Cpk (Process Capability)
Cpk is the difference between a supplier who inspects out defects and one whose process does not make them. Automotive customers ask for it because sorting is not a quality system.
Cpk is a process capability index measuring how well a process fits within its specification limits, accounting for both spread and centring. A Cpk of 1.33 means the process is capable enough that defects are statistically improbable.
- Measures
- Capability including centring
- Automotive benchmark
- 1.33 minimum
- Cp
- Spread only, ignores centring
- Requires
- Stable process and real data
Cpk (Process Capability) at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Measures: Capability including centring
- Automotive benchmark: 1.33 minimum
- Cp: Spread only, ignores centring
- Requires: Stable process and real data
Cpk Versus Cp
Cp measures only spread — whether the process variation could fit inside the tolerance. Cpk also accounts for centring, so a tight process aimed off-target scores poorly on Cpk while scoring well on Cp. Cpk is the more honest number.
Why 1.33
A Cpk of 1.33 corresponds to the process spread occupying about 75% of the tolerance band with the mean centred, leaving margin for normal drift. It is the widely adopted automotive benchmark on special characteristics.
Topics covered on this page
- what is Cpk
- process capability index
- Cpk 1.33 meaning
- Cpk vs Cp
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What does Cpk 1.33 mean?
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The process spread occupies roughly 75% of the tolerance band with the mean centred, so defects are statistically improbable rather than merely inspected out.
What is the difference between Cp and Cpk?
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Cp measures spread only; Cpk also accounts for whether the process is centred on target, making it the more meaningful figure.
Do you provide capability data?
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Yes, on special characteristics for automotive customers as part of PPAP submissions.