Discrete ManufacturingFree calculatorPDF, Excel, Word exportCNC Machining

CNC Spindle Power and Torque Required Calculator

Before you push a roughing cut, you want to know if the spindle can carry it. This calculator turns depth, width, feed and specific cutting force into material removal rate, cutting power, motor power and torque.

Your numbers

Axial depth of cut (ap)

Depth of the cut along the tool axis.

Radial width of cut (ae)

Width of the cut across the tool.

Table feed rate (vf)

mm/min

Specific cutting force (kc)

N/mm2

Typical: aluminium 700-900, mild steel 1800-2200, stainless 2200-2600, titanium 1400-1700, hardened steel 3000+.

Machine efficiency

0.8

Spindle drive and gearbox efficiency, usually 0.75 to 0.9.

Spindle speed

rpm

Used only for the torque result.

Your results

Spindle motor power required

4.1667

kW the drive must supply, including losses.

Spindle torque required

13.2625

Newton metres at the spindle for the entered rpm.

Material removal rate

100

In cm3 per minute.

Cutting power at the tool

3.3333

kW delivered to the cut.

Motor power in horsepower

5.5875

Power per unit removal rate

0.0417

kW needed for each cm3/min removed.

This is a first-order estimate. Real kc varies with chip thickness, rake angle, tool wear and material condition. Compare the result with the power and torque curve of your machine at the chosen rpm, not just its peak rating.

Save results:

Motor power required vs feed rate

Sweeps Table feed rate (vf) from 100 to 3000, holding your other inputs fixed.

Pull spindle load and job data into your ERP

Optional: a short review of how to connect machine data to your ERP job records.

Optional. The tool and every export work without this. No spam.

How to use this calculator

Enter the axial depth ap, radial width ae and table feed vf from your program. Pick kc for your material from the help text or your tooling catalogue, then set the drive efficiency and spindle speed. The primary results are the motor power in kW and the torque in Nm. Compare both with your machine's power-torque curve at that rpm. If the torque needed is higher than the spindle delivers at that speed, the cut will stall or the spindle will load out even when the peak kW looks fine.

Use the real vf from the program, not the programmed override.
Use a catalogue kc for the material and chip thickness you expect.
Check the torque at your rpm, not only the nameplate power.

The formula behind it

Material removal rate is ap times ae times vf, divided by 1000 to get cm3 per minute. Cutting power is Pc = ap x ae x vf x kc / (60 x 10^6) in kW, where kc is the specific cutting force in N/mm2. Motor power is Pc divided by efficiency. Torque follows from power: T = P x 9549 / n, with P in kW and n in rpm. For example, a 5 mm by 20 mm cut at 1000 mm/min in steel with kc 2000 removes 100 cm3/min and needs about 4.2 kW at the tool, or 5.2 kW at the motor with 0.8 efficiency.

Reading the result

A good rule is to stay under about 80 percent of the continuous rating of the spindle at the chosen rpm, to leave room for tool wear, which raises kc by 10 to 30 percent as the edge dulls. If you are over, lower feed first, then width, then depth, because power scales linearly with each. The chart shows how motor power grows with feed so you can find the feed that fits your machine.

Power is linear in ap, ae and vf.
Torque rises sharply at low rpm on geared spindles.
Worn tools need more power than new ones.

Common mistakes

The most frequent error is using the machine's peak horsepower rating without checking the rpm band where it is available. Many mills reach full power only above 4000 rpm. Another is using kc from a different chip thickness, since kc rises when the chip gets thin. Finally, people forget that high-feed and trochoidal strategies change the effective ae, so use the engaged width, not the stepover of a different strategy.

Frequently Asked Questions

What is specific cutting force kc?

It is the cutting force per unit of chip cross-section, in N/mm2, for a given material and chip thickness. Tooling catalogues publish kc1.1 values and a correction exponent. This tool uses a single average value, which is accurate enough for a power check but not for precise force prediction.

What efficiency should I use?

Direct-drive and belt-driven spindles are usually 0.85 to 0.95, while geared spindle heads are typically 0.7 to 0.85. If you do not know, 0.8 is a conservative default for a milling machine and slightly overstates the motor power needed.

Why does torque matter as well as kW?

Power is torque times speed. At low rpm a spindle can only deliver limited torque, so a large cutter at low speed can stall even if the motor has enough kW at higher rpm. Always check the torque result against the machine curve.

Does this work for turning and drilling?

The same power idea applies, but the removal rate differs. For turning, use ap x feed per rev x cutting speed. This page is set up for milling with ap, ae and table feed.

If your CNC machine data lives in an ERP or MES, we can help you pull actual spindle load and job data into one view. This calculator needs none of that.

Talk to an Expert Explore More Free Tools

Related Tools

Discrete Manufacturing

More Discrete Manufacturing tools

See all

Back to All Tools