|
Machining uses two common ways to talk about speed, and they are easy to mix up. SFM (surface feet per minute) and RPM (revolutions per minute) both describe motion, but they describe different kinds of motion. The difference becomes much clearer when you focus on what is moving: SFM describes how quickly the cutting edge moves across the material, while RPM describes how quickly the machine rotates the tool.
These measurements are connected, but they are not interchangeable. A tool can rotate at a certain RPM while its cutting edge travels a different distance depending on the tool’s size.
Think of it this way: A large wheel covers more distance per rotation, so it spins more slowly. A small wheel covers less distance per rotation, so it must spin faster. Machining works the same way: larger tools need lower RPM, while smaller tools need higher RPM.
| 
