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Martin DM: Bridge Reglue


1. A Lifting Bridge is serious business. As the bridge/top glue joint becomes smaller, the string tension is distri- buted through an ever more concen- trated area of the top. This will even- tually cause damage to the top, bridge and bridge plate if left unattended.	2. Removing the Saddle. With the assistance of the shop's flush cutting end-nippers. This is the same tool that we use to remove frets. This guitar is equiped with an under the saddle pickup. I'll have to remove that in order to take off the bridge. Also, this original factory micarta saddle has got to go. Look at all that wear from years of play. I'll replace it with a new bone saddle.


3. Scoring the Finish. Most contemporary guitar manufacturers actually leave about 1/8th" of finish underneath the bridge all the way around the outside perimiter of the bridge. When regluing a bridge that was built this way, the finish beneath the bridge must be removed in order to insure a good glue joint. I'm using a sharp exacto knife to score the lacquer as deeply as I dare with- out cutting into the softwood fibers beneath the finish. This will make removing the finish under the bridge easier after the bridge is removed. On older instruments with a brittle lacquer finish, I'll use lacquer re- tarder to soften the finish in order to prevent chipping of the finish. On a newer guitar like this, it is not usually always necessary to soften the finish before scoring it.	4. Softening the Glue with a Heat Lamp takes about 7-10 minutes. The heat shield is protecting the fin- ish on the top of the guitar.


5. Removing the Bridge with a Spatula. I'm starting disassem- bally from the back of the bridge. I'll first free the rear of the bridge pushing the spatula only as far as the pin holes. Then I'll work my way around the bridge.	6. Bridge Removed. The bridge came off nice and clean. Note the finish around the perimiter of the bridge's footprint. This finish is much lighter in color as it has not been exposed to sunlight as the surrounding finish has.


7. Shaping the Bottom of the Bridge. This jig does a great job stabalizing the bridge while I contour the bottom of the bridge to fit the top. I start this process with a block plane frequently checking the bridge against a straight edge. Next I'll spot scrape the rosewood with a scraper. I want the bottom of the bridge to be ever so slightly concave from end to end. This will help stabalize the top with- out flattening it too much. If a top is too flat the tone will change.	8. Scoring the Bottom of the Bridge. This improves the strength of the glue joint. Next I'll prepare the top of the guitar by removing the old glue with a scraper and the scored finish with a sharp chisel. I'll lightly score the spruce too.


9. Bridge Reglue Clamping Cauls. Cauls help distribute the force of clamps to both protect the workpiece and ensure even clamping pressure (this is the same general purpose that washers serve when used in conjunction with a bolt). The inner caul is made of hardwood and has notches cut into it to accomodate for clamping around the top braces.	10. Heating the Bridge for Gluing. I'll be gluing the bridge with hot hide glue because hide glue won't creep like alaphatic resin glue does. Hide glue only has an open time (time in which it can be clamped) of about 10-15 seconds. Heating the parts to be glued increases the glue's open time.


11. Applying Hot Hide Glue to the Bridge. I've heated the glue in a double boiler. I prefer to use hot hide glue for just about all load bearing, wood to wood glue joints. Hide glue is superior to alaphatic resin and polyvinyle glues for instrument repair because it creates a stronger glue joint, is easy to dissassemble and doesn't creep. Though inferior to hide glue for guitar repair/construction purposes, A.R. and P.V. glues are quicker and easier to use than hot hide glue which is why these synthetic adhesives are used by the major guitar manufacturers.	12. Clamping the Bridge. The two thumbscrews on the outer caul ensure that the bridge is under even clamping pressure.


13. Cleaning up Dried Glue Film with a Damp Towel. Since hide glue is water soluble, a lightly dampened towel will easily remove any glue film from the finish and the bridge. I cleaned up the majority of the glue squeeze out right after I clamped the bridge, but there's always a little film left behind.	14. Cleaning and Sealing the Bridge will make it look new again and help protect the rosewood from both fluctuations in humidity and dirt and oils from the hand of the player. I first scoured the bridge with #0000 steel wool. Then I cleaned the bridge with naptha. I'm now using mineral oil to seal the bridge. This cork padded, steel sheet protects the top from the steel wool and any excess mineral oil.


15. Cleaning Out the Pickup Lead Access Hole in the bridge must be done since some cured hide glue has clogged it up.	16. Reaming the Pin Holes is necessary to clear the glue that squeezed out into the pin holes. All it takes is one or two turns to get that excess glue out of there.


17. All Done. A meticulously fitted joint glued with hide glue ensures that this bridge will stay put for a good long time.	18. A Reglued Bridge with a new bone nut, compression refret and new bone saddle has this guitar playing better than ever!