Ring gages and measurement error

[[Ja...]]

I am looking for some insight into measurement error of different size ring gages measured on a manual shop floor CMM with touch trigger probe.

I have two ring gages one with Ø1.99998 and other with Ø6.69190 class X; with everything cleaned and probe qualified with sigma value of .0001 in.
Now, following are gage measurements obtained: Ø2.00001 and Ø 6.69148 repeatable within .0001 in. I don't understand why would there be a difference in error on different diameter gages.

Then another thing, with gage correction, I expected measuring a part with its ID near Ø7.000 will show measurement error of (+.0005 in) which matched up when the same part was check on Zeiss CMM in temp controlled settings. But then I expected that any OD measurement would show the opposite error (-.0005 in) and surprisingly it showed measurement error in the same direction as ID (+.0005 in). Now this has me baffled. Could this be scale error at larger diameters? Any possible explanations?

[[DW...]]

4 tenths 20 millionths is quite the difference on a class X gage. And smaller to boot. This doesn't pass the common sense test - How could the gage have more material? Ring gages used daily might only get larger by say 30 millionths in a year..and certainly not get smaller year over year.

When is the last time this ring gage was calibrated? What numbers are on the certificate?

I am not sure but the Ø 6.69148 number is very suspect, even if repeatable. Something is quite askew...

[[Ja...]]

The ring gages are calibrated every year and the numbers on cert match within the specified gage tolerance.

The problem here is the manual shop floor CMM (brown and sharpe Gage 2000) that is showing these numbers when checking the standard ring gages. 1) the difference in measurement error when measuring bigger dia's and 2) the reason not being probe dia measurement error during qualification otherwise gage correction on OD and ID would have been opposite in nature. Right now, on larger dia parts both OD and ID; CMM reading is "actual dia - 0.0005" for diameters near 7 inch range while parts near 2 inch dia's are matching up between both CMMs and with ring gage as well.

The ring gages measured on Zeiss CMM (contura 2014) in the inspection room match with the gage size within the gage rolerance. So ring gages are in good condition. SO I assume the parts measured on Zeiss CMM are correct reading. I am comparing the difference that I am getting on the shop floor CMM which just doesnt make sense in what could be the reason behind this differences.

[[DW...]]

When was the last time the manual shop floor CMM was calibrated?

[[Ja...]]

Few weeks ago. I do not have cert details yet but he said everything is okay at the end. The problem persisted even before the qualification so it is not something that calibration guy did.

[[DW...]]

Are you using the same probe for both rings?

[[Ja...]]

Yes

[[DW...]]

It would appear that the shop floor CMM is the problem.. what exactly is going on with the CMM I can't begin to guess.

If you measure other things with the shop floor CMM, do you get "bad" results? Or just with these rings?