Specification Limits, Capability Indices, and Process Centering in Assembly Manufacture

The problems with using Cpk indices for components to control the capability of an assembly process are well known. Suppliers of components may, for example, actually decrease the yield of conforming product at assembly by raising their own Cpk's. We present formula relationships which link component capability indices (Cp, Cpm, Cpk) and specifications at the supplier level with capability indices and specifications at the assembly level. The minimum level of Cpk (representing a bound on yield) and the minimum Cpm (representing centering on target) at the assembly level are derived.