Chapter
Three

Making
the Sound Box Shell

Most harps use one of three types of
sound boxes, or shells.

Cross Section of a square back, round back
and stave back harp shell

In this chapter I will lay out the methods
that I use to build each of these shell types.

When I design a new harp, the size of
the shell is dictated by the string band and string angle.   Before I start to build, I will
have decided on a few critical dimensions;

-length of Soundboard

-width of the Soundboard at the bass
and treble ends

-angle of the cap (top) of the sound
box relative to the soundboard

-angle the bottom of the sound box (the
base) relative to the soundboard

Critical
dimensions of the sound box- Length, Widths, and angles of the cap and base.

These dimensions are common to a round
back, stave back or square back shell.

I think of the sound box as a
foundation for the sound board.
Like the foundation for a building, the part underneath has to be the
right size, and it needs to be strong enough to take the load the strings
impose on the neck, pillar and sound board.

Option #1 Building a Square back

The Square back is probably easiest
type for a beginner to build, and some argue that it makes for the best sounding
harp.   It can be as simple as
four boards screwed together into a long trapezoidal frame and a plywood back
tacked or glued into place.   A
square back is certainly easier to clamp when you want to cut sound holes,
attach the sound board, or add an edge treatment for the soundboard.

Really
simple square back construction.  The cap and base angle are 90°.  Because the box is trapezoidal, the end
of each board has to be cut cross cut at the appropriate bevel (about 7° in
this case).    The sides are
cut from “one by” stock
(3/4 thick) The sides are tapered from 4” wide at the base to 2.5
at the top.   The boards are
attached with a pair of counter sunk screws at each corner.   The back is made from a piece of
¼” plywood with two oval sound holes cut into it.   This is actually easier than
building a dog house.

I have tried book matching two planks
of solid wood for the back (resawn to ¼ thickness).    They invariably crack near
the center seam on the bass end.
12” seems too wide given the normal changes in humidity we
experience here in Maryland.

Planning for stresses

The string tension will make the neck
push down on the top of the box with about 40% of the overall tension.   The foot of the pillar will be
pushing down with the other 60%.
On all but the most lightly strung harps, I like to make a cap that
bridges across the sides so the joint is in compression.   If the design has a pillar that
pushes down on the base of the box (instead of being bolted to the string
band), you will need to ensure that the base is sturdily attached to the sides
of the box, string ribs and bass end of the sound box.

A refinement for a square back

A
sound box made with ¾” sides is going to be fairly heavy.   To lighten it, I will resaw the
sides to ¼” from solid wood.   This saves lumber and allows me to
book match the sides.
Resawing the sides and adding the liners takes a significant amount of
time, but the lighter box usually improves the instrument’s volume and
tone (to my ear).   I worry
about making sides too light though.
They have to be able to take the load imposed by the soundboard

The joint between the sides, soundboard
and back will need additional glue area, usually a full inch on the bass
end.   Pine liners are
typically added.     They
can be made square, but some builders use triangular liners.   I clamp triangular liners using
small notched blocks and PVC ring clamps every 3-4” along the length of
the liner.    I usually
taper the liner so it gets thinner at the treble end.

Option #2, Building a
Stave Back

My first two harps had been square back harps, and I
had been mulling over different techniques to produce a stave back.  With two simple jigs, I was able to turn
a quarter sheet of ¼” BC fir ply into a pleasing stave back shell.  If you are nervous about the process, I
suggest you do a trial run using plywood first.

The Ying and Yang of harp design

Some harp builders get really anal about planning every dimension and
mitered angle very precisely.
It works well for square backs but gets unmanageable on stave backs
where Nstaves > 5 or so.    As I developed this process,
I had to suppress the rigid Teutonic right brain engineer/builder that tries to
dominate my mind while building.
An appendix at the end of the book Reference points,
“Critical” dimensions, and Fudging can help A-type control
freaks to overcome the fixation on precision just long enough to try this
approach.    Freeing up
the creative side has provided me with many of the best “leaps forward”
on the harp building and design process.

Making the Stave jig:

Start by building the table
saw stave cutting jig. The cutting jig holds a rectangular plank while making a
long, true bevel cut to make two staves.

Jig cutting tapered
Bubinga staves on the table saw with wedges in place

First I lay out a cross section of the sound box, what
I want it to look like at the top and bottom.  This shows a 10 stave shell for a five
octave harp.  The shell will be
3½ inches wide at the top, and sixteen inches wide at the bottom.  The drawing will help me determine the
staves widths (.61 on the narrow end, 2.82 at the bottom) as well as the bevel
angle (10

I make my stave cutting
jigs from ½” scrap plywood or wafer board (construction site
dumpsters are a cheap source).  It
should be at least 5” wide and 2-3” longer than the staves to be
cut.  I glue a 1x2 along the edge to
stiffen it, and a block at the bottom to keep the plank from sliding back as it
is pushed into the table saw blade.
The hold down blocks secure the stave blank securely and control the
taper of the stave.   When I am
ready to cut the staves, I use small thin wedges of wood to clamp the plank
into place.

Cross Section Detail of
loaded Jig on the table saw

Glue the 1x2 stiffener
along the edge of a 5x50 board

1.
Glue on the tail
block

2.
Rabbet a 1x2 for
hold down blocks

3.
Run the board
through the table saw at a 10 degree bevel

4.
Attach top and bottom
hold down blocks at precise offsets from the edge.  For a 10-stave shell that will be
3½ ” wide at the top end and 16” wide at the bass end, these
offsets will be .62” at the narrow end of the stave and 2.83” at
the wide end.

5.
Use wedges to
secure test piece.

Preparing the staves:

The staves are ripped from
a plank that is ¼” thick, 2-3” longer than the sound box and
for the shell shown above, 3-3/4 wide.
On a 10 stave harp, each stave needs to be beveled 10° on each
edge.  I use a joiner to bevel each
side of the plank before cutting it into a stave.   It is important that the jig hold
the blank firmly, and that the top (narrow end) of the resulting stave be 0.62
± .03” Wide.  If the
stave is 1/8” too wide, the sound box ends up being ½” wider
at the top.  The width of the stave
at the bottom end is not as critical, but it should be about 2.8”
wide.   Set the table saw to
cut a 10° bevel, and adjust the rip fence, jig and hold down blocks till
you get a smooth cut and the correct dimensions.  It pays to be fussy here.

Dimensioned cross
sections of the stave blanks:

Making the Cradle for
the Shell

The next step is to make
the cradle for holding the staves.
It can be made from cheap scrap plywood or wafer board.  It basically consists of four
rectangular forms with a semicircle cut out of the top end, screwed down to
long trapezoidal base.

1.
Cut the sheet stock into four rectangles specified on
the table below.   The bass
(largest) form will be 18” wide and 10.5” tall

2.
On each form
strike a centerline then draw a semicircle of the diameter specified one
inch down from the top.  For the
bass form, this would be an 8” radius.

3.
Attach the forms
to a base board with 1x2 cleats and sheetrock screws

Form

Width

Height

Radius

Soprano

6

4.5

2

Alto

10

6.5

4

Tenor

14

8.5

6

Bass

18

10.5

8

It may be necessary to
adjust the four forms up or down along the base once you have your staves cut
and have placed them in the form for a trial fit.

Gluing the staves:

Arrange the 10 best looking
staves side by side into a fan pattern.
Each stave has two sides and the stave sides that are slightly wider
should be facing UP.  I take some
time match the grain and to place any blemishes where the sound holes will be
cut or on in the middle bottom (which will be cut away).  Tape each seam with 2” wide
packing tape (the cheap 3M stuff is fine).
It is important to keep pointed edges of the staves together and the gap
between the staves as small as possible.

Taping the Staves-An
extra pair of hands to hold the edges aligned is a big help.

These Bubinga staves
have been taped and flipped over.
They are now ready for glue.

Straight, smooth edges on the
staves make this a lot easier.  Once
the back of the staves are taped, flip the assembly over (it will be like the
handling the tambour for a roll top desk).   If this is your first time trying
it, you should probably check to make sure the taped staves fit; they should
slide into the jig with a little gentle pressure.

The staves will fit
“looser” into the jig if you place them towards the bass end,
tighter if you slide them in higher.    Inspect the joints between
the staves and check to see if they are tight.  If the joints open up at one end of the
cradle, you may need to adjust the location of the forms.  Once you are satisfied that the cradle
fits the staves you have cut, it is time to glue them together.

With the tape side down,
and the staves spread out flat on a work bench, there will be a narrow V shaped
seam between each stave.   Run
a bead of yellow glue down each of the V shaped seams.  I chase the glue with a tooth pick to
ensure the glue is worked down into the bottom of the seam.  Lift the assembly up by the edges so the
middle droops, and place it into the cradle.  Use a wet rag to clean out the excess
glue that squeezes out of the sound box.   Check to make sure each side of
the shell is about the same height in the cradle.

This is a nice point to pause and admire the
shell.  If the work has been done
carefully, there will be about 36 linear feet of a nice, tight ¼”
glue seam between the staves.  The
tape acts as a hinge, and as the shell is curled into its final form, it generates
sufficient pinching pressure to “clamp” the joint while the glue
cures.  I use spring clamps to hold
the top edges of the shell to the cradle as the shell will try to curl inward
as the wood dries and the glue cures.

Option #3, Building a
Round Back

Using a foam Mold and vacuum bag

The traditional approach for building round back harps
starts with a wood mold and 7-12 layers of veneer that are glued successively
to each other using long strips of rubber to generate pressure.   A good shell can be made with this
technique, but it takes a significant amount of time and skill to produce a
smooth, void free shell.

One
of my harp building mentors, Lee Gayman, developed a method using two layers of
1/8” bending ply and a vacuum press.   It is much faster than the
traditional method and allows me to make a much smoother, stronger shell than I
could with the traditional method.

A
round back shell over a foam mold, curing in a vacuum press bag

To
build shells this way, you will need a reliable vacuum bag system.   Most harp builders start by vacuum
veneering flat panels – soundboards or backs.   I really suggest that you start on
flat panels before attempting a round back (See the chapter on Soundboards)

You
will also need a mold for the vacuum press

You
can try to build a hollow wood mold, but realize a vacuum press will be
generating about four tons of pressure on a mold big enough for a 36 string
harp.   A solid wooden mold is
a possibility, but is obnoxiously heavy.   Lee found that airplane builders
made large wing coamings using structural foam and we made several molds using
the same technique the airplane builders do (see appendix D)

Once
you have a working press and mold, you will need to

1.
Gather
the materials

2.
Make
a veneer “Sandwich”

3.
Put
the sandwich in the press to cure

Gathering Materials

The
shell is made from two layers 1/8” birch bending ply and an outer layer
of veneer.

I
have also used 1.5 mm aircraft birch bending ply, but it is more
expensive.  There are a couple of
different kinds of 1/8” bending ply.   The cheapest bending ply is made
from three equal plies of a very blond, light wood.  It looks like aspen, and will bend down
to a diameter of about 6”.
The Birch bending ply (made in Italy) is made of two plies.  The thicker layer looks like Luan and is
covered by a thinner veneer of birch.   With a little coaxing, this will
bend down to a 1.5” radius.

You
may be able to find a local wood workers emporium that will special order the
bending ply.   If that fails,
then you will have to mail order it from Boulter (www.boulterplywood.com) or L.L.
Johnson (www.theworkbench.com).  1.5 mm Aircraft birch can be bought
through Wickes (www.wicksaircraft.com
).

The
size and shape of your plywood blanks will vary with the mold – I took my
measurements directly from the foam mold.
The plywood blanks for my shells are 48” long, 8” wide at
the treble end and 26” wide at the bass end.

Dimensions for bending ply blank

You
will also need adhesive and a layer of the veneer that you use to cover the
outside of the shell.  I used epoxy
for my first dozen shells, but found it bled through the veneer.   I now use DAP Weldwood Powdered
Resin exclusively.  I mix it with
water at a 10:4 ratio (by volume).
This glue cures faster than epoxy, cleans up with water, has good creep
resistance and has minimal bleed through.

Making a Veneer “Sandwich”

There
are a few challenges that you will face:

-
Developing
and maintaining even vacuum pressure over the entire surface of the shell

-
Keeping
the veneer from getting wavy or the adhesive from “pooling” under
the veneer.

-
Keeping
the excess glue that will be mooshed from between the layers from gluing the
shell to the mold or bag.

-
Keeping
the two layers of plywood and veneer aligned with each other in the press.   Most adhesives make the layers
slick, so they tend to slide around.

To
address each of these, I do the following:

I
distribute the pressure using a “breather layer” to channel air
from the shell to your vacuum connection.
Some use bubble wrap as a breather layer.  I use nylon window screen.   This is the very top layer of the
sandwich.

“Pooling”
is less likely if I spread a thin, even coat of adhesive.   Even then, I can get wrinkles,
seam puckers or waviness.  In flat
veneering, the builder can use a stiff platen (usually a ½” MDF or
ply) to ensure that the veneer is pressed flat to the substrate or core.  For round back shells, I use a third
sheet of bending ply as that platen.
The platen should be placed right under the breather.   You can use your platen as a
pattern for cutting the veneer and bending ply to size.   The surface of the finished panel
will not be any flatter or smoother than the platen, so I pick the nicest piece
for the platen,  turning the A side
(smoothest) towards the veneer.

Excess
glue can bleed though pinholes in the top layer of veneer, along the seam and
at the edges of the shell.   To
keep from gluing the shell to the platen, mold, vacuum bag or breather, I
contain the excess glue by wrapping the outside of the veneer in an oversized
sheet of freezer paper (this is like butchers paper, but it has a plastic
coating on one side).  I call this a
barrier layer, and the edges of the barrier sheet are folded over onto the
innermost layer of ply and taped into place.

Here is a “sandwich” ready to go showing
alignment screws, barrier layer (wax paper) and the platen.  Note the tape, and breather (window
screen) at the ready in the background. The barrier still needs to be taped to
the bottom of the stack before it goes into the vacuum bag.

Some
sandwich shops hold tall sandwiches together using oversized toothpicks piercing
the center.   I use two #6x
1¼ inch screws at opposite sides of the center seam to hold the ply,
veneer and platen into alignment during assembly and in the press.     Wood blocks are
epoxied into the mold (and reinforced with fiberglass) where the screws hold
the plywood and veneer into alignment.

Here
is the procedure I use to put this sandwich together –

1.
I
lay down two sheets of ply with the veneer on top.  Each is marked with a centerline so I
can line them up precisely.

2.
I
lay the barrier layer on top of the pile.
The edges of the barrier layer should extend 4” or so beyond the
edges of the bending ply.

3.
I
put the platen on top of the barrier layer.

4.
I
drill two small eighth inch holes though all four layers of the sandwich.

5.
I
place the 8d nails through the holes and I flip the entire assembly over.  This will leave the tips of the nails
pointing up.

6.
I
remove the two layers of ply and apply adhesive to the bending ply.

7.
I
stack the two layers of bending ply back onto the sandwich (Glue side down)
using the nails as locator pins.

8.
I
tape the edges of the barrier layer onto the inner layer of bending ply.

9.
Now
I have a neatly wrapped sandwich pinned together with two nails.  I flip the assembly back over and place
it on the foam mold.

10.
While
I hold the layers together I remove the nail and drive a #6 x 1¼ screw
through the sandwich and into the wood block embedded in the mold.    I repeat this for the other
end of the stack.

11.
I tape the leading edge of my breather on
top of the sandwich/mold and slide it into the vacuum bag.

12.
Seal
the closure, apply the vacuum.  As
the vacuum is applied, you may need to hold the sides of the bag out to the
side to prevent the bag from sucking back between the mold and underside of the
sandwich.

13.
Leave
mold and shell in the press until it is fully cured (6-8 hours)

Some
Tips

Even
Adhesive Coverage
– I use a thin nap 4” foam roller to spread the DAP Resin.  I use about 11 oz of glue for a
shell.    The glue is not
absorbed evenly into the wood, so three or four minutes after the initial
application, I go back to the sheet and play Robin Hood, redistributing the
adhesive from the rich areas to the starving patches.  I let the mixed resin sit open for about
5 minutes.  A good part of the water
evaporates off, eliminating bleed through.

Bag
Suck
– As the vacuum is applied, an oversized bag will have the tendency to be
sucked up between the mold and sandwich.
This prevents the sandwich from being pressed firmly to the sides of the
mold.   It can be mitigated by
using a bottom plate.

Eliminating
“Bag Suck” – the bottom plate

The
bottom plate is used in a conventional bag.  It is made from 3/4” MDF and
extend 6-8” beyond the footprint of the mold.   The bag needs to be large enough
extend another 4-6” beyond the bottom plate.

Removing
the Screws
- If you wax the screws before driving thhem into place they are less likely to
stick.   If you have trouble
removing them, heat them with a soldering iron till the adhesive softens and
pull them out.

Cracked
Shells
– I can usually torture the birch bending plywood down to a 1.5”
radius, but that is the limit.
I have used other plywoods and could achieve the following radii in the
vacuum press:

Substrate

Cost

Supplier

Minimum "in
bag" bending radius

5.2mm Luan plywood with
one face sanded off

$12

Most large hardware
stores

4.5 inches

1/8” Whacky Wood

Three ply Luan/aspen

$35

www.boulterplywood.com

3 inches

1/8” Birch Bending
plywood (two ply)

$35

www.theworkbench.com

www.boulterplywood.com

1.5 inches

Birch Aircraft Plywood

1.5mm (1/16”)

$90

www.wicksaircraft.com

¾” inch

I
found you can get slightly tighter radii if you thin the plywood (on the luan
side) by 1/3 or so to make the plywood more flexible.

Sharp
Edges
– To prevent sharp corners from puncturing your vacuum bag, Round the
corners of the backing plate, plywood and platen to a ¼” or
greater radius.

With
a completed shell, you are now ready to add the cap, bass and a few internal
parts.

Copyright ©
2007, Rick Kemper

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