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| Cracked Tenon, Help! http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22403 |
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| Author: | Joe Sustaire [ Sun May 17, 2009 4:13 pm ] |
| Post subject: | Cracked Tenon, Help! |
A while back there was a thread on reinforcing the Cumpiano neck tenon. This was made before that thread and I just got the finish on and was final fitting it to the body when I heard a small crack.  So preventing this is what that thread was all about.    I'm using a nail to pry it open a bit so it will show better in the pics. Now that I understand the need for maple sideplate reinforcements on future tenons.  What is the best way to fix this neck? Thanks, Joe |
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| Author: | Ken Franklin [ Sun May 17, 2009 4:45 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
Joe, I would work some glue (hide, PVA or epoxy into the crack and clamp it. When it's dry I would take 1/16 inch off of both sides and put in shims with the grain going perpendicular to grain of the tenon. Then I'd redrill the holes and Bob's your uncle. But I'd use inserts on the next one. |
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| Author: | woody b [ Sun May 17, 2009 5:35 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I don't use that type of neck so I'm not sure how it fits. Is there room between the end of the tenon and the back of the mortise? How about, after glueing the crack putting a reinforcing piece on the end of the tenon, instead of the sides? I believe that, along with gluing the crack should make it at least as strong as original. |
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| Author: | MRS [ Sun May 17, 2009 5:39 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
If you have a very fine glue syringe i would try to inject the glue. It looks like your pulling the cracks apart a little so you should be able to inject it to the cracks until it comes out the sides. i would also maybe re-drill the holes only larger, plug them and re-drill to corect size. This would add some reinforcement to the tenon. |
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| Author: | Ken Franklin [ Sun May 17, 2009 6:14 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I agree with Todd that you should wick in some CA to make the tenon stronger. I also think that if you make good layout lines it shouldn't be too hard to use a sharp chisel to cut out enough material for side reinforcements. I wouldn't trust a split tenon to last without reinforcement even if you make what seems like a good repair. Just my 2 cents. |
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| Author: | Rod True [ Sun May 17, 2009 8:28 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
That's the very reason I don't use that style of mounting hardware anymore. I had that happen on one my early guitars and since than I told myself I would use a different system. Anyway, I glued the seam with titebond and clamped it up. Than I drilled for a dowel down into the heel (1/2" dowel), glue the dowel in and than I re-drilled for the mounting hardware and bolts. |
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| Author: | Terence Kennedy [ Sun May 17, 2009 10:39 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I think I'd probably hold it snugly closed and flood with thin CA. You can tilt the neck down so it won't wick into the heel. Then I'd drill around a 3/16 hole (using a longer drill bit in a drill press) from the bottom to the top of the tenon on either side of the center line bolt holes. Then glue in wooden dowels or even metal rods to reinforce the tension side. The glue alone should be enough for the compression crack. Kind of a variation on what Rod said. Terry |
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| Author: | Joe Sustaire [ Mon May 18, 2009 9:23 am ] |
| Post subject: | Re: Cracked Tenon, Help! |
Thanks for the help guys! I'll work some pva into the crack and clamp it, I had to order some thin CA, so until it arrives I'll ponder shaving the sides of the tenon for reinforcing splints. It does make me re-evaluate this type joint. Thanks again for your help, Joe |
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| Author: | SteveSmith [ Mon May 18, 2009 4:21 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I do think this type of joint needs some reinforcing to prevent just the kind of failure you've experienced. Properly reinforced, I think the joint will be plenty strong. I epoxy in two 1/8" maple splines into the tenon, one on each side of the center. Admittedly a difficult fix after the board is glued on. I agree with Ken and Todd on a good course of action at this point. |
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| Author: | Dennis Leahy [ Mon May 18, 2009 4:39 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
SteveSmith wrote: ... two 1/8" maple splines into the tenon, one on each side of the center. Steve, I think you're onto something with a spline repair rather than an overlay repair. A couple of spline "dados" could be created with a handsaw very quickly, and whatever size the kerf is, just make the spline that thickness. With the hole exposed, Joe could use the hole as a drill guide to drill all the way through (again), this time just drilling through the splines. That also leaves the tenon the same width as before, so no futzing with that.Dennis |
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| Author: | SteveSmith [ Mon May 18, 2009 8:07 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
Dennis, I don't know if he could do this as a repair, I think he already has the fretboard on his. Attachment: DSCF0171-1.JPG
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| Author: | peterm [ Mon May 18, 2009 8:48 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I would plug the hole and flood with CA and then clamp it for a few mins. Then, drill for a threaded insert instead and go a bit deeper to get into more stable wood and use a long screw.... oh, plug the side hole too. |
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| Author: | Joe Sustaire [ Mon May 18, 2009 10:08 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
Still listening, yes the fretboard is on, so that makes splining tough at this point. I do like the spline idea on future tenons though, that's a good looking tenon Steve. So far I've just glued the crack and soaked the tenon with thin CA, thought I was out but found some. Peter your idea of plugging the hole and switching fasteners might be the simplest at this point. Pondering  Thanks for the help everyone, Joe |
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| Author: | Dennis Leahy [ Tue May 19, 2009 8:39 am ] |
| Post subject: | Re: Cracked Tenon, Help! |
Hi Joe, If you're plugging and switching to inserts, may I suggest that you may want to also drill up through the tenon, stopping before you hit the fretboard (  ), and glue in a dowel. That way, your insert will be grabbing much better than attempting to grab in end grain (a number of luthiers have documented failure of the fastener in end grain.)Revisiting the idea of a spline: if you have a radial arm saw and a 7-1/4" blade, you could still make slots for splines - with the blade set at 5/16" high over the table, fingerboard down and its end against the fence, pulling the saw out towards the heel. Then, you'd make spline pieces with a 7-1/4" radius on one edge. Same idea could be done on a tablesaw, obviously with the neck flipped fingerboard up. Is this the neck for your ladder-braced grand concert? Dennis |
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| Author: | Joe Sustaire [ Tue May 19, 2009 12:17 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
Yeah Dennis when I've used the inserts in the past I have used the dowel to give them some holding power. I've got a radial arm saw, I'll have to go check your idea out, that might just work!  And yes this is for the ladder-braced grand concert. Finish is on and ready for final fitting of the neck, once I get past this problem. Thanks for the ideas, Joe |
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| Author: | Joe Sustaire [ Tue May 19, 2009 6:24 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
No Dennis, I checked out your radial arm-saw idea and a 7 1/2" blade is too big a radius to do much good. I've got a 6" dado set and it would cut slots for splines that would cover the bottom hole, but it's a pretty corse blade and looks like it would be pretty tricky. I think I'm just going to plug the bottom holes and use a hanger bolt for the bottom bolt. By filling the existing holes with dowels and epoxy, I think the 1/4" hanger bolt will leave the most wood in the tenon and heel. Joe |
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| Author: | Daniel Minard [ Tue May 19, 2009 7:10 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
I always drill down near the back edge of the tennon & glue a 3/8" or 1/2" dowel in place to beef up the end grain. I do this prior to drilling the cross holes for the fasteners & I've never had a failure yet. If you want to try this, remove the barrel nuts & make dowels to plug all the holes before you attempt to drill for the re-enforcing dowel. Once the hole is drilled, remove the temporary plugs & glue in your re-enforcing dowel. I use CA, because it wicks through the entire joint & firms up the end grain at the same time. Don't worry if the truss rod is already glued in. Just mark your drill bit to stop before it touches the rod. You'll need to re-drill the old holes once the glue is dry. I find this isn't necessary with a laminated neck, but I still do it anyway... |
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| Author: | Dennis Leahy [ Tue May 19, 2009 8:01 pm ] |
| Post subject: | Re: Cracked Tenon, Help! |
Joe Sustaire wrote: No Dennis, I checked out your radial arm-saw idea and a 7 1/2" blade is too big a radius to do much good. I've got a 6" dado set and it would cut slots for splines that would cover the bottom hole, but it's a pretty corse blade and looks like it would be pretty tricky. I think I'm just going to plug the bottom holes and use a hanger bolt for the bottom bolt. By filling the existing holes with dowels and epoxy, I think the 1/4" hanger bolt will leave the most wood in the tenon and heel. Joe Hi Joe, I was just coming into your thread to post that I looked at it in CAD and found the 7-1/4" blade diameter too big to be effective. Yeah, you's need some dinky blade like the scoring blade on a sliding table saw or something you might see someone use to carve wildlife sculptures from logs, but nothing I would have around the shop. So, I guess the best answer is the end inserts. Dennis |
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| Author: | woody b [ Sun May 31, 2009 11:42 am ] |
| Post subject: | Re: Cracked Tenon, Help! |
Revival of an old thread. I was reading another neck thread and it got me thinking about this one. Cutting the tenon down to add reinforcement to the sides is difficult because the fingerboard is alread one. OK, how about making the mortise wider to accomodate the reinforcements. A wider tenon instead of cutting it down. I'm guessing the guitar is already fixed but this might help if someone has the same problem in the future. |
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