The cutting process in milling
Milling is an uninterrupted cutting operation wherein the teeth of the milling cutter enters and exits the work during each revolution. This uninterrupted cutting action subjects the teeth to cycle impact force and thermal shock for every rotation so that the tooth material and its geometry must be designed to withstand these conditions. There are two basic types of milling: down milling and up milling. In down milling the cutting force is directed on the work table so that thinner work parts can be machined with better surface finish. However, the stress load on the teeth is abrupt which can cause damage to the cutter. In up milling, the cutting force tends to lift the work piece. The working conditions for the milling cutter are more favorable because the cutter does not start to cut when it makes contact so that the surface has a natural waviness.
The geometric forms produced through milling
Peripheral milling - or plain milling as it is more commonly called is the process wherein in the axis of the milling cutter is parallel to the surface being machined and where the operation is performed by cutting edges on the outside periphery of the cutter. Peripheral milling has its advantages in lower power requirements and better surface finish.
Face milling – is the process wherein the cutter is perpendicular to the machined surface. The cutter axis is vertical however, in the new CNC milling machines, the cutter axis is horizontal. During face milling, machining is performed with teeth on both the end and periphery of the milling cutter. Again, up and down types of milling are available according to the directions of the cutter rotation and feed. An advantage of face milling is high production rates but surface finish is worse than peripheral milling and feed marks are always present.
End milling – the cutter in end milling is called the end mill and it has a diameter that is less than the width of the workpiece. The end mill has a helical cutting edge that is carried over into the cylindrical cutter surface. End mills are characterized by their flat ends and are mainly used to produce pockets and closed or end key slots.
Form milling – the form edge which is the cutting edge of the peripheral cutter has a special profile that it imparts to the workpiece. Milling cutters with various profiles are available for cutting a wide range of two-dimensional surfaces.
Profile milling – makes use of a conventional end mill to cut the outside or inside periphery of a flat part. The end mill works with the peripheral teeth and is fed along a curvilinear path that is equidistant from the surface profile.