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Sanding Dishes And Compound Curves
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Author:  LarryH [ Wed Mar 22, 2006 4:36 am ]
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I was having a thought late last night about go bars and sanding dishes and gluing backs and tops, using the dishes as forms, in order to achieve the compound curves that a dish would attempt to create.

Then I thought that there is no way to get a flat surface, which a glued back and top represents, to be formed into the compound curve that a dish creates. It seems that any time you are trying to get 2 curves, one longitudinally, and the other horizontally, one of those dimensions has to change in order to acheive this.

Is wood simply flexible enough for this to happen, which would require some compression at the neck and heel, or is th radius large enough and the effect 'close enough' to not sweat it?

Or am I missing a piece of the logic?
Author:  tippie53 [ Wed Mar 22, 2006 4:41 am ]
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   The braces will also help to create the parabolic curve. I use the disks on a vacuum clamp system and get s nice bowl shape every time.
   Before I got this set up the gobars did fine though I did overradius the bracing to help pull the top and back into the radius I wanted. I usually did 5 foot more on the braces than what I truley wanted.
I would do a 15 radius on the brace for a 20 foot back. This helped alot,
john hall
blues creek guitars
Author:  Michael Dale Payne [ Wed Mar 22, 2006 4:49 am ]
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Ok I am lost On my guitars the back dome is a consistant 15' dome radius just tilted to match the taper, and the tops are 28'. This is not a compound radii but a continious dome radius. The position of the domes center in reationship to the center of the guitar may vary but the radius is constant not compound. I handle the change of the center axis by moving the body forward in the dish to replacate the domes off center differance. So I am not following the compound radius you are refering to.
Author:  Phil Marino [ Wed Mar 22, 2006 6:53 am ]
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Larry -

I understand what you're saying, and I've been wondering the same thing for a while. To form a flat plate into a dome means you have to stretch it at the center and/or compress it at the edges. It's much more compressible across the grain, so it probably does, as you say, compress mostly at the neck and heel areas.

For those who don't see the problem here, try fitting a sheet of paper to the surface of a globe or a beach ball without wrinkling it.

It must be that the curve is gentle enough AND the wood is flexible enough that it can be forced into the dish. Although it obviously works fine, I don't like the idea of the wood being stressed to take that shape.

I mostly plan on following Cumpiano's text (at least for the first few instruments), and he doesn't use radiused dishes for either brace gluing or sanding. But, he clearly describes the compound curve on the back (page 217).

What I'm curious about is how much force does it take to get a top or back to fit into a dish?


Phil
Author:  Wayne Clark [ Wed Mar 22, 2006 7:09 am ]
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Welcome, Phil.

The amount of deflection to get the types of radiuses (radii?) is not really that much. Across a 2 foot dish its 0.2 inches for a 30 foot radius, 0.3 inch for a 20 foot radius. If you have a soundboard or back at a thickness of 0.100 inch, it will have plenty of flexibility. I don't know the force required to push the wood into the dome shape, but it seems very light.
Author:  Shane Neifer [ Wed Mar 22, 2006 7:17 am ]
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Phil,

Very little force to press into the dish! Cumpiano does, in effect, use a dish, he just does not radius it! When I was last at Bill's shop I actually asked if he still used the cork lined perimetre of his workboards. He took me into one of his back rooms and showed me the stack that he still uses! The only difference that I see between the Cumpiano (probably really Gurian) method and a radius dish is that with a dish you have better control/support over the radius you place on the top/back and if you use a go bar deck you need that support under the plate over the entire area, whereas with the corklined workboard you have an unsupported area and an inconsistent curve. As for the compression issue, the wood does indeed fold on itself, but, unlike your paper example (good by the way!) the wood fibres will compress rather than buckle but that is the idea, you are pre-stressing the plates to give them strength. They are much stronger in this condition than they are flat, this allows you the ability to decrease the thickness of the plates and make the instrument more acoustically responsive.

Hope this helps!

Shane
Author:  LarryH [ Wed Mar 22, 2006 8:05 am ]
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[QUOTE=Phil Marino] Larry -

I understand what you're saying, and I've been wondering the same thing for a while. To form a flat plate into a dome means you have to stretch it at the center and/or compress it at the edges. It's much more compressible across the grain, so it probably does, as you say, compress mostly at the neck and heel areas.

For those who don't see the problem here, try fitting a sheet of paper to the surface of a globe or a beach ball without wrinkling it.

It must be that the curve is gentle enough AND the wood is flexible enough that it can be forced into the dish. Although it obviously works fine, I don't like the idea of the wood being stressed to take that shape.

I mostly plan on following Cumpiano's text (at least for the first few instruments), and he doesn't use radiused dishes for either brace gluing or sanding. But, he clearly describes the compound curve on the back (page 217).

What I'm curious about is how much force does it take to get a top or back to fit into a dish?
Phil[/QUOTE]

I'm so glad someone can explain these things better than I. To make the dome or compound radius with no stress would require (I think) the back joint to have a very slight curve, wider in the middle and narrower at each end, with the gaps being closed in the dish and glued up there. Probably not worth the effort but sometimes it's the details that can create that great guitar.

It seems it would be very interesting to see a glued up back with no bracing that already had a curve glued in one direction. Might form a more perfect and uniform dome?

Larry
Author:  Shane Neifer [ Wed Mar 22, 2006 10:42 am ]
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Larry,

There a lot of guitars today with no back bracing. I have seen a Guild like this and I think that Martin is doing one now also, they are laminated into the arc. Your idea for the glue joint is good but only addresses a small portion of the overall back which is a continous curve. That begs the questions, why just concentrate on a small strip of the back? All of the rest of the back will be in either compression or tension and the same forces would probably at work on that glue joint. But hey, you shuld definately try it and see how it works, that's how great things get discovered.

Great Luck!

Shane

Shane
Author:  TonyKarol [ Wed Mar 22, 2006 10:56 am ]
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HI all,

Just my 2 cents - take that same sheet of paper and put it in the 28 foot bowl - it sits flat with no wrinkling - the curve is too soft for it to do so. As for the joint at the side/kerfing area, its essentially flat for the 1/4 inch thats there - the curve is almost not existant on that kind of measure. Some folks use a flat brace for the upper transverse - I dont - the ammount of radius you have to sand off to get the area flat under the FB is minimal - about 5 thou across the FB (and thats only in the center as well, it tapers to nothing at the Fb edges) , and about 10 thou in its length (again thats only in the middle) - then you glue the FB down to the top there anyway - it has lots of support when you are done.
Author:  MSpencer [ Wed Mar 22, 2006 1:09 pm ]
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Good point Tony, I was trying to reconcile the ball and paper to the 15-30' bowl myself.

Mike
Author:  tippie53 [ Thu Mar 23, 2006 12:13 am ]
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    You need to look at this from a strength of materials point of view . All materials will have yeild strength , point of Giration values and other engineering properties.
    What this means if the wood can change shape to a given degree without degradation of strength , so when used in a given application it would be expeted to perform.
    All materials have this. If they didn't things would break. Bridges move from temperature and load , so if the steel couldn't move the load would cause the bridge to fail , so be it the the wood on the guitars.
     Now with this in mind , the dome of the back will do 2 things , 1st as some strength and 2nd it will allow the wood to move. As humidity changes the wood will expand and contract and the dome will do this. So if you build a dead flat top or back , being that the plate is held flat , then as the plate expands and contracts the braces are put under a load of expansion or contraction and this can cause the surface to sink or rise. We usually see the sinking tops this time of year when the humidty is low.
    While this still will happen to a degree with the dome or radiused surface , you can see that the dome can allow this to happen without too much deflection.
   john hall
    
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