A router circle jig helps you cut circles with your router. This is by no means a new idea, but I have documented my build here so that you can see all of the steps. The jig is made from 3/8″ hardboard. The router is attached to the harboard and a is nail driven through both the hardboard and the piece of material being cut to act as a pivot point that traces out a crisp circle.
Start by ripping a piece of hardboard to the appropriate size for your router. Dimensions are not critical, but my jig is 6″ wide and 24″ long.
Next, using the base plate of your router as a template locate the appropriate locations to drill holes in the hardboard. The goal is to attach the router to the hardboard using the same harware that attaches the router base plate to the router.
Note that the slippery side of the hard board will eventually be placed face down on your workpiece. To prevent the screws that attach the router to the hardboard from marring the work piece, counter bore the holes on the shinny side of the hardboard. I used a 1/2″ counter bore bit.
Chuck up a 1/2″ straight cut bit in your router, and attach the router to the hardboard using the counter bored holes you just created. The next step is to create a hole for the bit to pass through the hardboard.
Secure the router and jig over the edge of your workbench and turn on the router. Slowly plunge the bit through the hardboard. The only remaining task is to create a pivot point for the jig to rotate around. This is accomplished by drawing a guide line down the center of the jig.
The effective radius of your work piece with this jig will be from your pivot point to the inside edge of the router bit. I needed a 12″ diameter circle cut in a piece of plywood, so I measured 6″ back from the router bit hole, along the guide line, and drove a nail through the hardboard.
To use the jig, drive a nail through the hardboard into your work piece. Clamp the work piece to your workbench and begin cutting. It’s not elegant, but it works well.