Also called Cp, Cpk, Pp, Ppk, capability study
What is process capability?
A number saying whether your process is actually capable of holding the tolerance on the drawing.
What else it is called
- Other names
- Cp, Cpk, Pp, Ppk, capability study
- In French
- capabilité du procédé
Related entries
Understanding process capability
Two related numbers that answer a question no amount of inspection can: is this process able to hold this tolerance, or are we relying on sorting?
Cp compares the tolerance band to the natural spread of the process and assumes it is perfectly centred. Cpk drops that assumption and measures to whichever tolerance limit the process is running closer to. The gap between the two is informative on its own. A high Cp with a low Cpk says the process is consistent but set off-target, which is usually a quick fix. A low Cp says the process is too variable for the tolerance, and no amount of adjustment will help.
Two cautions. The numbers mean nothing unless the process is stable, so control charts come first. And they assume your measurement system is trustworthy, which is what gauge repeatability and reproducibility is for.
How it is calculated
Cp = tolerance width ÷ natural process spread. Cpk = the same, measured to whichever tolerance limit the process mean sits nearer.
The natural spread is six standard deviations of the process. A Cpk of 1.33 means the nearer tolerance limit is four standard deviations from the mean.
In practice
A Cpk of 1.33 with a centred process means roughly 30 parts per million outside tolerance. A Cpk of 1.0 means about 2,700. The difference between those two numbers is the difference between shipping and sorting.
Frequently asked
What is the difference between Cpk and Ppk?
Cpk uses the short-term variation within a stable process and describes what it is capable of. Ppk uses all the variation over a longer period, including drift and shift changes, and describes what it actually did. Ppk is usually the lower of the two, and customers often want both.
What Cpk do our customers expect?
In automotive, 1.33 is the common requirement and 1.67 appears on safety-related and critical characteristics. Outside automotive it varies, so it belongs in the contract review rather than in an assumption.