CNC Milling

Process Definition & Working Principle

CNC (Computer Numerical Control) milling is a subtractive manufacturing process where excess material is removed from a raw workpiece (billet, bar stock, or casting) using a rotating multi-point cutting tool. Unlike CNC turning, where the workpiece rotates, in milling, the primary cutting motion (rotation) is executed by the tool mounted in the spindle, while the feed motion is generated by the linear or rotational translation of the workpiece (via the machine table) and/or the spindle head itself.

The physical principle of the process is based on plastic shear deformation. As the tool's cutting edge penetrates the workpiece material at a specific chip thickness $h$, high-concentration shear stress is generated along the shear plane. This leads to the crystallographic failure of the material and the formation of a chip, which is then evacuated through the helical flutes of the end mill. Extreme heat is generated in the cutting zone due to friction and plastic deformation; over 70-80% of this heat is carried away by the chip, while the remaining portion dissipates into the tool, workpiece, and environment (or is absorbed by cutting fluids).

Milling mechanics are inherently interrupted: each tooth of the cutter engages and disengages the material periodically. This cyclical thermal and mechanical loading requires tooling substrates and coatings with high resistance to fatigue and thermal shock.

Kinematics, Axes, & Machine Configuration

The kinematics of CNC milling machines are defined by the ISO 841 standard, which establishes a right-handed Cartesian coordinate system. The primary linear axes are determined as follows:

• Z-axis: Coincident with the spindle's axis of rotation. The positive direction (+Z) points away from the workpiece.
• X-axis: The principal axis in the plane of the machine table, perpendicular to the Z-axis. On vertical mills, (+X) represents motion to the right when facing the machine.
• Y-axis: Completes the right-handed Cartesian coordinate system. On vertical mills, (+Y) represents motion away from the operator.

The rotational axes are designated as: