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If we go by the SPC manual of TS16949,
CmK is a measure of machine capability. This is also called initial process capability. This is measured using50 or more continuous samples produced on a machine, keeping all influences other than the machine parameters constant. (like RM, Measuring, Operators, etc). Formula for calculating CmK is same as CpK. But here we need to use Standard deviation calculated for the50 data sets.
CmK = Tolerance Zone / Process spread = (USL-LSL)/(6*s) , s=standard deviation of50 data sets
Stability is verified by plotting this on a control chart to look at whether process is stable.
CpK is a measure of process capability. This is a measure of process performance over a period of time. We need to pick minimum25 subgroups for measurement, preferably5 data points per subgroups. Formula is similar to the one above. But, standard deviation is measured from the control chart as this is a measure of process performance.
s=Rbar/d2 is used for calculating standard deviation. Rbar from control chart and d2 is a constant.
SPC Manual is very detailed and gives lot more information on this.
