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DIY: How to Intonate a Flattop Guitar

DIY: How to Intonate a Flattop Guitar

Step 6

Carve a New Bridge Saddle
Though it had been played hard for years, Chapman’s guitar was in great overall condition. But the bridge saddle was worn and rather than spend time trying to intonate it, I decided to carve a new one instead. With a new saddle, I could simultaneously address the guitar’s action and intonation problems.

I chose a bone saddle blank because bone sounds great and is easy to shape with a file. (Important: Saddle slots are typically either 3/32" or 1/8" wide. Before buying a saddle blank, you need to measure your guitar to determine which thickness is required.) To carve the new saddle:

1. Remove the strings and the old bridge saddle.
2. Measure the fretboard radius (Photo 3). It’s very important to get this right, so you need to use a precision radius gauge. A multiradius tool or sets of individual radius gauges are available from such luthier supply companies as Stewart- MacDonald and Luthiers Mercantile.
3. Place your old saddle against the new blank. Using a mechanical pencil, trace the contour of the old saddle onto the new saddle blank. This will serve as a basic template for shaping the replacement saddle.

Note: If your original saddle was too tall, you’ll have an opportunity later to remove the extra material from the new blank. However, if your original saddle was too low, you’ll need to factor in additional height as you trace the contour onto the new saddle. To do this, simply slide the old saddle up a little higher against the blank before tracing its contour.

Tip: You can always sand the saddle lower, but you can’t add height back. When in doubt, err on the side of having your blank a bit too tall initially.

4. Check the line you just traced on the new saddle blank with the radius gauge to confirm the radius is correct.
5. Sand the new saddle blank to match the old one. When sanding the new saddle, it’s imperative that its thickness, height, and length are correct. Getting this right is harder than it sounds and requires frequent checking against the original saddle.
Using 80- and 400-grit sandpaper adhered to a flat surface, sand the thickness of the new saddle so it fits snugly into the saddle slot (Photo 4). Sand off just enough thickness that you don’t have to force the saddle in, but keep the fit snug enough that the saddle doesn’t rock inside the slot. Check your progress by trying to insert the new saddle into the slot (Photo 5).
Part of this process involves rounding the saddle’s left and right edges so they fit into the ends of the slot. An easy way to do this is to gently roll the end of the saddle blank as you sand it on a flat surface (Photo 6). The goal is to have each end of the saddle match the curved end of the bridge slot (Photo 7). The ends of the saddle shouldn’t bind or have any room to move from side to side.
6. Once the blank fits correctly into the saddle slot, use a radius block equipped with 80-grit selfadhesive sandpaper to sand the top of the saddle down to the line you traced onto the blank (Photo 8). This operation sets the basic saddle height while making sure the saddle follows the fretboard curve. Matching this radius ensures that the string height will be consistent across the fretboard. If you don’t do this, you may end up with some strings too low and others too high, making the guitar difficult to play.
I have several wooden blocks that are carved to various radii. The 914’s fretboard has a 14" radius, so I used the corresponding block and 80-grit sandpaper to sand the new saddle to the proper height.

Tip: I recommend placing the radius block in a vise and sanding the saddle blank upside down. If you attempt to sand the saddle blank while it’s in the bridge, you can easily slip and damage the bridge or the guitar’s top.

Lower the saddle height by sanding from the bottom of the blank—not the radiused top. Here I’m using 80-grit sandpaper attached to a flat wood block. To keep the bottom flat and square, I apply even pressure across the entire saddle.

Step 7

Adjust the Saddle Height
With the top of the saddle sanded to match the fretboard radius, it’s time to fine-tune the action.

1. Put the old strings back on, tune them to concert pitch, and measure the action. At the 12th fret, my target is to have the action measure 4/64" for the 1st string and 5/64" for the 6th string. If you use the correct radius block to shape the top of the saddle, the strings will consistently graduate in height from the 1st to the 6th string.
2. If the action is too high, loosen the strings, remove them from the bridge, and gently pry out the saddle. Using a mechanical pencil, mark a straight line along the bottom of the saddle corresponding to the height you want to remove.
Carefully sand off this amount from the bottom of the saddle (Photo 9).
On Chapman’s 914, the action was about 1/64" too high when I first strung it up. Following the procedure above, I sanded off that amount from the bottom of the saddle. Finally, I got the height right where I wanted it.

Tip: When sanding off material from the bottom of the saddle blank, use straight strokes and move in only one direction. It’s easier to keep the bottom of the saddle flat—which is what you want for the best sonic transfer—when you sand in one direction.

Step 8

Check String Height at the Nut
There was one more issue to resolve before I could finish the new saddle for this 914. The action at the 1st fret was still a little too high, so I needed to re-cut the string slots to correct this. Here’s the process:

1. Using gauged nut slotting files (one for each string), cut each string slot to the proper depth. Begin with the 1st string, which should measure 1/64" above the 1st fret.
2. Continue to measure and, if necessary, cut the nut slots for strings 2-6. Each string gradually increases in height from the 1st to the 6th, which should sit 2/64" above the 1st fret.
Once the action at the 1st fret is correct, you’re ready to begin the final and most challenging phase of this project— intonating the guitar.

10. Marking lines on the saddle to indicate the new contact points to be filed for each string. Here you can see the compensation marks for the high-E and B strings. 11. The B string’s new contact point will be at the rear of the saddle. 12. The contact points for the G, D, A, and low-E strings form a diagonal line, with the G close to the front edge of the saddle and the low-E 1/32" from the rear. 13. Filing away the excess bone behind the new contact points for the low-E, A, D, and G strings. The G string sits forward—it’s closest to the soundhole—while the low-E string sits at the rear edge by the pin holes.

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