Draw a circle with a square - Tack two nails to set the diameter you want, then rotate a framing square against the nails while you hold a pencil in the corner of the square. Rub a little wax on the square so it slides easily.
*Homemade Self Cleaning Blast Gate* I needed a couple of blast gates that could fit onto the Y of my dust collector. I did not want to use any of those plastic ones as they have problems in cleaning and leaking, so I decided to build my own.
Routers are one of the most versatile woodworking tools, useful not just for making trim and cutting edges, but for solving a whole range of woodworking problems. Learn how to cut curves, straighten boards, flatten bows and much more. - My Wood Crafting