Chapter
Four

Fitting
out the Shell

This chapter outlines the process for
fitting the parts that go into a round or stave back shell.   Square back sound boxes usually
have the cap and base of the sound box integrated into the frame.  When I am building a harp with a light
sound box and more than 800 pounds tension, I usually include some kind of
bracing.

Deformation under tension, Curved back
brace, straight bar brace

Planning for stresses

As
the strings pull UP on the soundboard, the soundboard will pull the sides
inwards.   If the sides of the
sound box are stiff enough, the deformations remain small.    As harps get larger, the big
bass strings introduce more tension.
The long sides of the sound box are springier.   If the sound board and/or sound
box deform too much, the sound board separates along the glue line, or the
spruce board may tear out around the mechanical fasteners holding it to the
shell.

After watching a shell implode or a sound
board “belly up” off the sound box, builders either stiffen the
sides of their shell or add internal bracing to make the shell more rigid.    Most of the stress and
deformation occurs at the bass end.
For five octave harps (~1300 lbs of tension), I stiffen the shell with a
single brace (the mid-brace) placed about 2/3 of the way down the length of the
sound board .

Marking out the
positions for the internal parts

For round or stave back
shells, the first task is to decide where to put the cap, bass and mid-brace
within the staves.

While I am marking out the positions for the internal
parts, I hold the shell onto the bench using a fitted cradle at the treble end,
and a clamp and two small edges at the bass end.

I used to try to mark the
locations for the cap and base using scraps of wood, rulers, string and a bevel
gauge, but it turned out to be a tedious, error prone process.   I found I could be much more
accurate using a marking jig and centering the sound box to mark the positions
for the cap and base.

The jig should be as long
as the length of the finished soundboard.   The end blocks on each end should
be cut to the angle specified in the harps design.   The width of the soundboard can
adjusted by moving the jig higher (or lower) within the shell.

Cap and base Jig - the dashed lines indicate where it
would rest on the upper edges of the shell when it is flipped over into
place.  The main plank is a thin
piece of plywood, 5” wide, sized to the soundboard (45.5” in this
case).   The cross piece on the
bass end needs to be wider than the bass end of the sound box.  The blocks on each end are cut to the
angle specified in the plans

The marking jig ready to be lowered into place.   The position for both the cap and
base are set by three marks – two on the upper edges of the sound box and
a third mark on the inside of the center of the shell back made by sighting
along the angled surface of the block.

The angle and position of
the mid brace is not as crucial.
Some builders use several braces to stiffen the shell.   If you use a U or T shaped brace,
you will need to make sure it does not interfere with the sound holes.    I simply use one U shaped
brace and place it about 2/3 of the way down the sound box.

The parts may not be circular
arcs.  The cap and base are usually
elliptical sections, so once the marks for positioning the cap and base are
made,  You can spile them
(description below) or use coat hanger wire (bent to the appropriate curve) and
cardboard templates to make patterns for the cap, mid-brace and base.

The cap and base are made
from good quality ¾“ birch plywood.   I will use ½” ply for
the mid-brace.  A good fit can be
achieved by spiling them to the edge of the shell.    To spile, I cut the part
oversize and place it an inch or so down the shell (toward the wider end).  Then I trace a line on the plywood with
a pencil laid flat along the length of the shell.   Using a band saw and stationary
sander, I remove stock right up to the pencil line, adjusting it to the bevel
by eye.    I return the
cap or base to the shell to fine tune the fit by marking, then sanding away the
high spots.    It sounds
tedious, but it only takes 30 minutes or so to neatly spile and fit all three
parts with gaps under an eight inch.

When I am satisfied with the fit, I scribe lines along
the upper and lower edges of the base and cap and mid-brace on the inside of
the shell.   These lines will
be used to cut the liners to the right length and angle.   After the liners are glued into
place the lines will also ensure that I will glue the cap and base into the
correct position.

Ready to trace the final
location for the cap, mid-brace and base in the shell

Liners can be rectangular in
cross section, but I like the look (and the economy) of triangular liners.  I rip liners from a clear pine board
with the saw blade tilted at 45 degrees.   I cut the ends to length and the
correct angle, and glue them into place with yellow glue, clamping with spring
clamps and PVC pipe ring clamps every 4” or so.

The Notched blocks
needed to clamp triangular liners in place

After the Liners are glued
into place, I taper them.  The
pulling force of the strings is concentrated at midrange and base end, and I
like to have a full inch of surface area to attach the sound board
(¼” shell + ¾” Liner).  At the treble end, the tension is much
lower, and a wider soundboard area will yield better volume and a fuller
response.  I taper the rib so the
width of the combined rib and sound box is about ½”.

Cut away view of the
shell, at the top and bottom ends showing fixtures to hold the cap and base
into place while the epoxy cures.
For the cap, a positioning block rests on the bottom of the shell.  It is made from a 1x3 that is cut to the
appropriate angle.  Double stick
tape holds the cap onto the block until the epoxy cures.  At the base end, three pieces of lath
are clamped to the shell (one on each side, and one on the bottom).  The base piece rests against the lath
stops.  I wrap the ends of the lath
with shiny plastic packing tape to keep any epoxy schmutz from bonding them to
the shell or base.

Even with careful fitting,
I end up with ¼” to 1/8” gaps around the cap, mid-brace and
base, so I glue them in using epoxy thickened with wood flour (fine sanding
dust).    It takes about
90ml of epoxy to glue all three into place.  I coat all the edges with neat (unthickened)
epoxy and let it soak in.  Then I
thicken the epoxy with wood flour to the consistency of peanut butter and glue
the cap, base and mid-brace into place.

For my first 5-6 harps, I
cut each sound hole by hand using Forstner bits, a jig saw and lots of
sanding.  Every other hole ended up
looking noticeably asymmetrical, and I realized I needed a jig that would allow
me to a quicker, tidier job of it.

Underside of the sound
-hole routing jig
>

Cross Section of the
shell, jig router and bar clamps, Set up on a shell.

To build the jig, I carefully cut a set of sound holes
into a trapezoidal piece of Luan that was 48” long, 12” wide at the
bass end, 5” wide at the treble end.
I glued two 1x2 runners along the edges of the template so they would hold
the plywood just above the shell.    A 16” piece of 1x2
(the clamping bar) is screwed to the top of the template between the second and
third sound holes.   Two bar
clamps are used to secure the jig to the shell.  Two additional spring clamps are used at
the ends of the jig (on the X’s shown on the jig schematic) to securely
hold the jig to the shell.
The holes are cut with a router equipped with a collar guide or a
ball-bearing pattern bit.

Next, I trim the tops of
the liners the cap and the base so they make a flat plane for the soundboard to
rest on.   Until I found a
fixture that would allow me to securely hold stave and round back sound boxes
to the work bench while I planed them, this was a daunting task.   After trying six or seven
elaborate frames to hold the shell, I came up with a system that would allow me
to quickly secure a wide variety of shell shapes firmly to the bench.

The fixture consists of two pieces.  One is a small cradle that fits
underneath the sound box on the treble end.   I screwed a ¾ x ¾ inch
stick that fits into a bench dog hole to the cradle.   The second piece is a crescent
shaped piece of wood cut from a 2x2 with a 3/8 x 10 inch carriage bolt epoxied
into it.   The crescent is cut
so the curve fits the inside of the sound box and it is just narrow enough to
fit into the middle sound box hole.
The parts of the cradle and crescent that contact the shell are padded
with thin foam.   Even though
1/3 of the harp overhangs the workbench, the fitted cradle and crescent provide
firm lateral stability for planing and routing operations.

Inserting the crescent
block through the middle sound hole

Trimming the cap and
base:

The shell will extend
beyond the cap and base end.
I use to trim to the cap using a power miter (chop) saw.   I set the shell onto a table and
set the double bevel on the saw to match the cap angle and make several
successive cuts to “sneak up” to the top of the cap.  There is a substantial risk of tear out on
the back end of the saw, so now I trim it with a bandsaw (about ¼ inch
high), then remove the excess on a stationary disk sander or the horizontal
edge sander.

Trimming the top of the
shell the old way

For the base, I use a band
saw to cut the shell so it stands ¼ inch proud of the base.    I lash the shell to my
workbench through the bench dog holes.
The shell is wedge shaped, so the lashing can be tightened by pushing
the shell down.

I use a router with a
straight bit and a block of wood screwed to the base to cut the shell so it is
flush with the base.

Router set up to trim
the shell flush with the base.  The ¾ MDF pad screwed onto the
router is the same curve as the edge of the sound box base.   While the pad rides along the
plywood base, the ½” straight bit trims the shell flush.

At this point the sound box
is almost ready for the sound board.

Copyright ©
2007 Rick Kemper

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