Spindle horsepower needed for a cut is the material removal rate multiplied by the unit power (specific cutting energy) of the material. Tougher metals need more horsepower per cubic inch removed.
HP = MRR × P
What the variables mean
| Symbol | Meaning |
|---|---|
| HP | Spindle horsepower required |
| MRR | Material removal rate (in³/min) |
| P | Unit power for the material (HP per in³/min) |
Typical unit power
| Material | Unit power P (HP per in³/min) |
|---|---|
| Aluminum | 0.3 |
| Cast iron | 0.5 – 0.8 |
| Mild steel | 1.0 |
| Alloy steel | 1.1 – 1.4 |
| Stainless steel | 1.3 – 1.5 |
| Titanium | 1.1 – 1.3 |
Worked example
Removing 3.75 in³/min of mild steel (P = 1.0): HP = 3.75 × 1.0 = 3.75 HP at the spindle. Divide by drive efficiency (~0.8) for motor HP.
Run it instantly with the Feeds & Speeds calculators.