FDM
SHRINKAGE
breakers, knives and pressure bar are
able to prevent snipe. Without this elevation, gravity will win, especially with longer lumber pieces. For short pieces that
are fed by hand, you could be tempted
to elevate the piece by hand, but safety
issues (getting your finger pinched between the lumber and bed plate) make
this an unacceptable procedure.
For snipe on the exit end, it’s the
same deal. If the pressure bar is slightly
mis-set or if the wood is soft enough
to compress slightly, and if there is
inadequate support on the exit end of
the piece of lumber, then gravity will
work to pull the leading down and push
the tail end upward into the knives.
Snipe will occur. Once the end is cut a
bit too thin, gravity will work to cut it
thinner as the piece moves through the
planer. Again, if the front end of the
lumber is slightly elevated as it proceeds
to exit the machine, that will eliminate
the force of gravity pushing the tail end
upward and then, without this extra
force, the pressure bar will be able to
do its job — unless it is mis-set.
As an added note: When the pressure
bar is too tight, you’ll have feeding problems. The piece will not feed easily, but
will get hung up in the machine. Hence,
many amateur planer operators will lift
the pressure bar way too high to make
sure that the lumber will never stall,
even after the knives are sharpened or
jointed. If the pressure bar is too loose,
you’ll see chatter and possibly a little bit
of snipe. In truth, the pressure bar setting is quite critical and requires skill to
set it at the correct opening.
Q:I was hoping you could give
me a simple explanation of
how lumber can be 100 percent
moisture content or more.
A:Many years ago, someone
decided that the moisture
content of solid wood would
be measured as a percentage
of its over-dry weight. So, if you have a
piece of lumber that weighs 10 pounds
and it has 5 pounds of water and 5
pounds of wood, the moisture content
is NOT 50 percent MC (which makes
sense because it is half wood and half
water). Instead, it’s 100 percent MC, as
the amount of water is 100 percent of
the weight of wood. With some species
(cottonwood, hemlock and redwood,
for example) there can actually be
more water than wood, so the moisture
will be more than 100 percent MC.
(For example, 5 pounds of wood and 6
pounds of water is 120 percent MC.) ●
Gene Wengert, “The Wood Doctor,” has
been training people in efficient use of
wood for the past 35 years. He conducts
many short courses and has written
hundreds of practical articles and books
covering all aspects of converting logs into
lumber and lumber into finished products.
He is presently an extension specialist
emeritus at the University of Wisconsin-Madison. Fax your questions to him at
847.390.7100 or e-mail to wooddoc@
uwalumni.com.
