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Process capability study

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  • #48019

    Sridhar
    Member

    I need clarification from others regarding the process capability measurement CPk.
    We have a manufacturing process Painting (Sheet metal components)to check the process cabability for this components /product. The coat thickness is identified for checking the capability.
    The design tollerence (70 to 120 microns) is high and in that case will it be appropriate to have this study?
    We need to do it on the process (test equipment / process equipment) or on the component?
    Sridhar
     
      

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    #160690

    Shan
    Member

    For a process like painting, plating, case hardness process capability is not a apt method to qualify the process..
    DOE is the right method of qualifying the process.
    1. Define the functional parameters affecting the process. Run the process based on DOE method and record your optimum setting.
    2. Fix the frequecy for performing the trial (yearly / half yearly)
    Process qualification can be done on the above process.
    shan

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    #160712

    annon
    Participant

    Dont fully understand. You are simply trying to determine the capability of a given process and you wish to use a process capability index such as Cpk to do this?  If that is the case, then as long as the underlying assumptions of stability and normality are intact, the tool should be applicable to your situation.
    Properly follow the Define and Measure phase elements to ensure the validity of your efforts.  Also, there may be other methods of quantifying your capability that are more appropriate for your effort and worth considering.
    Good luck.

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    #160726

    Sridhar
    Member

    Thanks for the clarifications / views _Mr.Annon / Mr.Shan. To give bit more information, we have a painting process (electrostatic powder painting_semi automatic line_manual touchup and auto spray powder) for sheet metal components and the specification minimum coat thickness 70 microns and maximum 110 microns. We monitor this specification random inspection in a day and process is going on. Some days the coat thickness goes up or down and further feedback instruction given to the powder sprayer to control. 
    Now i would like to know is it necessary to check the process capability for this process? Is it worth spending time, in that case how can i proceed? As you know in this process the paint thickness at various area various and can’t maintain say only 70 microns throughout area of the sheet.
    In that case how sampling / inspection data can be collected for calculating the process capability. For your information we have more variants in components run in a batch itself (More than 50’s of items having different profile)
    Please give your views regarding. If any one have sample excel data sheet done – similar please let me have.
    Appreciate your feedback.
    Regards,
    Sridhar 

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    #160731

    Shan
    Member

    Mr Srithar,’
    I need to understand your problem…Is the Quality rate of your process is the issue ? Is your rejection % is HIGH ?
    If it’s so yu can resolve the issue  by applying shanin DOE..
    Always remember to solve the problem at macro level rather getting in to more details…
    Good luck
    shan
     
     
     

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    #160776

    Sridhar
    Member

    Thanks for your comments. Regarding the problem, pls note paint more or less on the sheet metal component will not affected directly the function / performance. But the paint more may increase the cost.
    Please give some road map for doing DOE.
    Regards,
    Sridhar

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    #160787

    StefanL
    Member

    Hello Srindhar,
    From what I can understand, you want to decrease the variation in your painting process and use less paint on your components?
    If so, don´t do CPK on the full tolerance area, narrow it down to ~70% or less. Find out where your process stops being capable.
    How does your variables affect the outcome? Preassure, mouthpiece, programs and so on, are they under statistical control? Do they have control limits that are verified to be within the limits?
    Random data is good, but keep in mind that if you use short term data, your process is likely to shift ± 1,5 sigma. Try to measure it on a regular basis, and see where the data shifts.
    A DOE without D M A is not to be trusted, find the variables that you need to have in control and control them before doing DOE. Otherwise you can be in for a unpleasent surprise.

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