Imagine a large rotor with an ID of 12". One normally uses an adjustable bore gage, or perhaps an inside rod micrometer, to check the diameter. But in this case, a hub in the center of the part presents an obstacle. For this type of measurement we need a special type of gage, called a shallow bore gage (sometimes called a shallow diameter gage or shallow ID/OD gage). These gages may be used where the diameter is reasonably close to the face surface of the part (usually 3" or less), and where the face surface is flat and square to the feature axis. While some of these gages can measure diameters as small as 2" or so, larger dimensions are more common, and gage capacity can range up to several feet.
A rail usually serves as the frame of a shallow bore gage. An anvil post, with a reference contact, extends perpendicularly from the bottom of the frame. The sensitive contact extends below the rail at the opposite end, for the final leg of the "C." The sensitive contact is connected via a low-friction linkage to an indicator on top of the rail. Depending upon the specific gage, either or both contacts may be adjustable along the rail, and both are adjustable for depth. (For gages offering more than 3" of depth capacity, special bracing or heavier contacts are required to provide the necessary rigidity.) The contacts are usually back-tapered, to ensure true point-to-point measurements. Rest feet are provided to establish a reference plane parallel to the measuring plane of the part. These, too, are adjustable linearly to accommodate different diameters.
Most gages offer about 6" of capacity adjustment. On some gages, rails of different lengths may be interchanged, so that the range of adjustment may be virtually unlimited. Frames can be configured to avoid interference with protruding features in the center of the part. With so many adjustments possible, shallow bore gages are adaptable to a wide range of applications, including grooves, tapers, cylinders, and features recessed behind blind shoulders.
Because the diameters being measured are usually large, dial indicators with resolution of 0.0001" or 0.0005" are typical. For applications requiring higher resolution or data output, higher resolution indicators, digital indicators, or even electronic transducers can be substituted. The electronic devices, with their ability to automatically capture the "min" or "max" value during the sweep, may also be preferred as a means of reducing operator errors. Some gages provide the option of mounting the indicator either horizontally or vertically, to improve visibility in some applications.
In use, the gage is swept across the diameter to find the maximum or minimum reading, depending on whether the feature is an ID or OD. Some gages include a centralizing device, which can be helpful but does not eliminate the need to "rock" the gage.
Mastering is usually done with adjustable setting masters, which must first be set to the nominal dimension with a gage block stack or end rods. Gaging depth is usually set by eye, placing a steel scale beside the contacts as they are screwed in or out. Where depth is critical (as with tapered features), the rest feet are placed on gage blocks of the appropriate height, and the contacts are adjusted up or down until they just touch the reference surface. If the bottoms of the contacts are radiused, make sure the gage block height is raised accordingly.