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| Redwood’s Tendency to Split http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=2889 |
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| Author: | Michael McBroom [ Fri Aug 19, 2005 2:55 am ] |
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I have read before that one of the problems with working with redwood is its tendency to split or crack. Well, I've just discovered this tendency first hand on a top I'm working on after inlaying the rosette, cutting the soundhole and gluing down about half the top braces. This redwood comes from a perfectly quartersawn plank I found at a local wood supply, which I then resawed. I noticed that, through some areas of the plank, there were internal cracks that were not visible externally. When I mentioned this to a woodworker friend of mine, he called them "wind splits" or something like that. The theory is that when the tree was younger, it got whipped around by high winds which actually caused internal cracks. I'm thinking that, while there were no visible defects in the top set I selected, it had been weakened to the point of fragility. It's discouraging that I got this far before noticing this weakness. At the time I selected this set, I selected two others. So I turned my attention to them. One of the sets, I got each plate to split with only a minumum amount of flexing. The other set remained intact even when flexing with substantially more force than what caused the first set to crack. I'm thinking this one will be safe to build with. So, I guess from now on, if I'm building a redwood topped guitar, I'll have to give the top wood a flex test first. However, I really hate wasting that top, so I glued up the cracks with cyano. So far, it's holding well and no more splitting even when I flex it to a rather extreme amount. I'll not be using it on the build it was originally intended for, but I've half a mind to use it on something else, more or less just to see how well it holds up. Anyway, just thought I'd pass this along. Best, Michael |
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| Author: | Alan Carruth [ Fri Aug 19, 2005 5:16 am ] |
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Redwood has a split personality, if you'll pardon the pun: it's either fine or it's awful, and you really can't tell from looking at it. We used to call the crossgrain 'creases' you'd see from time to time 'wind checks'. It turns out they are the product of poor felling: they dropped the tree on something solid (like another tree) and it got mashed. Wood is much stronger in tension than in compression, and the tree tries to avoid compression loading when possible. One way it does this is to actually pre-stress each new layer of wood as it grows. After the cells are laid down, but before the fibers in the cell wals are glued together, they get an extra shot of pressure to make them swell up. Since there is a layer of fibers that runs spirally around the cell, as it swells up it gets a little shorter, or tries to. When it's in that state the tree 'nails it down', so each new layer of cells is in tension relative to the ones underneeath it. This helps keep the wood on the downwind side of the tree from failing in compression, but at a price. The wood toward the center of the trunk is progressively compressed as the tree gets bigger. This built in stress, added to the weight of the tree, can be enough to actually cause the wood in the center of a large tree to fail in compression. Usually the failure is in the form of microfractures inside the cell walls; invisible but problematic. Wood like this will often simply fail catastrophically when you try to bend it, at a relatively small load. Redwood trees do get big, and you can't tell what part of the trunk the wood came from, so I suspect this has something to do with the bipolar nature of the stuff. It's also one reason I tend to avoid stump wood. |
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| Author: | RussellR [ Fri Aug 19, 2005 6:22 am ] |
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Alan What a wonderful post. Thank you very much Russell |
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