A Precise Method for Drawing an Ellipse
An excellent and common method for drawing an ellipse is to use two push pins, a string, and a pencil. Trial and error is often recommended as the way to adjust the location of the push pins and the length of the string. These two things can be calculated very easily to give the exact ellipse that you want.
First, determine the length and height of the desired ellipse. The total distance between the push pins will be given by this formula: D = the square root of (L2 – H2), where L equals total length and H equals total height.
After placing the push pins on the major axis, cut a piece of string several inches longer than the total length of the ellipse, and form a big loop using a slip knot. Tension the loop with the pencil, slip the knot to make the correct size loop and then tie off the knot so it can no longer slip. The loop of string should be tied so that when it is stretched with the pencil, the loop is exactly half the total length of the ellipse plus half the distance between the pins.
Use a low-stretch string for the job, and when drawing, keep the tension on the pencil the same all the way around. A tiny grommet on the pencil point will help the string slide. They are readily available from hobby stores and electronic stores like Radio Shack.
From Fine Woodworking #121