To set the neck relief, first place a capo on the first fret. Next,
with one finger, press the sixth string (lower E) down onto the fourteenth fret.
The lower E string should now touch the first and fourteenth frets. Because of the enormous
tension of the string, the string gives you a perfectly straight line between the cusps of the
first and fourteenth fret.
The neck relief (forward curvature of the neck) that you're after can now be measured with a feeler gauge as a gap between the string and the cusp of the sixth fret. What most guitar manufacturers and luthiers recommend as a rule of thumb is a .010'' gap between the string and the sixth fret. I have yet to see a guitar and a playing style for which a .010'' neck relief did not work very well. I would recommend that you just go for that value and be done with it.
It is true that in rare cases, you can get away with a little less than .010'' of neck relief, and this will result in slightly better playability overall. However, unless you really want to become an expert at all this, I doubt that it is worth your time exploring different neck reliefs. If you really want to go there, you'll find some advice at the end of this page. For now, there is just one important thing to bear in mind: once you have chosen a neck relief and have completed the entire setup, you do not want to go back and decrease the neck relief. The reason is that decreasing the neck relief will lower the action at the first fret. To get the first fret action back to the correct value, you would have to raise the nut slots, and that is the one thing that's very hard to do.
If the neck relief is different from what you want, you change it by adjusting the truss rod according to your manufacturer's or luthier's instructions.
On many guitars, the nut that adjusts the neck relief is at the top end of the truss rod, inside the headstock. All you have to do is remove the little cover plate and adjust the nut. The picture below shows a Taylor 810 with a combination tool that usually comes with the guitar. Also, one of Taylor's many tech sheets explains everything about adjusting the truss rod on their guitars.
|On the Martin guitars that I have worked on, the nut was always at the lower end of the truss rod, inside the guitar's body. You need a suitable allen wrench to get to it.|
Once you've adjusted the neck relief to your preference, you should
perform the measurement at the sixth fret as described above for
the first (upper E) string as well. The measurements should come
out roughly the same, say, to within a tolerance of 10-20
percent. If there is a dramatic difference, then your guitar's neck
is seriously warped, and there is probably not much point in
spending any more time on setup.
As I mentioned before, it is often possible to get away with a tad less than .010'' of neck relief, but it's questionable if it is worth the trouble to find out. Unless you're a real stickler for perfection, skip the rest of this page and continue on to the next step.
If you really have to, here's how you go about finding the least possible neck relief:
Set the neck relief to a very low value, like .005''. If you're an uncurable optimist, you may even start with no neck relief at all. Then go to the next step of the setup, where you set the saddle height. As you try to find the minimal saddle height, you will probably find that string buzz occurs in the lower frets (second or third fret) way before it occurs higher up. That means you've got too little neck relief. Increase the neck relief a bit, then work on your saddle height again. Repeat until you don't find that buzz in the lower frets is significantly worse than buzz in the higher frets. By the time that happens, your neck relief will probably be right around .010'', which was the recommended value to begin with.
Now do the third and last step of the setup, where you set the nut slot depths. If you actually ended up with less than .010'' of neck relief after the above iterations, you may still not be good. The low neck relief may cause behind-the-fret buzz. Behind-the-fret buzz occurs when you fret a string at fret x (with your finger or with a capo), and then the "dead part" of the string between the string nut and fret x buzzes on one or more of the frets below fret x. One of the things that neck relief does is to prevent this buzzing by creating a tiny gap between the "dead part" of the string and the frets beneath it.
So if you have less than .010'' of neck relief and notice behind-the-fret buzz, it's back to the truss rod: increase your neck relief a tad, and go through the remaining steps of the setup (saddle height and nut slot depths) again. It is of course also possible to counteract behind-the-fret buzz by leaving the first fret action a bit higher, that is, by having less nut slot depth. But the consequences of a higher first fret action are so unpleasant that I very much doubt you want to go for that option. The bottom line is that the .010'' neck relief is hard to beat.