Chapter
Seven

Attaching
the soundboard

The sound board is under a lot of
strain, and if you are building a light and responsive board, it will
eventually rupture or start to come off.   The goal is to forestall that
failure for 15-30 years.
A broken soundboard is not the end of the world, but replacement entails
about half the work of building a harp from scratch, so I take some pains with
this part of the building process.

Many modern builders rely solely on
glue to hold their soundboard to the shell.   This allows them to create a very
slick modern look and eliminates the labor required for adding edge battens and
mechanical fasteners.

This chapter will describe how to

-prepare the face of the sound box

-glue the board into place

-apply mechanical fasteners and edge
treatments to secure the board

This approach presumes the foot of the
pillar will be attached to the string band with a lag bolt (a popular
configuration for many folk harp designs).   Other designs terminate the pillar
onto the base of the sound box, and this approach may require additional
reinforcement to transfer the strain from the foot of the pillar back to the
string ribs which are pulling down on the strings.

Cross sectional view
showing three ways to attach the foot of the pillar to the sound box

Bolted Foot, Pocket hole in Soundboard, Tie
Block.

The pocket hole is usually
used with a plywood sound board.
A hole for the foot of the pillar is cut into the board, and the bottom
of the board is nailed and glued to the base board.   In a tie block termination, a
chunk of wood is screwed to the bass and the string ribs.    Variations on the tie block
will use a metal strap or Fiberglass reinforcement in lieu of the wooden block.

Most string ribs are centered at the bass
end, and in the bolted foot design, pass underneath the pillar.   I cut a channel at the top and
bottom end of the sound box for the inner string rib.

Before
I cut the channel for the top end, I play around a bit with the lateral
alignment of the string rib at the treble end.  Most builders strive to make the strings
drop vertically down from the neck to the center of the string rib.   The strings can angle in from the
bridge pin, but if the angle is too sharp, it will be difficult to fit sharping
levers or the string may slip off the notch on the bridge pin.   Some builders are fastidious about
centering the string holes down the center of the board, and if this is the
look you are striving for, you will need to position the neck accordingly.   The alternative is to offset the
string rib to one side, which is done by number of harp builders and does not
seem to dramatically affect the instrument’s tone.    The offset will depend on
the width at the top of the sound box and the thickness of the neck.

I place the soundboard on the sound box
where I want it, and mark the edges of the inner string rib on the sound box at
the bass and treble end.  I remove
the sound board and use a square to extend those marks across the cap and
base.  With a handsaw, I cut a kerf
on top of the line to the approximate depth of the rib.  Then use a router set to the right depth
to clean out the waste between the kerfs.

After checking the fit of the inner rib
in the notches, I trace the inner and outer edges of the sound box onto the
sound board.  This will help
determine where glue is spread later.    At this point, it is
customary for the maker to sign and date the soundboard where it can be read
through the bottom sound hole.

Harp builders have used a variety of
glues for attaching the soundboard.
And after loosing a board or two, most builders have some pretty strong
opinions as to which glue is most appropriate for this critical joint.   I suspect some of these prejudices
have more to do with the builder’s familiarity with a specific adhesive,
their work habits and available clampage.   See the appendix for a more
complete discussion of glue choices.

If you cannot get a nice tight fit
between your shell and the sound box, or if you cannot provide even, adequate clamping
pressure, I would suggest you use epoxy because it can tolerate gaps and does
not require much clamping pressure.

Before you mix your epoxy, select a
clamping method and do a dry run so you have a good idea of how much time you
have before the adhesive sets.

I use about 60ml of epoxy to attach the
sound board for a 36 string harp.
I

Carefully measure the epoxy to the
specified ratio and thoroughly mix the epoxy for a full minute, scraping along
the sides and bottom of the mixing cup.   Then I use a cheap flux brush (50
cents) to spread the un-thickened epoxy along the face of the sound box and
between the pencil lines on the sound board.

<pic of thickened epoxy spread on
top of Sound box>

I mix fine sanding dust (wood flour)
with the balance of the epoxy to make a paste the consistency of catsup.  This is spread with the mixing stick
along the top edge of the sound box.

It is fairly easy to clamp a soundboard
to a square back harp.  Stave backs
and round backs prove to be more of a challenge.   Some builders solve this by using
mechanical fasteners as clamping devices.   The redundancy provides a backup
system if and when the glue loses its integrity (a belt and suspenders
approach).

The
simplest approach is to use a pneumatic stapler with ½ inch staples to
staple the soundboard down.
Care must be taken to ensure the force of the driving head does not push
the staples through the thin, soft wood at the treble end of the soundboard.   Most builders set the pressure
fairly low then go back with a hammer and punch to level any staples standing
proud.   Later the fastenings
may be covered with a decorative lamination of some kind.

Sprung Soundboard
Clamping Frame

I developed a jig with two curved beams
that allow me to clamp the sound board
to the sound box using one clamp at each end.   It works well with epoxy for stave
backs and round backs, and allows ample access to remove excess glue inside the
sound box through the sound holes.

Two curved 1x2’s are used
to distribute clamping pressure from the ends of the jig along their length to
the edges of the soundboard.
The curve is set by bending the 1x2 over a ½” block while
clamping each end to a sturdy work bench.   A line is scribed along the board
a half inch above the bench top.
Each 1x2 is then trimmed to the line.

The two curved 1x3 boards are screwed
to 2x2 blocks so that they are held apart at the widths of the treble and bass
end of the sound box.

Clamping the soundboard
– the jig is placed on scrap 2x4’s, and the soundboard positioned
on top of that.  Spread the adhesive
on the edges of the shell and place clamps at each end (indicated by arrows in
the last panel).  It is usually
necessary to screw a temporary block on the base plate to give the clamp
purchase.

Dressing
up the Sound Board Edge

There are a variety of treatments that
can be used to dress up the joint between the sound box and sound board.   I will describe the basic approach
for my favorites here.

Cross Section - Veneering the Sides

Steve
Green used to make kit harps, and he had the builder laminate the entire side
of the harp after the back and sound board had been glued into place.   I built a few of these kits for
clients.    First the
veneer is cut so it is ¼ - ½ inch wider than the side.   Next a thin but uniform layer of
yellow glue is spread on the side of the harp.    Then, the veneer is clamped
into place using a layer of carpet padding between the veneer and a
¾” board (backing caul).   The carpet padding helps
distribute the pressure evenly over the veneer.   After the glue dries, the excess
veneer is trimmed flush.

Cross section of Square back box with
thinner sides, liners and applied bindings

Bindings

A fancier method is to use a
binding.  Most guitars use a
decorative binding to cover the end grain and any gaps left by less-than
perfect seam between the sides and top of the instrument.

There are fancy varitions of this that
use layers of contrasting veneer between the binding and sound box.

To put bindings on a harp, I rout a
small rabbet (approximately ¼” wide and .125-.1875” deep)
along the edge.  Next, I cut a strip
of solid stock on the table saw.  I
cut it so it is 1/32” wider and thicker than the rabbet.    I glue the strip into the
rabbet, securing the binding every 2-3” with masking tape.  After the glue has dried, the binding
and excess glue can be scraped or sanded flush.   Because the edge of the instrument
is subject to a lot of dents and dings, the bindings are usually made from a
hard, dense wood.    You
can make your own, or get fancier bindings with fancy inlays strips from
guitar/Luthier suppliers like Stewart MacDonald.

The most secure method for reinforcing the
sound board/shell joint is to use a screwed batten.  The batten is secured every two inches
with countersunk screw into the liner and shell.  The top of the batten is covered with
veneer or an edge banding.
This is a little more labor intensive than the other methods, but is
cheap insurance against a blown board.

Cross section of a batten inlay edging and
Jack Yule’s L molding

Jack Yule uses a similar approach,
replacing the batten with an L-shaped molding.  Two shallow dados are cut into each face
of the molding strips.
Contrasting wood strips are cut to the width of the dados and glued into
place over the screw heads.

Here is the procedure I use for making
the batten inlay edge treatment –

After the sound board has
been glued down with epoxy, trim the edge of the soundboard with a laminate
trimming bit.  I use a scrap of
fiberboard to protect the soft spruce from drag marks and scratches from the
router base.

Preparing the Battens

The
battens are ¾” wide and 3/16” thick.   Here I’m drilling the screw
holes and a countersink for the screw head every 2 ½” along the
each batten.  A batten that tapers
at the treble end makes the treatment a look a lot lighter and less clunky.

Attaching the Battens

The
batten is rarely straight, so I pre-drill three or four holes and put in screws
at strategic points to position the batten so it overlaps the soundboard edge
by an 1/8” along its length.
Use #6 x ¾” wood screws.  Apply the glue, fasten the batten down
with three or four predrilled screws.
Wipe of the squeeze out and drill for the remaining screws and fasten
them down.

Route a Groove for the
inlay

The inlay bit cuts a groove 3/8” wide and
1/16” deep.  The inlay will
cover the raw edge of the sound board along its length, so the height of the
cutter has been set so it skims a sliver off of the underside of the
batten.   The bearing on the
bit has been wrapped with four layers of masking tape to make the channel
shallower.  The cherry veneer on
this harp is only about 1/28” thick, so the inlay band has to be flush or
slightly proud of the sound box.

Testing the fit of the
inlay in the channel:

Next, test fit the inlay into the channel

For the dry fit, the inlay should slide in
nicely.  Lay a bead of glue down and
smear it to the edges of the groove with a
finger.  Realize that as the
glue wets the veneer, it expands slightly.
Use tape over small sticks to hold down any veneer that tries to lift
off.

Iron on the Edge band
veneer -

Since
it is easier to trim an outside edge, position the edge of the veneer even with
the inside edge of the banding.
A wooden block can be used behind the iron to press it firmly down onto
the batten as it cools.  Trim the
excess using a utility knife with a fresh blade.

Finishing Touches -

Finish
with a scraper and fine sandpaper.
The heat activated glue on the veneer can gob up on the bottom of the
banding.  The scraper is a great
tool to remove the glue on either side of the batten and to round the edges
just a bit.   The eye is drawn
to the flashy inlay and the veneer seams are practically invisible.

Copyright ©
2007, Rick Kemper

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