Spindle Speed Calculator
Estimate the spindle speed and feed rate for lathe machining and milling operations using the spindle speed calculator.
Spindle speed2,292 RPM300 ft/min on 0.5 in diameter
Surface speed300 ft/min91.44 m/min
Metric equivalent2,292 RPMfrom Vc × 1000 ÷ (π × D in mm)
Feed rate36.669 in/min931.4 mm/min
Feed per revolution0.016 in
Feed per tooth0.004 in0.1016 mm
Material removal rate1.8335 in³/min30.05 cm³/min
Spindle power needed1.83 hpabout 1 hp per in³/min in steel; a third of that in aluminium
Tool tip travel per minute300 ftwhich is the surface speed, by definition
At half the diameter4,584 RPMsmall tools must spin faster for the same surface speed
The formula
RPM = surface speed × 12 ÷ (π × diameter)
Surface speed is the real variable
A cutting edge cares how fast it is moving through the metal, not how fast the spindle turns. That surface speed is fixed by the tool material and the workpiece — so a ¼-inch cutter must spin twice as fast as a ½-inch one to cut at the same rate.
The 12 in the formula converts feet to inches. In metric it becomes Vc × 1000 ÷ (π × D), with Vc in metres per minute and D in millimetres — the same relationship.
Feed per tooth matters as much as speed. Too light and the edge rubs rather than cuts, which work-hardens the surface and kills the tool faster than a heavy feed would.