he
mechanical properties are those that affect the performance of the wood
species and the ease with which you can work it. Is the wood
sufficiently hard, strong, and stable for your project? Can you cut,
assemble, and finish it easily? To answer these questions, refer to the
appropriate categories in the chart of “Mechanical Properties of Wood”
below. This chart includes the following categories.
SPECIFIC
GRAVITY
Although
specific gravityis technically a physical
property, it’s usually included with mechanical properties. It’s a
reasonably reliable indicator of the wood strength and ease with which
it can be worked. Woods with high specific gravities are generally
stronger and more difficult to work.
STRENGTH Because numbers so often
make little sense when considered by themselves, I've given each species
a simple comparative rating in these strength categories so you
can instantly understand where it falls in the entire range. If you need
the actual numbers, see the chart of "Relative
Wood Strengths."
The
hardnessis
the ability of the wood surface to resist damage. This is an
important consideration when choosing woods for projects that will
be subject to abrasion or heavy use. Ratings: Very Hard, Hard,
Medium, Soft, Very Soft.
Strengthis the sum
of compressive strength and stiffness. These determine the amount of
weight a piece of wood will safely support. Ratings: Very Strong,
Strong, Medium, Weak, Very Weak
"Bendability" or
bending strength
indicates both the ease with which a wood can be bent and the
percentage of its strength it retains after bending. Ratings: Very
High, High, Medium, Low, Very Low.
This is a simple "three-point" strength test for wood which, as it is
set up, measures both the modulus of elasticity and the modulus of
rupture for the species "greenheart."
This test was performed at the University of Cambridge in England.
MOVEMENT AND
STABILITY
Wood changes
dimensiontangentially (across flat
grain) and radially (across quarter grain). This movement is measured by
determining the percentage that wood shrinks from when it’s green
(freshly cut) to completely dry. The lower the percentage, the less the
wood moves and the more stable it is. Additionally, a big difference
between tangential and radial movement indicates that the wood is
susceptible to warping, twisting, and bowing.
WORKING
CHARACTERISTICS
Before buying a wood species that you’ve never worked before, it helps
to know how difficult it is to surface the wood with a hand plane or a
thickness planer. Does it lend itself to strong glue joints? Is it
incompatible with common finishes? These working characteristics depend
on many factors. If the wood is extremely dense, it’s harder to cut.
Irregular grain patterns make planning more difficult. The cell
structure of some woods makes them prone to splitting and chipping. Waxy
or resinous extractives interfere with cutting, gluing, and finishing.
The chart of “Mechanical Properties of Wood” rates the following:
The relative ease of
working a wood species with hand tools
The ease of working it
with power tools
The tenacity with
which common wood glues hold it together
The facility with
which it accepts a finish
COMMON USES
Finally, nothing helps so much in deciding whether or not to use a wood
species for a project as knowing how other craftsmen use the same wood.
In yellow birch (top), there is a large
discrepancy between the tangential movement (8.1%) and radial movement
(3.6%). Consequently, the species is prone to warping, twisting, and
bowing. In Honduran mahogany (bottom), on the other hand, the
discrepancy is very small (4.1% versus 3.0%). This wood is much more
likely to remain flat. (modulus of elasticity). This is a concern when
the wood must withstand high amounts of stress.*
Biodiversity
–
Many useful wood species, especially those from
fragile environments such as rainforests, are severely depleted from
overuse and poor management. These are specified as “Endangered”
in the Comments section of the chart of “Mechanical Properties of
Wood.” This designation is determined by CITES (Council on International
Trade in Endangered Species), the result of an international agreement
an the conservation of species that began in 1973. You can search this
list at:
CITES
International Environment House
11 Chemin des Anémones
CH-1219 Châtelaine, Geneva
Switzerland
Sometimes you get
lucky. Although there are 28,000 species of fauna on the CITES
endangered list and as many as 970 species of trees thought to be headed
for extinction, occasionally we get some good news. This is a Judean
Date, Phoenix dactylifera, a long-extinct palm tree important in
Biblical times as a source of food and medicine (see Psalms 92:12). The species was
revived, at least for a short time, from a 2000-year-old seed found at
the archaeological dig in Masada, Israel.
USING THE CHART
The chart of “Mechanical Properties of Wood” is divided into three parts
-- North American Hardwoods, North American Softwoods, and
World Woods, or woods from other places than North America. To
access any part, click on the title below. To see the properties for any
wood species, click on its name.
The extraction of salicin crystals from the bark of the Willow
tree by Johann
Buchner in 1828 led to the discovery of aspirin.
Marking tall white pines
in New England with a "broad arrow" to reserve them for the British
Royal Navy. This imposition was one of many that led American colonists
to revolt against Britain.
Redwoods are among the
largest (350 feet tall) and oldest (2,200 years) living beings on the
earth.
The resin of the Jelutong
tree, a favorite of wood carvers, is tapped to make chewing gum.
*Indicates that you
can enlarge a photo by clicking on it. To reveal the information in a "Superphoto,"
first enlarge it and then move the cursor over it.
"Abundant
to all the needs of man, how poor the world would be without wood."
Eric Sloane in Reverence for Wood
Wood and Woodworking
Materials/Hardwoods and Softwoods/Mechanical Properties,
part of the Workshop Companion,
essential information about wood, woodwork, and woodworking.
By Nick Engler.
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