Choosing the right spindle speed (S) and feed (F) on a CNC lathe directly affects tool life, surface finish and cycle time. The Mentor CNC calculator below focuses purely on turning: it calculates the spindle RPM to program and the feed per revolution (fn) from the machined diameter D and the cutting speed Vc, and generates ready-to-use G96/G97 code examples. If you do not know the Vc or fn value, general starting values based on the selected material and machining type are applied automatically. With the reverse calculation in the second section, you can find the actual cutting speed (Vc) from the diameter and RPM you are already using and compare it with the recommended value for the material.
CNC Lathe Speed and Feed Calculator
Calculate the CNC lathe spindle speed and feed from the workpiece diameter, cutting speed Vc and feed per revolution fn. If you do not know the insert values, general starting values based on the selected material and machining type can be used.
CNC Lathe Speed and Feed Calculation
Calculate spindle speed and feed from the Vc value
G97 Constant RPM Example
G96 Constant Surface Speed Example
G99 on some CNC controls and with G95 on others. Check your machine's programming manual.Calculate Vc from the Current RPM
Check the actual cutting speed from the diameter and RPM in use
How Is CNC Lathe RPM Calculated?
CNC lathe spindle speed is calculated from the cutting speed Vc and the workpiece diameter. At the same cutting speed, the required RPM increases as the diameter gets smaller. In G96 constant surface speed mode, the machine automatically adjusts the RPM as the diameter changes. In G97 mode, the programmed spindle speed stays constant.
How Is Feed Calculated on a CNC Lathe?
Turning feed is usually programmed in millimeters per revolution. For example, F0.200 means the cutting tool advances 0.20 mm for each revolution of the workpiece. The feed per minute is found by multiplying the fn value by the spindle speed.
What should I do if I do not know the Vc value?
You can select the workpiece material. The calculator uses the general Mentor CNC starting value defined for coated carbide inserts. The insert manufacturer's catalog or box value always takes priority.
What happens if I enter my own Vc value?
When you enter a value in the Vc field, the automatic cutting speed from the material list is not used. The calculation is based entirely on the Vc value you entered.
Why is the maximum RPM important when using G96?
As the tool approaches the center, the calculated RPM can rise very quickly. Therefore, a maximum RPM appropriate for the machine, chuck, workholding and workpiece limits must be defined.
The automatic Vc and fn values are provided for training and initial setup purposes. Insert grade, coating, geometry, material hardness, depth of cut, tool overhang, workholding rigidity, coolant and machine power can all change the real cutting values. The cutting tool manufacturer's current catalog data takes priority. The first trial should be performed under safe conditions; vibration, sound, chip formation and tool wear should be monitored.
Also check out our comprehensive calculator covering turning, milling and drilling with material removal rate, machining time and three-level (safe / normal / productive) recommendations: CNC Turning, Milling and Drilling Speed and Feed Calculator (Vc, n, fn, fz)