Gauging R&R

Nope, R&R doesn’t stand for rest and relaxation, or at least not here at Zebulon Solutions, at least not this quarter–we’re busy as spit.  It’s short for repeatability and reproducibility, a mouthful to be sure (hence the acronym), specifically Gauge Repeatability and Reproducibility, or Gauge R&R (or sometimes Gage R&R).

In layman’s terms, Gauge R&R is used to quantify the variability in measurement attributable to the instrument (nee gauge) and the operator.  If pick up a pair of calipers and measure a block of unobtanium 5 times, I’d like to think I’ll get the same result each time, or close to it. That’s repeatability.  And if Todd takes those same calipers and measures that same block, hopefully he gets close to the same result I did (OK, he’ll more than likely get a better result).  That’s reproducibility.   Oddly enough, Gauge R&R is about variance not accuracy, so in theory both Todd and I could be consistently measuring a 4 cm wide block as being 10 cm.  That’s bias, which can be addressed by calibration, a whole ‘nother can of worms, as Zeb would say.

In simple terms the total variance, sigma, is related to the variance of the part and the variance of the measure via a quadratic equation.

Again, without going into some really head-numbing math, basically we want to make sure that the errors in our measurements should be << the inherent tolerance of the part.  And we’d also like that the six sigma measurement error be a small fraction of the specification.

We’re currently working on setting up a Gauge R&R plan for a really complex piece of custom test equipment. It’s not made of unobtanium but our customer has some really brilliant engineers doing some really amazing things in terms of designing it. There isn’t really an operator component, but there are at least 7 axises of variation we will need to consider.  And an equally tough challenge in terms of creating and maintaining gold units–the standards by which the Gauge R&R tests will be run relative to.

As for unobtanium, we really don’t recommend designing with it.  (see Designing with Unobtanium to find out why)

Chuck


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