engineering principles for gcse engineering

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LSDA Local Network

GCSE in Engineering

Course Notes. 1. Workshop Safety 2. Handling Engineering Information. 3. Marking Out and Measuring Workpieces. 4. Common Workshop Hand Tools. 5. Bench/pedestal Drilling Machines, Tapping & Threading. 6. Off-hand Grinding Machines. 7. Common Engineering Materials. 8. Basic Turning 9. Basic Milling 10. Basic Heat Treatment

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1. SAFETY PRECAUTIONS IN ENGINEERING WORKSHOPS. Most accidents are due to carelessness, taking risks and disobeying rules. Students/trainees must therefore, be alert to the dangers in workshops, know the safe way to do their job, and why they must instinctively OBEY the safety rules. Remember that the following rules are for YOUR benefit.

RULES AND REGULATIONS 1. Students must NOT enter the workshop area unless accompanied by a member of staff. 2. LONG HAIR: students who are time-tabled for practical work in the workshops are not allowed to operate any

machine whatsoever, unless they are wearing a HAIRNET or a HAT into which their hair must be tucked INSIDE.

3. CLOTHING: Overalls must be provided by the student and worn in the workshop. The most suitable type is a

one-piece boilersuit of the correct size and in good condition. Overall buttons must be kept fastened. Torn overalls must be repaired, as they can catch in moving machinery or workpieces. Sleeves MUST be either fastened at the wrists or be tightly rolled up above the elbows. Loose bandages, ties, scarves; watches, or rings can also catch in moving machinery and MUST NOT BE WORN. Overalls must be removed before entering eating areas such as the refectory.

4. Sturdy boots or shoes preferably of the safety type MUST be worn. Students wearing unsuitable footwear will

NOT be allowed in the workshops. 5. CLEANLINESS: Wash your hands well and clean off any grease or oil before eating. Wash your hands after

going to the toilet. Try not to wipe your arms and hands on a dirty cloth or on your clothing while working, as metal particles may cause small cuts and abrasions and start infection.

6. SPITTING is prohibited as it can spread disease and is a disgusting habit. 7. FIRST AID: Even a scratch can become a serious matter if not properly treated. Always report cuts and

injuries and ensure you receive treatment at the first aid room. Never treat another student's injuries, especially where foreign bodies are in or near the eyes.

8. PROTECTIVE EQUIPMENT: Respirators, gloves and goggles will be issued to you for certain operations. The

DANGER is always there and your safety depends on using them correctly. Take good care of the equipment (for examples DO NOT place safety glasses with the lenses in contact with other equipment where they may become scratched), supplied to you and report any defects to your lecturer.

9. PRACTICAL JOKING AND HORSEPLAY: A practical joker in the workshop is a serious danger to his work-

mates, people have been killed in workshops by stupid jokes. NEVER throw anything to or at another person. NEVER be tempted to try out unfamiliar machines or equipment.

10. It is always dangerous to RUN in a workshop or in corridors, particularly at blind corners, and when entering

or leaving college. 11. HANDS OFF: Never meddle with switching controls, levers, valves, handles or taps, etc, or interfere in any

way with gas, water, air, electric or fire fighting equipment. On entering workshops which contain machines or equipment, it is not the signal for you to try out all the handles and controls, as serious damage could result if machines have been previously set. This is not only a danger to the machine, but even more important, it is a serious danger to the unsuspecting operator.

NOTE: 12. Students who, in the opinion of the lecturer are unfit (e.g. ill, intoxicated etc.), to be in the laboratory or

workshop will be refused admission. 13. COMPRESSED AIR: Compressed air is very dangerous if misused. The most serious injury and death can be

caused by quite low pressures directed on the body, especially on sensitive organs such as the eyes or ears. Compressed air is not to be used for cleaning or blowing down operations except those authorised by the staff.

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14. LIFTING LOADS: Do not try to lift components, which are too heavy for you, seek HELP or use the correct lifting equipment when it available. Learn the safe way to lift loads; use LEG and THIGH muscles NOT the arms and back to do the work.

15. METHOD - take a firm stance with the feet slightly apart to give good balance; bend your knees and grip the

load. Then, with your back straight and chin tucked in, LIFT by straightening your legs. Lifting with a bent back or snatching or jerking, can produce serious strains, hernias or injury to the back. It must always be possible to SEE over the load and round the sides of the load being carried.

16. FIRE PREVENTION Smoking is prohibited anywhere in the College. Careless disposal of cigarette ends and

matches can cause fire. Avoid the accumulation of inflammable waste material and make use of the disposal bins provided.

17. Check all GAS and AIR VALVES are closed and safe before leaving the workshop. 18. WARNING SIGNS: Watch out for signs warning you of DANGER POINTS. Red flags, Tiger Stripes and

danger notices highlight especially dangerous conditions; DO NOT ignore them and give a wide berth if you are not working there. If you do not fully understand what they mean - ASK. Do not interfere with such notices unless you have received instruction to do so from the staff.

19. OBSTRUCTIONS: Look where you are going and use the recognised marked-out gangways. Do not take

shortcuts between or under, or climb over machinery or barriers. Take extra care on stairways and use the handrails. Do not put objects down where they will be a danger to yourself or others. Keep gangways clear of machine tools, chucks, faceplates, bars etc., also beware of obstructions projecting from machines, such as long components on lathes and milling machines, etc.

20. CRANE AND TRUCK LOADS: Look out for moving cranes and trucks, as the driver may not be able to see

you. Never walk under or too close to a load being carried by cranes, pulley blocks, fork lift trucks, etc., watch out for the person whose job it is to give warning of an approaching load.

21. STACKING MATERIALS: Take care when stacking materials and watch out for any materials carelessly

stacked. 22. LADDERS: Treat ladders with caution, and if any doubt regarding their condition or safe use, inform the staff. 23. HAND TOOLS: The many accidents with hand tools are caused through using the wrong tool for the job, or

using one which is faulty. Defective tools must never be used as they can cause severe injury, particularly to the hands or eyes.

24. Make sure that handles are not split and are firmly fitted on all files, hammers, screwdrivers, etc. Ill fitting

spanners, hammers with chipped faces, chisels with MUSHROOMED heads must not be used. 25. Remember that ALL must be stored and transported safely. 26. POWERED HAND TOOLS: Half of all fatal accidents in industry involve the used of defective electrical

equipment. All machines should be checked before use and any defect such as frayed wires, loose connections etc., must be brought to the attention of the staff.

27. Students must not use portable electrical machines (i.e. drilling machines), near radiators, concrete floors,

sinks, water pipes or any earthed machine which provides low resistance paths to earth. This will minimise the risk of anyone being able to touch an electrical connection and an earth supply at the same time. Machines must NOT be connected to a 2-point electric supply or to lighting sockets.

28. No electrical apparatus, lead or connection should be touched with DAMP or WET HANDS, nor when

standing on a WET SURFACE. 29. BENCH WORK: Keep your bench and floor areas CLEAN and TIDY. Do not place heavy objects near the

edge of the bench and wipe up any oil or grease, etc. spilt on the floor. Make sure that workpieces are securely held in the vice etc. so that there is no possibility of it falling out or slipping while you are working on it. Defective vice jaws should be reported to the staff. Always use a chipping screen and goggles when carrying out chiselling or chipping operations. Do not carry sharp or pointed tools in your pockets.

30. MACHINES AND MACHINING OPERATION: When operating a machine, concentrate on the job you are

carrying out. a. Do not use a machine until you know how to operate the controls. b. Do not set in motion a machine with any guard removed.

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c. Do nor clean any SWARF or CUTTINGS with your bare hands or attempt to clean a machine while any part of it is in motion.

d. If you have to approach a machine operator, take care not to startle him or her so that attention is distracted from the work.

e. Never walk away and leave a machine tool running. f. Isolate machine tools, especially milling and shaping machines, when setting-up work. g. If oil, grease or coolant is dropped on the floor near your machine, make sure that it is wiped up as soon

as possible. Failure to do so might cause someone to slip and cause injury. h. Ensure that workpieces are held securely in appropriate holding devices before commencing any cutting

operations. i. Make sure that chuck keys are removed from lathes and drilling machines before starting. NEVER leave

these keys in the chuck at ANY TIME. j. When operating milling machines, ensure that the arbors and/or cutters are firmly held and running true

before starting up. Also make sure that the workpiece and/or dividing head vices are securely held to the table before cutting takes place.

k. When using grinding machines of the surface or cylindrical type, the workpiece must be securely held in the holding device before starting up the machine.

l. Make sure that NO-ONE including your self is in-line with the abrasive wheel on a grinding machine when it is started up.

m. Ensure that the work rest on off-hand grinding machines is not more than 2mm from the abrasive wheel before starting work.

n. Insecurely held work is the major cause of accidents on drilling machines. A secure method of work holding must be adopted, (i.e. use a vice or clamp to the machine table, an angle plate or Vee blocks). Parts being drilled MUST NOT be held by hand.

o. Furnaces and forge hearths must not be switched on or ignited by students unless under the direct supervision of the staff.

p. Molten metal may only be poured by students under the direct supervision of the staff. q. Perspex guards on machines must be cleaned before use.

31. EVACUATION PROCEDURE: In the event of an EVACUATION ALARM sounding, switch off all machines tools, leave the building via the nearest exit and assemble on the allocated area outside the main building, the appropriate area is shown in your student hand book. Report to your lecturer where the class register will be checked to ensure that all students have left the building and are accounted for. Do NOT re-enter the building until told to do so by the staff.

BASIC SAFETY RULES The basic points of the above safety regulations are as follows:

• Learn the safe way of doing each task.

• If you do not understand ask for an explanation.

• If you have not been taught or shown the correct methods then ask for instruction.

• Always use the safe method in practice

• Be constantly on guard against careless action by your self and others.

• Practice good housekeeping at all times.

• Cooperate promptly in the event of a fire or an accident.

• Report all accidents to the staff.

• Draw the attention of the supervising staff to any potential hazards.

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2. HANDLING ENGINEERING INFORMATION. The following section is intended to illustrate the main points necessary to understand and read simple engineering drawings. Engineering information may be presented in several different forms, i.e. graphical, tables charts or written. Perhaps the most common method used in the workshop is graphical forms such as, drawing and diagrams. Diagrams tend to be used for electrical and electronic wiring and or layouts. Engineering drawings are usually either assembly drawings or detail drawings.

Reading and Understanding Engineering Drawings. In order to produce components correctly it is important that all relevant information is extracted from the drawing and interpreted correctly. The engineering drawing also serves to provide a record of designs & it is important that each drawing is identified with a unique number & is carefully filed away for future reference. Large complex product drawings are drawn as general assembly drawings which may be broken down into several layers of sub-assembly drawings, the last level of drawing is the detail drawing, which is the type that you will normally be using during your initial training. Engineering drawing principles are governed by a set of recommendations contained in British Standard number 8888. This is contained in three parts, covering general principles, dimensioning and tolerancing, and geometrical tolerancing. BS 8888, and is often seen as the rule book of engineering drawing. Some companies employ their own drawing standards, but are usually based on BS 308 or other international standards. Drawing sheets in common use are referred to as the 'A' series sheets and start with A0 (which has a surface area of 1m², used in landscape format, i.e. wider than high). This is followed by A1 size sheets which are one half of the size of an A0 sheet. This process of halving the sheet size continues down to a practical limit of A4 (which is often used in portrait format, i.e. higher than wide) but smaller sizes may be encountered. Most of the drawings that you will use in the workshop at the college will be A4 format. Every drawing should have a title block, which should show certain minimum amount of information, such as;

• the company name,

• the drawing number, & drawing description,

• the date,

• who drew it, i.e. drawn by,

• the projection symbol (see later notes),

• the units of measurement (usually millimetres or older drawings in inches),

• the 'original scale', used on the drawing as enlargement or reduction may have taken place during reproduction or plotting,

• the scale should be shown in the form 1:2 etc. Any other supplementary information may also be included on the drawing such as the company sees fit, this may include information on revisions or different versions of the drawing, always check that you are using the latest version of a component drawing.

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Understanding Drawing Projection Systems. The principal projection method used for engineering drawings is that of multiple view orthographic projection. This system shows only one face of the component in each view. Each view is aligned with other views to give a full picture of the component, usually three views are sufficient for simple components, but more complex parts may require more views to show all necessary detail. Within this type of projection two systems are used, that of 1st. angle and 3rd. angle projection. Neither system has advantage over the other, but 1st. angle projection is the more common system used on British drawings. The method of projection system must be clearly indicated on all drawings, either in words or using a standard BS308 symbol.

Symbol

1st Angle

3rd Angle

A Typical Drawing Frame & Title Block Important Notes

General Tolerances Drawing Units & Projection System

Material Details

Draughtsman & Drawing Details

Symbol

1st Angle

3rd Angle

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Hidden Detail and Sectional Views. Features of components are not be visible in the normal views, may be shown as hidden detail using thin dotted-lines, but if the amount of detail is excessive then this method may become confusing. An alternative method is to use section views. Sectional views are invaluable to clearly show the internal detail of a component not normally visible in external views. Sectional views involve taking an imaginary saw-cut through the component, cutting through the feature which is required to be shown. The area of the cut face may be filled with a hatch pattern, usually a thin type line at 45°, spaced about 4mm apart, but some special hatch patterns may be used to indicate certain materials.

Symbols and abbreviations. Symbols and abbreviations are used on drawings to conserve space and time while maintaining a clear precise description. In order that all abbreviations are understood only accepted abbreviations should be used. Some of the abbreviations that you come across during the foundation training are; ø diameter preceding a dimension CBORE counterbore CSK countersunk DIA diameter EQUI SP equally spaced LH left hand PCD pitch circle diameter R radius preceding a dimension RAD radius or radii RH right hand SFACE spot face SQ square in a note � or � square preceding a dimension

A Comparison of 1st and 3

rd

Angle OrthographicProjection

3rd Angle OrthographicProjection

1st Angle OrthographicProjection

FRONTELEVATION

ENDELEVATION

PLAN

FRONTELEVATION

END

PLAN

AUXILIARY VIEWS

AUXILIARY VIEWS

ELEVATION

END

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Dimensioning & Tolerancing

With BS 308: Part 2:, recommendation are given for the general principles of applying dimensions and tolerances to engineering drawings for the manufacture of components relating to the condition in which they are to be used, including specification of any special surface treatments or finishing processes. It is important that you interpret sizes from drawings correctly. Note ALL dimensions are read from the bottom or from the right hand side of the drawing (this should avoid misinterpreting 6's and 9's). Typical features and examples of dimensions are shown in the figure below. In general, all necessary dimensions, tolerances and information to give a complete description of each feature should be given. It should be noted that all dimensions must have a tolerance, but may not necessarily appear with the dimension, i.e. tolerances may be assigned by the number of decimal places of the dimension, as given in a table in or near the title block. This method is used on most of the workshop drawings that you will be using. When dimensioning a component consideration must be given to the purpose of the component in order to decide which features affect the function of the component. Dimensions for functional features should be given directly & should not have to be found by adding and subtracting sizes from other dimensions. Tolerances limits and fits will be examined in more detail in the following section on marking-out and measuring workpieces.

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3. MARKING-OUT & MEASURING. Marking out (after process planning) is the first stage in producing a component and involves producing a visible outline of the component features on the raw material. This may involve scribing lines or marking the centre point of holes, which may be used for fitting purposes or for machining. The principle of selecting a datum (a starting point or reference point) is often used when marking out. A datum may be an edge, a point or a combination of these. In order to avoid errors it is important that all dimensions are taken from a common datum. When marking out the correct equipment should be selected, and checked before use for damage and then used correctly. The selection of marking out equipment will depend on the required accuracy of the feature of the component and the type of feature, i.e. a straight edge, a curve, or hole positions. Marking out should be carried out on a proper marking out table or on a surface plate (used on top of a normal work bench), both of which are usually made of cast iron and are made so as the top surface is flat to fine limits. When not in use the marking out table top or surface plate must be kept covered to prevent accidental damage. Before marking out, all surfaces must be clean, and kept clean throughout. The surfaces to be marked are covered with a marking medium, usually a thin coat of a blue substance known, strangely enough, as marking blue or by other various trade names. Other marking media may be used such as white wash on castings, or red-lead (which does not contain lead as it is now illegal to use for such compounds), even a permanent marker pen may be used for small components where only a small area needs to be covered. Some of the more common hand tools used in marking out are shown and described below. Scribers and straight edges are used together to produce straight fine lines. The scriber is a hardened steel pin which has a fine sharp point, which must be kept sharp to be effective and the straight edge or steel rule must be free from dents and should be checked for straightness on a marking out table from time to time. The engineer’s square consists of a blade and a stock which are set precisely at 90° to each other. When in use it is important to keep the stock of the square tight against the datum edge, both when marking out and checking for squareness. An engineer's square may be checked for squareness as shown below, when turned over the blade should line along the first scribed line.

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Dividers are used for scribing circles or arcs and may also be used to mark off lengths taken from the engraved lines of a steel rule.

Where radii are required which are larger than are possible with ordinary dividers then trammels may be used. These consist of two points carried on a long trammel bar. The points may be slid along the bar and locked in position.

Note when marking-out with dividers, trammels etc, a light centre pop mark is required to locate the divider point in the correct location. If the pop mark is out of position and needs to be moved slightly then if has been done lightly then this may still be done. A pop mark that is too deep cannot be easily moved and may mean that the job is scrapped at a very early stage. The pop mark may be deepened later if required for drilling by a second blow with centre pop and hammer. Odd leg callipers (or jennies) are used for scribing lines parallel to a datum edge. Another use is for finding the centre of circular bar, by marking arcs at several positions around the bar, the centre of the bar lies at the intersection of the arcs.

Scribing blocks (also referred to as surface gauges when fitted with a fine adjustment mechanism) are used to scribe parallel lines on workpieces, which should be held firmly against an angle plate. The point of the scriber should be drawn across the work such that the point does not dig into the work. An angle plate is a large cast iron block, which is machined to have faces, which are precisely 90° to each other. Scribing blocks are also used to set lathe tools to centre line height which you will come across later. Scribing blocks are usually set against a steel rule and there fore have an accuracy of about ±0.5mm.

When greater accuracy is required, then a vernier height gauge will need to be used. The vernier height gauge can be set to an accuracy of ±0.02mm. Care must be taken when handling this type of equipment as it is expensive and easily damaged if dropped. When not in use it must be returned to its storage box which should be kept closed. As well as being used for marking out the vernier height gauge is also used for measuring work pieces. Before use the vernier height gauge should be checked and zeroed to ensure accurate readings. This procedure will be demonstrated to you later.

Trammels

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Setting and reading vernier scales requires some practice this will be looked at in more detail later. In order to scribe angled lines, an adjustable angle bracket may be used. This very similar to a normal angle plate but has a pivot joint which allows the vertical plate to be rotated and locked in various positions. This may be set via a built-in scale or by using an adjustable protractor. A standard protractor may be accurate to ± 0.5°, where greater accuracy is required then a vernier protractor (accurate to ± 5') will need to be used.

When even greater accuracy in marking out angles is required, then a common method is to use a sine bar. The sine bar consists of a rectangular section of bar which has two matched, hardened precision steel rollers mounted an exact distance apart (usually multiples of 50mm) and so that a line centre between the centres of the two rollers is parallel to the top face of the bar. Using a little trigonometry and a set of slip gauges, it is possible to accurately set-up work for marking out at any angle.

Slip gauges are sets of steel block which are manufactured to extremely fine tolerances and high surface finish on the ends. They are used as standards for measurement by comparison (as opposed to direct measurement - see notes on Measuring Equipment & Techniques), by combining slips in plies they may be used to create a block of any height up to the maximum for the set. When using slip gauges they MUST be kept clean by wiping with a soft clean cloth and returned to the storage case when not in use. By using a technique called wringing the slips may be wrung together. As the surface finish on the slip is so fine by twisting and sliding the slips together all of the air is excluded from between the faces. As the air is excluded then atmospheric air pressure holds the slips together i.e. they are wrung together. Slip gauges are made in various grades of accuracy; British Standard 888 (Imperial) Workshop grade - for general purpose use in the workshop or production areas. Inspection grade - for highest accuracy in standards rooms. Calibration grade - used for the measurement and control of the manufacture of inspection grade

gauges. Reference grade - the most accurate and expensive gauges, very rarely used except by the likes of

the National Physics Laboratory (NPL). British Standard 4311 (Metric) Grades 0, I, & II - for general purpose use in the workshop or production areas. Calibration grade - used for calibrating other sets of slip gauges. Grade 00 - very rarely used, the most accurate and expensive gauges without using

calibration charts.

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The basic procedure for setting up sine bar is as follows;

• select a suitable size sine bar and ensure that it is clean,

• determine the sine of the required angle (from tables),

• calculate the height of the slip gauges required including protector slips, i.e. sine of the angle X sine bar centre distance,

• select, clean and wring the slips together,

• mount the sine bar on the slip gauges and then the work piece on the sine bar.

When the need arises to hold work which is round in section, then Vee blocks are used. These are usually made in matched pairs so that long workpieces may be held accurately. Work may be mounted horizontally or vertically. Vee blocks are usually made of cast iron or surface hardened and ground steel, and when not in use they should be returned to their storage box so that they are protected from damage.

To ensure accuracy when marking holes for drilling where the centre point is removed once drilling starts, holes are often scribed with the hole size or are boxed with a square scribed to the size of the required hole. This allows the hole position to be checked after drilling, i.e. the outer pop marks should be just visible and equally disposed around the drilled hole. In addition to this practice of marking holes, scribed lines are often made easier to see by lightly centre popping marks along the lines at intervals of 5~10mm. These pop marks are referred to as witness marks, and should still be just visible even when you have filed or machined the original marked line away.

Measuring Equipment & Techniques. There are two basic type of measurement, direct measurement and transfer of measurement (measurement by comparison). In direct measurement a scale or rule is positioned along side the workpiece and the size read from the rule. Alternatively a pair of vernier callipers or micrometer may be used to read a size directly. With transfer

sin A = a/c

a = slip heightc = roller centre distance

Oppositeside

slips

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measurement the workpiece is compared to a known standard either a template or pattern, using plain inside or outside callipers (the callipers themselves may be set directly from a steel rule), or a more commonly a pile of slips or using a dial test indicator. When the accuracy required is in the region of ±0.5 mm then a steel rule or tape is generally sufficient. In order to transfer sizes from a component to a rule, or vice versa callipers may be used. These come in a variety of types, but basically they are outside or inside types and may have friction joints or be spring types which have a fine screw adjustment.

When greater accuracy is required then a micrometer needs to be used. Micrometers come in a variety of sizes and types, the most common being the external micrometer. A simplified diagram of a micrometer which will measure from 0 up to 25 mm (0~25 micrometer) is shown here. Other sizes would be 25~50, 50~75 etc. each micrometer having a measuring range of 25 mm, (imperial micrometers would have a range of 1"). A standard micrometer will have an accuracy of 0.01 mm when used with care. When not in use they must be stored correctly in the appropriate case. Prior to use the micrometer should be checked that a reading of zero (or 25, 50, etc) is obtained when closed. If not the micrometer may be adjusted, seek assistance from the staff if this needs to be done. Other forms of the micrometer included depth micrometers, and internal micrometers. You may also find micrometer heads incorporated into specialist precision measurement devices. Looking in more detail at the micrometer barrel and thimble it can be seen that the barrel is marked in divisions of 1mm and subdivisions of ½mm. One revolution of the thimble moves the spindle 0.5 mm, therefore each division on the barrel is equal to 1/50th of ½ a millimetre i.e. 0.01 mm. The micrometer is read as follows;

• read the number of whole and ½ divisions visible on the barrel,

• read the value of the line on the barrel which lines up with the axial line on the barrel,

• add the two values together to obtain the full reading. The reading for the example shown above, for example is; 16.5 + (20 x 0.01) = 16.70 mm.

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Note: when using depth micrometers the scale on the barrel is reversed, but otherwise reads in the same manner. When measuring a workpiece ensure that both work and the faces of the micrometer spindle are clean. Measurements should be taken at several points to ensure that any high points are detected. When measuring round section work, make sure that readings are taken across a true diameter and at several points along the work to check for tapered work. Vernier callipers come in a variety of different styles, the type shown below have dual scales, i.e. metric and imperial. The vernier scale consists of two scales one fixed to one jaw and the second which is fixed to the sliding jaw. The fixed scale is graduated in millimetres or inches as a normal rule. The moving scale has the same number of divisions but the distance between each division is smaller. The common type of calliper found in the workshop has a metric scale graduated in 1mm division, with a sliding scale 50 divisions are spaced over an actual distance of 49 mm. The accuracy of this form of the vernier scale is given by 1 - (49 ÷ 50) = 0.02 mm. Vernier callipers may be used to measure external dimension, internal dimensions and by using the tail piece or the end step, shoulders and steps may be measured. Care must be taken to ensure that the faces of the jaws and the faces to be measured are clean before taking a reading and that when the jaws are fully closed they are parallel to each other (no light is visible between the faces) and the scale is zeroed correctly. As with the micrometer, readings should be taken in several places to avoid errors due to high spots on the work. Some vernier callipers incorporate a fine adjustment thumb wheel, some have locking screws and some have a spring operated clamp to retain readings when the callipers are removed from the workpiece, although it is usually better to take the reading while still in contact with the workpiece.

Note unless the vernier calliper is specifically designed for measuring internal diameters, i.e. the internal jaws are rounded to a known diameter, then they should not be used for internal diameters as there will always be a small flat on the measuring face of the jaw which will lead to errors. The vernier scale is read as follows;

• read the number of whole divisions on the fixed scale,

• read the value where the lines are coincident on the fixed scale and the moving scale,

• add the two values together to obtain the full reading.

��

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Examples. Where the moving scale is marked as shown the total reading is the value on the fixed scale plus the number of divisions on the moving scale x the accuracy of the calliper i.e. 0.02,

17 + (16 x 0.02) = 17.32 mm. Where the moving scale is marked as shown here the total reading is the value on the fixed scale plus the value on the moving scale,

17 + 0.32 = 17.32 mm. In this case the value is read directly. In addition to direct measurement, comparative measurement may be done using a variety of apparatus such radius gauges, feeler gauges and plug gauges etc. these will be demonstrated as and when necessary in the workshop.

Measurement Of Surface Texture. As well as the physical dimensions of a workpiece, the surface finish, or more precisely the surface texture may be important to the function of the component. Perhaps the easiest method is to use comparison plates (also known as Rupert blocks). By using a fingernail scratched along the workpiece and the comparison plate in turn, a fairly close reading may be obtained. More refined methods of checking surface texture are available using modern electronic devices, but these are quite expensive. The surface texture is expressed as a value representing the height of surface irregularities above and below an average surface centre line. This is called the Centre Line Average (CLA) number and is expressed in micro-inches (0.000001") in the imperial system. In the

metric system, this number is given a µRa number in micro-metres or microns (0.001 mm). Typical values obtained by normal workshop processes are shown in the table below.

CLA

Process 8

16

32

63

125

250

Drilling

Grinding

Reaming

Turning

Milling

Shaping

0.2

0.4

0.8

1.6

3.2

6.3

µRa

Limits and Fits. Due to variations in manufacturing processes ALL features of any component are subject to variations in size or deviation from the intended size. In order to ensure that satisfactory performance is obtained from the component, it is necessary to exercise some control over this variation. This is achieved by a system whereby an acceptable deviation from the true or ideal size is stated in the form of allowable limits or as a tolerance. In general terms a tolerance is the difference between the biggest and smallest acceptable sizes of any feature of a component.

Coincident lines

Coincident lines

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British Standard Limits & Fits, BS 4500: 1969. As with many aspects of engineering, in order to bring some uniformity to the application of limits of size a British Standard has been developed as a guide to this process. Without a consistent systems of limits and fits several problems were encountered:

• components had to be produced individually, which is prohibitively slow and costly;

• highly intensive skilled labour was necessary for assembly of parts which is also prohibitively costly;

• automation and the benefits that this may bring cannot easily be applied;

• replacement parts must be made to order in the same manner as the original and to the same size;

• some fits will be better or worse than others and with unknown variation. By employing a system of limits and fits all of the previously mentioned disadvantages are eliminated, and random selection and assembly of parts is possible. This principle is known as interchangeabilty and in itself offers other advantages:

• certain components may be produced by specialist manufacturers which allows reduction of manufacturing and production costs;

• production may be maximised by companies having various plants across the world;

• components measurement or checking may be performed by gauge, which simplifies inspection procedures and reduces operator costs;

• spare parts and part substitution is now possible and cost effective;

• by the introduction of batch production and 'just in time' philosophy, surplus stocks of parts may be reduced or eliminated which greatly reduces storage costs while ensuring correct assembly.

General tolerances. In cases where tolerances apply in general covering a range of dimensions, then a general tolerance note may be added to the drawing in the area of the title block. For example any sizes dimensioned to one decimal place may subject to a tolerance of 0.1 mm or if dimensioned to two decimal places may be subject to a tolerance of 0.01 mm. Where dimensions are required to be toleranced to values other than those shown as the default by decimal places in the drawing title block then dimensions may be toleranced in two basic ways, i.e. limits and tolerances.

Limits and tolerances. The extremes or limits of the variation in the basic size may be shown, with the numerically larger size being placed above the lower size, both being to the same number of decimal places. This makes a difference in the way in which internal and external features are toleranced. When specified in tolerance form, again both limits should be specified to the same number of decimal places. If one of the limits is zero then this may be shown simply as 0. Where values are selected from BS4500: Limits & Fits the selected tolerance may be shown alongside the basic size as in the form H7 or g6 etc. Where this method is employed then the drawing notes should make reference to the appropriate standard e.g. BS4500. sections of which are reproduced in various data books or tables such as Zeus books.

Fits. A fit is the relationship resulting from the difference, before assembly, between the actual sizes of the two mating parts which are to be assembled. General engineering fits are divided into three types:

• CLEARANCE FITS,

• TRANSITION FITS,

• INTERFERENCE FITS. A clearance fit is one where the shaft or male part will always be smaller than it's mating hole or female part, i.e. the largest shaft will always be smaller than the smallest hole. The size difference is referred to as the clearance. This type of fit may sometimes be referred to as a sliding fit. A transition fit is one where the shaft may be smaller or larger than it's mating hole, i.e. the largest shaft will be bigger than the smallest hole and the smallest shaft will be smaller than the smallest hole. A transition fit, therefore lies between clearance and interference fits. Interference fits are often applied to rolling bearings and keying fits, etc.

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An interference fit is one where the shaft is always larger than it's mating hole, i.e. the smallest shaft will always be bigger than the largest hole. The amount by which the shaft size exceeds the hole size is termed the interference. Mechanical or hydraulic presses are often necessary to assemble parts where interference fits are used, heating the female part and cooling the male parts may also be necessary to aid assembly, giving shrink fits when the temperatures become equal. Interference fits are often applied to valve seat inserts, cylinder liners, ring gears etc.

Angular Tolerances. Angular tolerances may be given in either decimal degrees or degrees minutes & seconds, the values of the limits should be expressed in the same units. Note this older imperial angle measurement systems is based on the fact that there are 60 minutes in a degree and 60 seconds in a minute. With the angle method tapers may be toleranced by tolerancing the angle or rate of taper independently of any tolerances on size. Tapered features may also be toleranced by fitting to a gauge or mating part. Measuring tapered features may be done using precision balls, slips and rollers, and a touch of trigonometry, these methods will be examined in more detail in the BTEC unit Manufacturing Technology.

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4. USE OF COMMON HAND TOOLS. One of the main purposes of using hand tools is to remove material to change the form or shape of a blank piece of material. In order to do this the workpiece MUST be held securely, this normally achieved using a bench vice. Apart from the standard type of vice some are fitted with a quick action mechanism. Care must be taken when using this to rapidly close vices, as several trainees have clashed their fingers when leaving them between the vice jaws and trapping them. In normal use the vice jaws should be checked for damage and that they are not loose. As most of the vice jaws have serrated faces, when holding work which has surfaces which are not to be marked then soft jaws or pieces of scrap should be positioned between the work and the jaws. The practical height of the vice may be judged to be correct when the vice top is roughly in line with the level of the users elbow. Of all hand tools files are perhaps the most commonly used. Prior to use the file should be checked that the handle is secure and free from splits. NEVER use a file without a handle, the tang may become imbedded in your hand/wrist should you slip. The file teeth should be checked, if the teeth are damaged or blunt then any work done with the file will be ineffective. Seek advice from staff if you are unsure as to the condition of any files. NEVER drop or clash files together, or use them as a lever as they are hardened and are also brittle and may easily shatter. Files come in a variety of shapes and grades, some of the most common are shown below. Some other files may have specialist functions such as sharpening saw blades.

When in use the teeth of the file must be kept clear of fillings. This is particularly a problem when filing soft materials with a fine tooth file. The file should be cleaned using a file card or a wire brush NOT by banging the file on the bench or vice.

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When filing you should adopt a balanced stance so that the downward force that you apply to the file may be varied in order to keep the file level. At the beginning of the stoke the weight (force) of the stroke should be on the point of the file, at mid stroke the force should be balanced about the centre of the file, at the end of the stroke the force will be transferred towards the shoulder of the file. When filing you should pace yourself i.e. so that filing is one with smooth steady strokes, trying to file like a bat out of hell’ is inefficient as you will tire quickly and start to make errors and will often result in scrapped work. There are three basic methods of filing as shown below.

The cross filing method is used for rapid removal of material, the straight filing method is used to achieve a good flat surface by keeping the file face in contact with the whole of the workpiece, and draw filing is used to achieve a

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good surface finish. When filing up to a square corner then a hand file may need to be used which has a safety edge which has no teeth and will therefore not cut into the corner. A note as regards to trying to achieve a good surface finish, there is no point in spending time draw filing on a surface which is still scratched and marked, draw filing will only serve to polish the high spots of the work and will not remove these deep scratches and marks. To some extent the same applies to the use of abrasive papers (emery paper or wet’n’dry) these will remove very little material and will NOT remove deep scratches etc. The quickest way to remove material is to use a saw, primarily a hacksaw or, for smaller sections a junior

hacksaw. Both of these rely on the blade of the saw being stretched (held in tension) by a frame. With the hacksaw the blade is tensioned by turning a screw, the junior hacksaw simply uses the elastic properties (bending) of the frame to apply the tension. It is important to check before using a saw to cut that the blade is correctly tensioned, as a slack blade will tend to flex and possibly snap or shatter. The blade must be set so that it is not twisted and that the teeth face in the correct direction before cutting starts. In order that the blade does not stuck in the cut, the teeth of the blade are set, each alternate tooth is angled slightly to one side then the other (on fine blades the whole blade is sometimes waved). This has the effect of making the blade cut slightly wider than the blade itself and so gives some clearance. Due to this clearance it is very difficult or even impossible to continue a cut with a new blade which has been started with a worn blade. Blades are generally 10" or 12" in length most hacksaw frames have some adjustment to cater for these different lengths. Coarse blade have around 14~18 teeth per 25 mm, fine blades have 20~30 teeth per 25 mm. Some blade are designed to be slightly flexible in that the blades are only hardened on the cutting edge, high speed steel blades are hardened all the way through and should NOT be bent. Blades with fine teeth should be used when cutting thin section material as a blade with coarse teeth tends to catch and jam on the front and rear of the cut. When cutting the saw should be angled at around 30°. Care must be taken when sawing to a marked-out line, it is generally better to keep the line in sight i.e. on the left hand side of the blade so that it can be seen while cutting is taking place. If you are trying to cut too fast then there is a much greater chance of the blade going off-line and possibly scrapping the job at the first cut. As with filing, smooth steady strokes will be much more effective than trying to cut as quickly as possible. If the cut does run-off then it may be corrected by slightly twisting the saw blade to the opposite side to bring the cut back on line.

Approx 30°

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Hand hammers and mallets also come in a variety of types and sizes, usually being classified by the weight of the head. Any hammer or mallet must be checked before use to ensure that the head is secure i.e. the wedge is fitted and secure, that the face is free from chips and the shaft is free from splits and cracks. Mallets usually have soft heads of rubber, hide (hard leather) or plastic, or may have inserts of soft metal such as copper or brass. Some types known as ‘dead blow’ have heads which are filled with lead shot so that when work is struck the mallet does not bounce and the majority of the force of the blow is transferred to the work rather than bouncing. Another common hand tool which is used to cut material is the cold chisel. Cold chisels are usually made from high carbon steel which has been hardened and tempered and again, these come in a variety of shapes and sizes, the main types being flat, cross cut, round nose and diamond point chisels. Before being used the chisel must be checked that the cutting point is ground sharp to the correct angle and that the head is not mushroomed, note this also applies to centre punches.

The mushroomed head occurs over time as the head is struck by a hammer, but must be removed by grinding before use as the mushroomed head may split or cause splinters to broken off which in turn may cause injury. When using chisels eye protection MUST be worn and safety screens used to protect others in the vicinity from flying chips. Flat chisels are generally used for cutting thin sheet, cutting small rods and bar, removing excess material from workpieces, removing rivet and damaged bolt heads etc. Cross cut chisels are used to cut grooves or to break up the surface of a workpiece into sections so that a flat chisel may be used to reduce the whole surface. Round nosed chisels are used to produce or to clean up oil or grease grooves in large plain bearings and bushes. Diamond point chisels are use to clean out sharp internal corners of jobs.

HeadPoint Body

Mushroomed heads

Are dangerous

60°120°

Center Punch point angle

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There are several methods which are in common use when holding the chisel, but the important thing to develop is the confidence to strike the chisel firmly and squarely without fear of striking your hand while holding the chisel, (if you think that youll hit your hand you probably will). To cut correctly the chisel must be held at the correct angle of inclination. This angle is may vary depending upon the material being cut, from 22 for aluminium to 40 for medium carbon steel. If this angle is too low then the chisel point tends to slip off the work. If the angle is too steep then the points tends to dig in and cut below the intended finished surface. Always take care to cut towards the fixed jaw of the vice as this is more robust than the moving jaw. If necessary place a block of scrap under the work to prevent it being pushed downward between the vice jaws.

Spanners and wrenches, although not always directly used in the manufacture of components are often used to adjust components, and in the maintenance of production equipment. The incorrect use of such equipment is often responsible for minor accidents and injuries. Spanners come in a variety of designs and are made to fit imperial, and or metric screw and bolt heads or they may be adjustable in design, they may also be single ended or double ended. As a general rule an adjustable spanner should be used only when the correct size spanner is not readily available. Spanners may be open ended, having two parallel faces, or ring spanners (where the head has six or twelve corners that fit around the bolt head), or may also be a combination of these two types. The size of a spanner may be given as the size across the flats (A/F) or by the nominal size of the bolt (which is more common with imperial sizes). As well as bolts and screw which have hexagonal heads, hexagon socket screws are commonly found in use in many areas. These have an internal hexagon into which a hexagon socket wrench (or commonly known as an Allen key) is used. These are defined by the size across the flats and may be either metric or imperial sizes. NEVER be tempted to use spanner which is not the correct size, damage will be caused either to the bolt, or yourself. Always pull with a steady force and try to direct the force of the spanner so that you pull towards your body, not by pushing away from your self, (you are more likely to slip in this way). If the nut or bolt is very tight don’t be tempted to use an improvised lever on the spanner or wrench to gain extra torque (turning force), or strike

the spanner with a hammer.

Angle of inclination too steep, thechisel point digs in.

Angle of inclination too shallow, thechisel point tends to slip off the work.

Angle ofinclination

Clearance

Rakeangle

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Another form of wrench may be found in common use, the pipe wrench. This tool is designed to so that the grip is tightened as the pipe is rotated. The serrated jaws will bite into the pipe surface. If the pipe is not to be damaged then it should be protected with packing material. In addition to these tool a range of specialist pliers and screwdrivers may be encountered. Pliers are designed to hold and or grip workpieces or some are intended to cut wire or cable. Electrician’s pliers have heavily insulated handles, but this does not indicate that is safe to handle live cable even using pliers. Other specialist uses include circlip pliers, wire strippers etc. Always use the correct tool as each has its own purpose, don't be tempted to improvise. Screwdrivers may seem simple enough, but many a screw head has been damaged by using a screwdriver whose blade does not correctly fit the screw slot. Always ensure that blade of the screwdriver used, fits the full width of the slot while not being too big. Care should be taken with Philips head screws and cross head screw, as they look very similar but using the wrong screwdrivers may cause damage to the drive slot and render the screw useless.

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5. USE OF DRILLING MACHINES, TAPPING & THREADING. Drilling is one of the most common machining operations carried out in the workshop, it essential that all safety precautions are followed to ensure that this is carried out safely.

• The machine MUST be electrically isolated when setting up the machine or when changing spindle speeds where this involves exposing and moving drive belts.

• Eye protection MUST always be worn.

• Guards MUST be used at all times when drilling.

• The drill MUST be held securely in the machine chuck or spindle.

• The work MUST be securely held to resist the cutting forces of the drill.

• Loose clothing or long hair MUST NOT be allowed to come anywhere near rotating parts of the machine.

• Care MUST be taken at the point when the drill breaks through the workpiece, especially with thin section material such as sheet metal.

• Swarf MUST be kept clear from the working area, by using a brush, NOT by sweeping with the hand Twist drills come in a variety of types, but the most common are the parallel shank twist drill and the taper shank drill. Standard parallel shank drills, also known as jobbers drills come in all sizes up to ½” diameter. It would very costly to stock every possible size, it is therefore normal to maintain stocks of drills of whole and ½ millimetre diameter jobbers drills. This type of drill is held in a Jacobs chuck which has three jaws which tighten around the shank of the drill, using a chuck key. The chuck key, itself must be a good fit in the chuck else it is liable to slip and then the chuck cannot be tightened correctly, seek assistance if you have problems with this.

Taper shank drills are located and driven due to a precise fit between the tapered shank of the drill and the internal taper of the drill spindle. The taper in general use is the Morse Taper, which comes in sizes from 0 ~ 7. You must ensure that the correct size shank is selected for the drill spindle or use sleeves to achieve the correct driving fit. The drill is removed from the spindle by a tapered drill drift placed through the slot in the spindle and giving it a sharp tap with a hammer, note that you MUST NOT handle the sharp edges of the drill with your bare hands. There are two basic types of drilling machines used in this workshop, the pillar drill together with a smaller bench mounted version, the bench drill and the radial arm drill.

Removal of a Taper Shank Drill

Twist & Taper Shank DrillTerminology

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Regardless of which machine is used, it is important that the workpiece is clamped and located securely to resist the cutting forces of the drill, several methods are commonly used to clamp the work as shown opposite. The work may be held in a machine vice which is in turn clamped to the drilling machine table, or the workpiece may be clamped directly to the table of the drilling machine. When clamping work in machine vices the following procedure should be followed.

• Make sure that the table of the drilling machine and the base of the vice are free from swarf.

• Carefully place the vice on the machine table and align the Tee slots with the slots in the vice base.

• Clamp the vice to the table using the correct Tee bolts, nuts and washers.

• Place two parallels under the work such that when the drill breaks through the work they are not damaged, lightly tighten the vice jaws on to the work when aligned with the drill point.

• Check that the work is seated on the parallels correctly, tap the work down lightly with soft-faced hammer if necessary.

• Tighten the vice jaws on to the work, hand tight should be sufficient, and check the parallels again.

• If necessary loosen the Tee bolts and reposition the work to line up with the drill, and then re-tighten.

• When clamping work directly to the drilling machine table the following procedure should be followed.

• Make sure that the table of the drilling machine is free from swarf.

• Place two parallels under the work such that when the drill breaks through the work the drill point does not damage them.

• Position suitable clamps and Tee bolts and nut to give maximum clamping area on the work.

• Align the work under the drill point and lightly tighten the clamping nuts.

• Make sure that the parallels are directly under the area of the work being clamped.

• Check that the work is lying flat and fully tighten the clamps. It is advisable to set the depth stop on the drilling machine to ensure that the drill does not protrude too far through the workpiece. Care must be taken to ensure that the clamps are positioned such that there is no bending load on the clamp bolts and that the maximum clamping force is exerted on the workpiece, not the packing block. Where work of an irregular shape such as some castings, has to be drilled, then an angle bracket may be used in conjunction with clamps to hold to work securely. Care must be taken in this case to ensure that the work is clamped squarely or at the correct angle.

The Radial Arm Drill

Speed Selectors

Sensitive Feed Lever

Radial Arm

Coarse Feed Lever

Longitudinal Feed Hand WheelSaddle

TableDrill Spindle

Radial Arm Lock

Table Lock

Drive Motor

Table Elevation Lever

Incorrect

Correct

Incorrect

Incorrect

Workpiece

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Where round section work is to be drilled then a Vee block may be used to hold the work which prevent it from turning. Again in this case it is necessary to use a square to ensure the work is aligned correctly. A common fault when drilling, is to try and drill a hole at the finished size all in one go. A method known as progressive drilling should always be used except in the case of small i.e. less than 4~5 mm diameter holes. The first stage is to use a centre drill, which allows any errors in alignment to be detected as soon as possible. If the original pilot hole is slightly out of line then it may be drawn back into position by chipping a small groove with a fine diamond point chisel, towards the true centre. When the pilot drill has been drilled in the correct position then the drill is changed for another larger diameter drill up to the final required size. This may be done in several stages depending on the size of the final hole. Ensure that the work is not moved between each stage or you will lose the alignment of the drill and hole. The correct spindle speed for a certain diameter drill cutting different materials may be determined using a simple formula. Spindle speed = surface cutting speed

π x drill diameter note the units must be in the same form, i.e. RPM = metres per minute

π x millimetres For example drilling a ø15 high speed steel (HSS) drill through mild steel, Spindle speed = 28 x 1000

π x 15 Spindle speed = 594 RPM As it is unlikely that the drill will have exactly this spindle speed, then the nearest available spindle speed would be selected. If this theoretical speed falls mid way between two available speeds then select the lower speed. This table gives spindle speeds for a range of diameters and common materials and assumes that the drills are made of high-speed steel (HSS). With deep holes it is good practice to withdraw the drill from the work periodically to

Surface cutting speed (S) in m/min

Diameter Steel Brass Aluminum

28 60 120

mm Theoretical spindle speed in rev/min

150 59 127 255

145 61 132 263

140 64 136 273

135 66 141 283

130 69 147 294

125 71 153 306

120 74 159 318

115 78 166 332

110 81 174 347

105 85 182 364

100 89 191 382

95 94 201 402

90 99 212 424

85 105 225 449

80 111 239 477

75 119 255 509

70 127 273 546

65 137 294 588

60 149 318 637

55 162 347 694

50 178 382 764

45 198 424 849

40 223 477 955

35 255 546 1091

30 297 637 1273

25 357 764 1528

20 446 955 1910

15 594 1273 2546

10 891 1910 3820

5 1783 3820 7639

4 2228 4775 9549

3 2971 6366 12732

2 4456 9549 19099

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allow swarf to clear from the work and to allow cutting fluid into the hole which cools the work and the drill and provides lubrication at the cutting point. This is sometimes called peck drilling. When a drill is cutting correctly i.e. at the correct spindle speed and feed, the swarf should leave the work as two neat spirals. If this is not the case then the drill may need re-grinding to ensure the cutting edges are correct. With experience you should develop the ability to recognise when a drill is cutting poorly or incorrectly. In addition to drilling the drilling machine is also used to produce common features such counterbores, countersinks and spot faces, as shown below.

A drill will produce a hole accurately only if the drill is correctly ground. Where a more accurate sized hole is required then a reamer needs to be used. Prior to using a reamer the hole must be drilled to a size about 0.5 mm less than the finished size. The work must not be moved between changing the drill for the reamer as the reamer will only follow the previously drilled hole and may snap if the work is moved of centre. Note that a reamer will not correct the position of a hole which has been drilled slightly off centre, it will only produce a hole of accurate size and shape (roundness). The spindle speed used for reaming is normally quite low, but the reamer is fed through the work quite quickly. Cutting fluid should be used to improve the surface finish, this may be normal coolant or a cutting compound such as Rocol.

Reamers (should) have very sharp edges and care must be taken when handling them. Reamers are available to cut parallel holes and tapered holes, adjustable reamers are also used (but these take a bit of skill to adjust correctly), which allow a range of hole sizes to be produced, in addition to non-standard diameter holes.

Tapping And Threading. A screw, is one of the most common mechanical fastening devices consisting essentially of an inclined plane (slope) wound spirally around a cylinder or a cone. The ridges formed by the winding planes are called threads, and depending on the intended use, these may be square, triangular, or rounded in cross section. The distance between two corresponding points on adjacent threads is called the pitch. If the thread is on the outside of a cylinder, it is known as a screw or “male” thread, and if it is on the inside of a cylinder, as in the case of a common nut, it is known as a “female” screw. Screws have a wide variety of uses. A screw jack, for example, makes it possible to raise a heavy object, such as a car, off the ground. A screw can also provide carefully controlled forward and backward motion relative to a connected machine member, as in a micrometer, which can measure distances to within 2.5 microns (1/10,000

Counterboring tool used to produce

counterbores and spot faces, requires a

pilot hole.

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in). The controlled motion is also used in various machine tools, such as a lathe, where the cutting tools can be advanced with a high degree of precision. The principle of the screw is also used in conveyors and in certain types of pumps. There are many different types of thread in use, but the metric coarse series is now perhaps the most common for fasteners. On older equipment there may still be found Whitworth threads, British Standard Fine (BSF), Unified National Fine (UNF), Unified National Coarse (UNC), on pipe work it is still standard to use British Standard Pipe (BSP) threads. Details of the form and diameters of these threads may be founds in tables such as in Zeus books.

Thread-Cutting. For cutting threads on bolts or on the outside of pipe, a thread-cutting die is used. It is usually made of hardened steel in the form of a round disc with a hole in the centre. The hole is threaded in the appropriate form and pitch, and the threads are cut away for part of their circumference, leaving longitudinal grooves in the die. These grooves form the cutting edges and give clearance for the swarf, (the chips of metal formed when a thread is being cut).

To cut an outside, or male, thread, the die is lubricated with cutting compound (Rocol) and then screwed onto an unthreaded bolt or piece of pipe, the same way a nut is screwed onto a bolt. The die is held in a stock which allows some adjustment of the die. For the first cut the outer screws are loosened and the inner screw tightened to allow the split die to open slightly. This has the effect of making the first cut easier. After the first cut the thread must be checked with the mating part where possible, or with a standard nut to check the fit. If the thread is tight then the die may be adjusted by

loosening the middle screw and tightening the two outer screws and then run the die down the job a second time. It will also reduce the force required to start the thread if the end of the component is chamfer slightly to give a slight lead. The die and stock must be kept square to the job throughout the threading process. It is necessary to keep the die clear of swarf and to ease the force on the die, once the first couple of threads have been cut, the stock and die should be turned back about a ½ turn until the swarf which is cut by the die can be felt to give slightly as it breaks off. The stock and die can then be turned again to cut another thread or two before repeating this process of clearing the die until the required length has been threaded. The corresponding tool for cutting a female thread, such as that inside a nut, is called a tap. Taps are used in sets, beginning with the first or taper tap, where the first 8~10 threads are tapered off, so that the thread is gradually formed. It is vital that the tap is presented to the work squarely, and kept square throughout the tapping operation.

The consequences of not observing this principle with result in an out-of-square hole which is then unlikely to align with its mating component. There is also a danger of the tap being overloaded and snapping. Extracting broken taps from components can be very difficult if not impossible and often results in scrapped work. Following the taper tap comes the

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intermediate tap which has the first 3~4 cutting teeth tapered, and then the plug tap. Each tap cuts progressively more material to form the full thread. It is important that the hole for a female thread has been drilled at the correct size. The tapping drill size may be found from tables, Zeus books etc. Note the same principle used to keep a die clear of swarf is used with taps. After the first two or three threads have been cut the tap is turned back to clear the swarf and so on. Before using any dies or taps they should be visually checked that they are the correct size, which is engraved or etched on the face or shank. In addition it should be checked that there are no broken or damaged teeth which may be an indication that the tap or die is about to fail. Always clean the tap, or stocks and dies before beginning cutting and use cutting compound such as Rocol. It is also good practice to clean any tools (particularly taps and dies) and other equipment prior to its return to the central stores If you have any problems when using the equipment described in this section seek advice from the staff.

6. OFF HAND GRINDING MACHINES. Grinding is the removal of metal by a rotating abrasive wheel. The wheel is composed of many small grains of abrasive, bonded together, with each grain acting as a miniature cutting tool. The process produces extremely smooth and accurate surface finishes. Because only a small amount of material is removed at each pass of the wheel, grinding machines require fine wheel control. The pressure of the wheel against the workpiece can be made very slight, so that grinding can be carried out on fragile materials that cannot be machined by other conventional devices. Off-hand grinding machines are used for sharpening a variety of hand and machine tools. Before using any grinding machine the following checks MUST be carried out. The grinding wheel must be visually checked for defects such as cracks or chips on the surface of the wheel. The tool rest must be checked to ensure that it is set correctly in relation to the wheel, as this will need adjustment

as the grinding wheel wears. Note do not attempt to adjust this yourself seek assistance from the staff. The wheel must be running true and even. This may be checked initially by rotating the wheel slowly by hand, but

only touch the grinding wheel when the grinding machine is isolated from the electricity supply. Safety glasses MUST be worn at all times when using or working near grinding machines. DO NOT stand in front of the grinding wheel when the wheel is first set in motion, stand to one side until the wheel reaches full speed before using. This is because the wheel may have been damaged such that it may burst when under the very high centrifugal forces which occur when the wheel is spinning.

If you have any doubt regarding the condition or setting of any grinding machine seek advice from the staff immediately. NOTE in accordance with the Abrasive Wheel Regulations 1970 only persons who have been trained and assessed as being competent, may change and or mount abrasive wheels. Off-hand grinding machines are basically two types, bench mounted or pedestal (floor) mounted, and may have one, or more usually, two wheels attached. The main parts of the grinding head are shown opposite. Work is held against the wheel as shown here.

You will have cause to use to grinding wheels for sharpening hand tools such as centre punches, chisels, drills and at a later stage lathe tools. When grinding a centre punch the punch should be held as shown, such that the tip of the punch is at an angle of 60° to the wheel.

Screen

Wheel

Tool rest Wheel direction

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The fingers are rested on the tool rest so that the tool may be held steadily As the point touches the wheel the punch should be rotated so that flat spots are not formed on the punch, keeping a light pressure on the wheel. Care must be taken to ensure that the tip does not get too hot by frequent use of the coolant. If this is not done then the may tip to soften.

It should be checked that the point of the punched is central to the body of the punch. Grinding chisels requires a similar technique, but in this case the point of the chisel is rocked slightly from side to side such that the cutting edge of the chisel is very slightly curved. Again the fingers are rested on the tool rest so that the tool may be held steadily. Each side of the chisel must be ground in turn, but ensure that the point is even and central to the body of the chisel. Grinding twist drills requires more skill to ensure that the resulting shape of the drill is correct.

The drill should be held similar to the manner shown opposite, with the fingers resting lightly on the tool rest. As with all grinding operations use coolant to prevent the tool from becoming too hot.

Grinding a centre punch

Coolant

Safety screen

Grinding a twist drill

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The action when grinding drills is to pivot the drill on the tool rest so that the drill is lightly pressed on the wheel in an upward motion. At the same time the drill is rotated slightly clockwise to give a clearance on the cutting lip. Keeping the hands in the same relative positions the drill is rotated 180° and the other cutting edge is ground in a similar manner. The lengths of the cutting edges (the lips) must be checked to ensure that they are both the same length and angle else the drill will not cut correctly. The lips may be measured directly with a rule or by using simple gauges as shown below.

7. COMMON ENGINEERING MATERIALS. Engineering materials are covered in depth through the BTEC unit - Materials. As a very brief introduction some of the properties and uses of the more common engineering materials are described in the following section. Common materials may be broken down into groups as shown below.

Ferrous Materials

Ferrous materials are all based on iron. Iron by itself is quite soft and has very little use in its natural form. When alloyed (chemically mixed) with varying degrees of carbon the result is steel. The amount of carbon greatly affects the properties of the steel and how the material may be subsequently heat-treated.

Lips are ofunequal length

Checking the lengths of the cutting lips, the arc scribed by the cutting tips must coincide.

59°

Engineering materials

Metalic Non-metalic

Ferrous Non-ferrous Natural Synthetic

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Incre

asin

g c

ost 7

77

77

7 Incre

asin

g h

ard

ness

Common description Carbon content %

Typical uses

Dead mild steel

0.01 ~ 0.015

Car body panel, general sheet metal. work, thin wire, drawn tubing.

Mild steel

0.15 ~ 0.30

General purpose steel, bar, rods, boiler plate, rolled steel sections (RSJ), angle sections etc

Medium carbon steel

0.30 ~ 0.05

Stressed components, i.e. crankshafts, forgings, axles etc.

0.50

Leaf springs, hammer heads, cold chisels etc.

High carbon steel

0.8 ~ 1.0

Coil springs, wood chisels

1.0 ~ 1.2

Taps and dies, files, drills, etc

1.2 ~ 1.4

Fine edged tools i.e. knives

Cast iron

3.2 ~ 3.5

Machine castings

Care should be taken to ensure that the correct type of steel is being used. Depending upon the manufacturer or supplier the ends of the stock are painted in single or two colour combinations, which is used to identify the grade of steel. It is therefore good practice to cut material from only one end of stock so that the colour identification is retained. Cast iron can be identified by its coarse open surface structure, it is brittle and will therefore snap easily if bent, but is very strong when under compressive forces. Plain carbon steels are simply iron and carbon. Alloyed steel by the inclusion of various other metallic elements, such as cobalt, tungsten, chromium, vanadium, cobalt and molybdenum are used for special steels such as high speed steel for cutting tools, and stainless steels. Each type of steel has a specific composition laid down by British Standards. As there are several relevant British standards in common use care should be taken to make reference to tables to ensure equivalent specifications where necessary. For example stainless steel is produced by the alloying of chromium with carbon steel.

Non-ferrous Materials. Aluminium in its pure form is very light, soft and easily worked. It is generally alloyed with other elements to improve its strength. It has excellent electrical conductive properties and is often used for overhead power lines with a copper core. Copper is soft and easily work in its pure form, it has excellent electrical conduction properties and is used extensively for electrical cables as well as having many uses in plumbing due to its excellent corrosion resistant properties. Brass comes in various grades but it based on an alloy of copper and zinc. It has good resistance to corrosion but its surface does tarnish quite easily. As it can be cast into shape it is often used for complex shaped components. It can be brazed or soldered and is easily machined. Brass also conducts heat and electricity well. Bronze is an alloy of copper and tin and is used where resistance to wear and corrosion are important, such as bearings, bushes and gears.

Non-metallic. Rubber is a naturally occurring material, and is commonly used for belting materials such as drive belts or conveyor belting. It is often moulded around other fibres to improve its strength in certain directions. Plastics are synthetic materials produced in a very wide range of grades, but in general they have good to excellent insulation properties, but they have come to have very many different uses. Plastics are primarily defined as being either thermo-plastic or thermo-setting. Thermo-plastic plastics tend to soften when subject to heat and pressure and may be reshaped. Thermo-setting plastics change shape under heat and pressure such as in moulding into a permanent form i.e. they cannot be reshaped. As an example, a commonly encountered plastic form is nylon which is often used for bearings or bushes.

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Plastics are often known by their acronyms such Polyvinyl Chloride (PVC). Other common types include, cellulose acetate (frame of safety glasses), polystyrene, expanded polystyrene (insulation foam), acrylics - polymethylmethacrylate (machine guards), fibreglass (insulation), phenol-formaldehyde resins (switch boards), acrylonitrile-butadiene-styrene ABS (vacuum formed containers), and synthetic resin bonded paper SRBP or SRBF synthetic resin bonded fibre, using phenol-formaldehyde resins (printed circuit boards). Care must be taken when machining some plastics as the dust formed may be hazardous if inhaled.

Forms Of Supply Some of the methods of supplying material and producing components such as forging and casting and moulding will be covered in more detail in the BTEC unit Manufacturing Technology. For the present the notes will only briefly describe the common forms of supply of material as used in a typical workshop. The most common form that steel is produced in is cold or hot rolled sections i.e. bar (rectangular) or rod (round). Cold rolled steel (often referred to as bright bar) is usually quite accurate in size and would be used when the finished size is suitable for the finished size of the component. When the component needs to be machined on all faces then is may be cheaper to use hot rolled (or black bar). Black bar is not as accurately to size as bright bar and has a black scale on the outer surface which is removed when machined. When produced by rolling, stresses are formed in materials. When one side of the material is machined it is common to find that these stresses will cause the material to bend. It is normal practice to machine both sides of the section in order to reduce this effect. If long lengths of stock are badly stored they may become bent due to their own weight, it is therefore important to check that the stock is not bent especially where long components are required. This may be done simply by sighting along the length of the bar or by rolling on a flat surface. It is also common to find material supplied in sheet form (when less than about 3 mm thick) or plate form when thicker. These plates may be of various sizes and may be 2 m x 3 m etc. in this case is usual practice to cut this material by guillotine or by oxy-acetylene burning torch. As well as rectangular section steel for fabrication of

structures is often supplied in various sizes of angle iron, ΙΙΙΙ beams and rolled steel joists (RSJ). The size of the steel section is expressed as the thickness x width, i.e. 10 x 50 mm or by the diameter with round bar, and in various lengths such as 2 ~ 4 metres. As metrication began in the 1970's you may expect that steel will be supplied as metric sizes, but you may still find that it is common to come across steel supplied in imperial sizes, i.e. d" x 2” would be the nearest imperial size to 10 x 50 mm, but the actual size would be 9.5 x 50.8 mm. This should not be a problem as in most cases the steel will be cut down to produce the component. It is important to check the section of the steel before cutting to avoid waste, do not assume that the material issued by the stores is the correct size, it is YOUR responsibility to check it.

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8. BASIC TURNING

SAFE WORKING PRACTICES When working with any machine tool ALL safety procedures must be observed at ALL times. This includes, personal protection and hygiene procedures, such as wearing safety glasses or goggles and washing hands after coming in to contact with cooling and cutting fluids. It is especially important to keep long hair covered to eliminate the danger of becoming entangled in rotating parts. The working area is kept clean & tidy at all times to reduce the danger of items falling when working. For the majority of the turning work that you will be doing in this College you will be using a Harrison 300 centre lathe. Lathes are usually defined by the swing and the length of the bed. The swing is the distance from the centre line of the spindle of the lathe to the level of the bed and represents the maximum diameter of work that may be turned in the lathe. Some lathes have removable sections (called gap bed lathes) which allow larger diameter workpieces to be accommodated. Before beginning work on any machine tool you must be come fully acquainted with the function and correct operation of all of the controls of the machine.

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Complete the table below and identify the controls indicated and their function.

Item Control – function Item Control - function

1 14

2 15

3 16

4 17

5 18

6 19

7 20

8 21

9 22

10 23

11 24

12 25

13 26

In addition to understanding the functions of the controls it is also important to keep the machine clear from cuttings and swarf before during and after use. This especially includes all slideways, the saddle, & spindle. Note that you should NEVER handle swarf with your bare hands. Before beginning work you must check that the system functions correctly, i.e. start the spindle at a suitable speed and operate the emergency stop/braking pedal. The spindle should stop almost instantly. If this is not the case seek assistance from the staff, DO NOT use the machine until this is rectified.

Common Work holding Devices. Before work may be turned it must be held securely aligned with the spindle. There are several Work holding devices and methods which are in common use, including three jaw self centring chucks, four jaw chucks, collet chucks, face plates and turning between centres. The particular method used in any given circumstances will depend on several criteria. The main criteria are:

• the type of work being carried out

• the shape and size of the workpiece

• the safety aspects of the operation

• whether the work is a one off, or batch production In all circumstances, the basic rules of secure work holding will apply, namely restriction of the component in relation to the Six Degrees of Freedom.

The Three-Jaw Chuck. The three-jaw chuck is self centring, this means that each jaw will move by the same amount such that each jaw is always the same distance from the centre line of the spindle. The jaws are moved centrally by a flat scroll into which the jaws are fed in rotation according to the number engraved on them. The chuck is usually equipped with two sets of hardened jaws, for holding external and internal work. Other jaws can be used which are not hardened and can be machined to suit specific workpieces, these are called soft jaws. The chuck is used to hold bright drawn material or previously machined work. In either case the work must be round, triangular or hexagonal. The work should not be removed from the chuck until all the operations required have been carried out, as once work is removed from the chuck it cannot be guaranteed to run concentric if replaced in the chuck. Note that black bar is not guaranteed to be round and should therefore NOT be held in a three-jaw chuck. Both three and four jaw chucks are tightened and slackened using a chuck-key. Only use the correct sized chuck-key for this purpose and NEVER leave a chuck-key in the chuck, as this could result in the chuck-key being thrown out at high speed.

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Four-Jaw Chuck. With the four-jaw chuck each of the four jaws is moved independently by a square threaded screw . The jaws are reversible so that they can be used for either very small or very large diameter work. The chuck is usually more heavily constructed than the three-jaw and has greater gripping power. Because of the four jaws, the chuck can hold both symmetrical and irregularly shaped work. The workpiece may be set to run either concentric or offset to run eccentrically. Eccentrically mounted work will need to be balanced to reduce machine vibration. All the jaws need to be set independently to hold the work in the desired position. These chucks are quite heavy, and to avoid injuring yourself or the machine, you must follow the correct procedure for mounting and removing them from the machine. In the first case you must protect your hands from swarf and from being trapped by the weight of the chuck and in the second case you need to protect the lathe bed by using a board of some sort. This may also be used to help to raise and lower the chuck into position. Setting work up in a four-jaw chuck requires skill and practice, especially when setting irregular shaped work as shown above.

Note each lathe has a matched three and four jaw chuck (indicated by stamped identification letters) DO NOT swap chucks from machine to machine!

To Set Cylindrical Work for Running Concentrically in a 4 Jaw Chuck. Set the chuck jaws to the approximate size of workpiece using indexing rings on the chuck face. Set work in jaws, check and adjust for concentric running by first using the pointer in a scribing block for a central position. If a more accurate setting is required then use a Dial Test Indicator (DTI) may be used for the final adjustment, when used on round work, but this should NOT be used on the surface of black bar or the probe of the DTI will be damaged. When setting up irregular shaped work such as shown below then it is necessary to use a setting bar held between the tailstock centre and a centre pop mark in the workpiece. Before using this method the alignment of the tailstock should be checked, this will be demonstrated later.

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Turning Between Centres. The method of turning between centres is often used when the workpiece is quite slender and has to be machined over most of its length. This method is often used for screw cutting. Before beginning turning the workpiece it is important to check that the tailstock is accurately aligned with the spindle centre line using a parallel test bar and a dial test-indicator. If necessary the tailstock must be adjusted to regain alignment. When turning between centres the work is located between the two centre drilled ends and a drive dog (or carrier) is fitted to the end nearest the chuck which is fitted with a drive plate and centre. A drive pin of the correct length must be fitted to engage with the end of the drive dog. When tightening the screw on the drive dog small pieces of soft (aluminium or copper) scrap plate may be used in order to protect a finished surface. The workpiece is held by bringing the tailstock centre up to the work and then locking it in place. The final pressure is applied using the tailstock handwheel which is then locked in place. When using a dead centre then lubrication MUST be applied and regularly checked so that the centre does not overheat. If a rotating centre is used then this lubrication is not required.

Face Plates. A face plate is a large circular plate, usually made of cast iron. It is located on the spindle nose in the same manner as the chucks. Slots are machined or cast into it to enable the work to be clamped to its surface. The face plate is used to hold large workpieces and any that cannot conveniently be held between centres or in a chuck. Components are often located and held by various types of angle brackets or fixtures mounted on the faceplate. The work is usually marked out to help with setting, and a setting bar may also be used as described for the four-jaw chuck.

Collet Chucks Collets are used to hold bright bar or previously turned components, there are two types in general use, collets with draw bar attachment and multibore collet chucks. But in the main draw bar collets are used in the College workshops. Each collet has a bore size to suit a particular diameter of work. Only a small variation on this diameter can be effectively held by each collet. A set of collets covering the standard diameters of work would therefore be needed to hold the normal workshop range of work. Special collets to hold hexagon, square or other non-standard shapes may also be used. The collets using the draw bar method fit into a housing placed into the lathe

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spindle nose. These collets have an external taper that mates with a taper in the housing. The draw bar passes through the headstock and is screwed onto the collets. The action of the draw bar is to pull the collet into the housing and put pressure on the tapers. Slits in the collet allow the collet to close onto the work as a result of the force on the tapers. By being located directly in the spindle nose ensures the collet and work runs concentric with the machine.

Lathe Tool Geometry. Successful tuning depends upon the cutting tool being ground correctly and being correctly aligned with the workpiece. Failure to observe this simple fact is usually the reason behind poor cutting and poor surface finish. The majority of tools in use in this workshop are ground from high speed steel (HSS). The tool steel manufacturer’s name/identification is usually etched on the surface of the tool. This needs to be checked as we use ordinary mild steel for practice tool grinding and these are sometimes accidentally picked up by trainees to use as cutting tools, this only makes a lot of heat and noise, take care!

The tool MUST be set such that the tip is level with the centre line of the spindle/workpiece. There are several methods in common use to set the height of cutting tools, but perhaps the most generally used is to rotate the toolpost such the tool may be positioned where it can touch a dead centre set in the tailstock. By measuring the height difference between the cutting point if the tool and the tip of the centre a pile of packing pieces may be selected which, when placed under the tool will raise it to the correct height. Care must be taken to allow for the fact that the tool will lower slightly when the tool post screws are tightened. Also some toolposts raise slightly in certain positions and care must be taken to ensure that the tool is correctly set before cutting takes place. As few pieces of packing as possible should be used, i.e. replace several thin pieces with one thicker piece. Ensure that the packing pieces fully support the tool in the toolpost, especially under the toolpost screws, as it is possible to put a bending force on the tool which may cause it to break when cutting. If for some reason the tool cannot be positioned next to the tailstock centre, then the centre height may be transferred to the tool tip by using a scribing block or surface gauge. One quick method of checking the tool cutting height is to place a parallel piece of packing strip between the tip of the cutting tool and the workpiece so that it is lightly held between the two. If the strip is exactly vertical then the tool height is generally OK. On up-to-date machines it is common to find quick change, adjustable height toolposts which much simplify this setting process, but these are an expensive add-on and unfortunately are not available for general use in our workshop. It is important to check the cutting geometry of the cutting tool prior to beginning cutting as setting the tool height on a tool which needs to be reground is waste of time. The cutting edge needs to be frequently checked during cutting as wear will take place and the surface finish will deteriorate. It generally good practice to touch up the cutting edge before taking a finishing cut. These basic angles on cutting tools are the rake angles and the clearance angles. The clearance angles need to be ground on the tool to avoid the tool rubbing the surface of the workpiece. Both the side and front clearance are generally kept to a minimum. If excessive clearance is allowed, the point of the tool (wedge angle) will become too sharp, it will then become weaker and lose its heat dissipating area. Both the clearance angles vary slightly with the material being cut and the side clearance angle may also be increased slightly for coarse feed rates. The clearance angle required for internal diameters i.e. when boring, has to be larger than for external diameters, especially for small diameters. Sometimes a secondary clearance angle is added. The top and side rake angles vary more than clearance angles, and the amount of rake angle will depend on the material being cut. In general, softer materials are cut using greater rake angles while hard/tough materials are usually cut using a small or zero rake angle. Rake angles may be positive or negative. Negative cutting angles are generally used on carbide or ceramic tools as used on CNC machines. The true rake angle on cutting tools is the angle resulting from both front and side rake angles, and falls away from the cutting tip. The swarf will follow this angle as it leaves the work.

A Quick Check for Lathe Tool Centre Height

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Different tools are ground to different shapes for different purposes and craftsmen generally have their own favourite way, based on experience, of grinding tools for different jobs and materials. This same experience will tell a skilled man when a tool is cutting correctly or not. At this stage if you have any doubt about the suitability of a tool seek help form the staff. The diagram opposite shows some of the principle tool shapes in common use.

Tool movement

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When selecting a cutting tool always visually inspect the tool angles and the condition of the cutting edges. If you are in any doubt about the condition of the tool seek assistance from the workshop staff. The condition of the cutting tool is directly responsible for the quality of the cutting action, and YOU are responsible for the condition of the cutting tool.

Using Drills & Reamers In A Centre Lathe. In a similar way that drill and reamers are used in pillar and bench drills they may also be used in centre lathes when mounted in a Jacob’s chuck in the tailstock of the lathe. The Jacob’s chuck is held by a standard Morse taper, care must be taken to ensure that the mating faces of the tapers are clean inside and out before inserting the chuck into the tailstock or assembling taper sleeves. The same applies when inserting fixed and rotating centres into a tailstock when turning between centres. The tang of the Jacob’s chuck must be carefully aligned in the tailstock before seating the chuck with a light thrust into the tailstock. When drilling into the end of the workpiece the end face of the job must be faced-off square, or else the drill may try to run-off and possibly break. Always begin drilling using a centre drill with the spindle speed set to suit the diameter of the drill in use. The tailstock should be brought close up to the work face and then clamped into position. The spindle should then be started and a supply of cutting fluid directed at the cutting point. The drill should be carefully wound into the workpiece to the depth of the centre drill and then retracted to allow the swarf to clear. Stop the spindle, unclamp and withdraw the tailstock from the working area, where the drill may be changed for the next size to be used when progressive drilling. When drilling deep holes ensure that the drill is periodically withdrawn from the job (peck drilling) to allow swarf and heat out and coolant/cutting fluid into the cutting area. In some cases it is beneficial to protect the centre hole when this is used for turning between centres. This is done by using an appropriate sized slot drill held in the Jacob’s chuck and taking a cut about 2~3 mm deep. Mounting reamers, taps, to achieve the required holes, threads etc. is done in a similar manner to drills. When using taper shank drills and reamers ensure that the correct sleeves (use as few sleeves as possible) are used and that they are free from swarf and dirt before assembly. The correct spindle speed for a certain diameter workpiece cutting different materials may be determined using a simple formula. Spindle speed = surface cutting speed

π x work diameter the table opposite gives spindle seeds for some of the most common materials. As it is unlikely that the lathe will have exactly the correct spindle speed, then the nearest available spindle speed would be selected. If this theoretical speed falls mid way between two available speeds then select the lower speed. When carrying out other operations on a centre lathe the spindle speed may need to be set differently. For example when knurling it is often best to set the spindle speed at its lowest value to give the knurling tool the chance to do its job fully. Similarly when screw-cutting the spindle speed needs to be set slow to allow the operator time to stop the tool in the correct positions i.e. before the tool collides with the chuck or shoulder of the job. Feed-rate is the term used to describe how fast or slowly the tool point passes along the job. This has a direct effect on the surface texture/finish of the cut. Feed-rate is usually expressed as either millimetres per minute (mm/min), or millimetres per revolution (mm/rev). The feed and the depth of the cut determine the volume of the metal removed. As a general rule it is better to remove material with big depths of

Surface cutting speed (S) in m/min

Steel Brass Aluminum Diameter 28 60 120

mm Theoretical spindle speed in rev/min

150 59 127 255

145 61 132 263

140 64 136 273

135 66 141 283

130 69 147 294

125 71 153 306

120 74 159 318

115 78 166 332

110 81 174 347

105 85 182 364

100 89 191 382

95 94 201 402

90 99 212 424

85 105 225 449

80 111 239 477

75 119 255 509

70 127 273 546

65 137 294 588

60 149 318 637

55 162 347 694

50 178 382 764

45 198 424 849

40 223 477 955

35 255 546 1091

30 297 637 1273

25 357 764 1528

20 446 955 1910

15 594 1273 2546

10 891 1910 3820

5 1783 3820 7639

4 2228 4775 9549

3 2971 6366 12732

2 4456 9549 19099

1 8913 19099 38197

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cut at a slower feed-rate. For reasons of productivity & efficiency, the feed-rates should be as high as is possible. When deciding the feed-rate to use the following should be considered,

• the power available to the machine tool.

• the quality of the machine tool, and it's condition.

• getting the maximum tool life.

• minimising tool breakage.

• the required surface finish.

• the accuracy of the component being produced.

• vibration. For finishing cuts, usually, the slower the feed-rate the better the surface finish will be, but this will obviously take longer. The main factor affecting surface finish is still the condition & setting of the cutting tool. Producing Components. The majority of features of engineering components are either flat (plane) faces, cylindrical surfaces or conical (tapered) faces, or are produced from a combination of these. In general surfaces are either generated or formed (this will be examined further in the BTEC Manufacturing Technology unit).

When generating;

• The feature is independent of the shape

of the tool

• Much longer profiles or features may be

generated than formed.

When forming:

• The accuracy of the feature is dependent upon the

accuracy of the forming tool

• Form tools often result in chatter marks on the

workpiece due to the relatively long cutting edge,

therefore cuts must be kept light

• Complex forms are difficult to produce as they required

complex tools

For example to produce a tapered surface several methods may be used, i.e. by angling the compound slide to the appropriate angle or by using a special tool. These methods are shown below.

The workpieces that you will be making in the workshop will incorporate a variety of turning techniques, these will be demonstrated to you as you progress through the training block. As with earlier work it is important to plan what you are going to do first, by producing planning/operation sheet. For the first one or two jobs this may be done for you, but from then on you will be expected to produce a planning sheet for each and every job. The log book that you will maintain towards evidence towards your qualification will need to include a plan for each job.

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As an example the planing/operation sheet for the first turning job is shown below.

OPERATION/PLANNING SHEET FOR DRAWING CSC-016

JOB TITLE - Turning Exercise

Op Operation Description Main Tools/Work holding Speed Feed

1 cut stock to length (Ø25 x 80) & deburr hacksaw

2 set work running true and face end square to give datum

4 jaw chuck - std turning tool 540

3 grip leaving 40 mm from jaws - rough turn to Ø22

4 jaw chuck - std turning tool 450

4 knurl for length 35mm 4 jaw chuck - knurling tool 40

5 finish turn to Ø20 to length 4 jaw chuck - std turning tool 540

6 turn 3 x 45° chamfer (rotate compound slide) 4 jaw chuck - std turning tool 540

7 centre drill to depth of cone 4 jaw chuck - centre drill 800

8 progressive drill to Ø 11.5 4 jaw chuck - various drills

9 ream to Ø 12 4 jaw chuck - Ø 12 reamer 40

10 reverse workpiece and set running true (protect finished diameter)

3 jaw chuck

11 turn to Ø 18 and length 38 3 jaw chuck - std tuning tool 540

12 turn to finished length (facing) 3 jaw chuck - std tuning tool 540

9. BASIC MILLING

Safe Working Practices The Ajax milling machines in use in our workshop can be operated as horizontal or vertical milling machines, there is one Bridgeport vertical/universal milling machine but this is used mainly by the staff for special purposes). Before beginning to use them you must become familiar with the controls of the machine. Note: you will come across many different types, regardless of what machine you are using, if you are unsure of the operation of the controls ask for assistance DON’T EXPERIMENT, serious accidents can and do happen due to careless use of machine tools. Before starting any machine tool make sure that you know how to stop it, this includes the emergency stop. Check the location of the nearest emergency stop device to your machine before starting. If a machine is allowed to become covered in swarf and cutting it becomes dangerous, clean the machine when you have finished with it, and return all cutters, collets clamps etc. to their correct location.

Take careful note as the controls are explained to you and complete the table for the milling machine in each set-up.

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Item Description Item Description 1 16 2 17 3 18 4 19 5 20 6 21 7 22 8 23 9 24 10 25 11 26 12 27 13 28 14 29 15

Item Description 1 2 3

Selecting, Checking & Mounting Cutting Tools For safety and for the accuracy of the finished component it is very important that the correct cutter is selected, mounted correctly and used correctly for the required operation. The following section gives a brief description of the main types and their uses. Milling cutters are classified by the shape of their teeth. Also each cutter is further described by diameter, width or length, number of teeth, spiral angle of the flutes, bore diameter and the 'hand' of the cutter, that is right hand (RH) or left hand (LH).

Horizontal Milling Cutters. Horizontal milling cutters come in various types, side and face cutters, or cylindrical type cutters such as face mill, slab mills etc. Side and face cutters are used to produce,

• vertical faces

• steps

• slots

• 90° Vees

• straddle milling Some cutters may only have teeth on one side only (a half side cutter), some have the teeth offset to alternate sides (a staggered tooth cutter). Some cutters have a Vee shaped profile for cutting angled slots and grooves. These cutters are a close fit on the milling machine spindle or arbor and are located by a key which fits into the keyway in the cutter. Other similar forms of cutters include slitting saws, which are thin cutters which are used to,

• cut material to length

• mill narrow slots

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• undercut corners

• removing excess material before finish cutting

An important safety point to note when using slitting saws is that the locating key MUST NOT be fitted. If the slitting saw were to catch or stick due to an unduly heavy cut then due to the thin section of the slitting saw it would be very likely to shatter with pieces being flung in all directions. With the key left out then the cutter would most likely spin on the arbor with less danger resulting. Cylindrical type cutters are proportionately longer than side & face type cutters. These are also sometimes referred to as slab or roller cutters, and are used to produce,

• flat top faces of workpieces

• shoulders They may be mounted in pairs or groups to cover very large surfaces. In this case it is usual practice to mount RH and LH cutters together so that the sideways cutting action of the helix of the cutters cancel each other out and result in steadier cutting.

Vertical Milling Cutters. Vertical milling cutters are generally either slot drills, end mills or face mills. Slot drills usually have two cutting teeth or flutes which extend across the whole of the end face of the cutter and may be used to,

• cut closed slots or keyways

• flats on cylindrical surfaces

• counter-bores or spot-faces

• realign incorrectly positioned holes. End mills usually have four teeth, but larger cutters may have six, eight, or more teeth. End mills are used to cut,

• shoulders

• angular faces

• internal and external faces

• flats and faces Note that end mills cannot be used to cut downwards into material as the teeth do not cut across the full width of the cutter and has a centre hole which does not cut. Serious damage may result if this point is not taken note of.

Staggered

toothSlottingcutter

Anglecutter

Equal Anglecutter

Concavecutter

Convexcutter

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Specialised forms of end mills include cutters for producing Tee slots and dovetail slots. Slot drills and end mills are mounted in several different ways, by straight shanks, flatted shanks and screwed shanks, as shown. The majority of slot drills and end mills used in our workshop are screwed shank cutters which located in quick-lock arbors.

Face mills are generally quite large in diameter and are used to produce,

• flat top faces on components

• milling shallow steps

• roughing deep steps on workpieces The general guidelines for selecting cutters for horizontal milling cutters are,

• select the smallest convenient diameter cutter

• use coarse or staggered tooth cutters when taking heavy cuts

• use cutters with opposite helix angles (RH & LH) when gang milling The general guidelines for selecting cutters for vertical milling cutters are,

• select the largest convenient diameter cutter when milling faces

• select the smallest convenient diameter cutter when milling shoulders

• use coarse tooth cutters when cutting normal well supported workpieces

• when cutting thin workpieces use finer toothed cutters

• use tipped face cutters for materials such as cast iron Before using any cutter it is important to carefully check its condition before use, and this means before mounting the cutter, there in no point in wasting time mounting and setting up blunt or damaged cutters. You must check that,

• the teeth are not damaged i.e. cracks or chips

• that the teeth are sharp, a visual check, don’t test this with your finger, a cutter in good condition should be

razor sharp and blood makes the cutters rusty

• the shank is not damaged i.e. the diameter and the screw thread are clean and free from score marks

If you discover any faults with any cutter report them to the staff immediately don’t simply leave it for someone else to find or use. Likewise if you damage a cutter in use (it does happen) report it immediately and change it. Ensure that any damaged cutters are handed to the staff and are not used again.

Mounting Milling Cutters When mounting or setting cutters the first step is to ensure that the machine is electrically isolated, such that there is no way that the spindle may be started when you are setting up cutters, workpieces vices etc. Before mounting cutters carefully consider the method work-holding & the location of the work. It is generally better to clock-in a machine vice etc. before mounting the cutters.

NEVER HANDLE THE CUTTING EDGES OF ANY CUTTER WITH YOUR BARE HANDS, ALWAYS

USE A CLEAN RAG OR LEATHER GLOVE

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Mounting Horizontal Milling Cutters

Arbor Assembly and Mounting The arbor locates into the machine spindle nose by means of a tapered shank. The tapers are intended to be non-locking unlike Morse tapers, and serve only to locate the arbor. The arbor is held in the taper by means of a draw bolt which passes through the column of the machine. The positive drive to the arbor is by two keys, called dogs, which fit into the face of the spindle nose.

Spacing Collars These are available in a variety of lengths and slide onto the arbor to fill the spaces on the arbor not occupied by the cutters. The various collar lengths enable the cutter or cutters to be positioned on the arbor as required. The collars and cutters are held tightly together by an end nut. The tightening of the end nut provides friction between the ends of the collars and the cutters, a key should be placed between the cutter and arbor to give a positive drive. To avoid the cutter and collar assembly being excessively tightened by the cutter turning forces (especially in the case of a jam up) the key may be placed in a collar between the cutter and the end nut. Engaging a Cutter on a Long, Type ‘B’ Arbor. Use a type ‘B’ arbor when the top surface of the workpiece will pass between the arbor flange and the steady.

When using arbor type B keep clothing clear of unguarded arbor nut. Type ‘A’ arbors are used to enable the top face of a long workpiece to be raised to just below the diameter of the spacing collars without fouling on the steady. If a ‘B’ type arbor were used in this instance exceptionally large diameter cutters would be required. The arbor nut may be either left or right-hand thread. Always select an arbor & nut which tightens in the opposite direction to the required cutter rotation.

• Engaging a Single Cutter an Long Arbor Type ‘B’

• Engage arbor in machine spindle

• Set machine in low gear.

• Clean taper bore of spindle and taper shank of arbor.

• Hold arbor approximately parallel to table surface.

• Engage arbor in machine spindle, rotating arbor to align drive dogs to arbor slots.

• Tighten arbor in machine spindle

• Hold arbor shank in position in spindle

• Screw drawbar well into arbor.

• Tighten drawbar locking nut with the correct spanner Decide cutter position

• hand wind the machine table in, to position the workpiece close to rear of the machine.

• visually check that the workpiece or work holding device will clear the front of arbor flange.

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• clean the faces of the collars and slide them on arbor pushing collars up to arbor flange.

• visually check build up of collars to see outer face is level with or protruding beyond, face to be machined. Fit key in arbor

• remove outer collar from arbor.

• fit key into arbor keyway, feeling close sliding fit of key.

• visually check that key will protrude into collar at each side of cutter.

• replace collar on arbor rotating it to align keyway to key. Engage cutter on arbor

• visually check cutter is facing in direction of spindle rotation and slide cutter onto arbor-

• rotate cutter, to align keyway to key, and push up to collars. Position arbor bearing collar

• check position of cutter to face to be machined.

• adjust table setting if necessary to correct positioning.

• slide further collars onto arbor rotating as necessary to align keyway to key.

• visually check the build up of collars, until outer face protrudes beyond front extreme of workpiece.

• slide steady bearing collar up to collars on arbor. the closer the steady bearing collar is positioned to the cutter

the greater the rigidity obtained when steady is positioned.

• slide further collars onto arbor until they just protrude onto threaded part of arbor. one or more of these collars

should be keyed to the arbor if the cutter has not been keyed. this ensures that the nut will not tighten if the

cutter should slip.

• visually check that sufficient thread protrudes for nut to be fully engaged. engage nut and finger tighten. Engage overarm steady

• loosen overarm securing nuts.

• slide overarm out visually checking that end protrudes beyond outer end of bearing bush.

• wipe mating parts of overarm and steady.

• visually align mating parts of steady and overarm.

• engage steady on overarm, sliding to position in which the front of steady is level with end of overarm.

• tighten nut to secure steady on overarm.

• slide overarm in, to position steady approximately central to length of bearing bush.

• tighten overarm securing nuts tightly. Finally tighten cutter on arbor. Position spanner on nut with handle approximately vertical and tighten nut securely. NEVER use a worn spanner or arbor nut. Check true running of cutter

• check that cutter is clear of workpiece.

• start machine spindle to run cutter at moderate speed.

• align vision to side of cutter and a fixed part of machine or workpiece to see if cutter wobbles.

• align vision to periphery of cutter and a fixed part of machine or workpiece to check concentricity. If the cutter wobbles

• check that the faces of the spacing collars, cutter, arbor and securing nut are clean and undamaged.

• check that the arbor is correctly mounted and the tapers are clean and undamaged.

• check that the faces of the spacing collars are parallel. If the wobble is still present;

• the arbor may be bent, check concentricity of arbor

• the cutter may be incorrectly ground Engaging Cutters on Long Arbor Type ‘A’ Cutters are engaged on this type of arbor in exactly the same way as on arbor type ‘B’ except:-

• no bearing bush is required the pilot diameter runs in a bearing which is built into the steady.

• check that the nut does not protrude on to the pilot diameter of the arbor.

• check that the bearing in the steady is clear of the arbor thread.

• check that the bearing in the steady does not protrude beyond end of pilot diameter.

• key one or more of the collars between the cutter and the securing nut, if the cutter is not keyed to the arbor

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The final stage before cutting can begin is to mount and set the guards. These vary in design, so check with the staff for the correct setting and location.

Mounting Vertical Milling Cutters Engaging Taper Shank Arbors in Machine Spindle The majority of vertical milling cutters in use in our workshop held using a form of quick locking or fast locking self-centring chucks. Select arbor

• use the shortest arbor possible.

• ensure that the internal taper is the same size as spindle taper. Engage the arbor in machine spindle

• clean both tapers.

• engage drive dogs in arbor slots.

• hold cutter in position applying upward pressure.

• screw drawbar firmly into arbor.

• switch on machine.

• rotate spindle at a moderate speed, to visually check cutter runs true. Engaging Cutters in Automatic Locking Chucks (Fastlock Chucks)

• engage arbor in machine spindle.

• engage low gear on machine spindle.

• clean component parts before assembly

• insert the collet into the bore of the sleeve, ensuring that the driving flats of the collet engage in the mating slot

of the sleeve

• insert cutter and screw into collet until it locates on the centre and becomes tight (protect your hands from the

sharp cutter edges).

• using the proper chuck spanner and give a final tighten to the sleeve Note: Cleanliness is essential. Ensure all parts of chuck are thoroughly clean before assembly and when returning to the storage boxes. Disengaging Cutters from Automatic Locking Chucks

• engage low gear on machine spindle.

• loosen locking nut with special spanner provided and remove nut from arbor.

• pull cutter out to remove sleeve and collet from arbor, again protect your hands.

• unscrew cutter and slide from collet

• clean all parts of chuck before storing away. Again when the cutter has been mounted then suitable guards must be fitted before any cutting takes place.

Workholding For Milling Operations The method of workholding that you chose is dependent on the size and shape of the component and the required accuracy of the finished component. Irrespective of the method chosen, the requirement of all workholding devices is that, all work to be machined must be securely held to resist the large forces exerted by the cutting tools. It is also very important that, if the work is to be accurately produced, it must be held either square or at right angles to the cutter, and this is directly dependent on the correct setting up of the workholding device. With all workholding methods the cleanliness of the component and the vice , clamps etc. is one of the most important factors. This means making sure that all the locating areas are CLEAN with ALL swarf removed, and also the faces and edges of the component are clean and burr free.

Arbor

Centre

Sleeve Nut

CutterCollet

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The Machine Vice The machine vice is probably the most common form of workholding device. The Fixed Jaw Always try to mount work in machine vices so that the cutting forces should be directed towards the fixed jaw as this is more stable than the moving jaw. For the same reason this jaw is used to set the vice parallel or at right angles to the cutter. The Moving Jaw This jaw moves along a slide to close onto and hold the workpiece. Because of the jaws clearance with the slide it is a little less stable than the fixed jaw. Clamping Slots The vice is clamped to the worktable by two Tee bolts in these slots. These slots have sufficient clearance to allow alignment of the vice with the cutter. Tenon Slot Tenon blocks may be fitted into this slot. These blocks form a close fit in the tee slots in the milling table and allow for the quick alignment of the vice jaws at right angles to the milling machine table. Parallel Packing These are parallel bars that are usually used in pairs, (they should be hardened and ground to resist wear) to identical sizes. The variety of bar sizes available, used either individually or in combinations, allow the workpiece to be raised to almost any desired level parallel to the datum surface of the workholding device. When setting workpieces in machine vices care must be taken to ensure that both work and the vice are free from swarf and dirt. The work should be lightly held by the vice at first then the work should be tapped with a soft-faced hammer. There will be an audible ringing noise when the work is tapped, until the work is seated squarely on both parallels. Check that the work is sitting squarely on both parallels i.e. there should be no movement of either parallel under the workpiece. When this has been achieved then fully tighten the vice and check the parallels again.

Setting The Vice Parallel And Square To The Cutter There are three basic methods of setting the jaws of vice parallel or square to the cutter,

• Using a square

• Using a dial test

indicator (DTI)

• Using tenons Using A Square The fixed jaw is used to set the vice or, alternatively, a parallel bar may be held in the vice jaws so that it protrudes above the jaws, making access much easier.

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This method is only suitable for setting the jaw at right angles to the table. A large square is held so that the stock is pressed against the flat face of the column of the milling machine and the vice adjusted to set the jaw parallel to the blade. Using A Dial Test Indicator (DTI) This method may be used for setting the jaws, both at right angles and parallel to the table, and when carried out correctly is more accurate than the engineer’s square method. This is commonly referred to as ‘clocking-in’. The DTI is supported on a magnetic base although clamping is an alternative. Using a parallel bar held in the vice jaws allows the plunger type of DTI to be used, this has the advantage of being more robust and having a greater range than the lever type. If the jaw itself is to be used, then the lever type will enter more easily. The method of setting up is to adjust the lever or plunger of the DTI until a reading is made in the centre of the parallel or jaw. The table is then operated to record a reading along the length of the jaw and to discover any variation. Adjustments are made to the vice alignment until a steady reading is recorded along the full length of jaw. Using Tenons Tenon slots are slots machined into the base of the vice. These run parallel and at right angles to the jaws. The tenon blocks are usually screwed or form a press fit into these slots. When the vice is lowered onto the table, the protruding part of the block makes a close fit into the tee slots of the milling machine ensuring alignment automatically takes place. Not all vices have tenon slots, and those that do may only have them running the length of the vice. The width of the tee slots may vary in the different makes of milling machine. This means that either the vice is kept for use on certain machines or a variety of sizes of tenon blocks are made available. For those vices that do have this provision, tenons are by far the quickest method of set up.

Important Points To Note On Setting Up A Machine Vice The machine vice can be heavy so always ask for advice and help before attempting to move or lift it. When first positioning the vice on the table, make sure the bolt slots are central to the tee slot in the table. This will ensure there is enough clearance between the bolt slots and the tee bolts to allow movement to set up the vice. To begin with, only tighten the tee bolt at the handle end of the vice allowing it to pivot about this end. Tighten both tee bolts when the jaws are correctly aligned, then recheck. You may find it quicker when setting the vice at right angles using the DTI, to first set it using the square.

Clamping Some workpieces, because of their size or shape, cannot be easily held in any of the normal workholding devices then clamping directly to the worktable is considered. Clamping may also be used to secure work onto angle brackets and milling fixtures.

The Clamp This contains a bar of metal of sufficient strength to resist bending under the clamping forces and which has a hole, or preferably a slot, to allow adjustment of the tee bolt.

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The Support Block This can be a convenient sized piece of scrap metal or a stepped block as illustrated. The block should ideally be of equal height to the workpiece, but in any case not lower, otherwise the clamping force on the workpiece will be adversely affected.

Tee Bolt These are made from high tensile steel. Note that a washer must be used, this is to allow the nut to be tightened without affecting the position of the clamp.

Clamping Force For the clamp to exert maximum force on the workpiece the tee bolt should be positioned as near to the workpiece as possible and the support placed at the greatest distance from the Tee bolt as possible. Care must be taken when using clamps to check for clearance to avoid them being hit by the cutter or arbor supports etc.

Swan Or Gooseneck Clamp This clamp allows the nut to be in a lower position to allow greater clearance.

Standard And Adjustable Angle Plates The standard angle plate is used for supporting work at 90° to the table. The work may be further set at an angle on the face of the plate by use of a protractor, sine bar or template. The work is usually clamped to the angle plate. To accommodate the clamping bolts a series of slots are provided. The adjustable angle plate differs from the standard in that it is hinged to allow the vertical plate to be swivelled to angles between 0° and 90°. Both types of plates are available in a variety of sizes.

The Standard Angle Plate The angle plates may be set parallel to the table by pushing the plate up to tenon blocks placed in the milling table tee slots, or set at right angles using a square or DTI in a similar manner as described for the vice. The plate may also be set at an angle on the table using a protractor.

The Adjustable Angle Plate The adjustable angle plate may be set to an angle to the vertical using the angular scale engraved on the swivel, or if more accuracy is required, by a vernier protractor.

Vee Blocks Vee blocks are used to support round work. They may be used singularly or as a matched pair. When choosing the size of block, the round bar should ideally rest halfway down the groove faces. Single Vee blocks may support work in a vice or by clamping. When used as a pair the Vee blocks may be set parallel to the table by pushing them up to tenons fitted into the machine table tee slots. The true alignment of the work may be checked using a DTI. The work is then held by a series of clamps. Care must be taken to ensure clamps do not foul cutters.

Selecting Suitable Spindle Speeds And Feeds The correct spindle speed for a certain diameter cutter for different materials may be determined using a simple formula, (this is the same basic formula for drill speeds and lathe spindle speeds).

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Spindle speed = surface cutting speed

π x cutter diameter note the units must be in the same form, i.e. RPM = metres per minute

π x millimetres As it is unlikely that the milling machine will have exactly this spindle speed, then the nearest available spindle speed would be selected. If this theoretical speed falls mid way between two available speeds then select the lower speed. Feeds rates on milling machines are generally expressed in millimetres per minute. The selection of a suitable feed rate that is dependent upon,

• the pitch of the cutter teeth

• the rigidity of the workholding set-up

• the condition of the machine tool

• the diameter of the cutter

• the surface finish required

• the type of material

• the width and depth of the cut Each tooth on the cutter will make an equal amount of cut as the work/worktable advances. Each tooth will therefore take a cut equal to the advance of the table per revolution; divided by the number of teeth in the cutter. This is known as the 'feed per tooth'. To work out the feed made by the cutter every minute you must multiply the feed per tooth by the number of teeth on the cutter and by the number of these revolutions made every minute RPM, i.e. using the simple formula, TABLE FEED (mm/min) = RPM x No of TEETH x FEED/TOOTH (mm/tooth) The recommended depth of this cut can be found from the cutter manufacturers' tables. In general a value of 0.1~0.15mm per tooth is usually a good point to start. Due to all of these factors setting the feed rate is often down to experience, seek the assistance of the experienced staff if you are unsure. It is better to start off with a lower feed rate and gradually increase this as long as the machine seems to be cutting efficiently without undue strain.

Producing Components The majority of components are produced from a combination of horizontal, vertical faces, slots holes angled faces, or curved faces. The first stage in making any component is to ensure that the basic component size is achieved and each face is square to the other. This is often referred to as blocking-up. Bright drawn mild steel (BMS) is produced by cold rolling and the rolled faces are usually reasonably square to each other. In this case there may be little work required to bring the block to its required size. Other forms of steel, i.e. black bar and particularly high carbon tool steels are hot rolled and the faces have a surface scale, which must be removed first. An additional problem with hot rolled steels is that the rolling process will usually leave high surface stress in the material, when the outer skin of the material is removed these stresses often cause the block to the bend. It is vital in this case that this outer skin is removed first, taking a light cut of 1~2 mm deep, (depending upon the material) and then squaring the block up. If the block is not square to start with, then the machine vice jaws will not grip the work squarely. In this case the work may be held using a roller (around 20~25mm diameter) between the moving jaw and the workpiece to allow the work to be pushed square against the fixed jaw. Before cutting can begin there a few points which must be understood in relation to the direction of relative movement of the cutter and the workpiece. These are referred to as up-cut milling and down-cut milling. This can apply in general to both horizontal and vertical milling machines, although some of the examples used may only apply to one type of machine.

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Up-Cut Milling The up-cut method of milling is the standard and most common method employed. In this method the rotation of the cutter meets the direction of the feed. The advantages of this method are:

• it is safer because the work cannot be dragged into the cutter

• up-cut can be used on machines not fitted with a backlash eliminator.

• it can be used on machines that are small or lack the necessary rigidity The disadvantages of this method are:

• the cutting forces tend to lift the component off the worktable or out of the workholding device

• the cutting forces oppose the rotation of the cutter

Down-Cut Milling Down-cut milling is where the cutter rotation is in the same direction as the feed. To machine using this method, the backlash eliminator must be engaged and the cutting must only be by power feed. The backlash eliminator reduces the clearance between the leadscrew and the nut and so ensures there is never any free movement in the table leadscrew/ nut motion, so that the table is always under the control of the nut and cannot be dragged along by the 'digging in' motion of the cutter teeth. The down-cut method is only used on the rare occasions when it is necessary to overcome certain problems and then only by experienced and skilled operators. The advantages of this method are:

• the cutting forces tend to hold the work down onto the worktable (horizontal). This makes the down-cut

method ideal for milling thin sheets and the face milling of work clamped direct to the table

• because the feed is in the same direction as the cutter rotation, this aids metal removal, allowing heavier cuts

to be taken

• ductile material such as aluminium may be machined leaving a better surface finish. This applies particularly

to vertical milling the sides of steps and slots The disadvantages of this method are:

• there is always a danger of the work being dragged into the cutter. This would cause serious damage to the

work, cutter and machine. The operator is also in considerable danger of being hurt by pieces of broken cutter

or work should this occur

• down-cut milling should only be undertaken by highly skilled, very experienced operators

• down-cut milling is not suitable when machining with fragile cutters such as dovetail or tee slot cutters, as the

forces involved will easily break these cutters

• down-cut milling can only be used on a machine fitted with a backlash eliminator, and then only on a machine

in good condition

• the cutting can only take place with a machine engaged in power. If attempted by hand, even only at the start

of a cut, the work will be pulled into the cutter with disastrous effects. Having the backlash eliminator engaged

will make no difference.

Up-cut milling Down-cut or

climb milling

Spindle rotation

Work FeedDirection

Work FeedDirection

Spindle rotation

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The Blocking-Up Process is generally as shown below. Ensure that between each stage the job is deburred and the vice & parallels cleared of swarf.

SAFETY

Deburr the work while still in the machine vice, taking care to wind the job clear of the cutter before doing so. Always clear ALL swarf from the work area between set-ups but NEVER with

your bare hands, use a brush.

The first two stages only require a clean-up cut to be taken, but the third and four stages, (and the cuts required on the ends) require that the block be machined to a specific size. When machining to specific sizes then care must be taken to establish a datum point by on the block and zeroing the machine slide dials in each of the three axes in turn (this may depend on the feature being machined. There are several methods in common use to establish a datum on the block. The simplest is to carefully bring the cutter into light (very light) contact with the workpiece. The dial on the relevant slide is then zeroed. This requires a little practice and careful observation (through safety glasses) and hearing, as you may often hear the cutter just touching the surface before you see it! Another method is to use a precision roller (it is easiest to use a roller of the same diameter as the cutter otherwise you need to do some maths to work out the difference in the diameters) in place of the cutter and bring this into position very close to the workpiece. The final distance is set using a feeler gauge. With this method the workpiece then needs to be moved an additional distance of the thickness of the feeler gauge to bring the cutting edge into the datum position. Don’t use feeler gauges between the job and a cutter, as this will damage the feeler gauges. Another method laying a piece of thin tissue paper (not thick paper towel) on the top of the workpiece which is wetted with coolant. When the cutter is brought into contact with the workpiece the tissue paper will be removed by the cutter. The thickness of the tissue paper is often thin enough to be assumed to be nil and that the cutter is just touching the workpiece.

1

2

3

4

4

3

2

1

4

3

2

1

1

4

2

3

R olle r

P ara lle ls

F ixed Jaw

M ov ing J aw

W ork p ie c e

C utte r

M ach ine face 1

R o ta te job, c lean deburr& m ach ine face 2

R o ta te job , c lean deburr &m ach ine face 3 to s ize

R o ta te job, c lean debu rr, rem overo lle r & m ach ine face 4 to s ize

Cutter

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Other methods for establishing datums use what is known as a wiggler or centre finder, or for very precise work optical centre finder devices may be used. As shown here there are many different features on components that may be produced on milling machines, these

will be demonstrated to you as required. Again as when you are working on lathes it is important to plan what you are going to do first, this involves producing planning/operation sheet (copies of blank planning sheets are available). You will be expected to produce a planning sheet for each job, remember the requirements of your log book! As an example the planing/operation sheet for the first milling job is shown below. This component was used in the first fitting section, so you should have the material already cut (almost) to size. The speeds and feeds have not been shown as these are dependent on the actual cutter used. You will need to determine these speeds and feeds for yourself.

Operation/planning sheet for drawing csc-006

Job title – milling exercise

Op Operation Description Main Tools/Work holding Speed Feed

1 MOUNT MACHINE VICE SQUARE TO AXIS OF THE MACHINE

2 OBTAIN BLOCK USED IN CHISELLING EXERCISE 50 X 50 X 50

3 MILL BLOCK SQUARE TO 47 X 47 MACHINE VICE, SLAB MILL

4 SET UP BLOCK IN HORIZONTAL MILLING MACHINE

MACHINE VICE

5 MILL TOP OF BLOCK SQUARE USING SUITABLE CUTTER & DEBURR

MACHINE VICE, SLAB MILL

6 MILL SLOTS TO DEPTH & DEBURR MACHINE VICE, SIDE & FACE CUTTER

7 ROATE BLOCK 90°

8 MILL SLOTS TO DEPTH & DEBURR MACHINE VICE, SIDE & FACE CUTTER

Narrow slots Vee Grooves

Closed Slots

Open Slots

ShouldersAngled Faces

Through & Tee Slots

Dovetail Slots

Typical Features Produced by Horizontal & Vertical Milling Machines

FEATURES ON UPPER FACEPREVIOUSLY CHISLLED

FEATURES ON LOWERFACE TO BE MACHINED

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10. BASIC HEAT TREATMENT. There are several basic methods used for the heat treatment of workpieces in the workshop,

• Hardening

• Tempering

• Annealing

• Normalising

Hardening. Hardening is carried out by two basic methods, surface hardening and through hardening.

Surface Hardening Steel can be classified as plain carbon steel or as alloyed steels i.e. steel with added alloying elements which are used to improve its mechanical properties. Plain carbon steel, is very broadly classified as: dead-mild steel – 0 ~ 0.15% carbon mild steel – 0.15 ~ 0.25% carbon medium carbon steel – 0.25 ~ 0.60% carbon high carbon or tool steel – 0.60 ~ 1.50% carbon Dead-mild, mild steel and medium carbon (depending upon the exact percentage of carbon) have too little carbon to be hardened directly and need to have carbon added before hardening can take place, by a carburising or case harden process. Case Hardening, is a beneficial processes for hardening the surfaces of steel used for certain products such as gears, axles, and other machine parts subject to much mechanical wear. This makes them more resistant to abrasion and wear, while leaving the interior soft and therefore tougher and more fracture-resistant. The hardening may be accomplished by dissolving carbon into the surface, a process called carburising, or by adding nitrogen, a process called cyaniding or nitriding. Steel may be carburised in the workshop by being embedded in charcoal, from which it absorbs carbon, in a furnace at a temperature of 800° to 900°C for periods varying from several hours to several days. The steel is then immediately immersed in cold water. The length of time that the component is heated for effects the final depth of hard case. In another process commonly used for steel articles which are case-hardened is to heat them until they are red hot in powdered potassium cyanide, which decomposes and gives out carbon to the steel. The carbon usually dissolves in the steel to a depth of 0.3 to 3 mm, depending on the length of treatment. Before hardening begins you must check the grade of steel being used to determine the specific hardening requirements.

Through Hardening. Through hardening of medium and high carbon steel may be done simply by using an oxy-acetylene torch or be using a furnace. Simple components with sufficient carbon content may be hardened successfully by heating to cherry-red then quenching in water. This will leave the components hard but very brittle. In most cases the component must then be tempered.

Quenching Media. The rate at which the component is quenched is governed by the medium that is used and may be generally;

• Caustic soda solution

• Brine (salt water)

• Cold water

• Warm water

• Mineral oils

• Animal oils (whale oil used to be a typical quenching medium)

• Vegetable oils

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These media are listed in order severity of quenching, it must be borne in mind that the more severe the quench the more the risk of the component cracking due to very high temperature stresses. Care must be taken not to use just any-old oil for quenching as this is a potential risk from the oil igniting if its flash-point is exceeded. The temperature at which a substance will catch fire and continue to burn is called its ignition point or its kindling point. A substance that can be ignited in the air is said to be flammable (or inflammable). The flash point of a flammable liquid is lower than its ignition point. The flash point is the temperature at which it gives off sufficient vapor to flash, or flame suddenly, in the air.

Tempering Hardened Steels. Tempering is a low-temperature process where a balance is obtained between the hardness and toughness of the finished component. Re-heating to a lower temperature than that used for hardening, decreases the hardness a little, but improves the toughness. The proper balance between hardness and toughness is controlled by the temperature to which the steel is reheated and the duration of the heating. This temperature may be controlled by an instrument for measuring high temperatures, known as a pyrometer, or, in simple workshop cases, by observing the colour of the oxide film formed on the metal during heating.

Tempering Colours For Carbon Steels

Temperature (°C) Colour Typical components.

220 pale yellow scrapers, hacksaw blades, light turning-tools

230 straw hammer faces, screwing dies for brass, planing and slotting-tool, razor blades

240 dark straw shear blades, milling-cutters, drills, boring-cutters, reamers, rock-drills

250 light brown knife blades, taps, metal shears, punches, dies, woodworking tools for hardwood

260 purplish brown plane blades, stone cutting tools, punches, reamers, twist-drills for wood

270 purple axes, augers, gimlets, surgical tools, press-tools

280 deep purple cold chisels, (for steel and cast iron), chisels for wood, plane-cutters for soft-wood

290 bright blue cold-chisels (for wrought iron), screwdrivers

300 dark blue wood-saws, springs

When lighting gas torches and furnaces the correct procedures for the lighting and extinguishing must be observed. It is important that the gauges are checked for the correct pressure before beginning. The fuel gas, usually acetylene is lit first and then the oxygen is introduced gradually until a suitable flame is produced. When extinguishing the flame the oxygen must be reduced and stopped be fore the fuel gas to avoid potential danger from blow-back. It is occasionally necessary to remove heat treatment stresses, or to machine hardened steel components before this can take place it is often necessary to reduce the degree of hardness of the steel normalising or annealing. There are several methods used to determine the degree of hardness that has been obtained from the heat treatment process. These hardness tests are named after the engineers who first devised them and are

• the Brinell test

• the Vickers test

• the Rockwell test

• the Shore Scleroscope. The Brinell test uses a hardened ball which is indented into the workpiece using a know force and the diameter of the indent is measured and the degree of hardness is calculated from this. The Vickers test is similar but uses a hardened pyramid to give the indent. The Rockwell test again uses an indentation to determine the degree of hardness, but will give a direct reading and is generally quicker and more convenient than the other tests. The Rockwell test uses different scales which use different indentors and loads for different materials and levels of hardness.

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The Shore Scleroscope works on a different principle from the other tests in that it uses a diamond tipped hammer held inside a graduated glass tube. The hammer is dropped onto the hardened surface to be tested and the height of the rebound is measured and is used to determine the degree of hardness.

Annealing and Normalising. Normalising is generally used to obtain a uniform structure throughout the steel component. This is carried out by heating to just above it’s upper critical temperature, the percentage of carbon needs to be know to determine this from graphs or tables. The component needs to be only held at this temperature for the temperature to equalise throughout the component before allowing the component to cool in still air. Annealing, process of heat treatment by which metals and alloys are rendered less brittle and more resistant to fracture. Annealing minimises internal defects in the atomic structure of the material and leaves it free from internal stresses that might otherwise be present because of prior processing steps. Ferrous metals (steels) are annealed by heating them to high temperatures and cooling them slowly. Large masses of metal may be allowed to cool within the heating furnace as the furnace cools and may take several days. Annealing time, varies widely according to the thickness of the individual piece. Annealing is often required as an intermediate step in metal-forming processes such as wire drawing or brass stamping in order to restore the ductility of the metal lost because of work hardening during the forming operation.

engineering principles for gcse engineering

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