The milling formula, and the units that trip people up
For milling, multiply the axial depth of cut, the radial width of cut and the feed rate. Keep all three in mm (feed in mm/min) and you get mm³/min — the volume of metal the tool clears each minute. It is the direct measure of how fast a job runs and how hard the machine is working.
Same idea, different operation
| Operation | MRR formula |
|---|---|
| Milling | depth × width × feed |
| Turning | depth × feed × cutting speed |
| Drilling | (π ÷ 4) × D² × feed |
| Grinding | depth × width × feed velocity |
Drilling carries the π/4 factor because the hole is round. Grinding uses the same shape as milling but runs the lowest rate of the four.
Typical rates by material (carbide tooling)
| Material | Typical MRR (mm³/min) |
|---|---|
| Aluminum | 4,000–15,000 |
| Copper | 3,000–8,000 |
| Steel (up to ~400 HV) | 2,000–6,000 |
| Gray cast iron | 800–2,500 |
| Hardened steel | 500–1,500 |
| Titanium | 200–1,000 |
These are working ranges, not limits. The safe number depends on tool, coolant and machine rigidity as much as the material.
What the number is really telling you
- Cycle time. Time = total volume to remove ÷ MRR. Clearing 50,000 mm³ at 2000 mm³/min takes 25 minutes, before tool changes and rapid moves.
- A ceiling, not a target. Too high brings chatter, poor finish and rapid tool wear; too low wastes machine hours. The best rate sits between.
- Roughing vs finishing. Rough at a high rate to clear metal fast, then finish at a low rate for accuracy and surface quality.
Common questions
What is the formula for material removal rate in milling?
MRR = radial depth of cut x axial depth of cut x feed rate. With depths in mm and feed in mm/min, the result is in cubic mm per minute. Divide by 1000 for cubic cm per minute.
How do I convert mm3/min to cm3/min?
Divide by 1000. So 5000 mm3/min is 5 cm3/min. This is the single most common unit slip in MRR work, because a factor of 1000 is easy to lose.
Can I use one high removal rate for every material?
No. Aluminum tolerates a very high rate, steel a moderate one, and titanium a much lower one because it holds heat. Pushing steel numbers into a titanium cut burns the tool and the finish.
