The spindle taper sets how tools mount and how much cut a machine can take. A 40-taper (CAT40/BT40) spindle is lighter, spins faster and suits general and high-speed work — the most common choice for VMCs. A 50-taper (CAT50/BT50) is bigger and more rigid, holding larger tools and taking heavier cuts in tough materials, at lower top RPM and higher cost.
Fast, lighter cuts and aluminum favor 40-taper; heavy roughing, big tools and hard materials favor 50-taper. See spindle taper and value machines at The Machine Blue Book.
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