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Patternmakers’ contraction allowances
Castings are always smaller in dimensions than the pattern from which they are made, because as the metal cools from its solidification temperature to room temperature, thermal contraction /shrinkage occurs.
Patternmakers allow for this contraction /shrinkage by making patterns larger in dimensions than the required castings by an amount known as the “contraction / shrinkage allowance”. This is done by making use of specially engraved rules, known as “contraction or shrinkage rules”, the dimensions of which incorporated a contraction /shrinkage allowance such as 1 in 75 for aluminium alloys, or 1 in 96 for iron castings.
Due to computer controlled machines being used for pattern-making shrinkage rues are sometimes expressed as a percentage allowance.