MILLING

A milling machine is a machine tool that removes metal as the work is fed against a rotating multipoint cutter. The milling cutter rotates at high speed and it removes metal at a very fast rate with the help of multiple cutting edges. One or more number of cutters can be mounted simultaneously on the arbor of milling machine.

Milling machine is used for machining flat

surfaces, contoured surfaces, surfaces of revolution, external and internal threads, and helical surfaces of various cross-sections. Typical components produced by a milling are given in Figure 1. In many applications, due to its higher production rate and accuracy, milling machine has even replaced shapers and slotters.

Figure 1 Job surfaces generated by milling machine

PRINCIPLE OF MILLING
In milling machine, the metal is cut by means of a rotating cutter having multiple cutting edges. For cutting operation, the workpiece is fed against the rotary cutter. As the workpiece moves against the cutting edges of milling cutter, metal is removed in form chips of trochoid shape. Machined surface is formed in one or more passes of the work. The work to be machined is held in a vice, a rotary table, a three jaw chuck, an index head, between centers, in a special fixture or bolted to machine table. The rotatory speed of the cutting tool and the feed rate of the workpiece depend upon the type of material being machined.

MILLING METHODS
There are two distinct methods of milling classified as follows:
1. Up-milling or conventional milling, and
2. Down milling or climb milling.
UP-Milling or Conventional Milling Procedure
In the up-milling or conventional milling, as shown in Fig. 2, the metal is removed in form of small chips by a cutter rotating against the direction of travel of the workpiece. In this type of milling, the chip thickness is minimum at the start of the cut and maximum at the end of cut. As a result the cutting force also varies from zero to the maximum value per tooth movement of the milling cutter. The major disadvantages of up-milling process are the tendency of cutting force to lift the work from the fixtures and poor surface finish obtained. But being a safer process, it is commonly used method of milling.

Fig. 2 Principal of up-milling

Down-Milling or Climb Milling
Down milling is shown in Fig. 3. It is also known as climb milling. In this method, the metal is removed by a cutter rotating in the same direction of feed of the workpiece. The effect of this is that the teeth cut downward instead of upwards. Chip thickness is maximum at the start of the cut and minimum in the end. In this method, it is claimed that there is less friction involved and consequently less heat is generated on the contact surface of the cutter and workpiece. Climb milling can be used advantageously on many kinds of work to increase the number of pieces per sharpening and to produce a better finish. With climb milling, saws cut long thin slots more satisfactorily than with standard milling. Another advantage is that slightly lower power consumption is obtainable by climb milling, since there is no need to drive the table against the cutter.

Fig. 3 Principal of down-milling

TYPES OF MILLING CUTTERS
Fig. 4 illustrates some types of milling cutters along with workpieces. Milling cutters
are made in various forms to perform certain classes of work, and they may be classified as:
(1) Plain milling cutters,
(2) Side milling cutters,
(3) Face milling cutter,
(4) Angle milling cutters,
(5) End milling cutter,
(6) Fly cutter,
(7) T-slot milling cutter,
(8) Formed cutters,
(9) Metal slitting saw,

Fig. 4 Types of milling cutters

TYPES OF MILLING MACHINES

According to general design, the distinctive types of milling machines are:

1. Column and knee type milling machines
(a) Hand milling machine
(b) Horizontal milling machine (Fig. 5)
(c) Universal milling machine
(d) Vertical milling machine (Fig. 6)

Fig. 5 Horizontal column and knee type milling machine

Fig. 6 Vertical column and knee type milling machine

2. Planer milling machine
3. Fixed-bed type milling machine
    (a) Simplex milling machine.
    (b) Duplex milling machine.
    (c) Triplex milling machine.
4. Machining center machines
5. Special types of milling machines
    (a) Rotary table milling machine.
    (b) Planetary milling machine.
    (c) Profiling machine.
    (d) Duplicating machine.
    (e) Pantograph milling machine.
    (f) Continuous milling machine.
    (g) Drum milling machine
    (h) Profiling and tracer controlled milling machine
Some important types of milling machines are discussed as under.

Column and Knee Type Milling Machine
Fig. 7 shows a simple column and knee type milling machine. It is the most commonly used milling machine used for general shop work. In this type of milling machine the table

 Fig. 7 A column and knee type milling machine

is mounted on the knee casting which in turn is mounted on the vertical slides of the main column. The knee is vertically adjustable on the column so that the table can be moved up and down to accommodate work of various heights. The column and knee type milling machines are classified on the basis of various methods of supplying power to the table, different movements of the table and different axis of rotation of the main spindle. Column and knee type milling machine comprises of the following important parts-
1. Base                            2. Column
3. Saddle                         4. Table
5. Elevating screw            6. Knee
7. Knee elevating handle  8. Cross feed handle
9. Front brace                 10. Arbor support
11. Arbor                       12. Overhanging arm
13. Cutter                       14. Cone pulley
15. Telescopic feed shaft.
The principal parts of a column and knee type milling machine are described as under.
Base
It is a foundation member for all the other parts, which rest upon it. It carries the column at its one end. In some machines, the base is hollow and serves as a reservoir for cutting fluid.
Column
The column is the main supporting member mounted vertically on the base. It is box shaped, heavily ribbed inside and houses all the driving mechanism for the spindle and table feed. The front vertical face of the column is accurately machined and is provided with dovetail guideway for supporting the knee.
Knee
The knee is a rigid grey iron casting which slides up and down on the vertical ways of the column face. An elevating screw mounted on the base is used to adjust the height of the knee and it also supports the knee. The knee houses the feed mechanism of the table, and different controls to operate it.
Saddle
The saddle is placed on the top of the knee and it slides on guideways set exactly at 90°
to the column face. The top of the saddle provides guide-ways for the table.
Table
The table rests on ways on the saddle and travels longitudinally. A lead screw under the table engages a nut on the saddle to move the table horizontally by hand or power. In universal machines, the table may also be swiveled horizontally. For this purpose the table is mounted on a circular base. The top of the table is accurately finished and T -slots are provided for clamping the work and other fixtures on it
Overhanging arm
It is mounted on the top of the column, which extends beyond the column face and serves as a bearing support for the other end of the arbor.

Front brace
It is an extra support, which is fitted between the knee and the over-arm to ensure further rigidity to the arbor and the knee.
Spindle
It is situated in the upper part of the column and receives power from the motor through belts, gears. and clutches and transmit it to the arbor.
Arbor
It is like an extension of the machine spindle on which milling cutters are securely mounted and rotated. The arbors are made with taper shanks for proper alignment with the machine spindles having taper holes at their nose. The draw bolt is used for managing for locking the arbor with the spindle and the whole assembly. The arbor assembly consists of
the following components.
1. Arbor                  2. Spindle
3. Spacing collars     4. Bearing bush
5. Cutter                  6. Draw bolt
7. Lock nut              8. Key block
9. Set screw

Planer Type Milling Machine
It is a heavy duty milling machine. It resembles a planer and like a planning machine it has a cross rail capable of being raised or lowered carrying the cutters, their heads, and the saddles, all supported by rigid uprights. There may be a number of independent spindles carrying cutters on the rail as two heads on the uprights. The use of the machine is limited to production work only and is considered ultimate in metal re-moving capacity.   
Special Type Milling Machines
Milling machines of non-conventional design have been developed to suit special purposes. The features that they have in common are the spindle for rotating the cutter and provision for moving the tool or the work in different directions.

SIZE OF MILLING MACHINE
The size of the column and knee type milling machine is specified by
(1) The dimensions of the working surface of the table, and
(2) Its maximum length of longitudinal, cross and vertical travel of the table.
In addition to above, number of spindle speeds, number of feeds, spindle nose taper, power available, floor space required and net weight of machine will also be required for additional specification.

DEPTH OF CUT
The depth of cut in milling is defined as the thickness of the material removed in one pass of the work under the cutter. Thus it is the perpendicular distance measured between the original and final surface of the workpiece, and is expressed in mm.

OPERATIONS PERFORMED ON MILLING MACHINE
Unlike a lathe, a milling cutter does not give a continuous cut, but begins with a sliding motion between the cutter and the work. Then follows a crushing movement, and then a cutting operation by which the chip is removed. Many different kinds of operations can be performed on a milling machine but a few of the more common operations will now be explained. These are:
Plain milling or slab milling
Fig. 8(a) illustrates the plain and slab milling operation. It is a method of producing a plain, flat, horizontal surface parallel to the axis of rotation of the cutter.
Face milling
Fig. 8(b) illustrates the face milling operation. It is a method of producing a flat surface at right angles to the axis of the cutter.
Side milling
Fig. 8(c) illustrates the side milling operation. It is the operation of production of a flat vertical surface on the side of a work-piece by using a side milling cutter.
Angular milling
Fig. 8(d) illustrates angular milling operation. It is a method of producing a flat surface making an angle to the axis of the cutter.
Gang-milling
Fig. 8(e) illustrates the gang milling operation. It is a method of milling by means of two or more cutters simultaneously having same or different diameters mounted on the arbor of the milling machine.
Form milling
Fig. 8(f) illustrates the form milling operation. It is, a method of producing a surface having an irregular outline.
End milling
Fig. 8(g) illustrates end milling operation. It is a method of milling slots, flat surfaces, and profiles by end mills.
Profile milling
Fig.8(h) illustrates profile milling operation. It is the operation of reproduction of an outline of a template or complex shape of a master die on a workpiece.
Saw milling
Fig. 8(i) illustrates saw milling operation. It is a method of producing deep slots and cutting materials into the required length by slitting saws.
T-slot milling
Fig. 8(j) illustrates T-slot milling operation.
Keyway milling
Fig. 8(k) illustrates keyway milling operation.
Gear cutting milling
Fig. 8(l) illustrates gear cutting milling operation.
Helical milling
Fig. 8(m) illustrates helical milling operation.
Flute milling
It is a method of grooving or cutting of flutes on drills, reamers, taps, etc,

Fig. 8 Various types of milling operations

Straddle milling
It is a method of milling two sides of a piece of work by employing two side-milling cutters at the same time.
Thread milling
It is a method of milling threads on dies, screws, worms, etc. both internally and externally. As an alternative to the screw cutting in a lathe, this method is being more extensively introduced now a day in modern machine shops.