If you were lining tuning problems up against a cop-station height chart, the NUT would be the prime suspect. He’s got motive and opportunity and his alibi is looking shaky.
That damn nut is probably the biggest cause of tuning problems.
Let’s start with what it’s made of. There’s tons of choice for nut materials and some are more troublesome than others.
I like bone. It’s traditional, it wears well, it looks well, and it sounds well. If it's properly cut, it tends to be pretty problem-free. It’s a winner all round.
If you’re opposed to bone for ethical reasons, the newer man-made composite materials like Tusq, Corian, and Micarta are pretty good. Personally, I think each loses a point or two against bone (in different areas) but, overall, they’re a good substitute.
What I don’t like:
Cheaper, softer plastics
No. Just no.
Some guitars still ship with soft plastic nuts. While there’s a tone argument to be made with these, my main beef is related to tuning.
Strings are under a lot of tension and can dig into the plastic. That’s a recipe for tuning woes. Strings can catch or bind and, depending on how the problem occurs can pull sharp or flat in diffrent circumstances.
Lift your strings out of the nut slots and examine the slots. Look for any evidence of the strings digging in or marring the plastics. This might be indentations made by the wraps under the wound strings or maybe some ‘mushrooming’ at the front or rear of the slots.
Expensive, softer plastics
Personal opinion here but I dislike some of the modern, permanently-lubricated nut materials — the ones impregnated with graphite or PTFE or whatever. I find these to be too soft and subject to the same digging-in and wear issues as some cheaper nuts.
It’s counterintuitive but these materials — meant to lubricate strings and allow them more free movement — can actually impeded strings if worn. And I feel their softness means they wear more easily.
Your mileage may vary on this. Lubricated materials make up a fair chunk of the nut and saddle industry and I’m sure they sell well. This is my two cents on the subject.
General nut problems
Materials aside, there are some issues that can impact any nut. These are pretty much all related to how the string slots are cut.
Slot too narrow
A nut slot that’s too narrow for the string will ‘pinch’ it. If the string gets pinched it can’t move freely and will cause problems getting it into tune and/or problems keeping it in tune.
Lift a string out of the nut slot and it should lift up freely without any ‘catching’. If it catches, you might need to widen the slot. Not too wide though…
Slot too wide
A string shouldn’t have any lateral movement in a slot. If it does, there may not be a single point it can return to — say after a bend. The string needs one ‘path’ in the slot.
Look down on your nut and see if there’s any side-to-side movement on your strings. If so, it might be time for a new nut.
A string slot should not be a ‘V’ shape. It needs a rounded or flat bottom. Gauged nut files are shaped for this but something like a triangular needle file might cause problems.
A V-shaped slot means the round string can catch and 'wedge' in the slot-walls before it sits on the slot-bottom.
Too steep a back angle will leave all of the string’s pressure on a small point of the slot-bottom. That can be a ‘catch-point’.
You want the bottom of your slot to slope down gradually, and ideally in a curve.
On a Fender-style neck, the string slots should be cut straight across the nut, giving the strings a straight path to the tuners.
On a 3-a-side neck, things are more complicated. The wider headstock means the strings flare out towards the tuners after the pass over the nut.
Typical practice is to cut the slots on these nuts just like on a Fender one — straight across (perpendicular to the front of the nut). Personally, I find it’s better and more stable to angle the slots towards their respective tuners. I put the string bend at the front of the nut rather than at the back.
My three-a-side nuts look like the one shown above, with string-slots angling towards the tuners. Usually, I’ll slightly flare the back of the each slot too. What I’m trying to do is — as much as possible — to avoid places a string can catch. This works well for me.
The Bottom Line
The nut. So often, it’s the nut.
For tuning problems, don’t replace your tuners until you’ve checked out your nut.
Some of this nut stuff is difficult for DIYers, I know. In many cases, working on a nut without a proper set of gauged nut-files is a good way to ruin it.
If you do this sort of maintenance on your guitars in any way regularly, nut files are a massively useful investment. They’re not the cheapest tools out there but they are pretty much indispensable for good nut-work.
P.S. I’m currently in the process of checking out a (possible/potential/maybe) alternative to a full set of nut files. I’ll report back with more info soon.