Yesterday morning at 5:00 a.m. the humidity was about 97% and it was raining. By the time I got home and into the workshop around 3 p.m. it was 60% and falling.
The last few days I worked on building the forms that will be used for bending the sides and the spreaders that will hold the sides snug against the mold. I finished them and looked at my hygrometer and the humidity was 45%--perfect gluing conditions.
So I got the jointer and joining jig out and got the back joined.
I put that aside to dry overnight and decided that I had just enough time before going back to speak at the 7 p.m. Refuge service to maybe get the x-braces glued on the top.
Steel-string acoustic guitars are sometimes called "flat-tops", and originally they were and sometimes are. The first two guitars I built the tops were flat, but it has become pretty standard to give the tops a very slight arch, which adds a little bit of strength. Whereas the back has a noticeable curvature--I use a 15' diameter arch for the back--the arch of the top is only a 30' diameter. On a finished guitar you wouldn't notice this unless you laid a straight-edge across the lower bout.
I have a couple of radius dishes with low-grit sandpaper affixed to achieve the arches, and I use them to radius the braces. Here's one of the x-braces on top of the 30' dish, and you can see that small gap under the middle.
After sanding the brace for just a couple of minutes it is radiused. The gap is gone.
I do this for both x-braces. I won't radius the braces in the upper bout; I want this area to be flat so that there is a good gluing surface for the fretboard extension. Some people radius all the braces on the lower bout, but apparently just doing the x-braces is sufficient, and that's what a lot of builders do.
Before I can glue them on the top, notches have to be cut in the braces where they cross.
After drawing the brace positions in pencil on the back of the top, I place the x-braces in position and mark on the lower brace where the upper one crosses.
I also mark about halfway up the height, and then place the brace in my vice at this mark. I use a small saw to cut the sides of the notch and a chisel to remove the waist. When it's even with the top of the vice, I know it is the right depth.
I repeat the process for the upper brace, creeping up on the depth of the notch so that the gluing surfaces are perfectly even. I want a tight fit, and I get it even though I actually cut the notch in the bottom brace a little wide. The top notch is tight, however, so after everything is glued I'll glue a wedge of brace material in the gap and sand it flush. This is more for the transference of vibrations than it is for structural integrity.
To clamp the x-braces would take some clamps with really deep throats--like around 8". I'm not even sure they make clamps like this. To get around this, someone invented a contraption called a "go-bar deck." This is just two thick boards held 2 feet apart by pipes. The clamping pressure comes from flexible fiberglass rods or "go-bars" that are a couple of inches taller than the deck. One end goes on the brace, and the other on the top of the deck, and you are able to get even pressure along the length of the braces.
The big factories like Martin and Taylor use rubber seals and vacuum pressure to glue all their braces. The vacuum actually speeds drying time. But this is how it was done for years and how most of the little guys still do it. A vacuum system would cost a few hundred dollars; I built this deck for maybe $30. At Vermont Instrument we used the bamboo stakes that are used in gardening as go-bars, and that's what I did for the last guitar, but they vary in thickness and tended to break, so I sprung for the fiberglass rods for about $2 each.
These are important braces, so they get an overnight dry. Next I'm going to work on the back braces so I can stabilize the zebrawood.
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