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So, in my first run of cutaways I've encountered a major problem. After only seven months the neck is collapsing into the soundhole, mostly on the treble side. I think this stems from the sharper radius that I put into the the thumb beam to account for the short distance between the fingerboard and the edge. It has me wondering about thumb beam ends in general. I'm starting to think that radiusing the ends by using a sanding drum to bring the ends down to 1/8th may not be the way to go and that they can only provide as much resistance as their weakest point, out there at the edge.
Can anyone think of a way to reverse this issue, somehow jack up and reinforce the thumb beam? Any thoughts would be appreciated as otherwise it's a great guitar.

What's a thumb beam. Are you refering to the upper transverse brace?

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Jim Watts
http://jameswattsguitars.com

Yup, call it what you will...

I would think that a cutaway should actually be stronger in the heel area than a non-cutaway. The whole cutaway side is a rigid beam, resisting the forces applied to the neck.
I could be dead wrong on this, but that's the way it looks to me.
I would check for glue failure between the treble side & the heel block.
On my deep cutaways, I angle the upper transverse braces (top & back) to bring the treble end much closer to the waist... Just looked like the right thing to do.
Whatever caused it, Bummer! Have to chock this up as one of those "opportunities for learning"
I guess. (I hate those...) Please let us know what you discover, when you dig into the problem.

How about a photo? Especially useful if you have one of the braced top. So the upper transverse brace ends just past the edge of the fingerboard/inside of the cutaway, rather than going all the way across to the upper bout below the horn of the cutaway? In the future, I'd make sure it goes all the way across like normal, even if it means angling the brace a little to make it past the deepest point of the cutaway. Can't say how to go about reinforcing this one without seeing the exact situation.

It goes into the part of the cutaway that is more or less at right angles to the centerline. Todd, am I to understand that you tuck your thumb beam? Er, upper transverse?

Cutaways aside, I really do not understand why some people scallop the ends of the UTB so much. Carving a deeper pocket in the lining only takes one more minute. If you feel there is not enough lining left, just carve all the way down and glue a larger support block.

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Build log

I meant tucked as in not continuing completely through the sides. You must cut it a little short of the sides but still into the lining?
Mine goes through the side and rests on the side/lining pocket, but I'm guessing the small radius is causing it to be too weak right at the edge.

The UTB brace is structurally one of the most important braces. Also a bit of a popsicle to help control the shear of the neck onto the top. In the old days Martin didn't have one ,but remember the neck block was 1/4 inch thicker than what is used today. You don't need a big hunk of one but at least one that covers the sides of the fretboard.
If you don't get a good tie in, you are asking for trouble as Todd points out

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I agree with daniel, I would think that this area on a cut out would actually be a lot stronger and therefore suspect slippage. FWIW my UTB's are full sized till about 1.5 inches where they taper to 1/4" on most of my guitars.

I have done some after-the-fact shaving on the ends of that brace and found that it does have some effect on tone which makes sense if you believe the treatment of that end of the X-brace effects tone.

The structure of that part of the guitar is pretty complex and I'm betting that there is something else going on besides a weak UTB. The UTB should not be the main element in preventing neck block rotation. It's effect on that is inefficient at best.

Photos of the construction and caving might help. I'm wondering about glue creep. Is there anything to indicate things slipping?

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http://www.chassonguitars.com

I'm also in agreement with Daniel about the cutaway area being stiffer.

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Jim Watts
http://jameswattsguitars.com

I'm in agreement too... except in this case where it's collapsing.
anyone have any thoughts on how to restore it?

I hate to ask ask as I'm sure you've probably checked, but your certain the neck joint is solid?

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Jim Watts
http://jameswattsguitars.com

Would that it were so easy! Though I don't see how a loose dovetail ( in this case) would make the top collapse. I'm pretty sure it's the end of the thu...upper transverse. Question is, what to do about it? Or I have I used my precious tonewood to make a most glamorous archery target...

I'd rather just build another guitar then do all that

Is the action so bad that it is not playable? IT may have settled in and won't creep much more. If the action is adjustable to a reasonable playability then I'd play it up until something really needs to be done, like build another guitar

Although if there is a curious scientist in you then removing the back could reveal a lot of clues.

FWIW after I read Sloan's classical construction book some 20 years ago I always place a vertical brace end on the UTB and the lower cross strut in the case of classical guitars to lock the ends of the brace in place. I think it's stronger then tucking into the lining and frankly it's easier too.

Well I'm pretty good going forward. Going in reverse tends not to work out so well for me. Especially since I'm the client in this case. I never seem to be too excited about working on my own behalf. Well, I guess I'll just get medieval on it sometime in the future.

You need to take it apart and determine the root cause of the failure so you can avoid making the same mistake again - even if you don't rebuild it.
If you had good documentation (pictures) of the build, maybe we could help.

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

How thin is the top ? how thick/thin is the UTB ( never heard of thumb brace before ) did you also put a popcicle brace ? like others said, it sounds strange that it would give in like this ! perhaps the UTB was bad/weak/to thin from the start or not put in correctly.
Are you sure no one sat on the guitar while you went to get another beer
+1 on what Todd said, that to document things untill the next build !

Lars

The UTB is 5/8h x 1/2 w. Top is .093ish. Thinner than I usually go but not unheard of.

If it is the soundhole area that is folding up as it does on some of the old gut strung guitars that are strung with steel strings, you may be able to reinforce this area by glueing in fairly massive bracing around the soundhole, between the upper arms of the X and the UTB. Fit them in tight against these braces and don't skimp on the glue. You should be able to add them through the soundhole without having to take the guitar apart. Some people notch the UTB and carry these braces into the neck block, but at this point that may not be practicle.
Another approch might be that of the "Tilton patent improvement" - running a dowel from the neck block to the end block, not a very elegant solution.

Hmmm... If I could use turnbuckles instead if a dowel to get it back into shape and then butt the UTB against a side brace ....

Once you get things back into position you could run braces from the top of the neck block to the back/sides at the waist similar to what Rick Turner, Sam Littlepage and others have done. The Mimf library has some information under "flying buttress bracing" and "frame guitar". Again this work could probably be done through the soundhole.
Unless the guitar is folding along it's centerline I would be surprized if the failure is due to the end attachment of the UTB.