Impossible to achieve NDC (Number of Distinct Categories) greater than 2

J

Jirka

#1
Hello everybody,

I have read many of your posts, and no one has solved my problems, so, instead of giving to you my example and make this thread so long such as the others threads which I have read, I had the idea to ask oppositte:

Does anybody can attach me please a R&R study excel sheet using the following data which we usally use in our factory?:

1- Dimension to be measured: Across flats 17.73 - 18.00 mm
2- Tooling used for measuring: Milessimal Micrometer (0.001mm)
3- The studies used are 10 parts x 2 appraisers x 2 trials
4- R&R study has to be performed considering the % of the tolerance (GRR)

The idea that I have is if somebody can fullfill me that sheet with random dimensions in order to get the NDC more greater than 2 and of course with the GRR% lower than 30%.

I can subbmit you my excel file, but I'm afraid that maybe has some mistake, so I prefer if somebody can attach me your excel file and then I would check with my data to find where is the mistake (if exist) because I have tried to create "the ideal" R&R study and it's impossible to achieve a GRR% lower than 30% and at the same time a NDC greater than 2.

We need to give an answer to our auditor next month of why our R&R studies doesn't get the ndc's... :eek:
Thank you for your kind attention! :thanx:
 
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B

Barbara B

#2
Re: Impossible to achieve NDC greater than 2

Jirka,

could you please give us at least the data so we could see for ourselves why there's a problem with your metrics?

%GRR and ndc could be converted into each other. To get a ndc>=5, %GRR has te be smaller than 23%. For %GRR=30% the ndc is calculated as ndc=3.3 (see AIAG MSA 4th, p.122-123).

Regards,

Barbara
 

bobdoering

Stop X-bar/R Madness!!
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#3
Re: Impossible to achieve NDC greater than 2

Your device should be sufficient for measuring the characteristic you are looking at....but there are some things that crop up the way the calculations work.

ndc is calculated based on PV, or (more importantly) the variation you presented to the device as a part of the study. If you have little variation and yet it represents the variation over time you expect, then ndc expects even more resolution or less gage error to achieve a reasonable value. Even 5 is a weak number if you wish to use SPC. If the data has less variation that what you expect to see over time, then you need to address that. PV is often replaced with the variation from long term data (since it is more real than a handful of specimens).

ndc is not related to tolerance - at least the calculation provided by AIAG. So, you can get a good % tolerance GRR but bad ndc. But, a bad ndc should relate to a high GRR as a % of the total variance.

This is why your data and the rest of the statistics would be handy for us to look at. Generating an academic data set will not be as beneficial. It is far easier to relate to a real situation.
 
J

Jirka

#4
Re: Impossible to achieve NDC greater than 2

Thank you so much for your fast response!

I must apologize cause I did a mistake with my explanations, usually the problem to achieve the proper NDC are with the pitch Ø.

So, I attach you an example of real R&R test. If according the 4th revision of the MSA we must achieve 5 NDC's, how we can reach that? It is real severe for us. I know that one solution could be to buy new measurement toolings which can read 0.0001 because the pitch diameter usually it's in millimess, but I'm afraid that this will not be still enough.

Anyway, what we are worried about is the answer to be given to our auditor...

Thanks again for your support
 

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bobdoering

Stop X-bar/R Madness!!
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#5
Re: Impossible to achieve NDC greater than 2

Thread measurement is always a painful topic, so don't feel bad - it's not just you.

As far as your data, you have good news and bad news. The good news - if it is true - is that you have little variation between and within your measurements. The average range is .004. The bad news is that ndc looks to divide that range up into statistically significant pieces. The resolution of the device is .001, so if you had no gage error your ndc would be about 4. With error, it is nothing but down.

So, here is the issue. You could increase your ndc by increasing your variation...but it seem your customer may just prefer to have tight data and your current measurement technique. You only need a higher ndc for SPC to sense process variation. But .004 out of a .105 spec would not support more expensive gaging for typical measurements - especially if the variation is truly what you can expect over time.
 
J

Jirka

#6
Re: Impossible to achieve NDC greater than 2

Thanks for your support and most of all thank you to tell me that I'm not alone in that kind of thread issues :D

Yes, we really have a very accurate data by measuring the pitch diameter after many years of training of measuring in the same way, but although we have that accuracy, you can see that the GRR result it's more near to 30% than to 10%. In coated pitch diameter R&R's I would never trust a result of less than 10% because it's really really difficult to achieve, at least measured in 0.001 devices.

About the NDC, if we should have more variety for get a better NDC, then the GRR would increase, so it is a really neverending story.

In fact, any of our customers has ever complaint about NDC, it's just our auditor which wants to know why we can't achieve the 5 NDC's requested... so, what would you suggest me to explain him? That in this case NDC's would have more sense in SPC and that our customers are good with our GRR results and accuracy?

I hope to find a good "defence" and then be able to handle it properly...

Thanks again for your support.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
Re: Impossible to achieve NDC greater than 2

I don't have the MSA book handy today - but I will in a day or so. But, I recall there are some tidbits buried in there that will help. The biggest defense is that the MSA book is a reference, not a requirement - so the auditor should not see the recommendation of ndc=5 as a mandate. I need to verify it does not crop up in the PPAP book under the MSA section - because that would make it a mandate (although misguided one).

Assuming it is not in the PPAP book, the justification is that the variation of your process is extremely small compared to the specification (very capable.) If you are not required to do SPC, then there is no point gaining a higher degree of statistical resolution (which is what ndc represents). If it is not specified verbally, it is very clear from the calculations used to develop ndc if the auditor was familiar with them.

Other misguided requirements come from customers that mandate this ndc requirement across the board in the CSRs - without having any idea what they are doing. After training the customer on this issue, they may have to provide a written waiver of their own requirement - which would be all the auditor would need.
 

Jim Wynne

Staff member
Admin
#8
Re: Impossible to achieve NDC greater than 2

I don't have the MSA book handy today - but I will in a day or so. But, I recall there are some tidbits buried in there that will help. The biggest defense is that the MSA book is a reference, not a requirement - so the auditor should not see the recommendation of ndc=5 as a mandate. I need to verify it does not crop up in the PPAP book under the MSA section - because that would make it a mandate (although misguided one).

Assuming it is not in the PPAP book, the justification is that the variation of your process is extremely small compared to the specification (very capable.) If you are not required to do SPC, then there is no point gaining a higher degree of statistical resolution (which is what ndc represents). If it is not specified verbally, it is very clear from the calculations used to develop ndc if the auditor was familiar with them.

Other misguided requirements come from customers that mandate this ndc requirement across the board in the CSRs - without having any idea what they are doing. After training the customer on this issue, they may have to provide a written waiver of their own requirement - which would be all the auditor would need.
All good advice. :bigwave:

As far as the PPAP manual is concerned, there's a note in MSA section (page 6 of the 4th edition) that says "Gage R&R acceptability criteria are defined in the Measurment Systems Analysis reference manual."

What that means is anyone's guess, given the fact that the MSA manual emphasizes the fact (as you point out) that it offers guidelines, not requirements. There will always be exceptions to the "rules," and we can only hope that customers and auditors will listen to reason. Otherwise, things like this become instances of customers unwittingly asking suppliers to make stuff up so the right numbers appear in the little boxes.
 

bobdoering

Stop X-bar/R Madness!!
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#9
Re: Impossible to achieve NDC greater than 2

As far as the PPAP manual is concerned, there's a note in MSA section (page 6 of the 4th edition) that says "Gage R&R acceptability criteria are defined in the Measurement Systems Analysis reference manual."

What that means is anyone's guess, given the fact that the MSA manual emphasizes the fact (as you point out) that it offers guidelines, not requirements.
I was suspicious that was the case, and, as you mention. MSA book is emphatic that there is no one way or criteria to approve a gage - especially in the section with the 10/30 criteria. :rolleyes:

I admire their intentions, but they on occasion trip over their own feet.
 

Jim Wynne

Staff member
Admin
#10
Re: Impossible to achieve NDC greater than 2

I was suspicious that was the case, and, as you mention. MSA book is emphatic that there is no one way or criteria to approve a gage - especially in the section with the 10/30 criteria. :rolleyes:

I admire their intentions, but they on occasion trip over their own feet.
That's because they don't have actual feet, just guidelines.:tg:
 
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