Metrology
Metrology
Sliced Bread And The Limits Of Air Gaging
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SLICED BREAD AND THE LIMITS OF AIR GAGING
George Schuetz, Mahr Federal

"Air gaging is the greatest thing since sliced bread," a friend once told me.  And he was right — air gaging is good.  It's fast, high resolution, non-contact, self-cleaning and easy to use.  For use in a high-volume shop, it's hard to beat.  But that begs the question, "If air gaging is so good, why would you ever consider going back to contact type gaging?"

The answer is that while air gaging does provide all of the benefits listed above, it and everything else that obeys the laws of physics, has some limitations.  There are, in short, some trade-offs and for every advantage you gain in the measuring process with air, you will have to pay the price and sacrifice something else.  The real question is, "What are those limitations and how can you best work with them?"

Air gaging gives you a fast measurement device that provides superior reliability in the dirtiest shop environment.  But you give up things like measurement range and a clear delineation of surface.  Air gaging has about 10 - 20% of the range of a typical electronic transducer with similar resolution.

The response of air to surface finish, however, is more complicated.  Think of an air jet.  The measurement 'point' is really the average area of the surface the jet is covering.  Now consider the finish, or roughness, of that surface.  The measurement point of the air jet is actually the average of the peaks and valleys the jet is exposed to.  This is not the same measured point you would have if a contact type probe is used.  This difference is a source of real gaging error, and one which is most often apparent when two different inspection processes are used.     

For example, let's say we have a surface finish of 100µ" on a part, and we're measuring with an air gage comparator and two-jet air plug that has a range typically used to measure a 0.003" tolerance.  The typical gaging rule says you should have no sources of error greater then 10% of the tolerance.  In this example, that's 0.0003".  If we used this plug on the 100µ" surface, the average measuring line is really 50µ" below the peak line.  Double this error with two jets and you get 0.0001" or 30% of the allowable error.  That's pretty significant and air would probably not be a good choice for this part.  As a general rule, the limit for surface finish with an air gage is about 60µ", but it really depends on the part tolerance.

This source of error should also be considered when setting the plug and comparator to pneumatic zero.  If the master and the part have similar surface finishes, then there is little problem.  Most master rings are lapped to better then a 5µ" finish.  However, if the gage is now used on a 200µ" finish part, there would be significant error introduced.  For most applications, there should be no more then 50µ" difference between the master and the part the gage is measuring.  Even this can be significant if the tolerance of the part is as little as 0.001".

In some applications air gaging can be the best thing since sliced bread.  In others, you can get in trouble with the butter.  When measuring porous surfaces, narrow lands, and areas extremely close to the edge of a hole, stick with a fixed size, mechanical plug with probe contacts.

However – there is a twist as long as you can do some testing and make a few assumptions. The fact is that air gaging is a comparison measurement. The assumption is that the surface finish is a consistent and repeatable result of the manufacturing process. If this assumption is taken testing between the air gage and a fixed mechanical gage can determine an offset value. This offset value can now become part of the air gaging result to project the size as if it were measured by a mechanical gage. But – this is a topic for another article.