February 2017
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DIY: How to Intonate a Flattop Guitar

DIY: How to Intonate a Flattop Guitar

Preliminary Evaluation
Always begin any project by taking essential measurements. This information serves as a baseline for any adjustments and also helps pinpoint any problems. Write these measurements down, so you can refer to them at any time during the setup process.

Here are the four preliminary measurements:

1. Action at the 12th fret.
2. Neck relief.
3. Action at the 1st fret.
4. Intonation.

Let’s go through these measuring procedures, one at a time.

Before measuring the action, clamp a capo directly on top of the 1st fret.

Step 1

Measure the Action
A guitar’s action—how far the strings sit above the fretboard—determines its playability. Our journey begins here:

1. Tune the guitar to concert pitch. If the strings are totally shot, restring first.
2. Clamp a capo on top of (not behind) the 1st fret (Photo 1). By doing this, you create a “zero” fret and temporarily remove the nut from the action equation. This allows you to initially focus on saddle height and neck relief.
3. Use an action gauge (or precision ruler) to measure the string height at the 12th fret. Measure from the bottom of the string to the top of the fret.
On Chapman’s 914, the height was 6/64" on the 1st string and 7/64" on the 6th string. This is very high action!

Measuring neck relief with an action gauge.

Step 2

Measure the Neck Relief

1. With the capo still clamped on top of the 1st fret, hold down the 6th string at the 14th fret.
2. Measure the greatest distance between the bottom of the string and the top of the frets (Photo 2). The largest gap typically occurs somewhere between the 7th and 9th frets—essentially in the middle of the neck.
The relief was .022" on this 914, which meant the neck had a little more forward bow than necessary. Given Chapman’s precise playing style, I knew I could reduce the relief and make the guitar easier to fret up and down the neck.

Step 3

Measure the Action at the 1st Fret
Another factor in playability is how high the strings sit in their nut slots. If the strings are too high, the guitar feels stiff. If they sit too low, you’ll get a buzz when you play the open strings.

1. Remove the capo and measure the distance between the bottom of the 1st string and the top of the 1st fret.
2. Repeat the process for all six strings. When the guitar is set up properly, the gap should incrementally increase from the 1st to the 6th string to accommodate their progressively thicker gauges.
At the 914’s 1st fret, the 1st string was 2/64" above the fret and the 6th string measured just over 2/64". Again, this is rather high, especially on the treble strings.

Step 4

Check the Intonation
Next, I use a strobe tuner to check the intonation. Here’s how it works:

1. Tune each string with the strobe using 12th-fret harmonics.
2. Starting with the 1st string, play the 12th-fret harmonic as a reference and then fret the same note. The goal is to have the fretted note match the harmonic. If the fretted note is sharp or flat, write down how many cents (plus or minus) it deviates from the reference harmonic.
3. Repeat the process for strings 2-6. Double-check the tuning of each reference harmonic as you work your way across the fretboard.
On Chapman’s guitar, I found most of the strings were sharp when played at the 12th fret, and the amount varied from +2 to +6 cents.

Step 5

Adjust the Relief
Once you have your measurements, you’re ready to begin the process of changing the action.

1. Remove the truss rod cover at the headstock. (On some flattops, you access the truss rod through the soundhole.)
2. Adjust the truss rod. Turn the wrench clockwise to tighten the rod and reduce forward bow, or counterclockwise to loosen the rod and reduce back bow. Go slowly, making very small adjustments and checking the results each time you move the rod.
By tightening the 914’s truss rod, I reduced the relief from .022" to .015". This is the proper amount of relief for Chapman’s playing style. Any less relief and the strings would be likely to rattle against the frets.

Tip: If you have any doubts about how to adjust a truss rod, get a guitar repair book or study the manual that came with your instrument. Many manufacturers offer free online instructions for adjusting the truss rods on their guitars.

3. Using a gauge to determine the fretboard’s radius or curvature. 4. Sanding the saddle blank to fit in the saddle slot. 5. Frequently check the thickness of the saddle blank as you sand it down. It should fit snugly in the saddle slot, but don’t force it in. 6. Rolling the ends of the saddle blank to round them off. 7. Checking the rounded ends of the saddle blank against the curved slot ends. 8. Using 80-grit self-adhesive sandpaper and a radius block that matches your fretboard radius, sand down the top of the saddle to the line you traced on the bone blank. This allows you to reach the desired height while maintaining the correct radius.

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