3 steps to routing large circles
With just a router, a straight bit, and a router trammel, you easily can form large, perfectly circular parts such as this project's tabletop. No trammel? Don't worry. In a flash, you can make the one shown in the drawing, below, that will let you cut circles up to 72" in diameter.
Here's how to put the setup to work: Insert and fasten the trammel's rods in your router's edge-guide mounting holes. Then, set the radius of the circle by measuring from the inside of the straight bit's cutting edge to the center of the trammel's pivot pin. Now, lock in the radius by tightening the trammel's knobs.
Drill a hole, sized to suit the trammel's pivot pin, in the center of the workpiece's bottom. Make the hole at least 1⁄4 " deep but not through the workpiece. Or drill a hole through the scrap pivot block's center, and reattach the block to the tabletop's bottom.
With the workpiece rough-cut to within 1⁄8 " of the finished diameter, insert the jig's pivot pin into the drilled hole. Now, plunge the router bit, and slowly rout the workpiece, as shown in the photo, above.