Application of Process Capability Indices to Measure Performance of a Multistage Manufacturing Process

A process is a unique combination of manpower, machines, methods and materials in providing a product or service. Process capability indices have been used in the manufacturing industry to provide quantitative measures of process potential and performance. High quality production provides advantages such as cost saving, reduced scrap or remanufacturing, higher yield and increased customer satisfaction and market share. Process capability indices (PCI) are extensively used in industry to evaluate the conformation of products (process yield) to their specifications. Conventional univariate process capability indices such as Cp and Cpk are applied to measure performance for single quality characteristic. In modern manufacturing when product designs are complicated and consumer’s requirements are changeable day to day, multiple quality characteristics must be simultaneously evaluated to improve product’s quality and also to consider correlations exist among the quality characteristics. In this paper process capability indices (both univariate and multivariate) are applied to measure performance in a multistage ‘locomotive wheel’ manufacturing process. The wheel manufacturing process has three stages namely press forging, rolling and heat treatment. Process capability indices are analysed for the above mentioned multistage manufacturing processes and the results are compared to identify the most accurate multivariate process capability index to evaluate multiple quality characteristics for the wheel manufacturing. The results show multivariate process capability indices (MVPCI) proposed by Taam et al. (1993) [MCpm = 0.7923] gives higher capability compare to Chan et al. (1991) [Cpm = 0.2342] and Shahriari et al. (1995) [CpM = 0.56].