Appendices

Appendix A

Reference points, “Critical” dimensions
and Fudging

After building a few harps I noticed that I
was spent a lot of time measuring and worrying about how the parts were going
to fit together.   I found that
I could eliminate a lot of the worry and wasted time if settled on some key
reference points and dimensions to build from.   Boats are usually built form a
keel centerline or wire strung between the stem and stern posts.  Buildings are laid out from the cornerstone.

The soundboard is the heart of the
harp.  I use it as the reference
plane that everything else is built from.   When I design harps, the client
specifies some key parameters (the number of strings, and spacing).   Depending on the sound and tension
they want, I set the angle of the strings to the sound boards, and the rest of
the instrument is fleshed out from there.

So, as you build sound boxes, realize that
you are simply making the foundation that lay under the soundboard.   As I build, I key in on 5 basic
parameters:

-Sound
board length - it needs be long enough

-Sound
board width at the treble and bass end

-The angle
of the cap plate has to be right so the neck fits without any big gaps.

-The base
plate needs to be close to a certain angle so the harp does not topple forward
or back.

Even significant errors on these parameters
can be compensated for by

Fudging

I try to stick to plan, but errors
inevitably creep in from time to time.     As I examine more and
more work from custom builders, I have begun to suspect that they are good at
avoiding errors in the first place.
But, they are even better and fixing errors and making adjustments ex
post facto.    Here
are some of the tricks I have used to adjust what was built to the planned
design.  In the engineering world,
they are often called field modifications.

-Angle of the cap: An additional piece of
wood (I call it a collar) can be glued to the top of the sound box.  In a contrasting wood it looks
decorative, even deliberate.  A
miter saw set to a compound bevel can cut the collar to an angle that matches
the bottom of the neck.

-Angle of the base:  I fine-tune the balance of the harp
right before finishing by trimming the feet.  You can shave a ½” or so,
which gives a surprising amount of latitude before the base begins to look too
queer.

-Length of the soundboard: It is OK to be an
inch or two long, the foot of the pillar just hits the soundboard a little
higher.   If that bugs you aesthetically,
you can  add space between the
strings on the bottom octave.
If the shell is too short, you can add up to 2” by adding an apron
(under the base) or a collar (on top of the cap).   I did this on one of my first
stave backs, and most of my clients actually WANT them to make the harp look
fancier.  Go figure.

Appendix B

Ten Things Ma shoulda told me about Epoxy

Epoxy is wonderful stuff.
It tolerates significant joint gaps and requires little clamping
pressure.   You can mix in
additives to make the schmutz perfect for the bonding requirements.   But you can screw up, so here are
some lessons learned from 10 years of working with the stuff.

1.
Measure
carefully.  An unjust mix will not cure,
and is practically impossible to clean up to do again.

2.
Mix
thoroughly.  A partially mixed batch
will not cure, and (c’mon now, everyone together) is practically
impossible to clean up to do again.

3.
Pre-coat joining
surfaces, especially ply end-grain with the neat (un thickened) epoxy.   End grain really sucks the stuff
up and can result in a glue starved joint that will fail.

4.
Wood flour (or
baking flour for that matter) is thixogen, and when enough is added to the
epoxy, it will make the mixture thixotropic, like a good thick peanut
butter.   This is a very useful
property; learn to use it to your advantage.

5.
When in doubt,
mix a smaller batch.  When the
hardener is mixed with resin, the reaction gives off heat, which accelerates
the reaction giving off more heat.
It will get away from you.

6.
Uncured epoxy can
be cleaned with Vinegar

7.
Cured Epoxy can
be removed with hours of sanding and scraping.   Maybe.

8.
Work clean, use
gloves, and mask off areas you don’t want epoxy on.

9.
Don’t test
or move the joint till the residue in the mixing pot has cured.

10.
The phone will
usually ring right after the resin has been added to the hardener.

Appendix C

Setting up a Vacuum bag and
mold

Get a working
Vacuum pump, regulator and bag

I
started by setting up a reliable vacuum bag and system.  After several false starts, I found
www.Joewoodworker.com.   Using
the directions from his website, a salvaged vacuum pump and about $200 of odd
parts, I was able to make a fool proof system in about 10 hours.  That included a big heavy duty 30 mil
vinyl vacuum bag.  I veneered a few
soundboards and box backs till I was familiar with the press, my adhesives (PPR
resin and Raka Epoxy).  I practiced
with a veneer saw, a sanding  block
and veneer tape till I could consistently get a nice, tight veneer seam.

Home
built vacuum pump, resevoir and regulator,

From www.joewoodworker.com
website.

The vacuum sensor switch provided by Joe Woodworker works
well for many pumps, but the starting current demanded by my pump seemed to
overload the relay or the vacuum sensor and it would eventually not shut off
cleanly.

As I thought about the function of the vacuum pump and
controller, it occurred to me that this was all an elaborate way to evacuate
the bag, then running the pump for 10-15 seconds on 10 minute intervals while
the glue cured.  There is a much
cheaper and less complicated way to get there.   So this is the system I would
recommend:

A cheap, effective Vacuum
System

The
Vacuum bag and vacuum reservoir are built using the methods and materials
provided at www.joewoodworker.com.  In place of the vacuum sensor, relay,
manifolds and MAC valve Joe recommends, I have substituted a $85 repeat cycle
timer.  Repeat cycle timers are used
to turn pumps and sprayers and fans on and off and you can get one rated for 10
amps for under $100.  I use the
Encore timer sold through hydroponics suppliers.

To
run the system, I simply plug the pump into the wall outlet till the pump has
evacuated the bag to half an atmosphere (7.5 psi or 15 inches of Mercury).  Then I plug the pump into the repeat
cycle timer.  That will cycle the
pump on and off until the adhesive has cured.    The settings for the repeat
cycle timer will vary depending on the efficiency of the pump, the leak rate in
the pneumatic system, the size of the vacuum reservoir and how high a vacuum
you want to maintain in the system.

My
reservoir is made from a six foot section of 3” PVC pipe (about 500 cubic
inches) and I use a 6 amp Rietschle Thomas pump.  When I set the repeat cycle timer to run
the pump for 15 seconds every 10 minutes, the pressure in reservoir remains
between 12 and 15 inches of Mercury (in Hg).

The
stand-up cost for the system, including a reliable pump and large trapezoidal
bag for harp shells is under $400:

Item

Cost

5.5 Amp Vacuum Pump

135

Vacuum Gauge

12

3/8 ID Vacuum Tube

7

Check Valve

9

PVC caps, pipe for Vacuum Reservoir

15

Brass Fittings

10

Repeat Cycle Timer

85

5 yds 30 mil Vinyl for Bag

80

H66 Vinyl Cement

6

Valve Stem Assembly

7

Total

$366

Joe
Woodworker’s prices are pretty competitive, and I bought everything but the
PVC, Pump and Repeat Cycle timer through him.

Appendix D

Building a Foam
mold for Vacuum Bagging

Round Back Shells

Lee
Gayman found that airplane builders made large wing coamings using structural
foam.  Builders cut the foam using a
heated nichrome or stainless steel wire.   For $170, we ordered enough large
cell polystyrene foam from Aircraftspruce.com to build four nice molds.   Later on, I found that model
aircraft builders will build up foam blocks by gluing together thinner
laminations of insulation foam from Home Depot.   This is the same blue or pink
stuff used to insulate walls.  It is
cheaper and easier to get, and is dimensionally stable in the press.  The literature on the internet indicated
that either foam would give off toxic, noxious fumes as it is cut with a hot
wire, but the aircraft spruce foam is supposed to be less toxic.

If
I had to build a new mold, I’d probably just buy a sheet of the rigid
2” pink foam from Home Depot and glue successive layers of it together
then refine the shape with a big long sanding/fairing board like the surf board
builders do.
Sawing/sanding a mold is a bit more tedious than using the hot wire, but
is probably quicker for building one or two simple molds.

The
first challenge was to build a looooonnnnngg hot-wire foam cutter.

The
tricky part was making a fixture to hold a 4’ length of hot wire under
tension.  We used a frame similar to
a bow saw, but had to make a few modifications to keep the wire from breaking
as it cooled and contracted after the current was turned off and the wire
cooled.

Frame
for the cutting wire

The
spring and turnbuckle allow the cutting wire stay in tension when it is hot
without breaking when the current is turned off and the wire cools and
contracts.   The spring
deflected an inch or so, and I estimate we had about 70-100 lbs of tension on
the cutting wire.  We cut slots into
the ends of the main beam that would allow the end arms to pivot.   The stainless cutting wire (.040
stainless) and top tension wire (bailing wire) were simply twisted back onto
themselves to fix them to the eyebolts.

The
wiring is simple, using a doorbell transformer and dimmer to regulate the
current going through the wire.

Wiring
diagram from http://www.dansworkshop.com,
“foam cutter”

Before
we connected the leads from the transformer to the eyebolts of the lower
(cutting) wire, we did some thinking about safety:

-We
turned on the shop exhaust fan so that it would draw fumes away from us.

-We
had a fire extinguisher on hand.

-The
idea of a very hot stainless cutting wire breaking and snapping back onto one
of us while cutting was a little disconcerting, so we wore face shields and
long sleeved jackets.

-We
realized we were working with a long unshielded wire with current running
through it, so we cleared the area of metal and worked on a wooden bench.   We agreed on a safe place where we
could put down the frame without burning anything or shorting the wire to
ground.

-We
did a dry run (with the current off) rehearsing the motions of the cut and
where we would put the frame down, who would cut the power in an emergency,
etc.

We
did a few test cuts, and found a setting that kept the wire hot enough to cut
through the foam at a reasonable pace.   If you are going to cut through a
4’ section of foam, make a practice cut on a four foot section of
foam.   If you move too fast,
the wire does not stay warm enough and will drag and bend.  If you move to slow, it burns away excess
foam.  We found feed rate of about
6-10” a minute (on the bass end of the mold) worked well.

The
shape of the mold is governed by at the ends by two end-plates of Luan.  We cut them to shape and secured them in
place to the end of the foam block with two 8d nails.

The shape of the endplates
and side view of a mold

The
hash marks on the end plates allow the cutters on each end (this is a two
person job) to keep in sync with each other as they drag the hot wire along the
edge of the end plates.   The
wire needs to move a lot faster at the bass end.   It will need to move about 26
inches while the treble end only covers 8 or so.

If
the molds have some surface roughness, don’t worry – ours did.   We found it easy to smooth with a
random orbital sanders or simple fairing boards which allow you to make the
molds baby-butt smooth.

Lee
thought that fiber glassing the mold would make it last longer.   A fiberglass coating will allow
you wax the mold (which keeps you from expoxying the shell onto your mold).  I told him we needed a mold for building
a harp, not a surf board.  I simply
use a barrier layer (butcher’s paper) to keep from gluing the shell to
the form.

Appendix  E - Sourcing Wood for Sound Boards

Burke
Girvan (253-952-4310, Sitka Spruce Sales, 10114 Taylor Street East, Edgewood,
WA 98371)
is the most consistent source I have found for harp sound boards.   If you give him the width and
thickness you need, he can supply you with some of the nicest Sitka Spruce
I’ve seen.  He charges about
$50 for the stock to build a board that is 48” long, 16” wide and
5/16” thick.  It can take
several weeks for an order to arrive here on the East Coast.

There
are lots of other sources and alternatives, and if you are willing to invest
some time and effort, you may be able to save some money.

You
can find a number of suppliers that cater to the Luthier Trade on the
internet.  Billets for Guitar and
Cello Tops are traditionally made of top grade Sitka Spruce and have worked
well for me.  They can be pricey
though.  You may ask if they have
unmatched sets or blemished boards that you could use in a harp to get a price
break.  I have also heard that there
are some piano repair suppliers that supply spruce for sound board
replacements, but I have never been able to identify a contact.

Aircraft
Spruce sells 20 lbs of Sitka Spruce cut in a $25 “bargain
box”.  These are cutoffs from spars,
the lumber they saw up into boards used in airplanes from nice clear, tight
grain spruce.  Some of the pieces
have significant run out, or are too narrow to use.  There will be more waste, but if you
have your own band saw and time, you should be able to get three to five
soundboards out of a box.

With
spruce becoming outrageously expensive or impossible to find, harp builders
will have to try some other wood species.   I have used redwood 2x6’s
found in the bowels of an old lumber yard.
I have seen and heard excellent results from builders that use Western
Cedar, Sassafras, Engleman spruce, Douglas fir or Spanish cedar.  If you are going to use other species in
a design that specifies Sitka Spruce, you may want to look up its properties
and make the appropriate adjustments.
The U.S. Forrest service has an excellent publication that you can
download on the internet called the Wood handbook--Wood as an engineering
material.  Chapter 4, Mechanical
Properties of Wood, provides reams of data on wood.  Most builders shoot for a light, stiff
board when they build Nylon or gut strung harps, so if you want something that
sounds like spruce look for substitutes with similar properties.

When
I consider lumber for a soundboard, I ask myself these questions.

Is it well seasoned?  Many
violin builders insist on billets that have been air dried and seasoned for at
least seven years.  They feel that
it has superior acoustic properties over Kiln dried wood.   The prolonged drying process and
humidity cycling of the seasons does make for a more stable wood.    I have not had problems with
wood that has been seasoned for three years or so.  If you can not find out how long it has
been seasoning, you can use a scale, a small sample, and the kitchen oven set
at 175 degrees Fahrenheit to determine the moisture content.

Is
it Rift or Quarter Sawn?  Quarter of Rift
sawn will be more stable than flat sawn lumber.   This reduces the likelihood of
sound boards cracking along the seams between the boards.    Customers will also expect a
quarter or rift sawn board.

Look at the grain on the
end of the board to identify

Quarter
Sawn                         Rift
Sawn                        Flat Sawn

How
much run out and waste is there? You will need to discard sections of a board with
sap pockets, knots, checks, or wavy grain.
If the grain of the board does not run along its length (significant
run-out), it will need to be corrected while re-sawing and there will be more
waste.

What
is the ring count?  Some builders insist on certain ring
count (20 or 40 growth rings per inch).   As a general rule, trees that grow
slowly make for stronger, more stable lumber, but I do not use ring count as
hit-or-miss requirement.  Many
commercial makers build fine instruments using boards that range as low as ten
to fifteen rings per inch, so I try to consider ring count as one parameter in
balance with the other characteristics I am seeking.

Appendix
F,

Selecting
Adhesives

Different builders have their favorites
– and this prejudice is usually a result of familiarity.    Glues work best when the
builder is familiar with the adhesive’s working properties and how it
will perform.   As I built more
and more harps and tried different glues, I found I would vary my choice based
on how well it would perform for the task at hand.

I rely on Yellow glue for many
“general construction” applications – gluing string ribs to
sound boards, knee blocks, and square back sound boxes and gluing staves into a
shell.

For critical joints or joints that require
gap filling, I will use epoxy.
This includes gluing the cap and base into stave back and round back
shells, attaching sound boards, and splines for reinforcing the neck or joining
the neck to the pillar.

I use powdered resin in the vacuum press
because it is more economical than epoxy and does not bleed through the veneer
like epoxy usually does.

I use Cyanoacrylate (Superglue or CA glue)
for quickly repairing reattaching lifted veneer, torn out splinters or leveling
cracks that need to filled prior to varnishing

Here is a basic rundown of the pros and cons
for these and other glues that are widely used by harp builders.

Yellow Carpenters Glue – it is widely
available and most builders have used it so they are familiar with its working
properties

Pros

Cons

Economical ~$20/gallon

Needs a tight joint (~1/64 inch)

Convenient to use

Needs moderately high clamping pressure

Cleans with water before cure

Can creep under sustained stress

Non-Toxic

Softens at temperatures above 120° F

Sets in an hour, good strength in 2-4
hours

Shelf life of about a year

Hide
Glue – Traditional instrument builders laud hide glue because it is fairly
easy to take it apart and reassemble it for repairs.

Pros

Cons

Good
history (Still holding up in articles that are thousands of years old

Needs
to be mixed fresh every 2-3 days,

Repairable

Requires
thermostatically controlled glue pot (~$90)

Sands
well

Requires
a tight joint (~1/64 inch)

Non-Toxic

Requires
Moderately high clamping pressure for 4-8 hours

Moderately
priced, ~$11 quart

Cools
quickly so open time is limited.  Challenging
to do a large or complex assembly

Good
shelf life in flaked form

Can
soften in humid environments and temperatures above 120° F

Powder
glue is mixed with water

Pros

Cons

Reasonable
cost ($20 for a 5lb pail)

Must
be mixed, batches only last 2-3 hours, excess must be thrown out

Non-Toxic
Cleans up with soap and hot water

Needs
a tight joint (~1/64 inch)

Good
thermal, creep resistance, even in damp environments

Needs
moderately high clamping pressure

Shelf
life 18 – 24 months if stored in cool dry place

Should
be clamped for six hours, full strength in 24

Epoxy

Pros

Cons

Does
not need require tight fitting joints

Expensive
~$40-80 per gallon

Does
well with light clamping pressure.

Must
be measured accurately, mixed thoroughly.

Adaptable
– color, consistency and strength can be tailored for application with
additives.

Hard
to clean up, most use latex gloves

Long
shelf life

Pot
life is 20-30 minutes, excess must be discarded

Should
be clamped for 4-8 hours, full strength in 24 -48

Large
globs of adhesive will clog sand paper.

Cyanoacrylate
(CA) glue

Pros

Cons

Very
strong

Fairly
toxic

Good
Shelf Life

Very
expensive, $3.25 for ½ oz.

Sets
quite rapidly – hold assembly together, no extended clamping
required.  Great for quick repairs

Drips
cure very hard, glue lines must be kept thin to prevent blotches in the
finish

You
can glue your fingers together

Appendix G – New Design Checklist

Aesthetics

Do the lines of the bridge pins and
tuning pins sweep in a nice fair curve?

Ergonomics

1.
Is
the String Spacing reasonable?
For modern gut/nylon strung harp, this will be no closer than
3¼” in the top octave and 4½” in the bottom
octave.   Reproductions
historical harps can be tighter, i.e. the wire strung Clarsach may by much
tighter, as close as 2¾” per octave.

2.
Are their any unusual looking gaps
between the strings (strings too far apart or too close together)

3.
Is the harp’s weight centered
over the feet?   The center of
gravity is usually just about in the center of the triangle formed by the neck,
pillar and sound box.   If the
box is particularly heavy that center may shift an inch or to towards it.  The center of gravity should fall half
way between the tips of the front and back feet.

4.
Are the feet/base wide enough and deep
enough to provide adequate stability for the instrument?

5.
The soundboard width should be less
than 3.5 inches at the treble end to allow the harper to access those strings
without bending the wrist excessively.

6.
Is there adequate clearance to
comfortably get the right hand thumb onto the highest string?

7.
Do the sound holes provide adequate
access for stringing and tightening the internal bolts securing the pillar and
neck?

8.
Are there any big jumps in the
Tension-to-Length ratio for the strings?

Functional

1.
Is there clearance between the Bass string
and the pillar?  When you look at
the profile view, the pillar can go beneath two of the bass strings, but the
pillar should be beyond the bottom ¾ of the bass string.

2.
Are the eyelets adequately sized to
allow the knots on wound strings through them on the bass end?

3.
Does the neck extend below the bridge
pins an adequate distance to mount sharping levers?   I develop a table for each lever
type and check this distance once each octave

For Truitt Levers

String
Length

Distance
required below bridge pin

5

3/4

10

1

19

1-5/8

30

2
-1/4

40

2-3/4

49

3

Structural
Rigidity

1.
Is the neck thick enough? The thickness
required for a stable neck is a function of the harp’s tension, the
distance it spans, and the species of wood used.   Some rules of thumb.  A 36 string harp with 1200lbs of tension
will need a cherry neck that is at least 1½” thick.  A 26 string medium tension harp will
need a cherry neck that is 1¼” thick.   You may be able to use a slightly
thinner neck for lightly strung harps.   Softer woods will require a
thicker neck.

2.
Is the pillar thick enough?  The thickness required for a rigid
pillar is also a function of the harp’s tension, its length, and the
species of wood used.   If in
doubt, use a T-section neck or laminate an extra layer of material to the outer
edge.

3.
The sound board will move up and the
pillar will move down as the harp is brought under tension.   Have appropriate allowances been
made for this movement?   If
the design does not show an outer string rib, the strings will be a bit
shorter, requiring an additional allowance.

4.
If the pillar is not bolted to the
string ribs, is the connecting structure (base, sound box) strong and rigid
enough to transfer the load from the foot of the pillar to the bass end of the
soundboard?  Can it withstand this
strain for an extended period of time?

Acoustics

The acoustics will depend a lot on what
the sound and volume you are trying to achieve.  In general I ask myself,

1.
Do the scantlings of the soundboard and
string ribs match the string tension and soundboard width?  High tension on a lightly built board
can sound good, but may implode.
Conversely a heavy board on a light set of strings will sound dead.

2.
Are any of the strings too long for the
desired pitch (leading to string breakage)?  Too short (poor or dead tone)?

Copyright ©
2007 Rick Kemper

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