[Neil K Walk]

Does the donut have to have a beveled edge? Would a simple nylon washer work?

The way I see it, the way this differs from a simple edge router guide attachment is that has a wider surface (the dumbbell) to true against the side of the guitar and a smaller surface (the donut) to simply keep the depth of the cut consistent as the surface being cut moves along. I’m guessing the focus is to maintain even pressure on the edge guide (the dumbbell) to keep things square?As long as the plunge base on the router stays tight the donut will keep the bit from cutting downward any further than you set it?