An experienced foundryman can estimate the metal contraction that will occur on any dimension, but only trial by actual production will show precisely how a specific part will shrink. A toolmaker works from a supplied drawing in constructing the tool but the tool then dictates the final dimensions of the part. The tool will dictate the exact mean dimension that may, or may not, correspond with the desired drawing dimension when processing of the part is complete. Tolerances for the production of a single or small lot of castings, therefore, tend to be liberal. In contrast, castings produced in large numbers present the opportunity to make changes to the tooling and manufacturing process to compensate for abnormal casting contraction behavior. For this type of casting, variation will be minimized, but a variation will still exist.
A quick rule of thumb for investment castings is a variation of +- .010” for the first inch of dimension plus an additional +-.005” for each additional inch of dimension. This rule applies to almost every dimension measured, length, flatness, roundness, etc. Example, if a part is 7” long then a variation of +-.040” can be expected.