Official Luthiers Forum!

Jeff, thanks for your continued input. Much appreciated. Your point about weakening the structure with the installation of the bolt-on hardware is well taken. What I'm actually thinking I might do, if I go the route of the ebony "through-spline" I described above, is install barrel nuts in the spline before assembling the heel, rather than using threaded inserts. Then, the holes I'd have to drill through the face of the spline would be much smaller - just big enough for the bolts to pass through. I like barrel nuts better, anyway.

Andy, thanks for the cool idea. That makes a lot of sense. The problem with it that I see, though, is that this bolt would get in the way of the truss rod, and possibly the CF rods in the neck as well. If the head of the bolt were set down in far enough that the truss rod would go over top of it, I'm not sure it would work, because the part of the heel that needs the reinforcement the most in this elevated-FB design is the uppermost part. Hmmm...

_________________
Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Instead of washers, use steel plates with holes drilled in them. Instead of one rod, use two: one on each side of the truss rod. They don't have to be to thick. Angle them if you have to.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Andy, thanks again for your ideas. Much appreciated.

Giving some more thought to this, I think I see a potential problem with it. Maybe your experience or knowledge of this pre-stressing technique with wood will override my speculation about it, but here's what I see happening. The bolts would be tight when initially installed, but after some cycling of higher and lower moisture content in the heel wood (due to changes in humidity), the wood will become compressed slightly and the bolts wouldn't be tight anymore, so they would lose their effectiveness.

Background info for anyone who may not know this: any time you confine a piece of wood (in the cross-grain orientation) tightly between stationary objects, the wood will try to expand when its moisture content rises, causing it to partially crush/compress its own fibers, so that when the moisture content drops again, the wood is smaller than it was before, and small gaps open up on either side of it. The heel, being oriented cross-grain to the proposed bolts (and confined by the washers/plates at each end of the bolts), and being a rather long stretch of cross-grain wood at that, would be quite vulnerable to this, I think.

On the other hand, if the bolts were used in addition to a spline with grain running vertically up and down the heel, that might work out well.

As an aside, there's always a risk, when gluing cross-grain to long-grain, that the joint will fail eventually as a result of the expansion and contraction of the cross-grain wood, but I'm confident that a well-made glue joint between the spline and the cross-grain portions of the heel would prevent this from happening.

_________________
Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I've never actually heard of pre-stressing wood, the concept comes from working with concrete. Concrete has excellent compressive strength but terrible tensile strength. It also has nearly zero plastic deformation. By "pre-stressing", usually using cables and big washers, the concrete is put under so much compressive load that under normal usage, any deflection shows up as a decrease in compression and never becomes a tensile load. Spoked wheels work in a similar way.

I think your concern about the long term compression of the wood is a valid one. I think you should take some scraps from your next neck build and stick a bolt through them. Tighten it down and let it sit for a while and see what happens. The degree of this effect would be dependent on the thickness of the heel and it's stability WRT humidity.

I like the idea of laminating a piece of hardwood cross grain in the neck as well.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

For anyone who's interested, I've decided what I'm going to try on two instruments I'm building right now. I'm going to make an ebony "through spline" like I described above. It will be 3/4" thick. The grain will run vertically up and down the heel, from the underside of the FB down to the heel cap. It will extend back into the neck just beyond where the curve of the heel straightens out into the shaft. The joint will be a butt joint, bolted into a birch ply neck block using barrel nuts in the spline. After all the parts of the heel and neck are assembled, I'll carve the heel in my usual way, making a fairly deep (in the dimension parallel to the length of the FB) heel for added strength. The ebony spline will end up showing as a 3/4" wide black stripe running up the middle of the heel and terminating with a semi-circular end just beyond where the curve of the heel straightens out into the neck shaft. I think that will look elegant enough, and, when people ask about it, it will provide the opportunity to tell them about my cutting edge heel construction technology.

I'm fairly confident the ebony "through spline" would make the heel rigid enough as is, but, as insurance, I'm going to do a variation of Tony's idea (thanks, Tony!). Before installing the spline, I'm going to rout 2 grooves in its sides, one on each side, at a 45 deg angle, running from the edge that mates with the guitar body, about 1/2 way down the heel, up and back to near the upper back corner, under the FB, and inlay rectangular cross section CF rods into these grooves. The barrel nuts will go through the spline above and below these CF rods. The CF rods in the neck shaft will pass right along side (to the outside) the ends of these angled CF rods in the heel; that means spacing the CF rods in the shaft 3/4" apart, but I've drawn a cross section of the nut end of my neck and there's plenty of clearance for that spacing (or I could splay the CF rods in the shaft so they get closer together at the nut end of the neck, but I don't think I'll bother doing that). I suspect that spacing the CF rods in the shaft relatively far apart is a good idea, anyway - seems like they might be more effective at stabilizing the neck against twisting that way.

It seems to me that making the heel super rigid like this will likely have sonic benefits as well as solving a structural problem that arises with the elevated and floating FB. If you believe (as I do) that stiffening the neck, in general, has sonic benefits, then this is further stiffening in a way that should be beneficial as well. And it dovetails (not literally, but you know what I mean!) right into the stiffening of the body through the use of the internal CF struts and the CF capped back bracing. The soundboard, of course, is designed to be as flexible - actually more flexible in the upper bout - as a conventionally made guitar.

_________________
Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

One more thought I had on the pre-stress idea: One doesn't necessarily have to use bolts to get this concept to work. If one were to use the properly sized wires, they would just stretch with seasonal changes of the wood.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com