Larry, it could be one of the reasons why the SJ bracing pattern has a star brace on the back, Michael Payne would be the one to ask, Michael?
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| Author: | LarryH [ Tue Dec 26, 2006 11:42 am ] |
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Sorry about the title and am having a hard time finding the words for the question. I've built a couple of kit guitars and have taken a year off but would like to start a little more difficult project and always come up against the same internal question about 'radiusing' tops and backs. I just don't get it. I don't get how you can get a 'radius', using a dish - with wood! The dish is trying to create a compound curve isn't it? Flat, single pieces of wood don't do compound curves, do they? I can see the bout side to side curve that is formed when back braces are 'radiused' and the back glued to match those radii, but how can you get any curve in the longitudinal direction? And if you did get any curve in that direction what form of brace would hold that curve? Certainly not the back strip or back reinforcement? Then I checked out the Lowden pages referred to in a previous post and see that someone apparently isn't using a dish but just the radii formed by 4 radiused forms. Picture 8 at the link below. http://www.georgelowden.com/photos/makingof/steelstring/inde x.html So someone at Lowden doesn't use a dish to form a compound radius as a dish would suggest. When I assembled my 2 kit guitars I had to apply a bit of pressure to the top and back to get them to reach the neck and tail blocks because I had only a radius in one direction, but I read somewhere that was not such a good thing and that the top and back should fit the sides with very little effort. But I don't see how to preserve a bend or arch in 2 directions like a dished radius would suggest. So I'm still stumped and hope my frustration doesn't come through too strongly as I respect this forum and all its members immensely. But if someone could help a rookie mind understand the radius and dish/go bar concepts I would really appreciate it. Thanks in advance Larry H |
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| Author: | gozierdt [ Tue Dec 26, 2006 12:18 pm ] |
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Larry, I'm only a beginning luthier, but at one time I was an engineer also. It's not true that wood cannot bend in two directions. It is true that it bends more easily in one direction- across the grain, than longitudinally, but it can be bent in either direction, or both at once. What happens is that the bending stresses in the wood work in both directions, and combine, so the overall stresses are higher, but it can be done. Since the stresses are higher, the minimum radius is limited (if the stresses go too high, the wood will rupture), but in the range of wood thicknesses and radii we're talking about, all the woods used in guitar building are capable of being bent in either or both axis at once. I've built one classical and one steel-string guitar so far (pictures coming soon), and both had spherical profile on both front (25'), and back (15'). I created these profiles by first radiusing the braces- fan brace on the classical, X-brace on the steel string, then gluing the braces to the top/bottom in a radius dish in the go-bar deck. |
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| Author: | LarryH [ Tue Dec 26, 2006 12:35 pm ] |
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Wow, Very helpful as usual. I kinda get it. And also need a dish of some kind if I intend to create a dome that I didn't think was possible. Is the top the most important, or even the only possibility given the X braces? Is that why some backs are X braced? And if not X braced then not really likely or perhaps not that important? Thanks again Larry |
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| Author: | Serge Poirier [ Tue Dec 26, 2006 11:16 pm ] |
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Larry, it could be one of the reasons why the SJ bracing pattern has a star brace on the back, Michael Payne would be the one to ask, Michael? |
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| Author: | Colin S [ Tue Dec 26, 2006 11:28 pm ] |
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I also X brace most of my backs, mainly for this reason to hold the dome in place. Don't forget that you are also gluing it to a rim that has been sanded in a radius dish. Colin |
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| Author: | TonyKarol [ Wed Dec 27, 2006 12:28 am ] |
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There are those who build with a cylindrical top radius - the radius is cross grain in direction - why - because they, like us who use a dome, are trying to strengthen the top by flexing it into this shape, plus try to counteract the effects of possible future dryness by making the crossgrain measure of the top longer, thus giving the wood somewhere to shrink without cracking (hopefully). On a back that is ladder braced, the most important radius to maintain is again crossgrain (with the braces) - to prevent cracking. An X brace here may maintain the lengthwise dome better, but its not as crucial. As someone mentioned above, sanding the rim to a dome helps keep the dome intact even though there are no braces lengthwise that do this. And yes, you need to flex the back into place more here when gluing it down. |
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| Author: | RobLak [ Wed Dec 27, 2006 5:51 am ] |
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As a newbie yet to start my first, i am hoping to make my big blunders here in the posts rather than on my baby... so here goes. This discussion intrigues me because I have had the similar thoughts to LarryH. The compound bend doesn't have me stymied, but all(?) pictures i have seen for the backs, the underside of the braces (that is glued to the back) is never radiused, so i imagine when using the go-bar deck that the braces are forced down into the curve of the dish. To me, that means that the braces are spring loaded and the only thing stopping them from popping off is the hide glue. I don't know what the forces are but it seems like something one would not want to do. Have i missed the pictures of people shaving the braces before they are glued on? Or perhaps the tension acts to load the back tension and make it more responsive? (or maybe it just doesn't make a dang of difference?) Also, if it is a compound curve, are the braces tilted a bit (towards the imginary center of the 25' circle) to compensate for the curve or is there a little glue filled gap? Lastly, the picture above (to me) seems that the braces are a bit beefy. This discussion seems to imply that those braces mainly prevent the back from splitting, aren't they overly large for that? I see bias tape used for the sides for the same purpose. (I know, i have to go read some of the books first, and i do plan to. Just that this discussion caught my eye.) Obviously the backs take some banging, but i imagine the stresses are nowhere near those that the top endures due to the string tension... or am i waaaay off? They just seem to be overly designed. (and not just this pic but all guitars). What are the stresses the back braces are designed for? Here's where i am coming from. I have no engineering background, but i worked with a bunch of kids once in a thing called oddessy of the mind. They had a competition where they had to make a tower 18" high that had to support weights placed on top. After every third weight was placed on top, the judges used a lever to twist the tower. The tower was constructed entirely of balsa wood strips 1/8" x 1/8". You could not glue them together side by side to make a laminated strip. They used CA glue to make something that looked like an old oil derrick or the Eiffel tower. That sucker held over 700 pounds through the twist. The winners held almost 1600 lbs!... I read that the string tension is something like 90 lbs. Are luthiers just a bunch of sissies afraid to brace according to what is needed? Has anyone actually tested design principles to determine exactly what thickness of brace is needed for the desired result? Sometimes i think that guitar design is a bit like that old recipe for pot roast that first calls for cutting the roast in half... not because it helps it to cook better but because that's the only size roasting pan mom had.... Ummmm... luthiers have a sense of humor, right? Rob |
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| Author: | crazymanmichael [ Wed Dec 27, 2006 11:16 am ] |
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rob, there is a school of thought that favors forcing a straight brace and the plate into the dish or curved gluing caul and relies on glue to hold the curvature. far more common is the radiusing of the brace to match the final radius of the plate and only the plate is bent into the radiused shape, into the dish and around the brace. and one should be trying to ensure that there are no voids to be filled with glue. as to your comments regarding the bracing of the back above, to me it is overly heavy as well. better builders are well known to try to minimize their bracing, particularly of tops, to increase responsiveness. overbracing is something which factories and inexperienced builders are prone to. and testing of bracing is something which goes on constantly. |
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| Author: | LarryH [ Wed Dec 27, 2006 1:44 pm ] |
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[QUOTE=TonyKarol] There are those who build with a cylindrical top radius - the radius is cross grain in direction - why - because they, like us who use a dome, are trying to strengthen the top by flexing it into this shape, plus try to counteract the effects of possible future dryness by making the crossgrain measure of the top longer, thus giving the wood somewhere to shrink without cracking (hopefully). On a back that is ladder braced, the most important radius to maintain is again crossgrain (with the braces) - to prevent cracking. An X brace here may maintain the lengthwise dome better, but its not as crucial. As someone mentioned above, sanding the rim to a dome helps keep the dome intact even though there are no braces lengthwise that do this. And yes, you need to flex the back into place more here when gluing it down.[/QUOTE] VERY helpful as usual. I have a feeling that this topic has been visited a few times but the generous input and ideas are really appreciated. I have another question about various bracing schemes like one shown I used on my second Martin Jumbo kit. 
I just have this intuitive feeling about the fan type bracing and will probably use it again but I see no way to preserve any kind of radius with the braces aligned as shown. The guitar is not dead flat but I don't know how the radius got there. Is the 'X' enough to preserve a radius when using bracing other than 'tone bars' (of which I do not understand to this day) which seem like they would help immensely? |
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| Author: | crazymanmichael [ Wed Dec 27, 2006 3:30 pm ] |
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your two A braces if radiused would help preserve the top radius. most schemes i've seen akin to this use a transverse brace between the x at the back of the brdge plate. the profiled rim will also help retain the dome. |
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| Author: | TonyKarol [ Thu Dec 28, 2006 12:10 am ] |
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If your upper braces are radiused, they will certainly preserve the radius up there. Even if the lower X and the fans are not, you will still get a radius in the back over time - why - string pull will do it for you. Pretty much all the true disciples of Larrivee use top braces whcih are radiused from the X intersection forward to the neck, and flat to the back. Larrivee's reasoning is that the top wants to cave in front of the bridge, so the radius up there either keeps it radiused, or flat at worst, and the flat behind the bridge will pull up with tension anyway. |
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