Metrology
Metrology
The Real Dirt on Gages
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The Real Dirt on Gages

George Schuetz, Mahr Inc.

 

The human tendency is to blame the gage or instrument for an incorrect measurement when more times than not the trouble lies somewhere else. Measuring is subject to many variables that need to be eliminated before we finally take the gage to task, although some gage designs do invite trouble. So it's good to occasionally refresh our memories concerning some of the conditions that interfere with precise measurement or that prevent the gage from doing its best work.

 

Back when a machinist needed to work no closer than to .010", the question of gage accuracy cut much less ice. But with tolerances aiming at .000010" as they are today, the slightest error in measurement, no matter what its cause, looms up big in proportion.

 

Plain old dirt is probably the biggest contributor to these errors as far as just the gage is concerned. Dirt attains such a high ranking because it's -- well -- it's everywhere. Dirt is so common and everyday, we forget it and often ignore it.

 

As a "quickie" experiment to observe how detrimental dirt can be to accurate measurement, leave your high-resolution digital micrometer out of its box for a few hours, sitting next to a machine tool with the spindle turned back about a quarter inch. Then check it for zero setting. Next clean the anvils on a slip of clean paper in the customary manner and blow the lint away. Check the zero setting again. Some dirt accumulation is certain. If you can see this type of error with dirt on a digital micrometer, imagine what the effect is when working to microinches.

 

We did a similar experiment with a plain master disc size .7985"XX. We set up a precision length machine to this figure and let the master stand unprotected for a number of hours on a workbench out in the shop. We brought it into the temp control room and let it cool off, but we took special pains not to touch it or in any way clean the natural dirt off it. Then we measured it. The length machine measure was more than .0003" larger than the nominal .7985" setting.

 

Then we carefully thoroughly cleaned the master with alcohol and measured it again. This time the reading was only .000004" from nominal. Finally the now clean master was wiped "clean" again, this time in the conventional workman's manner with the palm of the hand. (Nobody, but nobody, ever seems to take a precision measurement without first wiping off the workpiece, the gage blocks, or the gage anvil with his finger or the palm of his hand.) When we measured it again, the reading had gone up to .000013". The master had lost half of its normal tolerance thanks to that wipe off with the palm of the hand.

 

To compound these fairly clean "dirt" errors, just think about the assortment of gages you see on the grease and grit-laden apron of the average machining center. The first thing a service man does when he starts on a repair job is to clean the gage, be it an indicator or the most intricate automatic gage we make. Simple cleaning is often about all the "repair" a gage needs.

 

One of the great advances for gages, especially hand tools and digital calipers over the past 10 years or so has been IP ratings. The IP rating defines how susceptible the gage is to liquids and physical contaminations. Often, photos will show the hand tool covered with coolant, being used to measure a part in a similar condition. My heart almost stops when I see these application photos, as they tend to give the impression that because of the IP rating the gage has suddenly become immune to the effects of dirt on the measurement -- a false sense of security for sure. Certainly the gage can withstand the affects of dirt and liquids on its operation, but dirt will certainly affect its measuring performance.

 

Even the lowly caliper is subject to the influence of dirt, as it often contains some pretty big particulates, which can deflect caliper jaws. So the moral is that every piece of precision measurement equipment, from the caliper to the lab gage, needs to be cleaned properly whenever dirt is in the area -- which happens to be everywhere.

To illustrate IP Ratings, photos will often show a hand tool covered with coolant, being used to measure a part in a similar condition. These tend to give the impression that because of the IP rating the gage has suddenly become immune to the effects of dirt on the measurement -- a false sense of security for sure.