Feed Rate for CNC Turning
Understand what feed means, calculate tool travel per minute, and choose a feed that fits your insert, material, and setup.
Feed per revolution vs. feed per minute
Feed per revolution (f)
Feed per revolution is the distance the cutting tool advances along the programmed path during one spindle revolution. In turning and boring, it is commonly expressed in inches per revolution (IPR or in/rev) or millimeters per revolution (mm/rev).
Feed per minute (Vf)
Feed per minute is the tool’s travel per minute, expressed in inches per minute (IPM or in/min) or millimeters per minute (mm/min). It depends on both feed per revolution and spindle speed.
| Quantity | Symbol | U.S. customary units | Metric units |
|---|---|---|---|
| Feed per revolution | f | in/rev (IPR) | mm/rev |
| Feed per minute | Vf | in/min (IPM) | mm/min |
| Spindle speed | N | rev/min (RPM) | rev/min (RPM) |
Calculate the feed
Feed per minute
Feed per revolution
Use matching units throughout the calculation. To convert in/rev to mm/rev, multiply by 25.4.
| in/rev (IPR) | mm/rev |
|---|---|
| 0.002 | 0.0508 |
| 0.004 | 0.1016 |
| 0.006 | 0.1524 |
| 0.008 | 0.2032 |
| 0.010 | 0.2540 |
| 0.012 | 0.3048 |
| 0.016 | 0.4064 |
| 0.020 | 0.5080 |
Conversions only—not recommended cutting settings.
Example
At 1,000 RPM and 0.012 in/rev, the tool advances at 12 in/min. The equivalent values are 0.3048 mm/rev and 304.8 mm/min.
This example demonstrates the calculation; it is not a recommended cutting setting.
Choose a feed for the cut
Start with the insert manufacturer’s recommended feed range for your material, grade, chipbreaker, and depth of cut. Then account for the required finish, nose radius, workholding, tool overhang, and machine capability.
Roughing
Use a feed within the insert’s working range that supports chip control and productive material removal without overloading the setup.
Finishing
Balance feed with nose radius and surface-finish requirements. Reducing feed can improve the theoretical finish, but going too low can cause rubbing or poor chip control.
Boring
Pay close attention to bar overhang, rigidity, and chip evacuation. A feed that works in external turning may not suit a less rigid boring setup.
When the feed needs adjustment
| Feed condition | What you may notice | What to check |
|---|---|---|
| Too high for the setup | Heavy cutting load, deflection, chatter, or edge chipping | Check the insert’s feed range, depth of cut, workholding, and tool overhang. |
| Too high for the required finish | Pronounced feed marks or excessive surface roughness | Check feed against nose radius, insert geometry, and the required finish. |
| Too low for the insert and cut | Rubbing, unstable cutting, or long chips that do not break as intended | Check the chipbreaker’s minimum feed and depth-of-cut requirements. |
| Feed appears inconsistent | Changes in tool travel as spindle speed changes | Confirm whether the control is using feed per revolution or feed per minute. |
These symptoms can have more than one cause. Check tool condition, cutting speed, coolant, and setup rigidity before changing feed alone.
Check the control mode
Confirm the active feed mode before running the program. Feed-per-revolution and feed-per-minute commands vary by control and machine configuration. With constant surface speed, RPM changes with cutting diameter; the active feed mode determines how tool travel responds.