Gage R&R (GR&R) Brain Teaser

W

wmeister

#1
Brain Teaser for all you readers out there:

If you start a process (variable-continuous) and determine the process is capable. Then do a process capability study with which includes the production gage which shows an acceptable Cp & Cpk (lets say 2.0 - 3.0+).

Pick One:

A. Now I do a gage r&r study.
B I did a gage r&r before I did the process capabilty study
C. Don't run a gage r&r study
E. What the heck is a gage r&r

--> Tell me why!

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Miner

Forum Moderator
Staff member
Admin
#2
Re: Gage R&R Brain Teaser

The correct approach is B.

The measurement variance (s^2) is additive to the actual process variance (s^2). So, your observed process capability as shown by your study includes the measurement variation. Ideally, you should verify that the %GRR (not P/T Ratio) of the gage is low enough to provide an accurate estimate of the true process capability.
 
B

Benjamin28

#4
Re: Gage R&R Brain Teaser

I'd delegate answering the question to the lead auditor and resume reading dilbert cartoons.

If, however, the lead auditor chose other than option B.) I would put him on a conference call with Miner and let them debate the matter.
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#6
Re: Gage R&R Brain Teaser

I pick C because I really don't have the resources and time to do gauge R&R studies except for very special cases - in which case I have to do all the legwork and number crunching myself.
 
D

Duke Okes

#7
Re: Gage R&R Brain Teaser

If the process is capable, why run an R&R. The capability study includes variation due to gage and operator (if done correctly), and if the process is capable then the measurement system is not contributing enough variation to worry about.

This is an applications viewpoint, not a theoretical one. We need to put our resources where they will be most valuable, not do something because it is the typical way.
 
J

John Nabors - 2009

#8
Re: Gage R&R Brain Teaser

If I had a crappy calibration program and poorly trained inspectors I would say B and do an R&R every time a capability study was performed, but since I have a very robust calibration system and an experienced and well-trained crew, I have to go with C with one exception: if I were still incarcerated in the accursed automotive industry and I was PPAP'ing a new part.
 
C

cuadra - 2010

#9
Re: Gage R&R Brain Teaser

I agree with Miner, the answer is B.

The measurement system variation always underestimates the process capability, but in the other hand, one cannot assume that a good process capability implies the measurement system is capable of achieving the target quality objectives.

The analytical approach to the answer is simple. First, one can find the equation that relates P/T as a function of both %R&R and Cp. Second, from the equation, one can easily conclude that a capable process doesn’t always imply that a capable gage was used. A nicely centered process with a small process variation could be measured with a lousy measurement system and one can still achieve acceptable process capability indexes (Cp and Cpk).
 
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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Re: Gage R&R Brain Teaser

A nicely centered process with a small process variation could be measured with a lousy measurement system and one can still achieve acceptable process capability indexes (Cp and Cpk).
This would be true with a device with bad resolution. It would make most parts appear exactly the same - where as a device with better resolution would show more significant variation - maybe not so centered. Take a yardstick, for example... :notme:

Fact is, Gage R&R specifically answers the question: Is this the correct gage for the application? If you have not suitably answered that question before you used the gage for any reason, then you are already starting at quality level TCE. :cool:
 
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