Minor NC - NDC of 0.4 yet the system had been accepted with no investigation

C

cruss

#1
I have the folowing case :

the result from the GRR study is 16,66% . The NDC indicator shows 0,39 ! This was marked as a minor NC and the explaination was
TS16949 clause 7.6.1 states“... The analytical methods and acceptance criteria used shall conform to those in customer reference manuals on measurement systems analysis. Other analytical methods and acceptance criteria may be used if approved by the customer...”
· the report for Micrometer showed an NDC of 0.4 yet the system had been accepted with no investigation into this low figure.
My opinion is that if you use the 1:10 rule and your parts are almost perfect with insignificantly small part variance , the NDC is expected to be less than 5 , simply because it uses the formula PV/GRR. And tbh i can`t understand what is the relation between the text mentioned from the auditor and the observed situation.My customer don`t have any reference manual regarding the MSA requirements at all.And i doubt that the auditor use his brain at all .for example :

the nominal value is 3,35mm with tolerance +/- 0.05
the results obtained are:
3.331
3.335
3.332
3.334
3.337
and so on...
the machine producing this part is set to limits 3,3~3,4 mm

so if somebody can prove me that this tool is not good for this measurement task please hit me in the face.Any comments are welcome.
 
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harry

Super Moderator
#2
Cruss, I moved your post to form a new thread in order to attract the attention of our regular users and experts in this area.

Pending further replies, I would like to invite you to read Miner's blog.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#3
The point of needing the ndc to be 5 or more is to show that the variation detected from the resolution of the gage is statistically real. The variation you have from the specimens you provided to the Gage R&R was too much gage error variation in relationship to the actual product variation.

One key question is this, when you run this process, will it always be 3.330 and 3.338? Or do you expect the variation to be larger over the lot? Are you doing any SPC?

If you expect there to be more variation over the run for each lot, then you need to present that variation to your gage R&R to make its results valid.

If you do not expect there to be any more variation, then the point of the low ndc is that you can not tell if the variation you are observing is gage error or process change, and therefore you can not determine if the process itself is providing and clue of suddenly changing. It would not be acceptable for SPC, as the data that would flag an out of control condition would be too "chunky".

So, it depends on the use.

The new 4th edition of the MSA book has the criteria (not specification) on page 78, with a bold CAUTION: The use of GRR guidelines as threshold criteria alone is NOT an acceptable practice for determining the acceptability of a measuring system.
 
D

debyang

#4
Marc,

If your NDC is calculated per the AIAG MSA manual, then the NC might come from the reason that when your GRR is only 16.67% then the NDC should be around 5, not 0.39(0). :notme:
 
C

cruss

#5
The problem is that I (my person as msa responsible and my expirience ), can take a decision based on my knowledge , but the same auditor (he is coming every year for the renewal of the certificate) gives me each time different statement about this issue . However thank you for your comments , critics are always welcome because for taking a good decision , you need good logic and strange arguments.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#6
Here are the key points for anyone reading this thread. ndc>5 answers the question: is your resolution sufficient for your variation? 10:1 answers the question: Is the resolution adequate for my specification, if I ignore all gage error?

For SPC, you are interested in your resolution in relationship to your variation. For simple inspection, resolution in relationship to you specification would be adequate, but to ignore the gage error buy using the simple 10:1 back yard rule is not recommended. Gage R&R is a much better approach.

Now, remember, the above approaches are for those people with some interest in understanding their process. If you have no interest in understanding your process, and care to justify your gage usage by historically never sending bad parts, then a simple go/no go gage would be adequate. However, most customers, and the automotive industry in general, expects the measurements to be used to understand the process. That is why adequate resolution to be able to detect process variation is recommended, and will likely be sought by knowledgeable auditors.
 
Last edited:
C

cruss

#7
dear Bob i see that you love auditing. However , in my opinion auditor`s job is to check if everything is conforming with the rules and procedures. As i can see , you are trying to convince me that i have to use the tool for what the auditor is telling me , even this rule for NDC > 5 is not even a standard , because the MSA manual is for reference only. And also you are trying to convince me that even i am using crimping monitors for thousand euros , i can`t use a stupid micrometer for part inspection? And all this , because i`m checking tolerance range of 0,1 mm with a tool resolution 100 times greater ? Excuse me but i can not accept this.
 
D

debyang

#8
Dear Cruss:

You have mistaken my point. Actually, I never buy the rule such as GRR<10% or NDC>5 , sometimes I even doubt MSA is really needed. But I just can not figure out that if your GRR% is only 16.66%, how come your NDC indicator indicated 0.39?
If I were the auditor, I will suppose that there is calculation mistake in NDC calculation, that's why I had previous reply. Sorry for my misunderstanding.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
dear Bob i see that you love auditing. However , in my opinion auditor`s job is to check if everything is conforming with the rules and procedures. As i can see , you are trying to convince me that i have to use the tool for what the auditor is telling me , even this rule for NDC > 5 is not even a standard , because the MSA manual is for reference only. And also you are trying to convince me that even i am using crimping monitors for thousand euros , i can`t use a stupid micrometer for part inspection? And all this , because i`m checking tolerance range of 0,1 mm with a tool resolution 100 times greater ? Excuse me but i can not accept this.
I was answering your question concerning whether the gage was adequate for the job, not commenting on the auditor's job or adequacy of his response. It is adequate for inspection, not for SPC - at least with the information you have supplied. You may also want to consider GRR as a % of tolerance rather than as a % of the variation you supplied, which should be more than adequate.

Of course, it helps to understand the process. Now that you have added that it is crimping, it depends on what you are crimping. If you were crimping electronic terminals to stranded wire, for example, I would say that your resolution is fine for such a rudimentary measurement technique for that process. Other crimping applications may require far less scrutiny.
 
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