Preparations for the most important glue seam

The bridge of the acoustic guitar is pretty crucial for the performance of the whole instrument.

The strings go through it and sit on top of it, on the saddle made of bone. The bridge area spreads the vibrations of the strings around the top. It's the place where the physical vibration turns into acoustic waves, music. Light (0.012) strings at standard pitch have 72 kilos of tension. This causes many kinds of torsions and strains to the guitar, mostly the bridge area. This tension is also the thing that "drives" the acoustic guitar top, makes it an acousto-physical amplifier. Make the top too stiff, and the strings won't be able to move it, and you end up with a lifeless, dull guitar. Make it too flexible, and it warps, distorts and collapses in due time. But will probably make some loud, cardboardy music before that. Make it just right, and it stands the rigors of the road and is able to sing like a bird or growl like a lion when needed. And developes a "belly", a slight bulge behind the bridge, not unlike the ones seen above the belts of soon-to-be middle aged men... A flatpick guitarist Ricky Skaggs has been reported to say that one should "never trust a guitar without a belly." If it doesn't have a belly, It's probably overbuilt.


Anyhow, the bridge with all the tension it has to take in its lifetime, is attached to the top with just glue. No screws here! Titebond seam is said to be stronger than the wood around it, but in this case, there are some difficulties. We have two totally different wood species, thicknesses and sizes. Spruce top is large, thin and soft. Ebony (or rosewood) bridge is stiff, small and hard. The grain direction of the bridge runs perpendicular to that of the top's. All this means that these two objects are going to be having lives of their own when it comes to climate changes. This means the joint of these two has to be as perfect as possible in this fallen world (I believe the glue joints started to separate only after the fall. There will be no bridge re-gluing issues in the New Creation).

The bracing of the top is shaped to give the top a slight arch. No "flat top guitar" is actually flat topped. The arch will increase with the pull of the strings. This means, the bottom of the bridge has usually to be shaped to match the arch, to make the seam perfect and to prevent the bridge from popping out under tension. I do this by sanding the bridge bottom against a paper taped to the top. I sand until all the  pencil markings in the bottom are gone. I use a little something inside to give the top a slight lift, to imitate the string pull.

I weaken the tack of the tape before attaching it to the top.
Just tape it a few times on your blouse before use.

This little jack gives the top a little push, so it won't give in under sanding strokes. 

The underside of the bridge is carefully marked by a
 tattooed apprentice using clearly visible colours. 

ready to start

Using very subtle, short and controlled strokes, the bridge
is sanded until all the color is gone.
Feeler gauge's thinnest strip (20th of one millimeter) is used to
check the fit before putting any glue in between.

Finally there!

I glue the bridge using a special caul that I saw somewhere and made my own version of. The V. 2.0 is designed to spread the pressure evenly throughout the bridge. Any problem spot can be given its own special treatment with the 6 thumbscrews around. With all this compression power at the tips of my fingers (and hex wrench), I must be careful not to exhaust the seam by squeezing out all the glue...

The 4mm bolts that keep the whole sandwich together,
go through the bridge plate caul, bridge plate and top, then through
the bridge. The outside caul has two parts to it, so that
the compression bolts won't mar the bridge.
Not test driven yet, it seems like a nice idea.

And here is the test drive. Neat, no top bending heavy
clamps and easy to clean the glue spill outs.

Absolutely airtight!

Ready for glue. Just scrape the shellac off the top under the bridge

Gluing down the bridge, bolting the fingerboard

The last glue seam has been done for Jyrki's guitar. Here's a few pictures and some explanations to show how I did it this time. 

My big weakness in luthiery has been the bridge. Every bridge I've glued has had to be re-glued at some point after the first assembly. Now I believe I got it right from the start. Time will show. 

I've used bolt on attachment of the neck in every guitar. Two threaded inserts are inserted in to to heel of the neck, and the neck is bolted on to the body (the heel block) with 6mm furniture brass bolts. This time I decided to use tiny threaded inserts and 3mm bolts to attach the fingerboard extension as well. This way the neck will be fully removable, should there ever be a need for that. 

Here we go

The underside of the fingerboard received a pair of threaded inserts, with wood screw in the outside and 3mm thread in the inside. 

I reinforced the inside of the fingerboard attachment area with a 4mm birch plywood. The holes are not nearly perfectly alingned, but symmetry isn't crucial here. Everything will be hidden.

The outline of the bridge scribed on the shellac at exactly the  place.  

I used low-tac tape at the bottom (not to tear out splinters of top once the tape is removed),  and on top of it double sided tape to attach the 100grit sandpaper to follow the profile of the top. The bottom of the bridge is marked with a crayon to show how fitting is advancing.

Using very small movements, the bridge is sanded to fit the curvature of the top

This little stick gives the bridge area just a little lift - just like the strings will.  It also prevents the top from giving in under my sanding strokes.

Getting there 

Now the whole bottom is covered with sanding scratches and the crayon is gone. Time to make a dry fir

With the bridge clamped, I'm testing the fit with a 0,05mm feeler gauge.  If the 1/20th of a millimeter won't fit in between anywhere, it's close enough for me. 

Time to cover up and scrape off the shellac. A razor blade will do the stripping, with the help of a small 220 grit sanding block. The bridge is always glued wood to wood.

This is for real! The excess glue is coming out all over, with means a good fit. Here the glue is already wiped of with a damp rag.

At least it seems to fit!

Here's a photo of my improvised bridge slotting jig. Just a little pocket to keep the chunk of wood in place, and a steel ruler to steer the cut.