The allowance for finish is the amount added to a pattern to allow for machining in the machine-shop. As all castings are more or less rough when they come from the mold sand, and as they warp or spring in cooling as well as contract or shrink unequally, it is necessary to allow for their being machined to the required size and shape. No set rule can be laid down to cover the allowance for these irregularities. On small iron or brass castings, an allowance of 1/16 to 1/8 in. is usually sufficient. Under the modern method of mounting small patterns on a plate and molding them by machine, this allowance may be reduced to a minimum.
The allowance for finish on steel castings must be greater in all cases than for iron castings because steel has a greater shrinkage, contracts unequally, and warps. The allowance should be from 1/4 to 1/2 in., but in many cases more is allowed. In every instance, however, much of the allowance for finish is made necessary by the distortion of the casting in cooling.