shrink-rule

from The Century Dictionary.

• noun A shrinkage-rule; a rule or graduated scale used by pattern-makers, which is a fraction of an inch longer per foot than a standard rule. When used for iron, ⅛ of an inch allows for the shrinkage of the casting in cooling, since every dimension is longer than the nominal or standard one in that proportion. For brass pattern-work it is of an inch longer in each foot; for steel it is ¼ of an inch larger.