Measurement Systems Analysis - Effect on Process Decisions - MSA Manual page 19

[Yew Jin]

Referring to the MSA reference manual third edition by DaimlerChrysler Corporation/Ford Motor Company/General Motors Corporation on effect on process decision page 19, the relationship between indices of the observed process and the actual process as

(CpObs)^2 = (CpActual)^2 + (CpMSA)^2

But I can't understand the example given as "if the measurement system Cp index were 2, the actual process would required a Cp index greater than or equal to 1.79 in order for calculated (observed) index to be 1.33."

Anybody can help to interprete the solution in the example given as the reference manual?

 

[Miner]

Re: MSA - Effect on Process Decisions

I do not have my MSA manual available to review the specific page, but I think that I can explain the concept.

The Cp index is the tolerance divided by 6*StdDev of the process variation. The measured process variation is a combination of the actual product variation PLUS the variation of the measurement system.

Standard deviations cannot be added, but the VARIANCE can be added. VARIANCE = StdDev^2.
Therefore, VARIANCE (observed) = VARIANCE (parts) + VARIANCE (MSA).
Expressed in terms of StdDev, this becomes: [StdDev(observed)]^2 = [StdDev(parts)]^2 + [StdDev(MSA)]^2

Since the Cp index determined from a capability study is based on the StdDev (observed), the greater the measurement variation becomes, the smaller the true product variation must be to obtain a target Cp of 1.33.

 

[Yew Jin]

Thanks Miller, I understand the concept. But I can't get the answer that given in the example above. :truce:

 

[Miner]

Have you checked the Errata for the 3rd edition manual? Some of the printings had a number of errors in the examples.

There is a link directly to the errata posted here in the forums, or you can try the www.aiag.org website.

 

[Yew Jin]

Thanks guys.....at last I found the answer through the errata......