Tools and Materials:
Go Board Gluing Tool
Titebond II or III for Domestic Hardwood
2-Part Epoxy for Exotic Hardwoods
1/2″ Chisel for Glue Cleanup
Acetone for Epoxy Cleanup
Rubber Gloves for Acetone
Respirator for Acetone
The most challenging trick with bracing the Back plate in to attain a consistent arch on the back plate. The reason for the arch is two-fold.
The Back Arch Acts To Reflect Sound:
First, the arch acts as a reflector of the sound waves that are produced by the top plate. As these sound waves reach the back, they excite the back to resonate as well. The arch acts like a concave magnifying glass only with sound waves in lieu of light waves. The shape focuses the waves and directs them upward toward the top plate and through the sound hole.
The Back Arch Gives The Instrument Added Strength and Stability:
Secondly the arch gives the back, and the whole instrument for that matter, greatly increased structural strength, resisting tension that is exerted from the strings. The arch also gives the instrument some “breathing room” for humidity changes. As the instrument is introduced into a space of extremely low humidity, the wood shrinks and some or all of the arch flattens out and saves the back from severe cracking. Also the back plate wood is not nearly as forgiving as the top plate wood and will crack far more easily. Refer to our article on Humidity in the Shop for more humidity control information.
The bracing needs to be selected for the back and acclimated to the shop. Refer to the article on Guitar Bracing for more information.
Preparation of the Back Plate:
The use of exotic wood back/side combination for the instrument construction, care has to be used to to remove the resin that is inherent in the wood. This resin will prevent glue from adhering to the back plate and the sides. Wear rubber gloves and a respirator and put a liberal amount of Acetone on a rag and rub the entire surface of the surface of the back plate that will receive the braces. When doing this, I usually clean the entire surface of the back plate to minimize differences in color tone of the wood. Occasionally look at the rag and keep up this procedure until no colored residue comes off on the rag. You are then ready to glue.
If you purchased Guitar Building Plans, from Georgia Luthier Supply, refer to the bracing profile for the back plate.
Make a reusable template for the back arch. This template is from the Back Arch Template on the Template Sheet of the plans. The back arch template can be constructed of Plexiglas, Lexan or thin hardwood. Mark the centerline on the template and carefully sand or plane the arched edge on the bottom of the template.
Lay the template on the bottom of each brace and trace the arch outline on each brace, being careful to keep the template centered on the brace and equal at each end.
Next take a small plane, disc sander or belt sander and sand right up to, but not through the arched line on each brace. Check all the time that you are sanding square and that the arch face is exactly perpendicular to the side of each brace. Once all the braces are shaped and cut to length they are ready to go to the Go Board Gluing Jig for gluing the bracing. Install an Arch Mold in the GoBoard Jig that matches the arch for your particular instrument.
I prefer to use 2-part epoxy for exotic hardwoods and Titebond for domestic hardwood. Also, you will find that it makes life a lot easier if you perform most of the glue squeeze-out cleanup prior to the glue setting up completely. For Titebond I prefer to remove the glue when it is getting a bit crusty. Use a sharp 1/2″ chisel and run it down the face of each brace and the glue will zip right off. Epoxy is a bit harder to remove. Still remove most of the residue with the chisel and follow-up with an Acetone-soaked rag.
To get all of the back braces consistently arched and flat on the gluing surface, assemble them together as one group, clamp them together and sand the surface of all the braces at one time with a flat sanding block. It is far easier to keep the surfaces of the arched gluing surface perpendicular to the sides this way and you are also forming all of the braces exactly the same.
Another method is to cut the braces out of a single piece of bracewood. Cut out the arch on a band saw, then carefully sand the gluing surface arch on a stationary belt sander. Sand right up to your line or mark on both sides of the piece of wood. Next cut the bracing to width on a band saw or table saw and finish sand the sides.