Refinements in using Historical Process Data to Optimize Tolerance Allocation

Tolerance allocation for new products calls for experience and judgment. Whenever products are manufactured using production processes that are similar to existing products it is likely that the trend in production tolerances for both the new and existing products will be similar. It must be assumed that there have been no improvements to the tooling, modifications to the machines being used, or a change in process stability. The data employed to understand and exploit these trends is commonly referred to as process capability data and it is stored in a Process Capability DataBase (PCDB). There are several challenges to be addressed when creating a truly useful PCDB. A useful PCDB is of great benefit to companies because it helps them to reduce time to market by getting things “right first time” and eliminating a trial and error approach to tolerance allocation. A simple example to illustrate how a PCDB can be used to evaluate the capability of a design is presented and this shows how process capability data can be used as an integral part of the design process. The paper builds upon an experimental PCDB and describes the authors’ continuing experiences in creating a PCDB for use by a multinational company that manufactures electromechanical devices. The authors’ work in developing strategies on what data to include in a PCDB and prioritizing the geometry list to be used for data entry to make it as useful as possible is described. The method of indexing data in a PCDB for efficient retrieval by design engineers is explained. By correctly using a PCDB a product designer can more easily predict whether a tolerance is achievable or not based on historical process capability information.