4 January 2017

What is milling machine?

The first milling machine came into existence in about 1770 and was of French origin.

The milling cutter was first developed by Jacques de Vaucanson in 1782.

The first successful plain milling machine was designed by Eli Whitney in 1818.

The universal milling machine was invented by Joseph R Brown a member of the Brown and Sharpe company in 1861.

A milling machine is a machine tool that removes metal as the work is fed against a rotating multipoint cutter. The cutter rotates at a high speed and because of the multiple cutting edges
it removes metal at a very fast rate.

The machine can also hold one or a number of cutters at a time so the milling machine finds wide application in production work.


Types of Milling Machine :

According to the general design of the milling machine are :

1. Column and knee type
For general shopwork, the most commonly used is the column and knee type where the table is mounted on the knee casting which is turn in mounted on the vertical sides 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 according to various methods of supplying power to the table, different movements of the table and the different axis of rotation of the main spindle. 
Milling machine

  • Hand milling machine
The simplest of all type of milling machine is the hand milling in which the feeding movement of the table is supplied is the hand control.
The cutter is mounted on a horizontal arbour and is rotated by power. 
This machine is particularly suitable for light and simple milling operation such as machining slots, grooves and keyways.

  • Plain milling machine
The plain milling machine is much more rigid and sturdy than hand millers for accommodating heavy workpieces.
A plain milling machine having a horizontal spindle. In a plain milling machine, the table may be fed in a longitudinal, cross or vertical directions.
The feed is longitudinal when the table is moved at right angles to the spindle.
The feed is cross when the table is moved parallel to the spindle. 
The feed is vertical when the table is adjusted in the vertical plane.

  • Universal milling machine
A universal milling machine is named because it may be adapted to be a very wide range of milling operation. A universal milling machine can be distinguished from a plain milling machine in that the table of a universal milling machine is mounted on a circular swivelling base that has degree graduations, and the tale can be swivelled to any angle up to 45 degrees on either side of normal position. In the universal milling machine, in addition to three movements as in a plain milling machine, the table may have the fourth movement when it is fed at an angle to the milling cutter. this additional feature enables it to perform helical milling operation.
It may be also produced spur, spiral, bevel gears, twist drills, reamers, milling cutters including all conventional milling operation. so a universal milling machine essentially a tool room machine designed to produce very accurate work.

  • Omniversal milling machine
In omniversal milling machine, the table besides having all the movement of the universal milling machine can be tilted in a vertical plane by providing a swivel arrangement at the knee.
The additional swivelling arrangement of the table enables it to machine taper spiral grooves in reamers, bevel gears etc.

  • Vertical milling machine
A vertical milling machine can be eminent from a horizontal milling machine by the position of its spindle which is vertical or perpendicular to the work table. The machine may be of plain or universal type and has all the movements of the table for proper setting and feeding the work.
The machine is adapted for machining grooves, slots and flat surfaces. 

Vertical milling machine

2. Manufacturing of fixed bed type
The fixed bed type milling machine is comparatively large, heavy and rigid and it different from column and knee type milling machine by the construction of its table mounting.
The table is directly mounted directly on the ways of a fixed bed.
The cutter mounted on the spindle head may be moved vertically on the column and the spindle may be adjusted horizontally to provide cross adjustment. It can be classified below :
  • Simplex milling machine
  • Duplex milling machine
  • Triplex milling machine
The name simplex, duplex and triplex indicate that the machine is provided with single, double and triple spindle heads respectively. 
In the duplex machine, the spindle heads are arranged one on each side of the table.
In the triplex machine, the third spindle head is mounted on a cross rail. 
The feed cycle of the table includes the following: start, rapid approach, slow feed for cutting, rapid traverse to next work piece, quick return and stop.

3. Planer type
The planer type machine is built up for heavy-duty work, having spindle heads adjustable in vertical in transverse directions. It has a cross rail capable of being raised or lowered carrying the cutters their heads, and the saddles, all supported by rigid uprights.
The use of the machine is limited to production work only and is considered ultimate in metal removing capacity.

4. Special type
  • Rotary table milling machine
This machine construction is a modification to a vertical milling machine.
The face milling cutters are mounted on two or more vertical spindles and a number of workpieces are clamped on the horizontal surface of a circular table which rotates about a vertical axis.
The cutters may be set at different heights relative to the work so that when one of the cutters is roughing the pieces the other is finishing them.

  • Drum milling machine
The drum milling machine is similar to a rotary table milling machine in that work-supporting table which is called drum and it rotates in a horizontal axis.
The face milling cutters mounted on three or four spindle heads rotate in a horizontal axis and removal metal from workpieces supported on both the faces of the drum.
The finished machined parts are removed after one complete turn of the drum, and then the new ones are clamped to it.

  • Planetary milling machine
In the planetary milling machine, the work is held stationary while the revolving cutter or cutters move in a planetary path to finish a cylindrical surface on the work either internally or externally or simultaneously. 
This machine is adapted for milling internal or external threads of different pitches.

  • Pantograph, profiling & tracer controlled milling machine
A pantograph machine can duplicate a job by using a pantograph mechanism which permits the size of the work piece reproduced to be smaller than, equal to or greater than the size of a template or model used for the purpose.
A pantograph is a mechanism that is generally constructed of four-bar which are joined in the form of the pantograph. Pantograph machine is available in two dimensional or three-dimensional models.

Profiling machine duplicates the full size of the template attached to the machine. This is practically a vertical milling machine or bed type in which the spindle can be adjusted vertically and cutter head horizontally across the table.
The movement of cutter regulated by a hardened guide pin.

Tracer controlled milling machine reproduces irregular or complex shapes of dies, moulds etc.
The feeding motion of the machine is controlled by means of a stylus that scans a profiled template or a contoured model which is to be reproduced.

Comparison between Plain and Universal Milling

Principle parts that are used in milling machine are described below :

milling machine parts

  • Base
The base is made of grey iron casting accurately machined on its top and bottom surface and serves as a foundation member. It gives support and rigidity to the machine and also acts as a reservoir for the cutting fluids.
  • Column 
The column is the main supporting frame mounted vertically on the base. The top of the column is finished to hold an overarm that extends outward at the front of the machine.
  • Knee 
The knee is a rigid casting mounted on the front face of the column. The knee moves vertically along the guideways and this movement enables to adjust the distance between the cutter and the job mounted on the table. The adjustment is obtained manually or automatically by operating the elevating screw provided below the knee. The top face of the knee forms a slideway for the saddle to provide cross travel of the table.
  • Saddle
The saddle rests on the knee and constitutes the intermediate part between the knee and the table. The saddle moves transversely, i.e., crosswise (in or out) on guideways provided on the knee.
  • Table
The table rests on ways on the saddle and travels longitudinally. 
The table rests on guideways in the saddle and provides support to the work. 
The table is made of cast iron, its top surface is accurately machined and carriers T-slots that accommodate the clamping bolt for fixing the work. 
A lead screw under the table engages a nut on the saddle to move the table horizontally by hand or power.
The worktable and hence the job fitted on it is given motions in three directions:
Vertical (up and down) the movement provided by raising or lowering the knee.
Cross (in or out) or transverse motion provided by moving the saddle in relation to the knee.
Longitudinal (back and forth) the motion provided by hand wheel fitted on the side of the feed screw.
For universal milling, the machine table may also swivel 45° to either side of the centre line and thus fed at an angle to the spindle.
  • Overarm
The Overarm is mounted at the top of the column and is guided in perfect alignment by the machined surfaces. The Overarm is the support for the arbour. The arm is adjustable so that the bearing support may be provided nearer to the cutter.
  • Front Brace 
The front brace is extra support that is fitted between the knee and overarm.
The front brace is slotted to allow adjustment of the height of the knee relative to the overarm.
  • Spindle 
The spindle of the machine is located in the support part of the column and receives power from the motor.

  • Arbor support 
The arbour support is fitted to the Overarm and can be clamped at any location on the Overarm. Its function is to align and support various arbours. The arbour is a machined shaft that holds and drives the cutters.
  • Elevating screw
The upward and downward movement to the knee and the table is given by the elevating screw that is operated by hand or an automatic feed.

Operating Principle of Milling Machine :
In a milling machine operating principle machine tool used for the shaping of metal and other solid materials. By use of table and different work holding devices, the work piece is affixed and the milling cutter is rotated about the spindle axis. Milling operation involves movement of the rotating cutter sideways as well as in and out. The cutter and work piece move relative to each other producing a toolpath along which material is removed. Generally in all case of milling multi-tooth cutter is used so that the material removal rate is high and cutting action of the multi-tooth provides a fast method of machining. The machined surface may be flat, angular or curved but the most usually flat surface is produced by a milling machine. Many types of milling machine are used, ranging from relatively simple and versatile milling machine are used for the general purpose of machining to highly specialized machine for mass production. The milling machine may be operated manually or by CNC or NC machines.

Milling Machine Operations :
  • Plain Milling 
  • Face Milling
  • Side Milling
  • Straddle Milling
  • Angular Milling 
  • Gang Milling
  • Form Milling 
  • End Milling
  • Saw Milling
  • Profile Milling
  • Keyways, slots and grooves Milling
  • Gear Cutting
  • Helical Milling
  • Cam Milling
  • Thread Milling

Cutting Speed :
The speed of the milling cutter is its peripheral linear speed resulting from rotation. 
Cutting speed of milling machine expressed in meters per minutes.

 v = πdn/1000 metres per min 
Where    v = cutting speed
               d = diameter of the cutter
               n = cutter speed 

Feed :

The feed-in milling machine is defined as the rate with which the work piece advances under the cutter. There are different three methods to expressed feed.
  • Feed per tooth Sz
  • Feed per cutter revolution Srev
  • Feed per minute Sm
Feed per tooth, feed per revolution and feed per minute are related by the formula given below :

Sm = n * Srev = Sz * Z * n

Depth of cut :

The depth of cut in milling machine is the thickness of the material removed in one pass of the work under the cutter.

Number of Cutter Teeth :

The number of cutting teeth should be properly designed on a milling cutter for effective machining operation.
The number of cutter teeth is calculated from equation below :

Z = Sm / Sz * n
The course teeth cutter having lesser number of teeth on their periphery which are efficient in metal machining.
Advantages of a course teeth cutter :
  • Greater chip space may be provided
  • Cutter tooth cross-section may be increased 
  • Increasing strength
  • Less power required to drive the cutter
  • Less sliding friction 
  • Longer life of cutter
Safety Measures in Milling Operation :
Milling machine operators must be extremely careful in running this machine tool therefore some of safety points should be required to know milling operators.
Following safety points must be followed by operators :
  • Learn to operate controls before operating the machine.
  • Work piece must be rigidly held on the worktable.
  • Keep hands and body away from the revolving cutter.
  • Do not change spindle speed when machining is running.
  • Do not remove or tighten the milling machine arbour nut while the power is on.
  • Do not measure work while continuous operation.
  • Do not remove guards while machining.
  • Do not remove chips when the machining is running.
  • Wear snugly fitting clothing.
  • Do not lean on the machine when it is running.
For the use of lubrication life, any machine increases so we can also know about Milling machine lubrication 

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