The geometric shape accuracy of precision castings is determined by the machining allowance, the deviation of casting size and weight with the same amount. In other situations with the same conditions, the geometric shape accuracy reflects the advanced level of the precision casting processing process from the aspects of machining workload and metal consumption.

The allowable deviation of size and weight, as well as the machining allowance of castings, are determined according to the GB6414-86GB/T11351-89GB/T11350-89 standard and the technical conditions of the order, while the actual deviation is determined based on the data from the technical inspection department. The so-called casting size control is to control the degree of dispersion of the measured values under the premise that the measured average value matches the nominal size of the casting. This type of dispersion is usually caused by random changes or systematic errors in production technology conditions and raw material characteristics. Precision castings should be judged and corresponding measures should be taken based on the nature of errors to improve dimensional accuracy and meet standard requirements.

Sometimes, there are only one or two rough dimensional tolerances with stricter requirements on the entire casting, so it is not necessary to specify tolerances for all dimensions according to the same grade. To improve the accuracy level of castings, metal molds can generally be used. If significant improvement is needed, the molding method needs to be changed, such as using shell casting or other precision casting processes. However, this will inevitably lead to increased mold costs, extended production preparation cycles, and a doubling of casting costs.

When evaluating the geometric shape accuracy of castings, in addition to specifying the machining surface allowance and casting weight deviation, representative tolerance ranges or deviation ranges for wall thickness and rib thickness should also be given. The necessary wall thickness for the structure is determined by the casting designer, and the allowable wall thickness is determined by the casting process engineer. In most cases, the decision of the precision casting designer forms the wall thickness indicated on the drawing. The allowable deviation between the designed wall thickness and the actual wall thickness is an important indicator of the geometric shape accuracy of the casting.