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| Brace expression through my top http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=12745 |
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| Author: | Marc [ Fri Jul 06, 2007 11:57 pm ] |
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I can feel the expression of my fan braces through my tops. I brace at R.H. of 45 using hide glue and maintain a R.H. of 45 +- 5 always in my shop. Seems to be caused by the glue hydrating the wood during bracing then the top shrinks around the brace. Any tips on how to minimize this effect? |
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| Author: | WaddyThomson [ Sat Jul 07, 2007 1:25 am ] |
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That is a good question. Someone should have an answer for you, but it won't be me. |
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| Author: | old man [ Sat Jul 07, 2007 1:59 am ] |
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Seems to me it would indicate too much clamping pressure and/or too thin a top. Ron |
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| Author: | peterm [ Sat Jul 07, 2007 2:24 am ] |
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Never had that happen.... I would agree with too much pressure or a thin top. I braced a very thin top before but used an Aliphatic Resin glue instead and it was fine. Maybe you want to try that instead of hide glue? |
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| Author: | JohnAbercrombie [ Sat Jul 07, 2007 2:30 am ] |
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Sanding with a RO sander or a soft sanding block will do that as well. The top is more rigid at the braces, stands up to the sandpaper, and there is more wood removed over the braces. Between the braces the top just moves away from the sanding block and is sanded a bit less. Cheers John |
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| Author: | Shawn [ Sat Jul 07, 2007 6:11 am ] |
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Marc, First of all...is it a classical or a steel string? When you say fan braces it sounds like your building a classical... When you say that you can feel the impression of the fan braces through the top can you also see it on the surface? Does it look like it is sunken between the braces or just that the wood is slightly not flat in areas? In classicals guitars this is more common than steel strings as the goal is to have the top as thin as possible without tempting fate... There is nothing wrong with using hide glue for gluing bracing. You are also being careful to maintain R.H. which is always good. How thin is your top and what material is the top made of (spruce, cedar,..)? How are you gluing in your braces? In a radiused dish in a go bar deck... in a solera (classical)... It could very likely be a combination of factors...a too thin top of Western Red Cedar, Englemann Spuce or Redwood that is sanded too aggressively... Once we know more the answers can be more concise. |
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| Author: | John How [ Sat Jul 07, 2007 6:26 am ] |
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I would tend to agree with John. When you sand, be sure to use a fairly hard sanding block so that it doesn't distort the top but rides on the top. |
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| Author: | Marc [ Sat Jul 07, 2007 6:31 am ] |
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My tops are contoured from 2.4 mm on the edges to 2.7 in the middle. Here is a picture of a similar effect from Neil Ostberg's site. The effect on my tops is less profound--can't see the undulations but they can be felt. I don't use a sander on my tops after bracing and only minimal scraping because I don't want to change the thickness. I'll try fewer go-bars next time, or maybe the answer is thicker tops. |
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| Author: | Rick Turner [ Sat Jul 07, 2007 7:20 am ] |
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Maybe it's an indication that your top is not too thick! I see braces telegraphing through on practically every fine guitar that I think sounds good, including my own 1987 Ramirez 1a. Look at any truly vintage Martin and you'll see it. I see it on my steel strings and on our ukes. I don't consider it to be a major issue. |
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| Author: | Marc [ Sat Jul 07, 2007 7:53 am ] |
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Wow! Thanks Rick, now that's some good news I needed to hear. I have almost zero experience actually handling really fine instruments, if it's not at the guitar center then I haven't played it or seen it first hand. |
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| Author: | Marc [ Sat Jul 07, 2007 8:07 am ] |
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Shawn, I build classicals on a solera, I radius the lower bout 25' on a dish with gobars. Thickness 2.7 to 2.4 mm from middle to edges, seven fan brace. Occurs in both WRC and spruce but less so with WRC. The effect is subtle not really visible but I can feel it and see some slight undulations if view at an angle in glancing light. I think it is sunken between braces. |
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| Author: | Shawn [ Sat Jul 07, 2007 11:25 am ] |
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That is the common effect I mentioned that is common in Classicals...as long as the top is not losing structural integrity then it is fine. The reason why I asked if you were using WRC or Englemann is because they have much less cross grain strength than Euro, Adirondack or Sitka Spruce so the effect is greater so the top ends up needing to be kept thicker. 2.7-2.4mm in the middle of the lower bout is a safe thickness for most Spruce and should hold up well. 25' radius is the right range for doming on a classical so you seem to be in good shape all around. The way that Spanish builders judge if the top is the right thickness is to hold the lower bout in their two hands facing them and with the thumbs flex the top and back to see how much it deflects...ideally there should be some slight give but not too much as it means that eventually the top will fail, either physically or sonically (which would make it acceptable for recording but not loud enough for concerts). |
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