I've got a set of data from the process engineer with its Ppk calculated to be 1.80. However, it turns out that the set of data are a combination of four sources, each of which from ID measurement of an individual hole. (As usual, they can't be distinguished anymore)
The point of Ppk is to see if the process is centered and falls within the specifications. That said, how does combining four sources represent a "process"? What question would that answer? A "Grand Ppk" of a part? I suppose if it passes, the lesser levels of capability should be academic. But a process needs to first be analyzed with time-order data plotting (both the highest and lowest of each part's diameters, in the case of a circular feature), then generalized with capability indices – if applicable.
The other point that arises from data being presented in this manner is the training of the process engineer on capability. It sounds like they barely know the basic concept of rubber stamping Ppk, and very little about the underlying concepts. Ultimately, they are not using the tool to make good decisions.
In this case, what can be said about the stdev and Ppk for those individual holes ? Should the Ppk for each hole be ~1.8 at worst case ?
I believe that would be the case, but the other question is whether Ppk is even applicable. What is the process making the hole? How is the process controlled (SPC)…or is it? Is the measurement system accurate to truly detect the variation?