All data is as-is, reference only.
This formula applies to straight turning on a lathe, drilling, and milling. It converts surface feet per minute (the speed on the outside diameter of a spinning cylinder) to RPM (revolutions per minute).
The surface speed is often referred to as the cutting speed and is abbreviated Vc. The diameter can be a spinning workpiece as on a lathe, or a spinning cutter such as an end mill or drill bit. The diameter is often abbreviated D, and refers to the diameter of the cylinder where the speed applies, usually the periphery. The RPM is sometimes abbreviated as n.