Official Luthiers Forum!

A few months back I built a classical guitar in Spain with a 15 foot radius on the top and back and thought it'd be interesting to replicate this on a steel string. Has anyone tried this? How might I go about establishing a proper neck angle and flattening the upper bout so the fretboard extension sits flush if not building on a solera?

Haans Brentrup builds with a 12' cylindrical radius. I think Laurent Brondel uses something around there as well. But I don't know of any builders using a spherical radius that tight.

One surefire way to avoid fingerboard extension problems is to use a raised neck/cantilever fingerboard. But personally I don't like the added weight or tendency to tip forward.

You could just put a shim under the fingerboard extension, fitted to fill whatever space is left after you get the neck angle right with the fingerboard extension touching the soundboard at one point.

Or maybe do one of those fancy adjustable angle necks, where there's a cavity cut into the soundboard/headblock that the fingerboard extension (with additional support wood glued to the underside of it) sits down into.

Or build it more solera style, using highly arched braces but fitted to a flat perimeter. Then you can vary the upper transverse brace arch height to control the angle of the neck joint to soundhole span. And if you use an L shaped headblock, that will guarantee flatness in that area. This style would probably sound significantly different than a 15 foot domed perimeter though, so depending on what you're going for it may or may not be a good option.

Not sure if this suits you, but for a 25' radius I set the neck angle by first flattening the upper body to aim for 2.5-3mm above the saddle placement point and then use an adjustable bevel to transfer that angle between the flattened upper bout and the neck block area to the neck itself for cutting the tenon by hand before flossing.
Best thing is to draw it out full scale, this would enable you to assess the offset and calculate a radius to use for the UTB (I use 60' for a 25' top radius)

_________________
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.

Before I quit, I settled on 10' cylindrical radius on the top and 10' dish on back. With laminated bracing on both the top and back, never had a problem and "flat" out easy to set the neck on the top.

It works much the same as using any other radius. One thing that occurs to me is that the inherent stiffness added by the dome may require some counteraction by building slightly lighter elsewhere.

One thing that can help is to draw out the side view so you can see how all the relationships need to work out to get your fretboard to body geometry right.

Thanks for the replies! The main reason for my wanting to do such an extreme top radius is for the additional stiffness; building such a lightweight classical guitar in the granada tradition has me interested to see if I can incorporate what I've done into my steel string building.

Based on the drawings I've made it looks like if I just ride the bus and incorporate a spherical shape into the rims I'll end up with a little over 3 degrees of neck angle and wind up about 5mm (!!!) above the saddle position. Would it be unfounded to establish a neck angle of 1.5 degrees and then add the radius to the rims below the waist while using a flat UTB?

15' would look near flat compared to the old Gibson "flat tops" of the 30's and 40's. I've measured some of their old small bodies in the 10'-12' range.

_________________
Eschew obfuscation, espouse elucidation.