AMTMA Perspectives: 'Better sure than sorry': Are the calibration settings of adjustable thread ring gages you use valid?

For the calibration of ring gages to be truly valid, you must have a set plug—or everything that's based on it is false. And when it comes to gage calibration, there should be no other way.

By Gerald A. Flannery
Mercury Gage Co.

Have you considered the possibility that when an adjustable thread ring gage is calibrated and sent to the end-user, the original setting may no longer be valid? Unless the proper set plug is supplied with the ring, there's no way the end-user will know—for sure—that the ring gage has maintained its original calibration.

Here's how this can happen: The industry knows that if an adjustable ring is dropped, abused, or experiences radical changes in temperature, the locking assembly can shift—thus affecting its original setting. It may turn out to be bigger or smaller—that's simply the nature of the AGD adjusting and locking assembly.

Although I'm not suggesting that this always happens, it is a common problem. Lack of a set plug means the ring gage cannot be monitored (after the user receives it) for altered settings or wear while it's being used in the shop. It also means there's no simple way of detecting intentional tampering with the setting. A pile of scrap parts may be the first indication that there is a problem, but by that time, it's too late.

European practice avoids these problems because the preference is to use non-adjustable or solid thread ring gage. Thus, European gage companies manufacture rings to go/not-go set plugs. The wear of the rings is monitored by using a wear check plug made to a specific size, as specified in the ISO or DIN standards. So when the wear check plug enters the ring, the gage is considered out of tolerance and is scrapped.

The point is this: An adjustable ring gage without a setting plug may have a calibration report that does not reflect reality. Companies that calibrate gages will tell you that the "sizes" shown in their reports apply only at the time and place of calibration. They know a lot of things can change when gages leave the lab.

And, could there be product liability in this situation? I don't know, but perhaps gage calibration companies should include a disclaimer on all certifications to make end-users aware of the risks they take if they don't have the proper setting plugs for the rings they send out for calibration.

About the Author
Gerald A. Flannery, the president and CEO of Detroit, MI-based Mercury Gage Co., has spent 28 years in the manufacturing and metrology of both threaded and plain gages. He is president and a member of the board of the Michigan Tooling Association and is a past president of the American Measuring Tool Manufacturers Association. He can be reached during normal business hours at 313-366-0880.
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NOTE: American Measuring Tool Manufacturers Association Perspectives is presented quarterly by member companies of the American Measuring Tools Manufacturing Association.