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| Walnut back deflection http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=28564 |
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| Author: | Chris Ensor [ Mon Aug 09, 2010 12:25 pm ] |
| Post subject: | Walnut back deflection |
I am building a Clairo Walnut and Englemann spruce concert guitar currently. The back is what I would call "rift" sawn. It has not been final sanded yet, so it will be getting thinner still. It is currently at about .135" or just over 3mm thick. I am using standard parallel back bracing. The back has been glued to the sides and with it in the mold, the back can be pushed down quite a bit- somewhere in the range of 1/8" or more. I usually don't have much or any deflection in my backs- even the other walnut guitar I have built. Has anyone else experienced this? I am currently thinking of removing the back, the braces and the center strip- then laminating the back with another piece of similar walnut and rebracing it. Any other suggestions before I go to this extreme measure? |
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| Author: | John A [ Mon Aug 09, 2010 3:12 pm ] |
| Post subject: | Re: Walnut back deflection |
The back sounds pretty thick to me as it is. When you say you can push down on it - is this at the brace locations ? or between the braces ? Is it possible you have a cracked brace or weak braces ? I would not laminate - but check your braces and their bond to the back. What did you brace you back with, spruce, cedar, hog ? |
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| Author: | Chris Ensor [ Mon Aug 09, 2010 5:06 pm ] |
| Post subject: | Re: Walnut back deflection |
The back is braced with african mahogany. The braces all are making good contact across their entire length. And I can push the back a tiny bit on the brace (nothing too worrisome). But most of the movement is in between braces. |
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| Author: | Chris Ensor [ Tue Aug 10, 2010 5:02 pm ] |
| Post subject: | Re: Walnut back deflection |
Any other thoughts please?! |
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| Author: | JohnAbercrombie [ Tue Aug 10, 2010 5:21 pm ] |
| Post subject: | Re: Walnut back deflection |
Chris Ensor wrote: Any other thoughts please?! What 'quality' is the walnut? Is it straight-grained quartered stock, or flat-sawn/curly/spalted/burl etc...? I've laminated the back on an instrument (Yellow cedar inner layer) when the back was very curly wood. The result sounded very good (to me and the recipient, at least..) Cheers John |
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| Author: | Chris Ensor [ Tue Aug 10, 2010 6:22 pm ] |
| Post subject: | Re: Walnut back deflection |
It is a super curly back that is "rift" sawn. In other words, the end grain is at about a 45 degree angle. |
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| Author: | StevenWheeler [ Tue Aug 10, 2010 7:07 pm ] |
| Post subject: | Re: Walnut back deflection |
Curly and rift sawn, that's got to be the most flexible combo you could come up with unless you added spalt. When you go to sand that back your braces are going to telegraph big time. Have you thought about adding some lattice brace panels? Might look wierd, but it would work. Steve |
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| Author: | woody b [ Wed Aug 11, 2010 9:17 am ] |
| Post subject: | Re: Walnut back deflection |
It sounds to me like the wood isn't stiff at all. (.135" is really thick for a back) I use Mahogany (both African and Honduran) for back braces sometimes, but they're not usually as stiff as Spruce. I suspect this compounded your excess flex. If you think it will be structually sound I bet it will have a nice bassy tone. One of the things I do when I want more bass is thin the back some to make if more flexible. |
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| Author: | Doug Powdrell [ Wed Aug 11, 2010 9:48 am ] |
| Post subject: | Re: Walnut back deflection |
'Standard parallel back bracing'.....how deep are your back braces?? Since they are the main supports, and are flexing, something's not working properly (depth or grain ). Does the back have any built-in radius? (additional stiffness)... On two recently completed walnut classicals, my back braces are approx. 1/2"-5/8" (+/-) parabolic cross-section..with no significant deflection..... |
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