Process Capability Index (Cpk) quantifies whether a process can consistently produce parts within engineering tolerances. A Cpk of 1.0 means the process spread just fits within the specification limits. A Cpk of 1.33 is generally considered capable, and 1.67 is excellent. Below 1.0, the process is producing a significant percentage of out-of-spec parts even when running normally.
Cpk differs from Cp in that it accounts for process centering. A process can have a narrow spread (high Cp) but still produce defects if the process mean is shifted toward one specification limit. Cpk penalizes this miscentering, making it the more practical metric for actual manufacturing performance.
Automotive customers (under IATF 16949) and aerospace primes (under AS9100) routinely require suppliers to demonstrate minimum Cpk values for critical dimensions — often 1.33 or 1.67. Meeting these requirements means collecting measurement data from production runs and performing capability studies. Automated measurement data collection from CMMs and in-process gauging makes this achievable without burdening operators with manual recording.