Feed rate is where finish, tool life, and cycle time all meet. Set it from chip load — the bite each flute takes per revolution — rather than guessing, and you get predictable results across cutters and machines. This calculator converts between feed rate and chip load in both directions and offers per-material starting points.
How it works
The core formula multiplies the spindle speed by how many edges cut and how much each one removes:
feed (IPM) = RPM × flutes × chip load per tooth
chip load (IPT) = feed (IPM) / (RPM × flutes)Because every flute removes its chip once per revolution, a 4-flute cutter feeds twice as fast as a 2-flute cutter at the same RPM and chip load.
Worked example
A 4-flute end mill at 4,000 RPM with a 0.003 inch chip load gives a feed of 4000 × 4 × 0.003, or 48 IPM. If you later drop to a 2-flute cutter and want the same load per tooth, the feed must halve to 24 IPM.
Material starting points
Chip load varies significantly by material. A rough guide for a 1/4” end mill:
| Material | Typical starting chip load (inch/tooth) |
|---|---|
| Aluminum (6061) | 0.003 – 0.006 |
| Mild steel | 0.001 – 0.003 |
| Stainless steel | 0.001 – 0.002 |
| Cast iron | 0.002 – 0.004 |
| Titanium | 0.001 – 0.002 |
| Plastic / Delrin | 0.004 – 0.008 |
Scale these down for smaller diameter cutters — a 1/8” end mill in aluminum might run 0.001–0.002”. Always consult the tool manufacturer’s recommendation for your specific cutter geometry and coating.
What affects chip load choice
- Radial engagement — slotting (full width) demands a lower chip load than a light side-cut (adaptive toolpath). At full slot, heat and force peak; cut chip load by 30–50%.
- Axial depth — very deep cuts also raise heat; reduce chip load proportionally.
- Tool diameter — smaller tools are more flexible and break more easily; run lighter chip loads.
- Coatings — AlTiN and similar coatings allow higher chip loads in harder materials by reducing heat transfer into the tool.
- Rigidity — a long stick-out or a lightweight machine demands lower feeds to avoid chatter.
Reading chip appearance
Chips tell you whether the feed is right. In aluminum, curly silver chips carrying heat away are healthy; fine powder means the feed is too low and the tool is rubbing. In steel, blue chips signal too much heat — slow down or add coolant. On plastics, melted or welded chips indicate the feed is too low; raise it so chips clear before the material re-melts.