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I've done about 25 full bolt-on necks using the information from Sylvan Wells and John Mayes. The first few I used threaded 8/32 inserts as recommended by Sylvan and later switched to small T-nuts as John uses as they seemed more stable. This is the first complication I've had.
This guitar is about a year old and the owner had two humidifiers in the case and the environment was about 70-75%. The end of the fretboard warped a little, convex on the bottom. He had one of the new Oasis soundhole units plus a soap dish/sponge type.
He could hear a small crack occasionally when he put the guitar in the case. The action had come up because of the humidity and when I took the neck off I found the fretboard tenon had lifted on the bass side. This was glued with Golf Club shafting epoxy. Sylvan has pointed out that there is significant loss of gluing surface with the T-nuts vs. threaded inserts. This had T-nuts. Bolts were not too long.





I injected Smith epoxy under the tenon with a syringe and it looks solid. We'll see.



I'm going back to the threaded inserts for more gluing surface. Anyone seen anything like this?

Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

IMHO the high humidity was the culprit. Don't question your construction techniques, question your customer.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

I would stay with the T nuts. That is what I use. they cannot be unthreaded out. I have always epoxied my tenon down to the fretboard. So far none have had what happened to you.

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Andy Z.
http://www.lazydogguitars.com

I would prefer threaded inserts over T-nuts anyway, but what gluing surface are you referring to?

As to the problem you're dealing with, I'd be more curious to see the top/shoulder condition, and the block you were bolting the neck down to. Is there any buckling, or does the fit of the fingerboard block not fit tight against the facing surface in the neck/shoulder block?

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Eschew obfuscation, espouse elucidation.

I was referring to the joint between the tenon and the fretboard David. The countersunk T-nuts remove some of the gluing surface between the tenon and fretboard. The fit of everything was good out of the gate but when it came back the end of the fretboard had some convexity with the corners lifted slightly. I was thinking the humidity warped it and the glue joint failed.
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

You may want to consider using solid wood for the block rather than ply. Then it could move with the fb when people mistreat it.

It's too bad that there seems out be an idea out there that humidity is good, no matter how much. A friend of mine had a guitar come back that was in very bad shape and had signs of being over-dry and over-wet. Turns out it had gotten dry and a split started and so the owner put 3 humidifiers in the case.

I wonder if it's because of that Taylor video where they dry out the guitar till it starts to implode and then throw a bunch of humidifiers in it to pump it back up.

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http://www.chassonguitars.com

You can't really make something strong enough to cope with the stupidity of this owner.
It is probably just as well that something as easy to repair as this failed before serious damage to the box occurred.

Thanks for the help everyone. Good thoughts. David I forgot to mention that yes, with this joint there is a very small gap between the bottom of the tenon and the mating surface of the mortise. The owner is actually very intelligent and a really nice guy but didn't quite grasp that too much humidity can be bad too. If that was indeed the cause of this failure then we both learned something and no major damage was done.
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Terry,
That joint is one of the reasons I go with threaded inserts.
The other is that I can drill the alignment hole through the fretboard, ply section and the neck block extension after the neck angle is set so that alignment is never an issue.
If you epoxy or CA them in place, they don't back out.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

FWIW Dana Bourgeois uses T-nuts and builds about 500 a year and I've never seen this problem unless there is some serious over tightening to compensate for a poor fit.

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John Mayes
http://www.mayesluthier.com

How do you lower humidity in a guitar, I know you can raise it with the sponge thing...but I never thought about how to lower it....

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Here is what a Parlor Guitar is for!
http://www.youtube.com/watch?v=oEa8PkjO6_I

Did Dana use epoxy to attach the tenon John?
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I've gone through a half dozen or so builds with Sylvan and it is my impression, and I'm sure Sylvan would concur, that under normal conditions there will never be enough upward pressure at the end of the fretboard to pop the inserts if installed properly. I've also noticed that on some guitars there is very little gluing surface on the tenon, so the inserts do allow much more wood to wood contact. I see no benefit to using T-nuts over inserts. But I do see the benefit of the inserts over T-nuts. That being said, there could be several reasons why this happened, certainly humidity issues, bad glue, or just not enough glue.

titebond

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John Mayes
http://www.mayesluthier.com

This is a common problem with bolt-on FBs, I've seen it a lot.
The FB extension becomes concave under tension, it does not happen to every guitar. Maybe it depends on the ebony used. The solution I think is to shim the tenon so it is flush with the top, minus 10 thou for tightening.

Dana uses 2 x hard maple "skis" CA'd to the FB, with 2 x brass inserts each. So the FB extension is bolted with 4 bolts to the neck block tongue.

I've never been convinced by this system BTW, while I do not think it changes anything in terms of tone, in the context it is not as stable as a glued FB extension.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/