MEASURING AND MARKING OUT

Accurate measuring and marking out of metal before cutting, drilling or shaping is as important as with any other material, since any errors will be difficult to rectify. However, metalwork in the home will not require the same degree of accuracy as that in commercial use.

RULE An engineer’s steel rule can be bought in lengths of 150-1000mm. A stainless steel measure is ideal since it reduces the chance of staining and rusting, although this should not happen if you take the precaution of lightly oiling any metal tool before storing. Straight-edge A metal straight-edge, with the working edge bevelled, is often easier to use than a steel rule when scribing long lines. MEASURING TAPE When measuring lengths beyond the scope of a steel rule, you should use a good quality steel measuring tape. Ideally it should have a top sight – a see-through panel in the centre of the

You will be able to tackle most measuring and marking out jobs with a steel rule, an engineer’s try square and a metal scriber — although with ingenuity and care you can often improvise. tape housing for accurate reading – and a combined stop-hook, which is self-adjusting for inside or outside measuring. The easiest type to read has black markings on a yellow background.

TRANSFERRING MEASUREMENTS

When a direct measurement cannot be taken with a rule, you will have to use calipers, dividers or trammels and transfer measurements to the work. CALIPERS These are mainly used for measuring the external and internal diameters of shafts and tubes. Odd-leg or jenny calipers have one leg with a right-angled step and the other carrying a point; these can, for example, be used to mark out lines parallel to the edge of a workpiece. Firm-joint calipers are useful for large work because they have stronger legs, but they can be difficult to set by hand and you may find it easier to tap one leg against a hard surface. Spring calipers have a central screw which makes them easier to adjust.

Practice is necessary to develop the right ‘feel’ for calipers, which should never be forced over or into the work.

VERNIER CALIPERS An accuracy of 0.02mm is possible with the vernier calipers, which consist of a fixed-rule scale with A fixed jaw arranged at right-angles to one end of the scale. Mounted on the scale is a sliding jaw which has another scale engraved on it. When the caliper is closed the zero marks on the two scales coincide. With the jaws apart the position of the zero mark on the sliding scale in relation to the fixed scale gives the distance between the jaws to an accuracy of one division on the fixed scale. By counting the number of divisions on the sliding scale from zero to the point at which a line on the sliding scale coincides with a line on the fixed scale, it is possible to measure to a fraction of one division on the fixed scale. The usual metric vernier calipers have divisions of 1mm on the fixed scale. The sliding scale has 50 divisions and it is therefore possible to measure to ^ of 1mm. On Imperial calipers the fixed scale has divisions of2o in and the sliding scale has 50 divisions, giving an accuracy of rooo in.

Most vernier calipers are so designed that the can be used as inside or outside calipers and many also have a depth gauge attachment. This makes them a versatile measuring tool and one to meet most demands. Models engraved with both metric and Imperial scales are also available. DIVIDERS The metalworker’s dividers are equivalent to the draughtman’s compasses. The legs have hardened points and are adjusted by a centre screw. Their main use is for circle-marking and ‘stepping off measurements along a line – for example, if you wish to space holes at equal distances. This marks the centre of the holes with more accuracy than by measuring with a rule. TRAMMEL Basically a large pair of compasses.

Below A selection of tools which can be used for measuring and marking out your metalwork 1 Firm joint inside calipers 2 Firm joint odd-leg calipers 3 Feeler gauges for measuring clearances 4 Sliding bevel gauge 5 Firm joint outside calipers 6 Screw pitch gauge 7 Centre square 8 Metal straight-edge 9 Surface plate 10- Angle plate for use with a surface plate 11 Vee block and cramp for holding circular or irregular shaped work 12 Toolmaker’s clamp used when marking out two pieces of work at the same time 13 Vee block 14 Steel measuring tape 15 Engineer’s try square 16 Protractor with locking steel arm 17 Engineer’s steel rule

It consists of a wood or metal bar which has one fixed and one movable point: the only limit on the radius that can be drawn is the length of the bar which carries the points. Professional trammels are quite expensive, but you can easily make your own with any straight, rigid bar and simple clamping devices to hold the fixed and moving points. Engineer’s try square There are a number of tools for measuring angles, but the most important is the engineer’s try square. This is a heavy steel stock with a thinner blade fixed at right-angles. A woodworker’s square may be used, but it is often too large for many metalworking jobs. The try square is used for marking out 90 degree angles and for checking work is square. It is essential the edges of the blade are parallel and the working edges of the blade and stock remain undamaged otherwise inaccurate work will result.

Centre square This is also called a centre-finding gauge and may consist of two arms at right-angles to each other and a third arm which bisects the angle between the other two. When the outer arms are placed against the end of a metal rod, ajine scribed along the bisecting arm will pass through the centre of the rod. By holding the gauge in two or three different positions around the rod, the scribed lines will intersect at the centre point. The gauge can be used for finding the centre of a disc in the same way.

Sliding bevel gauge Two slotted arms are clamped together by a knurled adjusting screw. By loosening the screw the angle between the arms can be set to any desired position, using a protractor. This is used for measuring angles or for transferring previously measured angles to the work: it can be made quite easily by cutting a slot in two straight pieces of metal and joining them with a screw and wing nut. Protractor Simple protractors consist of a steel semi-circle or a steel circle, sometimes with a steel arm which can be locked at the centre of the protractor. Simple protractors measure to the nearest decree: for more accurate work use one with a vernier scale.

ENGINEER’S COMBINATION SET This versatile set, a combination of four tools, consists of a heavy steel rule with a slot machined along the centre of the rule. In the slot you can mount a centre square, a protractor with spirit level, and a square head also fitted with a spirit level. The heads can be bought separately; if they are all available, the tool can be used as a try square, as a centre square and as a depth gauge. It can also be used to set out true verticals and horizontals and to measure any angle. The square head has one edge at 45 degrees so it can be used to set out mitres. The rule can. of course, also be used on its own.

CLEARANCE AND THREAD GAUGES Clearances can be measured accurately with feeler gauges, which comprise 20 or more different blades of varying thickness. Threads are measured with screw pitch gauges – a set of blades with notches at different spacings; the notches fit into the screw thread and the blade with the notches which coincide exactly with the screw thread tells the pitch of the screw.

Using PRECISION TOOLS

The tools already described should be more than adequate for the home workshop. High-precision engineering is costly in terms of the equipment and time needed to complete the work. If you wish to operate within fine limits, it may be worth investing in a micrometer and/or a dial test indicator.

MICROMETER This consists of a horseshoe-shaped frame which carries a hardened-steel anvil inside one end. A hardened-steel spindle at the other end moves backwards and forwards in a barrel. The spindle is moved by a thimble, which rotates around the barrel. On the barrel is a scale, which on metric micrometers is divided into divisions of ]mm – the distance each complete turn of the thimble moves the spindle. On the edge of the thimble there are 50 divisions, which enable you to measure the distance moved by the spindle to 50 of one division on the barrel – or 100 mm. On Imperial micrometers the barrel is graduated in divisions of 4 and this indicates the maximum size of work it will accept.

INSIDE MICROMETER AND DEPTH gauge It is possible to buy a micrometer to measure inside holes; but since the smallest hole which can be measured in this way is 50mm. few home workshops will have the equipment for this size of boring. You can buy micrometer heads with extension rods for measuring depth of holes and grooves, but for most work the vernier calipers or combination set will be more than adequate.

DIAL TEST INDICATOR All the measuring tools previously described give a direct reading. The dial test indicator is used to compare the object being measured with some previously set measurement. It consists of a clock face with a large and a small hand. Inside the clock is a plunger which, when moved upwards, causes the hands of the clock to rotate and indicate the distance moved by the plunger. One complete rotation of the large hand coincides with a movement of one division on the small hand. The zero position on the gauge can be set by releasing a locking screw on the side of the gauge and rotating the face so the zero coincides with the large hand. The gauge requires accurate setting. It is usually held in a clamp, and if incorrectly mounted, large errors could be made.

MEASURING ACCURATELY

With all measurements you will be working to specified degrees of accuracy and never to absolutely exact dimensions. The degree of accuracy depends on the equipment and the skill of the metalworker. Therefore, if you make a series of measurements along a line, you will stack up your errors as you work. If. for example, you are measuring to an accuracy of,oo mm. there may be a maximum error of 10 x,,1>0 mm after marking out ten successive lengths. To avoid this it is good practice to make two edges of the metal fiat and square and take all measurements from these.

The tool most often used for marking out is the scriber. a metal rod with a hardened-steel point at each end. One point is straight and the other end is cranked. Care must be taken that lines are not too deeply scribed or they may spoil the appearance of finished work. Avoid scribing bend lines because the scribing may weaken the metal and cause it to crack when bent. Use a pencil line instead.

A firmly drawn scribe line will make a good guide for cutting or filing or for indicating the centre lines for drilling holes and there are several methods of improving the visibility of this line. Metal with a scaly surface should be chalked; metal with a smooth or polished surface should be coated with engineer’s blue – a slightly oily dye. sold in small tubes, which is lightly smeared on the surface to be marked. Another method for smooth metal is to coat it with copper sulphate solution, which produces a surface on which scribed lines stand out.

Always work with the scriber point against the rule or straight-edge to avoid inaccuracy.

SURFACE PLATE The important principle in marking out is to work from two edges which are known to be perfectly flat and at right-angles to each other. Once this has been achieved, usually by filing, place the work on a flat cast-iron plate to complete the marking out. If a scribing block is used, you can ensure all lines scribed are parallel to your datum edges. A good substitute for a cast-iron surface plate is a sheet of thick plate glass.

ANGLE PLATE IN order to hold work vertically on the surface plate, it is usual to clamp the work to a cast-iron angle plate, which has two flat surfaces at right-angles to each other. Usually the plate is slotted so the work can be clamped on. An angle plate is essential for high accuracy; for less critical work a piece of angle iron can be used.

BLOCK THIS IS designed to hold circular or irregular shaped work during marking or drilling. It is a rectangular block, three sides of which are normally machined flat and square to each other. A 90 degree vee is machined into the fourth side. Slots are machined into two sides of the block to take a clamp to hold the work in the vce. Blocks are supplied in pairs to ensure the work is held perfectly horizontally.

You can improvise with two lengths of rod of approximately the same diameter as the workpicce. using toolmakers clamps to hold the rods.

TOOLMAKER’S CLAMP This consists of two metal jaws which are opened and closed by two screws arranged at right-angles to the length of the jaws. Since the screws can be rotated independently, the jaws can always be adjusted to be parallel to each other and so exert the most effective grip on the workpiece. The clamp is used when two or more workpieces are being marked out simultaneously or if you want to hold work to the angle plate.

MARKING AND DRILLING HOLES

The centre of a hole should first be marked by scribing two lines at right-angles to each other so the lines intersect at the desired centre. Next use a centre punch to produce a starting point for the drill bit. This is quite suitable for small diameter holes: but for larger holes a pair of dividers should be used to scribe a circle equal to the required diameter after the centre-punching. Follow this by making light punch marks, using a centre punch with a 60 degree point. If the hole is correctly drilled, parts of the light punch-marking will be seen all round the hole. The same technique can be used when cutting straight lines.

WARNING

When drilling, the workpiece must be clamped down and not hand-held: if not -and the drill jams – serious injur’ could be caused. Accurate drilling is difficult to achieve with a hand-held drill because the hole is unlikely to be exactly at right-angles to the work surface. You should always drill on a stand.

Sorry, comments are closed for this post.