The American Measuring Tool Manufacturers Association (AMTMA) is an organization whose members manufacture, supply, and/or calibrate precision gages and measuring instruments. If you use this type of equipment, the odds are it came from one or more AMTMA member companies.
For over forty years AMTMA members have grappled with measurement problems and disputes, sharing experience and expertise in an attempt to resolve them. Despite these efforts, disputes over measurements continue to plague the industry. Some members say it’s getting worse rather than better due to declining levels of technical expertise in this area of metrology.
In the past, the data from a number of AMTMA measurement studies was circulated among the companies that participated in them. However, a review of the data indicated that gage users and independent calibration laboratories needed to be aware of the realities involved. It was generally agreed that this information could help to reduce disputes over measurements that no one could prove one way or the other at the commercial level.
Around 1990, the AMTMA decided to find out how good or bad industry was when it came to calibration. ‘Industry’ in this case meant gage manufacturers and independent calibration laboratories. They conducted an informal round-robin study within the industry and focused on the measurement that was at the center of most disputes: pitch diameter of thread plug gages. When the results were analyzed, the spread of the readings was much greater than all but a few old timers thought it would be: +/- .0001” or more on average.
In an effort to determine the causes for such a wide range of readings, another round-robin was set up. At that time, most pitch diameter measurements were made using bench micrometers set to a gage block buildup. For this new study, a master and the appropriate thread wires were supplied to participants along with the sample gages. The results became available in 1995 and included readings from twenty-five participants. The range of readings dropped to half the previous value: +/-.00005.”
While the reduced spread was more comforting, it was noted that the study did not reflect reality and thus was not a useful number. But at least it highlighted where half the problems came from: the masters and the thread wires that were used. This study, and others since, included NIST and covered plain plug and ring gages as well as thread gages
In 2001, a study with 32 companies and another one in 2009 with 16 participants brought forth similar numbers. When it comes to thread pitch diameter measurements using proper equipment and techniques, variations between several people calibrating the same gage will range up to +/-.000l” or +/- .0025mm. Readers should remember that participants in these studies spend the better part, if not all, of their working day taking such measurements so the skill levels are usually quite high.
For those who question these numbers, it is interesting to note that NIST’s uncertainty for thread gage measurements using similar instrumentation is .00007” or about .002mm. NIST’s uncertainty indicates that the results of the studies do reflect reality for this type of calibration.
One example of the problem created by such high variations is shown in the calibration of a 1/4-28 thread gage. The pitch diameter readings varied by up to .00025” for a gage having a tolerance of +.0003”/-0. A look at the claimed uncertainties for this measurement ranged from .00001” to .00010” which indicates that many of the participants had no objective evidence of the uncertainties being claimed. With this sort of spread in readings for a class X gage, class W thread gages with as little as half that tolerance would have to go to NIST for calibration that has any credibility.
In recent years the Boeing Company followed a similar process with 50 participants, a study that took several years due to the number of labs participating, and included plain plug and ring gages as well. The results turned out to be not much different than the AMTMA’s results.
Despite what these studies show, measurement disputes continue. During discussions over the studies, Jim Popovic, AMTMA’s managing director, summed it up: “We still have people knocking their brains out over differences in readings of a few millionths of an inch.”
In part two of this three-part series, I’ll look into one critique by a non-participant and the results from other gages that were also tested.