Cutting Speed vs Spindle Speed in CNC Milling

Cutting speed and spindle speed are closely related, but not the same.

Cutting speed (Vc) refers to the speed of the tool's cutting edge relative to the material surface, measured in meters per minute (m/min). Spindle speed is simply the number of revolutions per minute the tool makes.

The relationship between cutting speed and RPM

The formula is:

rpm = (1000 × cutting speed) / (π × tool diameter)
    

You can automate this calculation using the CNC RPM Calculator.

Why cutting speed matters

• Higher cutting speeds can reduce cutting forces
• Too high cutting speeds generate excessive heat
• Tool manufacturers specify ideal Vc values per material

Always adjust spindle speed based on tool diameter. Larger tools require lower RPM to maintain the same cutting speed.