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Speeds and feeds

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" by machinists. Its tangential linear equivalent at the workpiece surface (m/min or sfm) is called the "cutting speed", "surface speed", or simply the "speed" by machinists. The "feeds" may be for the X-axis or the Z-axis (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 constant surface speed to counteract that natural decrease, which speeds up the spindle as the tool plunges in.
Photo of a milling cutter during a cutting operation. Arrows show the vectors of various velocities collectively known as speeds and feeds. The circular arrow represents the angular velocity of the spindle (rev/min), called the "spindle speed" by machinists. The tangential arrow represents the tangential linear velocity (m/min or sfm) at the outer diameter of the cutter, called the "cutting speed", "surface speed", or simply the "speed" by machinists. The arrow colinear with the slot that has been milled represents the linear velocity at which the cutter is advanced laterally (usually mm/min or inch/min for milling; may also be measured as mm/rev or inch/rev). This velocity is called the "feed" by machinists.

The phrase speeds and feeds or feeds and speeds refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Each, however, can also be considered and analyzed in its own right.

Cutting speed (also called surface speed or simply speed) is the speed difference (relative velocity) between the cutting tool and the surface of the workpiece it is operating on. It is expressed in units of distance across the workpiece surface per unit of time, typically surface feet per minute (sfm) or meters per minute (m/min).[1] Feed rate (also often styled as a solid compound, feedrate, or called simply feed) is the relative velocity at which the cutter is advanced along the workpiece; its vector is perpendicular to the vector of cutting speed. Feed rate units depend on the motion of the tool and workpiece; when the workpiece rotates (e.g., in turning and boring), the units are almost always distance per spindle revolution (inches per revolution [in/rev or ipr] or millimeters per revolution [mm/rev]).[2] When the workpiece does not rotate (e.g., in milling), the units are typically distance per time (inches per minute [in/min or ipm] or millimeters per minute [mm/min]), although distance per revolution or per cutter tooth are also sometimes used.[2]

If variables such as cutter geometry and the rigidity of the machine tool and its tooling setup could be ideally maximized (and reduced to negligible constants), then only a lack of power (that is, kilowatts or horsepower) available to the spindle would prevent the use of the maximum possible speeds and feeds for any given workpiece material and cutter material. Of course, in reality those other variables are dynamic and not negligible, but there is still a correlation between power available and feeds and speeds employed. In practice, lack of rigidity is usually the limiting constraint.

The phrases "speeds and feeds" or "feeds and speeds" have sometimes been used metaphorically to refer to the execution details of a plan, which only skilled technicians (as opposed to designers or managers) would know.

  1. ^ Smid 2008, pp. 74, 85–90.
  2. ^ a b Smid 2008, pp. 74, 91–92.

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