Metric CNC Feed Rate Calculator
Calculated Feed Rate
Understanding Metric CNC Feed Rate Calculations
In CNC machining, calculating the correct feed rate is critical for optimizing production time, extending tool life, and ensuring high-quality surface finishes. For shops operating in metric units, mastering the relationship between Spindle Speed (RPM), Number of Flutes, and Chip Load is essential.
This calculator determines the Feed Rate (F) in millimeters per minute (mm/min), which dictates how fast the cutting tool moves through the material.
The Metric Feed Rate Formula
The calculation is based on the following fundamental physics of machining:
Feed Rate (mm/min) = RPM × T × CL
- RPM (N): The rotational speed of the spindle (Revolutions Per Minute).
- T: The number of teeth or flutes on the cutting tool.
- CL: Chip Load, or feed per tooth (measured in millimeters).
Why Chip Load Matters
Chip load is arguably the most important variable in this equation. It represents the thickness of the material removed by each cutting edge during one revolution. If the chip load is too low (rubbing), it causes excessive heat and premature tool wear. If it is too high, it can break the tool or damage the spindle.
Common Metric Chip Load Examples
While specific chip loads depend on the tool diameter and material hardness, here are general starting points for a 6mm – 12mm Carbide End Mill:
| Material | Typical Chip Load (mm/tooth) |
|---|---|
| Aluminum (6061) | 0.025 mm – 0.150 mm |
| Mild Steel | 0.015 mm – 0.080 mm |
| Stainless Steel | 0.010 mm – 0.050 mm |
| Plastics (Acrylic/Delrin) | 0.100 mm – 0.300 mm |
| Wood / MDF | 0.150 mm – 0.400 mm |
Optimizing for Surface Finish vs. Roughing
When calculating your metric feed rate, consider the operation type:
- Roughing: aim for the higher end of the chip load recommendations to maximize Material Removal Rate (MRR) and evacuate heat via the chips.
- Finishing: Reduce the chip load slightly or increase the RPM to achieve a smoother surface finish (lower Ra value).
Troubleshooting Feed Rate Issues
If you hear a high-pitched screaming sound, your feed rate is likely too low relative to your RPM (rubbing). Increase the feed rate or decrease the RPM. conversely, if you hear a low-frequency hammering sound, the chip load is likely too aggressive for the machine's rigidity or the tool's strength.