Milling Machine – Types, Working, parts, Operations, Cutting parameter

A milling machine is a machine tool that removes metal as the work is fed against a rotating multipoint cutter. The milling cutter rotates at high speed and it removes metal at a very fast rate with the help of multiple cutting edges. One or more number of cutters can be mounted simultaneously on the arbor of the milling machine. This is the reason that a milling machine finds a wide application in production work. Milling machine is used for machining flat surfaces, contoured surfaces, surfaces of revolution, external and internal threads, and helical surfaces of various cross-sections.
The milling machine is the main machining operations. In this process, workpiece is fed beside a turning cylindrical tool. The turning tool has of several cutting edges. The milling process is notable as of other machining operations on the foundation of direction between the tool axis with the feed direction, though, in other operations similar to drilling, turning, and so on. The tool is fed in a way parallel to the axis of rotation. Milling machines can also use for slotting, drilling, make a circular profile with gear cutting by having proper attachments.
The cutting tool used in milling operation is called milling cutter, which has of numerous edges call teeth. Machine tool to make the milling operations by produce necessary relative movement among workpiece and tool is call milling machine. It gives necessary relative motion under extremely controlled conditions. 

Working Principle of Milling Machine 

  • Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth as illustrated in figure Cutting action is carried out by feeding the workpiece against the rotating cutter. Thus, the spindle speed, the table feed, the depth of cut, and the rotating direction of the cutter become the main parameters of the process. Good results can only be achieved with well balanced
    settings of these parameters.
  • In this work is rigidly clamped on the table of the machine while revolving multi teeth cutter mounted either on the spindle or on an arbor. The cutter revolves at high speed and the work is fed slowly past the cutter. The work can be fed vertical, longitudinal, or cross direction. As the work advances, the cutter teeth remove the metal from the work surface to produce the desired shape.
working principle of milling machine
working principle of milling machine

Milling Method :

There are two distinct methods of milling classified as follows:

1. Up-milling or conventional milling
2. Down milling or climb milling.

Up-milling or conventional milling

In the up-milling or conventional milling, the metal is removed in the form of small chips by a cutter rotating against the direction of travel of the workpiece. In this type of milling, the chip thickness is minimum at the start of the cut and maximum at the end of the cut. As a result the cutting force also varies from zero to the maximum value per tooth movement of the milling cutter. The major disadvantages of the up-milling process are the tendency of cutting force to lift the work from the fixtures and poor surface finish obtained. But being a safer process, it is a commonly used method of milling.

up milling and down milling
up milling and down milling

Down-milling or climb milling

In this method, the metal is removed by a cutter rotating in the same direction of feed of the workpiece. Chip thickness is maximum at the start of the cut and minimum in the end. In this method, there is less friction involved and consequently less heat is generated on the contact surface of the cutter and workpiece. Climb milling can be used advantageously on many kinds of work to increase the number of pieces per sharpening and to produce a better finish. With climb milling, saws cut long thin slots more satisfactorily than with standard milling. Another advantage is that slightly lower power consumption is obtainable by climb milling since there is no need to drive the table against the cutter.

Parts of Milling Machines :

milling machine parts
milling machine parts

Milling machine Cosists following parts :

1. Base

It is a foundation member and it carries the column at its one end. In some machines, the base is hollow and serves as a reservoir for cutting fluid.

2. Column

The column is the main supporting member mounted vertically on the base. It is box-shaped, heavily ribbed inside and houses all the driving mechanism for the spindle and table feed. The front vertical face of the column is accurately machined and is provided with a dovetail guideway for supporting the knee.

3. Knee

The knee is a rigid grey iron casting which slides up and down on the vertical ways of the column face. An elevating screw mounted on the base is used to adjust the height of the knee and it also supports the knee.

4. Saddle

The saddle is placed on the top of the knee and it slides on guideways set exactly at 90° to the column face. The top of the saddle provides guide-ways for the table.

5. Table

The table rests on ways on the saddle and travels longitudinally. A lead screw under the table engages a nut on the saddle to move the table horizontally by hand or power. In universal machines, the table may also be swiveled horizontally. For this purpose the table is mounted on a circular base. The top of the table is accurately finished and T -slots are provided for clamping the work and other fixtures on it

6. Overhanging arm

It is mounted on the top of the column, which extends beyond the column face and serves as a bearing support for the other end of the arbor.

7. Front brace

It is an extra support, which is fitted between the knee and the over-arm to ensure further rigidity to the arbor and the knee.

8. Spindle

It is situated in the upper part of the column and receives power from the motor through belts, gears, and clutches and transmit it to the arbor.

9. Arbor

It is like an extension of the machine spindle on which milling cutters are securely mounted and rotated. The arbors are made with taper shanks for proper alignment with the machine spindles having taper holes at their nose.

The arbor assembly consists of the following components.
1. Arbor
2. Spindle
3. Spacing collars
4. Bearing bush
5. Cutter
6. Draw bolt
7. Locknut
8. Key block
9. Set screw

Classification Of Milling Machine: Types Of Milling Machine

The four most common types of manually controlled milling machines are listed below in order of increasing power (and therefore metal removal capability):
1. Column and knee type milling machines
Used for general purpose milling operations, column and knee type milling machines are the most common milling machines. The spindle to which the milling cutter is maybe
(a) Hand milling machine
(b) Horizontal milling machine
(c) Universal milling machine
(d) Vertical milling machine
2. Planer milling machine
Planer machines are similar to bed type machines but are equipped with several cutters and heads to mill various surfaces.

3. Fixed-bed type milling machine

In bed type machines, the worktable is mounted directly on the bed, which replaces the knee, and can move only longitudinally. These machines have high stiffness and are used for high production work.

(a) Simplex milling machine.
(b) Duplex milling machine.
(c) Triplex milling machine.

4. Machining center machines

Various milling machine components are being replaced rapidly with computer numerical control (CNC) machines. These machine tools are versatile and are capable of milling, drilling, boring, and tapping with repetitive accuracy.

5. Special types of milling machines
(a) Rotary table milling machine.

Rotary table machines are similar to vertical milling machines and are equipped with one or more heads to do face milling operations.
(b) Planetary milling machine.
(c) Profiling machine.
(d) Duplicating machine.
(e) Pantograph milling machine.
(f) Continuous milling machine.
(g) Drum milling machine
(h) Profiling and tracer controlled milling machine

Tracer controlled machines reproduce parts from a master model. They are used in the automotive and aerospace industries for machining complex parts and dies.

Milling machines whose motions are electronically controlled are listed in order of increasing production capacity and decreasing flexibility:
1. Manual data input milling machines
2. Programmable CNC milling machines
3. Machining centers (tool changer and pallet exchange capability)
4. Flexible Manufacturing Cell and Flexible Manufacturing System
5. Transfer lines

Column and Knee type milling machine:

Milling Parts: Function of Milling Machine Parts 
1. Base: It is a heavy casting on which column and other parts are mounted. It may be bolted to the floor strongly.
2. Column: there are guideways on the front face of the column, on which the knee slides. It houses power transmission units such as gears, belt drives and pulleys to give rotary motion to the arbor. The drive mechanisms are also used to give automatic feed to the handle and table.
3. Knee: It supports the saddle, table, workpiece, and other clamping devices. It moves on the guideways of column. It resists the deflection caused by the cutting forces on the workpiece.
4. Saddle: It is mounted on the knee and can be moved by hand wheel or by power. The direction of travel of the saddle is restricted towards or away from the column face.
5. Table: It is mounted on the saddle and can be moved by a hand wheel or by power. Its top surface is machined accurately to hold the workpiece and other holding devices. It moves perpendicular to the direction of the saddle movement.
6. Arbor: Its one end is attached to the column and the other end is supported by an overarm. It holds and drives different types of milling cutters.
7. Spindle: It gets power from the gears, belt drives, to drive the motor. It has provision to add or remove milling cutters on to the arbor
column and knee type milling machine
column and knee type milling machine

Difference between Vertical Milling machine and Horizontal Milling Machine :

ParameterVertical millingHorizontal milling
1. Position of arborIt is mounted verticallyIt is mounted horizontally
2. Spindle and worktableSpindle is vertical and perpendicular to the work tableSpindle is horizontal and parallel to the work table
3.Cutter movementIt can be moved up and down.It can be moved up and down.
4.spindle tiltingIt can be tilted for angular milling operationsIt cannot be tilted
5.OperationsAngular milling, slot milling, T- slot milling, flat milling, etcPlain milling, gear cutting, form milling, gang milling, etc

Milling operations

1.Plain milling:- For producing a plain, flat, horizontal surface parallel to the axis of rotation of plain milling cutter.
2. Face milling:- To produce flat surface face milling cutter rotated about an axis perpendicular to work surface
3.Side milling:- Producing flat vertical surface on the side of the workpiece
4. Straddle milling:- Producing flat vertical surfaces on both sides of a workpiece by using two side milling cutters mounted on the same arbor
5. Angular milling:- Producing angular surface on the workpiece other than at right angles to the axis of the milling machine spindle.
6. Gang milling:- Operation of machining several surfaces of a workpiece simultaneously by feeding the table against several cutters having the same or different diameters mounted on the same arbor.
7.Form milling:- Operation of producing irregular contours by using form cutters.
8. Profile milling:- Operation of reproduction of an outline of a template or complex shape of a master die on a workpiece.
9.End milling:- producing a flat surface which may be vertical, horizontal, or at an angle in reference to the table surface.
10.Saw milling :- Producing narrow slots or grooves on the workpiece.
11.Milling keyways, Grooves, slots:- Producing keyways,Grooves, slots in workpiece
12. Gear cutting:- To produce gears
13.Helical milling:- Producing helical flutes or grooves around the periphery of the cylindrical or conical workpiece.
14. Thread milling:- Producing threads by using a single or multiple thread milling cutter.

Different types of Milling Cutters :

Classification of Standard milling cutter:
1) Plain milling cutter
a) Light duty
b) Heavy duty
c) Helical
2) Side milling cutter
a) Plain
b) Staggered teeth
c) Half
d) Interlocking
3) Metal slitting saw
a) Plain
b) Staggered teeth
4) Angle milling cutter
a) Single
b) Double
5) End milling cutter
a) Taper shank
b)Straight shank
c)Shell
6) T-slot milling cutter
7) Woodruff key slot milling cutter
8) Fly cutter
9) Formed cutter
a) Convex
b)concave
c)corner rounding
d) gear cutter
e) thread milling cutter
10) Tap & reamer cutter
11) Face milling cutter

Indexing

Indexing is the operation of dividing the periphery of a workpiece into any number of equal parts. For example if we want to make a hexagonal bolt. The Head of the bolt is given hexagonal shape. We do indexing to divide circular workpiece into six equal parts and then all the six parts are milled to an identical flat surface. If we want to cut ‘n‟ number of teeth in a gear blank. The circumference of gear blank is divided into ‘n‟ number of equal parts and teeth are made by milling operation one by one. The main component used in indexing operation is the universal dividing head.
Function Of Indexing Head : 
The function of dividing head is to divide the periphery of a workpiece into any number of equal parts( Indexing). To provides a continuous rotary motion to the job during the helical milling. To set the workpiece in a desired position concerning the machine table. Acts both as a holding as well as supporting devices for the work during the operation.
Indexing methods
There are different indexing methods. These are:
1. Direct indexing
2. Simple indexing
3. Compound indexing
4. Differential indexing

Plain Indexing: –

In plain indexing the dividing head spindle is moved by turning the index crank. As the shaft carrying the crank has a single-threaded worm that meshes with the worm gear having 40 teeth, 40 turns of the crank are necessary to rotate the index head spindle though one revolution. To facilitate indexing to fractions of a turn, index plates are used to cover practically all numbers. Index plates with
circles of holes are as follows:-
Plate No. 1 – 15, 16, 17, 18, 19, 20
Plates No. 2 – 21, 23, 27, 29, 31, 33.
Plate No. 3 – 37, 39, 41, 43, 47, 49.
With the three index plate supplied, simple indexing can be used for all divisions up to 50, even numbers up to 100 except 96. The formula for index crank movement is given below:-
Index Crank Movement = 40/ N where, N= number of divisions required.
universal dividing head
universal dividing head

Specification Of Milling:

Along with the type of milling machine, it has to be specified with its size. Commonly size of a typical milling machine is selected as given below:

1. Size of worktable with its movement range table length x table width like 900 x 275 mm.
Table movements: Longitudinal travel x Cross x Vertical like 600 x 200 x 400 mm.
Over travels point to maximum movement in a direction.
2. Maximum longitudinal movement and cross feed of the table.
3. Number of feeds presented (give their values).
4. Number of spindle speeds (give their values).
5. The maximum distance the knee can travel.
6. Power of the main drive motor.
7. Number of spindle speeds.
8. Floor space necessary.
9. Total power existing.
10. Spindle nose taper.
11. Net weight.

Cutting Parameters in Milling:

a) Cutting speed:

The cutting speed of a milling cutter is its peripheral linear speed resulting from rotation. It is expressed in meters per minute and is given by formulas
= π d n / 1000 m/min

where d is the diameter of the cutter in mm & n is cutter r.p.m.

b) Feed:

It is defined by the distance the work advances under the cutter. Feed is expressed by – feed per tooth, feed per cutter revolution, and feed per minute. They are related with each other by a formula-

Sm = n * Sr = Z * n * Sz

where Z is no. of teeth, n is r.p.m.

Sz – Feed per tooth in mm

1) Feed per tooth ( Sz):- The feed per tooth is defined by the distance the work advances in the time between engagement by the two successive teeth. It is expressed in mm/tooth of the cutter.

2) Feed per revolution (Srev):- The feed per cutter revolution is the distance the work advances in the time when the cutter turns through one complete revolution.

3) Feed per minute ( Sm):- The feed per minute is defined by the distance the work advances in one minute. It expressed in mm/min

c) Depth of cut:

It is the thickness of the material removed in one pass of the work under the cutter. It is the peripheral distance measured between the original distance and the final surface of the workpiece. It is expressed in mm

Calculation of Machining Time:

The time required to mill a surface for any operation can be calculated from the formula-

T= L / ( Sz*Z*n )

where T – time required to complete the cut in minutes.
L – Length of table travel to complete the cut in mm
Sz – Feed per tooth in mm
Z – No. of teeth in the cutter and n – the r.p.m. of the cutter

Along with the type of milling machine, it has to be specified with its size. Commonly size of a typical milling machine is selected as given below: Type of milling machine


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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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