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Cm & Cmk

Discussion in 'Capability - Process, Machine, Gage …' started by Emmyd, Feb 29, 2016.

  1. Emmyd

    Emmyd Member

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    One of our customers has made some changes to the initial capability requirements and I'm having some difficulties in interpreting what they are looking for and how to calculate it. It used to be that they looked at Cpk (process) for initial cape studies. Now, they want Cm & Cmk (machine capability) for initial, with Cpk for an increased production run. I've calculated Cm before in Minitab, but it requires a target - which for 1 sided tolerances you have to input the target and a lower limit - I've found this to be counter intuitive to a good capability. For a single sided tolerance, there should be no nominal as you are trying to get closer to zero.

    Anyway - how to correctly calculate the Cm & Cmk in Minitab? If it is the same as the calculations for Cp & Cpk, why not report Cp & Cpk? I have a meeting with this customer this week and I'll be asking this information, but I would like to learn more about this before I go.

    Thanks for any help!
    M
     
  2. Plague Doctor

    Plague Doctor Member

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    What I was told this is the same calculation as for Cp & Cpk only the principle of sampling is different. For Cmk you need to produce 50 or more pieces one by one using the same machine setup, parameters and so on.

    Not sure that this is correct though. I also willl appreciate any information about this topic.
     
  3. Miner

    Miner Moderator Staff Member

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