Milling Machine - Definition, Operations, Parts, Cutter, Process



Milling machine definition


Milling is a machining process which is uses to remove material from the job by using multi point cutting tool. A machine which is used to perform various operations like horizontal milling, vertical milling, angular milling is named milling machine. Milling machine parts is given as 
below :



Milling Machine Parts


1.                Base
2.                Column
3.                Knee
4.                Gearbox
5.                Spindle
6.                Saddle
7.                Table
8.                Over arm
9.        Arbor
10.     Arbor support   


1.              Base:  The base of a machine works as a foundation for all other parts which rests upon it. It is made of cast iron. 

2.             Column:  The column is the main parts of milling machine which is mounted vertically on the base. The column houses have all the driving mechanism for the spindle and table feed. The main motor is incorporated in the column. The top of the column is finished to hold an overarm that extends outward at the front of the machine. The lower part of the column is a sturdy base, which incorporates the cutting fluid tank.

3.             Knee: The knee is of rigid casting that slides up and down along a precision machined guide-ways. The knee houses the speed mechanism of the table and the different controls to operate it. The feed motor and gearbox are usually incorporated in the knee.

4.             Gear box: The gear box for the spindle drive comprises, shafts with bearings and gears, and controls for the setting of the spindle speed.

5.              Spindle: The spindle is housed in the upper part of the column and receives power from the motor and transmits it to the arbor. The front end of the spindle projects from the column face and is provided with a tapered hole into which various cutting tools and arbors may be mounted.The accuracy in machining depends on the rigidity of the spindle. The speed of spindle can be selected by the speed gearbox, and the feeds can be selected through the feed gearbox. The spindle is arranged horizontally in the horizontal milling machine.

6.              Saddle: The saddle is placed on the top of the knee which slides on the guide way set exactly at 900 to the column face. A cross feed screw near the top of the knee engages a nut on the bottom of the saddle to move it horizontally for applying the cross feed.

7.             Table: The table rests on the guide way on the saddle and travels longitudinally. T slots are provided on the table to mount the work pieces directly or to mount the work holding devices. The longitudinal feed stops are located on the front of the table. These disengage the machine feed at a set position. The table is also fitted with a hand wheel for hand feed in the longitudinal direction and a lever for locking the table. There is a gutter around the around the edges of the table to collect the cutting fluid.

8.              Over arm and brace: The over arm is mounted on the top of the column above the spindle and is intended as a support when milling with an arbor. The arm is adjustable so that the bearing support may be provided nearest to the cutter. The brace provides extra stability to the machine. It must be loosened before the table elevation setting is altered.

Milling machine


Milling machine diagram




Milling Machine Cutter

Milling machine cutter can be classified as:

1. Plain milling cutter -

(a) Light duty plain milling cutter -   Helix angle= 25 degree
(b) Heavy duty milling cutter - Helix angle = 25-40 degree
(c) Helical duty milling cutter - Helix angle = 45-60 degree

2. Side milling cutter -

(a) Plain side milling cutter 
(b) Staggered teeth milling cutter - Used for large depth cut

3. Metal slitting saw cutter milling cutter - Suitable for cutting small cut and to divide the job in two parts

(a)  Plain metal saw milling cutter - 0.3 mm - 6 mm
(b) Staggered teeth slitting milling cutter - 6-7.5 mm

4. Angle milling cutter -

(a) Single angle milling cutter
(b) Double angle milling cutter


Milling cutter



5. Formed cutter -

(a) Convex milling cutter
(b) Concave milling cutter

6. Thread milling cutter

7. Wood ruff key slot milling cutter

8. Fly cutter

9. Tap and reamer cutter

10. Corner rounding milling cutter

11. Gear cutter

12. Sprocket cutter

13. Spline cutter

14. bolted cutter



                                                  

Material For milling cutter


1. High carbon steel (H C S) - 

2. High speed steel (H S S) - Tungsten = 14-22 % , Venedium = 4%

3. Sintered carbide- Mixture of tungsten carbide, cobalt and titanium                                carbide 

4. Ceramics

5. Diamond                                              

Angle of milling cutter

  
A milling cutter tooth is more or less identical to that of a single point tool. The rank angle of a milling cutter is the inclination of the tooth face and it is measured from a line joining the cutter of the cutter and the tooth of the tip. The clearance angle of a milling cuter is the relief given to the portion AB of the cutter. This clearance relief is given to from the cutting edge, and it avoids rubbing the rubbing the work piece while machining. It is given two stages. First it is ground to a small angle 0 and this angle is called relief angle. This angle should be very small as any increase in this angle would reduce the strength of the tooth. The portion TP of the tooth up to which the relief angle extends is called the land. After the land the tooth is further relieved t any angle 0 and angle is called the primary clearance angle which is about 150. The angle 02 is called the secondary clearance angle and this defines the shape of the chips space which is call gas. The tooth of the chip is radius to help cutting of the chips. This round portion of the chip space is called fillet. Primary clearance angle, relief angle, secondary clearance angle are all measured from a tangent drawn at the tool tip.
          
The positive rake angle is used for general purpose, and issued specially for milling materials, which produce continuous chips. In the positive rake, the weakest point, the weakest point is presented to cut first. The reaction   forces tends to pull the cutter into the work and the cutter forces tend to lift the work piece .The positive rake cutting edges peal the chip away from the work, and hence there is a likely hood of breakage of cast iron parts, especially those having thin and fragile sections.
         
A negative rake makes the tooth stronger and is suitable for roughing cuts. In the negative rake, the strong area is presented to cut first. The reaction forces tend to push the cutter away from the work, and the cutter forces tend to push the work against the fixture.
          
Zero rake angle is ideal for milling material like cast iron, brass etc. which produce broken chips.


MILLING PROCESSES


Several operations are performed on knee type horizontal milling machine and vertical milling machine .Some are as given

(1)           Plain milling or Slab milling: It is the production of a horizontal flat surface parallel to the axis of the milling machine. The work piece is held in a vice or fixture, or fastened directly on to the table.

(2)          Side milling: It is the process of machining a vertical flat surface perpendicular to the axis of the milling machine arbor. The combined this action of the peripheral and side teeth on a side and facing cutter perform this operation.

(3)          Face Milling: It is done to produce a flat surface perpendicular to the spindle axis of the machine. This is done by means of a face-milling cutter mounted on the vertical milling machine spindle. Face Milling may also be done in a horizontal milling to produce vertical flat surfaces.

(4)          End Milling: It is an operation similar to face milling but done using a much smaller cutter. Cutting is done on the end of the cutter as well as on the periphery. Its operation is used for facing small surfaces, milling slots, grooves, producing internal recesses and for truing the edges of a work piece.

(5)    Straddle Milling: In this type milling two vertical face of job is prepared simultaneously. Two side and face cutter is used on arbor in a single operation.

(6)    Gang Milling: When two or more surfaces is prepared simultaneously in milling machine is called gang milling.

(7)    Angular milling: To prepare the angular face of the job is used angular milling.

(8)   Form milling:   Form cutter is use in this process. Irregular contour is performed by this process.

(9)   Profile Milling 

(10)  Key way and Slot Milling 

(11)  Saw Milling

(12)  Helical Milling

(13)  Gear milling

(14)  Thread Milling

(15)  Cam Milling   



Up milling and - Down-Milling


The two operations differ by the directions in which the teeth of the cutter and the feed of the work piece move.


Up-Milling


The most commonly used method of feeding is to being the work against the direction of rotation of the cutter. This is also known as conventional milling. This is most commonly used method. In up milling the removal of the chip by each citing edge starts at the thinnest part of the chip and progresses to the thickest part. The cutting edge slides in the material before it starts to cut. This scraping causes a good deal of heat and wears on the cutter. The cutter and work pieces suddenly spring apart the machine chatters, and ripples flaw the material surface. The spindle for the milling is pensioned in the direction of feed. It is feasible on any milling machine, Chances of cuter breakage are less.

Up Milling


Down milling

It is also known as climb milling. In it the feed moves in the same direction as the rotation of the cutter. Chip removal starts at the thickest part. The cutter cuts into the material straight way and doesn’t slide. As a result less heat is developed and there is less wear on the cutter. Work piece is pressed down on the worktable by the cutting pressure and thus prevents the work piece from lifting. This Method is not recommended unless the machine table is fitted with a backlash eliminator.

Down Milling


Read Other Milling Topic -

Read About Vernier Caliper -






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