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Process capability for a guard banded process.

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  • #39743

    Tim
    Member

    An automatic balancing machine typically removes material from a workpiece in order to put it within the range of acceptability.  The machine uses an in-process gage to measure the magnitude of the unbalance.  A typical balance specification is a unilateral tolerance; (bounded by zero) and some upper specification limit.  The % tolerance due to the measuring system can be determined by conducting a gage R&R study.   From my experience the % tolerance will usually run between 10% and 20%.  I will normally establish the setpoint for a balancing machine at a value that is below the upper specification limit by an amount equal to the gage R&R % tolerance (a method commonly called “guard banding”).  My question is this; for the purpose of calculating the process capability index (CpK) should I use the upper specification limit defined by the customer or a reduced upper specification limit that coincides with the process setpoint?

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    #121568

    Mike Carnell
    Participant

    Tim,
    The guardband is on either side of the specification limit (units within the %tolerance above the spec limit can be good as well as the one inside the spec limit being bad).
    If i were doing the capability study for the customer I would use the customers specification. If I were doing it for myself (internally) – I would use the guardbanded number.
    Just my opinion.
    Regards

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    #121575

    Savage
    Participant

    “If i were doing the capability study for the customer I would use the customers specification. If I were doing it for myself (internally) – I would use the guardbanded number.”
    Why?  Isn’t the customer spec always the focus?  Could you elaborate?  I’m interested.
    respectfully
    Matt 

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    #121581

    Mike Carnell
    Participant

    Matt,
    No. When I choose to run my process with guard bands I am compensating for a poor measurement system (at least in this case of 10-20%). The customer only cares what I am delivering and if I am guardbanding it will make me appear more capable because I am actually building to a tighter specification that they require.
    Internally I want to be capable to the guardband because there is an associated risk and cost with anything that falls inside the guardband. Some companies scrap them, some bench them and some just run multiple tests to reduce the error. Either way it is additional cost and it is more efficent to just not produce there. I want to drive capability inside the guardbands.
    Just my opinion.
    Regards

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    #121588

    Vinay
    Member

    Internally or externally, where ever Customers dpeccification is the Gospel for process capability as, customer pays for the product and thus the organization generates revenue.
     
    Vinay

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