The Milling Machine
The milling process is characterized by solid work material, two-dimensional forming (one-dimensional forming may be used in a few cases), and a shear state of stress. The workpiece (W) is clamped on the table (6), which is given a translatory feed (D, that together with the primary motion (/?) of the cutter (V) provides the many geometric possibilities. The milling process—through the various types of cutters and the wide variety of machines—is a versatile, high-production process. Through various accessories (dividing head, attachments, etc.), many different special shapes can be produced. The milling process comes close to turning in extensive industrial use, since the geometric possibilities are enormous and the removal rate high. See Figure 15.35 for examples of a horizontal mill on the left, and a vertical mill on the right.
The hardness of the material should not be too high (HB 250-300), and a minimum of ductility is advisable. The obtained tolerances are normally good ( ±0.05 mm), and the surface quality high (I < R < 10 fim). A wide variety of milling machines is available: for example, the plain column-and-knee type (general purpose), the universal column-and-knee type, the bed type, and the planar type.
Much of the previous discussion on lathes applies to milling machine progress. In a sense, the milling machine is the opposite of the lathe, in that it provides cutting action by rotating the tool while the sequence of cuts is achieved by reciprocating the workpiece. The sequence of consecutive cuts is produced by moving the workpiece in a straight line, and the surface produced by a milling machine will normally be straight—in one direction at least. A milling machine, however, uses a multiple-edged tool, and the surface produced by such a tool conforms to the contour of the cutting edges. If the milling cutter has a straight cutting edge, a flat surface can be produced in both directions. The workpiece is usually held securely on the table of the machine, or in a fixture clamped to the table. It
is fed to the cutter or cutters by the motion of the table. Multiple cutters can be arranged on a spindle, separated by precision spacers, permitting several parallel cuts to be made simultaneously. Figure 15.36 shows some of the typical cutters in use.
The material is moved in an x-y direction for each pass, and can be moved toward or away from the cutting tool to change the depth of cut. which swivels on top of the saddle. Thus, the table can also be rotated in the horizontal plane. This arrangement permits cutting helices, for milling flutes in twist drills or milling cutters. With an indexing arrangement, it is useful for cutting gear teeth.
The vertical milling machine derives its name from the position of the spindle, which is located vertically and at right angles to the top surface of the table. The vertical milling machine is especially adapted to operations with end mills and face mills; for profiling interior and exterior surfaces; for milling dies and metal molds; and for locating and boring holes. Figure 15.38 shows one of the "old standard" vertical mills.
There are special-purpose production milling machines of various types. A duplex mill, for example, has two horizontal spindles mounted on independently adjustable spindle carriers, which slide on two headstocks placed on opposite sides of the bed. Two identical or two different milling operations can be performed simultaneously on one or more workpieces.
The improvement history of milling machines is much like that of the lathe, and perhaps even more features and capabilities are now available. Figure 15.39 shows a modern vertical milling machine. It is truly difficult to identify today's machining centers as either "a lathe" or "a milling machine." The question becomes more one of the capability of a particular machine to make a particular part, versus the cost of producing the part on another type of machine. Of course, the lathe still primarily rotates the material to be cut, and the mill primarily rotates the cutters.
For years the milling machine has been considered a three-axis machine, with the cutter fixed (either horizontal or vertical), the table moving past the cutter in the „v axis, toward the cutter (90° to the table motion) as the v axis, and up and down movement of the table called the z axis. With today's machines, it is possible to tilt or rotate the table, or tilt the cutter spindle—or both—creating five- and six-axis machines. Some grinders have six axes of motion. The use of multiple axes simplifies the number of cutters required and permits smooth transitions, but greatly increases the complexity of the programs and the training of the programmers. Figure 15.40 shows the definitions of milling, and mathematical relationships of the different elements of mill work.

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