Kerf:
Using a Saw -- power or hand -- the wood
removed by the teeth as the cut is made.
The diagram on the left -- from Mark Duginske's
Bandsaw Handbook, 1989, crude as it
is, illustrates not only "kerf width" on a
Bandsaw,
but such other factors that, in one way or another,
contribute to kerf. These factors include the
Blade's
Gauge,
the Set
of
the Blade's
Teeth, Side
Clearance.
For more on
Bandsaw Blades, please see section "D"
("Blade Anatomy and Terminology") of my
Syllabus on the Bandsaw
(Research on
Blades -- e.g., origin of -- for other
Tools (e.g.,Carbide Tips for
Table Saw
blades) is a future project.
Kiln-Drying Wood:
Drying Methods
Methods of
drying or seasoning lumber vary with the use
requirements. Today, because of the time saving
and control factors, nearly all upper grades are
kiln dried. Sometimes lumber is first
air dried and then
kiln dried.
In air drying,
the lumber is simply exposed to the outside air.
It is carefully stacked with stickers (wood
strips) between each layer so air can circulate
through the pile. Boards are spaced well apart
in the layers so air can also move vertically.
Fig. 25-9 shows lumber air-drying in a sawmill
yard.
The rate of
air drying lumber can
be partially controlled by varying the spacing
between individual boards and the size of the
pile. Because of the seasonal variations in
climate and local weather condition, it is
difficult to approximate the air-drying time for
any particular species or thickness. Lumber that
might become dry in 30 to 60 days during an
active drying period may re-quire more than 6
months under unfavorable conditions. The
moisture content of thoroughly air-dried lumber,
reduced during the spring, summer or early fall
will be about 12 to 18 percent.
Lumber is
kiln (often pronounced "kill") dried by placing
it in an oven where the temperature and humidity
are accurately controlled. The boards are
stacked in about the same way as for air-drying,
Fig. 25-10. When the green lumber
is first placed in
the kiln, steam is used to keep the humidity
high and the temperature is kept at a low level.
Gradually, the temperature
is raised and the humidity is reduced.
Fans are used to keep
the air in constant circulation over the
surfaces of the wood. See Fig. 25-11.
A kiln schedule is a carefully
complied set of temperatures,
humidities and
timings which are
followed by the kiln operator. They will vary
de-pending on the size (cross section) and kind
of wood, and its initial moisture content.
One inch lumber can
usually be kiln dried to a level of 6 to 10
percent in about three or four days.
Improper
drying, either air or kiln,
can result in such seasoning defects as splits,
checks, warpage,
loosened knots, honeycomb, and internal stresses
called case-hardening. Most of these are caused
by drying the wood too rapidly. In
case-hardening the surface layers dry, shrink
and be-come fixed or set before the inside
portion of the board. As the interior then dries
below the fiber saturation point and starts to
shrink it pulls on he outside, shell," creating
both compression and tension forces. When the
board is cut these forces are released causing
the kerf to close on (pinch) the saw blade or
the stock to warp in various directions.
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