Larry Ciesla

ellipse jig

Shown is a jig to create ellipses or ovals that is fairly simple to make. 

The jig starts with a base made either from high quality plywood or melamine.  In this particular example I'm making a base that is 18 inches square.  The size of the base will effectively determine the size of the ellipse or oval that you can create.  Once you understand the basic principle you can scale this either larger or smaller depending on your needs.  Also note that in the actual jig I created in the photo above I clipped the corners to facilitate travel of the router around the jig.  This may or may not be necessary, again depending upon the size of the jig you end up with.  For simplicity I'm not showing the clipped corners in this general discussion on how to make one of these.
The next step is to make and attach four quads that are attached on top of the base as shown.  You can also make these out of the same material as the base.  The quads are sized such that a 3/4 inch track is created running as shown in the picture.  You'll end up with a track that is 3/4 inch wide and 3/4 inch deep running in both directions.  These will act as guides for the runners that I'll explain in the next step.
Two slides are created to precisely fit into the tracks created by the quads.  These slides must be slightly less than 3/4 inch thick so they will float just above the plywood base.  Their width must be just slightly less than the width of the tracks to allow them to easily slide thru the tracks, yet not allow any significant slop.  In this particular case, I made these 4 inches long and drilled a hole in the center of each slide.  I waxed both the slides and tracks to allow the slides to easily move thru the tracks.

Next you will make a "router carrier" that will hold your router as the carrier is rotated around the base.  The precise dimensions of the router carrier will vary depending on the ellipse you're trying to create.  In this example the carrier is 4 3/4 inches wide by 30 inches long and is made from 1/2 inch plywood.  Making the carrier out of 1/2 in material makes it easier for the router bit to protrude enough such that it is able to penetrate the work piece you'll be cutting the oval in.  Note that two holes are drilled in the carrier to which you'll attach the slides.  I used a machine screw that I inserted from the bottom of each slide.  I drilled a larger hole in the bottom of each slide to allow the screw head to be buried in the slide so it would not interfere with the motion of the slide thru the tracks.  Attach the other end of the screw just tight enough to hold the slides to the carrier, but allow each slide to move and rotate.

The distance that you drill the holes in the carrier are with respect to the edge of the router bit.  Take the major and minor axis of the ellipse you want to create, and divide each of these measurements in half.  Drill the holes in the carrier along the center line of the carrier that distance from where the edge of the router bit will contact the work piece, then attach the slides.
Here is an imaginary view of the jig with the base removed.  It is shown at the position that represents the major axis.  As the router is moved around the jig, the slides will cause the router bit to cut a perfect oval or ellipse in the work piece.
Here is the completed jig with the router attached.  Note that I've attached a block to the end of the router carrier that keeps the router-end of the carrier above the work piece surface that same height as the jig itself.

Please note that all the dimensions I've shown are arbitrary and you can scale dimensions for your jig larger or smaller depending upon the size of the ellipse you want to create.  There really is no substitute for simply building one of these and experimenting with it yourself.  I've been able to make a couple of different sized bases using the same router carrier to make different sizes of ellipses.  You can simply drill new holes in the carrier to match the axis dimensions of different ellipses.