Cracked soundboard

[Nelson Howe]

So I recently finished two ukuleles following the Hana Lima 'Ia construction process, and now, with winter abruptly upon us, I have a cracked sound board. The sound board is old-growth douglas fir, with very tight growth rings, quarter sawn. I would describe it as splintery to begin with, which is likely a good bit of my problem. It cracked right where it joins to the tail block. Following the Hana lima 'Ia process, I installed the tail block with the grain running vertically front to back, i.e. perpendicular to the grain of the birds eye maple sides. The sides, being maple, which moves a lot with seasons, have shrunk, but the tail block did not, as it is sapele, and wood doesn't shrink lengthwise much anyway. As a result, the edges of the tail block are visible as a hump telegraphing through the sound board and the crack which runs to the bridge.

First question then: Is it the usual practice to orient the grain of the tail block vertically in spite of cross grain movement properties of the wood? If this were case work, I would not do it that way. Is there an acoustical or structural reason for doing it that way?

Second question: what would you luthiers suggest for corrective action?

My inclination is to try to remove the soundboard (yikes!) and replace it with a less splintery wood (probably birds eye maple or, perhaps, pau rosa). I am also tempted to chisel out the tail block and replace it with a piece of maple with the grain oriented parallel to that of the sound board and back.

I suspect that humidity is fairly constant in Hawaii (I read a comment that they can't use hide glue because it is too humid), but it varies wildly here in the Northeast, and I plan to travel with this thing a lot as well.

Thanks,

Nelson

[Anthony Scira]

Yeah every plan I have seen has the grain running parallel for just that reason.

[John Coloccia]

Has the top come unglued from the tail block? The idea of vertical grain on the tail block is to resist spitting of the sides, particularly on an acoustic guitar, or perhaps something in the violin family, where a drop on the end pin can easily split the sides. The downside is that it presents an end grain surface to the top and back for gluing. On a violin, it's not a big deal. Removing the top or back for a repair or adjustment is common and it's designed to do that. Ukes and guitars are not. Some people do it anyway, but the vast majority orient the grain the same as the sides and live with a potential split.

How badly do you want to play this particular instrument in it's current form? If you fix the crack properly now, and by that I mean removing the bridge, cleaning the edge of the crack, chamfering the edges, and then inlaying a little V shaped sliver, followed by reinforcement diamonds glued to the back of the crack, there's a good chance that it won't give you any further problems, though you'll have significant problems keeping the top glued to the end grain tail block if that joint came apart (old glue...end grain...fuggedaboudit). You'll probably just have to repair it with CA at the tailblock if it's popped loose and hope for the best. What was the RH when you glued everything up? If you were much more than 45% or so, you were begging for trouble no matter what you did.

You might be tempted to do the repair without removing the bridge, but that joint is under a lot of stress now too. You could do all that work and then find a crack in the sound board on either side of the bridge someday, or maybe even find that the bridge has simply popped off on it's own.

I'd probably attempt a repair first because it's least intrusive. You can always put a new top on later, but that's an enormous amount of work and it's not easy to do if there's binding. It's not the end of the world, but if you can avoid it, I would avoid it


Anyhow, that's just my opinion.

[Nelson Howe]

I hear what you're saying about trying to repair first, but with the tail block telegraphing through the soundboard as much as it is, I don't see how gluing it in its current configuration would work.

So you're saying that the majority of people orient the tail not in the vertical, the biggest risk therein being splitting the sides from a drop? For what I have going on so far, that seems like a better risk, as I already have so much wood movement that it's tearing itself apart.

And thanks for your opinion. I certainly helps.

Nelson

[Nelson Howe]

Well, in spite of John's excellent advice, I believe I will go ahead and replace the top with a maple one, chisel out the tail block and replace it with one with the grain oriented with that of the sides. The bulge that the current tail block makes top and bottom tells me that the douglas fir top will never be happy now.

Never having done this before, I wonder if anyone has any suggestions to help keep me out of too much trouble as I proceed?

Everything so far has been glued with titebond III. If I'm careful to sand and scrape in the tail block area, should I be okay to use the same glue again, or would it have soaked deep enough into the wood to preclude a good bond?

Thanks,

Nelson

[John Coloccia]

Well, in spite of John's excellent advice, I believe I will go ahead and replace the top with a maple one, chisel out the tail block and replace it with one with the grain oriented with that of the sides. The bulge that the current tail block makes top and bottom tells me that the douglas fir top will never be happy now.

Never having done this before, I wonder if anyone has any suggestions to help keep me out of too much trouble as I proceed?

Everything so far has been glued with titebond III. If I'm careful to sand and scrape in the tail block area, should I be okay to use the same glue again, or would it have soaked deep enough into the wood to preclude a good bond?

Thanks,

Nelson

Just scrape or sand the old glue down to bare wood, and you're good to go. Look online for some good techniques to re-top acoustic guitars. Since linings are already cut for the binding, there will be some precision and fiddling involved to get everything just right.

[Nelson Howe]

Thanks. Will do. At least there is no binding on this one.

Nelson

[John Coloccia]

Oh, that makes it MUCH easier.

[Kevin Looker]

Most people use old original Titebond because it can be released with heat for a repair.

I dont' know what type of bracing in is on your Uke (X-brace, ladder bracing?) but you want to glue the bracing to the top and the top to the rim when it's at around 40 to 45% RH and it's been at that RH for at least a few days. This should prevent future cracks assuming the instrument doesn't get extremely dry. Having an arch in the top as opposed to flat will also reduce the chances of future cracks.

[george wilson]

There is every reason to have the grain of the tail AND neck block running the same direction as the grain of the sides. As you have discovered,the tailblock will stick up too high in the winter,and if you pare it down to be level in the Winter,it will shrink below the level of the top and back in the Summer.

Another bad thing is you are gluing the top and back to END GRAIN. Hardly the best way to glue something on and have decent strength.

True,violins use a vertical grain neck and tail block. But,they are so thin in depth they can get away with it. And,good violin makers would apply hot hide glue to the end grains and let it dry,then scrape it evenly before applying more glue to actually glue the top and back on. This is the way violins have always been made. Even Strads.

You should only ever use HIDE GLUE to make instruments. It is reversible,and doesn't slide over time,and it transmits sound better than synthetic glues,being more crystalline in nature when hard. When I was young and stupid(and ignorant),I had even the tightest dovetailed necks creep out a bit at the bottom of the heel because the white glue would yield some.

[Nelson Howe]

Thanks George and Kevin,

I wasn't aware of the importance of rh at the time of glue up. It's winter here now, and drying out in a big way. I'm not sure what rh I can get when I'm ready for the glue up this time, but I will try to pay attention to that.

I'm glad of your advice about the tail block, George. Thanks. It makes more sense to me in light of what I have learned about working with wood. I will change out the tail block when I replace the top. This will also make both the heel and the tail block move with the sides. I am replacing the douglas fir with a birdseye maple one, as I am a little shy of how splintery the fir seems, and I'd like to avoid another crack.

Would liquid hyde glue be an adequate alternative to hot hyde glue? I've used it with success on furniture pieces, but have not used the hot stuff before.

Here they are before cracking, anyway.

IMG_0173.jpgIMG_0175.jpg
Thanks,

Nelson