Official Luthiers Forum!

You know my answer already....

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

and a flat top will definitely be my first but I want to understand what is the value of the different methods

I also figure people ought to know why they do the things they do even if it is just that's the way I was taught.


By the way I only made a back Radius dish not one for a top.

Here's a similar question I posed a while back viewtopic.php?f=10101&t=18692&hilit=tone

I use on my ukes I've built 13' on the tops and 10' on the backs, I did this as Dave White advised me on the bracing of my first uke and this is what he uses it projects very well and sounds good to me and friends on my first guitar and only guitar I used 25' and 15' on the braces and it wasn't built in a radius dish and it works fine too

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Formerly JJH

I learn more from my mistakes than my successes

I use a 10' radius on the back and a 15' cylindrical radius on the top. Larson bros. used high radius' both top and back, and the big radius on the back looks beautiful. The cylindrical radius on the top makes neck setting easy and tops really don't need to be stiffer along the grain.

Hmm... how does the cylindrical radius make neck setting easier? I thought the longitudinal curve was what made it easier, because it curves upward at about the same angle as the fingerboard extension when the neck is tilted back to get 1/2" string height at the bridge... Do you sand an angle into the ribs at the upper bout to get the proper tilt?

I'm still just using the arching from the Cumpiano&Natelson book. Sort of a compound radius, you arch the UTB to 1/16" and arch the X braces to 1/8", spring the top to them with cam clamps, and glue to flat rims. Tone bars are arched to about 1/16" too, but the exact amount shouldn't be too critical since they're past the string-to-bridge section. The rest of the braces are short enough to glue flat without problems. I think it works out pretty close to the common 28'. The 1/16" rise over 3" or so from edge to UTB results in an angle of 1.2-1.5 degrees, similar to that radius.

After all my ponderings on neck angle geometry, I've come to the conclusion that the end result of all of them is to increase the string height at the bridge by 1/16". With the Cumpiano style geometry, the distance from edge to UTB is about 1/3 of that from edge to bridge, so every bit of arch at the UTB makes 3 times as much difference as the arch in the X... so at 1/16" and 1/8", you gain 3/16" by the UTB arch, and lose 1/8" by the X arch, resulting in the desired 1/16". Which means that in theory, you could use a flat X and arch the UTB to 1/48"... or possibly use a true flat top and put the angle into the fingerboard thickness (i.e. fingerboard about 1/16" thinner at the nut than it is at the 12th fret, projecting to the desired 1/16" increase at the bridge. Maybe I'll try that sometime on a flat or cylindrical top.

Anyway, back to the original topic. From all I've read, more arch = stiffer, tighter sound, better projection. Also, I think John is on the right track that flat rims vs. radiused rims is a significant point, as that area is already naturally very stiff compared to the center, and the braces are short there so the top itself is playing a larger part in the stiffness than it does toward the center.
My instinct is that flat rims = more open like a flat top, radiused rims = tighter, need to sand the top thinner around the edge to loosen it back up... however, you can instead sand the whole top thinner before bracing, and use taller braces in the center to make up the stiffness, therefore resulting in significantly less weight than a flat top. Big advantage in the Somogyi school of thought.
So my flat rim, sprung braces style is more just for humidity tolerance and neck geometry. Stiffness wise, it just results in slightly shorter braces in the center, which makes little difference to weight. But probably a slightly different tonal character nonetheless.
Take with a grain of salt though, that's all just theoretical, as I'm still only working on my second and third guitars.

Maybe next time I'll try planing the spherical radius into the sides (just eyeballing it) before gluing them to the top, and see how that sounds. Or I could suck it up and buy a radius dish and build a mold to hold the sides steady while I sand the radius into them properly.

To my mind the radius does allow movement through extremes of RH changes and helps prevent catastrophic failure should things get very dry. But I also think that the top does stiffen as the radius is increased and that this could indeed have an effect upon the tone of the instrument. I would imagine, as Woody has described, that more bass is likely from lower numbers as the top is naturally more relaxed, it has somewhere to go so to speak so the oscillation of the monopole is larger and therefore slower, where as more treble is likely from higher numbers as the top is more restricted and the Hz are increased accordingly.

I cannot say that for certain but just by handling an unbraced top and flexing it, I can feel to wood come into tension the more it is flexed into an arch, and I would imagine that tension would only continue to build the more the wood was flexed until the top finally splits in two. With this in mind, perhaps the bowed saw analogy is not that far from the mark.

Oh by the way, I use 18' on the back and 25' on the top. I used these numbers only because I considered them adequate to allow for swings in relative humidity across most of Australia.

Cheers

Kim

I use 25' on the top and 12' for the back. I learned from Charles Fox and that's what he uses so I purchased those dishes for my production. I tend to brace the back pretty lightly because I want the back to move in response to the top, not remain rigid and reflect the waves, so the 12' radius seems to make better structrual sense to me.

Chuck

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"To live a creative life, we must lose our fear of being wrong"
- Joseph Chilton Pearce

Not flat nor radiused but both plates arched laterally, like within a 5' cylinder. As Haans mentioned, the top is dead flat along the grain and it makes setting the neck a breeze and right 100% of the time.
I put a slight longitudinal arch on the back, maybe 15', so I guess the back has a compound radius: 5' across and approximately 15' or whatever along.
I am not well versed in engineering but my feeling is that putting a radius on the top weakens it longitudinally.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

I guess my other post rambled on too long and this question was missed, so I'll ask you as well... how is neck setting easier on a cylindrical radius? Doesn't it present the same difficulties as a flat top then? Do you sand an angle into the rims at the upper bout, or wedge the fingerboard extension up, or put a taper into the fingerboard thickness, or something else?

Excellent thoughts D Ramsey, very logical processes now lets all man up and answer his question lol Maybe we should ask the Pad he's probably built one already.

Steve

Man up? Oh brother...
Dennis, I really don't have much trouble at all with the neck set. I usually end up sanding a tiny taper at the F/B area, but my final leveling of the F/B is done after the instrument is one piece.

I hope it's okay to revive this thread, I spent some time searching and reading this morning and after reading this post a question came to mind.

When using level-topped linings with a radiused top, is it necessary to accommodate the small flat-ish "ledge" near the edge of the top in the brace radii, near the ends of each brace?

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-John

If you have a radius on your top and use flat linings your top will have a funky kink. The top won't lie natural.

Clinton

Disagree.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Yes, I called it a "ledge" but it could also be a kink. IIRC kind of like the edge of a violin top when viewed in cross-section - arched in the middle area and flattening out at the edge.

I hope I haven't misconstrued what John Arnold wrote in his reply.

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-John

I agree... with Howard.

How the back vibrates makes a lot of difference in the overall tone and volume of a guitar. Or at least that's my thinking and experience on the subject. By simple physics, a back with more or less curve to it will have more or less difficulties vibrating in resonance with the top. The more curve, the more difficult it is to make it vibrate.

That said, a flat back would indeed be more fragile, more prone to crack after the bumps and hits a guitar gets in that area. in addition to the fact that it would look weird...

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

I guess anyone who uses a flat utb would have some kind of compound radius. I don't quite understand how you put an end to end radius on the back unless you cut sides that way. One usually doesn't think of that dimension being radiused, but rather matching however you taper your sides (if I am not correct).

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

That is exactly how I do it. The side profile is generated to create the lengthwise radius. On the first 20 or so guitars I made, I did this graphically. Once I found out Martin did it in a similar fashion, I just copied the side profiles on existing Martin guitars.

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John

This thread got me thinking about designing a git with a 1 foot radius top.
The bracing would be interesting!