Lee Valley Tools    Woodworking Newsletter
   Vol. 4, Issue 6
   July 2010
 
   Jig for Drawing and Cutting Ellipses
 

  The completed jig
  The completed jig
Woodworkers 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.

Dovetail slots and keys move and control the trammel's two axes and rotation.
Dovetail slots and keys move and control the trammel's two axes and rotation.

  Dovetail slots and keys control the two axes of rotation.
  Dovetail slots and keys control the two axes of rotation.
For 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.
 
 
           
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