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.
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.
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 Cosists following parts :
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.
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.
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.
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.
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.
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.
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.
3. Spacing collars
4. Bearing bush
6. Draw bolt
8. Key block
9. Set screw
Classification Of Milling Machine: Types Of Milling Machine
(b) Horizontal milling machine
(c) Universal milling machine
(d) Vertical milling machine
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.
Column and Knee type milling machine:
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.
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
Difference between Vertical Milling machine and Horizontal Milling Machine :
|Parameter||Vertical milling||Horizontal milling|
|1. Position of arbor||It is mounted vertically||It is mounted horizontally|
|2. Spindle and worktable||Spindle is vertical and perpendicular to the work table||Spindle is horizontal and parallel to the work table|
|3.Cutter movement||It can be moved up and down.||It can be moved up and down.|
|4.spindle tilting||It can be tilted for angular milling operations||It cannot be tilted|
|5.Operations||Angular milling, slot milling, T- slot milling, flat milling, etc||Plain milling, gear cutting, form milling, gang milling, etc|
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 :
b) Heavy duty
b) Staggered teeth
b) Staggered teeth
a) Taper shank
7) Woodruff key slot milling cutter
8) Fly cutter
d) gear cutter
e) thread milling cutter
1. Direct indexing
2. Simple indexing
3. Compound indexing
4. Differential indexing
Plain Indexing: –
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.
Index Crank Movement = 40/ N where, N= number of divisions required.
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.
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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