CNC / machinist calculator
Spindle Power and Torque Calculator
Find out whether your machine can drive a cut before you crash into the limit. Enter the material and the material removal rate and this calculator returns the horsepower at the cut, the horsepower the motor must deliver after losses, and the spindle torque at your working RPM. Give it your spindle rating and it tells you how much headroom you have, or by how much you are over.
- Power at the cut
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- Power at the motor
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- Spindle torque
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How it works
Cutting power scales with how much metal you remove and how hard that metal is to cut. The cutting horsepower is the material removal rate in cubic inches per minute times the material's unit power, a published figure that runs about a third of a horsepower per cubic inch per minute for aluminum, around one for mild steel, and one and a half or more for stainless and titanium. Dividing by the drive efficiency, typically about eighty percent, gives the horsepower the motor must actually supply.
Torque is what stalls a spindle at low RPM, not horsepower. For a given horsepower the torque is inversely proportional to speed, so a heavy cut at a few hundred RPM can demand more torque than the spindle has even though the horsepower looks fine. The calculator reports torque in pound-feet at your working speed so you can check the binding case.
Compare the motor horsepower to your spindle rating at the RPM you are running, since many spindles lose power below a corner speed. If the cut needs more than the machine has, reduce the removal rate by taking a lighter or narrower cut, or accept a lower feed.
Worked example
Removing 0.5 in³/min of mild steel (unit power ~1.0) needs about 0.50 HP at the cut and 0.63 HP at the motor after 80% efficiency, well within a 3 HP spindle.
Frequently asked questions
How much horsepower does a milling cut need?
Multiply the material removal rate in cubic inches per minute by the material's unit power, then divide by drive efficiency. Steel needs roughly 1 HP per cubic inch per minute, aluminum about a third of that, and stainless or titanium more.
What is unit power or unit horsepower?
Unit power is the horsepower needed to remove one cubic inch of a material per minute with a sharp tool. It bundles the material's cutting resistance into one figure, so power is just the removal rate times unit power.
Why does torque matter more than horsepower sometimes?
Torque is what turns the cutter against the cut, and for a fixed horsepower it falls as RPM rises. At low spindle speeds a heavy cut can exceed the available torque and stall even when the horsepower rating looks sufficient.
Should I compare the cut to cutting HP or motor HP?
Compare it to motor horsepower, which includes drive and spindle losses, against your spindle rating at the RPM you are running. Many spindles deliver less than their peak rating below a corner speed, so check power at your actual RPM.
What do I change if the cut needs more power than I have?
Reduce the material removal rate: take a shallower axial depth, a narrower radial width, or a lower feed. Dropping the surface speed lowers RPM but not the power directly, though it can help with a torque-limited stall at low speed.
Related calculators
Sources
- Machining Doctor - machining power calculator and Kc reference
- Speeds and feeds - power requirements (Wikipedia)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.