Tom Gardner wrote:
I get to make a boatload of small accurate parts from 6061 Al, mostly 1/2"
stock. I sooo like Aluminum! I bought a bunch of different size stock bars
from McMaster, they deliver the SAME day, how cool is that! But, squaring
up my sawed pieces with very light cuts in the BP has me a bit confused as
to how I'm getting tiny amounts of swarf that are 'kinda welded on the cut
surface. It seems to scrape off easily. I'm using the side of a new 3/4"
HSS end mill without lube and clamping the pieces in the vise and cranking
the "Y" handle.
I can tell you are "conventional milling" it, where the side of
the cutter is moving against the movement of the work. This
causes the cutter tooth to approach the just-cut surface and
skate along it until the pressure develops to the point that it
penetrates the surface and starts to cut. This sliding at the
beginning is indeed welding the chips back onto the just-cut
surface, and is called "chip rewelding" and a number of other
bad words.

The opposite direction is called "climb milling", where the
cutter tooth moves in the same direction as the work feed, and
so it first hits the uncut surface and immediately plunges in
and starts cutting. If a chip is still attached to the cutting
edge, the abrupt impact with the workpiece just knocks it off.

The downside to climb milling is it can pull the work and table
into the cutter by the amount of backlash in the machine. If
you have a lot of backlash (0.010" is enough to make real
problems) you can only climb mill on light cuts, or you have to
apply a lot of drag with the axis clamps. If you always make a
light finish cut (like .005") in the climb-mill direction, you
will get rid of the rough surface.

If your machine is tight, then you can climb-mill all the time,
and extend your tool life. That skating at the beginning wears
out cutting tools very early.

Jon