Gage R&R %Tolerance NDC

D

dsagario

#1
Using %Tolerance on one of my GR&R study, I was getting a value of below 5 computation of NDC. My %GR&R was almost equal to one. Is the result of my study unacceptable overall or not? What is the significant of my NDC with respect to %tolerance computation?
 
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Miner

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#2
ndc is closely associated with % Study Variation, but has no association whatsoever with % Tolerance.

Acceptability of the gauge depends on the use:

  • Process control: ndc, % Study Variation (or % Process in Minitab)
  • Inspection: % Tolerance
  • Dual Purpose (Inspection and Process Control): ndc, % Study Variation (or % Process in Minitab) and % Tolerance
If you have not found them , I recommend reading my blog on MSA.
 

bobdoering

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#3
ndc is closely associated with % Study Variation, but has no association whatsoever with % Tolerance.

Acceptability of the gauge depends on the use:

  • Process control: ndc, % Study Variation (or % Process in Minitab)
  • Inspection: % Tolerance
  • Dual Purpose (Inspection and Process Control): ndc, % Study Variation (or % Process in Minitab) and % Tolerance
If you have not found them , I recommend reading my blog on MSA.
This is a great overview, however I will also add that 5<ndc<10 is only good for inspection, and that is should be greater than 10 for SPC (assuming the specimens statistically represent your process distribution.)
 
D

dsagario

#4
ndc is closely associated with % Study Variation, but has no association whatsoever with % Tolerance.

Acceptability of the gauge depends on the use:

  • Process control: ndc, % Study Variation (or % Process in Minitab)
  • Inspection: % Tolerance
  • Dual Purpose (Inspection and Process Control): ndc, % Study Variation (or % Process in Minitab) and % Tolerance
If you have not found them , I recommend reading my blog on MSA.
Thanks Miner,

I am using the instrument only for inspection that's why I utilized the %Tolerance for my R&R study.

ndc is closely associated with % Study Variation, but has no association whatsoever with % Tolerance.
What would be the best reference or documents to show our auditors that NDC is insignificant on %Tolerance study. I can't see this on the MSA Manual. Can you please clarify what's the rationale behind the insignificance of NDC on %Tolerance?

Second, does the acceptance criteria for %total variation applies also to %tolerance like the %GR&R(<10%, 10-30%, >30%) ? what would be the difference?

Please enlighten.
 
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Miner

Forum Moderator
Staff member
Admin
#5
What would be the best reference or documents to show our auditors that NDC is insignificant on %Tolerance study. I can't see this on the MSA Manual. Can you please clarify what's the rationale behind the insignificance of NDC on %Tolerance?
The MSA manual is pretty clear that ndc is used as a measure of the adequacy of the gauge for process control. However, another approach would be to show the structure of the metric itself. ndc is simply a ratio between the process variation and the measurement variation. Tolerance is not part of the equation. While % Tolerance is a ratio between the measurement variation and the tolerance. Process variation is not part of the equation. The only time both would be used would be for a dual purpose gauge (inspection AND process control).

Second, does the acceptance criteria for %total variation applies also to %tolerance like the %GR&R(<10%, 10-30%, >30%) ? what would be the difference?
% Total Variation = % Study Variation. The criteria is the same (<10%, 10-30%, >30%).
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#6
The MSA manual is pretty clear that ndc is used as a measure of the adequacy of the gauge for process control. However, another approach would be to show the structure of the metric itself. ndc is simply a ratio between the process variation and the measurement variation. Tolerance is not part of the equation.

% Total Variation = % Study Variation. The criteria is the same (<10%, 10-30%, >30%).
This is why it is important when performing a gage R&R to present a true representation of the process variation to the study for the results of the study to be accurate. If you can not do this, and choose to use % Tolerance instead, your ndc will not be accurate.

The point of ndc is to give you an idea of the statistical (not readable) resolution over the range it will be used (supposedly the variation you supplied). ndc of 4, for example, means you are using a gage with a much resolution as a typical fuel gage in a car. AIAG is trying to say that it is not enough - although it is more than an attribute gage.
 
D

dsagario

#7
This is why it is important when performing a gage R&R to present a true representation of the process variation to the study for the results of the study to be accurate. If you can not do this, and choose to use % Tolerance instead, your ndc will not be accurate.
.

You're correct Bob, but another problem that we encountered with the %Total variation GR&R, is the difficulty to obtain samples that would represent the whole spread of the tolerance. Since the gauge was merely used to inspect the incoming leadframes for the hole dimension with respect to the tolerance, we decided to use the %Tolerance.
 
#10
Can anyone tell me why, in the AIAG manual/4th ed. page 129, it shows an example of ANOVA with ndc=4 and states "Measurement System is acceptable for process control analysis."?
I thought this was a clear distinction that anything less than 5 ndc was basis for NOT approving a system for process control.
 
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