Cmk is a short term machine capability index derived from the observations from uninterrupted production run. Even though the formulae are as same as Cp&Cpk calculation,the standard deviation used here is sample standard deviation (RMS). The preferred Cmk value is >1.67.Usually, The long term process capability to be planned and studied after achieving the targeted Cmk Value.
I have seen the Cmk index defined in two different ways.
In the first, it is used to measure Machine Capability (the "m" stands for "machine"). Its formula is similar to the one given above for Cpk, but the data used to estimated mu and sigmaST come from a special study conducted in such a manner as to focus on only process variation originating from the machine under study. Thus, the same operator would be used for the entire study, the same batch of material, the same setup, etc. Be aware that some companies will replace the 3 sigmaST with 4 sigmaST.
In the second definition I have seen, Cmk is used to quantify the performance of a measurement system, so here the "m" stands for "measurement". Again, its definition is similar to that given above for Cpk, but the data come from a properly conducted gage study. To further complicate the issue, some companies use 5 sigmaST in place of the 3 sigmaST that is normally used.
As you can see, to really appreciate the difference between any two capability metrics, you must first obtain the formulas and then an understanding of how the data are to be collected. Without knowing this, there really is no way to correcttly answer the question "What's the difference between Cpk and Cmk?".
Links to Forum Discussion Threads
From the discussion thread Cmk and Cpk - When will we need to calculate Cmk?
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