Class X & Class XX Master Ring Gauge Verification using a Mitutoyo Legex 707 CMM

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bigherm95

Currently we send out all our ring gauges, to a vendor that uses a Pratt and Whitney Universal Labmaster.

We only use Class X and Class XX Master rings at our plant.

One of our managers is on a push to save money, and time, and wants us to verify the rings in house, using a Mitutoyo Legex 707 CMM, that is currently being used to check incoming parts. Nowhere can I find a published accuracy of the Legex, not even on the calibration cert.! However according to Mitutoyo's website a Legex 774 CMM has an accuracy of ± .45 microns, which i am assuming, would be more accurate than a 707?

According to all standards I can find on Class XX Master Ring 20mm their Tolerance is ±.5 micron, and since it is a Master ring the tolerance is divide, to +.25 micron, and -.25 micron. And we use to 4:1 for our Calibration Standards

I know that by my calculations that the Class XX ring is more accurate than our CMM. But our management doesn't seem to get it. They want some other type of proof.

I investigated calibration labs to find out what the use to verify ring gauges. Almost all use, Pratt and Whitney Labmaster, Helios ComSupra, or equivilent interferometer. Of these, according to websites the Pratt and Whitney LabMaster is the most accurate.

Finally the Questions:
1. Are my numbers correct?
2. Is there a statement, in NIST, 17025, or other publication that states that ring gauges can not be verified with a CMM? Or is there a Nationally accredited method to verify ring gauges?
 
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Frank T.

Here is a statement I came across awhile back.

Evaluating the suitability of a CMM for an application traditionally revolves around determining measurement uncertainty relative to the workpiece size and required tolerances. Generally the CMM should be 10 times more accurate than the tolerance it will verify.

This probably didn't answer your question but, thought I would still post it.
 

Hershal

Metrologist-Auditor
Trusted Information Resource
The best answer is to send your master rings to Oak Ridge National Lab, for this type of measurement they effectively are NIST, since they are a National lab.....

The CMM you describe will have at best a 1:1 ratio, which means your measurement uncertainty will be quite high.....send the rings to the National lab or to a lab accredited to ISO/IEC 17025, with the appropriate scope.....

Hershal
 
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