Metrology
Metrology
“SQUEEZE” GAGING
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“SQUEEZE” GAGING
George Schuetz, Mahr Federal Inc.

 Machinists working strictly in metal-working shops do not have many occasions to gage the thickness of soft materials.  But many of our readers work in supporting roles, helping to build or maintain the machines that produce textiles, plastic films, paper and other products that are compressible.  Even readers who work only with metals should realize they are not alone in their concern for accuracy:  the shirt on your back, the newspaper you read this morning and the garbage bag you tossed it in, were all produced to exacting thickness tolerances.

 Differences in gaging pressure translate readily into differences in the degree to which the sensitive contact compresses or penetrates the material being gaged.  It is, therefore, essential to use a standardized method when measuring the thickness of compressible materials.  Every industry and every kind of material has its own standards, most of which have been established by the American Society for Testing Materials or by Underwriters’ Laboratories.  These standards specify the size of the reference table and the measuring contact (or foot), the dial gradations, and the amount of force to be used.  (Gaging force is controlled by a weight, rather than with the springs found in most dial indicators, to ensure consistency of compression).  Such standards make purchasing decisions quite straightforward:  often, you need only specify which ASTM standard you wish to meet.

 Assuming you have the right gage for your material, making accurate measurements of compressible materials is not difficult if you follow a few basic guidelines:

The Anvils Must be Parallel
 
 If they are not, they will compress the material at an edge rather than across a flat surface.  To check for parallelism, close the contacts and try to shine a flashlight through them.  Parallel contacts allow no light to shine through; any lack of parallelism is readily apparent down to about 0.0001” (2um).  Check front to back, and side to side.

 It is difficult to hold larger contacts -- say those above 1/2” x 1/2” (12mm x 12mm)--in alignment with the light source.  And the visual method may fail to reveal problems such as an upper foot with a worn center or a chipped edge.

 If you need more accuracy or if visual inspection reveals a lack of parallelism and you want to measure it, use this method:  Place a precision wire of 0.010” or 0.020” (.2mm or .5mm) diameter under the front edge of the upper foot, and zero the indicator.  Then retract the contact, place this “master” under the rear edge, and repeat the measurement, noting the variation.  Repeat for left and right sides also.

 Some gages have machine screws to adjust the reference table into parallel with the upper foot.  On those gages that do not, the common solution is to file down either the boss that holds the indicator to the frame, or the shoulder against which the reference table support is clamped.  Easy does it!


Make Sure Contacts Are Not Contaminated

 This is a common source of error.  As in any gaging procedure, the contacts must be kept clean of dirt, lint or hair.  When measuring materials such as polyethylene film, which may have been in liquid form only minutes before, beware of the buildup of chemical deposits on the contacts.  As with any textile product, frequently clean lint off the contacts.

Check for Gage Repeatability By Checking Several Points on the Surface of a Known, Consistent Sample

 Most gages for thin materials do not need to be mastered.  Simply “zero” the gage against the reference table.  To avoid having to count dial revolutions when measuring thicker materials like acoustic tiles or carpet, use a gage block to set the zero at the nominal dimension of the material.

 If a gage reads “under”, the problem is most likely a lack of parallelism.  If it reads “over”, dirt is probably the culprit.  Of course, other factors common to all gaging, such as friction in the mechanism or a loose fixture, can also be at fault.