For all ordinary measurements, a 2-foot, folding standard rule. Fig. 81, will be sufficient, but this rule must not be used for laying out or for working patterns, or any part of a pattern or core bos, to the required dimensions. For all such purposes a shrinkage rule most be used. The reasons are that when a mould made from the wooden pattern in the sand is fillet! with molten metal, its temperature is very high, and as it cools and solidifies it contracts. Accordingly, to com-psnsate for this, the pattern maker must add to the size of the pattern. In order that this may be done, and exact relations nevertheless be maintained for all dimensions, a shrinkage rule is used. This rule is marked off exactly like an ordinary rule; but if the two are compared, the shrinkage rule will be found to be about 1/8 inch longer than the other for each foot of length.
The contraction or shrinkage of different metals in the moulds varies greatly, that for cast iron bring, as above stated, 1/8 inch to each foot. For brass, however, the shrinkage is 3/16 inch to the foot; and for many of the softer metals it is as great as ¼ inch per foot. Shrinkage rales, Fig. 82, are usually made of a single piece of boxwood or beech, those for cast iron being 24¼ inches long. for brass 24 3/8 inches long, and for other soft metals 24½ inches in length. They can also be bought made of tempered steel 12 1/8 inches, 12 3/16 inches, and 12¼ inches in length. In making use of the shrinkage rale, the workman will proceed just as though he were using a standard rule; and when the pattern is completed it will be found to be larger in all its dimensions, just in proportion as the extra length of the shrinkage rule makes it greater than the standard rule.