FDM
THE WOOD
DOCTOR’S Rx
fdmonline.com
Gene Wengert
wooddoc@uwalumni.com
Shrinkage in
finished items
Q:We’ve had some shrinkage problems with our finished items.
How can this be?
A:A finish slows the moisture
movement, but doesn’t stop it.
Multiple coats of some finishes
provide better protection (
better slowing) than other finishes. If the
initial MC is incorrect, a finish will not
prevent moisture related problems. Incidentally, a slow uptake or loss of moisture
is less of a problem than a rapid increase
or loss. However, many finishes are so
thin that they do very little good at all in
slowing the moisture change.
●
lumber down tightly against the bed
plate of the machine. Therefore, the
entering end of the lumber lifts up
into the knives and it’s planed too thin.
Because this entering end is now thin,
the pressure bar (which is behind the
knife) can’t hold the piece down to
the bed either, as it is properly set for
thicker lumber. The result is snipe.
Note: Certainly, the chip breaker
must help to hold down the lumber to
the bed, but its main job is to prevent
chip-out, chipped grain, tear-out or
torn grain — all the same defect.
That’s why the chip breaker must be
set very close to the knives. As the
knives wear or are resharpened or
jointed, or as the chip breaker fingers
wear, adjustment is required.
Many finishes are
so thin that they
do very little good
at all in slowing
the moisture
change.
●
Q:We have a planer that has
been giving us problems with
snipe. How can we control this
machining defect?
A:For those readers who are
unfamiliar with this defect,
it’s a reduced thickness of the
front end or trailing end of a
piece of lumber, where the planer has
taken off too much wood.
In the following description, I’m
going to assume that you have a single
head in your planer and this head is
on the top of the lumber. If it’s on the
bottom side, then you’ll have to reverse
the description and suggested cures.
Snipe on the leading end of the
lumber is caused when the front end of
the lumber is pushed upward into the
head. The chip breakers (which are not
exceptionally strong and not designed
to fight outside forces; they’re located
in front of the knife) cannot hold the
Low infeed support
Ask yourself why the leading end
is being forced into the top head. In
fact, the knives and the chip breaker
are trying to push the lumber against
the bed plate, but they can’t do it. The
answer is that the infeed support for
the lumber is too low. Gravity is pulling the tail end of the lumber downward and the lumber is pivoting on the
same point in the machine, forcing
the front end upward. So, to prevent
front end snipe, you must lift the tail
end of the lumber slightly. That is, the
lumber must approach the machine at
an angle that’s very slightly above level
with the bed plate of the machine.
[Note: The support table is even with
the machine where it joins the machine,
but the other end is slightly elevated.] It
doesn’t take much elevation, but once
the effect of gravity is removed, the chip
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