Gaging ID’s and OD’S

Gaging ID’s and OD’S

George Schuetz, Mahr Federal Inc.



            Without a doubt, circles are the most frequently produced machined form generated by many different processes including turning, milling, center-less grinding, boring, reaming, drilling, etc.  There is, accordingly, a wide variety of gages to measure inside and outside diameters.  Selecting the best gage for the job requires a consideration of many variables, including the size of the part, the length or depth of the round feature and whether you want to gage in-process or post-process.





            ID/OD indicator gages come in two basic flavors: bench top and portable, as shown in these figures.  Bench top gages are generally restricted to measuring parts or features not more than 1" deep or long, while portable ID/OD gages can go as deep as 5" or so.  If you need to measure hole IDs deeper than that, bore gages or plug gages are the tool of choice.  On the other hand, snap gages are commonly used for ODs on longer parts — shafts, for example.



            Getting back to ID/OD gages, the choice between bench top and portable styles depends mainly on the size of the part being measured, and whether the part will be brought to the gage, or vice versa.  If the part is large or awkward to manipulate, or if it's set up on a machine and you want to measure it there, then a portable, beam-type gage is required.  Beam-type gages are available with maximum capacities from 5" to about 5', the largest ones being used to measure bearings and castings for jet engines and similarly large precision parts.  Range of capacity is typically about 6", while the measurement range is determined by the indicator installed.


            Most portable ID/OD gages lack centralizing stops, so they must be "rocked" like a bore gage to find the true diameter.  When rocking the gage, use the fixed contact as the pivot, and allow the sensitive contact to sweep across the part.  Likewise, if the gage must bear its own weight against the part, make sure that weight is borne by the fixed contact, not the sensitive one.


            A special fixture with sliding stops at major increments is used to master for large ID measurements.  Gage blocks are inserted in the fixture to "build out" the desired dimension.  For OD measurements, calibrated "end rods" are often used. There is nothing especially fancy about these rods; they're simply lengths of steel, carefully calibrated for length.  When mastering and measuring at large dimensions, the gage, the master, and the part must all be at the same temperature.  Otherwise, thermal influences will throw off the measurement.


            Even so, don't expect very high precision when measuring dimensions of a foot or more.  Most indicators on these large-capacity gages will have minimum grads of .0005".  This is adequate, given the inability of most machine tools to hold tolerances much tighter than about .002" for parts that large.  Beware the gage maker who tries to sell you a 3-foot capacity ID/OD gage with .0001" resolution; it's probably not capable of repeatable measurements.


            Bench top gages are used for smaller parts (diameters ranging from about .25" to about 9" maximum), and they're capable of higher precision. (.0001" is readily achievable.)  There are two basic bench top configurations: T-plates, and V-plates.  A T-plate gage has sensitive and fixed contacts oriented normally at 180º from each other to measure true diameters.  An extra fixed contact, oriented at 90 º or 270 º, serves to aid part staging.  A V-plate gage has two fixed contacts offset symmetrically from the centerline, and the part is held against both of them.  This arrangement requires a special-ratio indicator, because motion at the sensitive contact is actually measured relative to a chord between the fixed contacts, not to a true diameter. 


            This three-point arrangement is useful if the production process is likely to induce a three-lobed condition on the part — for example, if the part is machined in a three-jawed chuck.  By rotating the part in a V-plate gage, one can obtain an accurate assessment of deviation from roundness.  If the process is expected to generate an even number of lobes, the T-plate layout is more appropriate to measure deviation.


            Because they are self-centralizing, bench top gages are capable of rapid throughput.  To further accelerate gaging with either bench top or portable gages, mechanical dial indicators can be replaced with electronic indicators.  The dynamic measurement capabilities of the latest generation of digital indicators enable them to capture the minimum or maximum reading, or calculate the difference between those two figures.  Operators are thus freed from having to carefully monitor the motion of a rapidly swinging needle on a dial indicator when rocking a portable gage, or checking for deviation on a bench top version.