Cutting

• 1). Turn your router over and remove the screws holding the base plate in place. Lay the baseplate on your plywood in one corner of the sheet and trace around it. Trace the openings in your router's base onto the plywood as well. Mark the mounting screw holes. Mark the center of the circle.
  
  
• 2). Draw a line from the center of the circle out 4 feet with a straight edge. Draw parallel lines to either side of the first line, 2 inches away from it to create a 4-inch wide "tongue."
  
  
• 3). Cut the plywood compass from the plywood with a jigsaw. Use a drill to drill holes through the cutouts you outlined from the router base, then cut them out as well with the jigsaw. Drill the mounting screw holes with a 3/6-inch bit and countersink them slightly.
  
  

Usage

• 1). Attach the compass to the base of the router, using the screws from the router's base plate or screws with a similar head and thread that are slightly longer, if required. Tighten the screws with a screwdriver until they are flush with the plywood.
  
  
• 2). Drill holes along the center line of the compass' tongue with a 1/8-inch pilot bit. Position the hole that is the correct distance from the center of the router onto your material where you want the center of your circle. Drive a wood screw down through it into the material to anchor it in place. Do not over-tighten the screw. Leave it loose enough so the compass moves freely.
  
  
• 3). Set the depth of the router as required by the project and start it. Run the router from left to right along the curve proscribed by the compass so that it creates a perfect arch.