A traditional symbol for surface feet per minute, a unit measuring the rate at which a rotating tool, such as a lathe or saw, processes material. The term surface foot refers to the length of the material, similar to the linear foot
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}