A knee mill is a type of vertical milling machine where instead of the entire table being stationary, it is able to move along the y, or vertical, axis, a direction in which the workpiece can move in relation to the milling tool shaping it. This is different from standard milling machines, where the table can only move along both x, or horizontal, axises, and the head, consisting of the usually stationary top slide and spindle, is responsible for all drilling and boring movements along the y-axis. The knee mill is also different from most milling machines because of its head, which can also move along an x-axis. Bridgeport mills are knee mills and, due to their movement capabilities, they are able to take on a wide variety of workpiece sizes.
Vertical milling machines all work and shape metal in a similar manner. Much unlike a wood or metal lathe, where the workpiece turns at high revolutions and is cut by a stationary tool, a milling machine does the exact opposite. This is where the workpiece is kept stationary and is clamped to the table, while milling tools spin at very high revolutions to make precise cuts. The differences between types of milling machines usually lie with how the position of the workpiece is moved.
Also like most milling machines, a knee mill is capable of performing an incredibly wide variety of cutting and forming processes. Some of these processes include surfacing, cutting, and slot machining. Such a wide range of forming capabilities is possible through the employment of milling tools, which are very similar to drill bits. These metalworking tools come in many different shapes and sizes, all of which cut different-sized shapes when used.
The knee mill can be very beneficial when it comes to machining larger objects. In some scenarios, a moving spindle will not provide enough torque to drill vertical holes, a process also known as boring. Knee mills, on the other hand, have stationary spindles, and all y-axis movement is performed through the table the workpiece is situated on. This arrangement results in a significant torque increase, making it possible to drill holes in harder material. Another benefit to the knee mill is also a result of the vertical movement of the table, as larger objects can also be machined, due to an increase in height capacity.