Shrinkage Allowance

Shrinkage Allowance Points : Contraction Allowance or Shrinkage Allowance The metals used for casting contract after solidification in the mould and the pattern must therefore be larger than the casting by an amount known as ‘pattern-maker’s contraction. The pattern-maker is equipped with a special rule or scale, called the pattern-maker’s contraction rule, which is used to compensate for the shrinkage value. There may be different contraction rules for different casting metals, but the rule usually used is the one that has two scales on each side, the total number of scales being four for four commonly cast metals, namely, steel, cast iron, brass, and aluminum. To compensate for shrinkage, the graduations are oversized by a proportionate amount, e.g. an 1 mm or 1 % scale, each 100 cm is longer by 1 cm.