Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool with multiple cutting edges (in rare cases, a tool with one cutting edge, called a fly-cutter, is used). The axis of rotation of the cutting tool is perpendicular to the direction of feed. This orientation between the tool axis and the feed direction is one of the features that distinguishes milling from drilling. For drilling, the cutting tool is fed in a direction parallel to its axis of rotation. The cutting tool in milling is called a milling cutter and the cutting edges are called teeth. The machine tool that traditionally performs this operation is a milling machine.
The geometric form created by milling is a plane surface. Other work geometries can be created either by means of the cutter path or the cutter shape. Owing to the variety of shapes possible and its high production rates, milling is one of the most versatile and widely used machining operations.
Milling is an interrupted cutting operation; the teeth of the milling cutter enter and exit the work during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force and thermal shock on every rotation. The tool material and cutter geometry must be designed to with stand these conditions.
Milling is a material removal process.
A rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool's axis of rotation. The speed motion is provided by the rotating milling cutter. The two basic forms of milling are peripheral milling and face milling.
Milling is a similar cutting process to that of drilling. As in the case of drilling, the main rotary cutting movement is produced by the tool. However, the feed motion in milling is not in the axial direction of the cutting tool, as with drilling, but rather vertical to the main axis of the tool. Roller milling and end milling are general purpose processes which are widely used for producing flat surfaces in any type of configuration. Hobbing is a major process for gear making. Form end milling is a significant process for the machine tool industry, because one can use this process to mill slides and other parts of machines tools. Die sinking is a milling process used to make dies for deforming processes. Gang milling is a process where a variety of tools are mounted on the same rotating axis in order to make a complex shape.
A universal milling machine, and the milling processes in general, provides a high degree of flexibility in terms of sizes, and shapes that can be machined.