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Machine capability subgroup URGENT PLEASE

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  • #45174

    Joseph
    Participant

    Hello my name is Joseph I am new in six sigma. I have a question about the sampling size of a machine capability study I am doing right now for two machines.
    I collected 620 consecutive parts from each machine no subgrouping was done. The data from one of the machines is non normal. From historical X-bar charts we now that these process are in control from both machines. How could I use this data to get an idea of my machine capabilities. Since no subgrouping was done is this data valid for a machine capability study?
    Thank You!
    Joseph

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    #146681

    Schmidt
    Participant

    You don’t need subgrouping to do a machine capability study. Sample size should be  > 50 of consecutive parts (100 recommended). But your data has to be normal !
    For a preliminary process capability study , you have to use subgroups usually 3-5 of consecutive parts, and at least 20 subgroups.
     
     

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    #146682

    Schmidt
    Participant

    What I forgot .. if you have historical data, why do you want to do a machine capability study ? Why not simply calculate your process capability ?

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    #147649

    Joseph
    Participant

    Thank you for helping me Marcus. I need to measure the capability of my machine not my process. Using historical data will lead me to measure other sources of variation. I want to see if my equipment is capable or nor capable to the job within specifications.My question is 620 samples is a good size since we produce 800 parts per minute?

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    #147650

    Theo
    Member

    This is incorrect.  Your data does not have to be normal to use XmR charts.
    Read Dr D Wheeler “Normality and the Process Behaviour Chart”. 

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