Checking Correlation Between Two Similar Gages Measuring the Same Parts

Jim Wynne

Staff member
Admin
#11
There's been some interesting and helpful discussion here so far, but I'd like to inject a word about bore gauges in general, which is just to say that unless you really need to know something about the interior shape of a hole, it's best to avoid using them. They are have notorious repeatability--and often reproducibility--problems, and require a good deal of experience and development of "feel" in order to be used with confidence. Not only that; often the actual measurement values are not as important as just discerning the lack of cylindricity present in the hole. If consistent shape is important, to a certain degree actual size may not be as critical.

In short, never use a bore gage when a pin will do.
 
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#12
In short, never use a bore gage when a pin will do.
Amen and add to that trying to use a CMM.

The biggest battle royale I ever was involved in was

bore gage vs. plug gage vs CMM.

It made Saturday night rasslin look tame!

PS wanna buy some bore gages, we have lots in boxes....
 

Miner

Forum Moderator
Staff member
Admin
#14
Friends,

Have you considered looking at the definition and verification of "Rule 1" in ASME Y14.5M-1994?

Stijloor.
The problem is that Rule 1 is more than a little difficult to apply in the real world. Not that it isn't valid, but nothing short of 100% scanning and mapping of the entire 3D surface will tell you whether you actually meet it.

Most practitioners do not have the resources in either equipment, manning or time to do this.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#15
Also paired t tests are not recommended for gage R&R - the intraclass correlation coefficient is inteded for use with measurement devices and methods. the standard deviations that are tested by a t test and the intraclass correlation coefficient are different. The paired t will only really only tell you if there is a bias between the 2 gages. it can't tell if you it has any practical importance. and if there is no bias there may still be still be no match between teh 2 gages but you'll never see it: if one ahs good repeatabilityand the other is lowsy.
Like wise, simple regression will totally miss bias as the correlation is based on variation about the best fit line and perfect measuremetn error will have a 1:1 line.

What I do is:

FIRST establish that teh gage is appropriate fo rteh intended use of teh part (see Jim's commment about pin gages and think about the difference between flow thru a hole (cross sectional area) and fitting a solid object (effective min diameter for 'egg shapes' or tri lobular shapes)
SECOND: each individual gage is testeed for repeatability; if a gage isn't repeatable to itself the bias study to a second gage is irrelevant. If teh 2 gages will each be used by different operators than a reproducibility study of each gage by the operators that will use them is also in order.
THIRD: I then perform the bias study.

I use the same parts for the R&R and bias studies...and I never use 10. we are tryign to estimate a standard deviation (measurement error) and standard deviations estimates are fairly inaccurate at n = 10. better to use 30 parts.

I also plot all the data on a scatter diagram with a 1:1 line drawn on it and teh X and Y axes have the same scale. MUCH more informative than JMP or Minitab output....

even better - get yourself a copy of Wheeler and Lyday's "Evaluating the Measurement Process" money well spent!
 
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