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GR&R Acceptance Criteria

#1
Hello all,

We've been challenged on our acceptance criteria for GR&R and a group of internal SMEs designed the following criteria:

•% Tolerance - <10% (above 10 to 30 is marginal, above 30 is unacceptable)
•% contribution - <10% acceptable, above 10 unacceptable (or nDC ≥5)

We have to meet both in order to have a successful GR&R.

They say %Contribution is the statistical equivalent for nDC, and they've included that requirement in order to know if the equipment (a MicroVu) is able to distinguish process variation.

Even though our aim is to discern between good and bad parts they keep asking for the system to also capture process variation, I think that's our main problem.

I'm trying to find information on the relationship between %Contribution and nDC and if %Contribution is truly an acceptance criteria tied to a GR&R which aims to discern between good and bad parts (%Tol).

Thanks
 
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Ninja

Looking for Reality
Staff member
Super Moderator
#2
No comment on your direct question...others here can handle that better than I...
I wonder, however, what your tolerance is? and what your normal process variation is?

I've used the MicroVu extensively...and to miss NDC>5 on a machine that can discern 50 millionths never came up in my work...
So I wonder if your samples used for GRR cover the spec range? Or is your spec range THAT tight that 50millionths is on the edge?
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
These ‘requirements’ aren’t horrible, but they are ‘blind’. Your SMEs should be able to answer your questions in simple terms, but since they have perpetuated the myth of the 10% of the tolerance I would say that they arent’ Experts at all - they are have simply memorized a limited section of a Guage R&R standard. If they were experts they would know that %tolearnce is a bogus mathematical calculation that is just fake math. Also the 10% limit is an arbitrary line - there is no reason for drawing the limit at 10% or 30%. The 10% of tolerance is a fairly conservative limit such that if you meet it your gauge system is probably pretty decent for acceptance testing. The difficulty is that it will unenessarily reject systems that are fine. My real issue with it is that it is mathematically incorrect and represents a lack of critical engineering thinking. People just accept it because that’s the way it is.

As for the 10% of product variation for SPC - that is total crap. IF the calculation is mathematically correct the %variation will be a real number but of they don’t use the correct formaula it will be just more fake numbers. It is essential to understand the actual contribution of measurement error to the product variation to properly monitor your processes through ‘SPC’. But there isn’t really a limit on that contribution - it will simply drive what rules and what sampling frequency you employ.

I suggest reading Miner’s blogs and my paper on MSA in the resources section of this forum. Then read the references cited by Donald Wheeler...then you can be a subject matter expert...
 
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