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, highvalue 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.