The previous two columns dealt with a study of thread plug gage calibration, the results of which will be discussed and/or argued over for some time. Not to be left out, the adjustable thread ring gage provides plenty of fodder for arguments about how ‘good’ their calibration is in practical terms.
The ASME standards to which these gages are made require them to be verified by their fit on an appropriate setting plug. There is no other option on the smaller sizes but different hardware for directly measuring pitch diameter is often used to get around the costs of a setting plug for non-stocked gages. Each method takes a different approach to the situation and because they are not the same from a metrology point of view, the ‘answers’ they produce are rarely the same either.
A screw thread has a number of elements that make it up such as major and minor diameters, pitch diameter, flank angles, linear pitch, etc. Direct measurement of pitch diameter as an isolated element ignores the others and thus gives a false indication of a ring gage’s ‘size.’ Any one of the ignored elements could mean a gage is out of tolerance even though a pitch diameter measurement indicates otherwise.
Setting plugs have the advantage of incorporating all of the elements of the thread so if any one of them is out of tolerance, the ring gage won’t fit the plug properly. It’s true that one element may mask another and the whole process is dependent on the fit between master and ring. However, despite these drawbacks, properly set gages will accept good threads and reject bad ones, which is what the overall game is about.
Proponents of direct measurement of these gages will argue that pitch diameter readings can be corrected to suit variations in related elements but that assumes the other elements have been measured by a suitably accurate method to provide meaningful values for the corrections. This is highly unlikely since far too many labs do not have adequate equipment to measure thread pitch—particularly on internal threads—to the accuracy required.
In the third paragraph I noted the elements that make up a screw thread and ended the listing with “etc.” What did I leave out? Helical path! I did this on purpose since there are probably less than half a dozen private labs on this continent capable of measuring this critical feature over the full length of a standard thread. If there is significant helical path error in the ring gage, direct measurement of pitch diameter will never find it but it will make a major difference to the ring’s functional size.
Okay, I’m nearly finished—I left the best part for last: roundness of a ring gage. Since the rings in question are adjustable, basic geometry tells us that you cannot adjust a circle and have it stay round. Those rings with three segments will produce a triangular shaped diameter when adjusted. You can measure pitch diameters on such rings ‘til the cows come home and never know this situation exists.
Rings with one or two segments will go oval when adjusted, a condition that can be detected by two diametrical measurements, one at ninety degrees to the other. Unfortunately, one of those measurements is not always possible because the adjusting slot gets in the way.
Adjusting a ring gage without lapping it creates a pitch error that is undetectable by direct pitch diameter measurement but gage users will not pay the extra costs involved in doing the job properly.
Once more, unmodified setting plugs will not detect these conditions but will ensure that the functional size of the ring gage created by their effects is within limits.
My comments respecting roundness apply to the direct measurement of the minor diameter of a ring gage as well although the tolerances are coarser than for pitch diameter. Labs that specialize in gage calibration often have plain plug gages on hand to verify the minor diameter where standard off-the-shelf gages are concerned, or they may use a three anvil internal micrometer.
Everything I’ve noted in this column is familiar to specialists in the field as are other factors that need to be considered about this type of calibration. But I’ll leave that for another day so no one gets a migraine over it all.