Before doing any routing on the frame, it's best to cut the circular insert first. The insert shown here starts out as a 14" square made by laminating a piece of white Formica to a piece of W plywood. However, before laminating, drill a V«" hole in the center of the plywood. (This will be the pivot hole for the trammel-point attachment.) Then bond the Formica and plywood with contact cement.
To cut out the circular insert, tack the four corners of the laminated square to a plywood backing board. Then I used a Sears router and trammel-point attachment to rout a 4'¿"-radius circle, Fig. 1,
There is one problem here. The stem of the trammel attachment is too long to fit in the shallow pivot hole in the plywood. This may cause the trammel attachment to bend as you're routing — thus creating a slightly beveled edge on the circular insert. This edge can be sanded square, or left as is; either way will work.
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