Modern milling machines look much the same as they did 25 years ago. However, they now must cut super alloys, titanium, and high-tensile steels to closer tolerances and at faster rates then previously. To handle these requirements, the new milling machines provide higher horsepower, greater stiffness and wider speed and feed ranges than before. In addition, more accurate lead screws, closer alignment, numerical control (NC) and computer numerical control (CNC) all result in faster work with better finishes and greater accuracy than ever before attained. A modern CNC vertical milling machine is shown below.
13.2 Types of Milling Machines
The many types of milling machines used in manufacturing have been grouped into three general classes.
* Column and knee machines
* Bed-type milling machines
* Special purpose machines
13.2.1 Column and Knee Machines
Column and knee milling machines are made in both vertical and horizontal types. The schematic diagrams below show both types of machines. Versatility is a major feature of knee and column milling machines. On a basic machine of this type, the table, saddle and knee can be moved. Many accessories such as universal vises, rotary tables and dividing heads, further increase the versatility of this type of machine.
Regardless of whether the machine is of the vertical or horizontal type, several components on all column and knee milling machines are similar, except for size and minor variations because of manufacturer's preference. These similarities are described in terms of general shape, geometric relationship to the rest of the machine, function and the material from which the components are made.
Column: The column, which is usually combined with the base as a single casting, is cast gray iron or ductile iron. The column houses the spindle and bearings, as well as the necessary gears, clutches, shafts, pumps and shifting mechanisms, for transmitting power from the electric motor to the spindle at the selected speed. The gears usually run in oil and are made of carburized alloy steel for long life. Some of the necessary controls are usually mounted on the side of the column.
The base is usually hollow and, in many cases, serves as a sump for the cutting fluid. A pump and filtration system can be installed in the base. The hole in the center of the base houses the support for the screw that raises and lowers the knee.
The machined vertical slide on the front of the column may be of the square or dovetail type. The knee moves up and down on this slide. The slide must be machined at a 90-degree angle to the face of the column in both the lateral and vertical planes. The tolerances are very close and are usually expressed in minutes or seconds of arc. The large hole in the face of the column casting is for the spindle. The hole is very accurately bored perpendicular to the front slide in two planes and parallel to the upper slide.
Spindle: On a horizontal milling machine, the spindle is one of the most critical parts. It is usually machined from an alloy steel forging and is heat-treated to resist wear, vibration, thrust and bending loads. The spindle is usually supported by a combination of ball and straight roller bearings, or by tapered roller bearings that absorb both radial loads and end thrust loads. Spindles are hollow so that a drawbar can be used to hold arbors securely in place.
The front of the spindle is machined to accept standard arbors. The two keys that fit into corresponding slots in the arbor do the actual driving of the arbor. The internal taper, which is accurately ground so that it is concentric with the spindle, locates the arbor.
Knee: The knee is a casting that is moved up or down the slide on the front of the column by the elevating screw. Two dovetail or square slides are machined at 90 degrees to each other. The vertical slide mates with the slide on the front of the column, and the horizontal slide carries the saddle. It contains the necessary gears, screws and other mechanisms to provide power feeds in all directions. The operator can select various feedrates through the controls mounted on the knee.
Saddle: The saddle for a plain milling machine is a casting with two slides machined at an exact 90-degree angle to each other. The lower slide fits the slide on the top of the knee, and the upper slide accepts the slide on the bottom of the table. The surfaces of the slides that make contact with the knee and the table are parallel to each other. Locks for both...