Simple circle-routing jig runs rings around other trammels
Unlike most shop-made circle cutters, this one has no limit to how small the cut circle can be, incorporates a fine adjustment, and doesn't require removing the router's subbase. And it's easy to make!
Build the jig as shown, but leave off the dowel rod (used as a pivot point) for now. Neither the bolts that hold the rods nor the fine-adjustment bolt require inserts. Simply drill their pilot holes, then put drops of instant glue into the holes and allow the glue to cure before tapping the holes. The threaded holes will hold up in a dense hardwood such as oak.
After installing a 1⁄4 " upcut spiral router bit, attach the jig to the router, slide the router against the inner rail, and plunge-rout a hole completely through the base. Glue the dowel into the hole.
To use the jig, insert the dowel into a 1⁄4 " hole 1⁄4 " deep in your workpiece. Turn the fine-adjustment bolt to dial in a stop the exact diameter of the circle to be cut, knowing that one full bolt turn equals 1⁄16 ".
—Keith Kroma, Menomonie, Wis.