Please clarify the Rule of 10 to 1 - AND - What is the ndc number?

The important thing is that specifications are based on what will work, and that everyone understands that it's just as bad to reject "good" material as it is to accept "bad."

Rounding should be used when rounding is a useful concept. I don't expect to find fractions of cents reported on my bank statement or pay stub, because I can't actually put my hands on a fraction of a cent. We shouldn't confuse academic mathematical expediency with manufacturing precision. The former might be useful in developing specifications, but once the specs are set,"no greater than x," means that I reserve the right to reject anything greater than x. There has to be an explicitly set limit.

Hi all,

does it mean, if i carry out a GR&R study with a sample that has distinct value, let say 10 distinct samples of data, my ndc will surely be >5 right?

But the argument here, which is posted by one of our external auditor is that, the data should be of homogeneuos sample. thus, the average mean of a sample should not be very big difference. or, i can just perform the R&R study with any heterogeneous data and get a good ndc instead?

second question, can anyone elaborate more on the statement in pg 46 of the AIAG MSA ref manual (3rd edition) which says "A measurement system will have adequate discrimination if it's apparent resolution is small relative to the process variation. Thus a resolution to be at most one-tenth of total process six sigma std dev instead of the traditional rule which is the apparent resolution be at most one-tenth of the tolerence spread."

thanks in advance!!!

Hi,
I know this is an old question, yet I appreciate someone can give me a hint.
1). Assuming there is a measurement with Tolerance of 12 (+/-6) cm. According to 1:10 rule, the readable unit should <= 1.2 cm, so that I can be convinced the resolution is okay.

2). Now assuming my process variation of measurement is happen to be 12cm (UCL-LCL=12cm), how do I be convinced that ndc >=5 is okay? eg. 12 can be devided into at least 5 categories (12/5=2.4cm)

In this case, can I say my measurement resolution is acceptable if ndc is greater than 5? But as u see that 1:10 rule is not fulfilled (2.4 > 1.2 cm).

Appreciate for clear explanation.
Clouds

To my understanding you are trying to relate apples with oranges, it does not work that way.

1) the 1:10 is NOT a rule, it is an good engineering guide for initial gauge selection. But if you have a good capable process, 1.67-2.33+, you probably will need 1:20 to distinguish NDC regions.

2) Very wrong, the NDC relates directly to part Variation (process capability) and GR&R and is NOT simply related in the manner shown to a tolerance band and as such your assumptions are ill grounded.

Sorry to pour water on your assumptions.

Re: Please clarify the MSA factor of % Tolerance

I am getting doubts from my employer about the results of Gage R & R and the less than or equal to 10% showing an acceptable gage system.

The current MSA (third edition) has nothing in the Average and Range method for Gage R &R with the Tolerance factored in---to see a 6.4% Gage R&R in the % tolerance but incorrectly determine that 16.8% % of total variation pushes the Study as unacceptable.

Who, here, has proof that the R&R Tolerance factored in is the most effective method? I can complete and ANOVA but have spent over 15 years with the R&R based on part tolerance (process variation), Equipment variation, and appraiser variation.

Am I wrong to reassure my employer about this %Tol: 6.4%?
Incidentally, the EV is 6.4%. From my experience the Study shows a good result.
Send me facts I can relay, PLEASE!
Rick, Illinois

Your question is unclear. Do you mean that the P/T Ratio is 6.4% and the %GRR is 16.8%?

The more capable your process, the greater this discrepancy between the two. What is this gage used for? %GRR is only used for gages used for process control or for statistical studies. If the gage is used strictly for inspection, use the P/T Ratio.

Also, a max %GRR of 10% is usually only required for critical characteristics. Most other characteristics can go to 30%. Is the 10% requirement customer-driven or is it an internal requirement? If it is internal, you should relax it to 30% unless it is critical.

In addition, I recommend that you review some recent threads in this forum concerning an article written by Donald Wheeler. This article discusses erroneous calculations in the AIAG MSA method coupled with the arbitrary 10% criteria that result in disqualifying acceptable gaging systems.

Here's the study, Miner.
The diameter is critical +/- .0005."
I call the gage, a CMM, acceptable at 6.4% R&R.
How significant is Total Variation %?
Rick, Illinois

The %Tol (P/T Ratio) = 6.4%. This is the measure for a gage used for inspection. As such this CMM is perfectly suitable for inspection.

The %TV (%GRR) = 16.4%. This is the measure for a gage used for process control (SPC) or for statistical studies such as capability studies. At 16.4% the gage should be adequate for non-critical dimensions unless you have a customer-specific requirement of 10% max on all dimensions.

What do you use the CMM for, inspection or SPC?

The CMM is used for inspection (First article, correlation checks, etc.).
The whole need for the Gage R&R is for Operational Qualification purposes on a Class III Medical Device project Validation here.
Can I safety present my customer with the Study @ 6.4 % R&R?
If not, please explain why not.
Thanks again,
Rick Tompkins

If the CMM is used for inspection, the P/T Ratio is the appropriate measure to use. In your report, P/T Ratio = %Tol = 6.4%. Unless your customer specifically states the other measure, I recommend reporting 6.4%.