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What means CP, CPK and PPM?

What means CP, CPK and PPM? How is it used in Enventive solutions?

Cp and Cpk

Cp and Cpk are capability index. They both characterize the performance of a process by representing the ratio of specification limits (SL) to process capabilities. Both indices assume ±3 Sigma manufacturing process capabilities:

Cp = SL / (3·sigma)

Cpk = MAX [(Mean-LSL) / (3·sigma), (USL-Mean) / (3·sigma)}

They represent the number of times the spread of the process can fit into the tolerance width. The higher the value of Cp, the better the process. Some indication of their values and corresponding interpretations:

  • Cpk < 1.0 : Indicate the manufacturing process cannot reliably meet the design goals
  • Cpk > 1.33 : Good performance of the process
  • Cpk > 3 : Loose specification of limits

The difference between Cp and Cpk is that Cpk  takes into account the possibility that the process mean may be shifted from the mean of the design limit range. In Enventive Concept, all the contributors have their respective input Cp values, while the output of the study is given in term of Cpk.

When calculating Cp and Cpk, Enventive Concept treats all contributors as if they were at their process limits. The contributors are in turn used to calculate the resultant Sigma range for the statistical analysis. As a result, it is important to verify that the tolerances on all contributors are greater than or equal to the process limits.

PPM

In addition, Enventive Concept characterizes the performance of the process by publishing the “Failure rate” and the “% in tolerance”. The failure rate is the number of times per million the design will be out of tolerance. It is expressed in “ppm” (Parts Per Million). The percent in tolerance is simply the opposite of the parts per million failure rate (1,000,000 – Failure Rate = Percent in Tolerance.)

Cpm

Other capability indices that are not calculated by Enventive also exists. Cpm is one of them. This indice automatically takes into account the bias from the target. Although the Cpm is used less frequently, it could be easily manually added in Enventive’s spreadsheet using the standard deviation reported by Enventive.  

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