calibrating ring gages

[george frick]

i was wondering how your company calibrates master ring gages? we check our ring gages on our zeiss cmm's and use that data in our calibration files. i have always ascertained that this was putting the cart before the horse. that a cmm should never be used to calibrate ring gages and that master ring gages should be used to verify the cmm not the other way around. using the 10 to 1 rule i don't see how you can use any cmm to calibrate master ring gages. i believe we should be sending our ring gages out to a lab like we do our gage blocks but this always falls on deaf ears. what do you think?

[eacosta]

you are right George,

unfortunate that kind of practices is very common on many companys

many times managing decisions are made thinkin only in $$$.

10 to 1 rule is very simple, but not for managers

wish yo luck!

[John Kingston]

I agree George. You can't really get form and size as well with the cmm. This has been deeply discussed in the old forum. I'm sure Matt, Hilton, and others will jump in on this discussion.

[Wes Cisco]

Yep, yep, yep, it is a downright boneheaded practice and is therefore all too common. You know the drill George, just run the program, post the results, and smile when they hand you your paycheck. Unless they want you to sign some kind of calibration/certification document saying you have verrified tracability back to the national standards. . . Then you have to stand up and say NO! There is plenty of technical proof and legal precedent to back you up on that. I have not seen a reminder on this forum lately, so this is a good time for one: People can and do go to PRISON for false, altered, and neglegent inspection practices and results. It is more likely to happen if you work for the government or a company that primarily supplies the government, but it can happen with almost anything you can get paid to inspect. If your boss asks you to do something stupid like "calibrate" a ring gage on the cmm and you are just handing him a report that you did not sign and you told him about the 10-1 rule, then you have no problem. You are just doing the job your idiotic employer is paying you to do. But if you are signing off to something you know has not been done, or done properly, or results you know to be inaccurate. . . it could land you in jail. Most of the time nothing will happen, but if for some reason the document you signed becomes an issue in a court of law. . . you just might want to save yourself some grief and find a different job instead of signing off, wink wink nudge nudge. Disertation done.

[bw_bob]

We do the same thing here. I have told them, but it falls on deaf ears. We reject gages with a tighter tolerance then the accuracy of our CMM's

[craiger_ny]

Maybe they have no idea what kind of tolerances ring gages are to be held to. 10 to 1 is industry convention for a sound measurement system but gage maker's tolerances are way beyond that. Here is a little executable I got from Vermont for gage tolerance. It uses the 10% rule but also displays the tolerance of the ring. I'd put these type of numbers next to data on the accuracy of the CMM. The impression I get of your leadership makes me believe it will be fruitless but not for a lack of trying. There is always the library.

Tolerance_Calculator.zip

[gage guru]

OK we do the sme dumb thing here as well. There is a way that it can be done properly with a CMM but for a person's time to do it properly it is cheaper to actually send out - shich we do not do to save money ha how ironic). See attachments.

[Hilton Roberts]

We calibrate master ring gages ( XXXX ) on an internal comparator (SIP ) and standardize the machine with a gage block stack.

Lower level rings (XX and lower) are compared with XXXX rings.

I know that Oak Ridge lab uses a Moore special CMM to calibrate gage blocks to NIST levels and have the data to support the claims.

To my knowledge, CMMs are just not accurate enough to calibrate ring gages.

Remember that if you do not really know the uncertainty of your measurements, you really should not be in the business of certifying ring gages. If you know the uncertainty of your CMM, I think you will find it is not capable at the level of precision you need for even a XX ring.

Just my 2 cents worth......

Hilton

[Sl33stak]

Can someone POST the 10 to 1 rule? I know I know, I have much to learn.

[cmmguy]

I don't know if it is so much a problem with accuracy(except xx, xxx, & xxxx), it is more that the CMM is just not the right method to check diameters. AHilton pointed out and many already know, the method used by ALL labs is a internal comparator that measures with TWO diametrically opposed points that are set with gage blocks or ring gages of a higher grade. In fact, I doubt highly that there is anyplace that is capable of truly calibrating QuadX rings.

I recall a round robin blind study done by the quality manager of a major aerospace hydraulics company that yielded dismal results. 5 different sized XXX rings were circulated among the top three fixed limit gage manufacturers and two different NIST facilities. The results were mind expanding

[John Kingston]

Can someone POST the 10 to 1 rule? I know I know, I have much to learn.

Typically, your gage should resolve to 10% of the tolerance of what you are checking. So, if you are building a gage, it's tolerance is 10% of the part tolerance.