Official Luthiers Forum!

I bought some 4/4 kiln dried black walnut. Made it into a scarf jointed neck blank, inserted a double action truss rod, glued down fretboard and cut out the basic headstock/neck shapes, flushed the neck wood up to the dimensioned fretboard, drilled tuning key holes and radiused the fretboard. At this point the neck was perfectly straight. All I had to do was carve it and it would be complete. It sat for several days before I carved it. Neck still perfectly straight with no tension on the truss rod. I carved the neck, routed my neck pocket in the body, set my neck angle and glued the neck in with Titebond. At this point the neck was still perfectly straight. Left the neck clamped into the pocket overnight. Came back in the morning and the neck had developed drastic backbow overnight. Not twisted - just perfectly symmetrical backbow. I set the guitar aside for about a week to see what else would happen and it's stayed about the same. Huge backbow. If I adjust the double action truss rod forward as far as it will go the neck will finally straighten out. I have not fretted the board - which will likely also introduce a very small amount of additional backbow.

I've built a number of guitars and I've never had this happen before so I'm looking for tips or advice from anyone. What would you do in this situation?

Sorry to hear that. Sounds like the way it was milled and kiln dried led to the problem. Uneven moister absorption is the culprit.

Since there is no twisting I'd guess it will eventually straighten out once under tension from the strings and truss rod.

If it is for a customer it would be a no-go but if you can string it up and keep it around for 6 months it might turn out fine.

_________________
Joe Beaver
Maker of Sawdust

So far my best idea is this:

I'm considering routing a 1/4" channel along the back of the neck contour and inserting a long piece of 1/4" hardwood (basically like what you see on one piece necks where the truss rods are inserted). I would glue in the 1/4" strip of hardwood while the neck is clamped down and held perfectly straight. I would then of course flush it up with the carved neck. This would hopefully act to conteract at least some degree of the backbow.

perhaps a piece that is bowed in the opposite direction?

I would take no drastic actions myself, at least for a few weeks. Finish it up and see how it acts under tension. You may be lucky. IMO, wood does what it wants/needs to and trying to fight it's forces is only asking for more problems. Of course, YMMV as they say. Good luck.

Good thoughts. But I'm totally jumping in and fighting with it. haha I clamped the neck down in the center of the fretboard and am forcing forward bow into the neck. I put small space heaters on either side of the neck for a period of time til everything got nice and warm. I've taken the heaters away and am going to leave it clamped until it all cools back down. Just a small little experiment to see how this effects it and if so if the changes hang around over the next few days. There is a highly viewed and rated video on youtube demonstrating putting a neck in the oven at 150 degrees while clamped against a steel beam to make it straight again and good results came of it. I just can't exactly put this entire guitar into an oven. So I figured I'd experiment with this approach a little bit.

All wood has some internal tensions of some form or another as trees usually don't grow perfectly straight up. Those internal stresses can show their ugly heads at the most inopportune times, especially when you cut into them. Over time it will relax, though how long it might take is anyone's guess. You're at least fortunate that it wants to back bow and not the opposite.

Also, it may be just a short lived thing where the moisture from the titebond works its way through the neck, creating the back bow as the part of the neck near the fretboard expands. If that's the case it may resolve itself in a just a week or so.

I'd be willing to bet dollars to donuts that those good results were short lived.

I had similar thoughts and that's why I let it hang out for over a week without touching it and if anything I think the backbow got sliiiightly worse. The guitar is too far along and too nice to give up on though. It will be fixed. It's just a question of how at this point. But I'm absolutely going to legitimately fix it in a respectable fashion.

Maybe. I don't know. https://www.youtube.com/watch?v=MtAJDbPrNnU

Before I went to town putting a channel down the back of the neck I would remove the fingerboard, put two channels in the neck, one on each side of the truss rod, and put in carbon fiber bars. It will give you much better results than putting wood in the bottom of the neck.

_________________
Joe Beaver
Maker of Sawdust

It's definitely a good thought and it had also crossed my mind. But man I don't know about removing the fretboard. I've never done a lot of repair work and separation of glue joints work. I removed a fretboard one time from a neck and it went pretty well. Was able to re-plane the surface of the neck and then just had to use a new fretboard. The old fretboard ended up being all twisted and warped from heating it with an iron and peeling it up. But this was a neck that had not yet been carved and was not glued to a body. So I had plenty of leeway with dimensions. This current neck is already glued into a body, fully carved, finish sanded, the proper neck angle is set and everything. I'd be pretty concerned about destroying the whole guitar if I removed the board. But I'm open to it. I need to think more about it.

So after leaving the neck clamped to my workbench for about 3 hours with a lot of forward bow forced into it (I'm talking like nearly 3/8" at the 7th fret) and space heaters warming the neck to probably 130 degrees or so to the touch for the first 1/2 hour of that time the neck now does not have any back bow and has a slight bit of forward bow. I was pretty amazed and was expecting to be disappointed. I'm letting it rest now in my 40% humidity shop to re acclimate. We'll see what it does in the coming days - that is the real test. The results I just had might not last at all. After that first 1/2 hour of heat I can now feel a bit of a lip where the fretboard meets the neckwood because of how the woods moved differently with the heat. I saw it as a good sign that the wood was shifting and changing properties. We'll see what happens. It's all an experiment at this point. Acoustic sides and so forth are bent in a similar manner so maybe a good portion of these changes will last. Even if only a half to one quarter of this initial movement lasts, I think that will be enough so that I can comfortably and confidently move forward with the guitar.

Sounds like a plan

_________________
Joe Beaver
Maker of Sawdust

The book "Understanding Wood" shows the theory and practice of turning trees into wood and explains how wood can end up with stresses in it, some of which can be fixed and some of which ruin the wood. Get a copy of this book to see some eye-opening pictures of how screwed up wood can be. I like your bold experimental approach. No matter what you do, I wouldn't breathe easy or sell the guitar until it had been stable for like four or five years.

Looks like you got most of it out.
I 've found fine carving or scraping after the neck is installed and under tension it will move quite a lot as the stresses are released with each pass of the scraper and the neck gets thinner.
I don't usually need to take a lot off.
But I suppose a lot would depend on the type of wood and cut and the profile limitations

_________________
You just pick up a chord, go twang, and you're got music. -Sid Vicious

Next time use epoxy to glue on the fretboard and the neck will be straight.

I would say that at the point in the process which you "routed my neck pocket in the body, set my neck angle and glued the neck " is where the trouble started.
Maybe when the glue dried in the pocket it got tighter and with the FB glued down to the top at the same time it pushed the head end back, ie the FB in a way got longer while the neck shorter in a sense. When clamping it might not have been obvious.
So a solution to prevent it in the future might be to not gluing the FB down until after the neck pocket is set and dried.
Just a theoretical observation

_________________
You just pick up a chord, go twang, and you're got music. -Sid Vicious