Editor’s note: This is the third in a three-part series on gage calibration.

Those of you who have been following this series of columns know the caveats that apply so I won’t repeat them this time around. If you don’t know them, buy, borrow, beg or steal the previous issues for an update.

Thread plug gages are calibrated using similar hardware to plain plug gages, but there are problems unique to threads. The first is measuring force. The standards are quite specific as to what it shall be for a given pitch of thread. If you are measuring gages to European specifications, your readings must be corrected to remove rather than include the effects of any measuring force used.

The next problem is the thread measuring wires that must locate somewhere on or near the pitch line of the gage. Any size variation in them delivers three times the variation on pitch diameter measurements. The accuracy of the most expensive measuring device in the world can be lost because of the thread wires used with it.

Some European calibration equipment uses nominal size wires or spheres to cover a range of thread pitches and, with suitable precautions, this can work well. However, few people take those precautions; coupled with the closer tolerances of North American gages this means additional errors included in the readings.

Now that I’ve beaten up pitch diameter requirements, it’s time to look at other hardware needed to calibrate the lowly thread plug gage. The half angle of the thread form requires verification whether the gage is new or used, probably more so if a used gage is involved. There are two devices available for this: the optical comparator or the toolmaker’s microscope.

Another element that must be verified is the pitch of the thread, often incorrectly referred to as lead. What you must have is a device specifically designed for this measurement or the appropriate add-ons for a universal gage calibration device. Hardware, no matter what form it takes, is expensive for this measurement. Some folks try to use a typical optical comparator but the accuracy is not good enough when you consider that errors in this feature affect the gage size by a factor of approximately 1.7 to 1.

If you’re not totally demoralized by the hardware involved so far, I’ll add one more piece of equipment a well-equipped facility will have-a helical path analyzer. These devices are few and far between, but give a chart showing the errors of pitch and path combined so the actual functional size of the thread plug is known. They also are handy for evaluating threads produced by computer numerical controlled machines when you’re having thread interchangeability problems or gage fights on your hands. And, yes, I do have one in my lab.

The toolmaker’s microscope or optical comparator mentioned earlier can take care of minor diameter measurements. Alternatively, whatever you are using for pitch diameter measurements also can be used for the minor diameter by measuring over thread prisms. As their name implies, these triangular-shaped bars have an included angle that clears 60 degrees to contact directly on the minor diameter. They should not be confused with other devices of similar appearance that claim to be able to measure pitch diameter.

I’ve said it before and written it many times-no, your handy digital micrometer is not suitable for calibrating thread plug gages; see the rule in part one of this series. And despite repeated warnings, I still encounter people using pitch mikes to calibrate thread plugs. They don’t have a chance of getting useful information using them no matter what digital readout they have.

My February 2007 column dealt with adjustable thread ring gages so I needn’t repeat myself. In case you missed it-shame on you-such gages are checked using setting plugs. Direct measurement of them is not very accurate in comparison and could lead to all sorts of problems.

For more information on gage calibration, I suggest that you get a copy of “Searching for Zero” from a member of the American Measuring Tool Manufacturers Association. Alternatively, call the association at (440) 974-6829 or fax (440) 974-6828.