Putting the "Plate" Gage to Use

Source (all photos): Mahr Inc.

The “plate” gage has been in use for decades. The term originally described a bench comparator that employed a slanted staging plate to rest and locate the part being gaged. These types of gages are primarily designed to measure IDs and ODs.

This bench comparator design was especially suited for rapid comparison gaging of flat, relatively thin parts, as typified by those in the bearing industry — inners and outers, whether roller or ball bearings. The gage design made it easy to determine where the measurement must be taken in a plane parallel to at least one side, square to the diameter. In their heyday, I’m sure tens of thousands were sold to manufacturers in the bearing industry.

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While it was called a “plate” gage, in a vast majority of those built, the part never really rested on the plate surface. The plate typically had line markings to help set the gage to the size being measured, but not to support the part. Rather, the part was raised off the plate and supported by the three anvils used to measure the actual diameter. In use, two of the anvil/contacts serve as position references, while a third sensitive anvil/contact makes the actual comparison of the part to the master setting.

These gages can be configured in either “T” format for direct two-point ID/OD checking or in a “V” format to emulate a three-point check. Of course, the purpose of these configurations depends on the manufacturing process of the part. If the process is apt to produce an even lobbing condition of the diameter, then a two-point/T arrangement would be used. If the manufacturing process is apt to produce odd-number lobbing, then the V arrangement is a must. As we have seen in previous articles, not selecting the proper gage contact configuration can result in the gage not measuring the true diameter, and, subsequently, parts not fitting together properly. This is another reason these gages were a go-to for the bearing industry.

Because these are comparative gages, a setting master is required to set a nominal dimension to which the parts being measured are compared. First, the anvils are set to the approximate size locations, then the contacts are adjusted to the proper height and fine-tuned to the master dimensions. All is locked in place. Note that the vast majority of plate gages use a retraction mechanism to open/close the measuring contact, which makes part placement and removal much easier.

Plate gages of the T format use one contact as the diameter reference, and the sensitive contact, usually linked to a digital indicator, provides the diameter comparison. They make part placement easier and improve operator use by positioning a third anvil/contact at 90 degrees to the diameter. This allows for faster staging of the part for diameter measurement.

There are two trains of thought for the use of this third/reference contact. Some users set the nominal diameter using the reference and sensitive contact, then bring the second reference contact into place to lock the master in place, which yields the true diameter.

However, in this scenario, if the part being measured has a slightly larger or smaller diameter, there is a chance that the gage is actually reading a chord to the left or right of the true nominal. Depending on the part tolerance, this small error is non-consequential. But it must be considered when setting up the gage. The other train of thought is to allow a little clearance on the second reference contact. This allows the user to sweep the part on the gage and search for the true maximum diameter and thus the true diameter. Of course, this takes more time, requires operator skill and user influence — but depending on the part tolerance, it may be beneficial.

On the other hand, the V-type gage uses two reference contacts on each converging leg. Therefore, these reference contacts must be set balanced to the centerline of the diameter — otherwise, again, a chord diameter could be read as the result. This double reference acts very similarly to a gage that employs a Vee stop, providing very precise positioning of the part and effectively locking it in place. When measuring a diameter in a Vee block, the gage is not measuring the diameter but rather the height of a triangle. If the triangle created is small, this could be inconsequential; but it is much larger in this application. Therefore, a compensation factor must be entered into the digital indicator to show the true change in diameter.

Today’s ID/OD comparators have evolved to incorporate adjustable tilting for the most convenient operator use, and digital indicators/probes provide direct diameter readings on the V-plate design. They are ideal for use at the point of manufacture, with friction-free transfer and easy part positioning for the user, and will continue to serve the industry for decades to come.