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| Advice on stiff cedar top http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22236 |
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| Author: | LiquidGabe [ Sun May 03, 2009 12:18 pm ] |
| Post subject: | Advice on stiff cedar top |
I'm putting together a l-00 with a cedar top. Its a really nice piece of wood, and extremely stiff. In fact, at least as stiff as any of the 7 or eight sitka tops I've handled and much stiffer than the other cedar. Across the grain it is really remarkable, but also very brittle. Giving it less flex than I would normally it snapped along the grain. I glued it with HHG and its just about invisible, but I'm at a loss for how to gauge thickness without fear of snapping it again. Its going on a l-00 from the MIMF plans, and I've got it at .13 now, but figure a bit thinner. Your thoughts are appreciated... |
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| Author: | Alan Carruth [ Sun May 03, 2009 12:44 pm ] |
| Post subject: | Re: Advice on stiff cedar top |
I figure that the thing that limits how thin you can get away with making a top is the long-grain stiffness. It's bridge torque that eventually pulls the top up, and it's hard to know how much cross grain stiffness contributes to keeping the bridge down. At any rate, wood 'cold creeps', so it seems to me that over the long term distortion of the wood across the grain will mean that the cross grain stiffness makes less of a contribution. Whehter or not this is true, it's a conservative approach, in that if you thickness every top as if it had no cross grain stiffness for structural purposes, you will probably make some tops thicker than they need to be, which might cost some volume, but you will also have fewer failures. If you buy that line of logic, then the thing to do would be to use some sort of deflection test to find the lengthwise stiffness of the top. Compare it with a top that you know worked, and you'll have an idea of how thin you can go. Remember that the stiffness of a plate goes as the cube of the thickness: twice as thick is eight times as stiff. If a new sample of wood has a Young's modulus that is twice as high as something that already worked, making the new piece about 25% thinner will yeild the same stiffness. |
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| Author: | LiquidGabe [ Tue May 05, 2009 12:02 pm ] |
| Post subject: | Re: Advice on stiff cedar top |
Thanks for your thoughts Alan. I build few slowly, but I can see how setting up some kind of standard test would be useful. |
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| Author: | Mike Collins [ Tue May 05, 2009 12:43 pm ] |
| Post subject: | Re: Advice on stiff cedar top |
Gabe; A 00 is not very wide. because of this you can use a thinner top then a dreadboat would need. I hope you graduate your tops (thicker in the middle-thinner at the edges) String guage will matter also. Lights pull 160-70# med.180-90# This makes a big difference for the longevity of the top. depth of the body and sound hole size will play a factor also. Brace it up and get the box together & test for the resonant frequency (R.F.)at that time. If it's higher than you want thin thinning the edges and working your way to the bridge area checking the R.F. as you thin. Check the R.F. of a similar guitar you like and try to get it as close to that R.F. as possible. Mike |
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