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Name ........................................................................................................ Company ...................................................................................................

Materials, Fabrication, Tools and Measuring Devices used in the Automotive

Environment Workbook

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Materials Used in Motor Vehicles

Have you ever wondered exactly what a car is made of?

We hear a lot about the parts that make up our cars, such as engines, transmissions, seats and so on. But we never really give much thought to the raw materials that are used in auto manufacturing to create these things.

The car industry uses a tremendous number of materials to build cars, including iron, aluminum, plastic steel, glass, rubber, petroleum products, copper, steel and others. These parts are used to create everything from those small things we don't think about, such as dashboard needles and wiring, to the big stuff, such as the engine block or the transmission gears.

These materials have evolved greatly over the decades, becoming more sophisticated, better built, and safer. They've changed as new automotive manufacturing technologies have emerged over the years, and they're used in increasingly innovative ways.

In this section, we'll discuss five of the materials used most in automotive manufacturing. First, we'll take a look at the one that makes automobiles so heavy.

Material Properties All materials will have differing properties that make them especially useful for a given task. Below is a description of each material property. Strength The ability of a material to withstand a force being applied to it without bending. A strong piece of timber as used in the floor joists in your home have high strength and will not bend when weight it put on it. Elasticity The ability of a material to absorb force and flex in different directions, returning to its original position. A length of steel rod will flex and return to its original shape when a force is applied to it. Plasticity The ability of a material to be change in shape permanently. If too much force is applied to the same length of steel rod in the previous example, it will bend and change shape permanently. Ductility The ability of a material to change shape (deform) usually by stretching along its length.A piece of soft material such as lead, will stretch and deform if a pulling force is applied from either end.

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Tensile Strength The ability of a material to stretch without breaking or snapping. Bolts and studs as used for wheel fixings have high tensile strength but if the torque setting is exceeded, the bolt will snap. Malleability The ability of a material to be reshaped in all directions without cracking. If a piece of steel is heated up it become malleable and can be reshaped permanently. Toughness A characteristic of a material that does not break or shatter when receiving a blow or under a sudden shock. A hammer and chisel need to be tough so that they don’t shatter when used. Hardness The ability of a material to resist scratching, wear and tear and indentation. Many tools used by tyre technicians and mechanics need to be hard to prevent wear, as do components such as wheel bearing surfaces etc. Conductivity The ability of a material to conduct electricity and/or heat. Copper is a good conductor of electricity and heat and is used throughout a vehicle’s wiring and cooling system.

Steel Steel is used to build the car's underlying frame of support.

On modern cars, most of the weight comes from steel. Consider that most cars now weigh around 1,360 Kilograms, and most pickup trucks weigh around 1,810 kilograms -- that's a lot of steel!

In cars, low carbon steel is used to create the underlying chassis or cage beneath the body that forms the skeleton of the vehicle and protects you in the event of a crash. Door beams, roofs and even body panels created during auto manufacturing are also made of

low carbon steel. Steel is also used in a variety of areas throughout the body to accommodate the engine or other parts. Exhausts are often made from stainless steel, for example.

Steel is used because it is reasonably strong, tough, malleable, ductile and easily welded.

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Steel manufacturing has evolved greatly, so carmakers these days can make different types of steel for different areas of the vehicle that are rigid or that can crumple to absorb different impacts. These innovations in automotive manufacturing help keep you safe on the road.

Steel is often classified by its carbon content: a high-carbon steel is serviceable for dies and cutting tools because of its great hardness and brittleness. low- or medium-carbon steel is used for sheeting and structural forms because of its amenability to welding and tooling. Alloy steels, now most widely used, contain one or more other elements to give them specific qualities.

Aluminium steel is smooth and has a high tensile strength. Chromium steel finds wide use in automobile and airplane parts on account of its hardness, strength, and elasticity, as does the chromium-vanadium variety. Nickel steel is the most widely used of the alloys; it is nonmagnetic and has the tensile properties of high-carbon steel without the brittleness. Nickel-chromium steel possesses a shock resistant quality that makes it suitable for armor plate. Wolfram (tungsten), molybdenum, and high-manganese steel are other alloys. Stainless steel, which was developed in England, has a high tensile strength and resists abrasion and corrosion because of its high chromium content.

Plastic Many parts of the dashboard, including gauges and dials, are created from plastic.

Today's cars now use tremendous amounts of plastics. They make up about 50 percent of the construction of new cars today. It's not surprising because plastics are durable, cheap to make and can be turned into just about anything.

Your dashboard, gauges, dials, switches, air conditioner vents, electrical cable insulation, door handles, floor mats, many gaiters, seals, seat belts, airbags and many other parts are all made from a type of plastic known as PVC (Polyvinyl Chloride).

In addition to the dashboard parts, many of the tiny parts inside the engine, such as the handle on the oil dipstick, are also made of plastic. Because of their lightweight

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nature, plastics are being increasingly used in body structures and in engines during automotive manufacturing

Aluminium

Aluminum is used to make parts such as wheels and hubcaps.

In the world of auto manufacturing, aluminum is kind of the new kid on the block. It's being used increasingly in the car world for its lightweight but tough nature. In 2009, aluminum components made up about 9 percent of the weight in most modern vehicles, compared with about 5 percent in 1990 and just 2 percent in 1970 [source: Aluminum Association].

Aluminum can be used in automotive manufacturing to create body panels for a lighter, more performance-oriented vehicle. Many supercars have been constructed out of aluminum, including the Audi R8. Wheels are also often made out of aluminum.

In addition, more automakers are switching from traditional cast iron engine parts to aluminum alloys. It tends not to be as durable as iron, but it’s lighter weight means a big boost in performance

Rubber

Tyres, in addition to several other car parts, are made from rubber.

What's the one thing all automobiles have in common? They all need tyres if they're going to get around. Tyres are one of those parts people tend to take for granted, but they're one of the most vital parts of any vehicle. This is where the importance of rubber comes into play in auto manufacturing.

Automotive manufacturing is the driving force of the rubber industry, as about 75 percent of the world's natural rubber production is used to make tyres for vehicles [source: Industrial Rubber Goods]. The rubber tyre protects the rest of the wheel and its internal parts from wearing down, which can be good for fuel mileage and road safety.

In addition to the all-important tyres, parts such as wiper blades, engine mounts, seals, hoses and belts are also made from rubber. As with plastic, it's a very durable, cheap and flexible material that has a wide array of uses in automobiles.

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Glass Glass is used for windscreens to protect passengers from the elements and potential flying objects.

As with rubber, glass is one of the unsung heroes of automotive manufacturing. It's also heavily linked to the auto industry.

Glass is used in many areas of cars. Obviously, its primary use is to create windscreens so you can see properly while remaining safe from any airborne objects. It's also used to create rear and side-view mirrors to boost your view of what's around you while driving. In addition, its cousin

fiberglass is also commonly used in auto manufacturing as an insulation material on cars.

However, as technology advances, glass is also being used to create more innovative parts on cars. For example, it can be used to create navigation screens and lenses for back-up cameras to allow drivers to have an even better view of what's behind them.

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Questions

1. What is the name given to a material that returns to its original shape after being deformed? (4 marks)

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2. Hardness is normally recognised as the ability of a material to resist certain

things. Name two of them (4 marks) ............................................................................................................................ ............................................................................................................................

3. If a material can be permanently deformed without breaking, it is said to be p............................ (4 marks)

4. List three properties of mild steel which make it suitable for use on a vehicle. (6 marks)

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5. This material is used for many crankcases, cylinder blocks and inlet manifolds. It is light and conducts heat well. What is it? (4 marks)

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6. PVC is used for seal and electrical insulation. What does PVC stand for? (6 marks)

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Basic Hand Tools Despite the wide range of machine tools available, and despite the high rates of materials that can be removed with modern machine tools, bench fitting using hand tools still has a place in modern industry, particularly in the motor trade where one off components can be manufactured or adjusted. Advantages Hand tools are relatively cheap and versatile for making small components. Small and delicate components may not be strong enough to withstand the

clamping and machining forces. High levels of accuracy and finish can be achieved with care and attention. No large investment required for costly machines. Hand tools are more easily maintained compared to machine tools

Disadvantages

The rate of material removed by hand is limited and the process is slow. Difficulty in achieving high levels of accuracy and finish with some more

specialised processes. The cost of producing individual components is high compared to a machined

component.

The workbench The term fitting covers those operations that can be performed by hand at the bench. The production of accurate components by hand demands levels of skill that can often take many years to acquire. The basic requirement of successful fitting is a properly designed work bench. There is no single design for an ideal bench, however, for accurate work it is generally accepted that:

The bench must be made of strong timbers on a strongly based metal frame so that it is as solid and rigid as possible.

It must be positioned so that there is adequate natural lighting supplemented as required by shadowless, artificial lighting.

The height of the bench should allow the top of the vice jaws to be in line with the underside of the fitter’s forearm when held parallel to the ground.

There should be adequate storage facilities

The Vice A fitter will require a parallel jaw vice of the type shown below. It is often fitted with a quick release device that frees the screw from the nut so that the vice can be opened or closed quickly. This saves time when changing between wide and narrow work. For accurate work, the vice must be kept in good condition as follows:

Oil the screw and nut regularly Oil the slideways regularly Ensure that the vice is substantial enough for the work in hand Heavy hammering should not be done on the vice. When chipping, the thrust of the chisel should be against the fixed jaw

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Never hammer on the top surface of the slide Vice shoes The jaws of a vice are serrated to prevent the work from slipping. However, these serrations can mark and spoil a finished surface. If the vice is only used for fine work and light cuts, the jaws can be surface ground flat and smooth. Alternatively, if the vice is going to be used for both rough and fine work, then vice shoes can be used. These can either be cast from a soft metal such as lead or they can be faced with fibre as sown. Using a vice The vice should be securely bolted to the bench using the largest bolts that will pass through the fixing holes in the base of the vice, and a length that will pass through the bench top. The vice should be positioned so that the fixed jaw is just clear of the edge of the bench. This allows long work to hang down clear of the bench. Work should be positioned in the vice so that the major cutting forces acting on the work are directed towards the fixed jaw. The work should always be held in a vice with a minimum of overhang as shown. There is always a possibility that work protruding too far out of a vice will bend under the force of the cut, and also that the work will vibrate and produce an irritating squealing sound.

Parallel jaw vice

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Cold Chisels Chipping is the removal of metal by the use of cold chisels. This process is used for rapidly breaking down a surface. It is the quickest way of removing metal by hand but the accuracy is low and the finish is poor. However, in some instances there are no alternatives.

Cold chisel types (a) and uses: (b) cutting an oil groove with a half-round chisel; (c) squaring out a corner with a diamond point chisel; (d) chipping a flat surface.

Safety When using a cold chisel:

DO NOT chip towards your work mates Always use a chipping screen Always wear goggles to protect your eyes from the flying splinters of

metal.

Cutting edge slightly curved

(a)

(b) (c) (d)

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Hammers Hammers are important tools and must be treated correctly and safely. Their main uses are:

Tapping marking tools such a centre punches Riveting Chiselling Driving things into positions.

Hammers are identified by their shape and weight. Most hammers are available in a range of weights, typically between 225gm (1/2 lb.) and 900gm (2 lb.). Bigger hammers than this are called sledgehammers and are used for heavy work. If the hammer is too big, it will be clumsy to use and proper control cannot be exercised. If the hammer is too small it has to be wielded with so much effort that again, proper control cannot be exercised. In both these instances the use of the correct size of hammer will result in an unsatisfactory job, possible damage to the work and possible injury to the user.

(d)

Hammer construction (a) and pein types; (b) ball; (c) cross; (d) straight

Uses of hammers: (a) correct grip; (b) hammer used with another tool; (c) hammer used direct

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Before using a hammer you must check it to make sure that the:

Handle (shaft) is not split Head is not loose Head is not cracked or chipped Never ‘strangle’ a hammer by holding it too near the head, hold it as

shown above. A hammer is usually used to strike other tools such as chisels, drifts and

centre punches. Sometimes a hammer is used to strike a component directly, in which case

great care must be taken not to cause damage. You must be careful when using a hammer that the components being struck are not bruised. Soft-faced hammers should be used when machined surfaces have to be struck. Soft-faced hammers (mallets) are faced with various materials such as soft metals like brass and aluminium and non-metals such as plastic and rawhide. Some are made of rubber moulded to the handle, however these tend to bounce and it is difficult to deliver a dead blow. An improved design is hollow and loosely filled with lead shot. This type of mallet will deliver a dead blow and also provide the protection against bruising. Alternatively a soft metal (brass, copper or aluminium) drift should be placed between the hammer head and the component being struck.

Soft faced hammers (mallet): (a) rubber and plastic; (b) hide faced

Filing Filing operations can range from roughing down to fine and accurate finishing operations. There is a wide variety of files, and to specify any given file you must state the length, shape and grade of cut. The main features of a typical file are shown below. For normal metal removal a double cut file is used. Double cut files tend to clog up when used on soft metals such as aluminium. An alternative is to use a Milnecut file as shown as (a) in the photo below. Here the teeth are milled into the blank and the ‘gashed’ along the length of the blank to break up the size of the chip produced and help

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prevent clogging. These files are only made flat. They are useful for heavy stock removal but they do not provide a good finish. The Dreadnought type file is also used for rapid stock removal. Again the teeth are formed by milling rather than chiselling and are curved in shape. They are very effective on soft metals and plastic materials and are available in flat and half-round sections; the shape of the teeth shown below (b) The grade or cut of a file depends upon its length. A long second cut file can have a courser cut than a short bastard cut file. The grades of file available in the UK are listed in the table below. The most commonly used are:

Bastard cut – general roughing out. Second cut – roughing out tough materials such as steels and for finishing

on less tough materials. Smooth cut – general finishing of precision components and fro draw filing.

Grade

Pitch (mm)

Use

Rough 1.8 – 1.3 Soft metals and plastics

Bastard 1.6 – 0.65 General roughing out

Second cut 1.4 – 0.60 Roughing out tough materials Finishing soft materials

Smooth 0.8 – 0.45 General finishing and draw filing

Dead smooth 0.5 – 0.25 Not often used except on tough die steels where high accuracy and finish is required

(a) (b)

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The shape of a file is governed by its application. Some of the more common are shown below.

Flat File – Tapers for the last third of its thickness. Double cut on both faces and single cut on both edges. Used for general filing of flat surfaces

Hand file – Parallel in width but tapers slightly in thickness. Double cut on both faces and single cut on one edge

Pillar File - Similar to hand file but is flat, thicker and narrower. Useful for work in narrow slots.

Warding file – Similar to a flat file but smaller and thinner. Used for filing flat surfaces in narrow slots

Half round file – Not semicircular but a segment of a circle. Double cut on flat side and single on curved. Used for concave surfaces and into corners.

Round file – Circular in cross-section and tapers in last third of length. Used for opening out holes and rounding

internal corners.

Square file – square in cross-section and tapers on all sides. Double cut on all surfaces and used for filing square and rectangular holes, slots and grooves.

Three-square file – Triangular in cross-section with all its angles at 60°. It is double cut on all three sides and tapers for the last third of its length. Used for filing corners between 60° and 90°.

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Use of a file A file can only be controlled if the fitter’s body is correctly positioned and balanced. It has already been stated that the vice jaws should be at elbow height for convenience. This is particularly true when filing. The diagram below shows the correct position for your feet and the way that your body should be balanced.

Use of a file: (a) top of vice should be in line with forearm held parallel to the ground; (b) position of feet and balance.

Equally important is the way the file is held. During each stroke, the weight must be gradually transferred from the front hand to the hand gripping the file handle. If this is not done correctly the file will rock and a flat surface will not be produced. The diagram below shows how a file should be held for various operations.

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Care of files Files should be treated with care. Files that are badly treated are hard to use and leave a poor finish and poor accuracy.

Keep all files in a suitable rack. Do not jumble them up in a drawer or keep them with other tools as this will chip and damage the teeth.

Keep your files clean with a special wire brush called a file card. Bits of metal trapped in the teeth reduce the rate of metal removed and scores the surface of the work.

Never use new files on steel. This will chip the teeth and make the files useless. Always ‘break in’ a new file on softer materials such as brass or bronze.

Never file quickly, this only wears out the file and the user. Slow, even strokes using the full length of the file are best.

Files only cut on the forward stroke. The downward pressure should be eased on the backstroke to reduce wear on the teeth. Do not lift the file off the work on the backstroke. Keeping the file in contact with the work helps to remove the particles of metal that lie between the teeth and also maintains your balance and rhythm that are essential to the production of a flat surface.

Rubbing chalk into the teeth of a clean file before starting work will prevent the teeth from clogging with metal ‘pins’.

Safety When filing:

Always ensure that the file is fitted with the correct size of handle and that the handle is secured to the file. Never use a file without a handle. The tang can easily stab into your wrist causing serious damage leading to the paralysis of your fingers.

A badly fitted handle or the wrong size of handle reduces your control over the file causing you to slip and have an accident.

A split handle does not protect you from the tang of the file. The hacksaw The diagram below (a) shows a typical engineer’s hacksaw with an adjustable frame that will accept a range of blade sizes. For the best results the blade should be carefully selected for the work in hand. It must be correctly fitted and correctly used.

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Figure (b) shows the main features and dimensions of a hacksaw blade. To prevent the blade jamming in the in the slot that it is cutting, side clearance must be provided by giving the teeth of the blade a set as shown in figure (c). There are two ways in which the set can be applied. For course pitch blades for general workshop use, the teeth are bent alternately to the left and right, with each intermediate tooth left straight to clear the slot of swarf. Some blades leave every third tooth straight. For fine tooth blades used for cutting sheet metal and thin walled tubes, the edge of the blade is given a ‘wave’ set. Both of these types are shown in figure (d). Hints when sawing

The courser the pitch of the teeth the greater the rate of material removal and the quicker the metal will be cut. However, there must always be a minimum of three teeth in contact with the metal as shown in (a) below.

Material Pitch (mm Solid metal

Pitch (mm) Tube and sheet

Ferrous metal Non ferrous metal

1.4 – 1.6 1.8 – 2.1

0.8 1.0 – 1.2

The hacksaw blade: (a) blade selection – (I) a wide component should be broken down in a series of short cuts, (ii) the pitch of the blade should be chosen so that at least three teeth are in contact with

the workpiece at all times; (b) use of the hacksaw

Thick material should be broken down into shorter surfaces ‘Rigid’ or ‘all hard’ high-speed steel blades give the best results but

tend to break easily in unskilled hands. ‘Flexible’ or ‘soft-back’ blades are best for persons who are not yet fully skilled.

The teeth of the blade should face the direction of the cut and the blade should be correctly tensioned. After the slack has been taken up, the wing nut should be given at least another full turn.

The rate of sawing should not exceed 50-60 strokes per minute. The correct way to hold a hacksaw in shown in figure (b) With use, the blade gradually loses its set and the slot cut will

become narrower. For this reason never use a new blade in the slot started by an old blade. It will jam and break. Always start a new cut with a new blade.

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Sawing sheet metal Sheet metal and thin walled steel tubing requires a hacksaw blade with at least 32 teeth per inch (TPI) to avoid snagging on the material. The depth to which a hacksaw can cut is limited to the depth of the frame, that is, the distance between the teeth of the blade and the inside of the back of the saw frame. Long, narrow cuts are often required in sheet metal and, for this purpose, the blade can be turned through 90° as shown below. It is not easy to exert downward force on the blade with the saw in this position, but this is not so important when cutting sheet material of limited thickness. An ordinary hacksaw blade is useless in cutting profiles and, for this purpose, a tension file should be used. This is a long, thin, round file that is kept in tension by the saw frame as shown below. It is held in the frame by means of adapter clips.

Cutting sheet metal: (a) blade tuned through 90° to cut sheet metal; (b) tension file – when the wing nut is tightened, the frame distorts and is put in a state of stress; in trying to spring back to its original

shape it exerts a tensile (pulling) force on the blade or file, which is now in a state of tension.

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Questions

1. Hacksaw blades are classified by length, type and number of teeth on the blade. What determines the number of teeth to use? (4 marks)

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2. What type of metal would need a hacksaw blade which has 32 teeth per inch (TPI)? (4 marks)

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3. When filing metal it is often common practice to rub chalk into the teeth of a clean file before starting work. Why is this done? (4 marks)

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4. List three essential safety checks that should be made to a hammer before use. (6 marks)

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5. State two advantages and two disadvantages of using hand tools to manufacture small parts. (4 marks)

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Screw thread terminology, identification and applications Screw thread terminology Sometimes it is necessary to cut internal and external threads using taps and dies. Before we consider such threading tools we need to know some more about the screw thread itself. The diagram below shows some typical thread forms and their names. Screw threads: (a) elements and types; (b) square thread; (c) V-thread; (d) acme thread; (e) buttress

thread V-thread This thread form has many advantages for fastening applications, which is why it is so widely used:

It is easy and cheap to manufacture It is easily cut with taps and dies It is the strongest form It is self-locking (particularly if it has a fine pitch) and only works loose if

subjected to extreme vibration. Square thread This thread form is used where rotary motion is to be transformed into linear motion. For example the traverse screw on an engineer’s vice.

Has less friction than a V-thread form as there is no wedging action It is not as strong as the V-thread form Is more difficult to cut than a V-thread

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Acme thread This is a modified square thread and is widely used for machine tool lead screws, particularly when used in conjunction with a split nut as on the lead screw of a centre lathe.

There is less friction when the split nut is being engaged and disengaged. The taper lets you ‘feel’ the nut engaging with the screw, thus avoiding

damage to the corners of the thread. Wear can be compensated for by moving the split nut deeper into

engagement with the screw. Buttress thread This form is only used where the axial force acts in one direction only.

It is much stronger than the square thread form It is used for quick release vice and balance machine wheel clamps.

Features of screw threads For the purpose of this book we are only interested in the V-form as shown in diagram (a) on the previous page.

The major diameter of the thread is the maximum diameter measured over the tops of the threads

The nominal diameter of the thread is the diameter by which it is known and specified. For most practical purposes it can also be considered to be the same as the major diameter.

The pitch (simple effective) diameter is, as its name suggests, the diameter at which the pitch of the thread is measured. It is also the diameter at which the thickness of the external thread and the thickness of the internal thread are equal.

The pitch is the distance from a point on one thread to an identical point on the next thread.

The thread angle is the angle of the ‘V’ that gives the thread its form. The angle is 60° for metric, UNC and UNF threads; 55° for BSW, BSF and BSP threads, and for 47½ ° for BA threads. These initials will be explained later.

The root is the bottom of the thread – usually radiused. The crest is the top of the thread – it may be radiused or flat. The flank is the side of the thread. Only the flanks of the thread should

make contact. Note – For general engineering, a full thread form and full engagement of mating threads is not required. By slightly reducing the major diameter of an external thread and/or the minor diameter of an internal thread to give 80% engagement, the thread becomes easier to cut with less wear and tear on the threading tool and less chance of breakage. The strength of the fastening is adequate for normal purposes.

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Specifying screw threads To identify a screw thread the following information must be specified:

The nominal diameter. This is stated in metric or inch units. The pitch or the TPI. For metric threads the actual pitch size is stated in

millimetres The type of thread form. There are various types of thread form and,

although metric screw threads should be specified for all new equipment, the older thread forms are still widely used.

Unified form (a) This has a 60° thread angle and is dimensioned in inch units. It is the basis of Unified Course threads (UNC) and Unified Fine threads (UNF). These threads originated in the USA but are also used in the UK. A typical example would be specified as 3/8 – 24 UNF – 2B indicating that the thread is Unified Fine, 3/8” nominal diameter, 24 threads per inch (TPI) and a tolerance classification 2B (general purpose). ISO metric form This is the same as the unified form but is dimensioned in millimetres. A typical example would be M10 x 1.50 indicating that the thread has a metric form (M), that its nominal diameter is 10mm and that the pitch of the thread is 1.50mm. British Association (BA) form (b) This was originally introduced for the small threaded fasteners used in instruments and later was widely used in electrical equipment. It has a 47½° thread angle and is dimensioned in millimetres. The threads are numbered. The largest is 0BA, which has a nominal diameter of 6.00mm and a pitch of 1.00mm. The smallest is 25BA, which has a nominal diameter of 0.25mm and a pitch of 0.0700mm. Threaded fasteners with BA threads should now only be used for maintenance purposes. New equipment should have ISO metric screw threads (miniature series). These have the 60° metric thread form described previously. A typical example would be specified as S-08 which is 0.8mm diameter, with a pitch of 0.2mm. Whitworth form (c) This has a 55° thread angle and is dimensioned in inch units. It is the basis of British Standard Whitworth (BSW), British Standard Fine (BSF), and British Standard Pipe (BSP) threads. A typical example would be specified as: ½” x 12 BSW indicating that the thread is to British Standard Whitworth form, with a nominal diameter of ½ inch, and there are 12 threads per inch (TPI)

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V-thread forms: (a) unified thread form – the basis of all modern V-threads; (b) British association form – used for screws in scientific instruments and small electronic components; the BA thread form has always been based on metric sizes and is still acceptable in a metricated system; it continues to fill a gap in the ISO system, which fails to provide instrument-size threads; (c) Whitworth form – this form has been traditional in Great Britain since it was introduced by Sir Joseph Whitworth in the nineteenth century; although now obsolete, it will continue to be used for many years for replacement and maintenance purposes

Cutting internal screw threads (use of taps) The diagram (a) below shows a section through a thread-cutting tap and how rake and clearance angles are applied to a thread cutting tap. Since the ‘teeth’ are form relieved, the clearance face is curved and the clearance angle is formed by the tangent to the clearance face at the cutting edge. The rake angle is formed by the flute, so we still have our metal cutting wedge. The flute may be straight or it may be helical like the flutes of a twist drill. The diagram (b) shows a typical thread-cutting tap and names its more important features. (c) shows a set of three taps:

The taper tap is tapered off for the first 8-10 threads and is used first. The taper helps to guide the tap into the previously drilled tapping size hole with its axis parallel to the axis of the hole. The taper also helps to increase the depth of cut gradually and helps to prevent overloading the teeth.

The intermediate or second tap has only 3-4 threads tapered to guide it into the threaded hole started by the taper tap. This tap can be used to

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finish threading a through hole. It can also help to cut full threads near the bottom of a blind hole.

The plug tap does not have any tapered threads and is used for cutting a full thread to the bottom of a blind hole.

Screw thread taps: (a) cutting angles applied to a thread-cutting tap; (b) nomenclature for taps; (c) set

of thread-cutting taps

Thread-cutting taps are rotated by means of a tap wrench. Various types of tap wrench are shown in figure (a) and figure (b) overleaf shows how a tap wrench should be used. The tap is rotated in a clockwise direction and it should be reversed every one or two revolutions to break up the swarf . It is essential to start and keep the axis of the tap parallel to the axis of the hole. Normally this means that the axis of the tap will be at right angles to the work as shown. If the tap is started at an angle other than a right angle, the tap will cut more heavily on one side of the hole than the other. At best this will produce a drunken thread, at worst it will cause the tap to break off in the hole. It is usually impossible to remove a broken tap and the component is scrapped.

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Before you can cut an internal screw thread, you have to decide on the size of the hole to be used. Theoretically this should be the same as the minor diameter of the thread to be cut. In practice, the hole is always somewhat larger in diameter than the minor diameter for the following reasons.

As stated earlier, a thread with 80% engagement is adequate for most general engineering purposes. This considerably eases the load on the tap, which is a fragile cutting tool that is easily broken if overloaded.

You have to use the nearest standard drill size available. You cannot use one smaller or the tap will jam and break, so you use one that is slightly larger for the reason stated above.

Fortunately you do not have to work out the correct size of tapping drill to use. Published sets of work tables provide this information for you but you can often find the drill size marked on the tap. Hints when tapping holes

Make sure the tap is sharp and in good condition (no chipped or missing teeth) or it will jam and break off in the hole, scrapping the job.

Use a cutting compound that has been formulated for tread cutting. Lubricating oil is useless since it cannot withstand the cutting forces involved.

Select the correct size of tap wrench to suit the size of tap you are using. The wrong size will inevitably lead to a broken tap. A range of tap wrenches should be available.

Make sure the tap is at right angles to the surface of the component. The following diagram shows two methods of ensuring correct tap alignment.

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Cutting external screw threads (use of dies) The diagram (a) below shows how the basic cutting angles are applied to a thread-cutting button die. You can see that a die has rake, clearance and wedge angles like any other cutting tool. Diagram (b) shows the main features of a button die, whilst (c) shows a typical diestock that is used to rotate the die. (d) shows how the die is positioned in the diestock.

Split button dies and diestock: (a) cutting angles applied to a thread-cutting die; (b) split-die; (c) diestock; (d) positioning of die in stock – the engraved face of the die is visible, ensuring that the lead

of the die is in the correct position

Screw thread dies are used to cut external threads on components. The split button die is the most widely used. The diestock has three adjusting screws, which engage in the slot and spread the die to reduce the depth of cut. The other two screws close the die and increase the depth of cut. As for a tap, the die must be started square with the work axis as shown below in diagram (a). It is difficult to control a screw-cutting die and any attempt to cut a full thread in one pass will result in a ‘drunken’ thread. It is better to open up the die to its fullest extent for the first cut. (This will also produce a better finish on the thread.) Then close the die down in stages for the subsequent cuts until the thread is the required size. The thread size can be checked with a nut. The diestock is rotated in a clockwise direction and should be reversed after every one or two revolutions to break up the swarf.

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Hints when using screw-cutting dies

The die has its size and the manufacturer’s name on one face only. This is the face that should show when the die is in the stock. This will ensure that the taper lead of the die will engage with the end of the work. A chamfer on the end of the work will help to locate the die. Start with the die fully open and gradually close it down to the

required size in successive cuts. Select the correct size of stock for the die. This is largely controlled

by the diameter of the die. A range of diestocks should be available.

Use a cutting compound that has been formulated for thread cutting. Lubricating oil is useless since it cannot withstand the cutting forces involved.

Miscellaneous thread-cutting devises Rectangular loose dies are also used but are less common than button dies. They have a bigger range of adjustment but require a special type of diestock. An example is shown in diagram (b) below. Die nuts as shown in diagram © are used for cleaning up bruised threads on existing bolts and studs. Screw thread dies: alignment and further types: (a) diestock must be aligned with the axis of the work;

(b) rectangular split dies; (c) solid die-nut

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Tools used in assembly and dismantling Screwed fastenings So far, we have only considered using bench tools to make components. However, another and very important aspect of a fitter’s work is the assembly and dismantling of components. Screwed fastenings are used by fitters as an everyday occurrence and some examples are shown below.

L

Locking devices for screwed fastening: (a) positive locking devices; (b) friction locking devices

Locking devices for screwed fastenings Locking devices are employed to prevent threaded fastenings from slacking-off in use as a result of vibration. A selection of locking devices are shown above. You will see that they are divided into two categories: those which depend upon friction and those whose locking action is positive. Positive locking devices are more time consuming to fit, so they are only used for critical joints where failure could cause serious accidents, for example the control systems of vehicles.

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Fastener Materials General

Fasteners are manufactured in a wide range of materials from common steel to titanium, plastic and other exotic materials. Many materials are further separated into different grades to describe specific alloy mixtures, hardening processes, etc. In addition, some materials are available with a variety of coatings or plating to enhance the corrosion resistance, or appearance of the fastener.

Fastener material can be important when choosing a fastener due to differences between materials in strength, brittleness, corrosion resistance, galvanic corrosion properties, and of course cost.

When replacing fasteners, it is generally best to match what you are replacing. Replacing a bolt with a stronger one is not always safe. Harder bolts tend to be more brittle and may fail in specific applications. Also some equipment is designed so that the bolts will fail before more expensive or critical items are damaged. In some environments such as salt water galvanic corrosion must also be considered if changing fastener materials.

Materials

Steel Steel is the most common fastener material. Steel fasteners are available plain as well as with various surface treatments such as zinc plating, galvanization, and chrome plating.

Steel fasteners are commonly available in 4 grades. Many other grades exist but are used far less often. The most common grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade 2, 5, and 8 are usually plated with a silver or yellow zinc coating or galvanized to resist corrosion.

Stainless Steel Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant for the price and because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.

It is a common misconception that stainless steel is stronger than regular steel. In fact, due to the low carbon content, stainless steel cannot be hardened. Therefore when compared with regular steel it is slightly stronger than an un-hardened (grade 2) steel fastener but significantly weaker than hardened steel fasteners.

Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic.

18-8 Stainless 18-8 refers to any stainless steel containing approximately 18% chromium and 8% nickel. This is the most common stainless designation for hardware.

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Alloy Steel Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.

Silicon Bronze Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze is used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance, and over brass due to its higher strength. Bronze is similar to copper in colour and is also sometimes seen in fine woodworking where it is used for its appearance. The main drawback of bronze is its high cost.

Brass Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.

Coatings

Zinc Plating Many steel fasteners are electro-plated with zinc for better corrosion resistance. Fasteners that have been zinc plated have a shiny silver or golden appearance referred to as clear or yellow zinc respectively. They are fairly corrosion resistant but will rust if the coating is destroyed or if exposed to a marine environment.

Hot Dip Galvanizing Galvanizing is another coating involving the application of a layer of zinc. Hot dipped galvanizing puts the thickest possible coating on the metal resulting in superior corrosion resistance. Due to the thickness of the coating hot dipped galvanized bolts are not compatible with other nuts. Galvanized nuts are tapped slightly larger than other nuts to accommodate this coating. Hot dipped galvanized fasteners are frequently seen in coastal environments.

Chrome Chrome is used in plating fasteners for its appearance. It provides similar corrosion resistance to zinc plating. The main drawback of chrome is the extremely high cost. If more corrosion resistance is required stainless steel may be chrome plated, preventing any corrosion should the chrome be penetrated.

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Determining Bolt Grades

Bolts of different grades are marked on the head to show what grade bolt they are.

Grade 2 Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Grade 2 bolts have no head marking (sometimes a manufacturer mark is present).

Grade 5 Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head.

Grade 8 Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.

Metric Bolts

With metric bolts, strength is described by “property classes.” A property class designation consists of two numbers separated by a decimal point. There are nine commonly encountered: 4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9.

The number before the first decimal point is approximately one-hundredth of the nominal minimum tensile strength of the bolt in Newtons per square millimeter N/mm² or megapascals (MPa, which is the same thing. (1MPa = 1N/mm² = 145 lb/in²)

The number after the first decimal point is approximately the ratio between the minimum yield stress and the minimum tensile strength, times ten.

Tensile Strength is the maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.

Yield strength is the maximum load at which a material exhibits permanent deformation, i.e., changes from the elastic (ability to stretch) to the plastic (stretched) state.

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Example A class 8.8 bolt has a minimum tensile strength of approx 800MPa and the minimum yield stress is 80% of 800MPa = 640MPa A class 10.9 bolt has a minimum tensile strength of approx 1040MPa and the minimum yield stress is 90% of 1040MPa = 940MPa Determining Torque Values

"What torque should I use to tighten this bolt?" This is a question that is frequently asked by fitters where no torque data is available. In theory, every nut and bolt on a motor vehicle should have a published torque value but generally, only the common ones are listed in technical data manuals, such as wheel fixing, cylinder head bolts etc.

So what do we do if we cannot find a torque value for a fixing, guess?? Well, an experienced mechanic, using the correct length of spanner would be able to tighten a fixing satisfactorily, but it is very easy to either over or under-tighten a fixing, both having potential catastrophic consequences.

So, what is the answer? The widely recognized engineering formula, T= K x D x P (explained later), was used to provide the chart's values, but it must be understood that every bolted joint is unique and the optimum tightening torque should be determined for each application by careful experimentation. A properly tightened bolt is one that is stretched such that it acts like a very ridged spring pulling mating surfaces together. The rotation of a bolt (torque) at some point causes it to stretch (tension). Several factors affect how much tension occurs when a given amount of tightening torque is applied.

The first factor is the bolt's diameter. It takes more force to tighten a 10mm bolt than to tighten a16mm bolt because it is larger in diameter. The second factor is the bolt's grade. It takes more force to stretch an SAE Grade 8 bolt than it does to stretch an SAE Grade 5 bolt because of the greater material strength. The third factor is the coefficient of friction, frequently referred to as the "nut factor." The value of this factor indicates that harder, smoother, and/or slicker bolting surfaces, such as threads and bearing surfaces, require less rotational force (torque) to stretch (tension) a bolt than do softer, rougher, and stickier surfaces. The basic formula T = K x D x P stated earlier takes these factors into account and provides users with a starting point for establishing an initial target tightening torque.

• T Target tighten torque (the result of this formula is in inch pounds, dividing by 12 yields foot pounds)

• K Coefficient of friction (nut factor), always an estimation in this formula

• D Bolts nominal diameter in inches

• P Bolt's desired tensile load in pounds (generally 75% of yield strength)

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The reason all applications should be evaluated to determine the optimum tightening torque is that the K factor in this formula is always an estimate. The most commonly used bolting K factors arc 0.20 for plain finished bolts, 0.22 for zinc plated bolts, and 0.10 for waxed or highly lubricated bolts. .

Thread Tensile Stress

Area SAE Grade 2 SAE Grade 5 SAE Grade 8

Size TSA 75% Yield Strength (PSI) - 43000 75% Yield Strength (PSI) - 69000 75% Yield Strength (PSI) = 98000

Plain Zinc Plated Waxed Plain Zinc Plated Waxed Plain Zinc Plated Waxed

Square Inches A. lb. Ft.Lb. Ft.Lb. Ft.Lb. Ft.Lb. Ft.Lb. Ft.Lb. Ft.Lb. Ft.Lb.

114-20. 0.0318 6 6 3 9 10 5 13 14 6

1/4-28. 0.0364 7 7 3 10 12 5 15 16 7

5116-18. 0.0524 12 13 6 19 21 9 27 29 13

5116-24. 0.0580 13 14 6 21 23 10 30 33 15

318-16. 0.0775 21 23 10 33 37 17 47 52 24

318-24. 0.0878 24 26 12 38 42 19 54 59 27

7/16-14. 0.1063 33 37 17 53 59 27 76 83 38

7/16-24. 0.1187 37 41 19 60 66 30 85 93 42

112-13. 0.1419 51 56 25 82 90 41 116 127 58

112-20. 0,1599 57 63 29 92 101 46 131 144 65

9116-12. 0.1820 73 81 37 118 129 59 167 184 84

9116-18. 0.2030 82 90 41 131 144 66 186 205 93

5J8-11. 0.2260 101 111 51 162 179 81 231 254 115

5J8-14. 0.2560 115 126 57 184 202 92 261 287 131

3/4-10. 0.3340 180 197 90 288 317 144 409 450 205

3/4-16. 0.3730 200 221 100 322 354 161 457 503 228

The only way to properly determine the optimum tightening torque for a given application is to simulate the exact application. This should be done with a tension indicating device of some type on the bolt in the application. The bolt is tightened until the desired P (load) is indicated by the tension indicating device. The tightening torque required to achieve the desired tension is the actual tightening torque that should be used for that given application. It is extremely important to realize that this tightening value is valid only so long as all of the aspects of the application remain constant. Bolt suppliers sometimes have customers say that their bolts are no good because they have started breaking while being installed. Thorough investigation commonly reveals that the customer has started lubricating the bolts to make assembly easier, but maintained to same torque as was used when they were plain finished

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The table shows that by using this formula a 1/2-13 Grade 5 plain bolt should be tightened to 82 foot pounds, but the same bolt that is waxed only requires 41 foot pounds to tighten the same tension. A perfect 1/2-13 Grade 5 waxed bolt will break if it is tightened to 81 foot pounds because the K factor is drastically lower. The bolts are fine, but the application changed. Suppliers need to understand this and be able to educate their customers to resolve this common customer complaint about breaking bolts. The chart is provided for quick reference by fastener suppliers and users for selecting an initial target tightening torque. This chart was derived by using the formula shown earlier. An example of the calculation is as follows:

Product: 3/4-10 Grade 5 zinc plated bolt

Formula: T= K x D x P

• K= 0.22 (zinc plated) • D= .750 (3/4-10 nominal diameter • P= 23.046 pounds

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Fastening Materials (Rivets)

Rivets are used to join plates together and they have been used for hundreds of years. Before the widespread use of welding, rivets were used in heavy industries such as ship building and steam boiler making. The steel plates used to build ships such as the Titanic and the naval Dreadnaughts of World War One were held together by steel rivets.

Rivets have largely being replaced by techniques such as welding and brazing. However, joining plates together with rivets is still a useful technique especially if the plates to be joined are quite small.

Countersunk Mushroom Pan Head Snap Head

Cold Rivets can be used to permanently join plates together. Steel rivets are used for joining steel plates whilst aluminium rivets are used for aluminium plates. Rivets are available made from other materials such as copper and brass. The material of the rivet normally matches the material to be riveted together. In the case of perspex, aluminium or copper rivets are the most suitable because they are relatively soft and easy to shape.

Stages of Riveting

Rivets are made from either aluminium, steel, copper or brass. They are supplied in different lengths and diameters and must be cut to the exact length using a junior hacksaw

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The head of the rivet is set to rest in the 'dome' of a ‘dolly’. A typical 'dolly' is shown on the left. This tool is held in an engineer’s vice and it supports the rivet’s head.

The plates are placed over the rivet, resting on the top surface of the dolly.

The plates are pushed over the rivet and in order to ensure that there are no gaps between them a rivet set is pushed down, over the rivet's shaft. The rivet's shaft fits inside a small hole at the base of the rivet set.

A ball pein hammer is used to tap the plates together. If this is not done, the plates may be move slightly, even after a series of rivets have been hammered in position.

The rivet set is made from hardened steel as it is tapped at the top with a ball pein hammer. This closes any gap between the two plates

The ball pein hammer is reversed and used to expand the ‘tail’ of the rivet. This may take several slight taps. At this point the rivet cannot be removed from the plates.

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Spanners and keys Blind Pop Riveting This type of riveting allows the fixing of materials in situ where access to the rear is not possible.

The ball pein side of the hammer is used to form the rivet head. This takes time and should not be rushed. The head should form a dome.

The final shaping of the rivet head is formed using the rivet snap. The plate should be permanently fixed together at this stage.

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Spanners To turn a nut or bolt you have to use various types of spanners, keys and wrenches. A typical selection is shown below. Spanners are proportioned so that the length of the spanner provides sufficient leverage for a person of average strength to be able to tighten the fastening correctly. A spanner must not be extended to get more leverage. Extending the spanner will over-stress the fastening and weaken it. Further, it will also result in strain and possibly damage to the spanner jaws so that they will not fit properly, which can give rise to injuries. Keys and spanners

Correct use of spanners: (a) when tightening correctly, the force exerted on the spanner tends to keep

the jaws on the nut; used wrongly, the jaws tend to slip off the nut; (b) pull towards your body whenever possible

Where the tightening of a screwed fastening is critical (such as wheel fixings), the designer will specify the torque to be used. Torque is the force exerted on the spanner multiplied by the leverage distance. That is, the distance from the point of application of the force on the wrench, to the axis of the fastening. To ensure that the correct torque is applied, a torque wrench is used.

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Adjustable spanners and wrenches Adjustable spanners and pipe wrenches should only be used as a last resort when assembling and dismantling components. Because the arms of the adjustable spanners and wrenches are of a fixed length, small fastening may be over-tightened and large fastenings may not be securely fastened. As they become worn, the jaws of adjustable spanners tend to slip and damage the corners of hexagonal nuts and bolts. This can prevent normal open ended, ring and socket spanners being used on them in the future. Adjustable tools: (a) spanner; (b) pipe wrench

Screwdrivers Screwdrivers must also be chosen with care so that they fit the head of the screw correctly. A variety of screwdrivers and their correct applications are shown below.

Screwdrivers: (a) types of flat blade screwdriver; (b) correct selection of screwdriver blade; (c) types of

crosshead screwdriver; (d) the correct type and size must always be used for crosshead screws.

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Screw Types Screws are the most common form of joining technology. Force is transferred to the screw by means of positive locking. For this reason each screw requires a counterpart fitting its profile in the form of a screwdriver or bit.

Flat/Slotted Screws: The slotted profile was one of the very first screw heads and continues to enjoy widespread popularity. A simple slot on the screw head holds the tool blade. Disadvantages are the lack of centering, which means that the bit tends to slip out of the screw, as well as its sharp edges, making it unsuitable for mechanical applications. It does not offer any special properties.

With the slotted profile, the force is applied to the diagonally opposite outer edges of the slot. This shape is not particularly suitable for the transfer of high torques.

Phillips/Cross Screws: The cross-recessed Phillips profile is characterized by two slots at right-angle to each other. This crossed version offers a number of significant advantages over the classic slotted shape. The force is applied along the four edges and this reduces user fatigue when turning the screw. However, with this profile too, the user has to counteract the cam-out forces by applying an axial force.

Since the force is applied over a narrow area, there is a risk that the high stress placed on both the tool and the screw could cause damage to the profile.

Attention: The Phillips profile is often confused with the Pozidriv profile. Wrong utilization can lead to damages and fast wear of both assembly tool and screw.

Phillips/Cross Tamperproof Screws: Prevents unauthorized tampering with the screws. The Phillips/Cross Tamperproof profile has the same properties as the classic Phillips/Cross; the difference is that the tool has a hole located in the tip and the screw head has a small central pin.

Attention: The Phillips profile is often confused with the Pozidriv profile. Wrong utilization can lead to damages and fast wear of both assembly tool and screw.

Pozidriv Screws: The profile with the two crosses. The Pozidriv profile is a further development of the cross-recessed profile and features two crosses offset at 45-degree angles. However, the additional cross is narrower than the main cross and not as deep. It offers additional stability and better resistance to slipping. Although the Pozidriv profile offers an improvement to the classic Phillips slot profile, it doesn’t overcome the problems inherent in the classic Phillips design, such as the cam-out forces.

Attention: The Pozidriv profile is often confused with the Phillips profile. Wrong utilization can lead to damages and fast wear of both assembly tool and screw.

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TORX® Screws: TORX® has a star-shaped, elliptically-based geometry with six rounded lobes. The TORX® can be used to transfer a higher torque than the classical screw profiles without damaging bit or screw. Thanks to the low radial forces, the tool life of both screw and driver is prolonged. The low radial forces are a result of the circular geometry of the lobes, the straight, vertical sidewalls and the drive-angle of just 15 degrees. This geometry produces virtually no stress concentration. Unlike the cross-recessed profile, for example, the TORX® driver does not require the application of force. Furthermore, there is no danger of tool slippage, as is sometimes the case with the slotted profile. Much higher torques can also be transferred whilst applying the same amount of force.

TORX PLUS® Screws: Similarly to TORX®, the TORX PLUS® has a star-shaped profile with six lobes. However, these are flattened rather than rounded. A real drive-angle of zero degrees thus guarantees maximum engagement between bit and screw. Distributing the driving forces over a large area excludes all damaging radial forces. Through the larger cross sectional area it is possible to transfer even higher torques than with the classic TORX® profile.

TORX® Tamperproof Screws: Prevents unauthorized tampering with the screws. The TORX® tamperproof profile has the same properties as the classic TORX®; the difference is that the tool has a hole located in the tip and the screw head has a small central pin.

Square-recess/Robertson Screws: This profile is virtually defunct in Europe, but very common in the USA. It is still used by electricians to open electrical switch cabinets or by motor mechanics working with vintage cars. The four surfaces are at right-angles to each other. The opposite sides are parallel. Due to the small drive area, the square-recess profile is not effective for transferring high torques. At the same time, the lever-arm is very small.

Hex/Allen Screws: It is one of the best known and most widely-used profiles. For nuts, there is, as yet, no alternative to the nut driver. It is hexagonal in shape with six faces, all of which are at a 120-degree angle to each other. This is not good news for the transmission of force as the stress concentration is very high. This could result in the deformation of the screw.

Socket Screws: It is one of the best known and most widely-used profiles. For nuts, there is, as yet, no alternative to the nut driver. It is hexagonal in shape with six faces, all of which are at a 120-degree angle to each other. This is not good news for the transmission of force as the stress concentration is very high. This could result in the deformation of the screw.

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Hex/Allen Tamperproof Screws: Prevents unauthorized tampering with the screws. Unlike the conventional hex-socket profile, the tamperproof model has a hole in the bit tip; its counterpart in the screw head fits into this hole, thus ensuring that only the matching assembly tool can be inserted into the screw head. Otherwise, the tamperproof model has the same properties as the classic hex-socket.

Torq-Set® Screws: The Torq-Set® profile consists of four edges that are shifted asymmetrically to the centre. Torq-Set® is a security profile with an offset cross; it is designed for use exclusively by professionals. This profile is primarily used in the aerospace industry.

TriWing® Screws: Tri-Wing® is a winged security profile consisting of three edges. Thanks to its unusual, three-wing design, it is intended exclusively for use by professionals. This profile is primarily used in household appliances and in the aerospace industry.

Spanner/Snake Eyes Screws: The Spanner is a security profile with two rectangular pins, otherwise known as “snake eyes”. The spanner bit is little used in trade and industry. The advantage of this profile is the surface of the screw head, which is broken only by the two small rectangular pins and is otherwise solid. At the same time, this profile prevents over-tightening of the screw. Force transmission is associated with high stress concentrations. However, due to the two pins, there is a lower stress on the individual faces. The spanner profile is not suitable for transmitting high torque.

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Pliers Pliers are also used for assembly and dismantling operations where they are useful for holding small components and for inserting and removing split pins. They can also be used for cutting wire and stripping insulation from wires. Pliers with special jaws are used for removing circlip fasteners. A selection of pliers and their uses are shown below. On no account should pliers be used for loosening or tightening hexagon nuts and bolts as the serrated jaws would damage the corners of the hexagons.

Pliers: (a) flat nose pliers; (b) combination pliers; (c) electrician’s pliers (insulation must withstand 10,000 volts); (d) protected finished surfaces; (e) special pliers – wire strippers and circlip pliers

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Questions

1. When referring to bolts used in automotive applications, what is meant by the term ‘yield strength’? (4 marks)

....................................................................................................................................... .......................................................................................................................................

2. What is the name of the correct tool for final tightening of automotive nuts and bolts? (4 Marks)

....................................................................................................................................... .......................................................................................................................................

3. What is the name of the type of fastener illustrated below?

........................................................ (4 marks)

4. What would the following tool be used for?

......................................................................... ......................................................................... ......................................................................... .........................................................................

(4 marks)

5. Why is important that hand tools are wiped down and lubricated after use? ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... (4 marks)

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Measuring Equipment Micrometer instruments The micrometer is a precision measuring instrument, used by engineers. Each revolution of the rachet moves the spindle face 0.5mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face. The rachet is turned clockwise until the object is ‘trapped’ between these two surfaces and the rachet makes a ‘clicking’ noise. This means that the rachet cannot be tightened any more and the measurement can be read.

Using the first example seen below:

1. Read the scale on the sleeve. The example clearly shows 12 mm divisions.

2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement can be seen on the bottom half of the scale. The measurement now reads 12.5mm.

3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm).

The final measurement is 12.5mm + 0.16mm = 12.66

Sleeve Reads Full mm = 12.00 Sleeve reads ½ mm = 0.50 Thimble Reads = 0.16 Total Measurement = 12.66mm

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Dial test indicator A dial test indicator (also called a DTI, Dial Gauge or Clock) is a precision and delicate instrument that has a plunger projecting out of the bottom and a dial commonly graduated in increments of 0.01mm. When the plunger is moved, a rack and pinion mechanism causes the pointer to rotate around the dial, showing the actual amount of plunger movement. A smaller pointer registers the number of whole rotations the pointer makes (usually 1mm of plunger movement per revolution of the larger pointer). DTI’s must be held firmly in a static position and a magnetic base is often used for mounting DTI’s as it allows flexibility in positioning the instrument. When used for checking components in a production situation, a cast iron clock stand would be used. DTI’s are used in a number of ways For checking that a component is within its

prescribed limits For checking roundness using a V block For setting work level on an angle plate For setting machine vices For accurately setting work in a lathe 4 jaw

chuck For checking run-out

To use a DTI to measure brake disc run-out

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Feeler gauges Feeler gauges are generally supplied as sets of shims (very thin pieces of high quality steel). Each shim has a different thickness and is inserted into small gaps to assess the width of the gap. Feeler gauges can be used to assess the amount of distortion in a steel bar after heat treatment by putting the work on a flat surface and then inserting a thin feeler gauge into the gap between the work and the flat surface. If the feeler gauge goes in, a thicker one is tried, the process is repeated until the thickest possible gauge or combination of gauges is found; this represents the size of the gap.

Engineers Rule An engineer’s rule is a simple but useful measuring device and can measure accurately to 1/64 inch imperial and 0.5mm metric. Care must be taken to ensure

that they do not rust and should be oiled after use. When using the rule, align one end of the item to be measured to the zero line and measure to where the item ends.

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For greater accuracy, it it sometimes good practice to begin the measurement from the 1cm mark and deduct 1 cm from your final reading.

2 1 3 4 5 6 7 8 MM

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Questions

1. Examine the micrometer readings below and enter your reading in the space provided (9 marks)

2. What is the smallest accurate measurement that can be made on a standard metric 25 – 50mm micrometer? (4 marks)

............................................................................................................................ ............................................................................................................................

3. Why is important that measuring devices are wiped down and lubricated after use? (4 marks)

............................................................................................................................ ............................................................................................................................

4. What is the smallest accurate measurement that can be made on an engineer’s rule? (4 marks)

............................................................................................................................ ............................................................................................................................

5. What is the measurement shown on the rule? .............................. (4 marks)

2 1 3 4 5 6 7 8 MM

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Principles of Electricity Electrical Terms In many ways electricity behaves similarly to water. Cables are like pipes and many electrical terms, expressions and components can be related to hydraulic systems. One obvious difference is that electricity cannot normally be seen, whereas opening a water tap readily shows the flow. Part of the mystery of electricity can be dispelled if its basic behaviour is understood: this study demands a clear understanding of electrical terms. Furthermore, this basic knowledge is essential before any electrical test meter is used on a vehicle. A comparison between water and electricity is used here to introduce and describe the basic terms and principles. Pressure Electrical ‘pressure’ is expressed in volts (V) and is measured by a voltmeter. This meter has two leads to enable it to show the difference of pressure between the points in the circuit at which the leads are connected. A voltmeter connected across a battery indicates the potential difference (p.d.) between the two battery terminals – a typical reading is just over 12V. Water pressure is measured by a pressure gauge. If a sealed water container is pressurised, the water inside will try to escape. This is similar to a battery; the ‘pressure’ of the electrical energy exerts a force called electromotive force (e.m.f.) which attempts to drive the electrical current from one battery terminal to the other. Flow The amount of electricity flowing through a cable is called the current and is expressed in amperes (A). It is measured by an ammeter, which is connected so that the current passes through the ammeter, as it flows around the circuit. Electrical current flow is similar to a flow of water, which is given in litres/second. Resistance This term indicates the opposition to current flow. The unit of resistance is the ohm – the symbol for ‘ohm’ is the Greek letter Ω (omega). Cable sizes affect the

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resistance. A thin cable has a higher resistance than a thick cable, just as a small pipe has a greater resistance to water flow than a large pipe. Resistance is measured with an ohmmeter or the ohmmeter function of a digital multimeter. Set to Ohms. Connect the test leads and check the meter for zero – calibrate if necessary. Power The power is the work done in a given time and is expressed in watts (W). The power can be calculated from the expression: Power = Voltage x Current Or Watts = Volts x Amperes Ohm’s law The relationship between the three electrical terms is given by OHM’s law; this may be written as:

One volt is required to force a current of one ampere through a resistance of one ohm.

The law can also be expressed as Voltage = Current x Resistance Or V = IR where V = voltage I = current in amperes R = resistance in ohms The table below summarises the various electrical units.

Quantity

Unit

Symbol

‘Pressure’ or potential difference

Volt

V

Current (flow)

Ampere

A

Resistance

Ohm

Ω

Power

Watts

W

Conductor A material that freely allows a flow of electrical current is called a conductor. The low resistance of copper makes this metal suitable to use as a cable.

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Insulator An insulator is a material, such as rubber or PVC plastics that resists or opposes the flow of electricity. A cable is covered with an insulation material to prevent electrical energy taking a shorter path than intended. Electrical symbols Diagrams of electrical systems are shown either in pictorial form or theoretical form; in the latter graphical symbols are used to indicate the various items that make up the circuit. Many separate parts are used in an electrical system of a motor vehcile, so a convention is needed to enable people to understand the various graphical symbols. In the UK the British Standards Institution (BSI) recommend that the symbols shown in BS3939: 1985 should be used. Some of the main symbols are shown on the following page. Electrical Circuit Symbols Circuit

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Water will not flow around a hydraulic circuit unless the pipework makes a complete path from the supply tank, around the system and back to the tank. This concept also applies to an electrical system – electricity will not flow unless there is a complete circuit. When a circuit incorporating a battery is formed, small electrical charges called electrons move around the circuit. These charges pass through the battery, along the cable and back through the battery, the charge movement being initiated by the battery. When electrical flow is taking place, all the electrons in the various parts of the circuit move simultaneously; this shows why a complete ‘pathway’ must be provided before electron movement can take place. The action is similar to the flow in a pipe bent to form a loop and filled with water. No flow of water can take place in one part of the pipe unless there is a corresponding water movement in the remainder of the loop. A simple electrical circuit is formed by connecting a lamp and switch to a battery. The diagram below shows two ways of illustrating this circuit: pictorial and theoretical (schematic). The flow of electricity is controlled by a switch; this breaks the circuit and stops the flow of current like a tap in a water system. Closing the switch operates the lamp and causes current to flow through a fine wire filament, this becomes incandescent and gives the illumination. The terms closed circuit and open circuit mean the electrical circuit is complete and incomplete respectively.

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Series and Parallel Circuits There are two main types of electrical circuits, Series and Parallel. By connecting a circuit as shown below, it can be seen that different voltages are achieved using the same power source. Series Parallel Earth return Using the metal body and frame of the vehicle as a part of the circuit reduces the length of cable and simplifies the layout. The frame is called earth (or ground) and is connected to one of the battery terminals. Normally the negative terminal is ‘earthed’ and in this case the polarity of the vehicle is described as ‘negative earth’. Use of the word ‘return’ is associated with the current flow in a circuit. When a lamp is connected to a battery by two cables, one cable is called the feed and the other the return. By substituting the vehicle body for the return cable, and earth return

12V

3V 3V 3V 3V

+ - Current flows from the positive side of the battery, to the positive terminal of the light bulb, through the light bulb to the negative terminal which is connected to the positive terminal of the next light bulb. Because there are four light bulbs in the circuit, the 12 Volt supply is divided by four and each bulb receives 3 Volts.

12V

12V 12V 12V 12V

+ -

Current flows from the positive side of the battery and feeds each positive terminal of the light bulb independently. The current returns to the negative side of the battery via the negative terminal of the bulb.

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system is formed. The alternative two wire arrangement called insulated return is seldom used other than for special-purpose vehicles, e.g. petrol tankers. It is essential that the battery terminals are connected to give the correct earth polarity. Serious damage to components, especially those containing electronic devices, will occur unless the equipment is connected to give the correct polarity. Circuit faults The two faults shown below which occur in a circuit are:

Open circuit Short circuit

Open circuit This is the term used to describe a break in a circuit. A switch is an intentional means for making an open circuit and is used to control the flow of current. Sometimes an open circuit occurs when it is not required; this fault is caused by a fracture in a cable or a poor connection. Short circuit Electricity always takes the shortest route as it flows from source to source, so if a sharp edge of the body cuts through the insulation of a cable the current will flow to ‘earth’ rather than travel around the full circuit. The short-circuit path increases the

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current in the supply cable and if this is excessive, overheating of the cable will cause the cable covering to melt and smoulder. Unless a fuse is fitted the short circuit may well cause a fire. The thickness of the insulation of a cable is dictated by the voltage that the cable has to carry – the higher the voltage, the thicker the insulation. Fuse A fuse is a short length of thin wire, which is designed to melt and break the circuit if the current exceeds the rated value marked on the fuse. Provided it is of the correct rating, a fuse reduces the risk of fire in the event of a short circuit. A number of fuses, or other forms of thermal circuit breaker, are fitted to protect the various circuits of a vehicle. Some electrical components require a comparatively large current, so in these cases a fuse having a high ‘amperage’ is specified. Main electrical circuits The battery, which is the energy source when the engine is not running, is connected to a number of separate circuits. The main circuits are: 1 Charging – supplies the electrical energy when the engine is running. Also this

system maintains the battery in a fully charged state. 2 Ignition – provides a spark to ‘fire’ the mixture in a petrol engine’s cylinders. 3 Lighting – gives the necessary exterior and interior illumination 4 Starting – enabling the engine to be cranked over at a speed sufficient for it to

‘fire’ and keep running. 5 Auxiliary – includes the various accessories such as windscreen wipers and

washers, direction indicators, horn, in-car entertainment etc. The Digital Multi-Meter The tool commonly used by mechanics to locate electrical faults is the digital multimeter. This is a useful tool and carries out many of the electrical measurements accurately and cheaply.

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Questions

1. Give one example of a good and one example of a poor conductor of electricity. (4 marks)

....................................................................................................................................... .......................................................................................................................................

2. Describe the checks that should be made to an electrical measuring device before attempting to measure electrical resistance. (6 marks)

....................................................................................................................................... .......................................................................................................................................

3. Name the electrical test instrument shown. (4 marks)

............................................................................................ ...........................................................................................

4. Describe why a fuse is required in an electrical circuit. (4 marks) ....................................................................................................................................... .......................................................................................................................................

5. Sate the meaning of the following electrical symbols. (4 marks)

V = ....................................................... A = ....................................................... W = ....................................................... Ω = .......................................................