SEQUENTIAL TOLERANCE CONTROL IN DISCRETE PARTS MANUFACTURING

The machining of complex parts typically involves a logical and chrono logical sequence of n operations on m machine tools. Because manufactu ring datums cannot always match design datums, some of the design spec ifications imposed on the part are usually satisfied by distinct subse ts of the n operations prescribed in the process plan. Conventional to lerance control specifies a fixed set point for each operation and per missible variation about this set point to insure compliance with the specifications. This approach is inadequate for complex, low volume, h igh-value added parts such as those found in the aircraft, nuclear, or precision instrument manufacturing industry. This paper introduces th e concept of Sequential Tolerance Control, an approach that uses real- time measurement information at the completion of stage j to exploit a vailable space inside a dynamic feasible zone and reposition the set p oint for operations j + 1 to n. The procedure is repeated at appropria te locations along the n operations so as to optimize the production o f an acceptable part.