Cutting conditions in milling

In milling, each tooth on a tool removes part of the stock in the form of a chip. The basic interface between tool and workpart is pictured below. This shows a only a few teeth of a peripheral milling cutter:

Three types of feed in milling can be identified:

feed per tooth : the basic parameter in milling equivalent to the feed in turning.

Feed per tooth is selected with regard to the surface finish and dimensional accuracy required.

feed per revolution: it determines the amount of material cut per one full revolution of the milling cutter.

feed per minute: Feed per minute is used to adjust the feed change gears.

Types of milling

There are two basic types of milling, as shown in the figure:

· down (climb) milling, when the cutter rotation is in the same direction as the motion

of the workpiece being fed;

· up (conventional) milling, in which the workpiece is moving towards the cutter, oppos­ing the cutter direction of rotation.

In down milling, the cutting force is directed into the work table, which allows thinner workparts to be machined.

Better surface finish is obtained but the stress load on the teeth is abrupt, which may damage the cutter.

In up milling, the cutting force tends to lift the workpiece. The work conditions for the cutter are more favourable. Because the cutter does not start to cut when it makes contact (cutting at zero cut is impossible), the surface has a natural waviness.

Milling Operations

Owing to the variety of shapes possible and its high production rates, milling is one of the most versatile and widely used machining operations. The geometric form created by milling fall into three major groups:

· Plane surfaces: the surface is linear in all three dimensions. The simplest and most convenient type of surface;

· Two-dimensional surfaces: the shape of the surface changes in the direction of two of the axes and is linear along the third axis. Examples include cams.

· Three-dimensional surfaces: the shape of the surface changes in all three directions Examples include die cavities, gas turbine blades, propellers, casting patterns, etc.