The previous section described a methodology for calculating process capability for new parts. If a process capability study was done with existing parts, and it was found to be unacceptable, the following suggestions might be followed to bring the process capabilities in compliance with six sigma or Cpk targets:
• Can specifications be amended (enlarged) and still meet system requirements?
• Can increased training，corrective action processes，design of experiments, or other quality improvement tools be used to increase process capability?
• If current processes remains not capable, can new equipment or outside suppliers be investigated?
This chapter showed how to handle the common problem of applying six sigma quality methodology to small as well as large production volumes. Statistical tools such as moving range and the z, t, f, and x2 distributions can be used to quantify the attributes of the population distribution for average and standard deviations based on samples taken. Many examples were given to demonstrate sampling techniques and their relationship to populations. Process capability as well as gauge capability were also demonstrated with formulas, examples, and case studies. Finally, the process capability applications in short- versus long-term production were also shown, with examples and strategies for handling process capability in the prototype as well as long-term production.