How to keep gage block sizes unknown for R&R factors of MU

[apestate]

Thank you all for the fantastic responses. I must admit that this thread was more of a cry for help on a general slew of issues more than what was specifically titled.

You guys are right on target with the response to the accuracy issue. Your recommendations will be incorporated into a test and training program for internal calibration staff, and we'll just have to swallow our pride and do it right.

I was extremely interested in Hershal's post. We obviously don't have the capability to calibrate TESA-hite digital height stands, 74" vernier calipers, Sunnen bore gages and setting fixtures, and these things will be outsourced to proper metrology professionals in the newly revised calibration system. These were done IN-HOUSE in the past, which is just irresponsible.

However, we are a fairly large and competetive manufacturer. We have many basic hand tools that should be within our capability to calibrate properly. It's a core competency of manufacturing companies to measure their product and operate their tools, and not being able to calibrate them is a weakness that spreads throughout the entire measurement system.

Hershal, you addressed something I have been wanting to inquire about. Is it really possible to have gage blocks poorly wrung with a film as thick as .00005?

[Hershal]

Actually the 0.00005" figure is commonly used and accepted in metrology labs. You may also double check in The Metrology Handbook, available from ASQ, but I believe it provides the same figure.

That can be a good justification for asking for larger blocks.

Hope this helps.

Hershal

[apestate]

Hershal, and to other metrologists

In regards the .00005" uncertainty contribution of wringings...

If this uncertainty contribution can be applied to the measurand, is the same contribution applicable to the measuring equpiment, in other words to the wring between the measurement anvil and the gage block surface?

I've often wondered about this when calibrating Mitutoyo and various Chinese off-brands which have extremely flat anvils. When I apply constant pressure and repeat measurements, the micrometer will come down and make contact, reading about +.0001, and then seat down right onto target. It seems like the anvil meets a film of surface tension and that it takes longer to push this air from between the anvil and the gage block. This may not be what I'm observing but it's very convincing!

In the case of this one wringing (since the .100 gage block is stuck to the bottom anvil) I could certainly see an expanded uncertainty of +/- .000029 being applied to the final measurement.

The question that raises in my mind applies when I'm measuring a stack of 2 blocks with a plain anvil micrometer, 4 wringings. The uncertainty contribution due to gage wringing, 4 x .00005, expanded uncertainty .000115". Is this reasonable?

How can this be reduced? For assessing calibration technicians, I think a total uncertainty of that amount would be large. Using just one block lets the operator know exactly what number to come up with.

It's hard to take an assessment of a person's skill and not know precisely what you're doing to measure it.

I will of course be doing research, but I will watch for a response.

[Hershal]

You have a good reason to ask for larger blocks. A Type B contribution (collectively) of 0.000115" from wringing is not unusual if multiple blocks are used. Reducing the number of blocks in turn reduces that contribution, but it is still there as you correctly ask about.

Hope this helps.

Hershal