High Speed Machining is, in a way, misnamed. The disciplines involved in high speed machining apply even to low speeds. High speed machining actually has to do with bringing more productivity to the machining process by making it more stable. Because the instability that threatens productivity is often more pronounced at higher speeds, many shops first come to high speed machining because of the challenges of using higher-rpm machining centers.
High speed machining has to do with milling. It has two different definitions applying to two different machining challenges: milling complex 3D forms, particularly for dies and molds, and milling large amounts of a metal such as aluminum, particularly to produce aircraft structural parts.
In the context of machining complex 3D forms, high speed machining involves taking milling passes with smaller-diameter end mills at very light depths of cuts, but taking these passes at high feed rates. The end mill is often a ballnose tool. The small diameter and very light depths of cuts allow the tool to machine complex features and surfaces precisely, and also to machine hard steels without excessive tool wear. Meanwhile, the high feed rates overcome any loss in productivity resulting from the light depths of cut. The overall metal removal rate in high speed machining is typically the same or better than the metal removal rate that could be achieved through a more typical process involving a larger tool taking heavier depths at a slower feed.
In the context of milling away large volumes of aluminum, high speed machining focuses on chatter. Every machining system chatters—that is, every machining system has some specific set of frequencies at which it inherently wants to vibrate. By identifying these frequencies and machining at precise spindle speeds that respect these frequencies, the machining process becomes inherently more stable, allowing the tool to take a heavier depth of cut than it could at other speeds. High speed machining in this context therefore does not mean machining at a higher speed, but instead means machining at a higher metal removal rate by machining at just the right speed.