Cpk is a capability index. It shows how well a stable process fits inside the drawing tolerance, taking account of both the spread of the results and how well they are centred.
Common practice is to ask for a Cpk of at least 1.33 on critical dimensions in ongoing production. Many auto customers ask for 1.67 on safety characteristics and in initial studies for new parts.
How Cpk is worked out
Take the distance from the process average to the nearer tolerance limit. Divide it by three times the process standard deviation. The result is Cpk.
A Cpk of 1.0 means the process just fits, with the nearer limit three standard deviations from the average. A Cpk of 1.33 means four standard deviations of room. A Cpk of 1.67 means five.
Cp is a related index that ignores centring. If Cp is high but Cpk is low, the process is narrow enough but sits off centre. A tool offset may fix it.
What the numbers mean for you
- Below 1.0, the process will make out-of-tolerance parts regularly, and you need full inspection.
- From 1.0 to 1.33, the process is marginal and needs close watching.
- At 1.33, a centred process with a normal spread makes very few parts outside tolerance, around a few tens per million.
- At 1.67 and above, out-of-tolerance parts become very rare, and reduced inspection may be justified.
Cpk and Ppk
Cpk uses short-term variation within subgroups. Ppk uses the overall variation of all the parts measured. Initial studies for a new part often report Ppk, because the process has not run long enough to prove it is stable.
If Ppk is much lower than Cpk, the process is shifting between subgroups. That needs investigation before you rely on the numbers.
When Cpk is meaningless
- When the process is not stable, because a moving average makes any single index misleading.
- When too few parts are measured. A study usually needs dozens of parts at least, and many customers ask for more than 100.
- When the gauge is not good enough for the tolerance, so the spread is mostly measurement error.
- When the data are not normally distributed and no correction is made.
How to use Cpk with a supplier
Mark critical dimensions on your drawing and state the Cpk you need in your specification or quality agreement. Ask for the raw data, not just the index. Check that the study was run on parts from a real production run, as part of a PPAP submission.
Then make sure the supplier keeps watching the same dimensions with SPC after approval.
Next step
We review capability studies, check the raw data and confirm the gauges are fit for the job. Book a free sourcing review to set the right targets for your parts.