Cutting speed of lathe machine

Cutting Speed :

The cutting speed of the lathe machine is cutting tool is the speed at which the metal is removed by the tool from the work piece.
In a lathe, it is the peripheral speed of the work past the cutting tool expressed in meters per minute.

Cutting speed = Πdn/1000 m / min

Where d = diameter of the work in mm
n = R.P.M of the work

Some example related to cutting speed : 

A steel shaft of 20 mm diameter is turned at a cutting speed of 40 m / min. Find the RPM of a shaft?