Versatility and precision built into an essential jig
When it comes to routing circles, you need a trammel. While you can quickly make one from a simple strip of plywood, having a more durable, finely-adjustable version makes good sense if you cut more than just the occasional circle. Here’s a design that has served me well over the years for cutting circles of all sizes. Its operation is based upon an offset auxiliary base that houses an adjustable sliding bar of any length that suits the job at hand. Unlike many commercial and shop-made models, this one is infinitely adjustable within its range, so it’s dead-simple to set up for a precise radius. And you can use either a pin or a screw for your pivot point, whichever suits your workpiece better. As with many jigs, the exact dimensions aren’t critical; adapt the design to fit your router and available materials. The jig is not hard to build, and I have provided a few construction tips to help.
Using it couldn’t be simpler: Just attach it to your router, install the appropriate bit, and then set the desired distance from the bit to the pivot point. Locate the pivot point on the workpiece, and rout the circle in several shallow passes, pivoting the router counter-clockwise.
Trammel Anatomy
The 5⁄8"- or 3⁄4"-thick base includes a T-slot that houses a wooden T-shaped slide bar of any length to suit the job at hand. Appropriately located holes drilled through the bar accommodate either a pin or screw as a pivot point. A studded lock knob extending though a clamp block and T-nut applies pressure against the bar to lock it in place. A nickel or a round electrical box knock-out sandwiched in between serves as a pressure plate.
Routing the T-Slot
Construction Notes
Begin with a rectangular plywood panel whose width matches your router base. Mount it to your router, trace the base profile, and mark the collet center. Dismount the base, and lay out and drill the 2"-diameter hole, centered on the collet. Then rout the T-slot as shown, centering it on the hole. Drill the 1"-diameter counterbore for the T-nut in the bottom, glue the clamp block to the top side, and drill a 9⁄32" hole through the clamp block at the center of the T-nut hole. Shape the profile of the base, tapering it as shown, and ease the edges with sandpaper.
Mill the slide bar material from maple or other stiff, strong hardwood, making enough for a couple long spares too. Then rabbet the material to fit the T-slot. For starters, drill one end of each slider for a 1⁄4"-diameter pivot pin, and the other end to accept a screw. Add other holes as particular jobs demand.
