Calculate lathe spindle RPM from cutting speed and workpiece diameter for turning, facing, boring and other rotary machining operations.
Enter cutting speed and workpiece diameter to calculate.
How do I calculate lathe RPM? For metric machining, multiply cutting speed in m/min by 1000 and divide by π times the workpiece diameter in mm.
Why does RPM change with diameter? The circumference travelled by the cutting edge increases with diameter, so fewer revolutions are needed to maintain the same surface speed.
What is cutting speed? Cutting speed is the surface velocity between the cutting tool and workpiece, commonly expressed as m/min or SFM.
What RPM should I use for turning steel? There is no single RPM for all steel. The correct value depends on steel grade, tool material, insert geometry, operation and desired tool life.
Should facing operations use the same RPM? On a conventional fixed-RPM lathe, RPM remains constant while the surface speed changes as the tool approaches the centre. CSS-capable CNC lathes can vary RPM to maintain programmed surface speed.
What happens at the centre during facing? The theoretical surface speed approaches zero at the spindle centreline, so constant-RPM and constant-surface-speed behaviour differ significantly during facing.
What is SFM? SFM means surface feet per minute and is the imperial equivalent of cutting speed expressed in metres per minute.
Can this calculator account for a machine RPM limit? Yes. Enter the maximum spindle RPM and the calculator will show the lower usable RPM and the resulting actual cutting speed.
Can this be used for boring? Yes. Enter the diameter at the cutting edge. For internal machining, verify the recommended cutting speed with the tooling manufacturer.
Can this calculate RPM for CNC lathes? Yes. It calculates the spindle RPM corresponding to the entered cutting speed and diameter. CNC CSS settings may require additional machine-specific parameters.