| Nut slots (with principles that apply
to bridge slots as well)
Here's a gnat's-eye view at the face of a nut as seen from the leeward side of the second fret. The slots for these two strings are cut so that they completely support the string.
The sketch above relates to fretted instruments, but the
principle is no different for violin family and other unfretted
instruments. I'll try to explain the clearance
in a minute.. Here's an idea of how it works on a bass: Having the slot cut too high above the
frets (or an unfretted board of some type) means that the act of
pressing the string down to the first few
frets actually stretches the string, raising the pitch and throwing the
intonation off in the process. Ideally the nut slot height is identical
to any other fret. Remember that strings can 'saw' themselves through a
nut just
through normal use. I ordinarily leave a nut slot a little higher than
necessary at first, to allow for the string to cut itself a little
lower. I also avoid synthetic and elephant ivory, both of which are too
soft and rubbery to make good nuts for steel strings. Here's a shimmed-up mess of a nut that has all the problems:
These slots are all too deep, but the B is still so high it doesn't play in tune, so someone shoved a piece of ebony under it to try and correct the intonation. Big "Ugh" for this one. People often comment on certain strings (mandolin A strings,
guitar G strings) being more troublesome, always seeming to go out of
tune during play. Mandolin A's are always the most troublesome because
they have to make compound
bends from the nut: back and to one side, and the length from the nut
to the post being the other important factor. And being plain strings,
they tend to bind if the slots aren't cut right. (The D's, being wound,
tend to refine their own slots.) When you tune, you always tune up to a note,
never down, right? Right. And with a badly cut nut, when you tune up, the
tension on the length of string between the nut and the post is greater
(per unit
of length) than the part you play that's between the nut and the
bridge. After getting the pitch just right, a bit of actual playing
works the string, making the tension on both sides of
the nut equalize, and voilà: you're out of tune in mid-phrase.
It has
nothing to do with the machines, but everything to do with setup,
particularly how precisely the string slots at the nut are cut. A quick word about creaking guitar G strings: this issue is
fading as elephant ivory nuts are fading. Bone is superior to ivory for
a nut material because it's harder and burnishes better. Ivory is soft
and actually registers the imprint of string windings. That creak is
the sound of the windings skidding over grooves inside the nut slot.
Setup is everything. (You can resurface string slots in ivory with
pearl, if you like.) The sketch below illustrates how - and how not - to shape a
slot for any string. Left: the nut is too high. When the string is way below the
top of the nut, you have great difficulty telling whether it's seating
properly. Next: a slot that's cut with a saw has a roughly flat bottom and also affords poor acoustic coupling. Saws seldom match the precise width of the string, which will roll side to side in the slot. Next: strings will work their way down a v-cut,
often bottoming out on frets (or the board, as the case may be with
fretless instruments). The
signal transfer is compromised because
of the limited contact, and the string sizzles on the fret or the
board. They also tend to bind and squeak. Right: the slot really fits the diameter of the string, does
not go above the halfway point of that diameter, and leaves a trace of
clearance above the fret or the unfretted board surface.
If you hold any string down on any fret of a well-setup instrument, you'll see that same preferred clearance at the next fret up. I'm reluctant to assign a measurement—it's very little. Before going further, here's how to
correct a string slot that's too low. Often it's wiser to repair a
blown slot than it is to replace the whole nut. Quick fixes like some kind of dust
(bone, acrylic, baking soda) with superglue are really temporary. It
takes little more effort to implant a little patch of bone (or even
pearl) into the nut and recut the slot. It's as good as the original,
and if done well, is quite invisible.
I have a couple of saws I use for widening and deepening in
preparation for an implant. One is a fine hacksaw blade in a short
handle, which is for wider strings. It leaves a nice flat-bottomed
slot. The other is a backsaw such as one would use to cut fret slots,
which does the same and is good for finer strings. Avoiding hitting the
first fret, assuming there is one, I cut down below the blown slot,
sometimes almost to the board itself, angling the saw back a bit. Then
I prepare the piece of bone (or whatever: ebony for a violin or cello)
by carefully filing a piece of the material with a fine flat file until
it slips snugly into the slot. I usually use old saddle scraps for
this. A drop of CA and a tap and it's in there: Trim and dress the nut as if it was new and uncut, then cut
the new slot.
I prefer to shape my slots in the shape of a horn's bell:  The point of this is to offer a smooth surface for the string
to
travel
from the tuning machine to the critical point of final contact at the
front of the slot, where it is held firmly to define the end of the
vibrating string length. Strings have to make a compound bend at the nut, and to make tuning easiest while ensuring complete firm contact at the front of the slot, this horn bell shape makes certain the string glides smoothly, no matter the angle of approach. Here's a treble side view:
The bell here is imaginary. The nut is in yellow, the fingerboard is dark brown. The string is the green line, and the tuning machines are off to the right somewhere. Notice that the string connects with a smooth curved surface, no corner or edge. Whether the string is coming from the top or the bottom of the string post, it will slide smoothly into the nut slot. The string is in complete contact with the front 30% of the nut. There's plenty of substance there to keep the string from sawing its way deeper into the bone. Here's the same slot seen looking straight down from above:
The string's other curve, from, say, the farthest peg on the bass side of the headstock, also elides with the inside of the bell-shaped slot, guided gently and firmly to the front where it's held firmly by its own tension inside the confines of a well cut slot. If the slot isn't properly angled back, several problems can
arise.
If it's too flat (some repair books actually advocate this!) the string
soon wears away the front of the slot and the functional point of
contact
is as much a 40% of the width of the nut back from the front edge,
which
can cause the note to ring poorly (because it's vibrating along a
surface, not held to a point) and perhaps cause intonation problems.
This is bad:
If the slot is angled back, but left a straight line, it will
bind on
the back edge, and the front edge will wear down from playing and the
string
is at risk for sizzling on the first fret or on the surface of the
board. This is also bad:
The precise shape of the slot at the front edge is extremely
important
for sound quality, stability of the setup, and intonation. More on bridges in due time, but the principles here apply to
bridge slots on the viol and violin families, guitars,
mandolins, and so on. |
