Metrology
Metrology
Inspecting Multiple Diameters
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Inspecting Multiple Diameters
George Schuetz, Mahr Federal Inc.

  During the past few months, this column discussed circular geometry gages at some length, and pointed out the rapidly growing popularity of these extremely useful instruments.  It is important to bear in mind, however, that most inspection gaging of round features is still performed with traditional indicator gages.

  When measuring multiple diameters on shafts and similar parts with two or more cylindrical features, fixture gages incorporating multiple indicators offer convenience, speed, and economy.  By building multiple gaging stations into a single fixture, it is possible to eliminate the expense of duplicate work-holding devices.  The gage user need only fixture the part once, and can quickly scan across the indicators or readouts, thus saving time and effort over the use of multiple gages that each measure just one feature.

  Gaging a single outside diameter is among the simplest of inspection tasks, but gaging multiple ODs simultaneously in a fixture gage can be deceptively tricky.  Even when using a gage designed specifically for the part in question, it is possible to get diameters, roundness, and concentricity mixed up.  Different gages or setups are required to properly measure each of these parameters.

  In gaging fixtures, workpieces may be held in V-blocks or between centers, or may be stood on end.  Some workpiece feature or features must rest securely against reference points on the fixture, to establish the proper position relationship between the gage's sensitive contact and the workpiece.  In the case of V-blocks as references, end-journals on the workpiece are commonly used.

  Problems arise when cylindrical features are assumed to be concentric with the end journals but are not in fact. Diameter A establishes the relationship between the part and the gage.  Because Diameter B is not concentric with Diameter A, its position relative to the gage contact is unknown, and it changes as the part is rotated.  Diameter B therefore cannot be measured for diameter or roundness.  On the other hand, this setup can be used to assess the concentricity of Diameter B relative to Diameter A.  Furthermore, because Diameter A is properly referenced, we could measure it for size and roundness simply by building a second indicator into the gage.  (For simplicity, we've shown only two features on the part, but some gages may measure many more.)

  If one wishes to measure Diameter B for size or roundness, the gaging unit must incorporate a pantograph-style gage or similar mechanism that is free to "float" around the non-concentric feature, while maintaining a fixed relationship between the reference and the indicator.  (Air gaging with opposing jets on a single circuit may also be used, if out-of-concentricity will fall within a limited range.  As the eccentric feature moves closer to one jet, it moves away from the opposite one, so that total pressure in the system remains constant and the gage reading remains unchanged.)
 
  We stated above that measuring a part with a single diameter is among the simplest of tasks.  There are conditions, however, where single-diameter work that is significantly out-of-round will appear perfectly round when using a micrometer, snap gage, or any other two-point method (such as that shown in Figure 2).  This is especially so with centerless-ground parts, in which three or five lobes may appear evenly spaced around the part's diameter, so that high and low points are all diametrically opposed.  In this situation, the diameter remains nearly constant, even though the radius may vary significantly.

  A circular geometry gage is required to detect and understand this condition.  Thereafter, it is possible to perform approximate out-of-roundness inspection on a production basis using indicator gaging.  The part is staged on a V-block, and a simple indicator is positioned over the V-block's centerline using a height stand or similar comparator device.  As the part is rotated in the V-block, the Total Indicator Reading (TIR) is noted, then multiplied by a constant.  For three-lobed parts, use a V-block with a 60° included angle, and multiply TIR by 3.00; for five lobes, use a 108° angle, and multiply TIR by 2.24; for seven lobes, use a 128°34' angle and use a 2.11 multiplication factor.  This method provides sufficient accuracy for roundness measurements within a few thousandths.  For greater accuracy, a true circular geometry gage may be required.