Using the results from GR&R (Gage R&R) Analysis - One sided dimension

Miner

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#11
If you are referring to the tab with the gage performance curve, Pa (probability of acceptance) is the probability that the gage will accept a part as within specification. This graph was based on the AIAG approach and was included for reference.
 
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QCAce

#12
...example - spec 0.1mm max. GR&R 10%. Therfore for inspection purposes the acceptance criteria should be 0.1mm - (10% of 0.1) = 0.09. This would then ensure no parts outside of the spec be accepted....
I am not 100% certain so I would want confirmation from some of our other Cover experts, but I believe that if you gage has an error of 10% of the tolerance range or .01, you may be able to assume that any reading could vary +/-.005. And therefore you could set your limit to .95.

Also remember that taking multiple measurements and averaging them can increase the confidence of your result. So parts falling above .95 could be set aside and have measurements repeated on them.
 

bobdoering

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#13
Also remember that taking multiple measurements and averaging them can increase the confidence of your result. So parts falling above .95 could be set aside and have measurements repeated on them.
...or save them for a slower, but more accurate measurement process.
 

Bev D

Heretical Statistician
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#14
yes...but's let's not forget that the 10% (or 20 or 30 or whatever) is mathematically incorrect as it is a straightforward ratio of 6*SD and the tolerance range. NOT correct as Standard deviations do not ADD, their variances do.

this is why I calcualte the number of distinct categories or Discrimination ratio using the Intraclass correlation coefficient formula (as Wheeler describes)

Then I divide the process variation range by the number of distinct categories. I divide this number in half and that becomes the guardband range. I too set aside those parts that are within the guardband range of the spec limit and remeasure each part 2 additional times. The average is used for pass/fail. (unless the characteristic of interest is functional AND the spec limits were actually engineered - then I scrap or rework...) Works very well!
 
S

swarbur4

#15
Bev, Miner,

I have been attempting to study and understand the 2 methods, MSA and Wheeler. They both use the constants K and D. I would like to understand how the constants were derived. If you could help or point me in the right direction it would be much appreciated.
Thanks
 

Miner

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#16
Both sets of constants are used to estimate the standard deviation from the average range. The average range provides a biased estimate of the variation of a subgroup. The size of the bias depends on the number of samples in the subgroup. This is why the constants depend on the subgroup size.

Regarding their derivation, here is a link (Control Chart Constants) with more detail.
 
S

swarbur4

#17
Miner,
Thanks for the explianation, I have 1 more question.
When calculating the %GR&R against TV it is 100(R&R/TV)
When calculating the %GR&R against Tol it is 100(R&R/(Tol/6))
Why is the Tolerance divided by 6
 
B

brahmaiah

#18
An interesting discusion came uip the other day when I was checking a PPAP / PSW submission.

For a SC (Significant Characteristic) the GR&R result was 10%. In production this dimension is inspected 100%. It is a one sided diamension. 0.1mm. The control plan and work instructions for the inspection descibed the actual spec for aceptance or rejection. example =<0.1 ok >0.1 NG.

The discussion was based on should the GR&R result be reflected.

example - spec 0.1mm max. GR&R 10%. Therfore for inspection purposes the acceptance criteria should be 0.1mm - (10% of 0.1) = 0.09. This would then ensure no parts outside of the spec be accepted.

What do you think? - has anybody used the GR&R result this way before.

Cheers
The acceptance critaria cannot be less than the specified tolerance.It should be 0.10 only.
V.J.Brahmaiah:agree:
 
B

brahmaiah

#19
Miner,
Thanks for the explianation, I have 1 more question.
When calculating the %GR&R against TV it is 100(R&R/TV)
When calculating the %GR&R against Tol it is 100(R&R/(Tol/6))
Why is the Tolerance divided by 6
Could you please tell us from where you got the information that %GRR against tol. is 100(R&R/(TOL/6).Did you get it from the IATF-MSA Manual?.Your information is wrong.
GRR IS 100(R&R/TOL) AND not 100(R&R/(Tol/6)
V.J.Brahmaiah
 
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