Part eight by David Carpenter

Well, you’ve started your apprenticeship. It is hoped you have made something, enjoyed the process, and are proud of what you have done. But now its time to look in a bit more depth at the business of actually starting your own project.

You have bought in the drawings, castings and metal stock. Where to begin?

Whatever the project, we need to be able to mark out the shape of parts, and the position of holes, accurately. And for that we need some simple equipment.

We need a perfectly flat base on which to work, and surface plates are available in either granite or cast iron. Small is beautiful here. A 12 x 9 inch surface plate will handle most jobs, and lifting it onto the bench does not require a Tarzan in the way that an 18in. square one does. After buying an 18inch one it will remain in the boot of the car until someone strong enough can lift it out! Anything larger will need a crane. An alternative to cast iron or granite is sheet glass. Modern plate glass is is very flat. Some thickish plate glass, say 1/4in thick, will do the trick. As a supplement to the small surface plate a long piece of plate glass, say an old shop display shelf two feet or more long, will come in handy for marking out large items such as locomotive frames, but will need to be used on a flat surface as glass does bend, thick chipboard should be alright, or a kitchen worktop.

Cast iron surface plates should be well looked after and protected from damage or rust. After use a light smear of grease or a thick oil will protect from rust and a wooden cover will prevent it from damage; if it is left on the workbench, as most are, it will be treated as part of the bench so should be protected from dropped and spilled items.

We need to be able to measure what we are doing. For most things all that is required is an engineer’s rule. For most marking out a 6 or 12inch (150 or 300mm) rigid rule one inch wide is best, and for general measuring use, keep a six inch flexible rule in your overall top pocket. You CAN afford to buy the best rules in the world – they are only a few pounds in small sizes. Or you can buy something from a market stall. Your choice!

Now we need to make some marks on the metal and the tool for that is the scriber. You do not need anything more complex than a piece of silver steel about 1/8 in  (3mm) diameter and ground to a point one end. However, professionally made ones are much better to use and are inexpensive.  But remember the silver steel alternative for when you can’t find your scriber.

To ensure that your scriber marks show up clearly, it is usual to coat the surface to be marked. Marking Blue is the norm. It is a spirit-based mixture that is brushed on and dries to a thin dark blue coating. A thick felt pen is an alternative. Copper sulphate solution was also used extensively in the past. Clever clockmakers coat brass items, such as clock plates, with a thin coating of primer paint, which can be cleaned off with solvent, saving a lot of abrasive cleaning to get those brilliant finishes on good clocks.

To mark out radii and circles, a pair of engineers’ spring-loaded dividers are required and, again, a top quality brand will not break the bank. If you need to mark out larger radii a tool called trammels or beam compasses will be needed, and you can make those for yourself, but a pair of 6 or 8in. dividers will be sufficient for most projects.

We need to be able to mark out lines that are precisely square to a surface, and the engineer’s square is the tool here. About a 3 or 4 inch one will meet most needs. You will probably need to be able to mark out angles at some time and protractors come in a range of designs, and a simple design will suffice for most.

When marking out it is usual to prepare two datum surfaces, which are machined or filed flat and square, and line positions are measured from those. Good accuracy can be obtained by marking the part out with a scribing block, aka surface gauge. You will need to hold your rule square to the surface plate, by holding it against an angle plate. This is another invaluable piece of equipment with many uses and one about 4in high is alright for most things. Adjust your scribing block until the scribing point is at the correct level on the rule. Lock it and it can then scribe the necessary line by dragging the block along with the sharp point on the workpiece. By standing the work on the second datum face another line can be scribed to intersect the first one to mark the position of a hole centre, for example. When marking out thin metal, it can be clamped to an angle plate.

This method is pretty accurate.  It is easily accurate to the finest division on the rule (normally 1/64 in. or 0.5mm) and using a magnifier, you should be able to work to around ).005in. or 0.2mm accuracy, which is good enough for most things. However, for greater precision a vernier height gauge is the answer. These are like vernier calipers attached to a base and fitted with a scribing edge. They read to 0.001in or 0.05mm. You can get even more accurate, but for that you need a jig boring machine.

Once hole positions are marked the point of the hole centre should be given a ‘centre pop’. Using a magnifier or jeweller’s loupe position the point of a centre punch at the intersection of the scribed lines, and give it a gentle tap with a hammer. Check with the magnifier to ensure the centre pop is in exactly the right place. If it isn’t, angle the centre punch to towards the right spot and give another tap. If that’s OK give the punch another tap with the punch held vertically. When you are happy the dimple is in the right place, give it a firmer tap (but not a whack!) to ensure that the position can be picked up by the drill, or centre drill.

A brief word on dimensions. On older imperial drawings you will see dimensions that only need to be accurate by rule, or holes that are drilled rather than reamed or bored, are shown as fractions of an inch. However, if instead of 3/8in the dimension is 0.375in., or 3.5mm is 3.50mm, that implied that it should be accurate to the ‘thou’ or to the hundredth of a mm, rather than by rule measurement. Unfortunately, today, many drawings  produced by CAD, often scaled down from full-size, and all dimensions are often shown to three decimal places because that’s what the computer came up with. That practice is born of designer laziness and most of the dimensions will not have to be accurate to three decimal places and could have been expressed as fractions or round numbers, with suitable allowances made. Watch out, too, for things like a drawing which showed the angle of inclination of model locomotive cylinders as 5.194deg. Yeah. Right! It’s unfortunate that these daft measurements are (presumably) put there for the benefit of CAD/CAM people, rather than those who will drill, saw and file their locomotive frames and relish the sense of achievement.

As far as possible, also do check drawings thoroughly for mistakes. Drawings published in magazines sometimes go through periods of clanger dropping. It’s a good idea to follow some way behind any serial and watch out for corrections.

That’s it.

It’s not like a five year apprenticeship in the days or yore, but hopefully provides a basis for continued learning for the beginner today.




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Editor: David Carpenter