[DonRotolo]

Cutting speed and spindle speed are both functions of the tool manufacturer's recommended chip load. Too little and too much chip load are both bad.

For example, I use an Onsrud single-flute cutter on a router. It needs to see 0.004-0.008 chip load. At 10,000 RPM, that means I need to move about 0.006 per revolution, which comes to cutting at 60 IPM. (With 3 flutes, you'd need to move 3x faster, or 0.018" per rev - so speed would be 3300 RPM for 60 IPM, assuming the same recommended chip load)

Now we consider cutting depth: My machine is definitely unable to cut at 1D (meaning 1/4" deep for a 1/4" cutter); instead I cut at 1/2D. If my machine was beefy enough, the tool can handle 2D.

That's for conventional milling around a perimeter. For slotting, I'd cut it down near the minimum 0.004" chipload.

Lastly, this includes either an air blast (wood, plastic) or air with mist coolant blast (for aluminum) to make sure all chips evacuate the cutitng path. Re-cutting chips is very bad. Also, there is a vacuum system (a 2.25" shop vac) that scavenges the chips and keeps the machine clean.

The bottom line: There is science behind selecting feeds and speeds, the tool manufacturer is your friend, so go and do the math.