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Martin Intonation Correction

1/2/2014

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1. A Martin D-18.  This one is from the 1970’s and has poor intonation, probably because it was built with the bridge in the wrong position.  As usual, the intonation is sharp so I will fill the saddle slot and rout a new one closer to the pin holes. 2. This Bridge Has Issues, aside from being glued in the wrong spot.  To avoid a neck reset, the bridge was “shaved” to allow the saddle to be lowered.
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3. Groovy Man.  The strings have been “ramped” via deep notches between the back of the saddle and the pin holes to give the strings some break angle over the extremely low saddle.  These notches have to be filled before I can move the saddle slot closer to the pin holes.

4. Rough Thicknessing the Plug. I carefully select a scrap of Brazilian rosewood that has the same color and grain as the bridge.  I rough-thickness the plug with a block plane.

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5. Thickness-Sanding.  I use the luthier’s friend sanding station on the oscillating spindle sander to sand the plug about .003″ proud of it’s final thickness.  I use the original saddle and a feeler gauge to set the fence.

6. Final Thicknessing the plug only takes a couple of passes with a block plane set up for a very fine cut.  I stop planing when the plug and the saddle slot have a snug fit.

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7. Rounding the Ends of the plug.  I use a square block of steel and the disc-sander without power to shape the end of the plug.

8. Fitting the Plug.  I lightly chamfer the bottom edges of the plug.  I glue the plug in with medium super glue applied only to the bottom of the plug in order to prevent squeeze-out and to ensure that the plug seated properly before the glue set-up.

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9. Planing the Plug flush to the bridge. I use my little Lie-Nielsen #101 violin maker’s block plane.  A chisel would also suffice.

10. Gluing the Plug. I apply water-thin super glue to the surface of the plug.  This ensures that the plug is properly glued in place along the walls of the saddle slot.

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11. Repairing the Notches.  I use a mixture of light-colored Brazilian rosewood dust and thin super glue.  I tape off the pin holes inside the guitar with masking tape to prevent super glue from getting on the bridge plate.


12. Sanding the Bridge with 80 grit PSA sandpaper on a paint-stirring stick gets rid of the excess glue.  I follow this up with 220 grit, 600 grit and 2,000 grit.

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13. Ready for Slotting.  During a restoration, moving the saddle for improved intonation should be done after a neck reset and refret.


14. Layout is Key.  I use Stew Mac’s “the intonator” and a digital strobe tuner to determine where the saddle should be located.  The intonator can also be used as a temporary saddle during a restoration that involves replacing the bridge.  Ideally, the bridge should be replaced, then the neck reset followed by the refret.  The final steps should be locating and cutting the saddle slot then fitting a new saddle.

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15. Drilling.  I Drill out the ends of the saddle slot with a 1/16″ diameter drill bit .046″ on center from the desired ends of the finished saddle slot.  These two holes make setting up the router jig much faster and negate the need to plunge the bit at the start of the first pass.


16. Routing the saddle slot with a palm-router instead of a dremel-tool is a huge quality of life improvement.  I make my first pass with a 1/16″ downcut bit set about .015″ shy of the final slot-depth.

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17. New Saddle Slot.  The 1/16″ bit removes most of the waste so that a final pass with the 3/32″ down-cut bit set to full depth (about 15/64″ between the D and G strings) creates a flat-bottomed slot with no tare-out.


18 A New Bone Saddle completes the repair.  The plug is nearly invisible and more importantly, the guitar plays in tune.