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more... Premier BlogHow-TosRepairGuitar Shop 101UpkeepFebruary 2014

Guitar Shop 101: How to Carve an Open-Slot Saddle

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Fig. 1. This 1937 Martin 000-18 has an open-slot saddle. Notice how the saddle’s ends are exposed,
rather than enclosed in a routed slot.

Carving a new saddle for any flattop guitar takes skill and patience, but crafting an open-slot saddle—the type that’s found on vintage Martins and many other older guitars—is especially tricky. Most modern flattops have a drop-in saddle that’s housed in a slot routed into the bridge. A drop-in saddle is surrounded on its front, back, and both ends by the bridge. By contrast, the ends of an open-slot saddle are exposed and tapered to match the contour of the bridge itself (Fig. 1).

A client recently brought in a beautiful sounding 1937 Martin 000-18 that needed a new saddle because the action was too low. This guitar provides us a perfect opportunity to discuss the process and challenges of creating an open-slot saddle. The owner wanted a “compensated” saddle to make the guitar play in tune, so we’ll cover that as well.

Before starting a project like this, it’s important to take four primary measurements: neck relief, action at the 12th fret, action at the 1st fret, and intonation. Gathering this baseline information will give you a sense of what the project will entail—what’s working and what needs fixing.

Remember: Always tune to pitch before taking these measurements.

Measuring relief. Start by placing a capo directly on the 1st fret. Next, hold down the low E string at the last fret and measure the greatest distance between the top of the frets and the bottom of the string. (The largest gap typically occurs somewhere between the 7th and 10th frets.) The easiest way to do this is with a String Action Gauge—a handy tool available from stewmac.com. If you don’t have one, a 6" machinist steel ruler will also work.

This 000-18 had .010" of relief, which is acceptable, but not ideal. For flattops like this, I prefer .015" of relief, but because this lovely old Martin was made in ’37, it doesn’t have an adjustable truss rod. So we’re going to have to live with this .010" relief—no change here.

Measuring action. When you’re carving a new saddle, you have to know how much taller or shorter it needs to be compared to the one you’re replacing. You determine this by measuring the action at the 12th fret with the original saddle installed. Again, use a String Action Gauge or precision steel ruler to read the gap between the top of the 12th fret and the bottom of the two E strings.

On this vintage 000-18, the high E measured 3/64" and the low E was 4/64". This action was too low—for the guitar to play without a lot of string rattle, the saddle should be taller. An action of 4/64" on the treble, and 5/64" on the bass side is ideal.

Once I knew how much taller the new saddle would need to be, I turned to the string nut. Here the action was perfect: 1/64" on the high E and 2/64" on the low E string.

Measuring intonation. The final step of the evaluation is to check intonation with a strobe tuner. First tune each string using its 12th-fret harmonic. Next, fret each string at the 12th fret and compare the fretted note to its corresponding harmonic. Using each harmonic as a benchmark, check the fretted note against it. If there’s a difference, determine the deviation in cents (a cent is 1/100th of a semitone) and write it down.


Fig. 2. Using 80-grit paper to sand the new bone saddle blank to the correct thickness.

In this case, the low E, A, and B strings were sharp and the G and D strings were flat. The good news for me was that none of the strings were off more than a few cents, so I knew setting the intonation on the new saddle wouldn’t be a problem.

Crafting the saddle. Begin by sanding the bone stock to the correct thickness of the saddle slot. I do this on a flat metal bar with 80-grit self-adhesive sandpaper (Fig. 2). Then I move to 600-grit paper to polish the saddle. If you don’t have a metal bar to mount your self-adhesive sandpaper, a 1/4" piece of glass will also work well. If you opt to use the top of your bench for this, make sure it’s dead flat, otherwise the saddle won’t fit properly.

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