Milling machines

Milling machines are used for milling operations. Milling is the process of removing material from work with a multi-toothed rotating cutter. There are various classes and types of milling machines in use, from small hand-operated types to fully automatic ones, the main of them being: column and knee-type, bed type, planer-type, and rotary type. Column and knee-type milling machines are made in three styles: horizontal plain, vertical and universal spindle milling machines. They are used for both toolroom and manufacturing work because of the ease with which they may be handled.

Fig. 52 shows the essential features of the horizontal milling machine. The machine is provided with a massive stream-lined column rising from a base which rests on a solid wooden or concrete floor which is sufficiently, heavy to withstand the weight of the machine. The base, hollow inside, contains a coolant tank with cutting fluid* that is delivered through piping by means of a motor-operated pump to the cutters and the place where the milling operation is performed. The centrifugal type pump is mounted on the pad located at the side of the base and is connected directly to the coolant tank. The cutting fluid flow can be regulated by means of valves connected to the outlet nozzles which can be swiveled for distributing a low pressure volume of cutting fluid to all diameter and types of cutters. A constant supply of clear oil is pumped to the column top, distributed through perforated pipes and flooded down over all gears, shafts, and bearings throughout the entire column and feed gearbox. In the lower part of the column there is a motor for driving the spindle and the change gears for the power feed. A set of change gears for driving the spindle at a proper speed required for any work being done on the machine is housed in the upper part of the column. The change gears are shifted either manually by a spindle speed change lever located on the face of the column or by power to form various trains for providing the necessary cutting speeds.

Fig. 52. Horizontal Plain Milling Machine:

A - column with speed gearbox and spindle unit; В - overarm; С - table;

D- additional link between knee and overarm; E - saddle; F - knee with feed gearbox;

G -base; 1- gearbox change lever; 2 -spindle speed change lever;

3 - longitudinal feed table handwheel; 4 - longitudinal feed table lever;

5- cross feed table handwheel; 6 - vertical feed knee hand lever;

7 - feed gearbox handwheel; 8 - gear train lever; 9 - vertical-cross feed table lever

The horizontal plain milling machine has a horizontal spindle rotating in anti­friction bearings in the column. The spindle is a hardened, ground, hollow shaft spaced horizontally from the front to the back of the column. In operation, the milling cutters are either attached to the spindle nose or carried on an arbor. Secured to the top of the column is an overarm consisting of one or two heavy steel bars. The overarm is provided with bearing brackets for supporting arbors or mandrels. The knee which supports the table and saddle units is mounted on the face of the column and can be moved up and down by means of an elevating screw for adjusting workpieces with respect to cutters.

During each milling operation the knee should be clamped: to the column, and in heavy-duty operations the knee is clamped to the overarm to ensure maximum rigidity. The upper part of the knee is provided with horizontal dovetail guides, which support the saddle unit mounted on them. The saddle, in turn, supports the table which is a heavy, semi-steel casing¹ sliding in the dovetail guides on the saddle. The table has T-slots of ample depth serving to fasten work-holding devices. All the three elements, table, saddle, and knee, may be either power- or hand-fed by screws taming in fixed nuts. By means of a hand lever on the knee, the machine can be set for continuous pendulum milling, or for automatic cycle operation, with power feed movement of the table in either direction. The machine is provided with a backlash eliminator, which enables climb milling to be performed with both right- and left-hand cutters and is automatically disengaged upon reversal

of the table. The eliminator can also be disengaged by movement of a lever when conventional milling is to be performed.

Vertical milling machines are similar to plain milling machines, but their spindle is positioned vertically.

Universal milling machines are also similar to plain milling machines but the saddle is mounted on and swivels on a clamp bed which in turn slides on the knee thus permitting the saddle to swing at an angle, and permitting table motion at other angles than 90° to the spindle axis.

Modern heavy-duty milling machines are equipped with a standardized spindle end which has a locating taper hole in the spindle. The arbor is seated by turning a draw-inbolt² which extends through a hole in the spindle, and screws into a threaded hole in the arbor. The arbor is driven by an adjusting key on the spindle nose which fits into slots in the arbor shoulder. The arbor support provides a cylindrical bearing for the pilot of the arbor, and in man) instances, an intermediate arbor support, serving as a bearing for an oversize collar, is employed. The arbor support is often connected to the knee by overarm braces for additional rigidity.

Cutting speeds on the milling machine depend upon the nature of the work, the type of cutter, the condition of the machine, and, in many instances, upon the experience and ability of the machine operator. Feed rates in, milling are expressed in two ways: in. per min³, or thousandths of an inch per revolution of the spindle. Delicate or fragile work requiring an accurate finish will need fine feeds, while heavy work, from which a considerable amount of metal is to be removed, can be subjected to coarse feeds. A good finish can usually be obtained by using a feed rate from 0.30" to 0.50" per revolution of the cutter. Finer feeds, such as 0.15" per revolution, will result in an excellent finish.

*American - cutter coolant

¹. semi-steel casting - отливка из сталистого чугуна

². the arbor is seated by turning a draw-in-bolt - оправка устанавливается поворотом втяжного стержня

³. in. per min - inch per minute - дюйм в минуту

Дата добавления: 2014-12-27