On Mon, 18 Dec 2017 10:59:34 -0700, Bob La Londe
wrote:

I spent most of Thursday and Friday setting up roller tables so I could
run the large pieces of bar stock into my bandsaw, and roughing out
blanks. I liked the way it saw cut.

Today I am truing up blanks on the Hurco mill for further machining.
Cut starts sound a little hard on my face mill, but the surface finish
on the final pass is really nice.

Higher RPM with the same feed reduced the pounding, and of course
improved the finish even on the roughing passes. Flood coolant seems to
work about the same as cutting oil, so I went with flood coolant since I
don't have to stand there with a brush. The face mill does not have
only has TiN coated inserts. They definitely did not work well dry. It
cut, but the sound made the hair on the back of my neck stand on end.

I am not looking forward to the sound of the detailed machining of the
embossing portions on the smaller mills at "high" speed, but the stuff
machines really nicely at lower RPMs on the big mill. My finish pass
isn't mirror smooth, but its close enough I know I could make it that
way if I needed to.

I like machining both 4140HT and 4340HT. Nice chips and the hot chips
smell sweet. You will need to make sure the chip load is high enough
to get under the surface hardened slightly by the previous pass. I
used to turn lots of both metals and roughing passes using negative
rake inserts worked well to break the chip and to keep most of the
heat of the metal removal in the chip. When turning to final
dimensions where I had to hold a couple tenths and of course get the
finish required by the close tolerances I used very sharp positive
rake carbide tools. Usually inserts modified by hand grinding with
diamond wheels and brass laps charged with diamond. But sometimes
brazed carbide tools too, also hand ground. I used high sulfur cutting
oil for the final high tolerance, low material removal cuts. I know
milling is different because of the interrupted cuts and the way the
chip changes thickness during the cut. But really sharp cutters are
important as is getting the chips away from the cut so as to avoid
re-cutting of the chips.
Eric