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So my first couple of guitars I radiused the UTB to match that of the soundboard. Having just purchased and watched Robbie O'Brien's online videos on this subject (well worth the $$ BTW), I learned that he keeps his flat. Are there others who radius the UTB? Pros and cons for both approaches? Thanks in advance!

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Doug

Don't let fear or common sense stop you from trying to build something

What is UTB?

I was using the top radius on the UTB and then sanding the fretboard area flat. But after reading Trevor's book I did one with a 60' radius and, as Todd stated, the geometry came out just right.

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Steve Smith
"Music is what feelings sound like"

+1

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Doug, I've used flat UTB (.625" tall x 1/2" wide) up to this point without issue. John Mayes mentioned it in his Advanced Voicing DVD, and I thought, cool, less work. The reasoning was no more involved or thought-through than that.



I've wondered about something related, and hopefully I can bring it up without stealing the thread. Folks in the Somogyi school tend not to feather the ends of their UTB's. I've seen it in a number of their pics, but the image I'm attaching is one Tim McKnight published on the AGF. Hopefully he won't mind me sharing it here.

On p.51 of The Responsive Guitar, Somogyi writes:


In general, I wonder what you all think of this argument for UTB's without feathered ends. Specifically, I wonder if not feathering the ends down to 1/8" or whatever would prevent this distortion?

I radius mine to 28' just like the rest of the top bracing. But I glue it all onto flat rims, and the geometry all seems to come together fine....just requires a little extra pressure to get the neck block glued...

I just realized that I'm doing an experiment on this as we speak. Like the picture above I stuck my UTB on in a similar way, only with feathered ends. I used a similar head block extension (without CF rods) and glued the UTB right to it at the proper angle to divert some pressure from the top to the sides without adding a bunch of weight. I also extended the head block for the back and radiused it at the same time as the sides the same way as the top.

Being my second-first build (the real first was a complete failure) I've got nothing to lose.

Sorry if the picture was misleading. The UTB was glued to the top. I think the pic simply shows it being fitted to the linings, but there was a pic of it being glued to the actual top after. I thought it was a pretty good example of the UTB's that aren't feathered.

I agree with Ervin on that. Feathering the ends is unlikely to make any difference to the sound, but most likely does make it less stable against long-term neck angle deformation. I do agree with Todd as well that full height isn't necessary, but scalloping to 1/8" or less like a lot of people do is way too much, especially with how far inward the scallops go by that point. I'd say 1/4" minimum, 3/8" better, 1/2"+ excessive but no harm in it.

As a side note, with the CF buttress style, personally I'd put some A-frame style braces on the back, going from the CF attachment points to the headblock... essentially making a tetrahedron of bracing, with the back waist brace being one edge, and the headblock being another. Probably the most rigid 3 dimensional shape you can make out of sticks.

As for the actual topic, I arch the UTB, but flatten the part that will be under the fingerboard. Provides a flat glue surface for the headblock (which I also glue to the UTB at the same time) and for the fingerboard, with some arching outside to remain convex in low humidity, and for neck angle purposes... although I still don't 100% understand the ideal fret plane-string plane geometry.

All very sensible.

He majored in English.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I've been going flat on the UTB for the last few years but am doing 60' on two current builds. The rest of the top remains 30'. I'll report back but am hoping to see a little less movement on string up.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I have been using a 60' radius on the UTB for the last 10 guitars or so with a 28' radius on everything else. Got this idea from Todd, and it works great for the geometry. You still maintain the arch for it's added strength and the fret board extension is in near perfect position, just requiring a little touch up block sanding.
As far as feathering the UTB, I feather to 1/8" on the ends, as Todd said, there is no need from an engineering standpoint for it to be any thicker.

Chuck

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That's not a brace, it's a truss, and a darn well make one at that.

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Joe Beaver
Maker of Sawdust

Have to agree with you there Joe - that UTB is pretty high on the cool factor...
Saw another one a few months back that had smaller to larger circles drilled out...
Not being a bridge engineer - are these stronger than a solid brace?

Less strong, less stiff than the equivalent dimensions (height, thickness) in solid, but higher stiffness to mass ratio.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

If you do a 60' radius on the utb and 30' on the rest, how do you fit the top to the sides? You can't just do it in a radius dish anymore can you?

Sent from my GT-S5830 using Tapatalk 2

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"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

Actually, you can, and that's the way it's usually done.

A larger radius on the UTB results in a localized flattening of the top dome when it's removed from the dish.

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

Ok, so you just do that by eye with a block plane do you? Or is there some other jig or dish you need to do it?

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"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

Hey Todd - With your radius system how much space do you get at the saddle area when you put a straight edge above the sound hole and run it down the center line (does that make sense!?)? Do you sand down the sides any at the heel block area, to achieve correct neck angle?

I have had success (though I haven't made many instruments) with using a flat UTB and getting the neck angle the way I want. The thought of using a slight arc (closer to flat than the normal top radius) to build in a little insurance from humidity changes appeals to me. I may as well use the 60 ft (or close enough) that you all are having success with. How much offset over say 24" would it take to approximate a 60 ft. radius. I'd like to just make a template of that arc and draw it on my stock, plane to the line and call it close enough.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Very handy tool for calculating arch offsets http://liutaiomottola.com/formulae/sag.htm

Handy indeed, thanks Dennis! I come up with 0.1 inches over a 24 inch span.

Oops looks like Todd sent me a fish while I was shopping for bait and tackle. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Bryan, I just take a couple of swipes on each end of the UTB with a block plane. Works for me.

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Steve Smith
"Music is what feelings sound like"

Yeah Steve, I knew the offset wouldn't be much but once I did the numbers It became obvious just how small the offset would be over the length of the typical UTB. I'll probably not bother with a template and just take of a few wisps as you described.

And I didn't properly thank Todd. Sagitta was the term I couldn't come up with when I tried to find a calculator on the internet.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I do it like Steve does. 1/16" off the ends with a block plane from a 2 1/2" flat center. Then I just slightly sand the heel block on a sanding board with a thin wedge propping up the tail block. Get's me .090" at the saddle.