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Top thickness / deflection
I'm done bracing the back on my first build and will soon start on the top. I have no context to evaluate whether I have thinned my top too much, not enough, or just right. I've tried Bruce Sexauer's "fwump" method, but having no experience with other tops, I'm not sure I actually got it right.
Some builders shoot for a particular thickness as a proxy for stiffness, while others tackle stiffness directly. If I wanted to measure stiffness by deflection, is there a rule of thumb (or math) for a) the weight on the top b) the distance between the fixed points c) how much deflection is optimal?
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Solo acoustic guitar videos: This Boy is Damaged - Little Watercolor Pictures of Locomotives - Ragamuffin |
#2
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The long answer is that you need to start doing deflection testing now and be like a scientist and record everything in your little note book. After having enough data you can start to make sense of it. FWIW I adopted Brian Howards method as seen here except that I deflect the joined top prior to cutting the guitar shape out: http://howardguitars.blogspot.com/20...ic-guitar.html |
#3
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Thanks for the link! Lots to read.
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Solo acoustic guitar videos: This Boy is Damaged - Little Watercolor Pictures of Locomotives - Ragamuffin |
#4
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The most common numbers I've found are 5lb with an 18" span.
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Thanks! Is there a target deflection amount for that?
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Solo acoustic guitar videos: This Boy is Damaged - Little Watercolor Pictures of Locomotives - Ragamuffin |
#6
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Not really, though it's kind of up to you to figure it out. Once you start getting a baseline of deflection readings (because every top will be different - some wildly so), you determine if you want to control your stiffness by top thickness, bracing accommodations, or a combination of the two.
Just to share how we do it (which is arbitrary but works for us). It used to be that we would leave floppier tops thicker and take stiffer tops thinner. However, we heard a wider range of tonal values than when we took all of our tops (some species excluded) down to about .100". From there we would use stiffer braces (yes we deflect those too) on tops we thought needed the support. It's worked out that this method for us got us a much more consistent tone that we were looking for than the variable top thicknesses. This could depend on many factors unique to our shop as we know many good builders who concentrate on exactly the opposite things we do. But, the moral is you have to start recording all your numbers now, so you can try to decipher what changes are making differences. |
#7
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Yes, of course there is, that's the point, but you'll need to determine what you think that is for yourself by experimentation.
I used to get miffed that people wouldn't say what it may be for them, but any number I give may not work for how you do you. I'd start by taking a Sitka top to .110 (general factory spec), measure the deflection, build a dread, adjudicate and adjust. No getting around wash, rinse, repeat. |
#8
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Thanks, guys, that's super helpful.
I also have two more tops to do some experiments with. Those are consecutive cuts from the same billet, so it'll be interesting to see how they perform if thicknessed / braced to different deflections. Cheers!
__________________
Solo acoustic guitar videos: This Boy is Damaged - Little Watercolor Pictures of Locomotives - Ragamuffin |
#9
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While I will not share my target values here is my process for deflection tuning my tops.
http://howardguitars.blogspot.com/20...ic-guitar.html |
#10
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Thanks good info Sent from my iPhone using Tapatalk Pro |
#11
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I use a different method of testing wood stiffness, but you can get very similar results using deflection testing. David Hurd covers it in 'Left Brain Lutherie' and so does Trevor Gore in the books he co-authored with Gilet.
My perspective is that lengthwise stiffness counts for a lot more than crosswise, although most folks concentrate on the latter. I vary my top thickness according to the long grain stiffness, and look at the ratio of long-grain to cross-grain stiffness to determine which top to use for a particular shape. Generally, the tops with higher cross stiffness go on wider boxes. I find I can get very consistent tonal results by varying the top thickness depending on the long-grain stiffness. Of course, you have to vary the bracing as well. Getting 'consistent' results is not saying that they're 'identical': that's a tough nut. |
#12
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I do believe the ratio of plate stiffness to bracing stiffness can be varied widely, and as long as the combined stiffness comes out about the same all over it should sound fairly similar, with the primary difference coming from the mass. But I don't have much data to back up the theory yet. Do you measure mass on the soundboards? Do the similar sounding ones tend to have similar mass? As for my own method, I cut the soundboard to shape and flex it along the grain by hand until I like the feel of it. I tend to go with stiffer plate and less bracing (plus some perimeter thinning) on small guitars where the mass will be low regardless, and thinner plate with stiffer bracing on large ones where mass tends to be high. This is backward from most builders, but makes more sense to me. Also, thinner tops get more braces so they're closer together, because otherwise there will be squishy spots between braces (although I'm not entirely sure that's actually a bad thing). 200 grams seems to be a good general maximum for the unbraced soundboard. 00 and smaller will usually be lighter by the time they're flexible enough. |
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Do you have like some sort of standard dimension to your braces before you test them to deflection and then glue them up and carve them out or do you actually shape the brace fully then do deflection and glue them up? That's the one issue I had thought about with deflection in general... What good is it when you are going to glue up braces anyway and then carve those by feel. To remedy that dilemma a bit I do deflection of the braced top as well, so in affect the whole unit can be compared to the next ones down the road. |
#14
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I have not been checking mass on the soundboards without braces. I usually stop thicknessing at a certain deflection. Travis |
#15
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However is there a range that can be shared? Or an order of magnitude? Say if I do 18-inch apart, 5-lb weight deflection testing, should I expect the top to move by .005? .05? .5? an inch? That's the part I'm stuck on--I have no frame of reference.
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Solo acoustic guitar videos: This Boy is Damaged - Little Watercolor Pictures of Locomotives - Ragamuffin |