CNC Cutting Speed and RPM

Cutting speed and spindle speed

Cutting speed is the tangential speed of the cutting edge relative to the workpiece. One revolution covers a distance of πD. Multiplying this by revolutions per minute gives the cutting speed. With diameter in millimetres, divide by 1000 to obtain metres per minute. A 20 mm cutting diameter at 1000 rpm gives 62.832 metres per minute.

Choosing the diameter

Use the machined workpiece diameter for turning and the outside diameter for drilling. With ball or round insert cutters, the effective diameter at cutting depth can differ from the nominal tool diameter. This calculator converts speeds but does not select suitable cutting conditions for a material. Recalculate the cutting speed using the actual spindle speed available on your machine.

Check using the circumference

With effective diameter 40 mm and speed 750 revolutions per minute, one revolution travels 40π mm. Travel per minute is 40π × 750 = 30000π mm, approximately 94.247780 m. Cutting speed is therefore 94.247780 m/min. Entering that speed and diameter 40 mm in the inverse mode returns 750 revolutions per minute. These values do not claim suitability for any particular material.

Halving diameter at unchanged spindle speed halves cutting speed. Maintaining the same cutting speed would require twice the spindle speed. Check the calculated speed against machine, workholding and tooling limits separately. This mathematical conversion does not determine those limits.

Entering radius as diameter changes the result by a factor of two. Cutting speed in metres per minute and feed speed in millimetres per minute are different physical quantities. Check which motion a value describes before transferring it between calculations. For a machine with stepped speed settings, recalculate using the actual selected speed. If diameter changes during cutting, this single diameter calculation describes only the chosen position.

Frequently asked questions

How do you calculate cutting speed for turning?

For example, the tangential speed equation gives 94.25 metres per minute for a workpiece diameter of 50 mm at 600 rpm. This preliminary calculation only converts diameter and spindle speed, and does not select suitable cutting conditions or verify machine safety.

Calculate with these values