It has to attach to the bottom of the router somehow. You probably will remove the base plate and screw the jig on in place of it. Then set the distance you want for the circle. This should be from a sharp-pointed pin on the jig to the outside edge of your straight bit.
This is actually an edge guide that when flipped over, can be a circle guide (Fig. 15). I have a Bosch that is somewhat similar. You mark a spot that will be the center of a circle, drill a hole (usually on the underside) that goes about half way through your work piece.
You use the rods to set the distance from the center of the pin hole to the inside of the bit you're using. That sets the radius. If you don't have a pin, you can use a drill bit upside down as a pin. Use the depth stop on a plunge base to set bit heights so you make several passes, each a little deeper, to cut the material. Take your time, the bit is going to heat up.
I would use a spiral bit for this cut. Cut about 1/8 to 3/16ths per pass. Make sure the pin fits fairly tight so it keeps the bit on track for each pass.
The beauty of a circle cutting jig is that it produces a pretty clean cut and that makes it easier to make it look great in a handwood top that is made up of narrower strips glued together. If you're cutting ply, it will be easier to cover with an iron on strip. That clean edge on the glued up top will allow you to add a nice edge profile.
While on the subject of circle cutting with a router. After decades of using many different jigs, I came across a YouTube video by Marius Hornberger demonstrating what I feel is one of the best I've ever seen!
I made a couple diiferent sized ones out of UHMW plastic, and I am extremely satisfied with their accuracy and ease of operation.
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