For an N-scale layout I built, I wanted to scratchbuild a turntable. The most obvious item that needs to be created is a large circle in the "Earth". The Earth is a piece of wood, in this case MDF. I needed a way to cut a perfectly round circle. I consulted "The New Router Handbook" by Patrick Spielman (ISBN 0-8069-0518-2, Sterling Publishing Co., Inc. 1993). At this point I only needed one circle, so a simple jig was all I needed (rather than buying a commercial jig). The jig is simply a piece of Masonite hardboard attached to my router. I needed a 5-inch radius (10-inch diameter) circle cut, so a piece of Masonite more than that length is required. I have a plunge router, which makes it easier to get the routing started. The router base has three threads for machine screws. I found some flathead screws that fit the thread. This was probably the most difficult part of the project! I'm test-fitting them in the router.
A piece of Masonite hardboard about 15 inches long was salvaged from the wood pile and sanded down. I also put a slight chamfer on the long edges of the board, this is to make it easier for the board to slide on the material being cut.
The trick is to get the three holes in the router's base marked on the board. I placed the router on the board and drove three nails through the holes (ones whose head was smaller than the holes in the router base). I could then remove the router and verify that I indeed had identified the location of the holes. I then drilled a small hole in each of the three spots in the Masonite, followed by a 3/8" drill to form the counter-sink for the screw heads. The smooth side of the Masonite hardboard is the surface that will be sliding over the to-be-cut material, so that is from where the screws are inserted, as shown in the next photo.
The next photo shows the jig attached to the router. Note that the screw heads are counter-sunk into the hardboard to prevent them from damaging the wood to be cut.
The above-board view is shown here. The advantage of using a plunge router is that the 1/4" straight-cutting bit can be used to drill a hole through the hardboard, and thereby giving me the exact location from where to measure the cutting radius.
This is shown in the next photo. The distance from the center of the pivoting hole to the far-right edge of the hole drilled by the bit determines the radius of the cut in the wood.
To start making the cut, I drove a nail through the pivot point in the hardboard and into the piece of MDF. It took several passes (about 5) to work the bit through the material before it completed the job. After each pass I removed the MDF dust from the routed groove with a dust collector.