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When metal is machined with an end-mill, a burr often
forms along the top edges of the cut. A similar phenomena occurs in
woodworking. In wood, this "burring" manifests itself as
splintering of the top edge following the grain away from the cut. The
degree of splintering depends on the type of wood, the speed of the
cutting edge, the chip-load (feed rate), and the sharpness and shape of
the cutting tool. If the tool and/or cutting parameters are poorly
matched to the wood being cut, splintering can be so severe that small
features can be totally obscured and the piece ruined.
The speed of the cutting edge and the chip load can be
tuned with great precision in CNC based woodcutters. The sharpness and
geometry of the cutting edge are another matter altogether. While you can
always start with an new, sharp tool, the shape and attack angle of the
cutter will determine how long the tools stays sharp and how well it cuts
as it inevitably wears down. Historically, one way to tell when a tool was
getting dull was to observe the degree of splintering and change the bit
when the splintering became too severe. It is probably not possible
to fabricate an end-mill that never splinters any kind of wood. However,
proper design and testing coupled with fine tuning rotational speeds and
feed rates can make it possible to fabricate tools that will reduce
splintering to such a degree that the splinters can be removed with a soft
tooth brush as with this test cut in black walnut ( 0.0625" dia.,
0.125" depth, 50 in./min. feed @ 40,000 RPM). |
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