Six Sigma process capability and product-dependance

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    Boris lipnik

     The situation: I succeeded to achieve Cpk=2 for a process.
    Let say that the process is machining of a steel (type 1) specimen while the spin  velocity is of 1000 rpm. I measured the obtained width  for 300 parts and got Cpk=~2.
    Then on the same machine, the steel (type 2) specimen is made, while the specified velocity is 1500 rpm, and when the target and spec limits are the same as above.
    It appears however that the measurements over 300 “type 2” specimens gave Cpk=1.2. (For both cases the process was considered as in-control). 
    Now, what is really my process capability?
    What is the approach should be, when the results are product-dependant ? At the end what we want to know is the PROCESS, not product capability!!!!!
    (For both described cases the PROCESS is considered to be the same).
     Should I mix the data from both specimens? What is the truly meaning of this situation? Should I declare that there are 2 different processes  and report them separately?


    Jim Johnson

    It appears to me that the speed at which the steel tube is machined makes the process different in some way.  Is this truly the same process? (regardless of how it is viewed).
    If it is the same process, then you need to make a combination measurement.  If they aren’t the same, then you need to separate them
    It may be that I am oversimplifying this process.
    I hope that this helps.

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