Tips for luthiers

Sterner TM

Depending on the design, instruments are easy or difficult to intonate. In some cases intonation has been impossible because of poor design. Here are a few thoughts and ideas based upon the problem I had to overcome while intonating different instruments.

Adjustable neck is not necessary - if it's stiff enough

The way I see it, it is unnecessary to make the relief adjustable by a truss rod. An instrument neck varies in width and thickness, so the effect of a truss rod is greatest where the neck is thinnest. Also a truss rod has no effect at all on the part of the fretboard that extends over the instrument body.

With suitable materials (steel, carbon fibres...) an instrument neck could be rigid enough to withstand the pulling force from the strings and altering humidity. There would be significant advantages:

  • Simplyfied neck manufacturing.
  • The hassle of adjusting the relief with the truss rod is eliminated. The relief you want can easy be made by sanding/grinding the height of the frets*.
  • Refretting will be easier because the neck won't bend back when the strings are removed.

* In this context I will recommend heavy fret wire. Big frets can be planed and crowned more times when they are worn. If you want relief it is way easier to grind it on the frets than to try to bend it with a truss rod. But above all, the instrument becomes much easier and faster to play! With thin frets you press the fingers against the fretboard as the strings dig into the soft fingertips. When the finger is stopped by the fretboard, you must press a bit more to get good contact between the string and the fret to get a clear note. With heavy frets you don't need to press that hard because the finger is not stopped by the fretboard. And there is no friction against the fretboard while sliding on the strings.

A heavy neck absorbs less energy from the strings

An instrument neck is a bad membrane. Therefore as much as possible of the strings vibrations should be channeled through the bridge to the membrane. The obvious way to do this is to make the neck heavy and rigid, preventing it from vibrating with the strings (= stealing energy from the strings).

On the banjos I've built, the necks have glued in steel bars instead of truss rods. Some would think the necks are heavy, but I'm used to it and I don't regret it. The instruments feel very stable. 

When making a nut extending over the fretboard, the extension should be supported by the hard wood in the fretboard. This makes the nut more silent and less energy is stolen from the strings.

Tilt the bridge bone backwards

On all acoustic guitars I've seen, the bridge bone is seated at a straight angle to the guitar top. This is bad design:
  • With a slightly loose fit the bridge bone will slant forward and execute a very strong pressure on the top front edge of the groove. Cracked bridges are common.
  • The top of almost all acoustic guitars will eventually bulge up behind the bridge and down in front of the bridge. This is normal, but it will cause the bridge to tilt more forward and shorten the length of the strings.
  • If the action is raised, the length of the strings will be shorter - WRONG!
    If the action is lowered, the length of the strings will be longer -

Far more better would be to tilt the bridge bone backwards when the groove is routed. Advantages:

  • The bridge bone leaning more in the direction of the resulting force would reduce the breaking force on the bridge and the bone considerably.
  • The vibrations from the strings ought to be easier transferred through the bone to the top, if the angle of the bone coinside better with the direction of the resulting force. Also the contact surface at the bottom of the groove between the wood and the bone will increase.
  • You get automatically a rough compensation:
    If the action is raised, the length of the strings will increase -
    If the action is lowered, the length if the strings will be shorter -

Leave enough room for the compensations

The compensation needed is often longer than one would imagine. In two cases (both on expensive Martin-guitars) the compensation needed for the low E-string at the bridge exceeded the space between the bridge bone and the bridge pin! As the bridge bone is slanted the room for compensation is shorter on the bass side - where the most compensation often is needed. Longer distance to the pins will also reduce the breaking force on the bridge. Cracks between pin holes are also less likely if the row of pin holes are slanted across the grain of the bridge.

                                      Bridge pins should be cylindrical
In this context I should point out that
conical bridge pins is a bad idea. They function as a row of wedges that want to split the bridge between the bridge pin holes. The bride pins should be cylindrical! That would radically lessen the risk of splitting the bridge, and manufacturing would be simpler. There is no risk that the pins will fall out or rattle, as they are fixated by the pressure of the ball end of the strings.

Seat the nut on the same plane as the fretboard

On some guitars the bottom of the nut has the same angle as the peghead. This complicates the making of a new nut. Also the force from the strings may press the nut away from the fingerboard.

The nut should be seated on the same plane as the fretboard!

  • The nut can be moved without the action changing.
  • The making of a new nut will be easier.
  • The force from the strings will press the nut against the end of the fretboard.

Why should the nut to press harder at the end of the fretboard? The wood in the fretboard is most often harder than the wood in the neck. If the nut is harder fixated against the fretboard, less energy from the vibrating strings is lost than if the nut is seated on softer wood (compare with the reasoning above about heavy neck). 

Copyright © Anders Sterner