How to Make a Router Circle Cutting Jig

Step 2 Layout

The base material could be made of any flat sheet good material, but preferably MDF or cabinet grade plywood.  Cut the sheet goods to 2-3" wider than your router base, and 5-6" longer than the largest radius you plan to cut.  I happened to have a scrap of 3/4" cabinet grade ply that was about 8" wide, and 29 3/4" long, which looked like a pretty good size to me.  It will allow me cut circles up to about 46" in diameter.

Draw a line down the center of the base lengthwise, and make marks on this line 1/2" from each end.

Remove the baseplate from your router, and position it centered on the line you drew, and 1/2" away from one end of the base.  Use a pencil to mark the screw mounting holes, the center hole, and trace around the outside diameter.

Make a mark on the center line one inch away from the outer edge of the baseplate.

My baseplate was round, but many will have a flat edge.  if yours has a flat edge, i would orient it towards the center of the base, which will enable you to cut samller circles than if it were orented otherwise..