SFM stands for surface feet per minute, the cutting speed at the tool edge, set by the work material and tool material. Spindle speed in rpm equals SFM times 3.82 divided by the tool or work diameter in inches, so a half-inch carbide endmill at 800 SFM in aluminum turns about 6,100 rpm. Metric shops use meters per minute instead.
That's HSS in 4140, keep it under eighty SFM or you'll cook the edge in one pass.
Rpm equals SFM times 3.82 over diameter; at the machine most people round it to four times SFM over diameter and adjust by ear.
Also heard as
- SFPM
- surface speed
- cutting speed
Related terms
Feeds and speeds
The spindle speed and feed rate chosen for a cut, set from material, tool and machine.
Chip load
The thickness of material each cutting edge takes per revolution, also called feed per tooth.
HSS
High-speed steel: the tough, hand-grindable tool steel used for drills, taps, lathe bits and some endmills.
Blue chips
Steel chips that come off straw to dark blue from heat, read as a sign of how hard a cut is running.