Ever wonder why flat bottom stand 4 fluted end mills so plunge operation worse than a standard hss drill bit and must rely sometimes on slower plunge feedrate? Simply just like hydrodynamics a travel through medium will acquire less frontal resistance force than and angulated less frontal surface object. The drill bit is designed with a good angle cutter plus a cone shape which penetrates easier and has a good lessened blunt cutting edge in addition to looks like less frictional surface area. End mill require a flat bottom most types for a top smooth cut operation. The rake angle of the bottom can change performance as well a cut channel for chip evacuation requirement. Optimal end mill can be numerically equate as have even hit impact under all circumstance and better cutting entry leading cut angle. Consider side stability maker from the metal itself as well as a smoother turn not catchy bottom profile. A 60% or other percent step over specialty end mill profiled end mill for that percentage operation. Big rigid over 1/2 will give better surface bottom end mill finish cut. For optimal surface cut should I feel not go below 1/2 for broad rand linearity. Too deep cutting channel will cut rigity. High rpm profile high temp high fluted end mills with little channel works looks like with 3/8 down to. An optimal CNC end mill should have a good leading edge bottom rake angle which generates the least amount of impact force. Seems to me high stability making greater fluted bottom better for irrational ring free cuts. My preference is for a 1/2 or greater size end mill for general duty and 4 flute plus. Rough end mills are recommended to reduce time if tool change doesn't knock off accurate position. Must keep end mill bottom planted either thinner, little extra feed rate to drag plant, proper dept side and bottom cut, higher percent step over cut, and don't forget a rigid Z stock. More rigid more cut deeper and faster due solely to stability and less vibrations. Key on flexable stock is keep final greater than your deepest ring pit cut light and thin and in multitude passes. Exit point indicates flex with irrationals. No problem with fly cutter-one point contact but must be linearly smooth rapids-try to swing it by hand. Face cutter at thinner higher rpm may produce a finer cut. Long shaft end mill better in the collet for stability and resonant vibration control than can chain react. Intelligent stiffening good but solid heavy has better characteristics for vibration control. in addition to types of alloy.-maybe semi malleable and stays accurate with hardened facing like high temp grounding and quenching may just suffice. Not sure but may avoid precipitation tempering on mill alloy. I see quite similarity of ductile iron and 1000 series steel like 1018c near identical spec. and characteristics. Brief synopsis.