For all ordinary measurements, a 2-foot folding standard rule. Fig. 60, 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 box, to the required dimensions.
For the molding dimensions of a pattern or core box a shrinkage rule must be used. The reasons are that when a mold made from the wooden pattern is filled with molten metal its temperature is very high, and as it cools and solidifies it contracts. Accordingly, to compensate 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.
Fig. 60. Standard Folding Rule.
The contraction or shrinkage of different metals in the molds varies greatly; that for cast iron being, 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 1/4 inch per foot.
Shrinkage rules, Fig. 61, are usually made of a single piece of boxwood or beech; those for cast iron being 24 1/4 inches long, for brass 24 3/8 inches long, and for other soft metals 24 1/2 inches in length. They can also be bought made of tempered steel 12 1/3 inches, 12 3/18 inches, and 12 1/4 inches in length. In making use of the shrinkage rule, 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.