Process capability ratio and process capability index: When is each appropriate? (Two case studies to illustrate appropriate use)

Engineers are challenged with determining whether a process is capable of producing a product that meet tolerances as specified in the product's design. Process capability studies provide the practitioner two measurements to determine if the process can produce to specifications. This paper uses two case studies to examine the appropriate use of the process capability ratio (Cp) and the process capability index (Cpk). The first case determined if a flexographic printing press was capable of accurately holding consecutive repeats that conformed to specifications. Although successfully achieving good Cp values with a specification range of +/- 1/8 of an inch, the Cp values for +/- 1/16 of an inch indicated that the press was not capable of meeting specifications. The second case determines assignable cause for burn-in failure for two prototype industrial grade compressors. Although the Cp indicated that the process was capable, Cpk's for the target settings were not being calculated and nominal means were consistently being set larger for shafts and smaller for bores than specified.