When Indicators Go Both Ways


George Schuetz, Mahr Inc.


 Measuring and gaging are two distinct forms of dimensional inspection.  Measuring is a direct-reading process, in which the instrument incorporates a continuous scale of units, against which the part is compared directly.  Examples of measuring instruments include steel rules, Vernier calipers, and micrometers.

 Gaging is an indirect-reading process, in which the instrument is first set to a standard or master, and then the part is compared to that setting (usually defined as zero).  This is usually done because the measuring capacity of the gage (i.e., the size of the workpiece it can accommodate) is greater than the measuring capacity of the indicating device (i.e., its range).  Gaging instruments include snap gages, bore gages, height and ID/OD comparators, and many others.

 There is a class of instruments that go both ways, blurring the distinction between gaging and measuring.  (Hey Gage, take a walk on the wild side...)  These are instruments with indicating devices whose range is equal to or greater than the capacity of the gage.  Instruments with long-range indicators can perform both direct and indirect measurements. 

 If zero is set at the reference surface on which the workpiece rests, the indicating unit will measure the part directly and display its actual dimension on its scale.  Alternately, zero may be set against a master, in which case the instrument becomes an indirect-reading or comparative gage, and displays part size as deviation (plus or minus) from a pre-set value.

 Long-range dial indicators that can be used in these "either-or" applications have existed for decades.  Although they are familiar and reliable, long-range dial indicators are not easy to read.  Workers often make errors when trying to interpret the rather complicated display, which, in addition to the main dial, includes one or two revolution counters rotating in opposite directions. 

 A long-range electronic indicator, with a digital display that eliminates the readability problem of dial indicators.  Long-range digital indicators typically have measurement ranges of 1/2"/12mm or 1"/25mm, and resolution of 20 u”/0.5um.  They are invariably equipped with lifting levers so that the contact point may be easily raised and lowered.  In comparison, long-range dial indicators have ranges from 1/2"/12mm to 3"/75mm or more, but resolution is rarely better than 0.0001"/0.0025mm, and sometimes as coarse as 0.010"/0.25mm.

 Long-range indicators are often used on portable thickness gages or bench height/thickness stands.  The portable gage provides flexibility for measuring a variety of parts anywhere in the shop, including parts on machines before, in the midst of, and after major stock removal, and multiple dimensions on the same part.  Likewise, a bench gage can measure whole families of parts or multiple dimensions on the same part, including thickness, shoulder heights, depths, and outside diameters.  Both types of gages offer potential time savings (from the elimination of mastering and setup changes) and cost savings (because one gage does the work of many).

 Rules for the use of portable, long-range thickness gages mirror those for snap gages.  Make sure the lower or fixed reference anvil is held firmly and squarely against the work, and do not allow the weight of the gage to bear on the sensitive contact.  Aside from that, most gages equipped with long-range digital indicators are easy to work with, and highly adaptable.