Casting Tolerancing

Dimensional Tolerances

A castings dimensional tolerance is developed from various factors. The tolerance of a raw casting is largely defined by the type of process used. An investment cast part will yield significantly better dimensional capability from a sand cast part. A sand cast part that has a sand mold compacted by an automated machine will yield a better dimensional capability than a sand mold created by hand.

There is some tolerance used up in the creation of the pattern as well. The pattern is built expecting a certain amount of contraction from the pattern to the final part. The correct amount of adjustment that is made to the pattern for the alloy being poured can be uncertain due to the part geometry and this can drive to a larger tolerance on the final part. If certain dimensions are critical, a small amount of capability castings, using the production process, can be poured before regular production begins.

The size and the weight of the part will determine the tolerance required on specific dimensions. Table 6 shows the casting tolerances (CT) achievable for several materials and mold making processes. Tolerance grades will differ for different casting processes. The tolerance table shows the different tolerances required for various dimensions.

Table 6 - CT Tolerance Grade

 

In a complex mold, there may be 4 or more components in the mold that need to be assembled before pouring. This as-sembly of the mold will also determine the dimensional variation of the parts. Critical dimensions of the parts should be designed so that they are molded in as few components as possible.

Cleaning and heat treating will also affect a parts dimensions. Heat treating specifically will affect flatness and straightness of a casting. Table 7 shows the achievable flatness for an investment cast part, with and without mechanical straightening.

Table 7 - Investment Cast Flatness Tolerance

Learn about the surface finish of a casting