To assign dimensions and tolerances to a part that is produced as a casting involves considerations of functional requirements of the finished part, and allowances for production requirements such as draft.
Casting designers cannot control processing variables that contribute to, or cause, dimensional variations. However, an awareness of their existence and an understanding of their nature will certainly help in specifying economically attainable tolerances.
In the process of specifying dimensions, the designer should allow the most generous tolerances consistent with the proper function of a cast component. For some parts, the dimensions and tolerances specified may represent a compromise between a castings function and the capability of the casting process.
Extremely tight tolerances that are close to, or exceed, normal process limitations, can be very costly in terms of pattern development. Metal contraction (shrinkage) during solidification and cooling can affect casting dimensions significantly. Texture, permeability, and hardness of sand molds, location of the parting line, and the use of cores can also affect dimensional accuracy of castings. Modifications of tolerances and coring practice can result in cost reductions with respect to machining, for instance.