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Assigning precise values to feeds and
speeds when cutting wood (soft or hard) with micro-tools is just about
impossible. The incredible variability in grain structure,
moisture content, and density, make any such assignment at best a
rough guide. Ebony from the same tree can vary in hardness and
density as much a 75% from point to point in a piece as small as 2" X
6".
Thermoplastics, although relatively isotropic and uniform in
density, present their own set of problems, many of which stem from
comparatively low melting points and poor chip formation. Many
polymers (cast and extruded) are also very abrasive leading to poor
cutter life and difficulty in removing debris from deep, narrow kerfs.
The family of composite materials span the range from glass-loaded
epoxy (fiberglass), to paper/phenolic, to natural mother of pearl. The
hardness and density variations are so extreme that no one family of
tools can be used to process them all. Each one must be handled as a
"special case" with its own, unique cutter geometry and processing
parameters.
The "other stuff" means just about anything else that someone, in a
fit of creativity (often alcohol induced), decides to cut with their
long suffering CNC router.
Adding to these difficulty are the variations in shop practice,
spindle run-out, and equipment backlash.
It is undoubtedly starting to dawn on you that there is not going to
be any magical formula that will guide you in cutting any random
material you might encounter. No "Grand Unification Theory Of Machining
Everything (GUTOME?)". However, where theoretical reasoning might have
failed us, empirical testing comes to our rescue.
Example: Suppose that you want to cut
a pocket 1/8 in. (0.1250 in.) deep in a piece of purpleheart using a
spindle turning at 30,000 RPM. Since you are trying for fine detail, you
decide to use a 1/32 in. (0.0313 in.) diameter bit. To determine the
minimum feed rate, find the tool diameter in the "diameter"
column. Since there are two 0.0313 in. cutters, select the one with the
depth of cut (DOC) that matches your requirements (in this case the tool
with a 0.125 in. DOC). Follow along that row until you reach the column
marked "purpleheart". Write down the number that you find in
that cell (0.00044). Multiply this figure by your spindle RPMs
(30,000).
The minimum feed rate for this
application is:
0.00044 X 30,000 = 13.2 in./min.
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