torstai 18. huhtikuuta 2013

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

tiistai 16. huhtikuuta 2013

This is how I make my bridges

Making an acoustic guitar bridge is a nice task. You take an ugly chunk of black wood and turn it into a graceful, silky object with beautiful contours and crucial significance for the operation of the whole guitar. This is how I make it. Sorry about the quality of some of the cell cam pics.

Here is the source of inspiration this time. My Bourgeois Vintage D bridge.
I took a photo and scaled it to exact measurements on computer screen,
then printed it out and glued it down to a cardboard backing.
...and here is the ebony blank to start the work with.
Madinter sells these (plus the fingerboard blanks) at a very reasonable price.
I start by belt sanding all the facets smooth.
Looks almost real...
With this template I trace the outline of the bridge on the sanded blank
I do a rough cut using band saw.
Then go down to the line with the belt and disc sanders.
The pencil lines help me figure out how the work is proceeding. 
The shape about roughed out.
radius of the top of the bridge is about 20 inches
Trying on the starting-to-shine blank on
the starting-to-shine top. I use these to bring ebony to it's shine.
My last secret weapon is an ordinary A4 sheet of printing paper.
Or a tearout of a newspaper. It brings the shine to its final level.
Oops. Not secret anymore...
Time to route the saddle slot. This handy little jig with cams
keeps the bridge still while cutting
And this handy little jig makes a safe and controlled ride for the router
A clean cut at exactly 3mm.
Trying in the saddle. Might need a tad taller one. But the fit is snug!
making pilot holes/starters for the bridge pin holes. Strongly recommended!
4mm wood bit is usually the right one. Remember to use a wooden backing
piece to prevent tearing at the bottom.
Here you should be able to count to six...
Making the countersunk holes for the pin heads.
The drill press really helps here to make the holes nice and even.
About done!
With this "poor man's tailpiece" I was able to get the first
sounds out of the guitar this evening.
The setup is still off in every respect,
but it does play like a guitar! And looks good too.
Well, not the tailpiece...

torstai 4. huhtikuuta 2013

Sunny stuff

It's always tempting to start shooting the shine once it starts to show in the beginning of the finishing process. Once again I gave in to the temptation... The spiderwebbing texture of the cocobolo is stunning!