This means it cannot account for deviations which are not expected, such as misaligned, damaged, or worn equipment.
A company produces axles with nominal diameter 20 mm on a lathe.
As no axle can be made to exactly 20 mm, the designer specifies the maximum admissible deviations (called tolerances or specification limits).
The process capability index is a measure for how likely it is that a produced axle satisfies this requirement.
If errors of the latter kinds occur, the process is not in a state of statistical control.
and the estimated variability of the process (expressed as a standard deviation) is
, then commonly accepted process capability indices include:
Fixing values for minimum "acceptable" process capability targets is a matter of personal opinion, and what consensus exists varies by industry, facility, and the process under consideration.
For example, in the automotive industry, the Automotive Industry Action Group sets forth guidelines in the Production Part Approval Process, 4th edition for recommended Cpk minimum values for critical-to-quality process characteristics.
However, these criteria are debatable and several processes may not be evaluated for capability just because they have not properly been assessed.
For instance, if the specification came from an engineering guideline without considering the function and criticality of the part, a discussion around process capability is useless, and would have more benefits if focused on what are the real risks of having a part borderline out of specification.
At least one academic expert recommends[3] the following: However where a process produces a characteristic with a capability index greater than 2.5, the unnecessary precision may be expensive.
If, after carefully monitoring the process for a while, it appears that the process is in control and producing output predictably (as depicted in the run chart below), we can meaningfully estimate its mean and standard deviation.
are estimated to be 98.94 μm and 1.03 μm, respectively, then The fact that the process is running off-center (about 1σ below its target) is reflected in the markedly different values for Cp, Cpk, Cpm, and Cpkm.