First, and most importantly it is for structural integrity of the back. As the guitar goes though environmental changes the arch relieves much of the stresses in the wood. Let’s say the guitar is constructed in a relative humidity level of 45 percent. At some point the guitar is subjected to humidity levels as low as 20 or 25% for extended periods of time.
You will notice the arch in the back, depending on the amount of arch, we flatten out. In the most extreme cases I have even seen them go completely flat or even go convex in shape.
The arch allows the wood to move an more or less relax as it shrinks. If the guitar remains in this environment of long periods of time the back will split, the center joint will split and you will also have other assorted problems. See the article on Guitar Humidity for more information on this.
Second, the back is arched to reflect back the tone toward and out the sound hole, much like an arched violin back, mandolin back or banjo resonator.
All of the plans we sell at Georgia Luthier Supply show back arch, for both acoustic and classical guitars and Ukuleles. Your plan should as well. If not visit our Plan Shop.
Making the Arch Template:
Most guitar arching uses the same arch radius over the entire length of the back plate. Ukulele backs on our plans are all the same as well. On my guitars and ukuleles, I also provide an arch from front to back or from head block to tail block. This type of arch does not affect your bracing arch though. You can make this arch by manipulating the taper on the sides.
Your plans should clearly show the back arch or have a diagram of it, brace diagrams etc. If you have this option, do the following:
Cut out the back brace template from the template drawings and glue it onto a piece of Plexiglas, or lexan or a thin piece of hardwood, such as mahogany. I use 3M spray adhesive for this. It releases and holds paper very well.
Cut the brace template to within 1mm of the line with your Band Saw or coping saw. You can also use a Dremel Rotary Tool if you don’t have a band saw.
The best way to finish the template in on a Stationary Belt Sander with a fence attached. This will make short work of this and you will be able to sand right to the center of the line. This step can also be finished with careful hand sanding with sanding sticks.
GLS Tip: Don’t have a brace or arch diagram? No worries, it is easy to replicate an arch without having to construct one with a string and a pencil. An old wood workers trick is to take a thin strip of wood, anchor both ends with stick pins or small finishing nails and push up in the center of the strip, until you have the correct radius. Then hold in place with another nail, nailed through the strip.
This will give you a perfect even radius every time.
Marking The Braces:
Now you can simply transfer your back arch to your bracing. Scribe the brace center-line on the template permanately. Also mark the center-line of the braces in a similar manner. Carefully mark the arch on the longest brace.
GLS Tip: Choose your brace wood and prior to cutting the braces to thickness, mark and cut the arch on the band saw and finish up on the Stationary Belt Sander. Now simple cut the brace widths on the Band Saw and run them through the Drum Sander and you are done! Quick and easy and all the braces have exactly the same radius on the bottom.