Structure of Wood - Marlington Local

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Transcript Structure of Wood - Marlington Local

Structure of Wood
• Composed of small cellulose fiber units
called Cells.
• Held together with natural adhesive called
Lignin.
• New wood is formed in the Cambium Layer
(near the bark)
• Springwood is early growth with larger and
lighter colored fibers.
• Summerwood grows later and is darker and
stronger.
• Both of these growth bands form the Annual
Rings.
• Age of tree can be determined by # of annual
growth rings
• Wood near bark is called Sapwood which
contains living cells
• As tree grows sapwood becomes inactive and
changes into Heartwood.
• Heartwood is usually darker in color due
to presence of gums and resins.
• Medullary rays run perpendicular to
annual growth rings and carry sap toward
pith (center) of tree.
• These are large in Oak, Beech and
Sycamore but small in most other woods.
Classification of Wood
• Two main classes of wood –
Softwoods & Hardwoods
• Softwood trees have needles
and produce cones. Also called
conifers. Examples are pine, fir,
spruce, redwood, cedar and
cypress.
• Hardwood trees have broad
leaves which are shed each year.
Also called deciduous.
Examples are oak, walnut, birch,
maple, hickory, ash and poplar.
Lumbering
• Two methods of harvesting trees –
Clear Cutting & Selective Cutting
• Clear cutting – all trees in an area
are cut at one time. Seedlings are
planted to replace cut trees. Used
mainly for softwoods because of
their rapid growth.
• Selective Cutting – used mainly
for hardwood. More expensive
process but less disruption to
ecology.
Steps in Lumber Manufacturing
• Logs stored in piles or ponds.
• Pulled into mill by a chain device
called a Jack-ladder.
• Log is washed with jets of water.
• Bark is removed.
• The Sawyer determines best way to
cut log and cuts it into a slab with
the head saw.
• Slab is then cut into desired lengths
by Trimmer saws.
• Other saws are used to cut boards to
desired widths.
Two most common methods of
cutting logs into lumber
• Plain or flat sawing – blade
cuts tangent to annual rings.
• Most economical – low waste,
fast.
• Quarter Sawing – cuts parallel
to the wood rays.
• More wasteful & time
consuming.
• Wood warps and checks less.
• Good for furniture making
Moisture Content
• Wood must be seasoned (dried).
• Greenwood (unseasoned) may have
30% to 200% moisture content (MC)
• Hardwoods are usually dried to 6% or
12%.
• Softwoods are usually dried to about
12%.
• Air drying is natural way of drying
wood outdoors (inexpensive but time
consuming). More prone to warping
and twisting.
• Kiln drying more controlled for
specific results.
• Kilns range in temperature from 120
to 130 degrees F.
Defects
• Two major classes of defects,
natural & defects after felling the
tree.
• Natural includes abrasions, fire
damage, growth defects, insect and
animal damage.
• Growth defects include knots,
shakes, and pitch pockets.
• Knots are result of branches &
limbs.
• Shakes are grain separation parallel
to the rings.
• Pitch pockets occur along the rings
and usually contain resin. (common
in fir, pines & spruces.
Grading
• Standards vary for softwoods &
hardwoods
• Softwoods are divided into 3
classifications, yard lumber, factory
lumber, and shop lumber.
• Yard lumber is divided into select
(finish) and common (utility)
grades.
• Select grades range from A thru D,
are used for siding, partitions, &
finish flooring.
• Common grades range 1 thru 5 are
used mainly as structural lumber.
• Softwoods are usually surfaced on
4 sides to finish dimensions and are
graded after surfacing.
Grading
• Hardwoods are graded
according to minimum sizes
and percentage of clear
surface cuttings that can be
made.
• First & Seconds is the best
grade for hardwoods.
• No. 1 common is lowest grade
for hardwoods.
• Hardwoods are normally sold
in rough form in random
widths and lengths. (yields
more lumber this way)
Veneer & Plywood
• Veneer – a thin piece of wood
sliced from a log 1/100 to ¼
inch thick.
• Three methods can be used to
slice veneer, rotary cut, flat
slice or quarter slice.
• Rotary yields a rippled grain
pattern.
• Flat yields more even grain –
is used for hardwoods.
• Quarter yields striped grain
pattern.
Plywood
• Made of veneers bonded to core
of composites, solid lumber or
cross veneers.
• Cross veneers (plycore) consists
of an odd # of layers, 3, 5, 7 etc.
• Five grades of veneers used in
construction grade (soft)
plywood – A, B, C, D. A = best,
B = worst.
• Grade will be stamped on worst
side using 2 letters. 1st. Letter
represents best side, 2nd Letter
represents the worst side.
• Glue type must be considered
during selection.
OTHER PANEL TYPES
• Usually made from scraps of wood
which saves money and resources.
• Are dimensionally stable, not
affected by humidity
• Types of panels are:
–
–
–
–
Hardboard
Particle board
Wafer board
Oriented strand board.
– All are manufactured with various glues
and resins giving them great stability
and strength.