You will know when they are too light when you string it up and it pops!!!
I would assume the 1.25 is millimeters, I doubt he would have any flex in the wood unless he drove a car over it.
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| Author: | gadgetman2 [ Fri Dec 20, 2013 12:19 pm ] |
| Post subject: | how light is too light for bracing |
I am trying chladni patterns for the first time on an om top. I have a partially closed ring at 250hz on a top for an om probably with not enough cross grain stiffness. The top is 1.25 thick. I believe the top is more lively than before trimming simply by watching the glitter dance but I don't have the ideal modes at all. I have trimmed the x down to 15 mm at the x then tapered down below the sound hole and trimmed the the tone bars to 7.5 mm max near the x and tapered. Cross grain is definitely grossly more flexible than long grain. I don't know how low I can go on the brace height. The x brace is 8 wide as are the tone bars. |
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| Author: | James Orr [ Fri Dec 20, 2013 1:29 pm ] |
| Post subject: | Re: how light is too light for bracing |
In terms of X-brace dimensions, we're usually looking at somewhere between .5" to .6" tall at the joint, by .25" wide. So you still have some wiggle room on the height at the joint, and it could make a dramatic difference. A picture would be great. We can't flex and tap the top, but sometimes looking can provide clues. At .125", the top is on the thicker side depending on how stiff it was to start. |
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| Author: | Jeff Highland [ Fri Dec 20, 2013 3:15 pm ] |
| Post subject: | Re: how light is too light for bracing |
gadgetman2 wrote: . The top is 1.25 thick.. 1.25 thick? Inches? millimetres? |
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| Author: | RusRob [ Fri Dec 20, 2013 3:34 pm ] |
| Post subject: | Re: how light is too light for bracing |
You will know when they are too light when you string it up and it pops!!! I would assume the 1.25 is millimeters, I doubt he would have any flex in the wood unless he drove a car over it. |
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| Author: | Clay S. [ Fri Dec 20, 2013 3:49 pm ] |
| Post subject: | Re: how light is too light for bracing |
1.25 mm would be rather thin, and 1.25 inches would be rather thick. Perhaps he means .125 inches -a.k.a. 1/8th inch? If the top really is slightly less than 1/16th of an inch thick this is in oud territory, and who knows what the Chladni patterns would look like. |
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| Author: | gadgetman2 [ Fri Dec 20, 2013 7:54 pm ] |
| Post subject: | Re: how light is too light for bracing |
i mean .125 /3 mm or 1/8 inch. I was told by a well know builder during thicknessing was best to stop at this thicknees. It is sitka by the way. I would definitely like to go lighter |
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| Author: | JSDenvir [ Fri Dec 20, 2013 8:52 pm ] |
| Post subject: | Re: how light is too light for bracing |
At .125 for your top, I think you can afford to go much narrower on those braces. The height is what delivers much of the stiffness. So start carving  As a profile, imagine the Eiffel Tower. And as you carve, tap and listen. Does the top seem to resonate more? Sustain more? Tap everywhere. Are some spots deader? Livelier? What do you think that means? The real challenge is figuring out when to stop carving. And to be honest, I think most of us are working towards figuring that out. |
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| Author: | Alan Carruth [ Sat Dec 21, 2013 5:55 pm ] |
| Post subject: | Re: how light is too light for bracing |
Thanks for the photos; that helps. Those braces don't look too low to me. I like to leave them taller at the bridge location; tapering from that as the high point, rather than the X crossing, but what the heck. I also find that reducing the height at the ends of the tone bars helps: you don't need a lot of strength or stiffness there. It can be a trick to get symmetrical patterns with an asymmetric bracing scheme, which is one reason I've gone to the double X pattern. Top thickness is best decided on by knowing the Young's modulus (E) of the wood along the grain, since that's what determines the stiffness at a given thickness. There's a pretty reliable correlation between density and E-long for all softwoods, so if you knew the density you'd be in pretty good shape to figure out the 'right' thickness. Of course, there's no substitute for actually measuring the E value directly. .125" is on the thick side for average Sitka, which tends to be denser than most spruces, and thus to have a higher Young's modulus. Chladni testing is, IMO, a method of balancing the brace and top stiffness. If the top starts out thicker and stiffer than it 'needs' to be then you'll end up stiffer bracing as well when the patterns are 'right'. The pitches of the modes will also tend to be on the high side, in that case. This is not a bad thing in a way: you can rest easy, knowing that the top is unlikely to fold up any time soon. You won't get the power you would have with the 'correct' thickness, and the sound may well be more 'treble balanced', but at least the tuning will help give a good, even, and clear sound. I'd opt for good patterns with high mode pitches, rather than trying to lower the pitches at the cost of messing up the patterns. |
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| Author: | truckjohn [ Wed Dec 25, 2013 11:36 am ] |
| Post subject: | Re: how light is too light for bracing |
So... Dumb question and all.. but what are the blobs on the ends of your bracing? Looks like globs of Epoxy? Since you are going through an exercise with Chladni - I would just keep working at it... The goal is a learning exercise.... If you end up going too far - plane off the braces and start over... but I wouldn't stop until I had it about "right" for what I was trying to do.... Thanks |
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