WOOD DOCTOR’S Rx
by Gene Wengert
gwengert@wattnet.net
Tear out on yellow poplar
Explaining ray fleck
and how it shows up in
quartersawn lumber
Q:What is ray fleck?
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A:Most of the small wood cells in a tree run up and down (or vertically) in
the stem, conducting fluids from the roots
to the leaves. A few cells run in and out
from the bark to the center of the stem (or
horizontally or radially). These cells, that
primarily store starches and sugars in the
tree, are called ray cells or sometimes ray
parenchyma cells. With a flatsawn piece of
lumber, the ends of the ray cells (the ends
appear like the cut ends of a handful of
soda straws) are exposed on the face of
the lumber and generally do not have too
much of an effect on the appearance of
the face: short vertical lines, if any effect.
However, on quartersawn lumber, the
sides of the ray cells are running parallel to
the face. This side view creates an interesting pattern in many species of hardwood
lumber, including maple, oak, sycamore
and beech. This side view or appearance
of the ray cells in quartersawn lumber is
called ray fleck. In fact, the standard definition of quartersaw or riftsawn lumber for
many species is that the ray fleck must be
obvious.
wood is at low MCs (even if the MC was
later raised a bit) and/or was dried at
temperatures over 160F. It is also worse
at high feed speeds and large depth of
cut. One solution is to use slower feeds
and perhaps higher spindle speeds to
effectively get a small cut with each
knife. With this defect, the knife is acting
like a wedge rather than a plow. The
wedge effect dominates with large rake
angles(small slender knives), while “
plowing” is common (and requires more hp
and sometimes will have the piece stall
while feeding and increases kickback
while improving quality) with small rake
angles. In either case, the knives must be
sharp. You could also consider making
two passes when moulding, with the first
cutting taking most of the wood, but then
Case hardening will
not cause tear out, but
it does cause some
warping after machining.
Q:Is the problem tear out or case- hardening?
A:First question. Tear out is caused when the knife is unable to cut a
chip from the wood as easily as it can
break a chip. The break follows the grain
into the piece, leaving a void space that
is called tear out, chip out or torn grain.
This defect will happen more often if the
a second cut with small stock removal
taking the final, quality cut. An addi-
tional comment about MC: You probably
measured the interior or average MC.
However, the knives are working on the
outside fibers, so when using a pin-type
moisture meter, try and get a surface MC
reading. Hopefully, this listing will provide
the information you need to determine
where your process can be improved.
Gene Wengert, “The Wood Doctor,”
has been training people in efficient
use of wood for 35 years. He is
extension specialist emeritus at the
University of Wisconsin-Madison.
