Gauge Blocks

This is chapter and verse:

**LINK**

5.2 Preparation and Inspection
It is essential that the gauge blocks being calibrated, as well as the standards being used, be clean
and free from burrs, nicks and corrosion if the calibration is to be valid. Therefore, it is necessary to
institute an inspection, cleaning and deburring procedure as part of every calibration. Such a
procedure will also protect the comparator working surface and the diamond styli.
5.2.1 Cleaning Procedures
Gauge blocks should be cleaned in an organic solvent such as mineral spirits. Solvents such as
trichloroethylene or benzene should be avoided because of health risks involved with their use.
Freons should not be used because of environmental hazards they pose. The bottom of the bath
should be lined with a waffled mat of neoprene rubber to minimize scratches and burrs to the gauge
block surfaces caused by grit or metal to metal contact.
Rectangular blocks should be immersed in the bath and a soft bristle brush used to remove the layer
of protecting oil or grease from all surfaces. Lint-free towels should be used to dry the blocks. A
second cleaning using ethyl alcohol is recommended to remove any remaining residue. Again, wipe
with lint-free towels.
Square type blocks are cleaned in the same manner as the rectangular blocks except that the center
hole needs special attention. The hole must be thoroughly cleaned and dried to prevent "bleeding out" of an oil film onto the contact surfaces and supporting platen when the block is positioned
vertically. A .22 caliber gun cleaning rod makes an ideal instrument for cleaning the bore of blocks
in the size range of 2 mm to 500 mm. A solvent moistened piece of lint-free paper towel can be
pushed through the hole repeatedly until all visible residue is removed.

Neil

It may be chapter and verse, but possibly from the wrong songsheet.

The quotation above relates to the preparation and cleaning of gauge blocks for comparative calibration using an electro-mechanical comparator with LVDTs. In the comparison process individual gauge blocks are compared with a block of known length. Wringing is not mentioned, or used in the comparison process.

Appendix B in the link given makes for interesting reading. The tentative conclusion is the wringing force is due to a liquid layer. And the thickness of this layer is accounted for in the calibration of the block.

Regards,

Andrew

LVDT = linear variable differential transformer - a pretty nifty linear displacement sensor capable of resolving down to nanometres, although I used them on the more prosaic task of measuring suspension ride height on racing cars.

I was always taught that they should not be oiled but used dry. The holding force is due to the very fine finish which is why wringing works. Oil will give a false reading and if I remember correctly from my apprenticeship a thin film of oil can take the equivalent of 40 tons per square inch to squeeze out. They will hold together with oil because of the viscous effect and tackiness of the oil but won't give a true reading.

Various sources suggest that sets should include pads, one for oiling for storage, and one for celaning off teh oil prior to use.

It's interesting that rubbing on a stone followed by a check on an optical flat is the approved way of reconditioning burred blocks!

Neil

Two slips wrung together WITHOUT oil give this result:

Wrung together with oil give this result:

The difference is there as can be seen - with oil we get +.00025" approx. I repeated this five times and got similar results each time.

Rik

Trouble is, Rik ... the Dial Test Indicator is not really a suitable tool for checking the thickness of Slips; so I'm not sure what your results actually demonstrate.

MichaelG.

Edited By Michael Gilligan on 01/11/2014 14:23:27

Andrew,

Did you know there are (and have been for a while, just not often mentioned) analogue output inductive sensors? They are more robust than LVDT's although they don't have the same resolution (as an aside, the resolution you mention is probably the sensor you used rather than the LVDT as they have infinite resolution only limited by the signal to noise ratio in the system/amplifier etc). They also do not have the same stroke range (measuring distance) as some long stroke LVDT sensors and they require a metallic target. 

Mark

Edited By Mark C on 01/11/2014 15:03:33

.

Not for the first time on this forum: I think we have more than one interpretation of apparently "obvious" descriptive terms [in this instance, those terms are "dry" and "oil film"].

Careful reading of Appendix B in the NIST document [referenced above] should clarify the matter.

... it's only a couple of pages; unless you follow all the references

MichaelG

Yes, what may appear to be dry to us, would indeed have a very thin oil film.

Mike C

Michael - If this method of measuring is good enough for the inspection department at Cincinnati Milacron machine tools then its good enough for me!

As for what the results demonstrate, I would have thought that was perfectly obvious.

Rik

I was taught that gauge blocks wrung together by air pressure, always believed that. No doubt someone will know better.

.

Rik,

Based on your pictures, I presume that you have measured the thickness of your applied oil film.

With the greatest respect: I can see no evidence that you have measured the thickness of the slips.

Please ... before replying, read Appendix B of the NIST document, and note my point about the differing interpetations of apparently simple terms.

MichaelG.

.

Gordon,

At the risk of labouring this discussion ...

NIST knows better, and has kindly given us Appendix B of the document referenced above.

MichaelG.

Yep read appendix B. Im still happy with the term dry as described by my teachers!! Appendix B appears to be talking about a surface that most definitely looks and feels dry in practice.

PhilH

.

Exactly my point

Perhaps we could put it to bed now.

MichaelG.

Michael - I tried to put it to bed but it's not completely exhausted.

"With the greatest respect: I can see no evidence that you have measured the thickness of the slips."

The object of the photographic exercise was to demonstrate the thickness of the oil film using the DTI as a comparator. The thickness of the two slips is merely an irrelevance as the same two slips were used for the checks.

"Please ... before replying, read Appendix B of the NIST document............"

Why?

Rik

.

Because it will describe what is meant, in the context of slip gauges, by an oil film.

[the thickness of your applied oil layer is, by comparision, like the cheese in a sandwich ... or whatever alternative takes your fancy]

MichaelG.

Edited By Michael Gilligan on 01/11/2014 18:45:58