both the 2 flute and 3 flute have excellent finish. I just use them for different types of tool paths. If I’m full width slotting @ 2x diameter DOC than it’s 100% going to be done with a 2 flute. Those chips are big and you need the larger gap between the flutes to be able to extract them at a high rate.
if I’m not full width slotting I’ll go with a 3 flute so I can get higher IPM. If I’m spinning 8,000 rpm and taking .003" per tooth with a 2 flute end mill that will = 48ipm if I keep the same rpm and chip load but add an extra flute it will = 72ipm. That will make a pretty dramatic change in your total cycle time.
I have the 3 flute roughing end mill but I haven’t tried it yet. I’m guessing I’ll be able to take more than .003" per tooth with that because it will chop the chip in half and allow it to be extracted faster.
Surface finish depends a lot on the chip load. If your chip load is .003" you’ll see .003" lines if your chip load is .001" the lines will be .001"
just think of it like your scooping ice cream. every time you take a scoop you leave a dished out line behind. If you take a bunch of shallow scoops close to each other the surface is less disturbed. If you take deeper scoops well…