Official Luthiers Forum!

Hi guys. In Cumpaino’s book he calls for a soundboard radius on the bottom of the UFG. However, shouldn’t that area be flat as possible? Do you radius the bottom of this brace?




Pages 150 and 154



Sent from my iPhone using Tapatalk Pro

A lot of folks glue up the two braces up there flat. That’s what I do as well.

Edited to add, also check Hesh’s tutorial on flattening the rims before putting the top on. It’s in the tutorials forum.


Sent from my iPhone using Tapatalk

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

I’ll try and find it. Thanks.
I do use a radius dish for the rims the. Use the flat side of the dish to flatten that bout. But, I’ll still look over it.


Sent from my iPhone using Tapatalk Pro

I do put a radius in the UTB even on the true flat top guitars I build. It's very minor but it's part of my system for setting the neck angle right and also it gives added strength to an area that could really use it. It's good that you are figuring this stuff out now becasue either way you choose to do it is fine just as long as you plan for it. THe Cumpiano and Natelson book has a lot of gray areas in getting the neck angle right. Well, they teach you how to set the neck correctly for sure but not how to build the upper bout to accept the with the appropriate geometry.

I use a neck angle jig from lmii for the general angle. But I’m not exactly sure which system radiused upper or flatter will serve that system better


Sent from my iPhone using Tapatalk Pro

What radius do you use? I think Woodie mentioned they use 50’. Is that what you use? Do you use a dish to sand it in or just plane?


Sent from my iPhone using Tapatalk

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

That works fine for setting the neck angle but the problem is in designing the upper bout to accept it. IOW you can build a regular old sound box then use your LMI jig or the C&N method to set the angle but then you might be left with excessive fall away or just the opposite. Ideally you want the fret board to be straight from the nut to the last fret at the sound hole which shoots a straight line to the bridge and the only way to do that is to engineer the sound box to accept that proper geometry. And as usual in guitar building there is more then one way to achieve that.



I forgot what radius I use but it's more then 50. I build top down on a solera which is more or less a 'dish' of sorts except that the full geometry is worked congruently into the top. Here are a few pics to better describe this one method for doing it.

This is the UTB arch:


The upper bout is angled from the head block to about the edge of the sound hole. It's hard to tell but from about inch mark 10 it's flat all the way to the tail block. Basically it's canted and the apex of the angle is right at the edge of the sound hole.:



And that produces the correct geometry so that the fretboard shoots a straight line to the bridge such tath a 1/8th inch protruding saddle will make a 1/2 inch string height over the sound board at the bridge.

I use a neck angle jig from lmii for the general angle. But I’m not exactly sure which system radiused upper or flatter will serve that system better

That will set the overall string plane in relationship to the top/bridge. However, what happens at the body joint is a separate story.

In other words, any way you do it will work with that jigs, as long as it's all part of a coherent system.

Blues Creek and Hesh both have good tuts about this subject which is actually quite important...

If Ken were still with us, I believe he'd chime in here. I follow the KMG method pretty closely, and have found Ken's description to be the most logical and mathematically sound that I've read yet:

Neck Set Angle: Myth, Magic or Mathematics?
http://www.kennethmichaelguitars.com/neckangle.html

I’ve actually been following this method. My first guitar was a kit from John, so I followed his method. I simply flip the top radius dish over so that it is flat and press down on the upper bout area, raising it off the bottom bout, and sand the area flat looks me that. It is just is just like the pictures of the Martin factory sander, only upside down.
Perhaps I haven’t done this “enough” because each of my guitars have had a fall away up until now, and the perfectly strait shot to the bridge has evaded me.
Here is the section of the article referenced above that I’m referring to.




Sent from my iPhone using Tapatalk Pro

And yes... I need to charge my phone:)


Sent from my iPhone using Tapatalk Pro

Now, back to the original post....
It seems that if one were to follow this method of flattening the area, a brace that is NON radiused would serve the purpose better than a radius...


Sent from my iPhone using Tapatalk Pro

Slight terminology detour: there is a difference between the upper face brace (which the content of this thread concerns) and the upper transverse graft. Your title uses graft, but you clearly meant brace. Just so we are all talking about the same thing.

I think the Cumpiano/Natelson goal of putting a radius in most of the top braces is to help the guitar survive seasonal changes better, and is not meant to be solely focused on how the neck angle will affect how the fingerboard extension sits on the top. I agree that a flat upper face brace, combined with a strategic, precise tilting of the rim from the waist to the neck block, helps the fingerboard extension sit at the proper angle more easily. But that is getting to a level of precision that goes significantly past the “first guitar” instructions the Cumpiano/Natelson book provides. There are all sorts of things we can do better than what we see in that book. Yet, it is still a valuable reference for many things.

For my builds, the Upper Transverse Graft (colloq. popsicle brace), is flat, as is is the center 2/3-3/4 of the Transverse Brace (No. 1 brace in Martin x-brace terminology I believe).

I aim for little to no lateral arch/radius in the upper bout, only an incline toward the sound hole, giving the fretboard extension a flat landing area at the proper angle on the sound board.

I'd always suggest aiming for perfection, and a full understanding of the geometry, from your first guitar to your last. You may not hit all the marks in the early builds, but you'll know what you're trying to do and why.

We aim high to hit the mark.
Ralph Waldo Emerson

There's nothing wrong with fallaway, as long as it's not too extreme. I build it into all my guitars. Or, rather, I don't build it out.

But the amount of angle you sand into the upper bout has to be a reflection of all the variable. So that 1 degree may work for Martin's style, but if you tried that on a Gibson style 25' top radius, it wouldn't work.

Lots of good info here. I will add that some fall away is important if you are expecting the neck to rise (or the combination of things which will almost certainly occur) as the guitar ages resulting in high action. This would help prevent fret-string interference in those upper end frets. DAMHIK.


Sent from my iPhone using Tapatalk

_________________
"It is easier to fool a man, than it is to convince him that he has been fooled"
Mark Twain

The Upper Transverse Graft (aka, popsicle brace, UTG) is not radiused; the Upper Transverse Brace (aka, upper face brace; UTB) is radiused for the Cumpiano method.

There are several other approaches for generating the desired fretboard extension fit and string height over the top (e.g., our preferred 60' UTB/28' radius method, the upper bout wedge, the Larson Brother's cylindrical top, and Messrs. Cumpiano and Natelson), but sticking with one particular approach through completion is very much a requirement for success, given the built-in dependencies on previous operations and steps of each of those methods.

If looking for consistent bridge thickness, saddle height above the bridge, and string height above the top, you might consider looking beyond Martin or Larrivee for consistent methods and solutions. We see Martins with variation in bridge thickness of over 1/8" to achieve minimum or maximum saddle heights between 0.080" and 0.180", which suggests that emulating their practices for controlling extension fit and string height over the top will achieve no better than that brand's results. Larrivees show less variation in saddle height and bridge height, but concave upper bouts are not at all rare in their larger bodied guitars.

_________________
For the times they are a changin'

- Bob Dylan

I have done them both ways. I think in the end it comes to the methodology you choose to control the neck angle. When you do the math there need not be much of a radius to support a 1 1/2 degree neck angle for the proper bridge height.
I do my neck angle into the sides and use a flat UTB and make the final adjustment at neck setting. So I doubt there is one best way , it comes down to matching the technique to the process your useing

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I think in the end there is no formula that will give a precise outcome time after time. If we were working with a steel or plastic guitar it would but we are working with wood under a couple of hundred pounds of tension.

I think you just have to pick a method, stick with it, see what your guitars do over time with your own particular neck joint and upper bout bracing and tweak as necessary.

After almost 80 guitars it still seems to me like each guitar needs a few unique touches to get things right.

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