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 boatbuilder’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:
- Mark the 3 points of the arc on the piece of stock to be cut into an arc; you’ll have 2 outer points and one inner point between and slightly above the others:
inner * point
(I usually put the inner point equidistant from the two outer points, but that isn’t necessary for the method to work.)
- Drive a small brad into the positions of the two outer points; the long compass assembly will “ride” along these as you draw the arc
- Take two straight sticks, and place them along the two lines connecting an outer point with the inner point, forming a very shallow upside-down “V”. (Put the sticks above the brads, so that the “crotch” of the “V” is right at the inner point – see the picture below.) The sticks should overlap a bit in the region of the inner point, so they can be fastened together, and each should be a little longer than the distance between the two outer points. Fasten the two sticks together so they will maintain the angle they make, i.e., so the angle of the “V” becomes fixed.
- The stick assembly is the “long compass”; the arc is drawn by putting a pencil into the vertex (“crotch”) of the “V”, and then sliding the “V” along the brads, always keeping it in contact with them. As the pencil travels in the crotch as the sticks are slid, it will trace out a perfect circular arc through the 3 points! Truly magic!
The following diagram illustrates the long compass in action:
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 lutherie! 😉