Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool with multiple cutting edges (occasionally, 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. In 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 conventional machine tool that 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 withstand these conditions.
There are two basic types of milling operations: peripheral milling and face milling.
In peripheral milling, also called plain milling, the axis of the tool is parallel to the surface being machined, and the operation is performed by cutting edges on the outside periphery of the cutter. Several types of peripheral milling can be performed:
- Slab milling – The basic form of peripheral milling in which the cutter width extends beyond the workpiece on both sides;
- Slotting – Also called slot milling, in which the width of the cutter is less than the workpiece width, creating a slot in the work (when the cutter is very thin, this operation can be used to mill narrow slots or cut a workpart in two, called saw milling);
- Side milling – Cutter machines the side of the workpiece;
- Straddle milling – The same as side milling, only cutting takes place on both sides of the work;
- Form milling – The milling teeth have a special profile that determines the shape of the slot that is cut in the work.
In peripheral milling, the direction of cutter rotation distinguishes two forms of milling: up milling and down milling. In up milling, also called conventional milling, the direction of motion of the cutter teeth is opposite the feed direction when the teeth cut into the work. In down milling, also called climb milling, the direction of cutter motion is the same as the feed direction when the teeth cut the work.
The relative geometries of these two forms of milling result in differences in their cutting actions. In conventional milling, the chip formed by each cutter tooth starts out very thin and increases in thickness during the sweep of the cutter. In
climb milling, each chip starts out thick and reduces in thickness throughout the cut. The length of a chip in down milling is less than in conventional milling. This means that the cutter is engaged in the work for less time per volume of material cut, and this tends to increase tool life in climb milling.
The cutting force direction is tangential to the periphery of the cutter for the teeth that are engaged in the work. In conventional milling, this has a tendency to lift the workpart as the cutter teeth exit the material. In climb milling, this cutter force direction is downward, tending to hold the work against the milling machine table.
In face milling, the axis of the cutter is perpendicular to the surface being milled, and machining is performed by cutting edges on both the end and outside periphery of the cutter. As in peripheral milling, various forms of face milling exist, among which are:
- Conventional face milling – The diameter of the cutter is greater than the workpart width, so the cutter overhangs the work on both sides;
- Partial face milling – The cutter overhangs the work on only one side;
- End milling – The cutter diameter is less than the work width, so a slot is cut into the part;
- Profile milling – A form of end milling in which the outside periphery of a flat part is cut;
- Pocket milling – Another form of end milling used to mill shallow pockets into flat parts;
- Surface contouring – A ball-nose cutter (rather than square-end cutter) is fed back and forth across the work along a curvilinear path at close intervals to create a threedimensional surface form. The same basic cutter control is required to machine the contours of mold and die cavities, in which case the operation is called die sinking.
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