which is a relatively good thermal insulator. Because dry wood or wood-based
products, such as MDF, have low thermal
conductivity, the cutting temperatures
are generally higher than expected.
Unlike many metal-cutting situations,
the workpiece, chip and/or liquid coolant cannot conduct heat away from the
cutting zone, and most of the heat generated goes into the tool.
Liquid coolants and lubricants have
been applied to metal, plastics and
green wood machining to reduce cutting temperatures or their effects on
tool wear. Due to the hygroscopic nature of dry wood or wood composites,
liquid coolants or lubricants are generally impractical for increasing tool life
when machining wood or wood-based
composite materials. However, cooling
tools with compressed and/or refrigerated air may reduce tool wear.
Compressed air cooling
Cooled or refrigerated air can be
produced for cooling a wood machining tool at down to -46C below the
compressed air supply with no moving
parts with a vortex tube. Compressed
air, normally 80-100 psi, is forced tan-
gentially into a vortex spin chamber.
The air stream revolves in an outer
vortex toward a hot end continually
expanding where some hot air escapes.
The remaining air, still spinning and
expanding, is forced back through
the center of the outer vortex and still
cooling. The inner stream gives off
more heat energy to the outer vortex.
The inner vortex continues to cool and
exits the vortex tube as cold air which
can be directed at a cutting tool.
The vortex tube appears to be a low
cost, reliable, relatively low maintenance solution to high-temperature
wood machining problems. Therefore,
a study was undertaken at Purdue University to apply vortex tube-cooled air to
solid tungsten carbide router bits while
A concern of tool wear testing is
obtaining meaningful results from
minimum testing. Consequently, the
tools need to be similar and represent
the population. Likewise, the workpiece material — such as MDF — has
to be relatively uniform. Randomiza-tion of the tools and a large sample
Wear and temperature
Temperature Wear Wear Total
degrees (F) Void (%) Scar (%) Wear (%)
70 32.07 44.09 76.16
40 25. 39 34. 64 60.03
20 25. 68 40. 63 66. 31
Wear void, wear scar, and total wear as a
percentage of the original clearance face area
for the average of two flutes.
of MDF can help the uniformity of
the respective populations. If the tool
and workpiece materials are similar,
respectively, then other treatments
such as refrigerated air should readily
exhibit a difference or no difference
in simple comparative tool wear tests.
Three double-flute, solid tungsten
carbide, 3/8-inch diameter router
bits were randomly selected to cut 22
sheets ( 4 foot x 8 foot x 3/4 inch) at
16,000 rpm and at a 1/4 inch depth of
cut per pass across the MDF on a CNC
router. The total length of cut was
more than 175,000 yards per flute.
One of three tools was cooled with
ambient compressed air temperature
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