Topnut Exposed by Chip Todd

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Topnut Exposed by Chip Todd

Post  Frank N. Peavey on Wed Dec 08, 2010 1:22 pm

Chip shared this great paper with me last night.

Topnut Exposed:

The topnut is one of the least-understood items on the guitar, yet is really one of the simplest to understand. Its filling and string-height adjustment is also the most noticeable of all the playing adjustments. In a production environment, it is also one of the most costly and labor-intense operations. In the case of acoustic guitars, manufacturers are loathe to get the strings the proper height, as potential buyers might taken an instrument off the wall hanger and commence scrubbing away at the strings, making them rattle against the frets. Then, they would tend to tell their friends that such-and-such instruments have fret buzz.

Fretted Instruments:

Embracing the understanding that fretting theoretically goes off into infinity past the topnut; the essence of fretting and topnuts tune the instrument to within several steps of standard pitch; preferably below pitch. The notch depths can be gauged with a foot-long piece of .010” thick steel rule, but that’s neither as easy nor as effective as gauging by eye. Except this premise, for the time being.

There must be some concave bow in the fretboard for the instrument to play without a fret buzz. So, the first step is to tune the instrument to pitch and check the bow in the neck. I prefer the clearance between a straightedge and the fret in the physical middle of the neck to be .008”, but anywhere between .005” and .015” to be fine. In lieu of a straightedge, the taut string can be used. (I never use a straightedge if the strings are on the instrument). Use a capo, or fret one of the strings at the first fret and the other hand to fret at the last fret. Then, check the clearance between the 8th fret and the bottom of the string. This should be between .005” and .015”. This is pretty easy to see, keeping in mind that most notebook paper is about .003” thick. To add bow (concavity) to the neck, loosen the truss rod (counterclockwise on the truss rod nut) to lessen the bow, tighten the truss rod nut. The same concavity applies to a fretless neck. With this done, we can start with the actual topnut work.

Place the guitar so that you are looking at the neck in the playing position, but with the top tilted bit toward your face. Finger the string you’re working on at the first fret. Tap the string above the second fret, noticing the distance it travels. This is easier if you can see the string’s shadow on the fret. Bounce the string against the fret several times, noticing the distance traveled by the string. Then, using the open string, tap the string over the first fret, comparing the distance it travels to that of the previous string travel. This should appear to be the same. The topnut notch may need to be filed to bring the strings to the proper height. Before you have finished the preliminary checking of all strings, the sight picture of the tapped string should have become clear to you.

--- If a string is already too low at the first fret, the topnut should be shimmed up until there is proper clearance or replaced. This is a good reason to do a rough check of all strings before doing any filing. ---

The notches in the string should, ideally, have vertical sides and a radiused bottom. The notch should be from .002” to .006” wider than the string; it will center itself if the bottom is a radius. The notch should also be at an angle of about 5 to 10 degrees down toward the peghead. It is essential for the string to last touch the topnut at the body side of the notch. If it doesn’t, there is a great propensity to have string buzz. (When checking for an elusive string buzz, touch a pin or knife point on the string at the body side of the topnut. If the buzz disappears, the notch isn’t slanted fully.)

Fretless instruments:

Though not quite as obvious, the topnut slots on fretless instruments have to follow the same rule. The string is inanimate and doesn’t know whether it’s on a fretted instrument or not. The topnut notch’s active edge should be down to the fingerboard level. This string cannot tell the difference between the straight line at fret level or at fingerboard level. If the notch is about 3 or 4 thousandths of an inch proud of the fingerboard’s surface, that’s also acceptable.

If I am not clear on a point, or if I haven’t addressed a point, please PM me or email me at: chiptoddguitars@gmail.com and I’ll respond fairly quickly.
Chip

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Frank N. Peavey

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