THE WOOD DOCTOR’S Rx
after machining are due to changes in
moisture. That is, the moisture of the
air is not equal to the moisture in the
wood, so the wood changes moisture
and swells or shrinks.
If the lumber is dried correctly, then
you need to keep it at this critical MC in
storage and during manufacturing. This
means the RH in storage and in your
plant should run at 38 percent RH (+ or
– 4 percent RH). This is indeed a tough
requirement, but your product (a 72-inch
door) is very sensitive to MC changes.
Is the lumber straight and flat when
you receive it? Some species, such as yellow poplar, are known to have stresses
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Wood Doctor’s Rx question and answers.
in the tree that causes the lumber to warp and cause large end splits.
It will be difficult to have straight narrow pieces from growth stress lumber. If
you do have warped lumber, the NHLA
grading rules, if properly applied will
typically lower the grade severely.
U V - Finishing
Wood will bow, as in your case, more
if the grain is not perfectly parallel to
the sides of the piece. That is, we need
lumber with zero grain slope. When
making narrow long products from
wood, such as pool cues, drum sticks
and even stiles, it would be best (maybe
it is a requirement, because we know
that wood will always experience small
moisture changes) if the sawmill sawed
the lumber parallel to the bark (also
called full taper sawing) and did not use
crooked logs. In the old days, sometimes
that log would be split to show grain
direction and then the wood would be
sawn parallel to the splits. Straight grain
material will have little or no risk of bowing when MC changes.
Good for You A nd G ood for T he E nvironment!
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No energy or time is wasted in warm-ups because the UV-X turns “ON”
and “OFF” instantly, giving you unmatched flexibility and increased
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Q:A customer indicated that
because we use a microwave
for measuring MC (we run it
on low power using a carousel
tray until all weight loss stops, and using
two-minute weighing intervals at the end)
that we can be off 6 percent MC or more.
True or false?
A:False. A properly operated
microwave (and you stated
the key operating parameters correctly) will, with
more labor but in a shorter time, get
the same results as a hot air oven. An
electric moisture meter used before
you start microwave drying should be
within 1 percent MC most of the time
between 7 to 28 percent MC, corrected
for temperature and species. ●
low temperature curing
more machine up-time
no resin bleed
the thrill of finishing
Gene Wengert, “The Wood Doctor,” 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 firstname.lastname@example.org.