axolotl wrote:
I am making some rack mounts out of (presumably 6063) 2x2x1/4 angle. My
results are not good. I am using a new, sharp, two flute 3/8" end mill
with WD-40 as cutting fluid. Spindle speed is about 2800; I started
slower and went up to get better results. I am cutting both sides of the
slot.
I am experiencing tearout, vibration and a lousy finish.
If it starts well, and then gets worse as the cut progresses, then the
work material is heating up. It doesn't take much warming at all for
some aluminum alloys to get gummy. My usual fix is to reduce the depth
of cut and increase the feed as much as I can. i have never liked
WD-40, and use wither thread cutting oil for light work or water-based
flood coolant for more concentrated cutting where the heat buildup is
more of a problem.

The idea of keeping the feed rate up is to spread the heat around the
material instead of letting one spot get hot.

2800 RPM for a 3/8" cutter might be too fast for this situation with
poor cooling.

When slotting, I always use an undersize cutter, plow down the middle
with relatively shallow step-downs in Z, no more than 1/2 the cutter
diameter, but in this case probably .050" per pass. When I get it cut
all the way through, I then move over and clean up the walls and widen
the slot to the desired size. I almost always "climb mill" where the
work is fed "with" the cutting edges, rather than against them. The
problem with doing this is that if the machine has a lot of backlash,
the table can be pulled into the cutter, breaking tools and damaging the
work. On a tight machine, though, you get MUCH improved surface finish
and less heating, as the cutter plunges into the uncut material rather
than sliding along the just-cut surface until it presses through.

Jon