Guitar Back with Bracing
Source: Guitar Building Articles/The Guitar Back
Tools and Materials Required:
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
Cotton Cloth
The biggest trick in bracing the Back plate in to get 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 set 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 Guitar Added Strength:
Secondly the arch gives the back, and the whole guitar for that matter, greatly increased structural strength from tension that is exerted from the strings. The arch also gives the guitar some “breathing room” for humidity changes.
As the guitar is introduced into a space of extreme 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:
If you are building you guitar with an exotic wood back/side combination you will have to remove the resin that exists in the wood. This resin will prevent glue from adhering to the back plate. Put on rubber gloves and a respirator and put a liberal amount of Acetone on a rag and rub the entire surface of the top. 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 Ultimate Guitar OnLine refer to the bracing profile for the back plate.
Make a reusable template that has the back arch indication on the bottom of it and a centerline for the brace scribed on the surface. Lay the template on the bottom of each brace and trace the arch outline on each brace. Also take the template that you made for your back plate and back plate brace layouts and transfer the lines representing the outside of each brace directly on the wood plate. Here is a little tip. If you are using dark wood for your back plate, use a sharp white artists pencil to mark on the dark top. Use only very light and fine lines.
Now take a small plane or disc sander or belt sander and sand right up to, but not through the arched line. 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 you are ready to go to your Go Board Gluing Jig and start gluing the bracing.
Since the bracing has a arch formed on the bottom of each brace and this is what is glued to the back plate, make sure you either have shims handy to place between the back plate and the Go Board Gluing Jig or a curved fixture that runs the entire length of the brace and has the same convex shape.
I prefer to use 2-part instrument 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 your 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. Just take 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.
Remove the back from the Jig. You are now ready to Install the Back Reinforcement Strip and Finish The Back Braces.
Ultimate Guitar Building Tip:
To get all of your 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 wood. 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.
Tags: acoustic guitar bracing, bracing the backplate, classical guitar bracing, Guitar Bracing, how to build a guitar, ultimate guitar building
