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I've measured all the set components in thickness with my caliper and
decided what thickness I need to take them down to with my abrasive
planer. The Rosewood back and side blanks measure a uniform .210 inches
thick. The Koa backs have arrived sanded to 0.12-inch; one koa side is
bandsawn to 0.19-inch and the other bandsawn to 0.14-inch. Those are two
figured koa soundboards in the background, waiting to be placed on Hawaiian-style guitars.
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One face of one of the koa sides is the original plank face, and it
displays considerable damage: several nail holes (worm holes perhaps,
but they're suspiciously identical in size) and a deeply pitted and
compressed surface, the result no doubt of an early encounter with a
dull planer. It's going to be touchy to remove most of the excess
thickness from the damaged face, and only the minimal amount from its
opposite face. I can work some of the holes outside into the waste
portion, but there's one that I'm not so sure--I may have to do some
cosmetic work on the guitar after its made.
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Final results: the sets are down to the selected dimensions. Although
the guitars will be otherwise identical, the harder rosewood will be a
trifle thinner than the Koa, to keep my "appropriate" proportions
(factoring in later scraping and sanding) relative to the OM guitar
size: 0.115 for the koa back vs. 0.110 for the rosewood; 0.085 for the koa
sides vs. 0.080 for the rosewood sides.
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My most common templates used are 000, M, and classical. So for these, I
bend the sides on a mold over a heating blanket. The molds consist
simple of multiple medium density particle board templates bolted
together. Two gadgets which I find indispensable are a good rheostat
and (just barely visible in the upper right hand corner) a digital
thermometer. The slot in the waist block help keep it aligned. The
sides are soaked in cold water in a galvanized steel trough for about
20 minutes, and then placed on top of the blanket when it has reached
275-285 degrees, and pressed down at the center first, sloooowly, and
then summarily the ends are brought down and pinned by the cauls and
cam clamps. It's left to "cook" for about a full minute or so and then
shut off. I find if I leave the whole thing sitting like this
overnight, I get virtually no springback later. That's why I have to
start bending early in the process because four sides will take four
days.
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The back plates have been jointed and joined on the same fixtures
that joined and jointed the soundboards. In the foreground is my
kerfing drawer and box of spring clamps. In the background,
above is an antique Puerto Rican tiple.
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After jointing and joining the back plates, we've routed a channel for the back seam decoration.
I've opted for a neat strip of herringbone. We don't want a gaudy strip to steal any thunder from
the dramatically figured koa back. The router is pressed against a clamped straight edge.
Taped against the straight edge are paper shims that are used to widen the slot in progressive
passes until a snug fit is achieved for the marquetry strip.
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My student gets the gaudy abalone back strip which better compliments the uniform grain
of the rosewood back.
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The arched back braces are glued on against a flexible backing slat,
just like the arched soundboard braces, so the back can curl around them,
imparting a slight permanent arch to the flexible plate.
When the back is glued to the longitudinally arched rim of the shell,
the back will take on a domed shape. This makes the back, and therefore the entire guitar soundbox,
extremely strong and ready for 180 lbs of constant string tension.
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All my braces are shaped longitudinally to a parabolic cross-section.
The stiff paper placed next to the brace protects the back against
cross-grain scratches caused by the plane.
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The plane marks are sanded off with this miniature sanding plane.
My old colleague Ivon Schmukler of Artisan Fretted Instruments taught me how to make these.
I have a dozen of these in all shapes and sizes.
They are very handy and keep your fingertips from getting scorched from vigorous sanding.
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The brace ends are trimmed down to 1/8" at their ends. How much to carve off?
Until I feel they're...just...right...
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After the headblock and tailblock are glued to the soundboard, the bent sides are fit,
the brace ends are trimmed back, and the ends of the sides are trimmed to their final length.
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The trimmed sides are then kerfed. Here we see the sides with the kerfing strips pinned down flush
to their top edges by numerous steel spring clamps. I had the foresight to buy these in 1975 when
they cost $1.50 each. I'd hate to have to buy as many of them now! I would have to use clothespins,
as many of my luthier friends do. But I get much better pressure with these.
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My tutorial student is a computer hardware developer from near Toronto,
here seen leveling the kerfed edge of one of his rosewood sides.
The side is pressed down with the hands and the arms to insure even pressure along their length,
and then dragged across the sanding board along a curved path,
which keeps them from flopping about while sanding.
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Kerfing chips are removed to clear the brace ends.
Several dry runs are performed in anticipation of the fairly complex glue-down procedure to come.
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The sides glued on. The glue-laden sides are manipulated to the template line
drawn on the top before the myriad clamps are tightening down,
pinning them faithfully to the template line traced on the soundboard.
The glue-laden ends of the sides are clamped to the tail and headblocks with cauls and c-clamps.
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At the end of the day, two guitar shells drying.
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A line is traced from about 3" behind the waist to the headblock
with a pencil held against a paper straightedge.
The sides are trimmed down to the line with a block plane.
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The rim contour is smoothed, leveled and equalized with a large sanding board.
The 80-grit sandpaper lining the sanding board grinds everything very rapidly
so care must be taken not to grind away the longitudinal arch we've imparted to the shell.
The precise shell contour that is desirable is described in a diagram in my
book.
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In order to keep the sanding board from ripping the headblock off the soundboard,
a workboard shoe has been fastened to the headblock and workboard with bolts.
The shoe is shaped so it rests on the apex of the x brace, the center of the upper transversal,
and snug up behind the back of the headblock, keeping the headblock upright at about 90 degrees
to the workboard. Not quite 90 degrees: actually the headblock must tip back towards the tailblock
by about 1/32". This accomodates the correct neck set later on, and should work together
with the arch in the upper transversal face brace to keep the fingerboard level along its entire length.
Kinda complicated series of factors that have to work together, and hopefully when the dust clears,
everything will come out right in the end.
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Another view of the headblock fixture. If you look closely, you'll see inside the headblock mortise,
the head of a lag screw which secures it to the workboard shoe.
It takes advantage of the hole that's already in the headblock, which will later clear the neck bolts.
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The curved top rim of the sides is lined with kerfing strips. It looks like a clamp convention, no?
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The braced back is placed on the guitar shell, and their centerlines are aligned.
In this photograph (I should have clicked the close-up function to get it into better focus...sorry!)
I mark on the sides AND on the braces, precisely where they intersect.
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Taking care not to stab myself, I notch the kerfing where it's been marked, and carve a 1/8"
pocket to accomodate its corresponding brace-end (which in turn has been trimmed to where it's been marked),
with my favorite antique 1/4 paring chisel.
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All is trimmed and ship shape. I've made a bridge block and glued a cross patch
of irish linen onto the apex of the x-brace--all much easier done BEFORE the guitar is enclosed.
Notice I've also glued full-height kerfing chips above every one of the major brace-ends.
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The back is held tight onto the curved rim of the guitar shell while gluing with a rubber "rope",
stripped from a truck inner tube. Other luthiers use upholsterer's tape, even orthopedic elastic strips.
Some use elaborately curved wooden spool-clamp arrangements.
The roping technique is simple, ancient, effective, elegant...
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My tutorial student Joe Coady ropes the back onto his rosewood guitar according to my instructions.
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At the end of the day, two enclosed soundboxes trimmed and ready for binding.
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