Figure 5. Crushing of hard maple fibers
and vessels after knife planing at low
moisture content may cause raised grain
when exposed to high moisture content
condition.
Too shallow of a depth cut can
also increase raised grain, particularly
with small rake and clearance angles
or dull knives. A shallow depth of cut
can be caused by too low of a machine
depth setting or by too small of a bite
per tooth caused by machine settings
previously discussed. If the depth of
cut and/or feed per tooth are too small,
the knife edges will do more rubbing
and crushing and less cutting of the
wooden workpiece, particularly with
dull knives. The additional rubbing
and crushing of the wooden surface
may also increase surface glazing and
burning as well as tool wear.
Moisture content
Minimizing a change of moisture
content below fiber saturation point
can also minimize raised grain of the
wooden workpiece. As with other
woodworking defects such as from
drying or gluing, wood seasoning to the
proper moisture content will minimize
wood machining defects such as torn
or raised grain. Wood products should
generally be dried, conditioned and
equalized to 6-8 percent moisture
content to minimize wood machining
defects such as raised grain.
Proper wood seasoning minimizes
the shrinking and swelling of the
wooden workpiece in service. Proper
wood drying or seasoning also
minimizes the amount of subsurface
crushing or compressive failures
which causes raised grain. Proper
wood seasoning is essential for wood
machining as well as for other wood
processing.
Recommendations for minimizing
raised grain of wood also generally
apply to wood composites with
varying or uniform densities.
Proper tool geometry, woodworking
machine settings, tool and machine
maintenance and wood seasoning
are all paramount to producing a
satisfactory surface. ▲
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