# Gage RR CPK

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• #43043

batman
Participant

The team is trying to justify a new gage system. Our current system has %study variation of 20. The new gage system has %study variation of 10. How does an improved Gage RR correlate to an improved cpk index?
We are trying to justify the cost of the new style gage. If there is another way to do this we would like to know as well.

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#136222

Chicagoan
Participant

Batman,
Following is the relationship between Cp and GRR:
Cp(Actual) = Cp(Measured)/sqrt(1-GRR^2). If there is no significant mean shift in your process, the above formula holds true for Cpk’s as well.
Cp(Actual) = True process potential (cannot be directly measured as the it is being effected by the measurement device that is being utilized)
Cp(Measured) = Measured process potential with the gage
Hope this helps.

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#136246

batman
Participant

Ok a little more help, what number from the GRR do we use in the formula?
%Contribution = 1.12
%Study Var  =10.6
or something else? When we tried the formula the CP was better than expected. Here is what we used
Cp(Actual) = Cp(Measured)/sqrt(1-GRR^2)
Cp (actual)= 1.33/sqrt(1-(0.106^2))

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