Machining power estimates help check whether a planned cut fits the available spindle capacity. The calculation begins with the volume of material removed per minute and the specific cutting force for the actual chip and tool conditions. This reference gives the turning equations in a consistent metric convention and shows how to continue from net cutting power to torque. It does not supply a universal cutting-force value for every alloy or insert.

Machining power and specific cutting energy

Start with the removal rate

For turning, Sandvik expresses metal-removal rate as Q = vc × ap × fn when cutting speed vc is in metres per minute, depth of cut ap is in millimetres, and feed fn is in millimetres per revolution. With those units, Q is in cubic centimetres per minute. The formula describes the ideal swept volume; interrupted cuts, entry and exit, tool nose geometry, and nonuniform engagement can make the actual process differ.

Specific cutting force kc is not a fixed property of a workpiece alloy. Sandvik distinguishes the reference kc1 from the actual kc and explains that chip thickness and rake angle affect the value. A positive rake can reduce the cutting force relative to a neutral insert, while a smaller average chip thickness tends to increase kc. Use the tool supplier’s method and operation-specific inputs instead of assigning one remembered kc number to every cut.

Calculate net cutting power

With metric turning units, the net power equation is Pc = (vc × ap × fn × kc) / (60 × 10³), producing kilowatts when kc is in newtons per square millimetre. As a dimensional check, vc × ap × fn first gives the removal rate in cubic centimetres per minute; multiplying by force per area and converting minutes and millimetres gives power. The divisor is therefore tied to these units. Changing to inches, feet per minute, or horsepower requires the matching conversion formula.

Consider an explicitly hypothetical cut with vc = 200 m/min, ap = 2 mm, fn = 0.2 mm/rev, and a separately selected kc = 1800 N/mm². Q is 80 cm³/min and the estimated net cutting power is 2.4 kW. The example demonstrates arithmetic only: it does not identify a material grade or claim that 1800 N/mm² is correct for a particular tool and chip thickness.

Convert power to torque and compare the machine

At spindle speed n in revolutions per minute, cutting torque is Mc = Pc × 30 × 10³ / (π × n), with Pc in kilowatts and Mc in newton-metres. For the hypothetical 2.4 kW result at 1000 rev/min, the estimated torque is about 22.9 N·m. At lower speed, the same power corresponds to higher torque. Check the spindle’s power-versus-speed curve instead of comparing power alone with a motor nameplate.

Calculated net cutting power does not include transmission losses, spindle acceleration, coolant pumps, chip conveyors, or other auxiliary demand. The motor’s rated power may not be available at every speed or at the tool. Keep a suitable engineering margin and confirm machine, holder, insert, workholding, and process stability limits.

Common interpretation mistakes

Do not confuse specific cutting force kc with a certified material strength, hardness, or a universal energy constant. The value is tied to a cutting model and chip geometry. Do not use feed per tooth in the turning equation where feed per revolution is required. Do not compare a kW result with horsepower without converting, and do not combine the formula numerator from one unit system with the divisor from another.

For drilling and milling, engagement geometry and feed definitions differ, so use the corresponding operation equations. If the process includes a interrupted cut, long overhang, thin wall, or unstable chip, average-power arithmetic may not describe peak loading. Verify unusual or high-risk cuts with the tooling supplier or an instrumented trial.

Sources and limitations

The equations and variable conventions are from Sandvik Coromant turning formula guidance. The discussion of kc1, chip thickness, and rake angle follows its specific cutting force reference. Treat the result as a planning estimate and verify current tool data for the exact material, insert, and operation.