Glossary & Reference
Cutting speed — also called surface speed or SFM — is how fast the workpiece surface is actually moving past the cutting edge. It's not the same thing as RPM, and mixing the two up is the single most common speed-and-feed mistake new machinists make.
Cutting speed is the tangential, or surface, velocity between the workpiece and the tool's cutting edge — how fast the material itself is passing the edge, measured at the diameter being cut. "Cutting Speed," "Vc," "SFM," and "Surface Speed" are all names for the same thing. In the US it's expressed in Surface Feet per Minute (SFM); almost everywhere else in the world it's meters per minute (m/min).
Imperial: SFM = (RPM × D) / 3.82, where D is the tool or workpiece diameter in inches. Metric: Vc (m/min) = (π × D × N) / 1000, where D is the diameter in millimeters and N is spindle speed in RPM. Both formulas describe the same physical relationship shown in the diagram — diameter and RPM together determine the actual surface speed at the cutting edge.
Recommended cutting speeds are really a material-and-tool-grade recommendation, not an RPM number. That means for a fixed target Vc, a larger-diameter tool or workpiece needs a lower RPM to hit that same surface speed, and a smaller diameter needs a higher RPM. This trips people up constantly: the speed chart never changes, but the RPM you dial in has to change every time the diameter does.
Cutting speed is one of the primary drivers of tool life, heat generation at the edge, and surface finish. Run it too slow and you risk built-up edge and a poor finish; run it too fast and you overheat and burn the cutting edge, shortening tool life fast. Recommended speeds vary a lot by workpiece material and tool grade or coating, which is exactly why manufacturers publish speed-and-feed charts broken out by material family.