The Long Compass

The long compass is a really simple and really useful device for accurately drawing a portion of a large arc (a short segment of a circle of large radius, to be precise). From what I've heard, it's an old boatbulider's technique. It's used to draw a portion of a circular arc given 3 points on the arc - generally only useful for small portions of arcs of large radius, like a 15-foot radius arc for which you need a 2-foot-long segment of the circle (e.g., for the back braces on a guitar).

The procedure for making and using a long compass is as follows:

                             inner point
   *                                                      *
 outer                                                  outer

(I usually put the inner point equidistant from the two outer points, but that isn't necessary for the method to work.)

The following diagram illustrates the long compass in action:

Long compass diagram

The compass thus made is only good for the one radius. You can of course make a general-purpose compass that's just two sticks with a hinged joint that permits the angle to be adjusted, so you can use it for multiple-radius circles.

Hope this helps to "round out" your luthierie!   ;-)

Jon Sevy