English: A line drawing showing some basic concepts of speeds and feeds in the context of lathe work. The angular velocity of the workpiece (rev/min) is called the "spindle speed" (light blue) by machinists. Its tangential linear equivalent at the tool-cutter interface (m/min or sfm) is called the "cutting speed", "surface speed", or simply the "speed" (dark blue) by machinists. The "feeds" may be for the X-axis (purple) or the Z-axis (pink) (typically mm/rev or inch/rev for lathe work; sometimes measured as mm/min or inch/min). Notice that as the tool plunges closer to the workpiece's center, the same spindle speed will yield a decreasing surface (cutting) speed (because each rev represents a smaller circumferential distance, but takes the same amount of time). Most CNC lathes have CSS to counteract that natural decrease, which speeds up the spindle as the tool plunges in.