Radius gauges measure the curvature of an object to determine the radius of the curve. The radius is the distance from the center of the curve to the edge. Why not simply measure across the circle? Because sometimes the piece is not a complete circle; some objects, such as stamped metal parts, might be only partial circles, or you may need to measure a curve molded or cut into a piece. Radius gauges come in several styles, from curved blades to adjustable-jaw types. They're made to measure a curve from the inside (concave) or the outside (convex).
Things You'll Need
Measure a Radius With a Blade-Type Gauge
Select the blade that appears to match the curve to be measured, and hold it against the piece. Blade-type radius gauges usually come in sets of blades with a range of radii, sized at specific increments. The blades have precise curves that will match a specified convex or concave curve. Use a strong source of light to look for a gap between the piece and the blade.
Try blades of different sizes until the entire contact fits perfectly, with no gaps. If the blade is too small, a convex piece will show a gap in the center of the contact, and a concave piece will show gaps at the ends of the contact. The blade can rock inside the curve. If the blade is too large, the convex curve will show gaps at the ends and the blade can rock on the outside of the curve. A concave curve will show a gap in the center.
Read the measurement marked on the blade. If you cannot find a perfect match, you may be trying to measure a metric dimension with a fractional inch gauge, or vice versa. Or, the blade set doesn't have exactly the right size for the piece. These are the weaknesses of the blade type of gauge.
Measure Radius With a Jaw-Type Gauge
Hold the gauge with the band against the piece. Jaw-type gauges use a flexible band with either a convex or concave curve. Unless the gauge happens to already be set to the correct size, the contact will show a gap as described with the blade-type gauges. Most designs include a lock nut, which will lock the gauge at a certain setting for convenience when measuring many pieces that should all be the same size.
Use the thumb screw or thimble (exact designs vary) to adjust the curve of the flexible band, until the contact is a perfect fit. This is the advantage of the jaw-type design; it is infinitely variable and it will fit perfectly at some setting unless the piece is too large or too small for the adjustment range of the gauge.
Take the reading. Some designs use a calibrated thimble around a calibrated sleeve, like a micrometer. For example, the sleeve might be calibrated in increments of 1 mm and the thimble calibrated in increments of 0.1 mm, and one full revolution of the thimble equals one mark on the sleeve. Therefore, a measurement of 10.5 mm would show as 10 on the sleeve and 5 on the thimble. Some designs use a dial gauge, and some give a digital readout.
Make sure to hold the gauge perfectly perpendicular to the curve you're measuring. If it's at an angle, the reading will be incorrect.