Gaging Tips

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3, 2, 1, CONTACT - MEASURING THICKNESS
George Schuetz, Mahr Federal Inc.

Thickness is one of the most frequently measured dimensions, and also one that is very easy to understand.  So you might think that someone would come up with a one-style-fits-all measurement approach good for just about every kind of thickness application, but it just isn’t so.  There are many approaches to measuring thickness, depending on the requirements of the part.
Some of the most common include: micrometers, thickness gages, air thickness gages and motorized gages with many variations of each.  They range in price and complexity from a few hundred dollars for a standard handheld tool, to a few thousand dollars for ones that are custom-built for the application.  One important consideration that makes each of these solutions different from the others is the way the gage makes contact with the part.
Whether it's the thickness of a piece of sheet metal, a silicon wafer, a latex glove or photographic film, we are usually talking about measuring the distance between two parallel surfaces.  Accurately determining the length of a line that is perpendicular to two parallel surfaces has everything to do with how the gage makes contact with the part.
Micrometer.  A handheld micrometer is a simple low-cost method for measuring the thickness of a piece of sheet metal, for example, which is relatively stiff and thick.  It certainly provides a lot of measurement range.  With flat and parallel contacts and constant gaging force applied with the friction or ratchet drive, the micrometer can self-align to the part for a fast and accurate reading.  A potential problem with the self-aligning flat contacts of the micrometer is that they can bridge across and "average out" minute variations in thickness.  So if higher resolution is required, you should look for a different approach.
Thickness Gage.  The portable thickness gage raises the ante on resolution by combining a flat anvil-type reference surface on the bottom with a radiused (ball) measurement contact on top.  Making a single point measurement eliminates the possibility of gage error that could be caused by faulty parallelism of the contacts.  This would be a good gage for a narrow strip of photographic film where a spot check of thickness is required.
Air Thickness Gage.  In some cases the part may be so susceptible to scratching or marring that any amount of gaging contact could destroy the part.  An air thickness gaging system directs thin, precision-aligned, opposing streams to each side of the part which is held perpendicular to the air streams on a ground base.  The gage measures back pressure on each of the air streams.  Since the back pressure is directly proportional to the distance between the contact point and the nozzle, it is easy for the gage to automatically calculate thickness.  This non-contact technique provides a means of sliding the part around for measuring thickness variation.  The air provides enough cushion to help float the part as it is repositioned.  The use of differential probing provides fast, accurate measurement regardless of where the part is positioned.  Most importantly, the part is not damaged by the gage contacts.
Custom Thickness Gage.  Many soft, compressible parts have rigid, standardized specifications for how they are to be measured.  In such cases the design of custom gages will frequently take into consideration the size and shape of the contact along with the gaging pressure applied to the part.  Usually the gaging pressure is defined as a dead load weight to assure constant force over the full range of measurement.  However, with some compressible materials, such as paper or latex rubber, the amount of time that the load is applied to the part will also affect the reading from the gage.  In these cases, the gage may incorporate a motorized measuring contact along with the cam actuated device to retract the contact after a specific period of time.  This uniform load and dwell measurement prevents the parts from deforming and setting.
As you can see, the method of probe contact is a very helpful way to consider how to set up thickness gaging systems across a wide range of applications.  Choosing the right approach can dramatically decrease the chance of inadvertently making a bad measurement.