Hey Doc
Take two aspirin and call me in the morning.
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| Author: | Andy Zimmerman [ Sat Nov 25, 2006 11:06 am ] |
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I just received a bunch of great cedar tops from the Zootman. The curly ones are not as stiff as the standard ones. Would most people just leave the top thicker or thin it as usual but brace it a bit heavier. I think I know the answer to this, but I would love to hear all of your input. Thanks |
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| Author: | Kevin Gallagher [ Sat Nov 25, 2006 11:26 am ] |
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Andy, I like to leave the looser tops a little thicker to give me stiffness consistent with that of the stiffer ones and then brace accordingly. The reason for my doing it this way may differ from those of some other builders. I've always felt that the top areas in between those supported by the bracing need to be stiff and stable on their own. I've seen a number of Cedar topped guitars from other builders and built years ago that exhibit distortion and twisting between braces and assumed that it was because the top was a little too thin and weak. This is just my preference and opinion and you'll likely find as many opinions as luthiers who reply. Regards, Kevin Gallagher/Omega Guitars |
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| Author: | Serge Poirier [ Sat Nov 25, 2006 11:42 am ] |
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Andy, what kevin said, i would leave them on the thick side as well and tap them out as you thin them down, mine was 0.130" thick and i'm glad i didn't sand down any further. even at that thickness, the top was very easy to flex across the grain and felt like i had to overbrace it at first which was not the case but you will like it better if left thick. |
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| Author: | Andy Zimmerman [ Sat Nov 25, 2006 1:12 pm ] |
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Thanks Andy |
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| Author: | Bobc [ Sat Nov 25, 2006 1:30 pm ] |
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Hey Doc Take two aspirin and call me in the morning.
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| Author: | John Mayes [ Sun Nov 26, 2006 2:47 am ] |
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ditto Kevin |
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| Author: | Scott van Linge [ Sun Nov 26, 2006 1:07 pm ] |
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Kevin, I always find your posts elucidating. Would you care to share more specific thicknesses for stiff and loose spruce and cedar that you consider reasonable limits? I've heard some top thicknesses mentioned somewhere that seemed awfully thin, and would appreciate hearing from your experience on this. Thanks, Scott |
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| Author: | John Mayes [ Sun Nov 26, 2006 1:22 pm ] |
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I'm not Kevin, but since I agreed with him I'll share some that I've gleaned from the hundreds I've built. For Spruce (it depends on the spruce, but in general) the thickeness will range from .135-.090 depending on the size of the guitar and the stiffness and weight of the top. I won't mess with it if the top is floppy at .135 or more. .090 thickness tops are reserved for the very stiffest of woods and smaller guitars (00 size and smaller). But I've built SJ's with .105 thickness tops that have held up for years and are really powerful crisp, and full sounding guitars. For cedar it normally ranges from .145-.120 give or take. I don't prefer Cedar too much in general, but those are the thickeness I've had work well for me. I built one parlor ( 00 size ) that had a .110 top on it and it reallllly bellied and I ended up replacing the top. |
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| Author: | Scott van Linge [ Sun Nov 26, 2006 3:16 pm ] |
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John, I'm sure we all appreciate your input here. Thanks, Scott |
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| Author: | Serge Poirier [ Sun Nov 26, 2006 3:20 pm ] |
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Yes, thanks John! Serge |
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| Author: | Shawn [ Sun Nov 26, 2006 3:38 pm ] |
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The thickness John mentioned are for steel string. For classical the same woods could be taken down further as the tension on the strings is much less but it is still a factor of the stiffness of the top determining how thin it can be taken without losing enough structural strength to cause the distortion and buckling that Kevin described given the type of guitar that is being built. I only build classicals and flamencos but when it comes to the thickness of the top I go according to the flex/stiffness of the wood both across the grain as well as along the grain to see if the particular top can be taken thinner. Torres when asked what the secret of his construction was was said to have responded by holding up his hand showing a small gap between finger and thumb...in other words he was implying that it was all about feeling the right amount of flex/stiffness as Spanish builders would press different parts of the top to see where they could take off material to get more sound out of a guitar. Because this is so subjective and depends on both the ear and experience of the builder, one good way to refine the sound of a guitar to know if you are taking enough material off or leaving too much on is to use "glitter" plate tuning (Chaldni) of which Alan Carruth knows more than almost anyone. Search the archives here for chaldni or glitter tuning and you will find Alan's threads on the subject...they are really interesting reading. I have always flexed the top as I have always built Spanish style but since hearing Alan speak at the ASIA symposium in 2003 on glitter tuning have been doing that as well to refine the voicing and to get better graduation of my tops. What it has given me is much more predictable sound, tone and volume as I can see visually how the material I take off is affecting the main resonant nodes of the guitar. |
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| Author: | Alan Carruth [ Mon Nov 27, 2006 2:27 am ] |
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The Chladni method gives you some information, but it's not as directly useful in determining proper top thickness as you might like. That's because you usually look at those patterns after the top is all braced. An exception to that is Mark Blanchard, who has data on top modes without bracing, and works on them to get the mode shapes and frequencies he wants at that stage, too. That's one reason he's the 'King of Consistancy'. Basically, the mode shapes tell you something about the balance of stiffness and mass distribution in the top, and the frequencies tell you about the ratio of stiffness to mass. If you have a 'ring+' mode, say, at 250 Hz, and the mass of the top is somewhere near that of one you knew was stiff enough, then this new top will probably work out alright. If the mass is much lower that that of a 'good' top, that implies that the stiffness is also lower, since the frequency is determined by the relationship between the two. In that case you've got a decision to make. A more direct way of finding the 'correct' thickness is to determine the actual Young's modulus of the the wood before you take it down to thickness. There are several ways of doing this. Some folks use a deflection test; supporting a top half on rods a known distance apart and placing a weigth in the center. You measure the deflection with a dial gauge, and calculate the Young's modulus from the known mass, length, thickness and deflection. I've been using my signal generator to find this information from resonant frequency data. It takes a little longer, but you also get some info on the 'damping factor', which may be important. In either case, the easiest way to do this is to measure stuff when it comes in. You label the wood, and keep a log. Once you know the Young's modulus you can calculate the proper thicness for a new top based on past experience; yours or somebody else's. The best way is to use your own experience to guide you, of course. I recently showed a student this method. I had built a Classical guitar that turned out very nicely, and, of course, knew how thick the top was, and the Young's modulus of the cedar wood. The stiffness of the top was therefore proportional to the Young's modulus times the cube of the thickness, an easy index number to calculate. We found the modulus of the tops he was going to use, and used the index number to figure out the thickness of the new wood that would yeild the same stiffness. His top wood was denser, and had a higher Young's modulus, so it ended up thinner, but a little heavier. This dropped the tap tones a bit in pitch, but still we knew that the tops were stiff enough. We managed to get a Red spruce top onto a box in the two weeks he had, and it sounds fine, if not quite as loud as the lighter cedar top, and it's not deflecting any more than the cedar did, even though it's a good bit thinner. The trick is that you need a baseline to work from. If you're making a 'standard' design, you can use 'standard' measurements and 'average' stiffness data as a start. Over time, with some record keeping, you can refine those numbers to take into account your own methods and designs. Wood, of course, varies a lot, and you might be surprised at how different things can end up and still be 'the same'. The usual rules of thumb about top thickness, of course, reflect the 'averages', and are fine as far as they go. However, there can be surprises lurking. Engelmann is usually a light and soft wood, but I just got in some Englemann tops that are as dense and stiff as any Red or Sitka tops I've tested. If they were left thick simply becuase they're Englemann, the results might be dissapointing. OTOH, I've got a Cedar top that tests out with the same density, long grain and cross grain stiffness as a Red spruce one: the points on a graph are right on top of each other. Those two _should_ make virtually 'identical' tops. We'll see if they sound the same. |
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