5.3 Milling and Milling Machine

Milling is the process of cutting away material by feeding a workpiece past a rotating multiple-tooth cutter. The cutting action of several teeth around the milling cutter provides a fast and smooth method of machining. The machined surface may be flat, angular, or curved. The surface may also be milled to any combination of shapes. The machine for holding the workpiece, rotating the cutter, and feeding it is known as the Milling machine.

There are several milling operations as follows:

Peripheral Milling. In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body.[4] The axis of cutter rotation is generally in a plane parallel to the workpiece surface to be machined (shown in Fig.5.2a).

Face Milling. In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter (shown in Fig.5.2b).

End Milling. The cutter in end milling generally rotates on an axis vertical to the workpiece. It can be tilted to machine tapered surfaces. Cutting teeth are located on both the end face of the cutter and the periphery of the cutter body (shown in Fig.5.2c).

Milling machines are basically classified as vertical or horizontal according to the orientation of their spindle axes. A vertical milling machine spindle axis is vertical and the horizontal milling machine spindle axis is horizontal. In addition, the vertical milling machine has a machine table that moves perpendicular to the spindle axis of rotation and the horizontal milling machine has a worktable that moves parallel to the spindle axis of rotation. These machines are also classified as column and knee type, ram-type, manufacturing or bed type, and planer-type.

Fig.5.2 Some basic types of milling operations

Used for general purpose milling operations, column and knee type milling machines are the most common milling machines. The spindle to which the milling cutter is clutched may be horizontal (slab milling) or vertical (face and end milling shown in Fig.5.3). The basic components are:

Column is the main casting which includes the base and supports all other parts of the machine. An oil reservoir and a pump in the column keep the spindle lubricated. The column rests on a base that contains a coolant reservoir and a pump that can be used when performing any machining operation that requires a coolant.

Knee is the casting that supports the table and the saddle that gives the table vertical movements for adjusting the depth of cut. [5] The feed change gearing is enclosed within the knee. It is supported and can be adjusted by the elevating screw. The knee is fastened to the column by dovetail guideways. The lever can be raised or lowered either by hand or power feed. The hand feed is usually used to take the depth of cut or to position the work, and the power feed to move the work during the machining operation.

Fig.5.3 The vertical milling machine

Saddle supports the table and can move transversely, which slides on a horizontal dovetail, parallel to the axis of the spindle, on the knee. The swivel table (on universal machines only) is attached to the saddle and can be swiveled approximately 45° in either direction.

Power feed mechanism is contained in the knee and controls the longitudinal, transverse (in and out) and vertical feeds. The desired feed rate can be obtained on the machine by positioning the feed selection levers as indicated on the feed selection plates. On some universal knee and column milling machines the feed is obtained by turning the speed selection handle until the desired rate of feed is indicated on the feed dial. Most milling machines have a rapid traverse lever that can be engaged when a temporary increase in speed of the longitudinal, transversal or vertical feeds is required. For example, this lever would be engaged when positioning or aligning the work.

Worktable is the rectangular casting located on top of the saddle. It contains several T-slots for fastening the work or workholding devices. The table can be moved longitudinally with respect to the saddle by hand or by power. To move the table by hand, engage and turn the longitudinal hand crank. To move it by power, engage the longitudinal directional feed control lever. The longitudinal directional control lever can be positioned to the left, to the right, or in the center. Place the end of the directional feed control lever to the left to feed the table to the left. Place it to the right to feed the table to the right. Place it in the center position to disengage the power feed, or to feed the table by hand.

Head contains the spindle and cutter holders. In vertical machines the head may be fixed or vertically adjustable. The spindle holds and drives various cutting tools. It is a shaft, mounted on bearings supported by the column. The spindle is driven by an electric motor through a train of gears, all mounted within the column. The front end of the spindle, which is near the table, has an internal taper machined on it. The internal taper (3 1/2 inches per foot) permits mounting tapered-shank cutter holders and cutter arbors. Two keys, located on the face of the spindle, provide a positive drive for the cutter holder, or arbor. The holder or arbor is secured in the spindle by a draw bolt and jam nut. Large face mills are sometimes mounted directly to the spindle nose.

Overarm is the horizontal beam to which the arbor support is fastened. The overarm may be a single casting that slides in the dovetail guideways on the top of the column. It may consist of one or two cylindrical bars that slide through the holes in the column. On some machines to position the overarm, first unclamp the locknuts and then extend the overarm by turning a crank. On others, the overarm is moved by merely pushing on it. The overarm should only be extended far enough to so position the arbor support as to make the setup as rigid as possible. To place the arbor supports on an overarm, extend one of the bars approximately 1-inch farther than the other bar.