Stable first, capable second
A capability index compares the spread of the process with the tolerance. But the index only says something about the future if the process is stable. If the process is affected by special causes, the index only describes the period when the data was collected.
That is why a capability study starts with a control chart. If the process is not stable, the next step is to find and remove the causes, not to calculate indices.
The measurement must be good enough
The spread used in the index is the observed spread, that is, the variation of the process plus the variation of the measurement system. A measurement system with large variation makes the process look worse than it is. So a measurement system analysis should be done before capability is judged.
Short-term and long-term spread
The spread can be calculated in two ways. The within-subgroup spread describes the variation between parts made close together in time. The total spread describes all the variation over the whole period, including differences between shifts, material batches and tools.
Indices based on the within-subgroup spread (Cp and Cpk) show what the process can do at its best. Indices based on the total spread (Pp and Ppk) show what the customer has actually received. The difference between them shows how much there is to gain by making the process more stable.
Machine capability and process capability
Machine capability (often Cm and Cmk) is measured over a short period with as few disturbances as possible, for example when a new machine is delivered. It shows what the machine itself can do. Process capability is measured under real conditions, with different operators, material batches and shifts. Machine capability is a precondition, but does not guarantee process capability.
Which requirements apply?
The requirement comes from the customer, and it differs between customers and industries. Always read the customer’s own requirements. As an example, the automotive industry’s manual for production part approval (PPAP) gives the following for an initial process study:
- Index above 1.67: the process meets the requirements.
- Index between 1.33 and 1.67: may be acceptable, but may require action or the customer’s approval.
- Index below 1.33: the process does not meet the requirements.
The levels apply to initial studies according to PPAP, 4th edition. Other customers and standards may have other levels and other requirements on how the study is done.
How a capability assessment is done
- Find out the customer’s requirements: which index, which level and how the study should be done.
- Show with a measurement system analysis that the measurement system is good enough.
- Collect data under real conditions and show in a control chart that the process is stable.
- Calculate the indices and compare them with the requirement. How Cp, Cpk, Pp and Ppk differ is described in the article Cp, Cpk, Pp and Ppk.
- If the process does not meet the requirement: centre it if Cpk is clearly lower than Cp, and reduce the variation between subgroups if Ppk is clearly lower than Cpk.
