Page 164 of 339

PUTTING IT ALL TOGETHER
deteriorates; then back off and decrease the
speed by around 10%.
Feeding Too Fast
When a feed rate is too fast, the end-mill evacu-
ates chips too quickly. This rapid removal pre-
vents chips from having enough contact with
the end-mill to absorb its heat. This reduced
contact causes the tool to retain heat and ulti-
mately fail. Decreasing the feed rate slows the
speed of the tool along the toolpath. This
decrease in feed increases the duration of con-
tact between the end-mill and chips, so the
chips adequately absorb heat before they are
ejected.
Feeding Too Slow
When the feed rate is too slow, the chip evacua-
tion doesn’t keep up with the chip formation.
These chips retain heat, and this buildup of
heat, in combination with the forces of feed, will
break your bit. Increasing the feed rate will
move the end-mill more quickly along the tool-
path, so that the rate of evacuation keeps up
with the rate of chip formation.
Don’t be afraid of pushing the feed rate; it’s
good to keep the tool moving quickly through
the cut. Most bit breakage comes from cut-
ting too slowly.
Decrease Cut Depth
Cut depth is another factor that can lead to
suboptimal machining. First, double-check
your settings, because it’s easy to enter the
incorrect numbers or put a decimal point in the
wrong place. If your depth of cut is agressive,
then dial it back to match your end-mill diame-
ter.
End-Mill Flutes
As the number of cutting edges increases, your
feed rate should increase to prevent burning
and premature tool dulling. Using more flutes
reduces chip load and improves surface finish,
if the feed rate remains the same.
Spindle Speed/RPM
You rarely need to adjust your spindle speed.
That’s probably around 12,000 RPM when cut-
ting plywood with a ¼″ diameter two flute tool.
It’s typical to pick a speed that works for your
material and then adjust the feed rate accord-
ingly, using the formulas in “Machining Vari-
ables” on page 145. However, speed is another
variable that you can consider. When the chip
load is too large for your tool to handle,
decreasing the RPM will reduce the rate of chip
formation. Alternatively, you can increase your
spindle RPM to allow the tool to form chips
more quickly.
In the next chapter, you’ll get hands-on with
CAM software, where you’ll be able to import
files, define toolpaths, and run machining simu-
lations.
162
DESIGN FOR CNC
    "