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| Best way to cap the X-brace?? http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=6841 |
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| Author: | Dread1916 [ Wed May 24, 2006 1:41 pm ] |
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From a previous post, I got a lot of recommendations to Cap my x-brace and I think Serge even sent a pic showing an example. I got to thinking about this and was wondering is Capping is the best solution. What is the difference between using the Cloth and a Cap? Who uses what? And...if a Cap is the best way to go...is it one piece and how is the grain oriented? I could really use some direction on this stuff... (pardon the oriented and direction pun) Thanks, JP |
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| Author: | Alain Desforges [ Wed May 24, 2006 2:40 pm ] |
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The cap is a lot more solid than the cloth. The reason behind putting the cap is that the small piece of wood makes the brace almost as solid as if it had never been cut. You're trying to add as little mass as possible. You just need one little piece running over the brace that was up-notched, say about 1.5 inches long and not too thick either. Adding the cap should tighten the X, adding sustain and clarity to your instrument, or so we hope. Hope this helps. |
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| Author: | Serge Poirier [ Wed May 24, 2006 3:13 pm ] |
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Like Alain says, really thin and not too long. It's just a matter of securing that very important joint and hopefully add more clarity. Serge |
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| Author: | Mattia Valente [ Wed May 24, 2006 5:31 pm ] |
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Sliver of spruce, very small, maybe 2"-3" long, tapered out, same grain direciton as the brace. Should stiffen up the joint a touch. I'm no expert, but here are a couple of pictures of mine, although I'm planning on making them a touch smaller for the next guitars: Actually shaved this one down a little more (ditto the braces) 
my first 
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| Author: | Dave White [ Wed May 24, 2006 11:12 pm ] |
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[QUOTE=ToddStock] Stiffness of a beam in bending is related to the cube of the depth, so this means that without the cap, one of the braces will be much stiffer than the other (beam depth of 3/8" for one brace and nearly 3/4" for the other...eight times as stiff (minus a bit for material removed next to the soundboard). [/QUOTE] Todd, One thing puzzles me deeply here. Both of the X braces have half of them removed where the X joint is. As I understand it the brace with the part removed next to the top keeps a lot (if not all ) of the stiffness as the top length of the brace isn't cut, but the other brace looses some stiffness as the top length has a gap. But with a snug fit X joint, the other brace now fills in this gap, and when glued in must bring back a lot of the stifness to the brace by making it full height again and effectively bridging the gap. Putting a cap across the top of the brace must largely be "putting icing on the cake" and extra insurance. Also the X braces are glued to the top when it is domed and under tension (if a domed top is used), and if the loss of stiffness was such a factor, surely you would have to keep the top firmly in its domed position before the cap is glued on - otherwise the top will pull out of the ideal shape due to the "not stiffened" X brace, so putting the cap on without doing so would be "bolting the stable door after the horse has bolted" so to speak. |
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| Author: | LanceK [ Wed May 24, 2006 11:34 pm ] |
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Ervin & Judy are demonstrating the cube effect here, this was at the ASIA Symposium 2005. 
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| Author: | Mattia Valente [ Wed May 24, 2006 11:53 pm ] |
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Dave: I simply don't trust end-grain glue joints, which is what the open part of the thing is. Ergo: cap. |
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| Author: | Phil Marino [ Thu May 25, 2006 12:04 am ] |
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[QUOTE=Dave White] [QUOTE=ToddStock] Stiffness of a beam in bending is related to the cube of the depth, so this means that without the cap, one of the braces will be much stiffer than the other (beam depth of 3/8" for one brace and nearly 3/4" for the other...eight times as stiff (minus a bit for material removed next to the soundboard). [/QUOTE] Todd, One thing puzzles me deeply here. Both of the X braces have half of them removed where the X joint is. As I understand it the brace with the part removed next to the top keeps a lot (if not all ) of the stiffness as the top length of the brace isn't cut, but the other brace looses some stiffness as the top length has a gap. But with a snug fit X joint, the other brace now fills in this gap, and when glued in must bring back a lot of the stifness to the brace by making it full height again and effectively bridging the gap. Putting a cap across the top of the brace must largely be "putting icing on the cake" and extra insurance. Also the X braces are glued to the top when it is domed and under tension (if a domed top is used), and if the loss of stiffness was such a factor, surely you would have to keep the top firmly in its domed position before the cap is glued on - otherwise the top will pull out of the ideal shape due to the "not stiffened" X brace, so putting the cap on without doing so would be "bolting the stable door after the horse has bolted" so to speak.[/QUOTE] You've got a point, Todd. But I think adding a small top piece as a bridge across the joint is still a good idea. Here is my take on it. The gap is, as you say, bridged by the other X-brace, but I see two possible problems. One is that the gap-filling brace has its grain in the wrong direction - wood is MANY times stiffer and stronger along the grain than across it. It's also much less stable with temperature and humidity, so the top may change shape slightly because of that. But the most important reason may be that glues hold very poorly to end grain. The joint ( between the cut ends of the notch on one brace and the sides of the other) may be solid and stiff for a while, but with time (and even a light blow to the top) the glue joint between the two braces may open up. Try gluing two sticks of spruce together end-to-end, ( or, the end of one to the side of another) and you will see how weak that glue joint is. Phil |
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| Author: | Dave White [ Thu May 25, 2006 12:24 am ] |
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Mattia and Phil, Thanks for this. Phil it was me and not Todd who was querying - Todd was rock solid supporting the use of the cap. So the X joint is for the stiffness and the cap is to keep the joint stable. So in a sense I was right, the cap is "insurance". Pedantic I know and please forgive me but in my head I found it hard to visualise that you glue on small strip and stiffness there immidiately shoots up. |
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| Author: | CarltonM [ Thu May 25, 2006 2:13 am ] |
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Dave, It's more than just the stability of the glue joint. That small strip does, indeed, shoot up the stiffness. As you probably know, it's possible to drill a hole through the side of a brace without reducing its strength or stiffness, but cutting out half its height at any point will lower its stiffness considerably. The cap works under the same principle. The brace with the gap functions as if it were only half as tall, right at a critical point. The cap completes the brace by restoring it to full height. It will then function like a brace with a hole drilled through its side. The other half of the X is functionally capped by the soundboard, so it's already at its full strength. I, too, always thought that the gap was functionally filled by the other half of the X, but Alan Carruth gave us a more technical explanation some time ago, and his logic was very convincing. |
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| Author: | letseatpaste [ Thu May 25, 2006 2:35 am ] |
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I did this sort of on a whim... I figured it would stiffen the notched brace, as well as stiffen the structure of the X-bracing. I don't know if it's any real advantage, and may be unnecessary mass. I mainly just wanted to make it a little harder to get a pick out of it if one is accidentally dropped in the soundhole. I wouldn't be offended if anyone wanted to declare this dumb/useless/detrimental, etc... 
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| Author: | CarltonM [ Thu May 25, 2006 2:39 am ] |
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It should work okay, as long as the grain runs parallel with grain of the gapped brace. If it's flatsawn, I'm not sure. |
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| Author: | Dave White [ Thu May 25, 2006 2:44 am ] |
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Todd, Thanks for taking the time and trouble to do this!! A couple of points/questions though. Under string tension the top domes upwards and so the gap in the uncapped brace will actually be pulled more tightly closed against the other brace. Then as the strings vibrate the top you will get movement away and back again but not to the extent of the tops' initial tensioning up under the string pressure - at least this is how I am envisioning it in my tiny brain. The forces shown in your diagram are largely pushing the top inwards (concave, but it could be that I am not interpreting it correctly - a commion occurrence with me  .
Also under what you are explaining shouldn't the brace be capped when the top is under full string tension, and the x brace joint at its' tightest to give the best joint protection and brace stiffness? Calton, I will go back and look at what Al had to say. |
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| Author: | Dave White [ Thu May 25, 2006 2:56 am ] |
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[QUOTE=ToddStock] The domed top has little to do with this issue. We pre-arch the brace to get the dome, so there is little pre-stressing of the brace. Loads on the brace after the guitar has been strung up are almost identical between arched and flat plates - in other words, the dome is not to pre-stress the bracing, but rather to bias the plate/brace assembly towards controlled distortion in one direction to accomodate expansion and contraction of the top. To see what a notch does to beam strength, take two sticks of 3/8" by 3/4" x 12" pine, notch each halfway through as in a X-brace, then glue each to a small piece of 1/8" x 2" x 12" thick scrap (to simulate the soundboard). Support each between some 2x4s or other rigid support, then have fun loading them up. The 'notch down' sample should fail at about around 1/4 to 1/6 the load on the notch up material (correction for the top plate and the notch).[/QUOTE] Todd, Sorry - you were posting while I was posting!! Yes I accept that the brace isn't pre-stressed and that is partly why I was puzzled - putting a cap on in this state does nothing. As you correctly pointed out it's about how the braces react from there when things happen to the top. On it's own - yes the notch down will fail quicker, but in the guitar it isn't on it's own, it is locked with the other brace and it depends where the relative force and direction of the pulling goes on it (see my other post). I suppose the true test is what happens to relative stiffness and top response with and without the cap. Getting your head around what happens in a guitar is fantastic and it could well be that what you say is correct - I just need to get my head fully around this that's all. Thanks to all for being patient and helping. |
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| Author: | Bill Greene [ Thu May 25, 2006 5:34 am ] |
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I am genuinely amazed at the level of technical expertise on this board. Some of you folks are scary knowledgeable...and I like to learn. |
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| Author: | Alain Desforges [ Thu May 25, 2006 6:51 am ] |
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Ya just pull out your Glock, hold it sideways, and Bang! Bang! Gangsta style!!! |
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| Author: | CarltonM [ Thu May 25, 2006 7:42 am ] |
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[QUOTE=Dave White] On it's own - yes the notch down will fail quicker, but in the guitar it isn't on it's own, it is locked with the other brace... [/QUOTE] But that's just it, Dave. It's not locked until the cap goes on. Without the cap, almost all the junction stress is carried by the notch-up brace, because the notch-down brace behaves as if it's only one-half its original height, leaving it weaker. Also remember, braces are not only for strength, but also for sound transference. Leaving the X uncapped creates an uneven distribution of both load and vibration, which, I think, would hurt the sound in the long run. Also, the string pull on the bridge does dome the top, but only behind the bridge. The area in front of the bridge, where the X is, is then compressed downward. You must have seen a lot of guitars with that typical "valley" between the bridge and the soundhole. That's compression at work. |
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| Author: | MSpencer [ Thu May 25, 2006 7:56 am ] |
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I do mine similar to Joe Kendall above, except mine are square. This is a part of the guitar that is not really seen, I do like the little crosses of hardwood and others I have seen, but a square or circle works fine. Mike White Oak, Texas |
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| Author: | Dave White [ Thu May 25, 2006 8:01 am ] |
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[QUOTE=CarltonM] [QUOTE=Dave White] On it's own - yes the notch down will fail quicker, but in the guitar it isn't on it's own, it is locked with the other brace... [/QUOTE] But that's just it, Dave. It's not locked until the cap goes on. Without the cap, almost all the junction stress is carried by the notch-up brace, because the notch-down brace behaves as if it's only one-half its original height, leaving it weaker. Also remember, braces are not only for strength, but also for sound transference. Leaving the X uncapped creates an uneven distribution of both load and vibration, which, I think, would hurt the sound in the long run. Also, the string pull on the bridge does dome the top, but only behind the bridge. The area in front of the bridge, where the X is, is then compressed downward. You must have seen a lot of guitars with that typical "valley" between the bridge and the soundhole. That's compression at work.[/QUOTE] Hi Calton, I'll leave it at this response as it's hijacking the original thread somewhat and that wasn't really my intention. My argument is that it is locked without the cap so long as the other brace is snugly in the void. This I contend fills in some of the missing gap and hence gives back the stiffness. Todd pointed out that if the forces/tension pull the sides of the notch away then you are back to the half brace tension position. This I agree with but was pondering whether the forces on the top under string tension would lead to this. My tops certainly dome up under string tension in the first few weeks/months after stringing up and not just behind the bridge with the front of the bridge compressed. In fact I am looking for this as part of the design/sound that I am looking for. I have to allow for this in the neck set and saddle height with for adjustment to get the action back to the desired position after the top has "stabalised". The reason I am a little sceptical on the sound front is that for my first 10 guitars I didn't use a cap and I did not hear a dramatic difference soundwise as I went from no cap to cap. Don't get me wrong - I'm all in favour of using caps and I do this as a matter of course now, but I'm still not convinced that you loose a half of the height to the power of 3 stiffness with an well fitted X brace minus a cap compared to a well fitted X brace with a cap. |
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| Author: | CarltonM [ Thu May 25, 2006 8:22 am ] |
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[QUOTE=Dave White]The reason I am a little sceptical on the sound front is that for my first 10 guitars I didn't use a cap and I did not hear a dramatic difference soundwise as I went from no cap to cap.[/QUOTE]Interesting. Well, you certainly have much more hands-on experience with this than I, so I really can't argue with you on this point. [QUOTE=Dave White]Don't get me wrong - I'm all in favour of using caps and I do this as a matter of course now, but I'm still not convinced that you loose a half of the height to the power of 3 stiffness with an well fitted X brace minus a cap compared to a well fitted X brace with a cap.[/QUOTE] Anybody know where to find Alan Carruth's treatise on this topic? He explained it more convincingly than I ever could. |
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| Author: | stan thomison [ Thu May 25, 2006 11:16 am ] |
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Has anyone actually had a lap joint at the X make a box fail or heard of that. I don't know If it makes a difference or not. If a builder thinks it does, use it, if not don't. I have used them on some and not on others, never had a joint fail or seem better in the long run either way. I have seen post where folks thought lack of one caused the bridge or bridge plate fail. That isn't dependent on a small piece of wood but that particular piece not glued well. Now sound, Come on. How can that cause a difference in tone, sound sustain or whatever. What next, the kind of wood used in the cap will make a differnce in voicing? This small piece of wood now appears to be the most basic part of the build to some, don't think so, but who knows. |
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| Author: | Alan Carruth [ Thu May 25, 2006 11:49 am ] |
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FWIW, I've lost track of the number of split X braces I've repaired over the years in cheap guitars, where they didn't use even a cloth patch. The string load on the bridge causes a torque that's pulling upward behind the bridge, and pushing downward in front, where the lap joint is. It doesn't matter if the whole top moves upward at first, there's still that down load acting on the lap. The worst of it is that the notch makes a stress riser. The stiffness changes suddenly from one value to another, much lower one. The bending is concentrated right at the edge of the notch, and the usual failure mode is that the down-notched brace splits at the level of the bottom of the notch. The cloth patch actually seems to work pretty well for 'normal' loads, but I suspect it's marginal. Even a fairly light blow to the top can start the brace splitting. We can go on all night about beam theory and acoustics, and I've done so in the past. Whatever you believe or don't believe about that, the _fact_ is that a small slip of wood, maybe 1/8" thick, 2" long, and the width of the brace, weighing a couple of tenths of a gram, will save you or your customers from a world of grief down the road. Why would you _not_ put it on? |
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| Author: | Bill Greene [ Thu May 25, 2006 11:54 am ] |
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...and there you are. 
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| Author: | Don Williams [ Thu May 25, 2006 12:22 pm ] |
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What Al said... |
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| Author: | Dread1916 [ Thu May 25, 2006 1:29 pm ] |
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Alright... I didn't realize that this was such a hot button topic. I agree the Cap should be there based on my knowledge of beam theory and aerospace structures (which brings me to a thought about the use of lightening holes in guitars...I might need to build an experimental box down the line...build 3 or 4 maybe) I unfortunaly do not have alot of spruce laying around as this is my 1st build...BUT I do have some left over rosewood strips that I used for the side reinforcements....any problems using 1 or 2 of these strips 1/16" x 1/4" x 2" ???? Otherwise...its time to hit up a vendor. Thanks for the great discussion...keep'm coming...I will repeat something stated above....Great Forum...we can get as technicle as you want in here... Good deal JP |
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