Arlington Fret Works

When a guitar has good intonation all the notes on the fret board sound in tune with each other.  Then chords played up the neck will play in harmony with the open strings.  For good intonation you need: properly aligned saddles, a cleanly filed nut, and frets with crowns in the middle.  

The saddle:

Steel strings guitars need compensated saddles in order to play in tune. Each strings has a small stiff portion at the contact point at each end. These dead ends don't move when the string is plucked.  The treble E string has about 1/32" dead ends so it's saddle needs to be moved back 1/16" (2 x 1/32"). The bass E string has about 3/32" dead spaces so its saddle is back about 3/16". The other strings fall in between.  Steel string acoustics have these measurements built in, so the saddle is installed at an angle.   (Nylon strings are very close to each other in stiffness, so classical guitars have straight saddles with equal compensation for each string.)  

Electric guitars have movable saddles so each string's length can be adjusted.  Usually the G string on an electric guitar is unwound, and is actually  stiffer than the thicker wound D string. It's saddle is placed slightly back of the D string. Here's a PRS saddle with the compensations built in: 

The nut:

The string slots in the nut need to be filed with a slope, front to back, so the strings break off the very front of the nut. Sometimes a slot gets worn and the break point moves farther back, away from the first fret. This creates tuning problems everywhere on that string. This problem is pretty common.  Usually filing the slot is all that's needed to fix it.

This brand new left handed Stratocaster had serious tuning issues. All the strings were breaking off the back of the nut, throwing off the distance to the frets on every string. You just couldn't tune the thing. These nuts must come out of the box sloped for right handed application, and someone at the factory had probably just flipped it around to go in lefty, without correcting the slopes. Then they intonated the guitar by matching the open note to the octave at the 12th fret and all the saddles ended up 1/8" too far back, matching the error at the nut ... wrong at both ends.

The string slots also need to be cut to the right depth. High action at the nut means you basically have to bend a note to reach the first fret. On these guitars you can tune so a G chord sounds good or a D chord sounds good, but not both.   One customer had a new Rickenbacker 12 string that wouldn't play in tune without a capo. Everything else about the set up was great, but the strings sat about .045 inch above the first fret. I lowered them to around .020" and the problem was solved.        

Frets:

Ideally the frets should be crowned, so the strings contact them in the center. If they're worn flat the contact points will be at the front of the flat area. It's like moving the frets away from the nut, throwing off the scale of the neck.  

This 1964 Gibson ES330 is a good example of a guitar with intonation problems. It had worn string slots in the nut, the saddles were adjusted wrong, and the first three frets had been worn flat.  The owner plays in different alternate tunings.  He's a very patient fellow, but was finally frustrated enough to have it looked at.

He uses a wound G string, so the saddle pattern should be 4-2, like on an acoustic. On the ES330 the patten looked like this:

The first thing I usually do is put a capo on the first fret and adjust the saddles so that the fretted notes match the harmonics at the 13th and 8th frets. This takes the nut out of the equation. Then I remove the capo and check the notes at the 12th and 7th frets. If these are in tune then I know the nut slots are ok. If a string plays sharp then its nut slot needs to be filed.  The dirt in the slots of this ES330 indicated this might be a problem, but the first fret was worn so flat I couldn't get an accurate reading with the capo on.

 
The first thing to do was to crown the flat frets, in this case the first three. 

These were so low that my crowning file wouldn't work, and I had use a triangle file.  

Now I could adjust the saddles with the capo on. After removing the capo I found that the A, D and B strings read sharp.  While filing them I could see that the front part of the slots were in fact lower than the middle portions. 

I like to file the slots at an angle about half way between the plane of the frets and the plane of the peg head. This assures good even contact across the nut and a clean take off point at the front edge.  I put a thin pencil line in the bottom of the slots as a visual reference while filing.  

Strings:

Even a new set of strings can have one that's defective. A customer with a Taylor acoustic said the intonation on the A string had "gone off". The saddle was fine, no chips, but the problem had persisted through several string changes. It turned out all the strings were from the same 3 pack, and all 3 sets had defective A strings. I installed a different brand and the problem went away.

Compensated nut:

I'm a big fan of the Earvana compensated nut.  I install them in all my personal and custom guitars.  Steel strings behave differently at the nut than they do at the frets, maybe because the angle is is different. Even though the math is correct, the notes at the first fret always play sharp. The Earvana nut simply moves the nut contact points closer to the first fret until the notes play true F, A#,D#,G#, C, & F.