Bench comparators consist of an indicating device, plus a height stand that incorporates a locating surface for the part.
On some stands—especially those used to measure large parts—the base itself serves as the reference surface. Bases may be either steel or granite, with steel being easier to lap flat when necessary.
For higher accuracy, it is usually desirable to use a comparator stand with a steel or ceramic anvil mounted to the base. As a smaller component, the anvil can be machined to a tighter flatness tolerance than the base—often to a few microinches over its entire surface. In some cases, the anvil may be so flat as to provide a wringing surface for the workpiece—an excellent condition for very critical measurements. The anvil is also easier to keep clean, and can be more readily adjusted to squareness with the indicator.
Some anvils have diagonal grooves milled into the reference surface. These serrations serve to wipe any dirt or chips off the part, and reduce the contact area across which contamination-induced errors may occur.
Accessory positioning devices may be used to increase the comparator's repeatability in various applications. A flat backstop permits lateral exploration of the part for variation, while a vertical vee used as a backstop permits rotational exploration of round parts. A vee can also be mounted horizontally, thus serving as a reference in two directions. Round workpieces may also be held horizontally between a pair of centers attached to the base for runout inspection. A round, horizontal arm may be attached to the post, below the arm that holds the indicator, to serve as a reference for measuring the wall thickness of tubes. And special fixtures may be designed to position odd-shaped parts without a flat bottom.
The post is the next important component, where bigger and heavier usually means more stability and less variability. Some posts have spiral grooves to reduce the chance of dirt getting between the post and the arm clamp, which is an invitation to part wear and "slop" in the setup.
The post should provide some kind of arm support beyond the arm's own clamp. Without it, you risk dropping the arm every time you loosen the clamp to adjust the height, which could result in damaged components and crunched fingers. At minimum, there should be a locking ring on the post. A better approach is a rack and pinion drive, which makes it much easier to position the arm, especially if it's a heavy one. Even these should be equipped with their own locking mechanism, so that the weight of the arm does not constantly rest on the drive screw. In some cases, the "post" may be a dovetail slide, which eliminates rotation of the arm in the horizontal plane. This can make setup easier when the anvil remains the same, but the arm must be raised or lowered to measure parts of different lengths.
When it comes to the arm, shorter is better to minimize flexing, although a longer arm may be needed for larger parts. A fine height adjustment screw is a valuable feature for accurate positioning of the indicator relative to the part. Also look for a good locking device that clamps the post to the arm across a broad surface rather than at a single point, as this could allow the arm to pivot up and down. An axial swiveling feature is available with some arms for special positioning needs.
As simple as comparator stands may be, there are hundreds of options, sizes, and levels of quality. Take the time to understand your application thoroughly, and make sure you buy enough capabilities for your needs. You'll end up with faster, easier, more accurate measurements, and less time spent on repairs and adjustments. It may cost more initially, but you'll come out ahead.