Go - No Go (Attribute) Gage R&R (Repeatibility and Reproducibility)

R

Richard

#1
Our customer ask us to do the gage R&R for a checking fixture (Go/Nogo gage). This gage was calibrated already by customer. It is used for our product length of dimension 1330+/-5mm in our in process 100% final inspection.
Obviously we have to use different formular for the variable measurements. I have no idea about go/nogo. How to do it? Which formulars should we use? Please advise.
regards
 
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R

Rick Goodson

#2
Richard,

There is a section in the Automotive Industry Action Group (AIAG) Measurement System Analysis (MSA) manual that describes the shiort and the long method for performing attribute gage studies. In essence the short study involves 2 appraisers, 20 parts, and replication. If you don't have access to a MSA manual you can purchase one through the Automotive Industry Action Group (AIAG)

Hope this helps.

Rick
 
R

Richard

#3
I have the MSA manual 3rd edition and start from page 125 attribute measurement system study. But I got lost as try to understand where those data derive from, such as reference, ref. value in table 12. Also confused by those cross-tab, Kappa. What is acceptance criteria for attrinute gage R&R?
Could you explain more detail?
Thanks.
 
C

Cristi?nC

#4
Attribute R&R

Richard:

If you are dealing with a measurement (lenght), why consider it as an "attribute"??

MSAs for Variables are more helpful for understanding how good or bad is our measurement process. It may help you to have an idea for the next steps in productive process improvements...

Attributes can only help us to simply detect no-go parts...

But...

Assuming that usage of go no-go gage and 100% inspection is demanded by your customer, I hope this simple concepts help you to perform an attribute study:

- Choose at least 25 or 30 "perfect" and "defective" parts and ask each of the operators to inspect them randomly, at least three times each part. Remember: randomly, and in a way that resemble their routine work.

- Write down their decisions in a check sheet and after that, obtain the following:


- Repe: Hability of the appraiser to "repeat" his/her decisions (agree with him/herself). Calculated as (# agreements / # parts inspected) and commonly referred in percentage. 90% is usually considered as "acceptable", but this is a "rule of thumb".

-Repro: Hability of all the appraisers as a whole to "repeat" their decisions among them. Calculated as (# agreements among all appraisers / # parts inspected). (90% again is acceptable)

Since you already know the exact condition of each part, then you can determine:

- Concordance: (# parts that agree with the standard / # parts inspected). 90% again.

- False Alarms: (# parts classified as "defective" when in fact is perfect / # of perfect parts). 5% is acceptable (AIAG)

- Wrong Classficaction (# parts classified as "perfect" when in fact is defective / # of defective parts). 2% is acceptable (AIAG).

- Mixed: (# of parts that were classified inconsistently along the inspections / # parts inspected)

You can use Excel.

Forget about Kappa, is useful for ordinal categories (extremely bad, bad, good, extremely good).


Hope this helps!
 
B

Bob_M

#5
CristiánC,

I am not sure where you got those basic formulas, but they sound very reasonable and easy to perform for a basic go-no go gage R&R type of test.

Thanks for the post.

Bob_M
 
R

Richard

#6
I spent whole day on it and realized chi square
knowledge which is key to the crosstabulation table.
Also I found some mistakes which have
been corrected on AIAG website for miss rate numbers on
page 132. Anyhow I got it .
Your comments give me the essence of that chapter.
Thanks for your help.


:)
 
C

Cristi?nC

#7
Source Repe & Repro calculations...

Bob_M:

Minitab 13.3.

Based on AIAG book, they simplified the math...
They don't use the repe and repro words, but, as you also conclude, the formulas refer clearly to this sources of variation...

Regards
 
A

Al Dyer

#9
Richard, may I ask the reason for 100% inspection on a gage supplied to you by the customer?

Has it failed capability studies, repeatability, reliability, bias, stability, etc?

Of course even a gage supplied by by customers has to go through some type of verification to ensure proper use during the manufacturing process.

With a fixture, take 100 actual production parts and run them through the attribute gage and use a P-Chart with a 4-5 part sample size. After that you know if the gage is stable? or do you?

Instead of a part use some standard, a calibrated piece of stock say (master) or some other calibrated and defined standard.

Just realize that all studies on calibration and verification will ultimately be your concern in the eyes of a customer, I.E. cover your:ca:

Al...
 
R

Richard

#10
This fixture is for pipe length 100% inspection. The length is not a Critical dimension according to drawing and also we are not asked to do any process control chart. We are doing the R&R study just for the PPAP submission. R&R study is not suggested for attribute data but it is asked by out customer.
Could you explain a little more about the reason of using P-chart? Do you mean we need do the P-chart control in our process control after we pass the R&R? What the customer concern will be? Please advise. Thanks.
 
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