Gage R&R @ 10% - Does that mean you should cut your tolerance

cpine

Registered Visitor
#1
During a quality counsel meeting recently, the facilitator posed a very interesting question that I am embarrassed to say no one could answer.

The question was in respect to GRR.

According to AIAG MSA percent GRR is acceptable at 0-10%, 10-20% and above20-30% requires corrective action.

The question, if you show a GRR @ 10% does that mean you should cut your tolerance by 10% to ensure product integrity?

Cpine
 
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Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
Re: Gage RR @ 10% - Does that mean you should cut your tolerance

Thread moved to correct forum.
 
D

Duke Okes

#3
Re: Gage RR @ 10% - Does that mean you should cut your tolerance

During a quality counsel meeting recently, the facilitator posed a very interesting question that I am embarrassed to say no one could answer. The question was in respect to GRR. According to AIAG MSA percent GRR is acceptable at 0-10%, 10-20% and above20-30% requires corrective action. The question, if you show a GRR @ 10% does that mean you should cut your tolerance by 10% to ensure product integrity?Cpine
That is insufficient information to warrant such a decision. GR&R only tells about measurement system variability, not process stability or capability. If process capability is very high, then further consideration would include issues such as: what would be the impacts on cost, competitiveness, etc. of cutting the tolerance?
 
J

justncredible

#4
You would not use the raw GR&R findings to adjust the tol, you can use the raw GR&R findings to calculate the measurement uncertanity and then use that for enginnering purposes. Page 61-63 of the MSA.
 

howste

Thaumaturge
Super Moderator
#5
The question, if you show a GRR @ 10% does that mean you should cut your tolerance by 10% to ensure product integrity?
I think what you're saying is that with 10% R&R, there's a slight probability of accepting bad product at the high and low spec limits. Because of this you might tighten the tolerance to reduce the risk of bad product. Is that correct? If so, I see two potential problems which I'll describe before I give my final answer.

First, if you tighten the tolerance, then the GRR value will actually increase because the measurement system variation is now a higher percentage of the (now smaller) tolerance. Then you'll have to tighten the tolerance again to be safe, and the GRR value will be higher still! Pretty soon you'll have zero tolerance and an infinite GRR %.

Second, if your process is capable, you shouldn't be near the limits anyway. When you do a capability study on a process, the variation of the measurement system is included. The product variance (std. dev. squared) and the measurement sytem variance add to make the total variance reported. So if your process capability study shows it's good, gage error has already been factored in.

Having said that, if your process is not capable (i.e. you're using the gage to sort out bad product), you may consider creating an "internal spec" that is used to reduce the risk of accepting bad product (as you mentioned) while keeping the same actual specification. Most GO/NOGO gage systems have wear factors built in which are already doing just that.
 
Z

zancky

#6
in my modest opinion,
GR&R @10% is telling You that, under the assuption of no bias error, performing just one measure the real value should be within ±R&R/2, or 5%of the tolerance, with a confidence of 99,7%.
therefore for just one measure to be sure at 99,7% that no bad part are extimated good you must reduce tolerance by10%.
But if you perform more than one measure, the mean value should be affected by a uncertainty of R&R/sqarerooth(numebr of measures).
 
E

e006823

#7
The question, if you show a GRR @ 10% does that mean you should cut your tolerance by 10% to ensure product integrity?

Cpine
Guard banding a measurement is a business decision and should be based upon the impact of accepting nonconforming product You would not alter the tolerance on the product but define an acceptance zone.

In the case where accepting nonconforming product can not be tolerted then you could define an acceptance zone of:

+/- Acceptance Zone = +/- Tol. * (1-GR&R%)

Where the cost of product is high and the cost of accepting nonconforming product is low you could define an acceptance zone of:

+/- Acceptance zone = = +/- Tol. * (1-GR&R%)

I hope this was of some help.

Regards,
 
Z

zancky

#8
in my modest opinion,
GR&R @10% is telling You that, under the assuption of no bias error, performing just one measure the real value should be within ±R&R/2, or 5%of the tolerance, with a confidence of 99,7%.
therefore for just one measure to be sure at 99,7% that no bad part are extimated good you must reduce tolerance by10%.
But if you perform more than one measure, the mean value should be affected by a uncertainty of R&R/sqarerooth(numebr of measures).
just a typeing mistake it is ±3/2R&R as R&R is one sigma


I agree with ee06823, we are saying the same thing
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
Except that %GRR is a mathematically incorrect calculation if you are using (measurement error standard deviation *6)/tolerance.

The real contribution of measurement error to total variation (the vector of the tolerance zone) is contained in the formula: SDtotal = sqrt(SDerror^2 + SDparts^2)

Typically, in my experience the %GRR calculated using the first formula over estiamtes the contribution of measurement error and guardbanding is typically not necessary.

I would recommend reading two articles from Dr Wheeler:
"How to Establish Manufacturing Specifications"
"Relative Probable Error"
free for downloading at his web site: http://www.spcpress.com/reading_room.php
 

Miner

Forum Moderator
Staff member
Admin
#10
Wheeler's articles are excellent and the Relative Probable Error is the simplest way to go.

Another alternative is to create the Gage Performance Curve. The AIAG MSA 3rd Ed. manual has a section on how to do this. The attached file will also create this curve. The guardbands should be set at the highest and lowest dimensions that still provide a Probability of Acceptance of 100%. Guardbands should only be used on important dimensions.
 

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