Gage R&R on a Single Sided Nominal - What is an aceptable method?

Do any of you out there know the acceptable method of dealing with a single sided nominal on the Gage R&R Study? Example: How do you introduce the numbers for the tolerance upon say a profilometer? You are looking for a minimum finish requirement of a 32 finish. Anything better than that is acceptable, so do you use the 32 as the upper end of the tolerance and give a tolerance range of 0-32? How about geometric controls such as flatness, true position, roundness, etc.? Is developing an SPC history and demonstrating capability the way to go?



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Dave S.

Since I asked this question and received no responses, I sought an answer from other sources and will share the response with you.
First, is there a special Gage R&R method for unilateral toleranced features? No. The methods are the same, but the interpretation of the data must be treated differently. "Unilateral features may produce "bounded data". Basically, that is any measurement data limited by an upper or lower value (e.g. runout, flatness, straightness, etc. where measurement values cannot be recorded lower than "0"). For example, a highly capable process could produce a diameter with a runout tolerance of .001" max that measures on the average .0002" with occasional measurement values up to .0012". This would be considered a capable process. If we tested the data it would not be normal (bell shaped curve)but appear skewed right.
When we violate the assumptions of any statistical tool, the tool becomes unpredictable and may indicate a false conclusion. What happens to unilateral data that is not normal, but treated or assumed to be normal? The measurement system appears unstable, the Gage R&R error is inflated. It makes the measurement system look worse than it is.
An advanced practitioner of GR&R understands the assumptions of the tool and remains a healthy skeptic (test the assumption). When you know that you are dealing with unilateral or skewed data - test it for normality. If it is normal data, proceed with the standard methods and evaluation techniques. If the data is not normal, you can usually transform it to act normal, then proceed evaluating the transformed data. If you can't transform, proceed with skeptic caution. You will rely on profound knowledge an practical experience with the measurement system.
Certain high-end measurement systems (circular geometry, surface finish, CMM, etc.) are best evaluated using a control chart method and measuring master artifacts on a regular schedule.

[Marc]

I want to thank you for taking the time to stop back and let us know what you found out. I assure you we all appreciate it!

[komlossy]

With all due respect, that doesn't seem to answer the question...

I have a measurement situation with one-sided tolerance, the spec. is that the parts must be less than -20 (this is an electrical characteristic). We do 100% checking, so this is a "product control" situation in MSA terms. In this situation tolerance limits should be used in the calculations. How should I calculate the GRR%?

[Bill Ryan]

Time for me to get thrashed (once again ) and learn.

Doesn't the tolerance only come in to play to give the "% Part Variation" in an R&R study? That should be a large number in a "good" R&R study. As I look at the output stats in a study, I don't see where the tolerance has any effect on the Repeatability or Reproducibility values.

I don't understand the original poster's response - what does normality of the data have to do with a Gage R&R? Even with a "naturally" bounded feature, if I measure flatness (runout, position, etc.) on a part three (or more) times, should I not get a fairly comparable reading each time? Regarding surface finish (where I believe he meant a maximum of 32), I can understand a little difficulty with interpretation due to the "nested" study as it is a "destuctive" test (that just sounds funny this morning - when you check a surface with a profilometer, you are actually improving the finish - not destroying it . Sorry, it's been a rough day already!) .

[Sebastian]

It is a second time I see, someone wants mix dimension tolerance in MSA.

If I understand MSA correct, You are also allowed to take only NG parts for analysis. See page 113 and 114 of MSA Manual 3rd Edition.

[Bill Ryan]

Originally Posted by Sebastian

If I understand MSA correct, You are also allowed to take only NG parts for analysis. See page 113 and 114 of MSA Manual 3rd Edition.

Why would you think you only use no good parts for a Gage R&R? ("NG" means "No Good"?????)

[Sebastian]

From my point of view, MSA should tell me if operator performs reccurent measurements and it is not important what kind of part he checks.
So, that's why I put this extreme example with only Not Good parts.
For sure no one use this (I guess), but it shows what is really important in MSA.
I see, some people treat this like dimensional report in PPAP and it is a reason, they put here dimesion tolerance