I wonder how many guitar technicians have heard that? Worse still, how many clients come in AFTER they've broken or stripped the "adjuster"? Truth is, very few acoustic guitars have adjustable necks, but almost ALL modern-day steel-string guitars have an adjustable "truss rod". Believe me, NOT the same thing at all.
Guitar set-up was on my mind this week, as I did the final set-up on my "Gemini" guitars -- matching 6- and 12-string dreadnoughts -- see the Gemini photo album. Personally, I never want to see one of my guitars come back with a damaged truss rod, so I thought I would blog a bit on the subject.
"So, just how does someone get a "good action" on a guitar?" To reiterate, you DON'T get it by cranking on that truss rod adjuster!! Of course, I'm assuming that the guitar doesn't have any fret issues or other problems that could cause noises. But even brand-new, high-end factory guitars in a music store will require some additional set up to play properly.
What is commonly referred to as "action" decribes the height of the strings above the fingerboard. If they're too high, it takes more effort to press the string down to "fret the note" and the guitar is hard to play. If they're too low, the strings will vibrate against frets and cause all manner of unpleasant noises. The height of the strings is determined by the height of the nut slots at one end, and the bridge saddle at the other end -- turning the truss rod adjuster doesn't affect either of these!
"So what DOES the truss rod do, and why is it adjustable?" The truss rod is embedded into length of the neck shaft and is used to fine-tune the straightness of the neck when it is under string tension. The picture show a neck blank where I've routed a slot to receive the truss rod assembly that is sitting loose on top. While the string tension will cause the neck to want to bend slightly, the truss can be adjusted to apply counteracting pressure.
"So, you adjust it so that the neck is perfectly stright, right?" Well, in fact, no. A small amount of "relief" is required so that the "action" can be set low, without the strings vibrating against frets. We generally check the truss rod setting with the strings tuned, and a capo placed on the first fret. We then fret a string on the 12th or 14th fret, and observe the "relief" or gap between the string and the 6th or 7th fret. If there's no gap, either the neck is too straight, or maybe even bent backwards! We're looking for a 0.010 to 0.020" (0.25 to 0.5 mm) gap, depending on playing style -- about the thickness of a standard business card. To decrease the gap, we turn the truss rod adjuster a little bit clockwise, to increase the counteracting pressure. We ALWAYS measure the change, and if it doesn't change, we don't just keep cranking -- a stripped adjuster or broken truss rod is usually a very expensive repair!
"My neck relief is within spec, but the guitar is still hard to play!" With the neck relief adjusted, the technician will then measure the gap between the strings and the 1st fret, and the 12th fret. These indicate how much the string slots in the nut need to be deepened (or filled!), and how much the bridge saddle needs to be raised or lowered.
The gap at the first fret should be around 0.010 to 0.020" (0.25 to 0.5 mm) -- again about the thickness of a business card, and the height above the 12th fret 0.060 to 0.090" (1.5 to 2.3 mm), depending again on playing style. If the gaps are way off, the adjustments must be made to the nut and saddle in steps, as lowering the saddle, will also tighten the gap at the first fret.
"OK, my guitar is now really easy to play, but never seems to be in tune -- what's wrong?" You may have noticed that saddle on an acoustic guitar is on a "slant". The length of the high E string is shorter than the low E string. We call this "compensation". When we fret a note, the string is slightly stretched. Compensation helps to adjust for the effect this stretch has on the pitch of the fretted note. Guitar makers use various methods to figure out the average compensation across all strings, and build this into the slant of the saddle. But, that alone is not perfect.
If you play the open harmonic at the 12th fret, and then play the fretted note, they should sound exactly the same -- I actually measure the pitch with an app on my iPhone. If they don't, we say that the "intonation" must be adjusted -- effectively lengthening or shortening the string at the saddle to fine-tune the compensation. I won't describe the method in detail, as it's not something a novice should attempt -- but if you really must do-it-yourself, drop me a line and I'll point you to a tutorial on the net.
After checking the intonation string-by-string, I ended up with my 12-string saddle with a bunch of guide marks, telling me where to file the "string-break" peak for each string:
I then removed the saddle, filed the break points, polished the saddle, and then reassembled the guitar. You'll notice that the break points for the high octave strings on the 3rd and 4th pairs are at the leading edge of the saddle, and the lower pitch strings "break" over peaks toward the back edge.
Now this is 12-string that is easy to play -- AND plays in tune!