Official Luthiers Forum!

Just wanted to say thanks to you all for your responses to my last post regarding your choices for FB radii. Very helpful.

Now, if you will humor me, I have a couple questions regarding compound radii.

I understand that the smaller radius is at the nut, but from other threads it appears that the large radius is set, at least theoretically, at the saddle. Is that how most of you do this and if so how do you set this up? Is everyone using a radius jig that is long enough to simulate the length from the nut to the saddle or are you doing the math to figure the required radius at the 12th or 14th fret? Also, do you continue the radius to the end of the FB or stop at the 12th or 14th fret?

Thanks,

Kevin

Kevin I don't do compound radii on acoustics, but even if I did I would continue the radius to the end of the fretboard. Every fret needs a radius in perportion to the saddle radius to control string height ratios over the frets.MichaelP38810.7142939815

Michael,

Thanks. What is your thinking behind not using compound radii on acoustics? I assume (and that's always a bad thing) then that you do use them on other guitars. Why the difference? Kevin

Mine are at the nut and at the end of the fretboard approximately. To each his own, based on their setup for doing it.

_________________
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Don,

Do you then radius your bridge/saddle at a correspondingly larger radius to continue the "string shaping" if you will?

Kevin

There are several discussions/articles on this in the GAL big red book volume 3; when people refer to the compound radii, they mean the first and last fret positions, at least that's what I assume unless told otherwise. The determination of the final fret position radius is based on the saddle radius selected, though, in theory.

no I do not think it is a bad thing I just have always built full radius boards on acoustics. really is that simple

[QUOTE=KevinA] I understand that the smaller radius is at the nut, but from other threads it appears that the large radius is set, at least theoretically, at the saddle. [/QUOTE]

[QUOTE=KevinA] Don,

Do you then radius your bridge/saddle at a correspondingly larger radius to continue the "string shaping" if you will?

Kevin[/QUOTE]


In theory, yes, the saddle would continue on with the section of a cone, and define the largest radius. But in reality ... consider this:

You radius your fingerboard, set up the saddle blank at the appropriate extrapolation of the fingerboard radius, and then what do you go and do? You fine tune the saddle and action, and end up reducing the height of the saddle for the thinner strings, getting them closer to the board. This, in effect, reduces the radius across the saddle top, and in the end you may end up with something like the following radii:

Nut: 12"
21st Fret: 20"
Saddle" 15"

It's always black and white, huh?

Pete,

That is exactly what I have been working through on my own. I like noodling through the math (must be the engineer in my background), but was also wondering about the level of precision one ought to strive for. As you say, once you have all the radii set, you start filing or sanding away on the saddle to get everything where you want it anyway.

Glad to have gotten everyone's thoughts though, since the aesthetics of consistent FB edge thickness are important as well in the finished product which wasn't something I had really considered.

Kevin