This simple jig works on a disc sander to create accurate, repeatable radii on workpieces.
Illustration of jig to round corners

Start by making a base from 3⁄4" plywood at least as large as the sander's table. To the underside of the base, attach a runner sized to fit the miter slot. Position the runner so the base rests against the sanding disc. 

To set up for sanding, for example, a 3⁄4" radius, draw a reference line parallel to and 3⁄4" from the edge of the base on the bottom. On this line, drive a 3-penny finish nail through from the bottom until the point protrudes approximately 1⁄8" on the top side. This becomes the pivot point for the workpiece. Snip the brad flush with the bottom face.  

To the left of the pivot on the top face, draw a line perpendicular to the disc and 3⁄4" from the pivot point. This line serves as a reference for locating the end of the workpiece before sanding. Draw a parallel line spaced the width of the workpiece minus the radius (3⁄4"), as shown above. On this line, near the front edge of the base, drive a finish nail into the top face, leaving 1" above the surface. This serves as a stop as you rotate the workpiece while sanding the radius.

To create the radius, align the end of the workpiece against the line closest to the pivot point with the edge butted against the sanding disc. Lightly tap the workpiece onto the pivot point. Turn on the sander and rotate the workpiece slowly clockwise until it contacts the stop.

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