My elliptical obsession (or why everyone needs a set of trammel points)

I’ll admit it.  I have an obsession.  I’m fascinated by ellipses.  I’m amazed at how many different ways you can create an ellipse.  And most of the regular followers of this blog know that I’ve carried on about the subject, ad nauseum.

I’m just going to throw one more method out there for your consideration.  Then, I promise that I won’t visit this subject matter again…

Most serious woodworkers are familiar with the “string and nails” method of making an ellipse.  It’s not a bad method, but it can be imprecise, as nail placement can be a little off and string can stretch.  In fact this method is many times referred to as the Gardener’s Ellipse.  It seems that elliptical garden beds were quite the rage and this became the preferred method of layout.  However, in less agrarian applications, there is another method that is both simple and very precise.  It does require a set of points and some type of marking device.  Usually these points would be manufactured trammel points, although they can be as simple as two nails.  So, here’s the method:

First, strike perpendicular lines;

Our goal is to create an ellipse with a major axis of 18″ and a minor axis of 12″.  Set the points at half of each axis from the marking device.  In this case half of the minor axis is 6″ and half of the major axis is 9″;

Secure any type of squared guide in one of the quadrants that you’ve drawn;

Carefully begin to rotate the device while keeping the points in contact with the square guide.  (BTW, most folks will refer to two trammel points on a stick as a “beam compass”.  A beam compass, of course, is used for drawing arcs and circles.  But with the addition of a third point, the beam compass becomes a trammel beam (also referred to as a “trammel rod” or simply “trammel”) and can be used to create elliptical lines, something that can’t be done with dividers or wing compasses, hence the layout of “two arc” ovals.  Many old texts show the trammel being used with a cross shaped guide.  This would have been a common device in engineering or layout departments.)

The following series of pictures demonstrates the actual travel of the trammel beam.

After one fourth of the ellipse has been drawn simply position the squared guide in the other quadrants and repeat the process.  The result will be a precise, repeatable ellipse.

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Tags: drawing ellipses, elipses

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