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



As I noted in last month’s column, this will be an overview of the most popular hardware used for calibrating gages. In case you are wondering why I didn’t mention the use of coordinate measuring machines for this work, and won’t, the answer is quite simple. Few of them are accurate enough for the tolerances involved, and those that are rarely exist outside of national measuring institutions.

Most plain ring gages are measured on specially designed comparators that have two arms with ball contacts on them. These units are set up with a gage block buildup for each size to be measured. Many models employ a means of positioning either the arms or the gage to eliminate cosine errors. Those that don’t require a very skilled inspector to manipulate the ring during measurement to eliminate errors.

This is a critical function that is often overlooked by designers of such equipment who may not be aware of the ring gage specifications in various parts of the world. A European instrument maker was exhibiting a semiautomatic device at a show I attended and was anxious to give me a demonstration. I should have asked for the price first and saved everyone a lot of time. But I didn’t and watched as this “technological breakthrough” went through its paces.

All you had to do was plop a ring on the worktable and push the start button. Lasers flashed while a computer crunched the data, and in less than a minute, the job was done and the machine opened up so the gage could be replaced with the next one. I thought I’d missed something and asked, “Do you measure and provide corrections for cosine error?”

The designer hastened to explain that it was not necessary as squareness of the ring bore to its faces was controlled by the standards it was made to-or should be. I suggested that while the European specifications for ring gages may cover this, North American specifications did not at that time. Thus, there were probably a million or more plain ring gages out there with squareness conditions that would make his machine useless. I never saw the device again.

Another feature comparators should have for this type of calibration is the ability to position the measuring contacts at various locations along the bore. And once again, the contacts should be long enough to cover gages made to our specifications that are longer than rings from Europe or Asia.

Modern versions of the metroscope-type calibration devices and related counterparts are being used more often for ring gage calibration. The reason for this is that a single setting enables the unit to measure up to 4 inches or 100 millimeters.

Like any comparative device, the devil is in the setting masters being used. This more significantly affects devices used for ring gage calibration, as either a zero point or a nominal value for one-to-one comparison has to be established.

Gage block build-ups are the only practical masters for one-to-one comparison but allowance must be made for the fact that the ball contacts are being set between two flat and, hopefully, parallel surfaces. Universal gage calibration devices can be set this way but most makers recommend the use of a ring gage. The uncertainty of the setting ring calibration now becomes a major element in the uncertainty of the calibration using it. The only way to reduce this is to use a ring that has been calibrated by NIST, the procedure followed by our laboratory where several such masters are used.

There are a number of instruments in the marketplace touted as being able to measure plain ring gages. The truth is that all manner of hardware can be used, but few can do so with anywhere near the accuracy required, despite  having digital displays that read to 10 microinches, or 0.00025 millimeter or better. And as far as hand tools are concerned, remember the rule: if you can hold it in your hand, it won’t be accurate enough to calibrate gages.