Official Luthiers Forum!

Last week I posted on reinforcing the tenon for the Cumpiano barrel nut neck attachment system. I have not had any fail but have heard it has happened. I've used it on all my guitars and was worried. I've done a little crude testing since and would like to report the findings.
I used a dummy neck block, a mahogany tenon with the wood grain as it would be in a real tenon, mounted on a backing board, and drilled for the system.
One was plain mahogany, one reinforced with carbon bars (glued with Smith's All Wood Epoxy), and one with side reinforcement of .100 maple with the grain running 90 degrees to the mahogany. (Glued with Titebond)
A big problem was attaching the tenon to the backing board tightly so that the joint would not fail before the tenon did. Simple gluing pulled off easily, glue with screws pulled off before the reinforced tenons broke, and finally glue plus 6-32 bolts and washers was the only thing that held. (and even then not 100%-see below)

Basic Setup - Glue plus screws.





Forgot I had a torque wrench but I would guess it only took 1-2 ft-lbs to crack a plain tenon







They seemed to fail in tension and along the grain lines

Then I tried the carbon reinforced. I had to use glue plus 6-32 bolts to hold the backing board. Used a torque wrench. At 7ft-lbs the tenon cracked above the reinforcing bars. Obviously a stress riser point.



Here's the crack, the attachment system held well.



The maple reinforced tenon never did fail. At 8 ft-lbs the bolts and washers started pulling through the wood and the threads of one of the bolts stripped



Conclusion:
This experiment does not exactly mimic a real neck and is a bit crude but I get the idea that without reinforcement and a heavy hand on the wrench, this system is pretty fragile by itself. If carbon bar reinforcement is used you should probably extend them the whole length of the tenon and then I think you'd be fine. With the cross grain maple reinforcement I think the tenon would pull off before the attachment system failed. Bottom line, I think you need to reinforce this system in some manner if you use it. I know I will from now on.
Thanks and please add comments positive or negative.

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.

Awesome, thanks a lot for posting this! Very relevant and informative.

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Jordan Aceto
Ithaca, NY

Terry, that's great, thanks for sharing your test with us.

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Jim Watts
http://jameswattsguitars.com

Terry... I haven't had any failures either yet but I think this is one of those Better to be safe than sorry kind of thing. I may use maple on my next tenon. Appreciate the test and thanks for sharing it !

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Anderson Guitars
Clearwater,Fl. 33755

I always worried about my tenons. The maple insert is a great idea. Thanks for sharing! Clinton

Interesting. Thinking "better safe than sorry", I soak mine with thin CA. Maybe I should run a test to see how it compares to an untreated tenon.

Thanks for doing that Terry, it is reassuring!

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

Isn't the maple option what on cumpianos website he recommends I thing he used mahogany, and somebody stood on the neck without it failing

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Formerly JJH

I learn more from my mistakes than my successes

Good test, thanks for posting the results. I've put the maple strip in mine (actually 2 strips), don't remember where I got the idea but I'm glad now that I did!

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Steve Smith
"Music is what feelings sound like"

Thanks for the comments. I'm guessing with a tenon soaked in thin CA the strength would probably be fine provided whoever was working on the guitar was well versed in dealing with bolt-on necks. I am mainly worried about the owner or repair person that thinks you really have to lean on these babies. Also the wood slowly deforming over the years. At any rate I don't really see a downside to adding some sort of reinforcement.
Here's another question, I've noticed that as my guitars settle in the first 6-12 months the bolts frequently need to be snugged up a little. Do any of you guys use Loctite on your threads? How's it work? I've tried compression washers but the tension required to flatten them was more than I wanted to put on the joint. I figured the loosening might be from a little wood shrinkage and the Loctite wouldn't help that.
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'm using the compression washers but don't have any long-term data. The neck block is maple so I'm not too worried about it. We use a lot of different thread locking adhesives at work and I wouldn't want to use them on my guitar neck joints but that's just my opinion.

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Steve Smith
"Music is what feelings sound like"

Sorry to sound like an iconoclast, but what is the advantage of this system compared to drilling for 2 simple end grain inserts? I've yet to have an issue but I'm probably missing something.
I always thought 2 more holes, and less material, would fragilize the tenon.

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

When I first saw this system 5 or 6 years ago the fact that the bolts could self-align to the neck angle appealed to me. Obviously other systems work just as well. I do think now that with little simple reinforcement it's pretty impervious to the ham fisted guitar mechanic. I know many recommend reinforcing threaded inserts with an implanted dowel. It would be interesting to do similar torture tests on inserts and/or hanger bolts. It may turn out they are even stronger.
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.

The self aligning thing is what i like, not so much for setting the neck the first time, but it is appealing to me that when the neck is reset, the hardware can pivot and seat squarely.

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Jordan Aceto
Ithaca, NY

The self-aligning aspect is not much of a factor from what I've seen. I've been testing an adjustable neck design similar to Doolin's that uses hanger bolts instead of barrel nuts. There's plenty of wiggle room in the hardware for an adjustable neck, much less a fixed one.

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http://www.chassonguitars.com

On the last couple I have been reinforcing the heel with a 3/8 CF rod straight down. I couldn't do that without the tenon.

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

I had one break a few years ago in shipping, the break was similar to Terrys third picture down.
although it was both bolt holes and the heel was broken off. Certainly over tightening can do this also.

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Jim Watts
http://jameswattsguitars.com

Todd,
What you raise about the location of the failure is very important.
We can never practically eliminate the possibility of failure from impact or abuse, but perhaps we should be making intelligent choices about where that failure occurs so that it is easily repairable.
Maybe we should allow the heel to break before damage occurs to the box?
Sort of like crumple zones in your car absorbing the impact so your body does'nt break.

And your earlier comment about hole positioning and size is very relevant too.
The big bolt holes on the test piece would have been a major factor in the failure.

Todd, Obviously it's hard to know. I think this was a quick shock based on the lack of damage elsewhere, and shocks can create a lot of force as you know. I think other things joints would've of broken too, based on the number of pieces I got back. The heel was broken into three separate pieces and the tenon a lot more than that around the barrel bolts.

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Jim Watts
http://jameswattsguitars.com