This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
Metals contract more or less in cooling; hence when the molten metal which fills a mould begins to cool, it also begins to contract, and when cold the casting is smaller than the mould. To obtain a casting of the size designated on the drawing, its pattern must be made slightly larger than the casting. This enlargement in the size of a pattern is called the shrinkage allowance. The amount of this allowance varies with different metals and with different-sized castings of the same metals, hence the allowance necessary for each pattern is judged more or less by the experience of the pattern maker. The usual shrinkage allowances average about as follows, viz.:
Cast iron....................... | 1/8 | inch | per | foot. |
Heavy brass....................... | 1/8 | " | " | " |
Steel....................... | 3/16 | " | " | " |
Thin brass.......................... | 3/16 | " | " | " |
Aluminum.................... | 7/32 | " | " | " |
Lead................................. | 7/32 | " | " | " |
In laying out a pattern from the drawing the pattern maker uses the shrinkage rule for measurements. This is a rule which has the shrinkage allowance added, as, for example, a two-foot shrinkage rule for cast iron would be 24 1/4 inches long, graduated in inches and fractions. Each standard inch and fraction is thus increased by an amount equal to the shrinkage allowance.
Castings which are to be machined to a definite size must not only have an allowance made for shrinkage, but a certain amount of extra metal must be allowed for finishing. Also that part of the casting which is uppermost in the mould frequently has a still further allowance of metal to contain impurities and air bubbles which float to the top of the casting and may be imprisoned therein when the mould is poured.
The shrinkage allowance for small castings is sufficiently provided for by the rapping given a pattern to loosen it when it is removed from the sand of the mould.
 
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