# 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

0
#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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