Gage R&R - Is there an upper bound for number of distict categories?

H

hmquijano

#1
hi!

this is my first time here....
could anyone help me .....
we've been doing R&R studies in our company...
we're looking at %Gage R&R and ndc.
Some results show that Total %GR&R (based on %Study Var) is less than 1% but the ndc is almost 400. (format of the study: 10 samples, 3 trials)
Just want to know if there's an upper bound for ndc.
I understand that based on AIAG manual, ndc equal to or greater than 5 is desirable but don't you think ndc's greater than 100 is too large?

Any comments?

Thanks!
hmquijano
 
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D

D.Scott

#2
First, welcome to the posting side of the Cove.

If I understand, you have over 400 distinct categories in your readings. I am assuming you are using a very precise measurement system on a part with a large tolerance?

I don't know of any limit on ndc but I am not one of the experts in this field. I am sure if we keep it on top, we will get some good answers.

My initial thought would be that you are probably "overkilling" the measurement. It appears (without knowing the details) you are measuring something with such an accurate piece of equipment that you would be hard pressed to have an error. If you are measuring to a total tolerance of +/- .5, you would normally use equipment capable of discriminating a measurement of .01 or a discrimination of 10 to 1. I am guessing that your measurements are recording in three or more decimals than your tolerance requires.

Dave
 

Miner

Forum Moderator
Staff member
Admin
#3
There is no upper limit. From a pure measurement system view, larger ndc is better. However, at some point, it does become economically important. You must make the judgement between the cost versus the benefit of the measurement system.
 
H

hmquijano

#4
Thanks Scott and Miner!

In this particular study, we're measurering width of strips. Tolerance is +/-.5mm. THe equipment can give measurements up to 2 decimal places.
Thickness of samples used were 96mm, 86mm, 94.45mm and 103mm. Any comments on these samples? Is it right to get samples like these? Or do we have to get only 1 type to strip width and to account for part to part variability, we will consider getting samples from different rolls of strip produced (of1 strip width only, e.g. 96mm only)....

Thanks!
hmquijano
 

Miner

Forum Moderator
Staff member
Admin
#5
You are selecting samples from several different nominal sizes of strip. The acceptability of doing this depends on why you are performing the MSA. There are two basic reasons to perform an MSA.

Reason #1) to determine whether the gage can distinguish a good part from a bad part for acceptance measurements. You need to compare the measurement error to the tolerance using P/T Ratio. In this case, your sample selection method is okay because the criteria is independent of the sample variation.

Reason #2) to determine whether the gage can distinguish one part from another part for statistical measurements such as used for process control (SPC) or process improvement studies (DOE, t-tests, etc.). You need to compare the measurement error to the process variation using %GRR. In this situation, your sample selection criteria will give you false results because the variation of the samples is vastly greater than the process variation. You should select sample using the same criteria that you would use to collect samples for a capability study because the standard deviation of your samples must match the standard deviation of the short-term process capability.

Since you are concerned about the ndc, I presume that you are using Reason #2 for performing the MSA. If that is the case, your sampling method is incorrect, and is the reason for the high ndc value. Collect new samples as described above and repeat the MSA.
 
H

hmquijano

#6
Miner said:
You are selecting samples from several different nominal sizes of strip. The acceptability of doing this depends on why you are performing the MSA. There are two basic reasons to perform an MSA.

Reason #1) to determine whether the gage can distinguish a good part from a bad part for acceptance measurements. You need to compare the measurement error to the tolerance using P/T Ratio. In this case, your sample selection method is okay because the criteria is independent of the sample variation.

Reason #2) to determine whether the gage can distinguish one part from another part for statistical measurements such as used for process control (SPC) or process improvement studies (DOE, t-tests, etc.). You need to compare the measurement error to the process variation using %GRR. In this situation, your sample selection criteria will give you false results because the variation of the samples is vastly greater than the process variation. You should select sample using the same criteria that you would use to collect samples for a capability study because the standard deviation of your samples must match the standard deviation of the short-term process capability.

Since you are concerned about the ndc, I presume that you are using Reason #2 for performing the MSA. If that is the case, your sampling method is incorrect, and is the reason for the high ndc value. Collect new samples as described above and repeat the MSA.

Thanks again for immediate response!
Yes, we're doing MSA study using Reason #2.
Thanks again. We might as well collect new samples and repeat the MSA study.

:thanx:
hmquijano
 
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