Official Luthiers Forum!

I suppose I was thinking specifically of the way I do both structures, in which case the headblock extension has a lot more wood in it, and can be taller (and thus stiffer) than the A frame braces, since it doesn't need to pass underneath the upper transverse brace. But actually, the A frame does have the advantage of being harder to mess up.

The headblock extension is so rigid, it focuses all the neck force onto the upper transverse brace, so that has to be rock solid to survive. And if you leave a gap between the headblock extension and UTB, a small section of soundboard wood gets put under a lot of stress. And if you don't glue it to the UTB, then it could conceivably still flex right at the meeting point, allowing the neck to rotate (though in reality it probably wouldn't happen, especially if the back is prevented from elongating).

And funny reading back on it, that Eric was talking about fully cylindrical backs, and I was talking about just the part between the headblock and first brace being flat in the longitudinal direction due to the foot. That is, assuming the foot is flat. Which I'm really not sure how often is the case, so I should probably just stop talking

Dennis K wrote:
"I suppose I was thinking specifically of the way I do both structures, in which case the headblock extension has a lot more wood in it, and can be taller (and thus stiffer) than the A frame braces, since it doesn't need to pass underneath the upper transverse brace."

Now I'M confused.

I actually used to extend the neck block 'chin' until it nearly touched the UTB, and still get it as close as I can. As you say, that puts a lot of stress on things, and the A brace was the solution for that. There's no constraint on how tall the end of the chin can be; I usually make it about 5/8". The A braces go THROUGH the UTB. They're about 5/16" square, depending. They plug into the neck block, and are just notched through the UTB and inletted into the upper arms of the X, or the waist brace in a Classical.

I know this system works. I've had a few gutiars I've made that have been knocked off stands or tables, hitting the back of the neck on the floor. Before I went to the A brace this would often fracture the top along either side of the fretboard, break the glue line on the UTB, and the neck would shift inward. After seeing that a couple of times (I'm a slow learner) I added the A brace. Since then I've seen at least two of my instruments involved in the same sort of accident. Although both had big gouges in the neck there was no other damage.

Eric:
That back arch looks a lot like what I get on dulcimers, and I have to wonder it's there for the same reason. When I fit the back on a dulcimer it's face down on a fixture, sort of like the Classical guitar on a solera. I arch the back braces (4 of them), and dress the edges of the sides to the proper slope between the upper and lower braces, but don't dome it lengthwise to speak of. The ends, above and below the bottom and top braces are more 'domed'. This is a natural way to do things with this construction, and preserves nearly level sides for most of the length as Eugene Clark mentioned in his talk at the GAL a few years back. I wonder if it's one of those things that just 'falls out' of the method, and then people look for 'reasons' that sound more convincing than "I just do it that way". I don't know if it's really any more stable: I haven't noticed any issues in my guitar building that I could attribute to the domed backs.

Alan, I'm adopting the a brace myself. The two I'm working on now will be my first attempt at the a brace. When you say they go "through" the UTB, do you mean the UTB is notched tightly for them and glued to them or do you mean they pass through the UTB more or less unfettered (some amount of space around the braces so they don't touch the UTB)?

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I inlet them as tightly as I can. They're on the top 'compression' surface of the brace, and I try not to leave any space for movement to get started.

Thanks, that is what I thought but when you said through it made me wonder if I was missing something. I'll also assume you glue them to the notch in the UTB.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Inspired directly by Al I also use the A frame bracing and inlet it into the UTB and neck block. Here is a pic of a guitar all ready to be glued up.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.