Router jig trims rough edge to a perfect circle
To build it, first rout a slot in the router table, in line with the bit and perpendicular to the front edge of the table. Next, cut a hardwood runner to fit the slot. Butt the runner against the end of the slot and mark the radius of the workpiece on the runner, measuring from the edge of the installed straight bit.
Drill a hole and mount a pivot pin (a short length of a 10d nail works fine) in the runner at that location. Then, on the underside of your workpiece, drill a corresponding hole in its center. Slip the workpiece onto the pivot pin and pull the runner out slightly. Fire up the router, push the workpiece into the bit, and rotate it 360° to get rid of roughness.
—Eugene Gustafson, Two Harbors, Minn.