ABSTRACTS

Abstract

Statistical process control charts, which are essential tools to process control and improvement, have been widely used for monitoring individual factory production processes. In the multiprocess environment where a group of processes need to be monitored and controlled, it could be difficult and time-consuming to evaluate overall performance for all factory processes. Singhal introduced a graphical chart C_pk MP_ZONE to group the processes into different capability zones, which provides an easy way for the factory engineers to interpret status of each process. Although C_pk is widely used in the industry as a process quality indicator, it is an appropriate measure of progress for quality improvement if reduction of variability is the only guiding principle and process yield is the primary measure of success. When process centering is also a guiding principle in measuring quality improvement, C_pk becomes inappropriate because it can fail to distinguish between on-target and off-target processes. Consequently, the groupings of the processes obtained from C_pk MP_ZONE may not reflect the actual situation of the process characteristics. In this article, we presented a case study to illustrate how C_pk MP_ZONE may be applied and difficulties encountered. We also proposed a modification to this chart using the indices C_pm and C_pmk to group the processes. Problems causing the processes failing to center around the target were identified, and problems were successfully resolved.