Process Capability Analysis of a Centrifugal Casting Process

The concept of process capability was proposed in early 1970 with the introduction of ‘capability ratio’ by Juran, a world known quality expert. Thereafter almost 40 years have been passed and many indices such as Cp, Cpk and Cpm were developed for measuring process performance. However, many researchers, for example, Kotz and Johnson [1-2] raised concerns about the application of the diversified indices owing to three reasons (i) too theoretical for practitioners, (ii) mismatch with performance values measured through different indices, and (iii) inadequacy to mimic the reality. This study therefore focused on the application of the different indices developed so far with a case study on a centrifugal casting process. Five important quality variables were considered and 12 UPCIs (such as Cp, Cpk, Cpm, ...., Spmk, and Ca) values were calculated based on data collected from the plant. Results obtained are compared with percent conforming products as well as amongst the indices themselves. The values differ considerably for some indices. The application shows that the developed indices can create confusion to practitioners unless a clear and concise implementation scheme is provided.