RPM to Surface Speed Converter

Surface speed depends on both RPM and the diameter of the rotating part.

Formulas:

• SFM = RPM x Pi x Diameter (inches) / 12
• m/min = RPM x Pi x Diameter (mm) / 1000
• RPM = SFM x 12 / (Pi x Diameter in inches)

SFM = Surface Feet per Minute (used in US machining) m/min = Meters per minute (used in metric machining)

Common cutting speeds (SFM):

• Mild steel with HSS: 60-100 SFM
• Aluminum with HSS: 200-300 SFM
• Mild steel with carbide: 300-500 SFM
• Aluminum with carbide: 800-1200 SFM

Example:

• Turning a 2-inch diameter bar at 500 RPM:
• SFM = 500 x 3.14159 x 2 / 12 = 262 SFM

The key insight for machinists is that the tool doesn’t care about RPM, it cares about surface speed, the rate at which the cutting edge actually passes through the material. That’s why a small drill bit and a large one cutting the same steel need very different spindle speeds: the big one covers more distance per turn, so it needs fewer turns to reach the same surface speed.

Get the surface speed right and tools last and leave a clean finish; run too fast and the edge overheats and dulls, too slow and it rubs instead of cutting. It’s also why facing a large disc on a lathe ideally slows the spindle as the tool moves toward the center, holding surface speed steady as the diameter shrinks, which many CNC lathes do automatically in a “constant surface speed” mode. The speeds above are starting points, since the right value depends on the material, the tool, coolant, and how rigid the setup is.