Rout Perfect Circles
You can cut large circles with a router, bandsaw, or jigsaw. Of those three, only a router with a straight or spiral bit ensures a true circle requiring very little edge cleanup. To do so, you’ll need a trammel that extends out from the center of the circle and replaces the router’s subbase. Make it from 1⁄4 " hardboard or plywood, about 8" longer than the circle’s radius.
It takes only about 10 minutes to make and install a trammel onto your plunge router, and once you make it, all you need to do for a different diameter is drill a new pivot-screw hole. Here’s how to do it.
Start with a center hole.
Glue up your blank and on its bottom face mark the center. Drill a pilot hole for the pivot screw. We drilled a 7⁄64 " hole for a #8×3⁄4 " screw, about 2⁄3 through the thickness.
Mount the router.
Remove your router’s subbase and use it as a pattern to mark mounting holes on the trammel. Drill and countersink those and drill a bit hole; then mount the trammel to the router.
Measure and mark the radius.
Install a spiral bit. (We used 3⁄8 ", but any size works.) Measure from the edge of the bit (inset) and mark the length of the circle’s radius on the trammel.
Attach the trammel.
At the mark you just made, mount the trammel onto the workpiece with a screw, being careful not to go through the top’s good face. Tighten the screw snugly, but be sure the trammel can rotate smoothly.
Rout a shallow circle.
Begin routing the circle by plunging 1⁄4 " deep and moving the router in a counterclockwise rotation. Complete the full circle and then shut down the router, but don’t remove it.
Back up for straight bits.
If you use a straight bit rather than a spiral, you must climb-cut (routing backwards) the areas of the circle prone to grain tear-out. Do this in 1⁄8 "-deep increments.
Cut away the excess.
With the router trammel still attached to the workpiece, cut through the middle of the groove. Rotate the router out of the way as you cut around the circle.
Elevate the workpiece on riser blocks tucked inside the circle (to avoid contact with the bit). Rout the shape in 1⁄8 "-deeper passes, stopping with about 1⁄8 " of material left to go.
Prevent good-face tear-out.
For the final pass, clamp a scrap “spoil board” beneath the workpiece. The spoil board prevents splintering on the underside, which will ultimately be the top. Reposition the clamps as needed to clear the router’s path.