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William Cumpiano's
String Instrument
Newsletter #4

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Fretboard arching

1998 William R. Cumpiano All Rights Reserved

My Austrian colleague and newletter subscriber Martin Koch (http://www.cis.tu-graz.ac.at/wb/martin/welcome.html) writes:

What I find one of the most challenging thing in guitarbuilding is making the fretboard radius right. I build my first guitars with a flat fretboard since I learned to play on a classical guitar and didn't mind having a flat fretboard. When I understand it right you describe in your book how to make a so called compound radius fretboard. The plane travels in the "lie of the strings.

Why do so many manufacturers (e.g. Martin Guitars, Fender, PRS ...)make cylindrical fretboards? On a compound fretboard I have a flat line under each string which should be prefered. I visited Martin guitars last year and I saw their sanding jig. It would be as easy for them to make one arm longer and to produce a compound radius. Or is there not much difference in praxis?

I think of making a jig for a big a belt sander. What do you think is it worth the effort or is it with some practice easy do plane the radius?"

I remember how scared I was to take a plane to a fingerboard and curve it in the early days. I do it now without a thought. What changed? Not my planing skills. I just lost my fear.

You can wipe out the biggest portion of that worry by practicing on random-specie hardwood blanks before you approach the ebony. You'll be surprised at how soon you'll get the knack of it. The only way you can really ruin the fingerboard blank is either putting a radius on the long edge by tilting the plane as you shoot it across the board's length, (causing the fret end to poke out when you attempt to fret it) or blow a big chunk of ebony out of the board by planing into a sworl with a badly sharpened/adjusted plane)

I soon realized, that assuming the blank starts at finished thickness (and flat):

1) the actual amount to be removed is very, very small
2) you have to take more off where the fingerboard is wider
3) you get into trouble when you tip the plane. In fact I've come to leave the plane virtually flat as I shoot it, but just "lean" or "favor" it in the direction that material needs to be removed. 4) the plane must be deadly sharp, or forget it. The results can be disastrous: chattering, skipping, or the grain blowing out. If your plane skills/plane quality leave something to be desired, making improvements in that regard should be your first priority...or buy your fingerboards pre-arched.

I got hung up for years trying in my mind to figure out whether I should visualize the arched fingerboard's ideal surface as

1) a tapered section of a cylinder, or
2) a tapered section of a cone.

Note: for the handbuilder, to achieve 1), the plane strokes should be more or less parallel to the centerline of the fingerboard. Note if you start from the wide end, the the plane will run off the board as you cross the taper for most strokes, until you reach the narrow end, where they will turn into full-length strokes.

To achieve 2), you simply follow the strokes to match in direction the edge of the fingerboard. They will be full-length strokes from the beginning. The result will be a more radical arch at the narrow end and a flatter arch at the wide end. (In both cases 1) or 2), when the bulk of the material is removed, the plane facets must be removed and the surface averaged out with a sandpaper plane)

As an acoustic guitar builder, I began worrying about the difference far less when I discovered that some master builders affirmed that it should be 1) and other equally great builders affirmed that it must be 2)

This quandary could be solved in my mind only if I let it go at that ON ACOUSTIC GUITARS the effective difference between 1) and 2) must be neglible.

For those who are consistency- and mechanically-minded you can shape your fingerboard by

1) sliding it across a shaper with a (frightfully expensive) custom-made convex cutter. The result is a tapered section of a cylinder. Necesarily, the edge that you see when you play will progressive narrow as the fingerboard widens. You can prevent that by sliding it across the cutter in such a way that the it cuts deeper as the fingerboard NARROWS. The result will be a consistent edge, but a fretboard surface that tips back towards the nut relative to the flat back of the fingerboard. That's ok too. In many cases that may allow you NOT to have to set the neck back a few degrees when you fit it to the body.

2) rotating the fingerboard past a moving (preferably oscillating too)sanding belt. This is what I do in my shop. You should see the Rube Goldberg contraption we've devised! In this case, the fingerboard is affixed to a backing board, and the two end of the board are attached by long "spokes" to a common pivot. This allows the fingerboard and backing to be swung in the desired arc against the moving abrasive surface. The neat thing about this is that if the "spokes" are the same length (and equal to the desired radius of the fingerboard surface), the result is a fingerboard which is a tapered section of a cylinder. If the spokes are different length (equal to the starting and ending radius of the fingerboard, as desired), the result is a fingerboard which is a tapered section of a cone.

Why you would want to vary the radius (usually flattening out as the fingerboard widens) is a subject which would result in a very interesting, although perhaps endless, discussion. I can begin the discussion by commenting that clearly the Fender Tele and Strat fingerboards are a case in point.

The Fender necks became very popular in the early days because of the radically radiused arch of the fingerboard. In those days the highly curved fingerboards were tapered sections of a cylinder and the many young rock and rollers, who were often in possession of limited technical skills, found the radical arch very very easy to barre for their insecure fingers. Early rock and roll required playing mainly a few chords in the low-numbered positions. After Chuck Berry, musicians began exploring arpeggion in the high-numbered positions near the body, and lead players found themselves not only playing up there 90% of the time, but also "bending" the strings--pushing them up towards the centerline of the neck to create one or two-step glissando effects-- once or twice per measure. That, in conjunction with the radical arch, and a frenzied low-action obsession, resulted in a train wreck: Every time they tried to bend the string, they would run into the hump of the fretboard and choke out. Then, makers discovered the virtue of fingerboards that were tapered sections of a cone.

Indeed, most electric guitar fingerboards are "compound radiused" nowadays. Clearly fingerpickers don't need it, but more eclectic players may prefer it, although in truth, greater acoustic guitar action heights and stiffer strings make the matter moot, in my mind at least.

The rule of (my) thumb is that players who practice a lot and are conscientious about building their playing strength and speed in a concerted and methodical manner like flatter fingerboards. Players who want to play fast and impressively with minimal practice and effort on their part REQUIRE radically arched fingerboards.

I have been frightfully busy lately: I've been teaching, jigging up for a production of wedge-shaped acoustic guitars, composing a script for a two-hour video documentary on the Puerto Rican cuatro, and creating several webpages besides. I will resume my intended exploration of neck angle geometry and ancillary topics next week.

I honestly appreciate the good wishes and increasing numbers that I am receiving from my newsletter "club." I enjoy doing this, and I hope I can make it equally enlightening and enjoyable to all of you too.

Best wishes to all!