Circle Cutting Jig

I needed to cut a 120mm circle for the back board of my cajon. I didn't want to shell out $50+ for a hole cutter that I would use once. Had the hole been smaller, I could've used my existing set, or had it been much larger, I could've used the existing fence attachment on the router and put an adjustable pin in it. But unfortunately, it was neither.

Here's how I did it.... 

The easiest way I could think of is to use a router. The router base would have a vertical pin, which is placed into a pilot hole drilled at the center of the desired circle. This constrains the router cutter to moving in a circular path.

An aluminium bracket was machined according to a rough paper sketch. The slot allows the position of an M6 screw to be adjusted. The screw is put into a 6mm diameter pilot hole that is drilled at the center of the circle.

I drilled two holes in the side of my router base to attach the bracket using 2 M4 screws. The radius of the cut can now be set by measuring from the center of the screw to the farthest cutting edge on the bit. In this case, the minimum radius is 61mm - close enough!

Here it is set up and ready to cut.