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Question on domed tops and backs

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Question on domed tops and backs

Postby Eric Knapp » Sun May 22, 2016 8:14 pm

Hello,

I'm seeing the current thing is to dome the top and backs of acoustic guitars. The normal technique seems to be to shape the braces with the desired radius and warp the top and back to the braces. Has anyone experimented with carving the dome instead? Is the tension in the wood that is bent by the braces a good thing, a bad thing, or immaterial? I'm hoping to make an archtop guitar or two eventually. The effort to shape a dome seems much easier than carving a top, although I'm seeing all this through rookie eyes.

Thanks,

-Eric
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Re: Question on domed tops and backs

Postby Barry Daniels » Mon May 23, 2016 1:24 am

The curve is so slight in a top that it is MUCH easier to bend flat wood in a radius dish and glue in curved braces with go bars. Skip all the older and harder approaches and go directly to the generally accepted, modern techniques to building guitars. Various people do it differently, but I use a 25' radius for the top and 15' for the back, which is not uncommon.

When you get a lot more experienced, you might want to try an archtop. I had built a dozen flattops before I was ready for one. (Archtops are about 3 times more time consuming to build that a flattop).
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Re: Question on domed tops and backs

Postby Eric Knapp » Mon May 23, 2016 10:06 am

Barry Daniels wrote:The curve is so slight in a top that it is MUCH easier to bend flat wood in a radius dish and glue in curved braces with go bars. Skip all the older and harder approaches and go directly to the generally accepted, modern techniques to building guitars. Various people do it differently, but I use a 25' radius for the top and 15' for the back, which is not uncommon.

This is good information. I was just curious how this technique came about and if experimentation was ongoing. I've seen many videos and sites using go bars and they look very interesting. That is a new kind of clamp for me and I'll probably get some as soon as the budget allows.

Barry Daniels wrote:When you get a lot more experienced, you might want to try an archtop. I had built a dozen flattops before I was ready for one. (Archtops are about 3 times more time consuming to build that a flattop).

My interest is primarily archtops but I can see the benefits of getting more experience first. The term "flattop" seems to be outdated since they aren't flat anymore. Does a domed guitar sound any different than a pure flat one?

-Eric
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Re: Question on domed tops and backs

Postby Barry Daniels » Mon May 23, 2016 11:24 am

You may find a lot of varying opinions but little evidence on the sonic effects of domed tops. I do it mainly for the structural and stability benefits.

Go bars can be made from various materials including: homemade wood strips, hardwood dowels (1/4"), and store bought fiberglass rods. They are not a big investment, but they really do work well.
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Re: Question on domed tops and backs

Postby Alan Carruth » Mon May 23, 2016 1:26 pm

My first go-bar deck was a piece of plywood screwed to the ceiling above on the of my benches. I got a pile of dowels (I like 5/16" but that's just me) and cut them to the right length to work with the low ceiling. I don't have a table saw, and cutting wood bars that are smooth and uniform on a band saw doesn't work well. I've always felt that the fiberglass ones were too strong: I'd rather use more with lighter pressure on each one. Anyway, even using dowels it's not much of an investment.
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Re: Question on domed tops and backs

Postby Todd Stock » Wed May 25, 2016 7:27 am

28' radius dish...X, toners, fingers, bridge plate radiused at 28' and UTB radiused at 60' and glued on the flat - gives flat extension without wedge or fall-away. Back gets 15' radius on 15" dish.

3/16" x 24" glass generates about 8 lbs each, so the 20 something bars that one leg of the X takes generates about 36 psi at the glue line...and considering Titebond's tech lit calls for 100 psi minimum on softwoods, with upper end at 300 psi (not much chance of starving the joint). We ignore glue line pressure much of the time because our joints are short and small, so well fitted, but the higher the pressure, the better for someone that might be new to both woodworking and building guitars and have a little less-than-perfect joinery.

Goodwinds Kites has best price on fiberglass rods and tips. If using wood, ripping up some straight grained ash works well.
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Re: Question on domed tops and backs

Postby Clay Schaeffer » Sat May 28, 2016 8:13 am

"Is the tension in the wood that is bent by the braces a good thing, a bad thing, or immaterial?"

As mentioned, "doming" the top adds some stability and crack resistance from humidity changes. When carried to extremes It can create a "tight" (almost percussive) sound.
I build Octave mandolins with strongly domed tops because I want them to have a little stronger attack and a little less sustain. I don't want them to sound like guitars.
Guitars can be built with flat tops, but if not braced under the right humidity conditions (low humidity) they can go concave and be rather ugly. A slight dome is insurance against this.
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Re: Question on domed tops and backs

Postby Gordon Bellerose » Sat Jun 18, 2016 11:31 am

Regarding a go bar deck, they can be built quite inexpensively.
I think mine cost around 20 or 30 dollars. Most of that was spent on threaded rods, washers, and nuts.
All you need for wood is 2 pieces of good strong (I used 3/4 inch) plywood. 24 x 24 inches.
I need your help. I can't possibly make all the mistakes myself!
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Re: Question on domed tops and backs

Postby Eric Knapp » Sat Jun 18, 2016 12:06 pm

Gordon Bellerose wrote:Regarding a go bar deck, they can be built quite inexpensively.
I think mine cost around 20 or 30 dollars. Most of that was spent on threaded rods, washers, and nuts.
All you need for wood is 2 pieces of good strong (I used 3/4 inch) plywood. 24 x 24 inches.

What do you use for the flexible clamping bars?

Thanks,

-Eric
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Re: Question on domed tops and backs

Postby Barry Daniels » Sat Jun 18, 2016 12:38 pm

You can make your own go-bars out of any good straight-grain hardwood ripped to about 1/4" by 1/2". Or you can buy hardware store wood dowels (1/4" and 5/16") both work. Or fiberglass rods work great. You can get 5/16" very strong fiberglass rods sold for driveway marker reflectors. But the best are 3/16" diameter fiberglass kite rods from Intothewind.com
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Re: Question on domed tops and backs

Postby Bob Gramann » Sat Jun 18, 2016 12:58 pm

My gobar deck is a piece of plywood attached to the ceiling over one of my workbenches (to reinforce the ceiling). I ripped my rods out of the clear South American pine available at Home Depot. My net cost for the whole thing was next to nothing.
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Re: Question on domed tops and backs

Postby Greg Martin » Tue Sep 06, 2016 8:52 pm

Ok question, why use 2 different radius dishes, is it tradition,or is there a tone or structure benifit.ive only built archtops and everything is different, so i really dont know much about it.
I was asking a well known builder about doming for a harp guitar and a couple others also said that 30 foot radius was fine,i thought they ment both top and back?
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