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This paper will define two factors which represent different sources of dimensional variation in complex assemblies. These two factors distinguish build variation by design from variation associated with manufacturing and assembly. The relationship of these two factors to the process capability (Cp) is defined, and their application to a typical problem illustrated. Utilizing Variation Simulation Analysis (VSA®) (Appendix) to predict build variation under ideal design assumptions, Cp may be expressed as the product of Design Capability (Cd ) and Process Deviation (Dp), using the relationship of Eq (1). Design capability (Cd) evaluates the design (component tolerances and assembly operations by design), while process deviation (Dp) measures the ability of manufacturing and assembly operations to achieve results equivalent to an ideal process.

This paper outlines the use of VSA Audit procedures to identify sources of build variation in an assembly operation. VSA Audit isolates the effects of component variation from variation incurred during the the assembly process. (wear of fixtures, fixture breakage, etc.) This approach provides significant advantages when troubleshooting dimensional variation issues in an assembly plant environment. VSA Audit uses measured component dimensions in a computer simulation of the assembly process. Initially the VSA model is correlated to the assembly operation such that output from the model matches output from the operation. As new shipments of components are received, measurements from each batch are used by the VSA model to predict the impact of the new batch of parts on dimensional variation in the final assembly. This activity identifies potential problems due to part quality prior to building the product.