MILLING MACHINE: Working Principle OF Milling Machine, Size and Specification of Milling Machine, Types of Milling Machines

MILLING MACHINE

Milling is a machining process of metal removal due to cutting action of a revolving cutter, when the work is fed past it. The revolving cutter is held on a spindle or arbor and the work clamped on the machine table, fed past the revolving cutter. In doing so, the teeth of the cutter remove the metal, in the form of chips, from the surface of the work to produce the desired shape.

It has a capability to perform large number of operations, which no other single machine tool can perform. It gives high production rate, with in very close limits of dimensions. that is why it has largely replaced other machine tools like shaper, planer, slotter etc for small and medium size jobs only. It is too slow for machining very long jobs. For small and medium jobs, the milling machine gives the fastest production with a very high accuracy. It has very wide application in mass production work.

Working Principle OF Milling Machine:

The metal removing operation on a milling machine, the work is rigidly clamped on the table of the machine and the revolving multi teeth cutter mounted on a spindle or an arbor. The cutter revolves at a high speed and the work fed slowly past the cutter. The work can be fed in a vertical, longitudinal or cross direction. As the work advantages, the cutter teeth remove the metal from the work surface to produce the desired shape.

Size and Specification of Milling Machine:

Size of the milling machine is usually denoted by the dimensions (length and breadth) of the table of the machine. Different manufactures, denote these sizes by different numbers 0,1,2,3,4,5,6 etc. each of these numbers indicates a particular standard size. Other main specifications are the Horse power of driving motor, number of spindle speeds, feeds, drive, taper of spindle nose, required floor area etc.

Types of Milling Machines:

A large variety of different types of milling machines are available, the broad classification of these machines is as follows:

1. Column and knee type milling machines.

2. Fixed bed type or manufacturing type milling machines.

3. Planer type milling machines.

4. Production milling machines.

5. Special purpose machines.

1. Column and knee type Milling Machines

These machines are all general purpose machines and have a single spindle only. They derive their name “column and knee” type from the fact that the work table is supported on a knee like casting, which can slide in vertical direction a long a vertical column. These machines depending up on the spindle position and table movements are further classified as follows:

a) Hand Milling Machine.

b) Plain or Horizontal Milling Machine.

c) Vertical Milling Machine.

d) Universal Milling Machine.

e) Omniversal Milling Machine.

Plain or Horizontal Milling Machine.


Column and knee type Milling Machines

a) Hand Milling Machine:

It is the simplest of all the milling machines and smallest in size. All the operations, except the rotation of arbor, are performed by hand. The table carrying the work over it is moved by hand to feed the work. This machine is especially useful in producing small components like hexagonal or square heads on bolts, cutting slots on screw heads, cutting key ways etc.

b) Plain or Horizontal Milling Machine:

The vertical column serves as a housing for electricals, the main drive, spindle bearing etc. The knee acts as a support for the saddle, work table and other accessories like indexing head etc. Over arm provides support for the yoke which in turn supports the free end of the arbor.

Plain or Horizontal Milling Machine

The arbor carrying the cutter rotates about a horizontal axis. The table can be given straight motions in three directions, longitudinal, cross and vertical (up and down) but can’t be swivelled.

For giving vertical movement to the table the knee itself, together with the whole unit above it, slide up and down along the ways provided in front of the column. For giving cross movement to the table the saddle is moved towards or away from the column along with the whole unit above it. 

A brase is employed to provide additional support and rigidity to the arbor, when a long arbor is used. Both hand and power feeds can be used for the work.

c) Vertical Milling Machine:

It derives its name from the vertical position of the spindle. This machine is available in both types, the fixed bed type as well as column and knee type.

It carries a vertical column on a heavy base. The over arm in this machine is made integral with the column and carries a housing at its front. This housing called head can be fixed type or swiveling type.

In fixed type, the spindle always remains vertical and can be adjusted up and down. In swiveling type, the head can be swiveled to any desired angle to machine the inclined surfaces.

Vertical Milling Machine

The knee carries an enclosed screw jack, by means of which it is moved up and down along the parallel vertical guide ways provided on the front side of the column.

The saddle is mounted on the knee and can be moved, along the horizontal guide ways provided on the knee, towards or away from the column. This enables the table to move in cross-direction. The table is mounted on guide ways, provided on the saddle, which are in a direction normal to the direction of the guide ways on the knee. By means of the lead-screw, provided under the table, the table can be moved in the longitudinal direction.

Thus the work gets up and down movement by the knee, cross movement by saddle and longitudinal movement by the table. Power feeds can be used to both the saddle and the table. Mostly face milling cutters and shell-end type cutters are used on these machines.

d) Universal Milling Machine:

It is the most versatile of all the milling machines and after lathe it is the most useful machine tool as it is capable of performing most of the machining operations. With its application the use of a large number of other machine tools can be avoided. It differs from the plain milling machine only in that, the tool can be given one more additional movement.

Its table can be swilled on the saddle in a horizontal plane. For this circular guide ways are provided on the saddle along which it can be swilled. A graduated circular base is incorporated under the table, with a datum-mark on the saddle, to read directly the angle through which the table has been swiveled.

This special feature enables the work to be set at an angle with the cutter for milling helical and spiral flutes and grooves. It’s over arm can be pushed back or moved and a vertical milling head can be fitted in place of the arbor to use it as a vertical milling machine.

e) Omniversal Milling Machine: 

This is a modified form of plain milling machine and is provided with two spindles, of which one is horizontal, as in plain milling and the other is carried by a universal swiveling head and can be fixed in vertical position or can be set any desired angle up to 90on both sides of vertical i.e. in a plane parallel to the front face of the column, and up to 45in a plane perpendicular to the former direction i.e. towards or away from the column. Another special feature of this machine is that it carries, in addition to all the possible adjustments provided in a universal milling machine, two more adjustments.

These adjustments are of the knee which can be swiveled about a horizontal axis to tilt the table and can be moved horizontally also. These special features make it a very useful machine tool for tool room work as it facilitates various operations to be carried out in different planes and at different angles in a single setting of the work.


2. Fixed bed type or manufacturing type Milling Machines:

These machines, in comparison to the column and knee type, are more sturdy and rigid, heavier in weight and larger in size. They are not suitable for tool room work. Most of these machines are either automatic or semi-automatic in operation. They may carry single or multiple spindles. The common operations performed on these machines are slot cutting, grooving, gang milling and facing. They facilitate machining of many jobs together, called multi-piece milling. Their further classification is as follows:

a) Plain type (having single horizontal spindle)

b) Duplex head (having double horizontal spindles)

c) Triplex head (having two horizontal and one vertical spindle)

d) Rise and fall type (for profile milling)

3. Planer type Milling Machines:

They are used for heavy work. Up to a maximum of four tool heads can be mounted over it, which can be adjusted vertically and transverse directions. It has robust and massive construction like a planer.

4. Production Milling Machines:

These are also manufacturing machines, but differ from the above machines in that they do not have fixed bed. They include the following machines.

a) Rotary table or Continuous type. b) Drum type.

c) Tracer controlled.

5. Special purpose machines:

These machines are designed to perform specific type of operation only

a) Thread Milling Machine

b) Profile Milling Machine

c) Gear Milling or Gear hobbling Machine

d) Cam Milling Machine

e) Planetary type Milling Machine

f) Double end Milling Machine

g) Skin Milling Machine and

h) Spar Milling Machine.


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