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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