There are at least three accepted methods for drawing an oval. One uses two nails and a string, which is quick but sloppy. The second uses a square and involves laying out the points of an oval (or ellipse). This is accurate, but it takes time. The best method is to make a cross-shaped jig. Using a trammel with two pivot points, you can easily lay out an oval in a couple minutes.
Start with a flat board, preferably plywood or particle board (I used scrap medium density fiberboard). Cut the corners off the square board to make an octagon; this will allow the pencil to draw the oval. Lay out a cross on the center of the board.
With a router, cut a sliding dovetail channel about halfway into the thickness of the board and right down the center lines of the cross. It’s OK to use a plain groove, but dovetail slots will hold the trammel parts better.
Use a compass square to figure out the angle of the dovetail (sometimes it is written on the shank of the bit). Set the table saw for that angle. Rip a piece of wood that is wider than the slot and about 3/4″ thick. Rotate the wood 180° and rip it down until it fits into the dovetail slot. This will yield enough wood for plenty of trammel sliders. Cut two 2″ sliders from this strip.
Clamp the trammel sliders in a hand screw and drill and counterbore for the 1/4″ x 1-1/2″ hex cap screw that will go through the bottom of the sliders and stick up through the trammel. With the screws in the sliders, insert them into the slots on the board.
The trammel is a piece of 1/2″ maple with a 9/32″ hole for a pencil. Lay out and drill a series of 1/4″ holes that are 1/2″ apart down the center of the trammel. The holes should start near the center of the length and run to one end. Mark these distances next to the holes.
Add rubber feet on the bottom of the jig to keep it from moving while in use. To use the jig, lay out a center cross on the object on which you want to draw an oval. Position the jig on tthe center of the cross. Figure out half the width and length of the oval. These are the distances that you will put the screws through on the trammel. Insert a pencil into the trammel and rotate. The result is a perfectly formed oval.
Jim Stuard is a former associate editor at Popular Woodworking.