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Thread: Cracked soundboard

  1. #1
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    Cracked soundboard

    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

  2. #2
    Yeah every plan I have seen has the grain running parallel for just that reason.
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  3. #3
    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.

  4. #4
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    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

  5. #5
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    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

  6. #6
    Quote Originally Posted by Nelson Howe View Post
    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.

  7. #7
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    Thanks. Will do. At least there is no binding on this one.

    Nelson

  8. #8
    Oh, that makes it MUCH easier.

  9. #9
    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.
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  10. #10
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    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.
    Last edited by george wilson; 12-05-2014 at 7:19 PM.

  11. #11
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    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

  12. #12
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    Nelson,

    I just joined the website and ran across your issue the other night. One other aspect you might want to consider: in my experience as a furniture builder I have found sapele to be a very unstable wood and inconsistently graded. I am not sure what it is about that lumber but it is one I will avoid using. Super frustrating to cut a 32nd of an inch off of a stile and rail cabinet door and watch it warp 1/8". Mind blowing actually. Anyway, I use honduras mahogany on my tail and head and I do orient the grain vertically. Although I've only built three ukuleles they have all each gone from a very humid environment where they were built(Kauai) to NYC with zero problems other than some minor joint creep. The first one I built was in a non temperature/humidity controlled workshop (RH reached 88% at one point - we were heating up the uke with a hair dryer so the lacquer could cure). I still expect it to blow apart at some point but it has traveled back to the island from NYC and has pretty much held together plus its tone gets better and better. Your first is always special I guess. The other thing I did was to purchase some Oasis humidifiers for them as well. They are a little pricey but well worth the dough. The other two ukes were built in a controlled environment around 45-50% RH. Only the butt end joint of one uke creeped a bit. We'll see how they get through the year.

    I'd be interested to see how the repair lasts with the changed grain direction of the blocking(if that is the route you go). Part of me thinks that is going to have some issues as well (the more experienced luthiers here would be the ones to talk with). If you do it in the winter with proper MC in your wood it will only swell in the summer months and you should have an even better sounding instrument.


    My experience with animal hide glue is zero. I have read up on and will be experimenting a bit with it in the future. It seems like amazing stuff. Different strengths, minimal clamping, dries harder than PVC glues, and you can take it apart. The only con I can see is that it appears a bit messy to deal with. I do know that builders in Hawaii don't recommend it due to the high humidity there. Other than that it seems like a really good thing.

    Good luck with your repair/redo.

  13. #13
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    I have been building since 1954. Orient the neck and tail blocks horizontally with the sides. If possible,check the humidity in your shop and try to get it at 55%. Use the same wood as the sides are made of if you're having that big a problem. That will give better insurance about wood movement. Stradivari used willow wood for his neck and tail blocks,as a bit of extra information. It might not be so prone to dimensional changes. He used willow in the reinforcing blocks in the 4 corners of his violins,too.

    Also,Fir is VERY easy wood to develop a split. If you cannot use spruce,can you use Koa? That ought to be a local wood,and many ukes,even Martin ukes,have been built with it as soundboard,back,and sides material.

    Your ukes look nice,though I REALLY do not like the angled tuning pegs. Listen to customers for comments on that. Builders sometimes get ideas in their heads and are stubborn about changing them.
    Last edited by george wilson; 02-27-2015 at 8:40 AM.

  14. #14
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    IMG_0196.jpgIMG_0203.jpgIMG_0206.jpgIMG_0200.jpgIMG_0208.jpg

    Here are some of pictures of the current state of the uke.


    Hi John,
    Thanks for your reply. I did the repair. Let's see, I had enough forethought to glue the fretboard on with liquid hide glue the first time just in case. The fret board had to come off to replace the top. I removed the cracked top, chiseled out the tail block, replaced it with birds eye maple to match the movement of the birds eye maple sides and reoriented the grain as well. I made a birdseye maple soundboard which I braced with maple (I think that might have been a mistake.) While I was in there, I discovered that some of my doug fir back braces had split, so I removed them and replaced them with maple as well (somewhere in the construction manual they suggest matching the soundboard material to the brace material.) Glued it all up. Refinished it. As a result of heat and glue on the neck, I developed a bit of a bow, which was driving me crazy. So I removed the fretboard again, routed out a channel for a carbon fiber rod, and clamped it back together again, being very careful to keep it straight. Much better. Refinished for the third time. Built like my fourth bridge, and installed that. I am getting better at dressing the frets and applying the shellac. I consider it all a learning experience and plan to build two more. So far it seems much more stable, and plays well, though I am only now learning how to play it.

    George, thanks for the input. I will stay away from my stash of doug fir for this project. Koa isn't local to me here in NH, but I do have a lot of lots of birds eye. I bought some lutz spruce for the next ones, and will see how that goes. I can understand your objection to the angled tuners. Having no frame of reference for what I was doing, I chose that head pattern of the two that were on the plans because it looked more interesting. I think that angled was the only way I could get the tuners to fit with that pattern. I will do something different on the next.

    Also, George, I did buy hide glue, and used it for my repairs. I like it, but find it challenging to get pieces placed and clamped before the glue cools and gels. I did the top and the fretboard with it, and have to admit to having a little glue bulge under the fretboard. Any suggestions would be welcome.

    Thanks,

    Nelson
    Last edited by Nelson Howe; 03-20-2015 at 8:41 AM. Reason: added pictures

  15. #15
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    Try wetting both gluing surfaces with warm water before applying glue. Apparently you also need to get faster at getting the top and back in place before the glue gels.

    Maple is not at all a good choice for the sound board. It is too stiff a wood. At least use a soft wood. I read you have gotten some spruce. Good. Use it.

    I misread that you were in Hawaii before,hence the koa recommendation. Koa in the states will be expensive and may be hard to find.

    Do orient your neck and tail block grain horizontally,or you will have problems for CERTAIN. Honduras mahogany is a good choice for these blocks.

    Do not expect a carbon fiber rod BY ITSELF to stop the neck from warping. If the neck on a uke is warping,you are doing something wrong. You are apparently making baritone ukes,so you may have to install a truss rod. Just make them out of 3/16" cold rolled steel. I use 3/16" drill rod in guitars. You'll have to make your own for ukes as they are not offered anywhere I know for ukes. You need to make sure the truss rod can't rattle inside the neck. Also,it is ESSENTIAL that the truss rod be curved CONCAVE beneath the fingerboard,so that it tries to pull the neck backwards as it is tightened. For many years Gibson put out a drawing of their truss rod,showing it curved in the WRONG DIRECTION,to fool imitators who did not have enough sense to see that it was wrong.

    You can saw a straight groove down the center of the neck. Place a slip of wedged shaped wood at each end of the saw cut. The wedges should be 1/16" thick on their thick ends,about 1 1/2" long,tapering down to nothing. Glue them down. When you insert the 3/16" slip of wood over the truss rod,it needs to be clamped down while the glue dries. It should have a mild concave curve to it. Plane it off flush when the glue is fully dry. I anchor the body end of my truss rods by drilling a 1/2" hole very close to the end of the neck. I tap and thread a hole in a piece of 1/2" round brass to screw the end of the truss rod into. The brass is about 1" long. Bend the truss rod slightly down before you push it into the groove,so it rests properly on the wedge shaped slip of wood. As many times as I have done it,I can't recall the thread I use. It is either 10/32 or 12/32 (senior moment!) use which ever thread fits the rod. You can buy brass hex nuts from Stewart Macdonald that will fit the proper thread. Gouge out the truss rod well in the headstock end. Under the nut,you need to take a washer that will barely slide over the truss rod. Cut some off of one side of the washer so that it doesn't stick up above the truss rod well. The rod will work just fine if you have done all correctly. Your neck is not thick enough to use the "double truss rods" that are sold. The double truss rod would,of course,be too long for a uke anyway. But,the double has the advantage to bend the neck both ways,which is useful,if your neck comes out with a backwards bend AS CAN HAPPEN!!

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