Milling Machine : Definition, Working, Type`s, Parts, Operation, Advantage

Milling Machine : Definition, Working, Type`s, Parts, Operation, Advantage

Milling is a process performed with a machine in which the cutters rotate to remove the material from the work piece present in the direction of the angle with the tool axis. With the help of the milling machines one can perform many operations and functions starting from small objects to large ones.

 Milling is the cutting operation that removes metal by feeding the work against a rotating, cutter having single or multiple cutting edges. Flat or curved surfaces of many shapes can be machined by milling with good finish and accuracy. A milling machine may also be used for drilling, slotting, making a circular profile and gear cutting by having suitable attachments.

Working Principle: The workpiece is holding on the worktable of the machine. The table movement controls the feed of workpiece against the rotating cutter. The cutter is mounted on a spindle or arbor and revolves at high speed. Except for rotation the cutter has no other motion. As the workpiece advances, the cutter teeth remove the metal from the surface of workpiece and the desired shape is produced.

Types of Milling Machines

The two main configurations of the milling machining operations are the types of milling machines. These are the vertical mill and the horizontal mill. They are further discussed below:

Vertical Milling Machines

The vertical mill has a vertically arranged spindle axis and rotate by staying at the same axis. The spindle can also be extended and performing functions such as drilling and cutting. Vertical mill has got two further categories as well: turret mill and bed mill.

The turret mill has got a table that moves perpendicularly and parallel to the spindle axis in order to cut the material. The spindle is, however, stationary. Two cutting methods can be performed with this by moving the knee and by lowering or raising the quill.

The other is the bed mill in which the table moves perpendicular to the axis of the spindle and the spindle moves parallel to its axis.

Horizontal Milling Machines

The horizontal mill is also the similar cutter but their cutters are placed on a horizontal arbor. A lot of horizontal mills have got rotary tables that help in milling in various angles. These tables are called the universal tables. Apart from this all the tools that are used in a vertical mill can also be used in the horizontal mill.

Parts of the Milling Machine

1.     Column and Base

Column and base provide support for the other parts of the milling machine. There is an oil reservoir and pump in the column to lubricate the spindle. The column rests on the base and also it has a coolant reservoir and a pump to provide coolant in machining operations.

2.     Knee

Saddle and table have the support of casting. The gearing mechanism is enclosed within the knee. The knee is fastened to the column by dovetail ways. It is supported and adjusted by a vertical positioning screw also known as an elevating screw. The use of the elevating screw is to adjust the knee up and down by raising or lowering the lever with the help of hand or power feed.

3.     Saddle and Swivel Table

The saddle is on the knee and supports the table. Saddle slides on horizontal dovetail on the knee and dovetail are parallel to the axis of the spindle (in horizontal milling m/c). Swivel table is attached to the saddle that swiveled (revolved) horizontally in both directions.

4.     Power Feed mechanism

The power feed mechanism is in the knee. The power feed mechanism is used to control in longitudinal (left and right), transverse (in and out) and vertical (up and down) feeds. For the desired rate of feed on the machine, the feed selection lever is placed to indicate on the feed selection plates. For some universal knee and column milling machine, feed is obtained by turning the speed selection handle till the required rate of feed is shown on feed dial. Almost in every milling machine, there is a rapid traverse lever which is used when a temporary increase in the speed of the longitudinal, transverse, or vertical feeds is required. This lever is used when the operator is aligning or positioning the work.

5.     Table

The table is a rectangular casting that is present on the top of the saddle. The table is used to hold the work or for work holding devices. There are several T-slots to hold work and work holding devices (jigs and fixtures). It can be operated by hand or by power. For moving the table by hand, engage, and turn the longitudinal hand crank. For move, it by power, engage and longitudinal direction feeds control lever.

6.     Spindle

The spindle is used to holding and drives the cutting tools of the milling machine. It is mounted on the bearings and has the support of the column. The spindle is driven by an electric motor via gear trains and gear trains are on the column. The spindle face lies near the table and having an internal taper machined on it. There are two keys at the front face provides a positive drive for cutter holder or arbor.

7.     Over Arm / Overhanging Arm

Over arm is a horizontal beam and it is on the top face of the column. It may be a single casting and slides on the dovetail ways which is on the top face of the column.

8.     Arbor Support

Arbor support is casting with a bearing that supports the outer end of the arbor. It also helps to align the outer end of the arbor with the spindle. Arbor support prevents the springing of the outer end of arbor in cutting operations. Generally, there are two types of arbor supports used in the milling machines. The first one has a small diameter bearing hole of a maximum diameter of 1 inch. The second one has a large diameter bearing hole of up to 23/4 inch.

9.     Ram

The ram is an overhanging arm in a vertical milling machine. The one end of the ram is mounted on the top of the column and the milling head is attached to another.

Milling Machine Operations:

The following Operation is performed by the Milling Machine:

·         Plain Milling or Slab Milling Operation

·         UP and DOWN Milling Operation

·         Face Milling Operation

·         End Milling Operation

·         Gang Milling Operation

·         Straddle Milling Operation

·         Groove Milling Operation

·         Gear Milling Operation

·         Side Milling Operation and

·         T-Slot Milling Operatio

 

Up Milling and Down Milling:

Up milling is a method of milling operation in which the cutter and the workpiece both moves in the opposite direction.

Down Milling is a method of milling operation in which the direction of the rotation of the cutter coincides with the direction of the work feed. 

Chip Thickness will be minimum in the beginning and maximum at the end in the case of up -milling. Whereas, the chip thickness will be greater in the beginning and lesser at the end in the case of down-milling.

Tool Life will is more in the case of down-milling as compared with the up-milling.

The surface finish will be more in the case of the down-milling as compared with the up-milling method due to which down-milling method is used for the finishing operations in the industries.

Accuracy will be more in the case of up-milling as compared with the down-milling process since in up-milling the workpiece is pulled against the table so the fasteners will be under tension and we know that under tension the backlash error will not have any effect which ultimately results in the better accuracy in up-milling.

Advantages of Milling Machine:

The various advantages of Milling are:

·         High speed

·         Better surface finish

·         Increase productivity

·         High accuracy