When I go through the topic of capability indices for unilateral tolerances in my training class or consulting sessions, I usually get surprised looks from quality practitioners searching for Cp values. The common reaction is where is the Cp index? How come it doesn’t show on the chart or on my capability report?
For unilateral tolerance, Cp does not apply.
Why?
Because, by definition Cp is estimates process capability for a centered process. For unilateral tolerances the target may not be at the center of the specification limits.
The formula for calculating Cp is USL - LSL / 6σ
For unilateral tolerances, we have either the upper or lower specification limit.
That is why Cp cannot be calculated.
So what capability indices can be used for unilateral tolerances?
Cpk, Cpkm can be used. For details on these indices, please download our white paper on capability indices.
Cpk is Cpu for upper specified unilateral tolerance and it is Cpl for lower specified.
How do you track capability of your processes?
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