Cpk (Process Capability Index) is a statistical measure of how well a manufacturing process is both capable and centered within its specification limits. Unlike Cp which only measures spread, Cpk also accounts for how far the process mean is from the center of the tolerance band.
Cpk is calculated as: Cpk = min((USL − μ) / (3σ), (μ − LSL) / (3σ)), where μ is the process mean, σ the process standard deviation, USL the upper specification limit, and LSL the lower specification limit. A Cpk of 1.00 corresponds to approximately 2,700 defective parts per million (ppm) at the tails of a normal distribution.
In aerospace, AS9103 defines minimum Cpk requirements: Cpk ≥ 1.67 for Key Characteristics (KCs), typically corresponding to Critical Characteristics in AS9102. A Cpk of 1.33 is the minimum acceptable for Significant Characteristics in many customer flow-downs.
Cpk must be distinguished from Cp: a process can have a high Cp (wide tolerance relative to its spread) but a low Cpk if the mean is shifted toward one limit. Both indices are required for a complete process capability assessment.
Concrete example
A pin diameter with USL = 10.02 mm, LSL = 9.98 mm, mean = 10.00 mm, σ = 0.003 mm gives Cp = 0.04 / (6×0.003) = 2.22 and Cpk = min(0.02/0.009, 0.02/0.009) = 2.22 — highly capable and centered.