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 Process Capability study through CP and CPk measurement.

Process capability and process performance is through the study of the statistical measurements of Cp and Cpk.

Definitions of Cp and cpk

Cp = Represent Process Capability. An indicator to the capability of a particular process.
Cpk = Represent the index of the Process Capability study. A tool to measure the Cp value for the effect of non-centered distribution.

The study of CP

A common Process Capability measure, Cp indicates how well the process distribution fits within its specification limits ( Upper and Lower limit ). In the examples below, processes (a) has Cp greater than one, capable for mass production (b) is equal to one and (c) is less than one. Higher chances to fall out of tolerance.



The problem with Cp is that it does not measures how well the process distribution is, the process may have high Cp but distributed off the center of its limits, which can result in many rejects. The solution to this is the study of it Cpk.

The study of Cpk

Cpk study is to measures where the Cp value distributed between the upper and lower limits. Cpk only considers the variation half that is closest to the specification limits, as in the figure below.

Cp and Cpk have to be studied together to give a measure of both the potential and centering of the process distribution within the specification limits.

The larger the index, the more stable the process is, the less likely that any item will be outside the specification. CPK value of 1.33 is the industry standard of minimum acceptable level.

 

CPK Value
Capability
Inspection and action needed
Less than 1
Incapable
100 % inspection may be needed for the production batch. Process improvement is highly recommended.
Between 1 to 3
Capable
Normal sampling plan is needed. Process is stable enough for mass production.
Greater than 3
Very capable
No inspection is required, process is very stable.

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