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Hi All
I haven't been building any guitars for over a year but one of my 12 strings came back with a neck issue. When I took the neck off I noticed the tenon was cracked. I would love some input on how to fix this. I have attached a pict.

BTW it's good to be back.

Andy

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

Cross grain side supports. Welcome back Andy.
Tom

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A person who has never made a mistake has never made anything!!!

how would you do that? I am also concerned that I need some support up front to support the bolt under tension

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

That right there is the reason I switched to inserts.

You could go to inserts instead. Dowel the holes and fix the cracked tenon the drill for a "vertical" dowel and glue on in. The drill the heal side of the tenon for the inserts. Put the insets I place and clamp back to the body.


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One of my students had the same problem on his first guitar, only worse. He was preparing to string it up for the first time and hit the head on a low beam on the ceiling when he picked it up. The heel fractured through the upper furniture nut, and the body fell on the floor.

Whenever you put a piece of hardware in wood there's a chance that you'll create a stress riser. Those cylindrical nuts can throw all of the stress onto a single line if they don't fit really tightly in the hole. In wood aircraft construction the FAA requires bolts used to secure wooden structures to be a tight enough fit to require being hammered in. This distributes the stress all around the hole instead of on a single line at the bottom. When the stress is so concentrated it produces a sideways load that can split the wood, especially if you over tighten the bolt a bit.

The crosswise nut tends to fail badly when that happens; that is, there's no backup to keep things from falling apart. Putting the nuts in vertically works better: I put them in the heel rather than in the tenon, covering the hole with the heel cap. It's still possible to split the heel, of course, but if you do the tenon is contained within the neck block, so it can't fall apart. With any luck you'll see the split and be able to fix it without further damage. There's also the advantage of being able to use a longer nut to spread the stress out more.

It is a bit perverse of the suppliers to make a 1/4-20 nut that's 10 mm in diameter, instead of some inch size. In any event, to get the proper tight hole you'll need to drill under size and then ream it out. Even then it might be a bit looser than you'd like, in which case flooding it with CA when you're sure it's properly aligned is a helpful work-around. I have not had any problems since I started drilling the holes vertically instead of horizontally.

I'd rabbet a ledge on both sides of the tenon and install length-wise strips using epoxy.

I've seen this many times, even reports that the failure occurred just pushing in the barrel nut. William's method used here is very imaginative but unless the mortise is very deep and the tenon reaches deep as well the basic grain geometry make this joint failure prone at this bottom location -- even with the deep mortise.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Andy: Just a comment or two. Your initial install had the barrel nuts close to the outer edge. You know now they should have been as close to the heel as possible. Quite amazed at the cross grain break at the bottom.
The finger board is still on so hard to work on tenon. It looks like you make be able to widen the mortise and put strips on either side of tenon . Run the grain up and down the heel. These would be cross grain. I think that is what Ken is suggesting also.
Tom

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A person who has never made a mistake has never made anything!!!

thanks for the info. Live and learn.
If I am going to place cross grain strips, would you use epoxy?
Would it be better to use another wood type instead of mahogany for the strips?

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

Here's a good thread on the subject. I did some testing on that joint a while back. With cross grain reinforcement it seemed plenty strong. It fails in tension and the reinforcement acts as a tension band.

Without some support it was not that hard to get the joint to fail.

viewtopic.php?f=10101&t=31203&hilit=Cumpiano+neck+bolts

I drill the holes with a 3/8" drill jig, chamfer the edge with a reamer, and then run in a 10mm bit in reverse. The inserts seem to fit just right. I usually flood the whole thing with thin CA too but not with the inserts in place.

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

Andy - I use this type of joint too so I'm wondering what caused it to crack. Do you know if the guitar suffered an impact? How much do you typically tighten the bolts when you do the final neck attachment? Just snug or do you tighten them down more than that?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Terence.. when you add the maple strips, do you use standard wood glue or epoxy?
J not sure if it had any impact. None reported. I just tightened them to hold the neck but not too tight

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

Just Tightbond Andy. So far so good.

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

For repair purposes and because of that fracture I'd use something like Smiths epoxy though.

I always thought the idea behind the barrel bolts was so that the barrel nut would roll into position if there was any discrepancy in the angle of the bolts through the neck block. But I hear that folks are gluing those in or putting them in so tight they won't move. Does that not defeat the purpose of the design?

Hey Andy!!!

I just saw a Macassar ebony guitar last week and it reminded me of your very cool creations! Hope that all is well with you too!

I'd be inclined to convert the neck to a two bolt butt joint meaning remove the tenon all together, get longer bolts, install threaded inserts in the heel and call it a day. Some pretty good builders use butt joints with their necks and they are nice and easy to do and understand.

There is another thread currently running about reengineering a neck joint for three smaller bolts. I wanted to post that I'm not all that keen on these barrel bolt systems and why in that thread but then thought to myself.... "Hesh, be nice..... avoid conflict.... "

So I'll post that here and hide....

Great seeing you on the forum bro!

In fairness to Cumpiano I do believe if the barrel nut joint is constructed as William proposes it is much less likely to fail. The OP subject example seems to be much different.

http://cumpiano.tripod.com/Home/Article ... block.html

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Ken Cierp

http://www.kennethmichaelguitars.com/

I agree, in my testing rig with a reinforced tenon I could not get it to fail, whereas the un-reinforced joint failed fairly easily.

I've used the joint on about 65 guitars over a 10 year period and have not been aware of a failure. The early ones were not reinforced ( just flooded with thin CA before placing the nuts). I have a couple I kept around and get a lot of play. I worry about the un-reinforced ones.

As I mentioned in the other thread I would definitely not recommend using it without reinforcement at this time. Personally I love the joint but it's been good to me. I'm sure I would love inserts, hanger bolts, or a vertical barrel nut or bar in the heel just as much if I got good at using it.

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

Converting to a butt joint is a good idea but I think if you got some of that thin 1/16" plywood they sell for RC airplanes at hobby shops and put a veneer on each side of the tenon as well as the front it would hold fine. I like the idea of epoxy for the repair. You would have to widen and possibly deepen your mortise a little. Creative use of your binding routing set with the right bearings followed by a short top bearing flush cutting bit could be a way to do that.

It would be tough to rout a slot in the tenon for a reinforcing strip with the fretboard on.

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

A sharp chisel could be used to cut the slots fairly easily even with the fretboard in place. I would use cross grain rock maple inserts instead of aircraft plywood because the plywood has at least one of the laminations with the grain running in the weak direction.

One other thing that would strengthen the joint would be to make it run deeper towards the back of the guitar. Andy, any chance that you could deepen the mortise and extend the tenon by perhaps a quarter inch?

Structural epoxy with a bit of fiber additive would be the strongest adhesive for this repair. I use West Systems.

Thanks for all the help. This is my first failure of this joint in over 11 years. My guess is in the future either I will reinforce it from the beginning or use inserts.

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

That sucks. FWIW I use barrel bolts also. I install a 1/2" dowel the length of the tenon. It's quick to do. Takes about 10 minutes using CA and seems to give it the structural integrity needed.

Good to see you back here Andy. Some of your work is pretty darn inspirational.