Unit 23
LAMINATING AND BENDING WOOD
Curved wood parts can be formed by cutting the
stock to the required shape or by bending, Fig.
23-1. Cutting the curve with machines like the band
saw is often wasteful of material and the part is
seldom as strong as a bent piece. Wood can be bent
either by PLASTICIZING (softening) it with steam
or hot water, or by laminating and then clamping
it in a form for drying and curing. While saw kerfs
are sometimes cut on the concave side, parts pro­
duced by this method have very little strength.
WOOD LAMINATING is the process of forming
parts (usually bent) by attaching two or more layers
of wood together with the grain of each ply running
in the same direction. Plywood is also composed
of layers of wood, but with the grain direction
running at right angles in each successive layer.
Laminated wood is used for the curved parts of fur­
niture, baseball bats, tennis rackets, golf clubs,
boats, structural beams, and many other products.
Fig. 23-1. The graceful curves in the legs of the table, the chair,
and the chair back were formed by bending the parts to shape.
(David Ebner, Courtesy, Pritam and Eames)
393
SELECTING AND CUTTING STOCK
Both hardwoods and softwoods can be laminated
and bent; however, hardwoods have somewhat
better bending characteristics. Hardwood species
commonly used in industry for either solid or
laminated bending include ash, birch, elm, hickory,
maple, oak, sweet gum, and walnut. Laminated
structural members are made from such softwood
species as Douglas fir, southern yellow pine, white
cedar, and redwood. More important than the kind
of wood, is the quality and grain structure of a given
specimen. Straight-grained stock that is free of
knots, splits, checks, and other defects should be
selected.
When a strip of wood is bent, the inside surface
is made shorter (compressed) while the outside sur­
face is stretched. The thicker the stock, the greater
this difference will need to be. Wood fibers are dif­
ficult to stretch, but fairly easy to compress,
therefore the greatest change usually occurs on the
inside surface of the bend. Wood will bend across
the grain or along the grain. Across-the-grain bends
are seldom used in laminated parts; they apply
mainly to curved or formed plywood.
In the school shop, veneers are often used to
produce laminated parts. VENEER is defined as a
thin wood sheet, 1 /8 in. or less in thickness. Wood
over 1 /8 in. thick is simply referred to as STOCK,
and is produced by resawing operations. Standard
hardwood veneer thickness is 1/28 in. Other com­
mon thicknesses include 1 /32, 1 /20, and 1 /16 in.
In general, to save time and material, you should
use layers of the thickest dimension that will still
bend easily to the minimum radius of the part be­
ing produced. When the radius of bend is too sharp
for the layer thickness, it may be necessary to
moisten the surface and preform the wood before
making the final assembly.
Laminating layers can be cut to rough size on the
band saw or jigsaw. If 1/16 in., or thinner, veneers
are used, the cutting can be done with a pair of
Modern Woodworking
heavy scissors or snips as shown in Fig. 23-2.
Cross-grain cuts are easily made; cuts along the
grain will tend to split the veneer. The splitting can
be minimized by dampening the surface. After the
plies are cut to size they should be arranged care­
fully, with the best surfaces on the outside.
APPLYING GLUE
Urea resin glue fills the requirements for most
laminating work done in the school shop. It has a
sufficiently long assembly time, is strong, and stains
the wood only slightly. Casein glue can also be
used. It is less expensive, works best on oily woods,
but has a staining characteristic that may be
undesirable.
Glue can be applied by various methods. Fig.
23-3 shows urea resin being applied rapidly with
a roller. As the layers are coated, they are stacked
in the proper position. When the entire stack or
laminate is coated, it is placed in a plastic (poly­
ethylene film) bag or covered with wax paper. This
is to prevent the excess glue or "squeeze-out" from
touching the form.
Fig. 23-4 shows strips for a tennis racket being
coated with adhesive and then formed in a special
metal press.
Fig. 23-3. Top. Applying urea resin glue with a roller. Bottom.
Placing the stack in a plastic bag.
CLAMPING FORMS AND DEVICES
Fig. 23-5 shows the laminate placed in a form
and pressure being applied with a shop-built press.
A wide variety of forms and clamping devices can
be designed and used. Some are fitted with machine
bolts that are used to apply the pressure. The forms
must be accurately shaped so pressure will be
evenly distributed. Male-female types must be
concentric when spaced for the given laminate.
Alignment pins may be necessary to insure that the
two parts come together correctly.
Surfaces of the form should be smooth and the
curves should be free-flowing (faired). It may be
helpful to use a rubber pad between the form and
the laminate, or to line the surface with light sheet
metal. Wood forms should be finished with a coat
of sealer and paste wax, for easy cleaning and
maintenance, Figs. 23-6 and 23-7.
CURING
Fig. 23-2. Top. Cutting veneer to rough size. Veneer that is
too dry (less than 6 percent moisture content) will splinter and
break excessively. Bottom. Assemble the rough cut layers with
the best surfaces to the outside.
394
Allow the laminate to remain in the form until the
glue has thoroughly set and is almost completely
cured. At room temperature, this will require about
Laminating and Bending Wood
Fig. 23-4. Left. Ash strips for a tennis racket emerge from a glue spreading machine. Right. Glue-coated strips in position around
center form and ready for side clamps (arrow) to be closed. (Wilson Sporting Goods Co.)
24 hours for urea resin glue. If higher temperatures
are applied, this time can be greatly reduced. For
example, urea resin will attain more than 90 percent
of its total strength within one hour at a temperature
of 140 ° F (60 ° C).
Since the glue will raise the moisture content of
the wood, it is usually best to lay the part aside
for
I
several days before performing final shaping and
finishing operations.
CUTTING TO SIZE AND FINISHING
The edges of the laminate can be trimmed and
shaped with various hand tools. The band saw,
jigsaw, or sabre saw can be used if certain precau­
tions are observed. Special jigs and fixtures may be
required to properly support the work.
Surfaces can be smoothed with scrapers, files,
and abrasive paper. Finishing coats can be applied
in the same manner as other wood products.
When trimming the rough edges of a
laminate, you should wear goggles or
safety glasses to protect your eyes from
flying particles of hardened glue and
wood chips.
MOLDED PLYWOOD
Fig. 23-5. Top. Laminate placed in the form. Bottom. Pressure
applied in a shop-built press.
395
Extensive use is made of molded plywood in
furniture construction, especially for chairs and
Modern Woodworking
Fig. 23- 7. Open view of special high frequency press shown
in Fig. 23-4. Bonding and curing of tennis racket has taken
place in less than one minute.
VENEER TO
MATCH KERF
MAKE-UP OF SKI BLANK
C
Fig. 23-6. Clamping forms. A-Machine bolts are used to apply
pressure and align the two parts of the form. Laminate con­
sists of 7 layers of 1 / 16 in. veneer. B- Table leg. Six 1 /8 in.
laminations. C-Water ski. Two saw kerfs cut into the end of
solid stock and then filled with veneer. Stagger the depth of
the cuts and use a resorcinol glue.
commercial seating. It is laid-up like regular plywood
with the grain turned at right angles in alternate
layers, and then pressed in special dies that form
the required curved surfaces. The surfaces can be
formed in a single-curved surface or a double-curved
surface, like the seat and chair back in Fig. 23-8.
Single-curved molded plywood can be produced
in the school shop with wooden forms and stan­
dard clamps. Fig. 23-9 shows a simple pressing
arrangement to form a plywood seat for a TV stool
from layers of 1 /8 in. plywood. Also see Fig. 23-10.
396
Fig. 23-8. Chair seat and back made of molded plywood.
!Herman Miller Inc.)
Laminating and Bending Wood
BENDING WITH KERFS
Thick stock can be bent easier if a series of saw
kerfs is cut on the concave side. This reduces the
size of the surface by removing some of the stock,
rather than compressing it. However, a part pro­
duced in this manner is weak and should be used
only in assemblies where it can be securely attached
to other supporting members. For example, such
a part might be used to form the trim apron of a
tabletop, where the top itself, or other members,
would provide the structural strength required.
Another disadvantage results from the fact that the
saw kerfs may telegraph (show through) on the out­
side surface. This can usually be avoided if the kerfs
are not too deep and if the part is given a coarse
sanding after the forming operation.
The depth, and spacing of the saw kerfs will vary
with the kind of material and the radius of bend. You
will need to experiment with a number of sample
pieces to determine the best solution for your work.
Fig. 23-11 shows a press used to form a part made
from a piece of lumber core plywood. In this par­
ticular piece, a strip of veneer was glued to the
inside surface. In some work the kerfed part could
be attached directly to the structure with glue
blocks set along the inside surface to hold it in place.
Fig. 23-9. Top. Plywood forming press. Bottom. Curved
plywood blank made from five layers of 1/8 in. (3 mm)
plywood. Individual layers of veneer could also be used.
ELECTRICAL PLATEN -----..,.
SWITCHES WITH
PILOT LIGHTS-;;;
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TEMPERATURE
INDICATORS
ELECTRICALLY
HEATED PLATENS
(WATER-COOLED)--\ji!:ialii.:::llal
REMOVABLE
RAM ADAPTOR
PRESSURE GAUGE
K
WATER CONTROL
VALVE
ADJUSTABLE
THERMO SWITCH
CONTROLS
---+-+-
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Fig. 23-10. Small hydraulic press for laboratories and school
shops.
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Fig. 23-11. Top. Forming press. Note sponge rubber pad.
Bottom. Kerfed part, veneer strip, and finished piece.
397
Modern Woodworking
BENDING SOLID STOCK
applied with a metal strap (galvanized iron or
stainless steel) that is equipped with end fittings.
This strap is applied to the convex side of the stock.
As the bend is made, it absorbs the tensile stress,
and the wood cells are subjected only to compres­
sion forces. See Fig. 23-13.
The work must be held in the clamps until it has
cooled and dried, or SET. On heavy work this may
take several days, even in specially heated drying
rooms. In production work, bent parts are removed
from the bending apparatus soon after they have
cooled and are then placed in special retaining
clamps to hold them in position until the set is com­
plete. The amount of time or drying that is
necessary to set a bend varies with kinds of wood
and types of work.
Curved parts can be formed from solid stock by
plasticizing the wood with moisture and heat, then
bending and clamping the wood in the required
shape until it cools and dries. This process is used
extensively in industry, Fig. 23-12. It is not easily
adaptable, however, to work in the school shop,
except for small parts.
Stock for solid bending should be selected with
about the same consideration as for laminated work.
Air-dried stock with a moisture content of 12 to 20
percent will work best. Machine the stock to size,
providing allowances for shrinkage and finishing.
The ends should be seal-coated to prevent ex­
cessive absorption of moisture during the steam­
ing process, and to minimize end checking during
the drying and fixing process. For severe bends, cut
the stock so that the annual rings are perpendicular
to the plane of the bend.
The length of time required for steaming or boil­
ing the wood varies with the kind of wood, initial
moisture content (M.C.), thickness of the stock, and
the degree of curvature required. In general, most
wood will need to be steamed or boiled for about
one hour for each inch of thickness. Plasticizing with
steam, at or near atmospheric pressure, until the
M.C. of the wood reaches 20 to 25 percent, will
normally produce satisfactory results.
There are two broad classes of bends: those
made with end pressure, and those made without
end pressure (free bends). Free bending is feasible
only for slight curvatures. Bending with end
pressure causes the wood fibers to be properly
compressed on the inside of the bend and reduces
tensile failures on the outside surfaces. Also there
is less tendency of "spring-back" after the work
is removed from the clamps. End pressure is usually
STRUCTURAL LAMINATES
The wood laminating process is applied to a wide
range of structural members used in large buildings
where it is necessary to have clear space, unob­
structed by supports. Laminated construction
allows the architect a wide latitude in creating forms
adapted to and expressive of the function and
purpose of the structure. In addition, laminated
construction greatly extends the use of wood-the
most abundant, beautiful, and economical building
material available. Fig. 23-14 shows curved arches,
formed by the laminating process and incorporated
into a dignified, functional design.
In addition to flexibility in design, wood beam
construction also provides a high fire resistance
factor. Wood beams do not transmit heat like un­
protected metal beams, which lose their strength
and quickly collapse under extremely high tempera-
Fig. 23-12. Bending solid wood furniture parts in special metal
clamps.
398
Fig. 23-13. Steam bent white oak strip. Note the smooth curve
with little or no distortion; this is due to compression.
Fig. 23-1 4. The curved support beams of this modern struc­
ture are laminated beams. They provide great strength and
freedom of design. (Potlatch Corp.)
tures. Exposure of a wood beam to flame results
in a very slow loss in its strength. It is weakened
only in proportion to the slow reduction in cross
section due to charring. This takes place slowly and
thus provides precious time that may save life and
material, Fig. 23-1 5.
Laminated beams and arches must be carefully
designed so that they will provide the strength
required. Fig. 23-16 shows a sample arch being
submitted to an extensive series of tests. Data
gathered from these tests will be compiled and used
in future design problems. The parts and general
design of a typical V arch are shown in Fig. 23-17.
Fig. 23-16. Scientific testing of a laminated arch design.
(Forest Products Laboratory)
12
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Fig. 23-15. This photo, taken after a fire, shows wood beam
supporting twisted steel I beams. (Forest Products Laboratory)
399
----4-
Fig. 23-17. Parts of a laminated V arch. (Unit Structures, Inc.)
Modern Woodworking
Most laminated structural members are made of
softwoods. They are manufactured in industrial
plants specializing in such production and then
shipped, prefinished and ready for erection, to the
building site.
Figs. 23-18 to 23-22 show in-plant views of the
fabrication of beams and arches. Lumber is carefully
selected and machined to size. To secure the re­
quired length, pieces must be end-jointed. Since end
grain is hard to joint, a finger joint is used. In large
laminates, a number of these joints may be required
in each ply. They are always staggered at least 2
ft. from a similar joint in an adjacent layer.
Waterproof adhesives are applied and the layers
are then clamped to forms. Because of the size of
the units, it is seldom practical to utilize heat in the
curing process. After the beam or arch has cured,
the edges and faces are machined to size. Today,
many of the beams and arches are finished in the
factory to specifications that will match the interior
of the completed building. Prefinished units are
carefully wrapped and handled so they will arrive
at the construction site free of damage.
Fig. 23-18. Top. Assembling laminations for a straight beam
65 ft. long. Bottom. Finger joint used to join ends of lamina­
tions. Individual laminations should not exceed 2 in. in net
thickness. (American Institute of Timber Construction)
Fig. 23-19. Gluing a laminated arch in a special clamping
device. (Forest Products Laboratory)
Fig. 23-20. Final sanding and inspection of giant laminated
arches. After coats of finish are applied, they will be wrapped
in a waterproof covering for shipment to the construction site.
(Weyerhaeuser Co.)
400
Laminating and Bending Wood
Fig. 23-21. Laminated beams provide a clear span of 48 ft.
Purlins, spaced at 8 ft., will support 4 x 8 ft. prefabricated roof
panels. Note the metal connections used to fasten the purlins
to the beams. (Boise-Cascade Corp.)
Fig. 23-22. Construction worker attaches beam brackets to
steel drum. The drum serves as a hub for assembling major
beams. (Koppers Co., Inc.)
TEST YOUR KNOWLEDGE, Unit 23
Please do not write in the text. Place your
answers on a separate sheet of paper.
1 . When a piece of wood is bent, the outside
surface is stretched and the inside surface is
2. Veneer is generally defined as thi sheets of
wood under
in. in thickness.
3. Why is polyvinyl glue not sat�factory for
laminating work?
,J/!
.
4. When making a curved male-female type form
for laminations, not only the curve but the
____ of the lamination must be considered.
5. The spacing and depth of kerfs for bending solid
stock will vary with the kind of wood and
____ of the bend.
6. Which type of stock is preferrable for steam
bending?
7. The length of time required for steaming wood
for bending will vary with the kind of wood, initial M.C., radius of curve, and _____
8. Wood beam construction provides flexibility in
401
design and a high ________ factor.
ACTIVITIES
1 . Prepare a proposal for experimental work in
laminating, using thin hardboard core stock and
veneer or plastic laminates for the surface
layers. Suggest articles or projects for which this
type of laminate would be appropriate.
2. Develop a design for a heated laminating form
that could be used in the school shop. Consider
the use of a heating pad or the heating element
of a discarded electric iron.
3. Secure descriptive folders from a company that
manufactures laminated beams, arches, and
trusses. Your local lumber dealer may be able
to furnish addresses. Prepare a written report
describing the various types, uses, design data,
and finish. Also include information concerning
appropriate roof decking materials, and methods
of application.