11. SPC – Process Capability

Process capability is used to determine whether the process is sufficient and stable, and the elements described below shall be considered to calculate process capability. (Note: this is only applicable to samples within the same subgroup)

      • Process Accuracy (Ca)
        • Indicate how accurate/close the sample mean is with respect to specification mean.
      • Process Precision (Cp)
        • Indicate process consistency while verifying variances within the process.
      • Process Capability (Cpk)
        • Cumulative calculation of process capability from both Ca and Cp.
      • Upper Process Capability (Cpu)
        • Process Capability based on one sided specification only (primary for upper spec limit).
      • Lower Process Capability (Cpl)
        • Process Capability based on one sided specification only (primary for lower spec limit).

The methodology of how to apply the Cpk, Cpu or Cpl is given in the following table and process flow chart

For actual examples, 30 data points were gathered for a CNC processed cylinder’s inner diameter and the respective Cpk analysis is given on the right hand side (In this case T = USL – LSL):


But when looking at the following process distribution for the data above, then we can observe some interesting facts:

Although the Cp and Cpk is relatively high, but judging the process stability by only checking Cp and Cpk could have certain risk. Since the distribution also indicate that the actual sample is falling slightly to the lower end of the specification. Therefore there are still room for improvement to bring the distribution back to the specification mean.

Based on this example, high Cpk does not guarantee a stable process. Additional evaluation of process trend, process stability and process accuracy is still required before having a final evaluation.

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