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