Just when you thought that the bracing tasks were complete there was more bracing to do. Whereas the back braces are a bit more structural in nature, the top actually has a more complex bracing pattern that keeps it strong but also allows it to vibrate giving the guitar its unique tone. Bracing the top was my most recent task in the workshop.
Laying Out the Brace Pattern
There are quite a number of braces that must be attached to the underside of the top. As I mentioned, because these braces are both structural and also directly affect the sound of the guitar, the braces must be carefully laid out and applied.
To do the layout, I overlaid the guitar top onto the full-size pattern. I was careful to align the center line of the top and the sound-hole with those on the pattern because all of the brace positions utilize these elements as a reference. There are several sets of braces – the main structural braces are the X-braces and they form and X across the top. In the lower bout of the guitar, stemming from the X-braces are the tone bars. Around the sound-hole are some reinforcement strips and above the sound-hole in the upper bout are the main transverse brace and another reinforcing brace. Last, there is a flat bridge plate just under where the X-braces cross.
To layout all of the braces onto the back of the guitar top, I started by transferring the locations of the ends of the X-braces to the top. With a long straightedge I connected these points giving me the X-Brace locations. Next, I marked for the ends of the tone braces and the upper transverse braces. By extending the lines of the shorter tone bars across the pattern and then replacing the top onto the pattern I was able to extend the end marks for the tone bars to the points where they intersected the X-Braces. The upper transverse braces were straightforward to mark across the top and the sound hole reinforcing strips and bridge plate were done in a similar manner to the tone bars. The picture (click for larger view) shows the completed layout on the back of the guitar top.
Notching the Braces
Before the braces could be applied to the top, some work was required on the braces themselves. A half-lap had to be cut into the X-braces and notches were required at the intersection points of the tone bars, sound-hole reinforcing strips and bridge plate. I started with the X-braces. With one brace clamped to the top, I marked the intersecting points of the other X-brace. Then, by overlaying the other X-brace I transfered the marked location to denote where the joint needed to be cut. From there it was a matter of some quick cross-cuts to half the depth of the braces and cleaning out the material with a sharp chisel.
In addition to the half-lap on the X-braces, the intersection points of the tone bars, sound-hole reinforcement strips and bridge plate needed to be notched into the X-braces. With the X-braces clamped to the top, I used the pattern to mark out the locations of all of the intersection points onto the X-braces. I then scored the lines with a knife and filed small chamfers at the marked locations. With a small chamfer applied to the ends of the tone bars, reinforcement strips and bridge plate I was ready to apply the braces to the top.
Applying the Braces
In the first picture you can see the top with all of the braces laid out, fitted, notched and ready to be glued to the top. I applied the braces to the top in a couple of stages. First, the X-braces were applied. In the second picture you can see that I again used the Go-bar Deck to clamp the braces. Because of the breakage of a couple of the wooden dowel rods I used the last time, this time I decided to use fiberglass rods as the Go-bars. These were originally sold as stakes to mark out driveways during the winter here so that the snow plow driver can see where the boundaries of the driveway are. The stakes came in 4 four foot lengths so, I just cut them to size and applied protective screw covers to the ends. This was a much cheaper alternative than buying the fiberglass Go-bars sold by the guitar specialty shops and it worked very well. Again, while clamping the X-braces you can see that I used small strips of stock under the corners of the top to maintain the radius of the top while gluing the X-braces to the top.
After the glue on the X-braces was dry, I moved on to applying the rest of the bracing to the top. I did this in several stages just to keep things manageable. First came the tone bars, next the sound-hole reinforcement strips and the upper transverse braces and last the bridge plate. In the picture you can see the clamping of the tone bars and how many Go-bars were used. Notice that I used some small blocks of scrap to hold down pairs of tone bars. When I did this I just left the X-braces clamped to keep the top in the same position and then carefully clamped the tone bars.
Trimming the Braces
As I mentioned, these braces are attached to the top of the guitar for structural reasons. However, the idea is to reinforce the top and make it stiff without adding weight or hampering its ability to vibrate. As you can see in the pictures, the most of the braces were already scalloped along their length. This scalloping minimizes their weight while retaining their strength. To further allow the top to vibrate, the ends of the braces were trimmed where they will eventually intersect the kerfed lining when the top is attached to the guitar sides. The ends of the X-braces and main transverse brace were trimmed to a thickness of .100″. The ends of the tone bars were trimmed to nothing where they meet the kerfed linings. In the picture you can see the top with all of the braces attached and their ends trimmed. Next up will be attaching the top to the sides.