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:
* Lead free form of pewter
|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|
From Stanley Folding Rules A History and Descriptive Inventory
by Alvin Sellen's