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  Measurement, Test and Calibration
  R&R on a single sided nominal

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Author Topic:   R&R on a single sided nominal
David Drue Stauffer
Forum Contributor

Posts: 25
From:St. Louis, MO63132United States
Registered: Feb 2000

posted 08 December 2000 11:08 AM     Click Here to See the Profile for David Drue Stauffer   Click Here to Email David Drue Stauffer     Edit/Delete Message   Reply w/Quote
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.

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
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posted 19 December 2000 02:26 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Anyone out there know anything about this? I haven't had to deal with this kind of R&R.

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David Drue Stauffer
Forum Contributor

Posts: 25
From:St. Louis, MO63132United States
Registered: Feb 2000

posted 03 January 2001 09:21 AM     Click Here to See the Profile for David Drue Stauffer   Click Here to Email David Drue Stauffer     Edit/Delete Message   Reply w/Quote
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.

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 03 January 2001 10:31 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
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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