Product Design and Process Planning in Using the Process Capability Index

In the past, the process capability index (PCI) was the only one used in on-line quality management; there were no researches on off-line applications, such as product design or process planning. In using conventional PCI for off-line application, designers normally established the process mean close to the design target, and minimized the process variance so that the PCI value would be maximized. The process variance is determined by the process tolerance, which affects the production cost. This factor must be considered because cost will increase as process tolerance is minimized. Simply averting small tolerance for the sake of cost reduction is not always a rational choice, because a great tolerance value generally results in poor quality. Hence, the conventional PCI value does not truly represent the measurement score for off-line applications during product design or process planning. In this regard, the off-line PCI expression is developed with consideration of the balance between product quality and production cost. The product quality is represented by quality loss function, and production cost is expressed by tolerance cost function. These two functions are simultaneously related to process mean and process tolerance. Thus, the new PCI expression can be used to determine appropriate process mean and process tolerance, as well as a measurement score for comparison and selection among candidates. Consequently, an economical and quality of product design and process planning can be achieved during off-line applications.