Shrink Rules
Molten metal will shrink as it cools. To allow for this, patterns
are made slightly oversized.
A pattern that is 12 1/8" long will produce a 12" long
part when cast in iron. Shrink rules are elongated rulers that
take shrinkage into account. (A one foot shrink rule with 1/8"
shrink per foot looks like an ordinary ruler. It is actually
12 1/8" long.) By using a shrink rule to measure parts for
a pattern, the pattern maker does not have to scale up the
dimensions of the desired casting.
Each metal has its own shrinkage rate:
Metal Shrinkage |
Britannia* | 1/32" per foot |
Tin | 1/12" per foot |
Iron | 1/8" per foot |
Bismuth | 5/32" per foot |
Brass | 3/16" per foot |
Aluminum | 3/16" per foot |
Copper | 3/16" per foot |
Steel | 1/4" per foot |
Lead | 5/16" per foot |
Zinc | 5/16" per foot |
* Lead free form of pewter
From Stanley Folding Rules A History and Descriptive Inventory
by Alvin Sellen's
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