A holistic application of process capability indices

Different capability measuring techniques have been proposed and are being used in industry today. Each one gives a certain portion of the quality picture, leaving out some equally important details about the process. There is no single index which addresses the whole quality of a production process on its own, hence the need to look at all the indices holistically. Each index has its own merits and demerits. In this paper, we consider a case study of a company which manufactures belts, and has been using only one index, Cpk . Its quality checks were indicating that the process was under control. On the other hand, customers have been complaining that the belts they are manufacturing are not strong and tend to breaking easily. This paper concentrates on addressing the production process of belts by looking at different capability indices and come up with a method or algorithm that addresses this problem holistically. Strengths and weaknesses of each capability index are analysed and sets of indices which address the full picture of the production process are used to check the capability of the process. Results show that customers were justified in their complaints as the new quality checks indicated that the production process of these belts was incapable of producing belts which meet customer satisfaction. Corrective measures were recommended to the company.