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It is easy to think that all measurement have to be of the highest accuracy possible. But this is usually the exception. In many cases a ruler is quite adequate. It is very quick. It does not need any expensive equipment.
A ruler is not only a means of measuring length it is also a straight edge. It is useful to have at least three rulers with good edges. These can be used to test against each other to check their straightness. Then any one of them can be used to check the straightness of anything else.
Often, on the milling machine, space is at a premium not only for machining but also for measuring. Accurate rulers are cheap enough that they can be chopped up to make very small rulers that can fit where nothing else can.
It can be useful to have a small ruler, say, 25-50mm long, glued at right angles to a thin handle that can reach almost anywhere.
A small mirror on a handle, as used by dentists, can sometimes be useful for seeing the scale on the ruler.
Most common metals, when they are cast, contract when they cool. If the pattern is made to the size required then the casting will be slightly too small. Consequently the pattern needs to be made slightly larger. Rather than make drawing for the pattern that are slightly larger the solution is for the patter maker make to make the pattern using the drawings but using a ruler with scales that are slightly larger. The pattern will be slightly larger. The metal when cast and hot will form the required object but slightly larger. But when it has cooled and contracted it will be the required size.
Such a ruler can have scales for one sort of metal or several different scales for different metals.
When a large number of casting might need to be made the pattern itself might be made of metal. In this case a ruler might have double contraction scales. The first pattern is made of word using the double contraction scale. The casting made from this will be slightly larger than the final casting. It is used as the pattern and the resulting pattern will be the right size.