Radiused Hollow Forms

RADIUSED HOLLOW FORMS

by Don Musser

When I first started building guitars, one of the most difficult tasks was the shaping of all the various top and back braces to a smooth uniform curvature. Doing that job free hand always resulted in uneven curves and flat spots that didn't seat and glue properly to the top or back.

One of my neighbors happened to be David Russell Young. When I questioned him about how he handled this problem I was shown two large plastic circles having concave dished curvatures that had been cut on a huge lathe. One had a 25 foot radius for top braces; the other had a 15 foot radius for back braces. With the use of some sandpaper, those forms al- lowed him to produce braces with even curves and a smooth flawless gluing surface. They also provided a back up surface against which the gluing and clamping of the braces to the top or back could be carried out.

I never ran into the guy with the lathe, but I did spend some time with a router mounted on a trammel and some particle board slabs spinning on a pivot and eventually I came up with some very useful hollow forms that radically improved and facilitated my assembly methods. Here is how they are made:

Take a 26" length of fir 2" X 4" and scribe an arc having the desired radius on it.

Cut the arc as smoothly as possible and then split the board into two 1" thick pieces so you end up with 2 pieces each having the same arc.

Next assemble a 26" square using the two arced pieces on opposing sides of the square.

A back up board (plywood or particle board) with a steel pivot rod positioned in a hole drilled in the center completes this part of the jig.

The next step is to build a trammel mount for a 1/2 hp router. Something like this:

The grooves are cut to fit the arched runners. There is an unseen hole cut through the board through which the router bit protrudes. This trammel then fits on the arched runner and allows the router to cut an arc into a board mounted within the square below.

Now take a piece of particle board (more a little further on about this particle board) and cut it into a circle having a 24" diameter. Drill a pivot hole in its center and mount it in the cutting jig on the pivot.

The fun now begins - start cutting. Use a 1/2" carbide cutter and make shallow passes. You'll have to turn the form after each pass and the depth of cut can, of course, only be changed after you've made a full rotation.

On my set-up, at a certain point and depth of cut I found that the cutting action actually started to rotate the form on its pivot. It turned into a lathe type operation as the form was propelled into a rotating motion as the cut progressed. You will have to feel it out as you go. It was sure a surprise to me though when the form started rotating with real force and speed and the chips started flying as the router took its bite.

My first attempts were done on just plain 3/4" particle board. For some reason the particle board tended to warp as the cut progressed. The problem was finally solved by using a laminate combination of 3/4" industrIal grade and 3/8" standard grade particle boards. The two pieces were laminated together using plastic resin glue. The 3/8" piece was then segmented with an 1/8" router cut as in the drawing at below:

The actual cutting was then made only on the 3/8" piece. The lamination combination along with the segmenting of the 3/8" piece was very effective in providing a stable form. There were no changes as the cut progressed and the forms have maintained their same curvature through five years and fifty odd guitars.

In addition to providing surfaces for the shaping and clamping of arched top and back braces, these forms can also be used as guides for the contouring of the guitar sides prior to the gluing on of the top and back.

Don first wrote this article for our catalog back In 1984. Since then he has continued building fine steel string guitars, and at his home in New Mexico, he mines silver for fun and profit.

Text From The Luthiers' Mercantile Catalog - 1993