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Aluminum is among the easiest metals to machine. And today, that's what makes machining it competitively a challenge.
Faster spindle speeds affect all metal cutting applications, but speed changes the rules for aluminum more than it does for other metals. In steels and harder alloys, the higher speed is combined with a light depth of cut, leaving the fundamental limits on the process unchanged. Maximum metal removal rate is still set by the performance limits of the tooling and the machine.
Aluminum, however, is unique among the most commonly machined metals in that the high spindle speed is often combined with heavy cuts. This combination—speed plus depth—makes chatter a far more significant concern. As a result, the shop hogging aluminum at high spindle speed faces not two process limits, but three. There are the machine and tool limitations just like always, and now there is also the limiting effect imposed by the harmonic characteristics of the spindle and tooling system overall.
The primary result of these harmonic effects is that the top speed is no longer the best speed as far as metal removal rate is concerned. Instead, any milling process run at around 15,000 rpm or higher is likely to offer some optimum spindle speed, a "sweet spot," where the cut is significantly more stable than it is at both higher and lower speed settings. This sweet spot rpm value may permit double the depth of cut compared to other speeds. It may permit triple the depth of cut. That's why effective high speed machining of aluminum calls for a type of knowledge—basic vibration analysis—the shop may never have needed before.