Let me start off by saying that Chuck gave a better explanation than I could have and there isn't a whole lot to add to that. The information I can give you is in the form that I use it (surface feet per minute, and feed per tooth) and is frequently not useful to folks manually turning cranks and going by feel. I can't go by feel, I go by numbers. I make adjustments based on appearance, sound and load meters. Well, occasionally by feel, if you count feeling the floor shake with your feet.
One small correction to Chuck, if you're breaking cutter, probably don't reduce RPM. Breaking cutters is frequently caused by too much chip load for conditions, which would require increasing RPM if conditions allow. Generaly, breaking cutters means the force required to feed it is higher than the strength of the cutter. A more shallow depth of cut is usually the answer. Feed and RPM are tied together, unless you're wanting to change your chip load, when you change one you change the other.
You might be using HSS cutters, they're more forgiving. My experience is with carbide. A middle of the road number for carbide in steel is 150 SFM. You can go up or down from there depending on what you're cutting. Very seldom do I go over 300 or under 50 in steel. HSS would be half these numbers. And because chiploads remain the same, HSS is therefore fed at half the feed, which is one reason why I don't use it.
A good middle of the road chip load of a 1/2 cutter range from .001 to .004 per tooth. Larger cutters use higher chip loads, smaller cutter use smaller chip loads.
An interesting effect of this is small cutters are fed at the same feed as large cutters. I might be running a 1/2" coated cutter in CPM 154 at 1200 RPM (157 SFM) 10 IPM (about .002 per tooth, on a 4flt) .100" DOC. I would then run a 1/4" cutter at 2400 RPM to achieve 157 SFM, and a .001 chip load would again give me 10 IPM, and .050" DOC. So, the difference between running a 1/2" cutter and a 1/4" cutter are RPM, and depth of cut.
To answer the question between 4 flt and 2 flt: I always try to run 4 flt because, all else being equal you can feed the four flute twice as fast and you can cut twice as much material before it dulls. There are exceptions. Some materials like aluminum and plastic aren't real picky about chip loads, and are real picky about recutting chips, so I'll frequently use a 3 flt on aluminum and plastic for better chip removal. Slotting in any material deeper than about 1X the cutter diameter you start re cutting chips, which leads to different problems in different materials. And plunging, it is much better to plunge with a 2 flt.
Some of the chip clearance issues of extra flutes can be worked around with cutter design and flute geometry and also high pressure coolant. A modern three flute cutter and a 3/4 HP coolant pump can slot and feed in aluminum and plastic at insane rates.
Short answer: use 4 flt where you can, keep 2 flt around for when trouble starts.
...hope this helps more than it confuses...
