Chapter
Six

Fabricating and Attaching String Ribs

In
this chapter I will outline the method I use to make string ribs then attach
them to the soundboard.   Some
harp builders omit the external string rib entirely.   If you are going to do this with a
solid spruce or fir board, you should take measures to reinforce the holes
around the eyelets as the tension of the strings is high enough to crush the
wood underneath the eyelets.
A hardwood veneer will help, but I usually back this up by laminating a
strip or two of fiberglass cloth underneath the decorative veneer (in epoxy).

Traditionally,
string ribs are made from a heavy dense wood (Hard Maple or better) which can
be difficult to cut and shape into long, gently tapered sticks.  Before I bought a thickness sander and a
made a simple sled for tapering ribs, I used a surface planer to make the long
tapers in pairs.  I will describe
this process now.

The clamping jig, a
soundboard and finished ribs ready for gluing

Fabricating
the ribs:

Start
by ripping a stick little over 1” wide from a piece of 8/4 Hard
Maple.  It needs to be as long as
the soundboard.    Run all
four edges of the stick through a jointer or planer to get them flat and
smooth.   For a conventional 36
string harp, the stick for making the two ribs should be 1” wide,
1¾” thick, and 48 long.

On
each end of the stick place a mark ¼ inch in from opposing edges and use
a straight edge to strike a line along the length of the board.  At this point I plug in a hot glue gun
(the cheap hobby kind) and take the stick to the band saw to re-saw it into two
tapered rib blanks.

Mark
and saw the stick into two tapered blanks

After
cutting the taper, the blanks will have three smooth sides and one rough
side.  Arrange the two blanks with the
smooth sides together and the two rough sides facing out.  Glue them together with the small bead
of hot melt glue at each end.
Gluing them together allows them to be fed through a thickness planer as
a single unit as the fourth, (rough sides) are planed smooth.

Glue
the blanks back together rough side out.

Gluing
the two blanks together also allows me to power plane the thin ends safely as
they are supported by thick end of the other rib.  I feed them through the thickness planer
till the thin ends are ~ 1/8” thick.
At this point, the thick end of each blank should be ¼ inch
thick.

If
I had enough foresight to make the ribs an inch or two long, I can simply cut
off the glued ends.  Otherwise, I
use a putty knife to pry the two ribs apart and clean off the glue with a sharp
chisel.

The
case for a narrow outer rib

I center the ribs as accurately as I can,
and lightly pencil the outline of the rib on the soundboard.  Wet glue and clamping can make the rib slither
around, and it is difficult to keep them centered.  If they are the same width, and they are
not perfectly centered over one another, the outer rib may begin to separate
from the soundboard.

The crack is not usually
a fatal structural flaw, but it can be an annoying cosmetic defect.

So, I make it a rule to keep the outer
rib ~¼” thinner than the inner rib, especially in the high stress
area where the bottom octave of strings pass though the sound board and ribs.    Some builders address this
by making outer rib ½ inch wide and about 3/8 inch thick along its
entire length.

Next,
lay out the taper across the width of each rib.  Cut close to the line with a band saw:

I
use a block plane and sanding blocks to true up the edges.  I cut the top 3-4” off the outer
rib and finish the tip to the classic fingernail profile.

Different
harp makers use different shapes for their string ribs.   Some use a much narrower outer
rib, or eliminate the outer rib entirely.
Unlike sound board thickness, rib dimensions are readily visible on most
harps, so you can study what other builders are doing.

Clamping Jigs

If
you only plan to make one or two harps, you can use weights and tape to clamp
the string ribs to the sound box while the glue dries.   A slightly more sophisticated jig
can be built from scrap plywood and “one by” lumber, see the
section on a production clamping jig
below.

Lee
Gayman’s cross bar jig:

For
my first dozen harps, I used a set-up similar to Lee’s.   It works well.  Beware that the string

rib
will tend to slip and slide around under clamp pressure before the glue
sets.  To prevent this, I clamp or
tape small sticks of wood onto each side to keep the rib centered on the sound
board and do a dry run with the clamps before
I cover the string rib with glue.
Once the rib has been covered with glue and clamped in place for 10-15
seconds, the glue grabs and I can remove sticks and use a wet rag to remove the
excess glue.

Gluing tip:

On some of my first harps, I found the
edges of the string rib pulling up and away from the sound board.  I made several futile attempts to close
the gap with wider cauls and more pressure.  I finally began to realize that the
water in the glue was making the wood swell:

I keep a small pail of
water and wet rag or sponge to glean up excess glue.   After the glue is spread and the
string rib in place, I wet the top with a damp sponge or cloth.

Since the top and bottom of the board are
both damp, the rib will stay flat and the edges will not lift up from the sound
board.   It also helps if you
make the rib from quarter or flat sawn stock.   Quarter sawn ribs cup less during
glue-up.

Production
Clamping Jig

For
my first dozen harps, I used a set-up similar to Lee’s.   It works well, but I thought it
could be improved by reducing the number of pieces to assemble and by providing
clear access to along the sides (making it easier to clean up excess glue and
inspect the joint).  The basic
concept behind the jig I use now is that of two beams sandwiching the board
with an eyebolt at each end:

The
main weakness of this set up is that the long beams bend, so most of the
clamping pressure is concentrated at the ends of the rib, and there is little
pressure in the middle.
Optimally, pressure should be distributed evenly over the surface of the
area being clamped.

A
second problem in gluing string ribs is that of an uneven or wavy sound board
and/or string rib.   Even with
careful fabrication, they are rarely perfectly flat and even.  Often, while I was cleaning glue up I
could see gaps resulting from these machining defects.   When I tried to close the gaps
with more pressure, I risked squeezing all the glue out of the high spots and
starving the glue joint.

I
found I could solve both problems and get even clamping pressure if trimmed one
of the beams to a shallow curve and used spongy ½” foam padding
under the sound board.

Trimming
the upper beam:

The
top schematic shows the pressure distribution on the sound board when using two
straight beams – it is concentrated on the ends.   To compensate for this, place
a  small block about midway between
the bolts and tighten them down fairly hard (but not super tight –
don’t strip the thread).  With
a compass, scribe a line up from the bottom beam (shown as a dashed line).  Remove the top beam and trim the beam
along the line.  When it is
reassembled and the bolts tightened, the beams will still deflect, but the
pressure will be much more even along the length of the board.  On my jig, I did not center the block,
but put it about 60% of the way down because I wanted to generate more pressure
at the bass end where my string ribs have more surface area.

To
address the second problem (gaps from slightly uneven or wavy boards and ribs),
I glue a long trapezoidal piece of foam onto the bottom beam.   To generate more pressure at the
bass end of the beam where my string ribs are wider,  I cut the foam to be proportional to the
width of the ribs.  If you do not
used string rib that is tapered across its width, you may want to use a
rectangular shaped pad and center the block when you strike the scribing line o
the upper beam instead.

I
use a “closed cell” foam – scraps left over from making harp
cases.  You could just as easily use
carpet padding.  If you ask a carpet
installer, he will usually give you his scraps for free, or you can retrieve
new material from construction site dumpsters.

As the bolts are tightened the foam pushes up on
the soundboard.  The foam not only
closes any small gaps, it helps distribute the pressure more evenly between the
board and rib, and will give you a better glue joint.

These
last drawings simply cover some of the details I used to make my clamping jig.
They are not a critical part of the construction, but they may give you some
ideas to work from.

When
I have built clamping beams like this for furniture case work, I found the
stiffer the beams, the less they will deflect under load.   I made the top beam in a T section
– the narrow base allows better access for cleaning out the glue.   The top of the T is a strip
3/8” plywood, about 3” wide.   A 3/8” hole has to be
drilled through the thin face of a 1 x 4 for the bolt, so on each end of the
upper beam I glued two 2.5 x 6” blocks of plywood on each end of the beam
for reinforcement .  Since my ribs
are less than ¾” wide at the treble end I also trimmed the lower
corners of the T on one end as shown in the drawing.

The lower beam holds the
head of the eyebolts, so instead of chopping mortises, I laminated it from two
pieces of 1x4 pine and a strip of 3/8 plywood.   The vertical strip of ply is cut
shorter than the 1x4’s so the eyebolts just slip in to place.  A second strip of ply acts as a bed for
the foam padding.

The
eyebolts are two 8” x 5/16” eyebolts with T-nuts embedded in big
wooden “wing-nut” handles.

Once
the glue is spread, speed is paramount.   I made one simple modification
that allowed me to open the jig up:

Cut
a slot into one of the bolt holes on the upper beam.

Opening
the jig will give you plenty of room to spread glue onto the board, and put the
rib into place (1).  Then, simply
close the upper beam (2), and swing the bolt up into the slot (3) and tighten
the wing nut.  I do a dry run first,
and pre-center the board into place before applying glue.

Copyright ©
2007, Rick Kemper

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