CNC turning speed calculator: RPM, cutting speed, and feed

RPM formula and variables

Metric theoretical RPM n = 1000 × Vc (m/min) ÷ [π × D (mm)]. Imperial theoretical RPM n = 12 × Vc (SFM) ÷ [π × D (inch)]. Vc is cutting speed and D is the workpiece diameter at the calculation position. The formula converts units; it does not recommend Vc.

RPM and feed per minute

Feed per minute = feed per revolution × spindle RPM. Metric units: mm/min = mm/rev × rev/min. Imperial units: in/min = in/rev × rev/min. Confirm the control’s feed mode and that the RPM represents the actual cutting position.

Diameter under constant surface speed

With constant surface speed, the control adjusts RPM as diameter changes, so theoretical RPM rises as diameter falls. Near center, maximum spindle settings, chuck rating, workpiece balance, stickout, and workholding all matter. A web calculator’s optional speed cap is arithmetic only; it cannot read or set the machine.

Selecting process data

Start with insert-maker data for the material group, grade, chipbreaker, and operation, then account for machine rigidity, coolant, workholding, and quality needs. Follow the shop’s process approval for uncertain workpieces or setups.

Review checklist

• Keep cutting speed, diameter, and feed units consistent.
• Use the diameter at the actual cutting position.
• Check spindle, chuck, workpiece balance, and stickout limits.
• Select data from the tool maker and validate it on the target control/machine.

Sources and scope

• NTK Cutting Tools — Calculation Formula for Turning — Source for RPM conversion; not a recommendation for a specific material, insert, or machine.
• Haas NGC Lathe Operator’s Manual (2024) — Haas NGC constant-surface-speed and speed-limit context only.
• Online RPM calculator — Browser-local unit conversion; it does not validate machining suitability.

Related reading

• G96 vs. G97: constant surface speed, fixed RPM, and limits
• Open online calculators
• Read the FAQ
• Get product support