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

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#### David Drue Stauffer

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.

D

#### David Drue Stauffer

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

##### Hunkered Down for the Duration
Staff member
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!

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#### komlossy

Problem solved? I don't think so...

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%?

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#### Bill Ryan - 2007

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

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

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#### Bill Ryan - 2007

Sebastian said:
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

Trusted Information Resource
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

K

#### komlossy

Sebastian said:
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
Really?

MSA page 73: "For Process Controll situations where the measurement result and decision criteria determine process stability [...] (i.e. SPC) [...] the availability of samples over the entire process range becomes very important"

The reason is simple: you will use study variation to estimate the unknown process variation. If you select only good parts (or only bad parts) the variation among the damples will be less than the varaition of the process...

Let me be more specific about my problem.

MSA page 73: "For Product Control situations where the measurement result and decision criteria determine conformance or non conformance to the specification (i.e. 100% inspection or sampling) [...] the assessment of the measurement system is based on the feature tolerance (i.e. %GRR to TOLERANCE)"

this is in line with page 116: "If the analysis is based on tolerance instead of process variation [...] %EV, %AV, %GRR and %PV are calculated by substituting the value of tolerance divided by six in the denominator of the calculations in place of the total variation TV"

This is clear to me. But what if we have no tolerance range, because there is only one tolerance limit? Maybe I go and check the SPC topics, they should face the same problem when calculating CPK...

#### Sebastian

Trusted Information Resource
komlossy said:
MSA page 73: "For Process Controll situations where the measurement result and decision criteria determine process stability [...] (i.e. SPC) [...] the availability of samples over the entire process range becomes very important"
I have to
Yes, You are right. My understanding of MSA was "to check measurement system only and not dimension tolerance".

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