Cpk is a number that tells you how well a stable process fits inside its tolerance, taking into account both how much it varies and whether it is centered. It is the distance from the process mean to the nearest specification limit, divided by three times the short-term (within-subgroup) standard deviation. A Cpk of 1.0 means the process spread just touches the nearest limit. Higher is better, and many customers ask for 1.33 or more on key characteristics.
Example: A shaft is specified at 10.00 ± 0.06 mm (9.94 to 10.06). The mean is 10.01 mm and the within-subgroup standard deviation is 0.01 mm. Upper side: (10.06 − 10.01) ÷ (3 × 0.01) = 1.67. Lower side: (10.01 − 9.94) ÷ 0.03 = 2.33. Cpk is the smaller value, 1.67.
Cpk tells a supervisor whether a characteristic can run with light sampling or needs close watching. A low Cpk from an off-center process is often fixed with an offset, while a low Cpk from wide spread needs a real process change, so knowing which one you have saves time.
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