Cracked neck tenon

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nkwak
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Cracked neck tenon

Post by nkwak »

Next time I'm not using barrel bolts. I overtightened them and cracked a little chunk out of the tenon. I tried to repair it with CA glue but now I'm wondering if the joint will ever be completely trustworthy?

Image

When not under tension the neck angle is spot on but when I tighten the bolts the end of the heel (lower bolt area) lifts up about 1/16".

The question I have is this: can it be repaired? If so, how? I put a LOT of work into this neck and don't want to have to toss it.
~ Neil
Ben-Had
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Re: Cracked neck tenon

Post by Ben-Had »

I'd fill the holes with a plug (hard wood dowel might work) and redrill for threaded inserts. Is that a crack through the lower heel?
Tim Benware
tippie53
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Re: Cracked neck tenon

Post by tippie53 »

Personally I hate these and have seen them used and fail often enough to be a concern. I much prefer the threaded insert. I used them and have note seen one fail. Martin also uses them and failures are very rare.
John Hall
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tippie53
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Re: Cracked neck tenon

Post by tippie53 »

that sounds like it would work. Also there are different inserts. I will have to post a pic of the ones I use and I agree that there are some inserts that are not worth the room they take up. how you apply it is also key.
Adding a spline adds a cross grain that will add a lot of strength Thanks for a different techique Rick. there are always more than one way.
But I still like my dovetails
John Hall
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B. Howard
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Re: Cracked neck tenon

Post by B. Howard »

tippie53 wrote:But I still like my dovetails
Yep, I agree with that. I look at a set up like this think "I hope I don't need to try and re-set that". I would plug the holes and put some nice inserts in and make a conventional bolt on out of it.
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tippie53
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Re: Cracked neck tenon

Post by tippie53 »

from an engineering standpoint. It is about the surface area of the fastener. A barrel bolt will only have the front half of the round in contact surface area. In a good threaded insert the surface area is greater and it is farther into the neck. This distributes the contact point into more of the stronger point of the wood.
https://www.google.com/search?q=brass+t ... l&tbm=shop
this link it to inserts that I would not use the outside thread is narrow and too close together.

http://www.highlandwoodworking.com/bras ... of-10.aspx
this is what I use. Also note the slot on the insert, THIS IS NOT FOR A SCREW DRIVER this end goes in first. You can use a shoulder bolt to drive them in. I have not seen one of these fail and I use them on some of my jigs also. They can take a great bit of torque and apply the stress to a joint very well.
I did a strength comparison on these and standard barrel inserts. These won hands down for my applications.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
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JLT
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Re: Cracked neck tenon

Post by JLT »

I've used barrel bolts in many guitars, mandolins, and a ukulele. But I use longer tenons than the one shown in the picture, so that there's at least 5/8" of "meat" between the barrel and the end of the tenon. Haven't had one break out yet.
tippie53
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Re: Cracked neck tenon

Post by tippie53 »

If the engineering of the joint is stronger than the applied stress it will work.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
nkwak
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Re: Cracked neck tenon

Post by nkwak »

Well, the inevitable happened: the crack in the tenon went all the way through and the bottom of the heel broke off cleanly. I'd plugged the holes with sections of an oak dowel but the installation pried the crack open wider and tapping in the new inserts made thing even worse. What did the heel in though was a combination of the insert not seating firmly into the opening which caused play in the whole neck set.

I'm still hoping to save the neck. The break was very clean and I used a modest amount of Titebond to reattach it. The tenon is still cracked though, so I'm thinking that it would be best to shim inside the mortise and once the neck angle is established I'm going to glue it all and not rely on the bolts to hold the joint together. For some reason now though I cannot for the life of me get the cheeks to come in 100% contact with the sides of the body.

BTW, I've also started using rubber washers on the bolts as an experiment against any shock. After looking inside a friend's Bourgeois I'm also thinking of bolting the fretboard extension down with 4 bolts.

Maybe I should just walk away from this though?
~ Neil
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