have used versions of this ellipse-cutting jig for generations. The one I built went through several modifications, with
a few mishaps along the way.
The distance from A to the center of the router bit is equal
to 1/2 of the minor axis of the ellipse. The distance from B
to bit center is equal to 1/2 of the major axis. The distance
between A and B determines the differential, or ratio, between
the axes. The size of this base (14 sq.ft.) limits the adjustment
range from 3-1/2" to 6". The adjustment range is logically
1/2 the finished oval. In plain English, the closer A is to
B, the fatter the ellipse and vice versa. Finally, the distance
the router is from the center of the base determines the overall
size of the ellipse.
slots and keys move and control the trammel's two axes
the base, I started by laminating a 15" square of 3/4"
MDF and then trimmed it to 14" square. Using a sharp knife,
I inscribed two perpendicular lines on the back and over the
edges. These lines must be accurate, as they are used for alignment
when employing the jig. I cut two dadoes at right angles and
1/2" deep. This makes it easier when cutting the dovetail
slots, which is done using a 3/4", 1 in 7 slope dovetail
bit set to 9/16" depth using a router table. I overlapped
the cuts, rotating the stock in order to center them. I cut
the square board round and then rounded over the top edge. Using
a table saw, I made two dovetail keys, each 2-1/2" long.
It's necessary to fiddle with this process and make several,
as the tightness of fit determines the overall accuracy. In
the center of each key, drill a pilot hole to take a 10-24 threaded
insert, which is installed from the back of the key to prevent
it from being pulled out during use.
slots and keys control the two axes of rotation.