly into previously tapped holes and are manufactured in brass, steel, stainless steel and plastics (usually nylon) and threaded their complete length. Various head shapes are available, as shown in Fig. 4.1. Depending on the style, thread diameters are generally available up to 10 mm, with lengths up to 50 mm. For light loading conditions where space is limited, a headless variety known as a grub screw is available. A typical application would be to retain a knob or collar on a shaft. Figure 4.1: Types of screw head 47 Although Fig. 4.1 shows head types with slotted head drives, these screws are available with a variety of head drives as outlined on page 39 with Phillips, Pozidriv and Torx the most common. 4.1.2. Socket screws Manufactured in high-grade alloy steel with rolled threads, this type of screw is used for higher strength applications than machine screws. Three head shapes are available, all of which contain a hexagon socket for tightening and loosening using a hexagon key, Fig. 4.2. Headless screws of this type – known as socket set screws – are available with different shapes of point. These are used like grub screws, where space is limited, but for higher strength applications. Different points are used either to bite into the metal surface to prevent loosening or, in the case of a dog point, to tighten without damage to the work, Fig. 4.3. Figure 4.2: Socket screw heads Figure 4.3:Socket set screws 4.1.3. Self-tapping screws Self-tapping screws are used for fast-assembly work. They also offer good resistance to loosening through vibration. These screws are especially hardened and produce their own threads as they are screwed into a prepared pilot hole, thus eliminating the need for a separate tapping operation. There are two types: - the thread-forming type, which produces its mating thread by displacing the work material and is used on softer ductile materials, Fig. 4.4(a); 48 - the thread-cutting type, which produces its mating thread by cutting in the same way as a tap. This type has grooves or flutes to produce the cutting action, Fig. 4.4(b), and is used on hard brittle materials, especially where thin-wall sections exist, as this type produces less bursting force. - More rapid assembly can be achieved by self-piercing- and-tapping screws. These have a special piercing point and a twin-start thread, Fig. 4.4(c). Used in conjunction with a special gun, they will pierce their own pilot hole in the sheet metal (up to 18 SWG (1.2 mm) steel) or other thin materials and are then screwed home in a single operation. Figure 4.4: Self-tapping screws (a) thread-forming (b) thread-cutting (c) self-piercing-and-tapping 4.1.4. Bolts Bolts are used in conjunction with a nut for heavier applications than screws. Unlike screws, bolts are threaded for only part of their length, usually twice the thread diameter. Bright hexagon-head bolts are used in engineering up to 36 mm diameter by 150 mm long. Larger sizes are available in high-tensile materials for use in structural work. 4.1.5. Nuts Standard hexagon nuts are used with bolts to fasten parts together. Where parts require to be removed frequently and hand tightness is sufficient, wing nuts are used, Fig. 4.5(a). If a decorative appearance is required, a dome or acorn nut can be fitted, Fig. 4.5(b). Where thin sheets are to be joined and access is available from only one side, rivet bushes or rivet nuts are used. These provide an adequate length and strength of thread which is fixed and therefore allows ease of assembly, Fig. 4.5(c). Available in thread sizes up to 12 mm for lighter applications, blind nuts of the type shown in Fig. 4.5(d) can also be used. The nut is enclosed in a plastics body which is pressed into a predrilled hole. A screw inserted into the nut pulls it up and, in so doing, expands and traps the plastics body. Spring-steel fasteners are available which as well as holding also provide a locking action. If access is available from both sides, a flat nut can be used, Fig. 4.5(e), or from one side a J-type nut can be used, Fig. 4.5(f). In their natural state these nuts are arched, but they are pulled flat when the screw is tightened. 49 Figure 4.5: Types of nut (a) wing (b) dome or acorn (c) rivet bush or nut (d) blind (e) flat spring steel fastener and (f) J type fastener 4.1.6. Washers Washers distribute the tightening load over a wider area than does a bolt head, screw head or nut. They also keep the surface of the work from being damaged by the fastener. Plain flat washers spread the load and prevent damage to the work surface. Flanged nuts are available with a plain flange at one end which acts as an integrated, non-slipping washer. The flange face may be serrated to provide a locking action but can only be used where scratching of the work surface is acceptable (Fig. 4.6). Figure 4.6: Flanged nuts 4.2. Screw threads Since 1965, British industry has been urged to adopt the ISO (International Organization for Standardization) metric thread as a first-choice thread system, with the ISO inch (unified) thread as the second choice. The British Standard Whitworth (BSW), British Standard Fine (BSF), and British Association (BA) threads would then become obsolete. The British Standard Pipe (BSP) thread is to be retained. The changeover has been extremely slow in taking place, and all these threads are available and still in use. 4.2.1. ISO metric thread This thread, based on a 60° triangular form, provides a range of coarse and a range of fine pitches. The threads are designated by the letter M followed by the diameter and pitch in millimeters, e.g. 50 M16 × 2.0. The absence of a pitch means that a coarse thread is specified, e.g. M16 indicates an M16 × 2.0 coarse pitch. 4.3. Locking devices Many ways are used in order not to allow the bolt to loose; which are i- Self-locking screws and bolts ii- Locking nuts iii- Locking washers iv- circlips 4.4. Riveting 4.4.1. Solid and tubular rivets Riveting as a means of fastening is used because of its speed, simplicity, dependability and low cost. Light riveting, used for general assembly work up to about 6 mm diameter, is carried out in industry using high-speed rivet-setting machines having cycle times as short as 1/3 second. Rivets are used on assemblies where parts do not normally have to be dismantled, i.e. permanent joints. They may also be used as pivots, electrical contacts and connectors, spacers or supports. The cost of riveting is lower than that of most other methods of fastening, due to the absence of plain washers, locking washers, nuts or split pins; also, the use of self-piercing rivets eliminates the need to predrill holes. Rivets are available in steel, brass, copper and aluminum in a variety of types. The more standard head types used are shown in Fig. 4.12. Solid rivets are strong but require high forces to form the end. In riveting, forming the end is known as clinching. Solid rivets are used in applications where the high forces used in clinching will not damage the work being fastened. Tubular rivets are designed for application where lighter clinching forces are used. Short-hole tubular rivets, Fig. 4.13(a), have the advantage of a solid rivet with easier clinching. These have a parallel hole and can be used for components of varying thicknesses. Figure 4.7: Solid-rivet head types Figure 4.8: Tubular rivets (a) short-hole (b) double-taper semi-tubular (c) bull-nose semi-tubular (d) self-piercing 51 Double-taper semi-tubular rivets, Fig. 4.13(b), have a taper hole to minimize shank expansion during clinching and are used to join brittle materials. Bull-nose semi-tubular rivets, Fig. 4.13(c), are used where maximum strength is required. The rivet shank is intended to expand during the setting operation in order to fill the predrilled hole in the work. This ensures a very strong joint. Self-piercing rivets, Fig. 4.13(d), have been specially developed to pierce thicker metal and clinch in the same operation. For metal up to 4.7 mm thick, the self-piercing rivet is made from special steel and is heat-treated to give the required hardness for piercing and ductility for clinching. One use of this type of rivet is automatic riveting in the production of garage doors. 4.4.2. Blind rivets Blind rivets, also known as pop rivets, are rivets which can be set when access is limited to one side of the assembly. However, they are also widely used where both sides of the assembly are accessible. Used to join sheet metal, blind rivets are readily available in sizes up to around 5 mm diameter and 12 mm long in aluminum, steel and monel (Nickel alloy). Plated steel rivets are used where low cost, relatively high strength and no special corrosion- resistance is required; aluminum for greater resistance to atmospheric and chemical corrosion; and monel for high strength and high resistance to corrosion. Blind rivets consist of a headed hollow body inside of which is assembled a center pin or mandrel. The rivet is set by inserting the mandrel in a tool having a means of gripping, and the rivet is inserted into a predrilled hole in the assembly. Operation of the tool causes its gripping jaws to draw the mandrel into the rivet, with the result that the head of the mandrel forms a head on the rivet on the blind side of the assembly, at the same time pulling the metal sheets together. When the joint is tight, the mandrel breaks at a predetermined load, Fig. 4.14. The broken-off portion of the mandrel is then ejected from the tool. Figure 4.9: Blind rivet 52 4.5. Welding In welding the metals being joined are locally melted and when solidified produce a solid mass, giving a joint strength as strong as the metals being joined. A filler rod is sometimes used to make up for material loss during welding, to fill any gaps between the joint surfaces and to produce a fillet. A flux is required with some metals and with some welding methods to remove the oxide film and provide a shield to prevent oxides from re-forming. The definition of a weld is: ‘a union between pieces of metal at faces rendered plastic or liquid by heat or by pressure or by both’. This can be realized by:  Fusion welding – where the metal is melted to make the joint with no pressure involved.  Resistance welding – where both heat and pressure are applied.  Pressure welding – where pressure only is applied, e.g. to a rotating part where the heat is developed through friction, as in friction welding. Fusion welding processes are distinguished by the methods of producing the heat, arc and gas. 4.5.1. Arc welding An electric arc is produced by passing an electric current between two electrodes separated by a small gap. In arc welding, one electrode is the welding rod or wire, the other is the metal plate being joined. The electrodes are connected to the electrical supply, one to the positive terminal and one to the negative. The arc is started by touching them and withdrawing the welding rod about 3 or 4 mm from the plate. When the two electrodes touch, a current flows, and, as they are withdrawn, the current continues to flow in the form of a spark. The resulting high temperature is sufficient to melt the metal being joined. The circuit is shown in Fig. 4.15(a). When the electrode also melts and deposits metal on the work, it is said to be consumable. 53 Figure 4.10: Arc welding (a) circuit (b) MMA Welding (c) TAGS welding (d) MAGS welding Electrodes made from tungsten which conduct current but do not melt are known as non- consumable. The most common arc-welding methods are manual metal arc welding, tungsten arc gas-shielded welding and metal arc gas-shielded welding. 4.5.1.1. Manual metal arc (MMA) welding, Fig. 4.15(b) In this process the arc is struck between a flux-covered consumable electrode and the work. This method is the most widely used form of arc welding and is used on all materials with the exception of aluminum. The flux produces gas which shields the surface of the molten metal and leaves behind a slag which protects the hot metal from the atmosphere while cooling and has to be chipped off when cool. 4.5.1.2. Tungsten arc gas-shielded (TAGS) welding, Fig. 4.15(c) In this process the arc is struck between a non-consumable tungsten electrode and the work piece. The tungsten electrode is held in a special gun through which argon gas flows to shield the electrode and molten metal from atmospheric contamination – the process is often referred to as TIG or argon arc welding. 54 Additional filler metal can be applied separately as rod or wire. The argon shield enables aluminum, magnesium alloys and a wide range of ferrous metals to be welded without the use of a flux. This method is used primarily for welding sheet metal and small parts and produces a high-quality weld. 4.5.1.3. Metal arc gas-shielded (MAGS) welding, Fig. 4.15(d) In this process the arc is struck between a continuous consumable wire electrode fed through a special gun. A shielding gas – argon, carbon dioxide (CO2), oxygen or a mixture of these – is also fed through the gun to shield the arc and molten metal from contamination. Using different filler wires and types of gas, this method is suitable for welding aluminum, magnesium alloys, plain- carbon and low-alloy steels, stainless and heat-resisting steels, copper and bronze. This process is often referred to as MIG or MAG welding. Using carbon-dioxide shielding gas for plain-carbon and low-alloy steels, this method is referred to as CO2 welding. 4.5.2. Electron beam welding (EBW) This is a fusion welding process in which the joint to be welded is bombarded with a finely focused beam of high-velocity electrons. As the electrons hit the work piece, their energy is converted to heat. The heat penetrates deeply, producing parallel-sided welds and making it possible to weld thick work pieces, with a typical maximum of 50 mm. Because the beam is tightly focused, the heat-affected zone is small, resulting in low thermal distortion. This gives the ability to weld close to heat-sensitive areas and the capability of welding in otherwise inaccessible locations. Weld face preparation requires a high degree of accuracy between the two mating surfaces. Almost all metals can be welded; the most common of these are aluminum, copper, carbon steels, stainless steels, titanium and refractory metals. The process can also be used to weld dissimilar metal combinations. It is used in the aerospace and car industries. The beam generation and welding process is carried out in a vacuum and so there is a restriction on work piece size. There is also a time delay while the vacuum chamber is being evacuated. A mode of EBW is available called non-vacuum or out-of-vacuum since it is performed at atmospheric pressure. The maximum material thickness is around 50 mm and it allows for work pieces of any size to be welded since the size of the welding chamber is no longer an issue. 4.5.3. Laser beam welding (LBW) This is a welding technique used to join materials together using a laser as an energy source. The laser is focused and directed to a very small point where it is absorbed into the materials being joined and converted to heat energy which melts and fuses the materials together. They can produce deep narrow welds with low heat input and so cause minimal distortion. They can be automated, producing aesthetically pleasing joints at fast rates. There are two types of laser commonly used,

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