Neck to Body Geometry on Classical Guitars

[Dean Harrington]

I know classical guitar builders have different methods that can all be successful to create the neck angle to body relationship. I'm curious to hear more methods that builders use to create the plane of the neck (minus fingerboard) to the top, and their reasons for choosing this method.

I've seen work boards that build in a forward neck set from the 12th to the nut. (the work board is lower at the nut). Then I've seen other work boards that keep a flat plane (with the exception of a dome at the bridge) from nut to tail block. I've heard some follow the forward neck angle work board method, but continue it into the body--to allow the fingerboard glue surface to be flat along it's entire surface, and possibly keep the same fingerboard thickness along its entire length. I recently saw a method that builds up the lower bout of the work board so it's higher at the tail block and then had a 25' dome that's scooped out to it's lowest point at the lower cross strut. Clearly these methods all work for people, so I'm not looking to criticize any method, but I would like to hear opinions pro/con for why people have chosen or abandoned a particular method. If you make the body and neck separately, I'm still interested in hearing how you handle the same issues.

[Waddy Thomson]

Regardless of whether you make the neck and attach it at the end or you build in a solera(as I do), the relationship of the neck and the top is the key. It is totally dependent on where you want the strings to be relative to the bridge saddle and the height above the saddle that you want your strings to be at the end, that will give you the correct action at the 12th fret. It's all about the geometry, which includes the fingerboard, whether or not it's tapered, and the height of the frets, and the height of the saddle. Not sure how you would do it without knowing all of those things.

[Douglas Ingram]

The neck angle has everything to do with the degree of doming that will be used. Recently I've been studying the Torres style guitar, and many of his guitars had a relatively high dome, about 5mm, and the front of the sides are flat and in plane with the neck. The neck angle from this geometry is zero.

When I used a dome of about 3mm, I find that a forward neck angle of about 2mm works. Basically, I find that dome (mm) + neck angel (mm) will equal 5mm and get you in the neighbourhood for final set up.

[Jeff Highland]

It's a little more complicated than that
I use a simple spreadsheet to work it out with inputs of - desired string height at the bridge, 12th fret action, dome height,fret height, fretboard thickness at nut and at 12th, and allowance for neck relief.
This spits out a neck angle value.
I use a solera with the nut end set down 3mm and then shim it to the desired value

[Dean Harrington]

Does anyone slope the neck/sides all the way to the sound hole to eliminate the angle change at the 12th fret, and to keep the fingerboard thickness the same across the entire board?

[Brad Heinzen]

This is all dependent on how much you dome your tops, fingerboard thickness & taper, and desired saddle height above the top. On my guitars, the neck angle is just about zero, which keeps things pretty simple. The neck extension on my workboards is sloped down a few mm, but I tape shims back on it to fine tune things. I usually shoot for a neck angle that gives me a fingerboard plane height above the top of about 3.75mm. If I were to dome my tops less, I'd have to go with more neck set 'forward,' or a thinner fingerboard to get where I want to be.

As for sloping the body from the neck to the soundhole, I think it's pretty typical for CG's to have a neck set 'forward,' which would give you an upper bout that slopes downward to the soundhole. Alternatively, you could taper the fingerboard from the 12th fret to the soundhole, and keep the upper bout flat.

[Dean Harrington]

I've heard of a couple builders who continue the forward neck set into the body to the sound hole, but I have yet to see any books that describe a method for this. I'm pretty sure I own or have read most of guitar making books. I'll have to give them another look, but if my memory is correct they all either used a zero neck set, which would automatically keep the glue surface of the fingerboard flat across its entire surface, or there is a negative neck set that ends at the body, so the fingerboard needs to have its glue surface angled from frets 12-20.

If the work board is to have a slope from the sound hole to the nut, then I image folks would either 1. install the linings a bit higher than the sides near the heel/head block and flush from the sound hole to tail block--to allow fitting the sides flush to the work board or 2. The sides would need to be fitted to the work board by planing the lower bout and then a slope could be sanded into the sides from the sound hole to heel block. or 3. I suppose the the upper cross struts could be made to have a concave arch so the fingerboard is lower than the plan of the sides, but this doesn't seem like such a good solution cosmetically. The 1st method seems the most practical to me, but if anyone has a different method, I'd be interested in hearing about it.

[Brad Heinzen]

I'm not familiar with builders who slope the soundboard as you describe, but that sure doesn't mean it doesn't happen. Zero neck angles are not uncommon, nor are fingerboards that have a little taper to them from 12 to 19/20. I like to keep things simple, so I go with one of these two options.

[Chuck Morrison]

Going back into ancient history (OK, the mid 20th century), Irving Sloane suggested a tapered fretboard on a neck parallel to the top in his first book on building a Classical guitar. I'm assuming a fair number of us remember that book. I still use this method on lightly arched traditional design classicals. I haven't seen a need for a 5+mm fretboard thickness over the top. Early (5 course and six string) guitars had the entire fretboard even with the top and even fretted the top itself when they needed more notes.