Lathe motor power calculator
Enter your cutting parameters to find out how much motor power your lathe needs. Uses the Kienzle specific cutting force method for accurate results by material.
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Required motor power
2.88kW
Net cutting power
2.12 kW
Power at motor shaft
2.31 kW
Cutting force
1,061 N
Specific force kc
2,121 N/mm²
h = fn × sin(κr) = 0.250 mm
kc = 1,500 × 0.250^(-0.25) = 2,121 N/mm²
Fc = kc × ap × fn = 1,061 N
Pc = Fc × Vc / 60,000 = 2.121 kW (net)
P_shaft = 2.121 / 0.92 = 2.306 kW
P_required = 2.306 × 1.25 = 2.882 kW
kc = 1,500 × 0.250^(-0.25) = 2,121 N/mm²
Fc = kc × ap × fn = 1,061 N
Pc = Fc × Vc / 60,000 = 2.121 kW (net)
P_shaft = 2.121 / 0.92 = 2.306 kW
P_required = 2.306 × 1.25 = 2.882 kW
Specific cutting force (kc) reference by material
kc1 is the specific cutting force at 1 mm chip thickness. The Kienzle exponent mc describes how kc rises as chip thickness decreases. Values below are from published machining data (Sandvik Coromant, Machinery’s Handbook); treat them as starting points and adjust if your measured power consumption differs significantly.
| Material | kc1 (N/mm²) | mc | kc at h=0.2 mm (N/mm²) |
|---|---|---|---|
| Aluminium (6061 / free-cutting) | 350 | 0.25 | 523 |
| Brass (free-cutting CZ121) | 750 | 0.22 | 1,069 |
| Copper | 1,100 | 0.26 | 1,672 |
| Mild steel (1018–1025) | 1,500 | 0.25 | 2,243 |
| Carbon steel (1040–1050) | 1,700 | 0.25 | 2,542 |
| Stainless steel (304) Stainless has a high kc. Use a rigid setup, sharp insert, and flood coolant. | 2,800 | 0.21 | 3,926 |
| Cast iron (gray) | 1,100 | 0.28 | 1,726 |
| Titanium (Ti6Al4V) kc for titanium spans 1300–2500 N/mm² depending on grade; treat this as a working estimate. | 1,800 | 0.23 | 2,606 |
| Hard plastic (nylon, acetal, ABS) | 150 | 0.20 | 207 |
Frequently asked questions
How do I calculate how much power a lathe motor needs?
The cutting power in kilowatts is Pc = kc × ap × fn × Vc / 60,000, where kc is the specific cutting force of the material (N/mm²), ap is the depth of cut (mm), fn is the feed per revolution (mm/rev), and Vc is the cutting speed (m/min). Divide by drive efficiency to get the power your motor must deliver, then multiply by a safety factor of 1.25 for continuous work.
What is specific cutting force (kc) and why does it matter?
Specific cutting force is the force needed to cut one square millimetre of chip cross-section, measured in N/mm². It depends on the material and the chip thickness. Harder or stickier materials like stainless steel or titanium have a much higher kc than aluminium or brass, which is why they demand more motor power for the same depth of cut and feed.
What is the Kienzle formula for kc?
The Kienzle formula is kc = kc1 × h^(-mc), where kc1 is the specific cutting force at chip thickness h = 1 mm (a material constant), mc is a dimensionless exponent that describes how kc rises as chip thickness decreases, and h is the chip thickness (feed per rev × sin(lead angle)). Thinner chips require more force per unit area, so kc rises as feed decreases.
What safety factor should I use when sizing a lathe motor?
A safety factor of 1.25 is standard for general turning: it adds 25% above the theoretical minimum to allow for tool wear, material variation, and occasional interrupted cuts. Use 1.5 if the lathe will run at or near full cut continuously, or if your material is inconsistent. A factor of 1.0 is only appropriate for short proving cuts.
How does drive type affect the motor power I need?
Every stage of mechanical transmission loses a small amount of power to friction and heat. A direct-drive or VFD-coupled motor is about 95% efficient, a V-belt loses around 8%, a gear head around 10%, and a back-gear arrangement can absorb 20% or more. The calculator accounts for this so the required motor rating always refers to the nameplate power, not the power that reaches the spindle.
My lathe motor is smaller than the calculator recommends. Can I still do the cut?
You can often still make the cut by reducing one or more parameters. Halving the depth of cut roughly halves the power requirement; reducing feed per revolution also helps, though less directly because kc rises as chips get thinner. Start conservative, watch your motor temperature, and increase the cut only when it is running comfortably.
Pc = kc × ap × fn × Vc / 60,000; kc = kc1 × h^(-mc), h = fn × sin(κr). Always add a safety margin and account for drive losses before choosing a motor.