Cut Perfect Circles with a Router Trammel
You need to cut a big arc or wheel out of a sheet of plywood, but you know that a handheld jigsaw will leave a less-than-perfect circle. Build a trammel for your router, and you can whip out perfect circles or rings anytime. The pivot base and beam allow a wide range of size adjustments.
Using 3⁄4 " plywood for the trammel, cut out one end to match the shape of your router's base. For length, construct the jig so the beam measures about 12" longer than the radius of the largest circle you wish to cut. Make the narrow part of the beam 4" wide and cut a 3⁄8 " slot through it as shown in the drawing at top.
On the bottom surface of the beam, use your router to cut a 1"-wide groove 3⁄8 " deep for the head of the bolt to slide in. Next, cut the pivot base from 3⁄4 " plywood and drill a 3⁄8 " hole for the carriage bolt 1" from the front edge.
Drive a screw through the base into the center of the circle to be cut and adjust the base until your router is correctly positioned. Tighten the wing nut on the carriage bolt and rout your circle with a straight bit. If you're cutting a ring, cut the outside diameter first.
—Wilton Elwick, Texarkana, Texas