Machine Tools

A machine tool is an electrically powered tool which is used to remove material, usually metal, at a controlled rate to achieve a desired shape or finish. A machine tool typically holds the workpiece and a cutting tool, and moves either the workpiece, tool or both to provide a means of machining the material to the desired shape. Machining, another term for metalcutting, is performed by shaving away the metal in small pieces called chips. An average machining operation can reduce the original workpiece weight by approximately 50%. The modern machine tool is a precision piece of equipment designed to cut metal and produce thousands of parts to an accuracy of millionths of an inch, which is approximately equal to 1/300 of the thickness of a human hair. Machine tools range from very small bench mounted devices to large complex machines weighing hundreds of tons. The major operations performed by machine tools are milling, turning, boring, planing, shaping, drilling, power sawing, and grinding.

Classifications

Milling machines can generally be classified according to the orientation of the spindle, either vertical or horizontal. Vertical milling machines can also have what is called "multiaxis" capability where the vertical axis can tilt and swivel to enable the machining of closed angles and contoured surfaces. Vertical milling machines are extremely versatile and can machine horizontal surfaces, vertical surfaces, angular surfaces, shoulders, grooves, fillets, keyways, T-slots, dovetails, and precision holes.

Horizontal milling machines are available in plain and universal types. Plain milling machines have tables which are fixed at right angles to the knee. Universal milling machines have a table which can be pivoted in a horizontal plane. This allows the machine table to be swiveled to different angles for milling helical grooves.

The universal milling machine is widely used by maintenance machinists and toolmakers because of its versatility. Computer numerically controlled (CNC) mills or "machining centers" are available in vertical and horizontal configurations and come with automatic tool changers which can store many different tools in "carousels." The major components of a typical milling machine include the following: base, column, knee, elevating screw, saddle, machine table, ram, head, and spindle. The base is the heavy foundation member of the machine which can also be used as a reservoir for coolant or cutting lubricant often used in machining operations. The base is a massive casting which helps to absorb and dampen vibration from the machining process. The column, which is either cast with the base or keyed and bolted on, supports the functioning members of the machine. Horizontal "ways" on top of the column support the ram and head while vertical "ways" on the column front face support the knee, saddle, and machine table. The knee moves along the vertical ways of the column and is the basic work-supporting member. The knee is equipped with ways on top to allow horizontal movement of the saddle to and from the column face. The elevating screw provides additional support for the knee and allows the knee to be raised and lowered. The saddle mounts on the ways of the knee and has horizontal ways at right angles to the knee ways to support the machine table. The machine table moves longitudinally on the ways of the saddle and supports the workpiece. Combined movements of the knee, saddle, and machine table allow for precise positioning and feeding of the workpiece left and right, in and out, and up and down. This is called "3-axis" movement (X = left and right movement, Y = in and out movement, and Z = up and down movement). A rotary table can be added to a 3-axis mill to give it 4-axis capabilities (typically rotation is about the longitudinal or X-axis), while 5-axis mills are able to tilt and swivel about the vertical axis. The ram is mounted on the horizontal ways at the top of the column and supports the head and provides horizontal movement and positioning of the head at varying distances from the column face. The head includes the motor, stepped pulley, belt drive (or in the case of heavier duty mills, the gear drive), and the spindle. The head assembly provides for rotation of the spindle and spindle feeding along the vertical axis using a quill. The spindle contains the tool-holding mount and drives the cutter.