Rochester Institute of Technology
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Theses
Thesis/Dissertation Collections
11-1-1978
Improved Methods of Plywood Carcass
Construction
George Berry
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Recommended Citation
COLLEGE
OF
FINE
AND
APPLIED
ARTS
SCHOOL
FOR AMERICAN
CRAFTSMEN
ROCHESTER
INSTITUTE
OF
TECHNOLOGYTITLE:
IMPROVED
METHODS
OF
PLYWOOD
CARCASS
CONSTRUCTION
SUBMITTED
BY:
GEORGE
W.
BERRYDATE:
THESIS
COMMITTEEChief
Advisor:Mr.
Douglas
SiglerAssociate Advisors:
Dr.
Robert SnyderThe
purpose ofthis
thesis
is
to
devise
plywood
joining
techniques
superiorto
those
in
use
today.
I
hope
to
addto
the
body
ofknowl
edge
surrounding
plywood as a major elementin
furniture
carcass construction. Iintend
to
make sample plywood carcass
joints,
test
them
in
the
Engineering
Department's strength ofmaterials
lab,
and usethe
information
to
build
DEDICATION
This
thesisis
dedicated
to"you;
to
anyone who takes
the
time to pullif
offthe
Rochester
Institute
ofTechnology,
there
waslittle
work
being
done
withflat
panel plywood;probably
because
plywoodhas
developed
abad
name. Inthe
40
's
and 50' sit
waslooked
on as acheap
substitutefor
real(solid)
wood.The
very
word veneeritself
still carries with
it
the
connotation ofcheap,
counterfeit or
structurally
unsound,mostly
due
to
industry's
tendency
to
balance
quality
againstprofit,
withthe
scales almostinvariably
tipped
to
the
side ofthe
dollar.
The
resultin
many
instances
wasthat
plywood's reputation wasdeserved.
The
mention of plywood stillbrings
to
many
mindsthe
picture offailing
glue andde-laminating
veneers;literally
woodthat
is
falling
apart.
The
advancesin
adhesivetechnology
brought
on
by
World
War II andputting
a man onthe
moonshould
have
laid
this
visionto
rest.I
wouldlike
to
quotefrom M.
L.
Selbo's
Adhesive
Bonding
ofWood
(p.
16,
Drake,
N.
Y.,
78;
originally
publishedby
the
Department
ofAgriculture
-2-The
durability
ofmoderately
alkaline phenol
resin,
resorcinol resinand phenol-resorcinol resin adhesives
is
essentially
similar. When these glues areproperly
used,
they
are capable ofproducing
joints
that
are about asdurable
asthe
wooditself
under various severe service conditionsstudied.
Properly
madejoints
willwithstand,
without significantdelam-ination
orloss
in
strength,
prolonged exposuresto
cold andhot
water,to
alternatesoaking
anddrying,
to
temperatures
up
to
those
that
seriously
damage
the
wood,
to
high
relativehumidities
wheremany
untreated speciesdecay,
andto
outdoorweathering
without protectionfrom
the
elements.The
joints
between
lumber
lami
nations or plies of plywood made withthese
adhesives will not separate whenexposed
to
fire.
The
glues are not weakenedby
fungi,
bacteria
or othermicro-organisms and are avoided
by
termites.
These
adhesives,
however,
do
not offerany
significant protectionto
the
adjacent wood.Consequently,
woodproducts glued with these adhesives
should
be
considered no moredecay
orinsect
resistant than solid woods ofthe
same species.Even
the
most avid solid wood craftsmanfinds
himself
using
plywood onflat
panels where movementand mass
to
strength ratios areimportant,
especially
if
only
one surfaceis
seen,
e.g.drawer
bottoms
andcabinet
backs,
not so muchbecause
plywoodis
less
expensive
(which
it
is)
,but
because
in
these
wood.
Artists
and craftsmen share atendency
to
command
material,
i.e.
to
have
the
capability
ofexercising
as much control as possible.For
the
woodworker
this
wouldbe
using
wood whichhe
has
felled,
milled anddried
himself.
This
is
certainly
the
mostgratifying
way
of working, andthe
only
way
ofexercising
any
control overthe
grainpattern.
When
using
plywood,
this
is
notthe
case.The
craftsmanis
more orless
forced
totake
the
material as
it
is.
Evenif
he
veneershis
ownplywood,
he
musttake
the
veneer asit
is.
This
loss
of control(dependency
onindustry)
simply
discourages
most craftsmen and preventsthem
from
utilizing
the
material withthe
same passion andinsight
they
arewilling
to
devote
to
solid wood(and
familiar
processes) . Iimagine
that
we wouldsee more
hand-crafted
plywoodfurniture
if
craftsmenhad
joining
techniques equalin
strengthto
those
ofsolid wood.
Plywood presents craftsmen with
many
otherproblems.
Among
these
are:1.
Withthe
inception
of stackedplywood,
-4-in
the
finished
piece offurniture.
2.
The
quality
ofthe
material variesgreatly
with
the
manufacturer.3.
Deep
interior
defects
in
core stock cannotbe
detected
untilthey
produce unwanted results.4.
If panels are not storedflat,
they
willgradually
warp.5.
Interior
plywoodaccidently
exposedto
exterior conditions
is
quickly
rendered useless.6.
Although
plywoodis
manufactured as amaterial of uniform
thickness,
changesin
humidity
affectthe
edgesbefore
the
center,usually
producing
a paneldomed
like
abar
ofsoap,
i.e.
slightly
thicker
in
the
middle.7.
Plywood
furniture
is
difficult
for
acraftsman
to
marketbecause
of plywood' sbastard
child reputation.If plywood can
be
thought
of as a material onits
own merits and not as a substitutefor
"real
wood",
then
several advantagesbecome
apparent:1.
Working
withit
is
less
time
consuming
because
the material comes ready-to-usein
awide range of sizes and
thicknesses.
and surface
defects
so commonin
solid woodare eliminated.
3.
The
strength of plywoodis
only
mildly
affected
by
graindirection
while solid woodis
relatively
strong
withthe
grain and muchweaker across
the
grain.4
.Changing
humidity
only
mildly
affectsplywood.
5.
Because
plywoodis
a manufactured materialand not a processed commodity,
it
is
readily
available,
i.e.
you canusually
just
go outand purchase
it
atany
lumber
yard.6
.The
use of plywoodactually
conserves ourwood supply.
There
is
more wastein
converting
a
log
into
lumber
andthen
into
furniture
(certainly
case goods)than
if
plywood werethe
intermediate
step.7.
Veneers canbe
matchedto
producestriking
results.
8
. Thebeauty
of a woodin
a giventree
canbe
seen
by
more people.9.
Plywoodconstruction,
evenin
high
quality
-6-While
planning my
early
graduate work atRIT,
I was approached with a commission
for
ten
large
(4
ft.
x8
ft.)
wallmounted,
glassfront
plywooddisplay
cases.Because
ofthe
difficulty
ofmaking
strong
plywood partitionjoints,
there
was somequestion
in my
mind asto
how many
partitionsit
would
take
to
support a150
lb.
safety
plate glassfront.
The
only
information
I
couldfind
on shortnotice
(from
industry)
wasthat
it
was reasonableto
expect a plywood
dado
to
support100
lbs.
perinch.
But
I
suspectedthese
figures.
I
wascertainly
dealing
with measurements made underlaboratory
conditions and
the
very
way
the
test
was structured(distributing
the
load
uniformly
along
the
length
ofthe
joint)
seemed meaningless when appliedto
my
design
problem ofsupporting
the
glassfront.
Iconcluded
that
thesefigures
werebiased
by
industry
to
insure
the
highest
possible result.I
decided
to
make some tests ofmy
own.I
madefour
test
samplesfrom
3/4" nineply
fir
coreluan
plywood;
two
dados
(one
nailed with sixpenny
cementcoated
box
nails and the other with six 3/8"dowels
on
opposing
20
degree
angles) ; andtwo
sliding
dove
tails
(one
3/8"to
experiment under conditionsthat
would approximatethe
stresses encounteredby
plywood carcasses.Therefore,
Idesigned
atest
that
wouldrip
a 12"long
dado
apartfrom
the
outside edge(Fig.
1
&2)
.With
one coat ofFranklin
titebond
yellowglue on each
surface,
the
naileddado
held
208
lbs.
before
it
snapped.The
3/8"deep
dovetail
held
250
lbs.,
the
5/8"deep
dovetail
held
299
lbs.
andthe
3/8"dowels
were strongest at331
lbs.
I was
intrigued
to
find
thatthe
dowels
were strongerthan
the
dovetails,
soI
tried
many
othermechanical solutions.
Among
these
were:different
angles of
dovetails,
bigger
dowels,
glueblocks,
double
rabbits,
locking joints,
endkerfs,
andfinally
a 3"
mending
plate(a
steelstrap
withfour
3"number
eight wood screws on
opposing
20 angles) .I
alsotried
kerfing
andv-grooving
to
give the uprighttooth
(something
for
the
glueto
grip) .I
tested
sixteen samples andfound
most ofthe
results around300
lbs.
However,
some werevery
interesting.
If
appearanceis
noproblem,
i.e.
whenthe
carcassis
only
seenfrom
the
inside,
the
mending
plate was strongest at
762
lbs.
Although
the
dowels
1-z
o
<
a:
<
Q.
w
make a nice
detail
and arevery
strong.The
outsidecan
be
kept
unbrokenby
using
short (1/4"deep)
band
saw
kerfs
in
the
end ofthe
upright.The
endkerfs
appeared
to
be
strongerthan
dovetails.
As
long
asI
wastesting joints,
I
thought
someinvestigation
of glueline
thickness wouldbe
reasonable. I made
three
more samples: one withthe
dado
sotight
that
I
had
to
drive
it
home
witha
hammer,
onesnug,
andthe
other quiteloose
(.016").
I was
very
surprisedto
find
the
loose
dado
over100
lbs.
strongerthan
the
tightest
one. Becauseit
is
desirable
to
test
varying
gluethickness
without
having
glue squeeze out ofthe
dado
(this
could
lead
to
assembly
problems) , I made seventeensamples,
keeping
the
dado
constant andvarying
the
thickness
of the uprightby
cutting
shouldersslightly
less
than
thedepth
ofthe
dado
on each side ofthe
upright. I
didn't
wantto
increase
the
strengthof the
joint
in
onedirection
and weakenit
in
another.
I
therefore madefourteen
additionalsamples
to
testfor
rigidity,i.e.
pushing
laterally
on the upright
(Fig.
3
&4)
.The glue
line
varied a greatdeal
(from
ILI
1->
Q
(J)
much
(from
367
lbs.
to
240
lbs).
After
re-evaluatingthe
experiment,
I
decided
that
it
had
been
structuredpoorly.
When
the
glueline
wasincreased,
the
gluedarea of
the
joint
decreased.
I made eight more samples. This
time
I
variedthe
width ofthe
dado.
I wantedto
repeatthe
original
test,
soI
tried
to
cutdados
that
fell
into
the
broad
categories oftight,
medium andloose.
I
alsotested
abutt
joint
(no
dado)
. Inthe
separation
half
ofthe
experiment,
the mediumdado
(loose
by
.010") was strongest.However,
in
rigidity,the
tight
dado
wasstrongest,
but
the
medium samplewas clamped
improperly
and could notbe
tested.
Eight
more samples weremade,
this
time
in
oakplywood.
The
mediumdado
(.011")
was strongestin
both
separation and rigidity.Although these
findings
arevery
crude andare
far
from
complete, I can see adefinite
indication
that
allowancefor
optium glueline
thickness
(under
for
Franklintitebond)
shouldbe
considered whendesigning
plywoodjoinery,
certainly
if
tensile
strength
is
theonly
consideration.By
optimizing
the
following
factors,
it
would
-10-into
a 3/4"by
12"dado
could support over a1000
pound
load
onits
outside edge withoutresorting
to
metalhardware:
1.
Glue
type,
its
method of application andthickness
2.
Depth
ofdado
3.
Size,
spacing
and angle ofdowels
4.
Size,
spacing
and angle of end-kerfs5.
Plywood
construction(lumber
core, uniformlaminate
orhardwood
inner
ply)My
experimenting
with plywoodhad
given me arespect
for
the
material and someintuitive
responseto
afew
jointing
techniques.
So,
when anothercommission came
along
for
an oak plywood stereocabinet, I
eagerly
acceptedit.
I wantedto
explore
the
possibilities ofcreating
strong
carcasscorner
joints
in
plywood case goods.It
seemedto
me that a
bending
technique would yieldthe
strongestjoint
because
the materialis
notfirst
separated andthen rejoined.
I
tried
thinning,
abending
processwhere enough material
is
removedin
the
area ofthe
bend
to
allow the plywoodto
bend
withoutbreaking,
it
flexible.
The
removed material mustbe
replaced,however,
with a shaped glueblock
correspondingto
the
curve ofthe
bend.
I
thinned
a piece of 3/4" oak plywoodto
3/32",
wet
it,
bent
it
90and
inserted
a piece of 3/4"quarter round
fir
moulding. This produced a roundedcorner about 7/8"
in
radius.A
7/8" radius was astight
as I wantedto
risk.For
longer
radii, thinning
creates problemsdue
to
the
logistics
ofremoving
suchlarge
amounts of wood andreplacing
it
with a
large
glueblock.
Itis
possiblethat
very
large
glueblocks,
2" or morein
radius, mightswell enough with changes
in
humidity
to
crackthe
veneer.
I
began
experimenting
withkerf
bending
(some
times
called"kerf
curving") . Inthis
process anumber of
deep
sideby
side cuts are madein
the
back
of a panelgiving
the
face
flexibility.
After
several attempts
I
found
that
sevenkerfs
1/8"wide
by
41/64"deep,
spaced 3/16" apart produced a1
1/4"radius
bend.
I
triedcutting
trapazoid
wedgesto
fit
in
the
kerfs
after the panel wasbent.
They
turned
-12-yellow and white glue with
sawdust,
planer shavingsand a combination of
dust
and shavings andpacking
it
into
the
kerfs
before
bending.
It wasreadily
apparent when
evaluating
the
resultsthat
I wastrying
to
strengthenthe
bend
in
successivesections
(a
chain,
which of course,broke
atits
weakest
link)
.Next
I
tried
reinforcing
the
bend
with a shaped glue
block,
but
because
the
inside
of a
kerf
bend
is
facetted,
the
glueblock
reinforced
joint
was notvery
strong.I
thentried
astrip
of canvas soakedin
yellow gluebecause
it
would conformto
thefacets.
I was amazedto
find
that
the
canvas and glue reinforcedbend
wouldsupport
my
weight(180
lbs.)
I wanted
to
use whatI
had
learned
in
one pieceof
furniture.
So,
in
the
oak plywood stereocommission,
I
thinned
the carcass cornersto
7/8"radius and
kerf
bent
the
counter edgeto
produce a2"
radius
(Fig.
5)
.I was
then
offered a commissionto
design
andbuild
a portabletyping
standthat
would concealthe
typewriter when not
in
use(Fig.
6
&7)
.I
liked
the
softline
thatkerf
bending
produced
-13-with
it,
but
I
also wantedto
develop
bending
techniques
for
other materialsto
usein
conjunctionwith
kerf
bent
plywood,
to
give me moredesign
options.
I
began
experimenting
withbending
formica.
I
used astrip
heater
andbent
a piece of 1/16"formica
20"
long.
Although
the
bend
was notcompletely
successful,
the
results were encouraging.After
experimenting
withthe
three
thicknessesof
formica
and varioustemperature
ranges, I was ableto
producetwo
consecutive 1" radius,3
6"long
bends
in
1/20"heat
formable
formica
by
raising
it
rapidly
(10
sec.)
to
300F.
(Fig.
8).
To
coverthe
edges,
I used 1/32" vertical gradeformica
which willconform
to
rathertight
radii(2")
in
thin
stripswithout
heat
forming
(Fig.
9)
.Since
the
pedestal,
which concealsthe
typewriter
when not
in
use,is
opento
the
front,
it
wasnecessary
to
skin theinside.
This requiredkerf
bending
aninside
corner. Inthis
processthe
kerfs
open
instead
of close whenthe
panelis
bent.
Kerf
bending
aninside
corneris
always moredifficult
than the outside corner
because
the
radiusis
smallerand
it
seemsto
be
easierto
stretchthe
face
veneercompress
them
in
aninside
cornerbend.
Cutting
thekerf
slightly
deeper
seemedto
help.
At
roughly
the
sametime
I wasworking
on thetyping
stand,
I
was given a problemin
my
Environmental
Design
minorto
design
a stereo cabinet,Naturally,
Ikerf
bent
it.
Thistime
I
used walnutplywood.
Since
walnut plywoodis
relatively
expensive,
Idesigned
the
cabinetfrom
one-half asheet (4' x
4").
Experimenting
further
with othermaterials,
I
used smoked plexiglassfronts
withclear plexiglass spacers
between
the
turntable,
amplifier and cassette player modules
(Fig.
10
&11)
.I
was pleased withthe
appearance ofthe
walnutstereo modules, so
I
decided
to
do
another walnutstereo,
this
timeto
include
speakers,
recordstorage and cassette storage. I used one sheet
of walnut plywood and 1/4"
black
styrenefronts
andspacers.
To
takekerf
bending
astep
further,
Iused
it
as apurely
visual element. I"golden-sectioned"
the
side ofevery
module with onekerf
(Fig.
12,
13
&14)
.I
tried afew
moretests,
kerfing
on achanging
angle
to
create a partial cone.I
rolled an edge
-15-have
rolledthe
counter's edge180
degrees
onthe
first
oak stereo cabinet.I
also experimentedwith a
fiberglass
reinforcedbend.
The result was anextremely
strong
corner,
much strongerthan
canvassoaked
in
yellow glue.This
fiberglass
reinforced corner wasstrong
enough
to
standby itself,
i.e.
abroken
loop
withno
front,
back
orbottom.
I usedit
for
a coffeetable.
I ripped a sheet of oak plywoodkerf
bent
one-half with an
inside
bend,
held
it
in
place,
layed
up
ten
ouncefiberglass
cloth with a specialepoxy
developed
for
the
boat
building industry
andsandwiched
it
in
between
the
other outsidekerf
bent
half
(Fig.
15,
16,
&17).
If
the
kerfs
are notdeep
enough the panel willbreak
whenbending.
Ifthe
kerfs
aretoo
deep,
they
will show
through,
anunhappy
event calledtelegraphing,
Oak
appearsto
resisttelegraphing
morethan
walnut.It seems that woods which steam
bend
well are morelikely
to
sufferfrom
this problem.My
previouskerf
bent
modular pieces seemedto
have
a machine madelook,
or a coldnessdue
to
the
flatness of the
fronts
andthe
fact
that
they
wereH
*
piece,
a modular armoire. I usedkerf
bent
inside
and
out,
fiberglass
reinforcedwalnut plywood
carcass,
and a compoundcurved,
vacuumformed,
Brazillian
rosewood veneeredfront.
I wanted
the
grain andthe
curves ofthe
front
to
matchthroughout
the
three modules,
sothe
vacuumform
moldhad
to
be
aslarge
asthe
front
(36"x
78")
.After
severalattempts,
I was successfulin
molding
four
layers
of 1/8"Italian
bending
poplar whichproduced a 1/2" compound curved panel veneered with
book
matchedBrazillian
rosewood.The
top
module,a sweater
chest,
waslined
in
upholstery
leather
with an aromatic cedar veneered partition.
The
centermodule
housed
ashelf,
sixleather
lined,
self-closing
drawers
andtwo
shallowjewelry
drawers
that
would remain open.
All
sections were of uniformlaminate
Balticbirch.
Alight
comes on whenthe
mirror
backed
doors
are opened.Opening
the
doors
also exposes
the
kerfs
in
the
carcass corners. Ifilled
them withblack
epoxy
(epoxy
mixed with carbonblack)
. Thebottom
modulehad
threedeep
drawers
onfull
extension(150
lb.)
drawer
slides(Fig.
18
&19).
The
carcasses were 24"deep;
thedeepest
kerf
bent
-17-The
longer
the
bend,
the
moreimportant
the
accuracy
ofthe
cut.A
piece of plywood 24" widecould
vary
in
thickness
.020", one edgecoming
from
the
thick
center of a sheet.This
variationin
thickness,
whentransferred
to
the
bottom
ofthe
kerf,
can cause severetelegraphing.
It wasnecessary
to
design
ajig
to
support a skill sawpassing
overthe
panel. Inthis
way,
the
depth
of cut could
be
controlled regardless ofthe
panel's
thickness.
I alsofound
that
by
making
the
saw
blade
wobble(tilted
onits
axis of rotation) ,producing
a wider and moreflat
bottom
kerf,
tele
graphing
couldbe
virtually
eliminated.Some
telegraphing
may
be
inherent
in
cutting
flat
bottom
kerfs,
but
it
canbe
controlledto
theextent
that
it
canonly
be
detected
by
atrained
eye,
and thenonly
on closeinspection.
In conclusion,
kerf
bending
affordsthe
home
craftsman or smallshop
owner an economicway
of
bending
wood,because
it
is
notnecessary
to
build
a mold. The panel can
be
kerfed,
bent,
f
iberglassed,
and
simply
held
in
place whilethe
fiberglass
hardens,
It
is
oftenimpractical
to
build
a mold which willbe
I
hope
to
develop
lighter
and stronger plywoodcarcasses
by
increasing
the
amount offiberglass
and
decreasing
the
thickness
ofthe
plywood,i.e.
1/8"of
fiberglass
sandwichedbetween
two
layers
ofkerf
bent
1/4"plywood.
My
future
research willbe
aimed atcombining
the
strength anddurability
offiberglass
with