Circle cutting is easy with a router and a shop-built trammel. And the trammel doesn't have to be anything fancy. In fact, on the rare occasion that I need to cut circles, I just use a scrap piece of hardboard. Then when the project is complete, the hardboard goes back into my scrap bin.
To make a trammel, first remove the router's sub-base, and trace the bit opening and mounting screw locations onto a rectangular piece of hardboard, as shown in the drawing. (I like to use a piece of hardboard that is slightly wider than the base of my router.) Bore these holes and make sure to countersink the holes for the head of the mounting screws.
Next, install a bit in the router (I use a ¼" straight bit for most projects). Then measure the radius of the hole you want to cut from the inside edge of the bit. Now drill the hole for the pivot pin. I drilled this hole with a ¼" bit and used the bit as the pin.
Always be sure to rout the circle counter-clockwise, which is in the opposite direction of the bit's rotation.
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