You will need one piece of plywood, 1/4" will work and a length slightly longer than the largest radius you will want to cut. In my case, I needed to cut a 4' radius maximum. You should make sure the plywood is true (which is easier to accomplish with plywood than regular wood, unless using hardwoods), then take a piece of laminated press board or any smooth sealed thin material. Glue this to the piece of plywood. Then take the cover off the base of your router. You will need longer screws of the same size, so you might want to pick them up from the store first. Mark the plywood from one end to the other through the center, creating what will be your ruler line later on.
Then mark across your board at the point the bit is located. this gives you a "t" and will facilitate later measurement markings. Take the plate and use it as a template, center it exactly side to side on one end of the board, mine required a board about 7" across. Mark the mounting screw holes and the center where the bit will hit. Take a drill bit the diameter of your screws and knock out the mounting holes and a 2-3" hole saw hole for the bit location. Now you should have a hole in the end of the board centered right to left and a few mounting screw holes. I used a small straight cut router bit about 1.5" long, all measurements will be made off of the cutting edge so it is important not to switch up the bit. Also it is imperative to counter sink the holes for mounting on the bottom side so your jig can slide easily. Also sanding down any roughness created during the drilling process is important, just enough to make it smooth, yet retain its ability to slide. Then mount your router to the board with the new bolts.
At this point you are ready to mark your center line with the basics. I chose to make the feet and half feet. When you want to cut a circle, drill a hole the size of a large nail at that distance, sand the under surface.
Clamp down the wood you plan to cut to a table, use a sacrificial piece if the table top is important to you, and measure where you want the center of your circle. Lightly tap the nail into the mark, remove, place in the hole you drilled in the jig, use the tip to find the hole again in the table and give it a pound or two, just enough to hold it. You are now ready to use the jig, safety glasses and hearing protection on, power on, and just swing the jig around the nail and presto you have a circle.
In my case I was making cathedral shaped doorways, so I only wanted part of the circle.The extra scraps of luan are to make the surface all one level due to using a scrap sheet and and also pulling a radius larger than the actual boards, basically you want to support your jig completely from nail to router. Any questions or additional thoughts to this are appreciated and welcomed. Take care, have fun, and go build something!!! Ha ha ha
