What is spindle speed?

A milling cutter must spin when traveling through the stock to allow for material removal. The rate at which the cutting tool rotates is called the spindle speed, measured in RPM (Revolutions Per Minute). The spindle is the part of the milling machine that holds and rotates the cutter.

Each cutter has its own spindle speed, depending on the type of material being cut and the size (diameter) of the cutter. When cutting the same type of material, the smaller the cutter, the faster it must rotate. This is because every material has an "Ideal speed". This is the optimum speed at which the material can be cut safely, in order to obtain a good quality of finish. These ideal speeds are calculated on the assumption that the material is cut with just one blade and that the blade moves in a straight line.

The speed at any point on the periphery (outside edge) of a cutter must always be equal to the ideal speed for the material for it to work at its optimum performance.

To calculate the RPM that a cutter should rotate, the simple formula shown below is used:

Spindle RPM = [Ideal Cutting Speed (Meters/Minute) x 1000] / [Diameter of the Cutter (mm) x PI] where PI=3.142
The only information we don't have at the moment is the ideal speed for a material. The table below shows a list of common materials and their cutting speeds, when using a H.S.S. (High Speed Steel) Cutter.

For example: To calculate the correct spindle speed for a 10 mm cutter machining plastic, the equation is as follows:
Spindle RPM = [Ideal Cutting Speed (Meters/Minute) x 1000] / [Diameter of the Cutter (mm) x PI] where PI = 3.142
Spindle RPM = [250 x 1000] / [10 x 3.142]
Spindle RPM = 7957 revs/min.