SFM ↔ RPM
Instantly convert cutting speed (SFM) to spindle speed (RPM) based on tool or workpiece diameter.
The Importance of Surface Footage
Why do we calculate speeds in SFM rather than just picking an RPM? Because the tool edge cares about linear velocity, not rotational speed.
A 0.250" endmill spinning at 3,000 RPM is dragging its cutting edge through the material at ~196 surface feet per minute. If you swap to a 1.0" endmill and leave the spindle at 3,000 RPM, the cutting edge is now tearing through the material at 785 SFM. For most steels, that will instantly burn up a carbide tool.
The Constant (3.82)
The number 3.82 in the formula is an approximation of 12 / π. It converts the diameter from inches to feet, and relates the circumference to the diameter.