>>> how important [is it] to have the string a bit above the top of the nut
In principle, you could have a nut with each slot exactly matched to the respective string diameter. In such a case, each slot could be as deep as exactly half the string diameter, with 1/2 of the string above the top of the nut. This is sort of the basis for the sometimes-stated ideal of the 1/2 diameter nut slot.
Also, in theory anyway, a deep and sloppy nut slot could allow for some string rattle within a very poorly cut slot. This may be part of the traditional aesthetic of a shallow, 1/2 string diameter, nut slot.
In practice, as you notice, this (1/2 diameter slot depth) is almost never done on guitars - particularly on the narrower treble strings.
Also, many guitar headstocks are designed with a significant side bend of the string as it exits the back of the nut toward the tuner posts. So in principle, a 1/2 diameter nut slot would need to be centered not under the bottom of the string, but offset to compensate for the (sometimes extreme) sideways turn toward the tuner.
In my opinion, it is not practical on many guitars to actually cut 1/2 diameter nut slots. Try it on one of those canoe paddle headstocks and watch those high strings pop out of the slots.
Further, it is difficult to exactly match the slot shape to a narrow-diameter string. A mismatch (even a slight mismatch) between the slot and the string can allow a string to pop out of a slot whose depth is 1/2 the string diameter.
If you are cutting a nut for a classical double-bass with Trans-Atlantic cable strings, and a relatively straight pull through the slots, then it is practical to finish the top of the nut at about the height of 1/2 the diameter of the strings. The absolute tolerances are relatively large, and few blues masters do extreme bends on a fretless double bess.
Some high end classical guitars, with their relatively large-diameter strings (and arguably sedate bending style) are built with the top of the nut cut down to look at least somewhat close to the 1/2 diameter of the strings.
But try doing this on a .012 high E (never mind the even narrower strings used by some), and it can be difficult to keep the string from popping out under duress. Side loads on the slot and slight diameter mismatches make a shallow nut slot arguably impractical.
Now, a super deep slot is considered a sign of a mass produced nut. Many luthiers take some pride is finishing the top of a nut nicely. This includes the aesthetics of a polished, and somewhat rounded surface with some of the diameter on the bass strings clear of the top of the nut.
But in practice, you could have a very deep nut slot that is cut well and works just fine. It might just look a little sloppy - especially if you look at how many modern nut files are designed and how this would shape the upper part of a very deep slot.
The classically "correct" way to make a stringed instrument nut is one thing, what works is arguably something else - in my opinion.
Last edited by NiAg : 04-17-2011 at 10:17 AM.
Reason: spelling and some cleanup, and more cleanupo
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