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Take off the sub-base of the router to determine how big the circular end of the jig should be. Cut the end of the piece of 36" x 14" x 1/4" plywood into a circle the same size as the router's sub-base.
Drill holes into the circular end of the jig to correspond with the holes in the sub-base. Use the sub-base to guide the drilling.
Screw the circular end of the jig into the router's base, aligning the circles on the sub-base with the holes drilled in the jig.
Drill small pivot holes into the arm of the jig, spaced 1 inch apart.
To use the jig to cut a circle, draw a line on the work piece from its center to the end of the desired radius of the circle. Center the router atop the end point of the radius. Drive a nail through the appropriate hole in the jig that corresponds with the center of the work piece (image 1). Set the router to the proper depth, and make a slow pass through the work piece, using the nail as a pivot point (image 2). Make deep cuts in several passes, lowering the router bit slightly after each pass.