A radius gauge is a combination tool used to measure or check convex and concave radius profiles. A typical radius gauge is a small, handheld tool used to measure machined radii in light engineering and hobby applications. It generally consists of a set of separate steel inserts or blades each having several cut-outs or projections machined into their outside edges. These anomalies each represent a known radius with a gauge set covering a defined range of radii. Radius gauges which employ complicated pantograph and sliding scale arrangements are used in heavy industry to check larger radius profiles.
Radius gauges are normally sets of thin steel inserts which each have a number of known radii machined into their edges. A set of inserts may measure only inside or outside radii or a combination of both. Each radius gauge set will cover a predetermined total radius range which may be supplemented with additional inserts or sets if needed. Each cut-out or projection will be marked with the relevant radius value. These sets may consist of loose inserts presented in a case or wallet or be of a combined, flip-out design similar to a tappet feeler gauge.
The radius gauge is most commonly used to check radius accuracy during machining operations. The devices can also be used to establish the exact radius of machined profiles on existing parts. To check the accuracy of a radius during machining, the relevant projection or cut-out is chosen and pressed against the sample profile. The mated surfaces are then inspected against a strong light source. If any light can be seen between the workpiece and the gauge surface, the profile is incorrect. To check an unknown profile on an existing part, the inserts can be applied in the same fashion until one is found which matches.
Large machine parts may be checked for accuracy by using more complex types of radius gauges. One type of large radius gauge consists of a flat, flexible strip which is attached at its ends to a series of pantograph linkages with its center held captive on the gauge body. These linkages are connected in turn to a sliding scale indicator. To check a radius, the center of the strip is pressed against the test piece and the scale slider moved up or down until the strip mirrors the test piece radius. The exact radius may then be read off the scale.