The cutting tool must supply force to plastically shear the workpiece material and overcome friction.

The resultant machining force is often resolved into components. In a simple turning interpretation:

• the cutting force $F_c$ acts mainly in the cutting-speed direction;
• the feed force acts along the feed direction;
• a radial or thrust component acts normal to the generated surface.

The mechanical power delivered to the primary cutting motion is approximately

$$P=F_cV,$$

where $V$ is cutting speed in metres per second and $F_c$ is in newtons.

For example, if $F_c=800\ \mathrm N$ and $V=2.0\ \mathrm{m/s}$,

$$P=800\times2.0=1600\ \mathrm W=1.6\ \mathrm{kW}.$$

A useful material/process measure is specific cutting energy $u$, the energy required per unit volume removed:

$$u=\frac{P}{\mathrm{MRR}}.$$

If the same cut removes $400\ \mathrm{mm^3/s}$,

$$u=\frac{1600}{400}=4\ \mathrm{J/mm^3}.$$

Specific cutting energy is not a universal material constant. It depends on material, tool geometry, chip thickness, friction and cutting conditions. At very small chip thicknesses, the apparent energy per unit volume can rise because edge and friction effects become comparatively important.

Force and power estimates are used to size spindles and drives, check fixture loads, anticipate tool deflection and compare feasible machining conditions.