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In a quality-control exercise for a machined shaft, the process capability report shows Cp = 1.67 but Cpk = 0.72. I initially read the Cp value as evidence that the process could comfortably meet the drawing tolerance, so the much lower Cpk has thrown me.
What should I investigate first?
- Does this gap suggest that the process average is too close to one specification limit, rather than that the spread is too wide?
- Would adjusting the machine offset be enough, or would that conclusion require more evidence?
- How should I interpret either index if the control chart shows drift during the sampling period?
I am looking for a practical explanation of Cp and Cpk in manufacturing: what each number tells an operator, and what it cannot establish on its own. The exercise assumes a roughly normal distribution, but it does not say whether process stability was checked first.
