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Calculating Cpk using Japanese Method

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  • #28550

    Kiong
    Participant

    Hi All,
    I had recently came across a method to calculate Cpk which was called the Japanese method. I really don’t understand…maybe anyone can shed some light. The so called Japanese method of calculating Cpk is using the following formula:
    Cpk = Cp (1-k)
    where k = 2(Process average – Tolerance centre)/Tolerance width
    Well I am still trying to understand this formula.
    Thks
    Kiong
     

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    #71326

    Sridhar
    Member

    Hi,
           This is just the manipualation of the formulas. I will prove the calculations.
          Cpk = min { USL-Average/ 3*sigma , Average – LSL / 3*sigma}
            Cp= (USL – LSL)/6*sigma
            sigma = (USL-LSL)/6*Cp
             Putting sigma in Cpk equation
    Cpk=2*Cp/(USL-LSL)*min{USL-Average, Average-LSL}
         = 2*Cp/(USL-LSL)*min(USL-T+T-Average,Average-T+T-LSL}
        where T = USL+LSL/2
      = 2*Cp/(USL-LSL)*min(USL-(USL+LSL)/2-(Averge-T), Similar}
     = Cp/(USL-LSL)*min(USL-LSL-2*(Averge-T), similar for this)
     = Cp*min{1-2*(Avergae-T)/(USL-LSL), Similar term}
    =Cp*(1-2*(average -T)/Tolerance)
    =Cp*(1-k)
    where k = 2*(Average – Tolerance center)/Tolerance width
    Hope can understand this calculation. If not send me your mailid.
     
    sridhar
     

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