Calibration of ring gages with a CMM

M

merrick65

#1
I've been asked to add the calibration of ring gages to our scope. Is it valid to calibrate a class X (or lesser grade) ring gage with a with a CMM? I don't think we would be able to do this. The CMM is a Tesa Micro-Hite3D with a resolution of 0.00001". The repeatability of the CMM is 0.000078" with an uncertainty of 3.5 micrometers. Based on the repeatability of the CMM it looks as though we would only be able to do class Z & ZZ. We might be able to do class X above 2.51" and Y above .825". But some opinions (boss) are that we go by the resolution of the CMM to determine which class of rings we can do. Any input would be helpful.
 
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bobdoering

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#2
I've been asked to add the calibration of ring gages to our scope. Is it valid to calibrate a class X (or lesser grade) ring gage with a with a CMM? I don't think we would be able to do this. The CMM is a Tesa Micro-Hite3D with a resolution of 0.00001". The repeatability of the CMM is 0.000078" with an uncertainty of 3.5 micrometers. Based on the repeatability of the CMM it looks as though we would only be able to do class Z & ZZ. We might be able to do class X above 2.51" and Y above .825". But some opinions (boss) are that we go by the resolution of the CMM to determine which class of rings we can do. Any input would be helpful.
A key question is what is the tolerances you are trying to measure with the rings. Were they bought at those classes because you need them, or because they were handy. Remember, calibration at the ring gage tolerances require technique - I like to refer to it as "white glove" calibration. You can't just whip them into a CMM, much like one may calibrate a caliper with gage blocks. They must be stabilized to temperature. clean, not influenced by the temperature of the person's hands touching them. But, that is if you need that tight of tolerance.

By the way, you cannot use the resolution of any device to determine anything. Just because its numbers show up, doesn't mean it measures that accurately in use (ever use a calculator to divide up a tip at a restaurant? What about those fractional cents?) That is why we do gage R&R to find the real resolution in the hands of the practitioners on the parts in question. For example, the CMM may read its artifact that well, but, a part wobbling in a crappy fixture (I would like to say I am exaggerating for effect, but in many cases it is not an exaggeration) will not truly be that accurate. How were you going to fixture the ring? Your "Bat-sense" (retro reference) is probably right - it may not be a good idea to calibrate ring gages in a CMM.

But, remember: the most correct answer to any question is "It depends.":cool:
 
J

justncredible

#3
You are 17025?

Can you do it, sure, well it be valid, no. Your boss seems to not understand what calibration is for. Your machine repetes at 80 millionths, it is not accurate to 80 millionths. With a MU of .035 microns your at .00014 accurate. Which for a CMM is good.

Me personally I would do it and let him explain it to a auditor or customers. It is his call. I would also find another job.
 
J

jfgunn

#4
Your basic question is the following: Can I add something to my scope with a really high uncertainty just to say that I can do it?

The answer is Yes.

You can calibrate anything with anything as long as you report the uncertainty. You could certainly report that you measured a ring gage and got a reading of 1.00000" with an uncertainty of 100uin. You could then make a statement that the ring gage meets Class XXX and that you ignored measurement of uncertainty in your pass/fail decision.

You could even get a reading of 1.00008" with an uncertainty od 100uin and say that the ring gage meets Class XXX based upon a Stringent Rejection/Relaxed Acceptance Policy where you expanded the acceptance window by an intervalequal to your uncertainty.

Even though you CAN do all of this, I do not think that you would be doing your customers a service. If I were competing against you on rings and saw this high of an uncertainty, I would make copies of your scope and give them out to all of your customers as I tried to convince them to switch to my service.

If your boss wants to use the CMM, why not use a caliper? Your uncertainty would be slightly higher at around 300uin but you could calibrate 30 or 40 rings gages in an hour. I am sure you could do them quicker with a caliper than a CMM.

Why not calibrate gage blocks too?

As a general rule, I would suggest going to A2LA or L-A-B or ACLASS or IAS or NVLAP to look at scopes of accreditation. If everyone else has an uncertainty of 4 to 20uin for rings, then you should too. Perhaps showing your boss a stack of scopes from currently accredited labs might convince him/her that the proposed capability of using a CMM for ring gages is not sufficient.

Hope this helps.
 
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