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What is a Radius Gauge?

By Paul Scott
Updated May 17, 2024
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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.

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Discussion Comments

By MrMoody — On Dec 09, 2011

@hamje32 - I’ve never seen a radius gauge. I have seen a ring gauge once, though, when I worked in a metal shop. It’s a similar concept, except that it’s used to measure the complete diameter of the circle not just the radius.

Also I believe it’s exclusively used for cylindrical objects and I think some ring gauges are fixed while others are adjustable. While the concept is similar to the radius gauge, I like the ring gauge better because there is no need for a light source.

By hamje32 — On Dec 08, 2011

@everetra - My friend is a mechanic and he uses this thing called a dial indicator to measure pistons and stuff like that. It’s similar to a radius gauge in that it’s used to measure the width of a rotating part.

The dial indicator has a prong that is pressed against the part as it’s spun around, and slight deformities in the part will create shifts in the radius. When that happens, the measurement is recorded on the dial indicator.

This is done for precise measurements. When you’re working with automobile parts you have very strict tolerances for the machine parts.

By everetra — On Dec 07, 2011

@Mammmood - I think the pantograph is a neat invention. You can find these utensils in hobby shop stores. If you do any stencil drawing you might find it particularly useful.

Basically it uses a radius gauge and criss crossing stencils to enable you to trace small figures and expand them to larger scales. You can adjust the size of the scales so you have some control over how big the final output will be.

Without the use of a pantograph, the only other alternative I can imagine would be to draw the picture at normal scale, scan it, and then stretch it to a larger scale. But in that case you might get pixilation in the final printed output.

By Mammmood — On Dec 07, 2011

While it doesn’t say, I think that measuring instruments like a radius gauge may be used to check things like eyeglass lenses too. I say this because they use the instrument to check convex or concave structures and eyeglass lenses would certainly qualify.

I suppose if the materials were neither convex nor concave then you wouldn’t need a radius gauge. You could just use a ruler to take a straight up measurement from the middle of the profile to the circumference to get the radius.

I think it’s the deflection of the materials that necessitates the use of the radius gauge to get accurate measurements.

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