A simple jig to draw an ellipse
Arched openings add a nice touch to a doorway, an alcove, or even a piece of built-in furniture. Architects and builders typically choose the easy-to-draw, equal-rise- and-run section of a circle for this application. Just swing an arc, and be done with it. But sometimes an arc doesn’t look right because the curve wants a softer transition to the intersecting wall. That’s when you need the harder-to-draw unequal-rise-run ellipse. The drawing shows a simple way to solve this layout problem.
Start by measuring the baseline. For an arch at the end of a hallway, for example, the baseline is the distance between the walls. Divide this number in half to get the run of the ellipse. Next, determine the height of the arch in relation to its intersection with the wall. That’s the rise of the ellipse. On a sheet of plywood, lay out the baseline, run, and rise of the ellipse, as shown in the drawing. Now drive 10d finish nails at points A, B, and C. Next, square a line from point B the distance of the rise to find point D. Drive a nail into it. Now take two strips of plywood, overlap them as shown in the drawing, and nail them together so that they are in contact with the neighboring nails. Remove the nail at point D.
To draw the ellipse, place a pencil at the intersection of the two strips, and slowly slide the strips, always in contact with the remaining nails, as you move the pencil from point A to point C. Repeat from point A to point B to scribe a perfect ellipse.
Watch Chuck Miller demonstrate how to draw an ellipse.
David Kalin, Kaneohe, HI