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William Cumpiano's
String Instrument
Newsletter #5

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Where the fingerboard meets the soundbox: a train wreck?

I've received some feedback: from Roger Labbe, which I will quote and expand upon.

"In your 3rd newsletter you specify that on a classical guitar the fretboard should slope so that a straightedge hits 1/16" below the bridge's front lip. On mine it's nearly 3/16 below. The slope of the straightedge when placed on the frets is not parallel to the top of the guitar. I.e. if the bridge was not in the way the straightedge would eventually hit the top. Is the neck supposed to be parallel to top of the guitar, or should it form a slight acute angle?"

First off, Roger, with your straightedge hitting your bridge so far down, you probably are suffering with a very low saddle and mondo extremo high action!

On classical guitars you have a situation that is essentially reversed from that of steel-string guitars. It is customary to build the classical guitar with a "positive" fretboard slope, and on the steel-string, with a "negative" fretboard slope. In this case, I've used the terms positive and negative relative to the level of the soundboard. "Positive" implies that the nut is above the level of the soundboard, and the fretboard slopes down to meet the soundboard, i.e. on the classical guitar. "Negative" implies that the nut is below the level of the soundboard and the fretboard slopes up to meet the soundboard, like on the steel-string.

By the way, I believe that the reason for this is since on both the steel and the classic guitar, the two traditions have come to expect: a) similar bridge thicknesses of and similar saddle protrusions over the bridge (with a few exceptions) for both guitars b) but different "normal" action heights (greater for classic, lower for steel)

With the added factor that classic has a relatively massive neck and low tension (little body-neck flex), and the steel a slim neck and high tension (greater body-neck flex), the end result is that classic fretboards are tipped up, and steel fretboards are tipped back.

IF NECK IS SET FLAT TO THE SOUNDBOARD You may attach the neck to the soundbox so that it's fingerboard-gluing-surface is on the same level as the soundboard. In that case, the fretboard surface will have to be planed into a slope so that when the straightedge reaches the bridge, it bumps into the front 1/16-inch below it on the classic; and on a (new) steel string 1/32-inch to 1/16-inch above it (depending on expected string tension: greater = leave 1/16-inch; lesser = leave 1/32") On an older well-settled-in guitar, reduce all those measurements by 1/32-inch. Note that if the bridge is not present during this procedure, a stand-in can be used, or you can simply use a ruler to measure the straightedge offset right at the saddle location of the top. My book discusses how you may favor the bass side of the fingerboard to yield a level saddle on the classic (you would otherwise have to have more saddle showing on the bass side of the bridge in order to impart the correct amount of extra bass-string clearance over the fingerboard, accounting for its greater travel.

Note that these measurements apply when you placing the straightedge directly on the frets. If, on the other hand, you are placing the straightedge directly on the fretboard surface (i.e., when you are planing the slope in by hand), you assume that the frets are nominally 1/32-inch high, so to account for that you should aim the straightedge yet another 1/32" below all the above measurements.

IF THE NECK IS SET AT AN ANGLE TO THE SOUNDBOARD Now, the result of setting the neck into the soundbox so that it is flat to the soundboard is that the fingerboard will, indeed, appear to taper from nut end to soundhole end. It is customary for classical players to see their fingerboards tapering this way--and on some long-scale Ramirez', quite dramatically so. This disturbs the aesthetic sensibilities of many builders, my teacher Michael Gurian among them. His solution was (he's retired from building now) to build the classic neck with a positive slope on its fingerboard-glueing surface already built in (this is accomplished by putting a 1/16-inch shim on the flat workboard right under the nut location when roping on the back--thus locking the neckshaft forever into a sloped-up configuration.

The result is that the edge of the classic fingerboard will appear to be of equal thickness along the length of the neckshaft, rather than tapered. But you run into trouble where the neck meets the body. Since the gluing-plane of the fingerboard changes at the neck/body join when you tip the neck shaft up relative to the body, now you must facet the bottom of the fingerboard to conform to that change of plane. And that is what many of the world's greatest classical luthiers do. But Michael Gurian taught me to do something quite unusual: by CONCAVING the upper face brace by a small amount (and slightly dipping the soundboard-gluing surface of the neck's headblock, he could achieve a SOUNDBOARD SLOPE in the fingerboard area matching the neckshaft slope. The result: an evenly-thicknessed fingerboard along it's full length. It seems like a lot of trouble, though doesn't it? But so do the other options. Come to think of it, it's ALL a lot of trouble. But who said this was going to be easy? If it was easy, we probably wouldn't be here now, would we?

On steel-strings, the neck is always set back to a negative slope. The plane change at the neck/body join is a similar headache for steel geeks as it is for classic geeks. But its GOT to be right. If you don't pay attention to it, the action will be dreadful up there, and will be instantly complained about by good players who play the end of the fingerboard all the time. Now you can attach the steel neck so that it is level to the soundboard and then plane a back-slope into fretboard surface to impart the proper inclination of the straightedge when it reaches the bridge. But then for the player, the neck will appear to thicken as he/she plays closer to the soundbox, and won't like that. So, you must maintain an equivalent fingerboard thickness. To accomplish that, you must tip the neck back at the heel, and RAISE the soundboard in the fingerboard area by CONVEXING the upper face brace and tipping the headblock glueing surface by the right amount.

There is another way to do this which is a real hip way developed by my dear friend Michael Dresdner when he worked for Martin several years back. He's free-lance now, and I'll email him a request for details, which I will then put in this newletter next time. You heard it here first! Essentially it entails a FLAT upper face brace, but you must shape the rim of the sides and the headblock so the entire top third of the soundboard (above the soundhole) slopes down towards the headblock, presenting a continuous plane to the sloped neckshaft surface. Clever, no?

Again, I've digressed from my original intent of following the set-up sequence to cover the rather large and complex subject of guitar action geometry. Well, this diatribe was nonetheless still strictly within the theme of action geometry so I'll forgive myself. This time. Bear with me.