Gage R&R on a Single Sided Nominal - What is an aceptable method?

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Atul Khandekar

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#11
komlossy said:
....

This is clear to me. But what if we have no tolerance range, because there is only one tolerance limit? Maybe I go and check the SPC topics, they should face the same problem when calculating CPK...
I've seen two views about this. One says that %Tolerance cannot be defined in case of single-sided tolerance (like Cp) and that you should compare GRR with Process variation that you get from ongoing SPC syudies.

The other view is that you can calculate the value of tolerance from process mean to the available specification and use the constant 3 instead of 6 (or 2.575 if you are using the 'traditional' 5.15 to represent the full spread of error)

Comments anyone?
 
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Howard Atkins

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#12
I am bumping this up again as I do not think we have a definitive answer.
It is possible to use part to part variance instead of the tolerance method and so there exists one solution.
If I have a requirement for say max 20, then can I say that the tolerance is 20?
If I have a requirement for say min 20, then can I use trlal and error to decide on a tolerance that is reasonable in relation to my measuring uncertainty?
Any other comments after a year to think about it
 

Miner

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#13
Atul Khandekar said:
I've seen two views about this. One says that %Tolerance cannot be defined in case of single-sided tolerance (like Cp) and that you should compare GRR with Process variation that you get from ongoing SPC syudies.

The other view is that you can calculate the value of tolerance from process mean to the available specification and use the constant 3 instead of 6 (or 2.575 if you are using the 'traditional' 5.15 to represent the full spread of error)

Comments anyone?
GRR as %Study Variation has taken the forefront as a more important measure of the gauge's suitability for process control or statistical analysis. If this number is acceptable and the process is capable of meeting tolerances, the gauge will automatically be acceptable for part acceptance to tolerance.

If the gauge will not be used for process control purposes, but only for product acceptance purposes is the only situation where GRR %Tolerance becomes important (unless the customer insists, of course).

The method described by Atul would yield differing results depending on a shift in the average. A better method for determining acceptability would be to develop a Gauge Performance Curve as described in the MSA manual. While it does not give you a single tidy number to base your decision upon, it does clearly show your Alpha/Beta risks as you approach the spec.
 

Atul Khandekar

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#15
From Minitab R14 Help:

%Tolerance: Percent of tolerance for each component.

If the tolerance (Upper spec - Lower spec) is given, percent tolerance is calculated by dividing the Study Var for each component by the specified tolerance.

If only one specification limit is given, percent tolerance is the one-sided process variation divided by the one-sided tolerance. The one-sided process variation is Study Var divided by 2. The one-sided tolerance is the absolute value of the given specification limit subtracted from the average of all the measurements.
 
L

llapanowski - 2007

#16
I think the MSA book is vague enough to cause all these unnecessary (should be if they were not vague) questions :( . I've been fighting this problem and have figured out what MiniTab does with unilateral (at least checks like flatness, runout, etc.) tolerances. This is in the Minitab help if someone needs further clarification. Take the metric (EV, AV, or GRR) and do the following:

%Tolerance(unilateral) = ((EV*6)/2)/(abs(AvgOfAllSamples - Tolerance))

I'm a programmer working for a Gage Company that has been trying to get our software to calculate GRR's as the AIAG MSA 3rd Edition manual specifies. I've downloaded a free trial of MiniTab to verify my results. If you believe Minitab calculates Gage Studies in accordance with the MSA, which I do, it is probably your best source for how to do the calculations, via the help files and creating samples. The only thing I really had to dig for is how to compute the p-values for the ANOVA method, but after many hours on the internet I did finally solve that problem. If your using Excel this is a non-issue due to the fact the FDIST and FINV function will compute the necessary values.:)

OOOOOPS, I missed Atul response - basically says same thing.
 
I

isotsadmin

#17
"this is in line with page 116: "If the analysis is based on tolerance instead of process variation [...] %EV, %AV, %GRR and %PV are calculated by substituting the value of tolerance divided by six in the denominator of the calculations in place of the total variation TV"

Hi.
How can I compute for the tolerance for the samples used when it covered the entire operating range as per average and range method ? will I need to get the maximum tolerance and the minimum tolerance for all operating ranges then get the range from there? Please enligthen. Im confused.
 
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