The simplest way to calibrate plain plug gages is by using a high resolution and high accuracy comparator. While there are mechanical instruments out there that may be suitable for all but the closest tolerances, most folks opt for an electronic system using a probe and amplifier. This offers the advantage of having multiple ranges to choose from and is switch-selectable between inch and metric readings. Typically, the resolution will be 5 or 10 microinches.

The probe is mounted on a comparator stand fitted with a hardened, ground and lapped worktable. People who use this system a lot usually wring a sub-table onto the worktable or even a carbide gage block to reduce the contact area. The stand may also include an intermediate contact system to protect the probe. The column on which the gage head carrying the probe is mounted will be well over 1 inch in diameter, with the better quality models around 2 inches in diameter. You can appreciate from this brief description that these stands have a lot of steel or cast iron in them for mechanical and thermal stability.

Calibration with this equipment is a one-to-one comparative process. You will need a gage block build-up for each gage size you will be checking and a normalizing plate to keep the gages and masters at a common temperature.

The downside of this system is that the inspector must have the skill to be able to find the right reading and be able to select and wring the gage block buildup together. Also, the inspector must be able to apply the appropriate correction factors from the block calibration report.

Other high accuracy comparators are available for such work and often are suitable for both internal and external comparisons. Some have the ability to manipulate the gage head to cancel out cosine errors.

Another method for plain plug gage calibration is to use bench micrometer-style instruments that are fitted with micrometer-type anvils or measuring faces. In this case, the gage can be placed on the worktable and finding the diameter is relatively easy. Errors occur if the end face of the gage is not square to the axis of the gage, requiring it to be manipulated for the correct reading. Some of these models incorporate a micrometer-type head but are used as comparators and, of course, some are available with laser-type measuring systems to measure over a long range.

There are instruments specifically designed for gage calibration that read down to a millionth of an inch. These devices incorporate concepts that have stood the test of time in laboratories around the world. Originally known as horizontal metroscopes, they have long since dropped the microscope that was part of them and now incorporate digital readout systems.

As one would expect, such equipment has some unique features. Two of the most important allow you to adjust the measuring faces for reduced parallelism error. They also have devices for mechanical positioning of the gage being calibrated to eliminate cosine errors. These units can typically measure up to 4 inches without resetting or using gage block buildups.

If you’ve been paying attention, you’ll notice I haven’t mentioned your trusty digital micrometer. The measuring uncertainty using one will nearly equal the gage tolerance; that means you stand a better chance guessing what the gage size is than measuring it. One way to avoid such problems is to remember that if you can hold the measuring device in your hand, it won’t be suitable for calibrating gages of any kind.