Understanding Plate Gages

You’ve seen the bench-mounted ID/OD comparative gage with the tilting stage plate to set and locate the part being gaged. This basic design, which has been around for more than 50 years, is convenient for fast, comparative gaging of flat and relatively thin-walled parts, such as ball and roller bearing rings, where diameter measurements must be made in a plane parallel to at least one of the faces, and sometimes at a particular depth on the ID or OD. Sometimes the location is the minimum or maximum diameter of the ball bearing race.

The gage consists of a plate that is ground flat, and it may incorporate some wear strips on which the part to be gaged is rested. In many cases, however, the plate is no more than a protective surface for the gaging mechanism. Instead of resting the part on the plate, which could cause it to wear and destroy the reference plane, the gaging surface is built into the sensitive and reference contacts of the gage. It is much easier and less costly to replace the contacts on this design than to replace or regrind a reference plate. This design also provides less surface area for dirt or chips to get into the measuring loop and potentially affect measurement results.

There are two types of contact arrangements in these plate gages: a “T” plate design and a “V” plate version. With either version, there are movable reference and sensitive contacts that are set close to the diameter to be measured.

The T plate design is the most common and probably the most familiar. Because the reference contact and the sensitive contact are in line, the gaging principle is the same as in a portable snap gage. There is a difference in the plate design, however. The contacts used on the plate gage are not flat and parallel, as in a snap gage. They are generally curved or donut-shaped, which calls for special consideration. The gage may not necessarily pick up the max or min diameter of the part every time. Some slight “swinging” of the part through the contacts is necessary to identify the min or max position. The second reference contact on the T can help locate the part. However, it should only be used to position the part close to the true diameter. It should be set to produce a reading slightly outside of the min or max value. Otherwise, if it is set to be exactly on the “zero” diameter, any other position would produce a chord reading and not read the true diameter of the part.

The other contact configuration is the V plate design. It incorporates two reference stops, one at the top of each arm of the V, that must be adjusted symmetrically to ensure that the part is staged on the center plane of the V. This double stop has a locating effect similar to that of a vee block and provides positive and precise location of the part on the gage. This greatly speeds up the measuring process, eliminating some of the operator involvement, and it is especially useful when the part might contain an odd lobing characteristic from the machining process.

However, there is a drawback to this type of contact arrangement. Because the sensitive and reference contacts are not in a direct line, there is not a one-to-one relationship between sensitive contact movement and the diameter. Thus, there are two special considerations that should be borne in mind when using this type of gage. The first is that the angle between the reference contacts determines the multiplier needed to determine the measurement, just like the multiplier used when measuring a diameter on a vee block. In most cases, this angle is 60 degrees and the ratio is 4:5. This means that for every four units seen by the indicator, five units come out (which is another way of saying the sensitive contact is multiplied by 1.25 to get the correct result).Plate gages are a mainstay in the bearing industry, and they can also be used anywhere that fast, accurate readings of ODs or IDs are needed.

These configurations work only for comparative readings and cannot be stretched into the “absolute measurement” world. This is because there is a window of accuracy wrapped around the angle setup for the reference contacts. But if the sensitive contact is moved significantly away from or toward the reference contacts—as would probably happen in an absolute measurement scenario—the angle relationship changes. This changes the multiplier needed to get correct results. To correct for this, a scaling multiplier based on the measurement size and the location of the contact would be needed. It could be done, but it’s pretty complicated for a bench fixture gage.

Fortunately, the gages take these angles, ratios and long-range measurements into account, and they have been doing so for a long time with proven success.