From Newsgroup: rec.crafts.metalworking

I really had this shown to me recently by accident.

I was doing some deep pocketing in aluminum, and before starting the
pocket I drilled for the thru holes to allow chips and coolant to drain
away a little better. For drilling I shifted the Tormach into low gear
which has a top speed of 2000 rpm. It did a satisfactory job of peck
drilling an 8X deep thru hole with a 1/2 inch jobber drill.

Then I starting the pocketing operation at 5120 RPM, 35.9 inches per
minutes, and 0.30 depth of cut with a very long half inch end mill and
lots of stickout. I was using a 10% trochoidal cut for the operation.
It was not fast, but it was as much MRR as I felt comfortable taking
with the setup and the machine. I had run those exact settings on an
earlier operation, turned the feed up until it started to chatter, and
then back it off until it didn't. That's where I got the feed rate.

It was cruising along ripping out aluminum just fine. It didn't sound
bad at all, but something sounded different. Then I realized I had
forgotten to switch the mill back into high gear. At 5120 commanded it
was still only turning 2000. It was niggling around in the back of my
brain, but somehow increased torque was making of for the lack of RPM.

The Tormach only has a 1.5HP spindle so it has limits. I really have
never managed to calculate more than about 1HP load and get good looking
and sounding cuts.

Now I will point out with flood coolant you CAN remove aluminum (6061
any) as fast as the horsepower and speed of your machine and the end
mill can handle.

Recently a lab tech at the NAU machine shop said the HAAS TL-1 wouldn't
turn fast enough for some material the engineering race team wanted
turned. 4140 shaft if I recall. It didn't make any sense to me, so I
walked a couple of the guys into the back of my shop and wasted some
4140 QT TGP.** I have a couple sticks of 1.375. It turned nicely at
755 RPM with about .002 FPR and 0.20 DOC (0.10 per side). Far less than
the 2000 RPM max of the TL-1. After making a number of test cuts
showing the guys facing grooving, and turning I parted it off at 460 RPM
with no trouble with a parting insert. (That surprised even me.) Then
they mentioned the lab tech had told them the TL-1 didn't have enough
power. WHAT?!? My PM1440ELB has a 3HP single phase motor and uses
gearing for a limited number of speeds. The TL-1 has a 7.5 HP 3 phase
spindle with a VFD for speed control, and it can over current the motor
for more than a few seconds if it starts to overload at rated max
current. No way!

I think the tech just didn't know how to turn it, or maybe didn't know
how to program it. Maybe, my 3HP machine makes up for some of that
because it has full power (less parasitic losses) at every speeds due to
being a gear head, but 3HP vs 7.5HP plus over current capability is a
big difference.

I was also surprised at how little exposure these engineering students
had to machining and manufacturing processes. What I was showing them I
felt was pretty basic knowledge learned by trial and error. Something
they should have learned and seen in a survey of manufacturing processes
or a beginning machine shop class that they should have taken long
before they were about to graduate with a 4 year degree in engineering.
Then I found they did take a CNC machining class atleast, but they
generated code and tested it on simulators. Almost none of them had
seen any of their code run on a machine, or had any concept of how much
you can do with a machine. After seeing my (big for me) lathe break
chips in 4140 one of the guys said he was definitely going to have a
lathe in his home shop when he gets his own place.

Now I am going to sound like one of those old guys who used to talk down
to me when I was learning. I think they would ALL benefit from a
semester or two of HANDS ON manual machine shop operation. I know that
may sounds a little condescending coming from me. A self taught guy
(with the help of all of you) who basically graduated from hand tools
and a grinder into "real" machining with little "real" experience and
started with a desktop CNC mill, well and a POS harbor Freight mini
lathe.

** QT means quenched and tempered. Its roughly the same as HT which
means heat treated. 29-32 HRC
--
Bob La Londe
CNC Molds N Stuff

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