Sequential tolerance control in discrete parts manufacturing

The machining of complex parts typically involves a logical and chronological sequence of n operations on m machine tools. Because manufacturing datums cannot always match design datums, some of the design specifications imposed on the part are usually satisfied by distinct subsets of the n operations prescribed in the process plan. Conventional tolerance control specifies a fixed set point for each operation and permissible variation about this set point to insure compliance with the specifications. This approach is inadequate for complex, low volume, high-value added parts such as those found in the aircraft, nuclear, or precision instrument manufacturing industry. This paper introduces the concept of Sequential Tolerance Control, an approach that uses real-time measurement information at the completion of stage j to exploit available space inside a dynamic feasible zone and reposition the set point for operations j + 1 to n. The procedure is repeated at appropriate locations along the n operations so as to optimize the production of an acceptable part.