The Router Trammel Attachment
To make a circle with a router, one must have an attachment called a trammel. This attaches to or takes the place of the sub-base of your router.
these jigs can be bought or shop made.
when buying a circle jig, you are limited to what size diameter you can cut. If you are capable of fabricating your own circle cutting jig, you are only limited by the space you have to swing an arc. I have made a radius as big as 16 feet which is a 32 foot diameter. Obviously this is a custom made jig, but with the resources and the room it can be done.
There are many materials that can be used to make a circle jig. Masonite, plywood, aluminum, sintra (sheets of PVC), and plexiglass to name a few, are just some of the more practical materials to use. I prefer plexiglass to make my jigs. It allows me to see my work and by using 1/2 inch material I’m able the insert a sliding adjustment mechanism.
I have 2 such jigs. One cuts circles from 12 inches to 50 inches in diameter, and the other, 24 inches to 122 inches. Router bases are permanently attached to both jigs. It’s just a matter of dropping the motor into the one I want to use.