Dial in spindle speed and feed rate for CNC milling using common metric formulas. Select a material preset or enter your own values.
What is feed speed in machining?
In CNC milling, feed speed (or feed rate) is how fast the cutter moves through material, usually in mm/min. Feed speed works together with spindle RPM, tool size, flute count, and material type. If feed is too high, you can overload or break tools. If feed is too low, you can rub instead of cut, generating heat and reducing tool life.
The formulas used by this feed speed calculator
1) Spindle Speed (RPM)
RPM = (1000 × Vc) ÷ (π × D)
- Vc = cutting speed in m/min
- D = cutter diameter in mm
2) Feed Rate (mm/min)
Feed = RPM × Z × fz
- Z = number of flutes
- fz = feed per tooth in mm/tooth
How to use this calculator effectively
Step-by-step workflow
- Select a material preset for quick starting values, or choose custom input.
- Enter your cutter diameter, flute count, and feed per tooth.
- Optionally enter max machine RPM to prevent unrealistic recommendations.
- Click Calculate Feed & Speed.
- Use output as a starting point, then tune based on spindle load, chip shape, and finish quality.
Example calculation
Suppose you run a 10 mm, 4-flute carbide end mill in mild steel:
- Vc = 120 m/min
- D = 10 mm
- Z = 4
- fz = 0.05 mm/tooth
RPM = (1000 × 120) / (π × 10) ≈ 3,820 RPM
Feed = 3,820 × 4 × 0.05 ≈ 764 mm/min
That gives a practical first-pass recipe. After a short test cut, you can increase or reduce feed by 5–15% based on chip color, vibration, sound, and spindle load.
Common mistakes to avoid
- Mixing units: m/min, mm, and mm/tooth must stay consistent.
- Ignoring flute count: doubling flutes doubles feed potential at the same chip load.
- Using catalog max values blindly: conservative starts are safer for unknown setups.
- Not accounting for machine limits: max RPM constraints can force lower cutting speed than target.
- Confusing feed per tooth with feed per revolution: they are not the same input.
Quick tuning rules after first cut
If you hear chatter
- Reduce radial engagement first.
- Lower RPM slightly and test again.
- Check tool stickout and workholding rigidity.
If chips are dusty or tool is rubbing
- Increase feed per tooth slightly.
- Verify tool edge is sharp and not worn.
- Ensure coolant/air blast clears chips.
If tool wear is too fast
- Reduce cutting speed (Vc) 10–20%.
- Keep chip load reasonable to avoid heat concentration.
- Use appropriate coating and flute geometry for the material.
Final thought
A good feed speed calculator helps you start from proven math instead of guesswork. Use these numbers as an intelligent baseline, then optimize with short test cuts and data from your actual machine. Over time, your own material-and-tool library will become more accurate than any generic chart.