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Hi, my name is Chris and I am a flat top builder....

I recently watched the video in this thread: http://luthiersforum.com/forum/viewtopic.php?f=10101&t=37145 and I have to admit that one of things that I have been avoiding about domed tops is the tendency of builders to then flatten the area under the tongue. This seems more complicated than it is worth. Plus, I also like the idea of a truly domed top as I feel this would maximize the strength of the top. My question is, if you were to build a top that is domed from neck to tail, how would the fingerboard tongue fit the top? Would it need to be flossed like the heel?

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I've been using a fully domed top for years. I think it does go some way toward increasing stability, but it poses some problems in building, too. If you set the neck such that the fingerboard comes out tangent to the top at the edge, the projection at the bridge will give you a saddle that's about 18-20mm off the top, which is too much torque, and you have to deal with that somehow.

When I set the neck, I make the heel angle such that the fingerboard projection will be correct at the bridge when the foot of the neck is a little proud of the top, and the bottom of the fingerboard just touches the top at the edge of the soundhole. I then fill in the space between the fingerboard and the top with a wedge of the same material as the neck. This eliminates the issue of fitting the fingerboard itself to the top, and any problems that might arise if the fingerboard becomes too thin at the end.

I did the math once to see how muich wood is being removed from a 28 ft radius top, ubnder the FB ... for a 4 inch length, its about 10 thou, across the 2.4 inch width, only 4 or so .. I build with a full 28 ft radius, and to fit the FB flat, I simply flatten the top. Its just that easy.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Tony,
Are you saying that you sand the top itself flat under the area where the FB will join? That might be the best way to go about it.

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Yep .. exactly.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

That's brilliant. Thanks.

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I take a block plane and flatten out the top. But the arch there is not very pronounced and I doubt you would even see a gap with slight finger pressure.

FWIW the last few guitars I built were true flat tops and I prefer the tone. You can built flat and make it plenty strong.

I love the tone I am getting out of my guitars even with their flat tops. I just fear that dreaded behind bridge bump.

If I were to change to a domed top, what sort of differences are we talking as far as tone?

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I have always used a 28' domed top and flattened the dome where the FB extension lays. Worked fine. On the last couple, I tried Todd's method with a 60' radiused UTB and there is less to level and just the right amount of fall away and still an arched UTB ( which I like for strength). For now, it's a 28' dome and 60' UTB.

Chuck

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I would like to try different radii (on the computer). How does one go about calculating a huge radius like this?

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If you check the archives there was a thread here years ago. People were choosing a 28' radius because of this very reason. The angles work out nicely when r=28'.

Looking up the formula on google .. you get this ...

r = (d2 + r2) / 2h, where inside the bracket d and r are squared, and d is half the chord length, ie, how large is your radius bowl, say 2 ft, so d is 1 ft, and h is the distance form the chord to the top of the radius, ie, how deep is the 2 ft bowl in the centre.

So you can do this one of two ways .. pick a radius, and use 2 ft as the chord, so 1 ft as d, and solve for h, or pick an amount of h, and solve for the radius.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

I have been building my small SStrings in the Bogdanovich style as it seems like such a sensible idea
I ony use the solera to brace the top not to fit the neck/heel

Interesting Todd ... I have no taper on the extension, at a full 28 ft .. maybe my bowl isnt quite 28 ft, but its close ... and I get about 1/2 inch string height at the bridge ...somehow it works for me

years ago Micheal Payne had computer drawn plans that showed the side view with a 28 ft radius top ..it all worked out perfectly IIRC ....

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Hard to explain in words. Seems to be not as tight sounding and has lots of sustain. It's just different. But as to your original question the radius of the top going under the fretboard in the same direction that the frets lay is inconsequential and will easily come out when you sand the guitar for it's finish.

I don't have any problem with sustain or tightness really. I think I might build a prototype with a radiused top to see what come out of it.

Does anyone use a program like sketchup to draw out a radius? I like to make my own dishes...

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I use a fully domed top, and flatten the area under the FB extension, as has been mentioned (BTW, a good way to do this is to use a "paddle" covered with sandpaper, extending from the bridge area, to establish the correct geometry for the FB). I made my radius dishes more that 10 years ago, so I can't say for sure, but I believe the one I use for tops is between 25-30 ft radius. My target string height above the top at the bridge is 12 mm (1/2"), and I usually don't have to thin (or add a wedge under) the FB extension to achieve this with this method. Anyways, whenever I've had to do it, the amount of material to be removed or added has been so small, that it is barely noticeable. This is no biggie...



I want to see a bit of belly behind the bridge, it tells the top is light enough in this area. The bridge will also rotate a little bit forward, a couple of degrees maybe, but the overall geometry should still be quite uniform, with no hump at the neck joint etc. Anyways, the belly nothing to be too worried about, unless it is too big, and increasing... I would be more concerned if the top started sinking in front of the bridge, and things began to dive towards the soundhole, the action went up etc. Those are not good signs for long term stability.

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Rian Gitar og Mandolin

go here:
http://liutaiomottola.com/formulae/sag.htm

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Thanks Joe! That's exactly what I needed.

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I use a slightly different approach. I build the upper bout flat and dome the lower bout to a 15' radius, starting at about the narrowest point of the waist. That gives me a 0° neck angle, fingerboard lying flat on the top, and about 1/2" string height at the bridge.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

Agreed, I am going flat from now on... just seems like less issue with fingerboard extension gap. Yes I can fit a wedge between the gap but its hard to get perfect fit.

All the Stellas (and similar like Regal and others) have true flat top and they sound GREAT.

As long as you brace at the right humidity you shouldn't have any issue with potato chipping.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
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I do use a partial radius. In engineering you can influence the stiffness and how a structure will withstand a stress. The one thing that this thread can show is that there are many ways that you can do this and be successful. I tried many style and this is the one I use. Where I used to work we have a device that could take a holographic picture and then you could apply stress to the structure to see the stress patterns.
It was interesting to see the way things moved and handled a load. I found that the 3 faceted top structure was very good and handling the stress , full dome the next and flat the least . This doesn't make the structure stronger but takes the plastic limit closer to the yield strength.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
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