MSA - GRR (Gage R&R) Requirement for CMM

Bev D

Heretical Statistician
Staff member
Super Moderator
#21
In general we must focus on what we are actually trying to estimate and what actions we will take.

for gage R&R the critical estiamte is the measurement error, whcih is expressed by the standard deviation. This comes from the repeated measures of each part. Estimating a standard deviation from a small sample size can result in large deviations from the true standard deviation. Howver, the improvement in the individual standard deviation (whether estimated by the range or the SD) from n=2 repeats to n=3 repeats is negligable. BUT if we increase the number of these repeats the average standard deviation will be less effected by a few large estimates of any individual set of repeats. So, we are better off using more parts (n=30) and fewer reps (n=2). Same number of data points, same 'burden' on the operators, better accuracy in our estimates.

Now, how many operators? it depends on the system. if there is really no operator effect then a single operator is sufficient. (be careful, some CMM measurements are more effected by how the operator 'fixtures' the part than by the CMM itself) If there is operator effect, then I typically use two or three. The estimate of the statistical variation of these operators is bogus. (in statistical language they are fixed effects not random effects) what you are looking for is if there is a difference that matters so the range of difference is where you really look. I recommend this becuase of what we do with the information. If there is a substantial difference, we will retrain the operators to 'behave' lke the best one and then we can retest each individual operator.
 
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#22
First off all, please excuse the grammar, since iam not a native english person.

For our new project here at our plant, we decided to buy an extra CMM to do an automated inprocess inspection. The cmm will beloaded by inspectors outside off the cmm. With a barcoding system the trays with the fixture will then automated loaded into the machine. The cmm will use the barcode also to choose the program.
At no time the operator will infect the inspection process. However we decided to do an Gauge R&R in acc to MSA 4th edition including appraiser and equipmet variation.
The reason we did it is to avoid unneccesary dicussion with auditors/costumers.........

The result was iimpression, or better , it was whta we expected

There is allways room for a diccussionj about it. But as long as you refer to nothing you are always vulnerable during an audit.

Since we did the MSA, we had 2 audits. There where recomendation about the MSA, but no dicussion at all.

And finaly you know what your system is cabable of
 
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