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7.1 INTRODUCTION

After lathes, milling machines are the most widely used for manufacturing applications. In milling, the workpiece is fed into a rotating milling cutter, which is a multi-point tool as shown in Fig. 7.1, unlike a lathe, which uses a single point cutting tool. The tool used in milling is called the milling cutter.

Fig. 7.1Schematic diagram of a milling operation

The milling process is characterised by:

(i)Interrupted cutting Each of the cutting edges removes material

for only a part of the rotation of the milling cutter. As a result, the cutting edge has time to cool before it again removes material.

Thus the milling operation is much more cooler compared to the turning operation. This allows for a much larger material rates.

(ii)Small size of chips Though the size of the chips is small, in view of the multiple cutting edges in contact a large amount of material is removed and as a result the component is generally completed in

a single pass unlike the turning process which requires a large

number of cuts for finishing.

(iii)Variation in chip thickness This contributes to the non-steady state cyclic conditions of varying cutting forces during the contact of the cutting edge with the chip thickness varying from zero to maximum size or vice versa. This cyclic variation of the force can excite any of the natural frequencies of the machine tool system and is harmful to the tool life and surface finish generated

A milling machine is one of the most versatile machine tools. It is adaptable for quantity production as well as in job shops and tool rooms. The versatility of milling is because of the large variety of accessories and tools available with milling machines. The typical tolerance expected from the process is about ±0.050 mm.

7.2 TYPES OF MILLING MACHINES

To satisfy various requirements milling machines come in a number of sizes and varieties. In view of the large material removal rates

milling machines come with a very rigid spindle and large power. The varieties of milling machines available are:

(i) Knee and Column type

(a) horizontal

(b) vertical

(c) universal

(d) turret type

These are the general purpose milling machines, which have a high degree of flexibility and are employed for all types of works including batch manufacturing. A large variety of attachments to improve the flexibility are available for this class of milling machines.

(ii) Production (Bed) type

(a) simplex

(b) duplex

(c) triplex

These machines are generally meant for regular production involving large batch sizes. The flexibility is relatively less in these machines which is suitable for productivity enhancement.

(iii) Plano millers

These machines are used only for very large workpieces involving table travels in meters.

(iv) Special type

(a) Rotary table

(b) Drum type

(c) Copy milling (Die sinking machines)

(d) Key way milling machines

(e) Spline shaft milling machines

These machines provide special facilities to suit specific applications that are not catered to by the other classes of milling machines.

7.2.1 Knee and Column Milling Machines

The knee(升降台) and column type is the most commonly used machine in view of its flexibility and easier setup. A typical machine construction is shown in Fig. 7.2 for the horizontal axis. The knee houses the feed mechanism and mounts the saddle and table. The table basically has the T-slots running along the X-axis for the purpose of work holding. The table moves along the X-axis on the saddle while the saddle moves along the Y-axis on the guide ways provided on the knee.

The feed is provided either manually with a hand wheel or connected for automatic by the lead screw, which in turn is coupled to the main spindle drive. The knee can move up and down (Z-axis) on a dovetail provided on the column.

Fig. 7.2 Horizontal knee and column type milling machine

The massive column at the back of the machine houses all the power train including the motor and the spindle gearbox. The power for feeding the table lead screw is taken from the main motor through a separate feed gearbox. Sometimes a separate feed motor is provided for the feed gearbox as well.

While the longitudinal and traverse motions are provided with automatic motion, the raising of the knee is generally made manually.

The spindle is located at the top end of the column. The arbour used to mount the milling cutters is mounted in the spindle and is provided with a support on the other end to take care of the heavy cutting forces by means of an overarm with bearing. As shown in Fig.

7.2 the overarm extends from the column with a rigid design. The spindle nose has the standard Morse taper of the suitable size

depending upon the machine size.

The milling cutters are mounted on the arbour at any desired position, the rest of the length being filled by standard hardened collars of varying widths to fix the position of the cutter. The arbour is clamped in the spindle with the help of a draw bar and then fixed with nuts.

Milling machines are generally specified on the following basis:

(i) Size of the table, which specifies the actual working area on the table and relates to the maximum size of the workpiece that can be accommodated.

(ii) Amount of table travel, which gives the maximum axis movement that is possible.

(iii) Horse power of the spindle, which actually specifies the power of the spindle motor used. Smaller machines may come with 1 to 3 hp while the production machines may go from 10 to 50 hp.

Another type of knee and column milling machine is the vertical axis type. Its construction is very similar to the horizontal axis type, except for the spindle type and location.

The vertical axis milling machine is relatively more flexible (Fig. 7.4) and suitable for machining complex cavities such as die cavities in tool rooms. The vertical head is provided with a swiveling facility in horizontal direction whereby the cutter axis can be swivelled. This is

useful for tool rooms where more complex milling operations are carried out.

The spindle is located in the vertical direction and is suitable for using the shank mounted milling cutters such as end mills, In view of the location of the tool, the setting up of the workpiece and observing the machining operation is more convenient.

Fig, 7.3 Vertical knee and column type milling machine

Fig.7.4 Some of the milling operations normally carried out on vertical axis machines

The universal machine has the table which can be swivelled in a horizontal plane at about 45o to either the left or right. This makes the universal machine suitable for milling spur and helical gears as well as worm gears and cams.

7.2.2 Bed Type Milling Machine

In production milling machines it is desirable to increase the metal removal rates. If it is done on conventional machines by increasing

the depth of cut, there is possibility of chatter. Hence another variety

of milling machines named as bed type machines are used which are made more rugged and are capable of removing more material. The ruggedness is obtained as a consequence of the reduction in versatility.

The table in the case of bed type machines is directly mounted on the bed and is provided with only longitudinal motion.

The spindle moves along with the column to provide the cutting action. Simplex machines (Fig. 7.5) are the ones with only one spindle head while duplex machines have two spindles (Fig. 7.6). The two spindles are located on either side of a heavy workpiece and remove material from both sides simultaneously.

Fig. 7.5 Simplex bed type milling machine

Fig. 7.6 Duplex bed type milling machine

7.3 MILLING CUTTERS

There are a large variety of milling cutters available to suit specific requirements. The versatility of the milling machine is contributed to

a great extent by the variety of milling cutters that are available.

7.3.1 Types of Milling Cutters

Milling cutters are classified into various types based on a variety of methods.

(i) Based on construction:

(a) Solid

(b) Inserted tooth type

Based on mounting:

(a) Arbor mounted

(b) Shank mounted

(c) Nose mounted

Base on rotation:

(a) Right hand rotation (counter clockwise)

(b) Left hand rotation (clockwise)

Based on helix:

(a) Right hand helix

(b) Left hand helix

Milling cutters are generally made of high speed steel or cemented carbides. The cemented carbide cutters can be of a brazed tip variety or with indexable tips. The indexable variety is more common since it is normally less expensive to replace the worn out cutting edges than to regrind them.

Plain milling cutters These are also called slab milling cutters and are basically cylindrical with the cutting teeth on the periphery as shown in Fig. 7.7. These are generally used for machining flat surfaces.

Fig. 7.7 Arbor mounted milling cutters for general purpose

Light duty slab milling cutters generally have a face width, which is small of the order of 25 mm. They generally have straight teeth and large number of teeth.

Heavy duty slab milling cutters come with a smaller number of teeth to allow for more chip space. This allows taking deeper cuts and consequently high material removal rates.

Helical milling cutters have a very small number of teeth but a large helix angle. This type of cutter cuts with a shearing action, which can produce a very fine finish. The large helix angle allows the cutter to absorb most of the end load and therefore the cutter enters and leaves the workpiece very smoothly.

Side and face milling cutters These have the cutting edges not only on

the face like the slab milling cutters, but also on both the sides. As a

result, these cutters become more versatile since they can be used for side milling as well as for slot milling.

Staggered tooth side milling cutters are a variation where the teeth are arranged in an alternate helix pattern. This type is generally used for milling deep slots, since the staggering of teeth provides for greater chip space.

Another variation of the side and face cutter is the half side milling cutter, which has cutting edges only on one side. This arrangement provides a positive rake angle and is useful for machining on only one side. These have a much smoother cutting action and a long tool life. The power consumed is also less for these cutters.

Fig. 7.8Special forms of arbor mounted milling cutters

Slitting saws The other common form of milling cutters in the arbor mounted category is the slitting saw. This is very similar to a saw blade in

appearance as well as function. Most of these have teeth around the circumference while some have side teeth as well. The thickness of these cutters is generally very small and is used for cutting off operations or for deep slots.

Special form cutters In addition to the general type of milling cutters described above, there are a large number of special form milling cutters available which are used for machining specific profiles.

Angular milling cutters are made in single or double angle cutters for milling any angle such as 30, 45 or 60o Form relieved cutters are made of various shapes such as circular, corner rounding, convex or concave shapes.

T-slot milling cutters are used for milling T-slots such as those in the milling machine table. The central slot is to be milled first using an end mill before using the T-slot milling cutter. Woodruff key seat milling cutters are used for milling as the name suggests, woodruff key seats Some other special form cutters are dovetail milling cutters and gear milling cutters.

End mills These are shank mounted as shown in Fig. 7.9 and are generally used in vertical axis milling machines. They are used for milling slots, key ways and pockets where other type of milling cutters cannot be used. A depth of cut of almost half the diameter can be taken with the end mills.

The end mills have the cutting edge running through the length of the cutting portion as well as on the face radially up to a certain length. The helix angle of the cutting edge promotes smooth and efficient cutting even at high cutting speeds and feed rates. High cutting speeds(转速?) are generally recommended for this type of milling cutters.

Fig. 7.9 Shank mounted milling cutters and various types of end mills There are a large variety of end mills. One of the distinctions is based on the method of holding, i.e., the end mill shank can be straight or tapered. The straight shank is used on end mills of small size and held in the milling machine spindle with the help of a suitable collet. The tapered shank can be directly mounted in the spindle with the help of the self

holding taper. If the taper is small compared to the spindle taper, then an adopter accommodating both the tapers is used.

The end teeth of the end mills may be terminated at a distance from the cutter center or may proceed till the center (Fig. 7.9 f). Those with the cutting edge up to the center are called slot drills or end cutting end mills since they have the ability to cut into the solid material (Fig. 7.9 g). The other type of end mills which have a larger number of teeth cannot cut into solid material and hence require a pilot hole drilled before a pocket is machined.

The cutting edge along the side of an end mill is generally straight and sometimes can be tapered by grinding on a tool and cutter grinder such that the draft required for mould and die cavities can be automatically generated.