Horsepower Calculator
Before you push depth of cut and feed to the limit, check whether your spindle actually has the power to sustain it.
Unit power constants used
| Material | Unit power (hp per in³/min) |
|---|---|
| Aluminum | 0.25 |
| Brass | 0.7 |
| Cast iron (gray) | 0.7 |
| Mild steel (1018) | 1.0 |
| Alloy steel (4140) | 1.25 |
| Stainless steel (304) | 1.3 |
| Titanium (Ti-6Al-4V) | 1.2 |
Representative midpoint values — actual unit power varies with hardness, condition, and how dull the tool is. Treat this as a sanity check against your spindle's rated power, not a precision spec.
Where the numbers come from
Material removal rate (MRR, in³/min) = depth of cut × width of cut × feed rate tells you how much material you're taking off per minute. Multiply that by the material's unit power — how many horsepower it takes to remove one cubic inch per minute of that specific material — and you get the horsepower actually being consumed at the cutting edge. Divide by machine efficiency (accounting for spindle motor and drivetrain losses) to get the horsepower your spindle motor needs to be rated for.
Why efficiency matters
A spindle motor rated at 5 HP doesn't deliver a full 5 HP to the cutting edge — belt drives, gearing, and bearing friction all eat into it, typically leaving somewhere around 75–85% available at the tool. Using 80% as a default assumes a reasonably efficient drivetrain; a older or direct-drive/geared head machine might run closer to 70%, meaning it needs more rated horsepower to deliver the same cutting power.
What to do if the number is close to your spindle's rating
If the calculated horsepower is at or above what your machine is rated for, that's a sign to back off depth of cut, width of cut, or feed before you stall the spindle or overheat the motor — this calculator estimates steady-state cutting power, and doesn't account for interrupted cuts or shock loads, which can demand more instantaneous power than the average value shown here.