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A good value for money 4 Scale contraction /Shrinkage rule.
(Both sides of rule shown in photograph)
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 rules are sometimes expressed as a percentage allowance.