Editor’s note: This is the third installment in a four-part series on common calibration mistakes.
Many of the mistakes made in the calibration of plain plug gages also are found in the calibration of plain ring gages so it is wise to review them. As with plug gages, the simple nature of plain ring gages leads to the mistaken assumption that the devices used for their calibration are as well. While the instruments may appear to be pretty basic, their technical characteristics are not always that obvious so it’s time to take a look at the equipment involved.
EQUIPMENTA mistake that is often made when selecting the right device for calibrating plain rings is to assume that a device that sells for $5,000 is as good as one that sells for $25,000 or more. Too often, technical criteria lose out to price and when the performance doesn’t measure up on the simplest of functions no one can figure out why.
Two different types of instruments with similar resolution are considered equals by some, a mistake that ignores the fact that the electronics are only part of the device. The mechanics are the most critical part, for without sound design and construction, the digital display merely presents impressive fairy tales.
Internal comparators are the most popular device for calibrating plain ring gages. Basically, they incorporate a table on which the ring is placed. A slot in the center of the table allows two contacts or probes to protrude above the table surface to contact the bore of the ring. The contacts can be adjusted for different sizes and are finally set using a gage block buildup for each size to be checked. Some models have a long-range measuring head to reduce the need for a buildup for each size.
A significant mistake in using this type of equipment is to assume that you can plop the ring on the table and, after centering it, measure away. This would be more or less true except for the fact that the bore of the ring may not be square to the face resting on the table-a very common situation. Instruments that are specially designed for this work incorporate a mechanism that tilts the probes so this condition can be detected and accounted for. Instruments without this function require the technician to manipulate the ring or simply assume the problem won’t arise-a big mistake in its own right.
The spherical contacts on all ring gage calibration instruments are crucial to accurate measurements but often only receive a cursory visual examination when the equipment is calibrated. This mistake can cause all manner of mayhem down the road. It could be argued that this won’t matter so much since the instrument is set to each size it will measure and any “flats” on the contact surface will be incorporated in the setting.
This is true but you have to remember that the instrument is being set using flat, parallel surfaces off a gage block buildup and will contact radial surfaces for measurement. Flats on them will span a portion of the diameter being measured and not give a true reading of size. Another problem in this area is misalignment of the contacts vertically and horizontally.
MASTERSAs mentioned earlier, a buildup of gage blocks is the setting master of choice for short range internal comparators. All of the notes regarding their use as masters for plain plug gage calibration apply when used for plain ring gage calibration, plus one other problem.
When setting an external measuring device, the blocks are being pushed together by the instrument they are setting. The reverse applies when an internal comparator is involved. Some technicians will use a gage block holder or cage to keep the buildup together. Nothing wrong with this except for the mistaken belief that wringing can be avoided since the blocks are going to be clamped together. Doing so introduces changes in the overall dimension of the buildup that are not good.
Avoid these problems by ensuring the blocks and any accessory jaws are properly wrung together. If the blocks won’t stay that way when used without clamping them together, you need better blocks or reduced measuring force-or both.
And don’t forget to leave the buildup on a normalizing plate after wringing so any heat generated by the process or handling of them can dissipate.