This attachment mounts in place of your router’s factory baseplate and is used for cutting circular disks from, or boring circular holes in, sheet stock. It pivots about a small pin rotating in a blind hole drilled in the center of the stock to be cut. This one is based on a design from Bill Hylton’s excellent book Router Magic.
As much as I like Hylton’s project, I wanted to design a DIY trammel that did not require any table-mounted tools or expensive router bits for its construction. My version is laminated from three layers of quarter-inch stock, and replaces Hylton’s custom-milled T-slot and matching arm with a short piece of aluminum 80/20 rail, which is inexpensively available. The arm can be reversed in the slot to cut small radii, and Hylton’s dimensions have been slightly modified to provide a continuous range of possible diameters of about 2-22″.