Introduction :
➤ Milling is the machining process in which the metal removal takes place due to the cutting action of a revolving cutter when workpiece is fed past it.
➤ The revolving cutter is held on a spindle or arbor and the workpiece is clamped on the machine table.
➤ During the process, to produce the desired shape, the cutter removes the metal in the form of chips from the workpiece surface.
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| Working principle of a milling machine |
➤ The machine tool on which the milling operation is performed is called as Milling machine.
➤ Also, the workpiece can be fed in a vertical, longitudinal or cross direction.
A large variety of different types of milling machines are available. A broad classification of these machines according to the general design can be done as follows :
( 1 ) Column and knee type milling machines :
➤ Hand milling machine
➤ Plain or horizontal milling machine
➤ Vertical milling machine
➤ Universal milling machine
➤ Omniversal milling machine
( 2 ) Fixed bed or manufacturing type milling machines :
➤ Plain type milling machine (having single horizontal spindle)
➤ Duplex head milling machine (having double horizontal spindles)
➤ Triplex head milling machine (having two horizontal and one vertical spindle)
➤ Rise and fall type milling machine (for profile milling)
( 3 ) Planer type milling machines :
( 4 ) Production milling machines :
➤ Rotary table or continuous type milling machine
➤ Drum type milling machine
➤ Tracer controlled milling machine
( 5 ) Special purpose milling machines :
➤ Thread milling machine
➤ Profile milling machine
➤ Cam milling machine
➤ Gear milling or gear hobbing machine
➤ Planetary milling machine
Column and Knee Type Milling Machines :
Column and knee type milling machines are most commonly used in machine shops and tool rooms. These machines are general purpose machines and have a single spindle only. The worktable is supported on a knee-like casting, which can slide in vertical direction along a vertical column. Depending upon the spindle position and table movements, they are further classified as follows :
1. Hand milling machine
2. Plain or horizontal milling machine
3. Vertical milling machine
4. Universal milling machine
5. Omniversal milling machine
Now,,,,,
( 1 ) Hand Milling Machine :
➤ It is the simplest of all the milling machines and smaller in size.
➤ All the operations are performed by hand except the rotation of an arbor.
➤ The table carrying the workpiece over it, is moved by hand to feed the workpiece.
➤ This machine is mostly useful in producing small components such as hexagonal or square headed bolts, cutting key ways and slots on screw heads.
( 2 ) Plain or Horizontal Milling Machine :
➤ In which the vertical column serves as housing for electricals, main drive, bearings, etc.
➤ For supporting the saddle, worktable and other accessories such as indexing head, knee is provided.
➤ Overarm provides support for the yoke which supports the free end of the arbor.
➤ The arbor carrying the cutter rotates about a horizontal axis.
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| Plain milling machine |
➱ Longitudinal direction ( ↔ )
➱ Vertical direction ( ↕ )
➱ Cross direction ( ● )
➤ For vertical movement of the table, the knee along with whole unit over it slides up and down in the guide ways provided on the column.
➤ For cross movement of the table, the saddle is moved towards or away from the column along with the whole unit over it.
➤ To provide additional support and rigidity to the arbor, a brace is employed.
( 3 ) Vertical Milling Machine :
➤ In this type of machine spindle position is vertical hence, called as vertical milling machine.
➤ This machine is available in two types i.e. fixed bed type and column and knee type.
➤ It consists of a vertical column on a heavy base and overarm is made integral with the column and carrying housing as its front.
➤ This housing is called as head which can be of fixed type or swivelling type.
➤ In fixed head type, the spindle always remains vertical and can be adjusted up and down.
➤ Whereas, in swivelling head type, the spindle can be swivelled to any required angle for machining inclined surfaces.
➤ The knee carries a screw jack with the help of which it is moved up and down along the guide ways provided on the column.
➤ The saddle is mounted on the knee which can be moved towards or away from the column, along the horizontal guide ways provided on the knee.
➤ This enables the table to move in cross direction.
➤ Similar to horizontal milling machine, in vertical milling machine also there are three motions for the table i.e. longitudinal, vertical and cross.
➤ Hence, with the help of knee the workpiece gets vertical movement, by using saddle cross movement; and longitudinal movement by the table.
➤ This machine is available in two types i.e. fixed bed type and column and knee type.
➤ It consists of a vertical column on a heavy base and overarm is made integral with the column and carrying housing as its front.
➤ This housing is called as head which can be of fixed type or swivelling type.
➤ In fixed head type, the spindle always remains vertical and can be adjusted up and down.
➤ Whereas, in swivelling head type, the spindle can be swivelled to any required angle for machining inclined surfaces.
➤ The knee carries a screw jack with the help of which it is moved up and down along the guide ways provided on the column.
➤ The saddle is mounted on the knee which can be moved towards or away from the column, along the horizontal guide ways provided on the knee.
➤ This enables the table to move in cross direction.
➤ Similar to horizontal milling machine, in vertical milling machine also there are three motions for the table i.e. longitudinal, vertical and cross.
➤ Hence, with the help of knee the workpiece gets vertical movement, by using saddle cross movement; and longitudinal movement by the table.
( 4 ) Universal Milling Machine :
➤ Universal milling machines are most versatile of all the milling machines and most useful because, they are capable of performing most of the machining operations.
➤ By using this machine, large number of other machine tools can be avoided.
➤ Universal milling machine is similar to horizontal milling machine, except that the table can be given one more additional movement.
➤ Its table can be swivelled in a horizontal plane and for this purpose circular guide ways are provided on the saddle.
➤ To read the angle of swivelling, a graduated circular base is provided under the table with datum mark on the saddle. Hence, workpiece can be set at an angle with the cutter for milling helical and spiral flutes and grooves.
➤ By using some attachments, this machine can be worked as a vertical milling machine.
➤ Universal milling machine is similar to horizontal milling machine, except that the table can be given one more additional movement.
➤ Its table can be swivelled in a horizontal plane and for this purpose circular guide ways are provided on the saddle.
➤ To read the angle of swivelling, a graduated circular base is provided under the table with datum mark on the saddle. Hence, workpiece can be set at an angle with the cutter for milling helical and spiral flutes and grooves.
➤ By using some attachments, this machine can be worked as a vertical milling machine.
( 5 ) Omniversal Milling Machine :
➤ Omniversal milling machine is a modified form of a plain milling machine.
➤ It is provided with two spindles, out of which one is horizontal and other is carried by universal swivelling head.
➤ The latter can be fixed in vertical position or can be set at any required angle up to 90⁰ on both the sides of vertical.
➤ One more special feature of this machine is that, it carries two more adjustments in addition to all the adjustments provided in a universal machine.
➤ Its knee can be swivelled about a horizontal axis to tilt the table and can be moved horizontally also.
➤ Hence, this machine is very useful for tool room work.
➤ The latter can be fixed in vertical position or can be set at any required angle up to 90⁰ on both the sides of vertical.
➤ One more special feature of this machine is that, it carries two more adjustments in addition to all the adjustments provided in a universal machine.
➤ Its knee can be swivelled about a horizontal axis to tilt the table and can be moved horizontally also.
➤ Hence, this machine is very useful for tool room work.
Principle Parts of Column and Knee Type Milling Machines :
The main parts of all column and knee type milling machines are similar but, only the movements are different. All these machines consists of following main parts :
1. Base
2. Column
3. Knee
4. Table
5. Saddle
6. Overarm
7. Arbor
2. Column
3. Knee
4. Table
5. Saddle
6. Overarm
7. Arbor
Work Holding Devices :
➤ For effective machining operations, it is necessary that the workpiece should be properly and securely held on the milling machine table.
➤ Following are the general method of holding the workpiece on the table :
1. T-bolts and clamps
2. Angle plates
3. V-blocks
4. Vices
1. T-bolts and clamps
2. Angle plates
3. V-blocks
4. Vices
Milling Cutters :
➤ The milling cutters may have straight teeth i.e. parallel to the rotation axis or in helical shape.
➤ The angle of helix may be right hand or left hand which will decide the direction of rotation of the cutter for cutting operation.
➤ A milling cutter may be made of single piece of steel or having cutting teeth inserted in a solid body.
➤ According to the shape of teeth, milling teeth are broadly classified as follows :
1. Plain milling cutters :
➱ Light duty cutters
➱ Heavy duty cutters
➱ Helical cutters
2. Side milling cutters :
➱ Plain cutters
➱ Half cutters
➱ Staggered teeth cutters
➱ Interlocking cutters
3. Metal slitting cutters :
➱ Plain cutters
➱ Staggered teeth cutters
4. End milling cutters :
➱ Taper shank cutters
➱ Straight shank cutters
➱ Shell cutters
5. T-slot milling cutters
6. Woodruff-key milling cutters
7. Fly cutters
8. Angle milling cutters
➱ Single angle cutters
➱ Double angle cutters
9. Formed milling cutters
➱ Convex milling cutters
➱ Corner rounding milling cutters
➱ Gear cutters
➱ Concave milling cutters
➱ Thread milling cutters
10. Tap and reamer cutters
➤ The angle of helix may be right hand or left hand which will decide the direction of rotation of the cutter for cutting operation.
➤ A milling cutter may be made of single piece of steel or having cutting teeth inserted in a solid body.
➤ According to the shape of teeth, milling teeth are broadly classified as follows :
1. Plain milling cutters :
➱ Light duty cutters
➱ Heavy duty cutters
➱ Helical cutters
2. Side milling cutters :
➱ Plain cutters
➱ Half cutters
➱ Staggered teeth cutters
➱ Interlocking cutters
3. Metal slitting cutters :
➱ Plain cutters
➱ Staggered teeth cutters
4. End milling cutters :
➱ Taper shank cutters
➱ Straight shank cutters
➱ Shell cutters
5. T-slot milling cutters
6. Woodruff-key milling cutters
7. Fly cutters
8. Angle milling cutters
➱ Single angle cutters
➱ Double angle cutters
9. Formed milling cutters
➱ Convex milling cutters
➱ Corner rounding milling cutters
➱ Gear cutters
➱ Concave milling cutters
➱ Thread milling cutters
10. Tap and reamer cutters
Now,,,,,
➤ Plain milling cutters are cylindrical in shape and have the cutting teeth on their periphery.
➤ The cutting teeth may be either straight or helical.
➤ Their end faces are either ground square with the axis or slightly concave to reduce the friction.
➤ Hence, by side faces no cutting action is provided.
➤ These cutters are used for milling flat surfaces parallel to the rotation axis.
➤ These cutters are available in diameters from 16 to 160 mm and width range from 20 to 160 mm.
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| Plain milling cutter |
(a) Light duty cutters :
➤ These cutters are available up to 20 mm width and carry straight teeth.
➤ These cutters are used for key way and slot cutting.
(b) Heavy duty cutters :
➤ These cutters are wider and used for heavy duty work.
➤ The cutters have few teeth on the periphery and this increases the chip space permitting them to take deeper cuts.
(c) Helical cutters :
➤ These types of cutters have coarse pitch and the helix angle of teeth ranges from 45⁰ to 60⁰.
➤ These cutters are useful in profile milling work because of smooth cutting action and are adapted for taking light cuts on soft steel or brass.
( 2 ) Side Milling Cutters :
➤ Side milling cutters have teeth in their periphery and on one or both sides.
➤ Also, for mounting them on the arbor, they are provided with central hole.
➤ When used in pairs they are called as straddle mills.
➤ These cutters are employed for removing metals from the sides of a workpiece.
➤ They are available from 50 mm to 200 mm in diameter and 5 mm to 32 mm in width.
➤ The different types of side milling cutters are as follows :
(a) Plain cutters :
➤ These cutters have cutting teeth on periphery and on both sides.
➤ They are used for face milling or cutting slots.
(b) Half cutters :
➤ Half side milling cutters have helical or straight teeth on the periphery and on one side only.
➤ The helix can be either right hand or left hand.
➤ Actual cutting operation is performed by peripheral teeth where finishing and sizing is done by side teeth.
➤ Mostly, they are used in pairs for milling two parallel surfaces simultaneously and the operation is called as straddle milling.
➤ These cutters are used for face milling.
(c) Staggered teeth cutters :
➤ These cutters have alternate teeth on the periphery only.
➤ These teeth have opposite helix angle, staggered from side to side, like teeth of wood saw and cut alternatively on one side and then other side.
➤ These types of cutters are mostly used for keyway cutting and slot cutting.
(d) Interlocking cutters :
➤ Interlocking cutters are formed by joining two half side milling cutters or staggered teeth side milling cutters.
➤ By inserting spacers between two cutters, they can be adjusted to acquire the required width.
➤ The spacers are used to make good reduction in width of the cutters due to wear and frequent resharpening of teeth.
➤ These cutters are useful for milling wider slots of exact width. Also widely used in gang milling.
( 3 ) Metal Slitting Cutters :
➤ Metal slitting cutters are also called as metal slitting saws.
➤ They appear like plain or side milling cutter but are of very small width.
➤ These types of cutters are mostly used for parting-off operation and for slotting.
➤ Metal slitting cutters are available in following varieties :
(a) Plain cutters :
➤ As compared to other milling cutters, these cutters are very thin.
➤ To prevent rubbing, their teeth carry some side relief.
➤ They are available in widths ranging from 1 mm to 5 mm.
(b) Staggered teeth cutters :
➤ These cutters are similar to the staggered side milling cutters but width is limited from 4 mm to 10 mm.
➤ These cutters are used for heavy work only.
( 4 ) End Milling Cutters :
➤ End milling cutters have cutting teeth on the end as well as on the circumference of the cutter.
➤ The teeth on circumference may be straight or helical and the helix may be left hand or right hand.
➤ These cutters are used for light milling operations such as cutting slots, machining accurate holes, profile milling, narrow flat surfaces, etc.
➤ The different varieties of end milling cutters are as follows :
(a) Taper shank cutters :
➤ They have a tapered shank or extensive on one end for mounting and driving the cutters.
➤ The cutters may have double flutes or multiple flutes.
➤ The taper shank end milling cutters are available from 10 to 63 mm in diameter.
(b) Straight shank cutters :
➤ For mounting and driving the cutters, the straight shank mills have round shanks.
➤ The cutter teeth may be straight or helical.
➤ The diameter of cutter ranges from 2 mm to 63 mm.
(c) Shell cutters :
➤ Shell end milling cutters are heavier and larger end milling cutters and are provided with central hole for mounting the cutter on an arbor.
➤ The cutting edges are on the circumference and at the end.
➤ The cutting teeth may be straight or helical and may be right handed or left handed.
➤ These cutters are generally used for face milling operations.
➤ The diameter of cutter ranges from 40 mm to 160 mm and width from 32 mm to 50 mm.
( 5 ) T-slot Milling Cutters :
➤ T-slot milling cutters are special forms of end milling cutters used for producing T-slot.
➤ In smaller size, it has the shank integral with the cutter, whereas large size cutters are mounted on a separate shank.
➤ During the operation, the narrow groove at the top is first milled by using a slotting cutter or end milling cutter.
➤ After that, T-slot milling cutter is employed for milling wider groove.
➤ In smaller size, it has the shank integral with the cutter, whereas large size cutters are mounted on a separate shank.
➤ During the operation, the narrow groove at the top is first milled by using a slotting cutter or end milling cutter.
➤ After that, T-slot milling cutter is employed for milling wider groove.
( 6 ) Woodruff-key Milling Cutters :
➤ It is a small type of end milling cutter which resembles plain and side milling cutters.
➤ Small size (up to 50 mm) diameter cutters have a solid shank, whereas larger size cutters are provided with a central hole for mounting on an arbor.
➤ Smaller size cutters have straight teeth on the periphery, whereas larger cutters have staggered teeth on the periphery and sides.
➤ Small size (up to 50 mm) diameter cutters have a solid shank, whereas larger size cutters are provided with a central hole for mounting on an arbor.
➤ Smaller size cutters have straight teeth on the periphery, whereas larger cutters have staggered teeth on the periphery and sides.
( 7 ) Fly Cutters :
➤ It is actually a single point cutting tool, which is mounted on a cylindrical body and held in a stub arbor or in a bar.
➤ Generally, fly cutters are the simplest form of cutters and used in experimental shops or tool room works.
➤ The cutting edge may be formed to reproduce countered surface.
➤ When standard cutters are not available, this cutter is considered as an emergency tool.
➤ Generally, fly cutters are the simplest form of cutters and used in experimental shops or tool room works.
➤ The cutting edge may be formed to reproduce countered surface.
➤ When standard cutters are not available, this cutter is considered as an emergency tool.
( 8 ) Angle Milling Cutters :
➤ The cutting edges of the angle milling cutters are formed at the conical surface around the periphery of the cutter.
➤ Its angular teeth are neither parallel nor normal to their axes.
➤ They are mostly used in milling V-grooves, notches, dovetail slots, reamer teeth and other angular surfaces.
➤ Two common types of angle milling cutters are as follows :
(a) Single angle cutters :
➤ They may have their teeth either only on the angular face or both on angular face and the side.
➤ The angle of the cutter is designated by an included angle between the conical face and flat face of the cutter.
➤ Their teeth may have an included angle of 30⁰, 45⁰, 60⁰, 75⁰, 80⁰ and available with diameter of 50 mm and width of 12 mm.
(b) Double angle cutters :
➤ These cutters have two angular faces which join together to form V-shaped teeth.
➤ The included angle of V is generally 45⁰, 60⁰ or 90⁰, but it is not necessary that the angle of both the faces should be equal.
➤ Double angle milling cutter is mainly used for cutting spiral grooves on a piece of blank.
➤ The cutters are available up to 100 mm diameter and width ranges from 12 mm to 36 mm.
➤ Its angular teeth are neither parallel nor normal to their axes.
➤ They are mostly used in milling V-grooves, notches, dovetail slots, reamer teeth and other angular surfaces.
➤ Two common types of angle milling cutters are as follows :
(a) Single angle cutters :
➤ They may have their teeth either only on the angular face or both on angular face and the side.
➤ The angle of the cutter is designated by an included angle between the conical face and flat face of the cutter.
➤ Their teeth may have an included angle of 30⁰, 45⁰, 60⁰, 75⁰, 80⁰ and available with diameter of 50 mm and width of 12 mm.
(b) Double angle cutters :
➤ These cutters have two angular faces which join together to form V-shaped teeth.
➤ The included angle of V is generally 45⁰, 60⁰ or 90⁰, but it is not necessary that the angle of both the faces should be equal.
➤ Double angle milling cutter is mainly used for cutting spiral grooves on a piece of blank.
➤ The cutters are available up to 100 mm diameter and width ranges from 12 mm to 36 mm.
( 9 ) Formed Milling Cutters :
➤ Formed milling cutters are also called as form relieved milling cutters or radius cutters.
➤ The formed cutters have irregular profiles on the cutting edges, to generate an irregular outline of the work.
➤ The teeth of formed milling cutters are provided with a certain angle of relief so that their forms and sizes are retained even after resharpening.
➤ The common types of formed milling cutters are as follows :
(a) Convex milling cutters :
➤ Convex milling cutters are the most commonly used formed cutters.
➤ The name convex does not indicate the shape of the surface to be produced, but the shape of the cutter teeth.
➤ It is used for producing concave surface on a workpiece.
(b) Corner rounding milling cutters :
➤ The corner rounding milling cutters have teeth curved inwards on the circumferential surface to form the contour of a quarter circles.
➤ These cutters are manufactured separately as single cutters or double cutters.
➤ These cutters are used for milling the edges and corners of the jobs to a required radius.
(c) Gear cutters :
➤ The gear cutters have formed cutting edges which reproduce the shape of the cutter teeth on the gear blank.
➤ According to the gear teeth profile, the shape of the cutter may be involute or cycloidal.
➤ They are used for milling gear teeth on a milling machine.
➤ This process is used for milling operations such as cutting teeth of worm wheel, helical and spur gears, spline shafts, etc.
(d) Concave milling cutters :
➤ Concave milling cutters are also mostly used formed milling cutters.
➤ In this also, the name does not indicate the shape of the surface to be produced, but the shape of the cutter teeth.
➤ These cutters are used for milling a convex surface on a workpiece.
➤ The concave milling cutters have teeth curved inwards on the circumferential surface to form the contour of a semicircle.
(e) Thread milling cutters :
➤ Thread milling cutters are used for milling different types of threads, mostly for worms and acme type threads.
➤ The cutters may have parallel or taper shanks.
➤ These cutters are available in diameters ranging from 8 mm to 20 mm and the length of threaded portion varies from 8 mm to 33 mm.
➤ The included angle of cutting teeth will correspond to the angle of the threads to be produced.
➤ The formed cutters have irregular profiles on the cutting edges, to generate an irregular outline of the work.
➤ The teeth of formed milling cutters are provided with a certain angle of relief so that their forms and sizes are retained even after resharpening.
➤ The common types of formed milling cutters are as follows :
(a) Convex milling cutters :
➤ Convex milling cutters are the most commonly used formed cutters.
➤ The name convex does not indicate the shape of the surface to be produced, but the shape of the cutter teeth.
➤ It is used for producing concave surface on a workpiece.
(b) Corner rounding milling cutters :
➤ The corner rounding milling cutters have teeth curved inwards on the circumferential surface to form the contour of a quarter circles.
➤ These cutters are manufactured separately as single cutters or double cutters.
➤ These cutters are used for milling the edges and corners of the jobs to a required radius.
(c) Gear cutters :
➤ The gear cutters have formed cutting edges which reproduce the shape of the cutter teeth on the gear blank.
➤ According to the gear teeth profile, the shape of the cutter may be involute or cycloidal.
➤ They are used for milling gear teeth on a milling machine.
➤ This process is used for milling operations such as cutting teeth of worm wheel, helical and spur gears, spline shafts, etc.
(d) Concave milling cutters :
➤ Concave milling cutters are also mostly used formed milling cutters.
➤ In this also, the name does not indicate the shape of the surface to be produced, but the shape of the cutter teeth.
➤ These cutters are used for milling a convex surface on a workpiece.
➤ The concave milling cutters have teeth curved inwards on the circumferential surface to form the contour of a semicircle.
(e) Thread milling cutters :
➤ Thread milling cutters are used for milling different types of threads, mostly for worms and acme type threads.
➤ The cutters may have parallel or taper shanks.
➤ These cutters are available in diameters ranging from 8 mm to 20 mm and the length of threaded portion varies from 8 mm to 33 mm.
➤ The included angle of cutting teeth will correspond to the angle of the threads to be produced.
( 10 ) Tap and Reamer Cutters :
➤ Tap and reamer cutters are special types of double angle cutters.
➤ These cutters are used for milling flutes on reamers and taps.
➤ The pointed end of the tooth is round and the tooth profile corresponds to the type of groove that is to be made.
➤ These cutters are used for milling flutes on reamers and taps.
➤ The pointed end of the tooth is round and the tooth profile corresponds to the type of groove that is to be made.
Milling Machine Operations :
A large variety of components can be machined on a milling machine which involves various type of operations. These operations are broadly classified as follows :
1. Plain or slab milling
2. Face milling
3. Angular milling
4. Form milling
5. Straddle milling
6. Gang milling
7. Slot and groove milling
8. Key way milling
9. Slitting or saw milling
10. Side milling
11. End milling
12. Profile milling
13. Gear milling
14. Thread milling
15. Helical milling
16. Cam milling
1. Plain or slab milling
2. Face milling
3. Angular milling
4. Form milling
5. Straddle milling
6. Gang milling
7. Slot and groove milling
8. Key way milling
9. Slitting or saw milling
10. Side milling
11. End milling
12. Profile milling
13. Gear milling
14. Thread milling
15. Helical milling
16. Cam milling
Now,,,,,
➤ Plain milling is used for machining a flat, plain, horizontal surface, parallel to the axis of cutter with the help of plain or slab milling cutter.
➤ During the operation, the workpiece and cutter are secured properly on the machine and by rotating the vertical feed screw of the table and machine, the depth of cut is adjusted.
➤ Face milling is employed for machining a flat surface which is at right angle, to the axis of revolving cutter.
➤ The operation is performed on plain milling machine and depth of cut is adjusted by rotating the cross feed screw of the table.
➤ Face mills under 150 mm are called as shell mills and are held on a short arbor.
( 3 ) Angular milling :
➤ Angular milling is used for machining a flat surface at an angle, other than right angle, to the axis of milling machine spindle. For example, V-blocks.
( 4 ) Form milling :
➤ Form milling is the operation of machining irregular contours by means of form milling cutter.
➤ The irregular contour may be concave, convex or of any other shape.
➤ The cutting speed of form milling process is 20-30 % less than of the plain milling.
➤ The cutting speed of form milling process is 20-30 % less than of the plain milling.
➤ Straddle milling is the operation of flat vertical surface on both the side of a workpiece simultaneously.
➤ For this purpose two side milling cutters are mounted on the same arbor.
➤ This process is mainly used for production of square and hexagonal surfaces.
( 6 ) Gang milling :
➤ Gang milling is the operation of machining several surfaces of the workpiece simultaneously.
➤ This method saves machining time and is widely used in repetitive work.
( 7 ) Slot and groove milling :
➤ Slot milling is the operation of producing slots in solid workpiece.
➤ Groove milling is the operation of producing grooves of various shapes like plain grooves, curved grooves, V-grooves, etc.
➤ According to the shape of slot or groove, the cutter is selected.
( 8 ) Key way milling :
➤ Key way milling is a common operation which is performed on milling machine.
➤ In this a groove is milled on shafts and spindles, which is known as key seat.
( 9 ) Slitting or saw milling :
➤ Saw milling is the operation of production of narrow slots or grooves on a workpiece by using a saw or slitting milling cutter.
➤ It is also used for complete parting-off operation.
( 10 ) Side milling :
➤ In this operation a side milling cutter is used for machining a flat vertical surface on one side of the workpiece.
➤ When two operation vertical flat surfaces are required to be machined, then two side milling cutter in pairs are used.
➤ By using spacer, the space between two cutters can be adjusted.
➤ Hence, this operation is similar to straddle milling.
Dividing Head or Indexing Head :
➤ The dividing head or indexing head is one of the most important of the milling machine accessories.
➤ The dividing head is a mechanical device used to divide the circumference or periphery of a job into a specified distance or angular separations.
➤ It is also used to hold the workpiece securely.
➤ The dividing head rotates the workpiece through a certain number of degrees or a certain fraction part of a complete circle for graduating the part. This operation is called as indexing.
➤ Following are the types of dividing head :
1. Plain dividing head
2. Universal dividing head
3. Optical dividing head
Milling Machine Attachments :
➤ Milling machine attachments are used to add to the versatility of the machine, high work capability and to increase the efficiency of the machine.
➤ A standard attachments used on milling machine are as follows :
1. Vertical milling attachments
2. Universal milling attachments
3. High speed milling attachments
4. Rack milling attachments
5. Gear cutting milling attachments
6. Rotary table
7. Dividing head
Cutting Speed, Feed and Depth of Cut :
Cutting Speed :
➤ The cutting speed of a milling cutter is the distance travelled per minute by the cutting edge of the cutter.
➤ It is measured at the circumference of the cutter and generally expressed in meters per minute. Mathematically it is given as,
V = ㄫDN / 1000 m/min.
Where, V = Cutting speed in m/min.,
D = Diameter of cutter in mm,
N = Cutter speed in r.p.m.
Feed :
➤ Feed represents the table travel in any direction. It is the rate at which the workpiece advances under the cutter.
➤ It is generally expressed in milli-meters (mm). It can be given to the table, either hand or through automatic means.
➤ It is expressed in milling machine by the following methods :
1) Feed per tooth : It is defined by a distance, the workpiece advances in the time between engagement by the two successive teeth. It is expressed in mm/tooth of the cutter.
2) Feed per revolution : It is defined by a distance, the workpiece advances in the time when the cutter turns through one complete revolution. It is expressed in mm/rev. of the cutter.
3) Feed per minute : It is defined by a distance, the workpiece advances in one minute. It is expressed in mm/min.
Depth of Cut :
In milling machine, depth of cut is the thickness of the material removed in one pass of the workpiece under the cutter. It is the perpendicular distance measured between the original and final surface of the workpiece. Generally, it is expressed in mm.
Alignment Test on Milling Machine :
➤ The surface produced after machining is achieved by both work and tool movement.
➤ Hence the accuracy of the finished product depends on the accuracy of the machine tool and its alignment with the parts of the machine tool.
➤ Various tests are performed on the machine tool to check its installation, flatness, perpendicularity of the various axes, parallelism of the various surfaces of machine tool and some practical tests.
➤ The instruments used for these alignment tests are dial gauges, test mandrels, straight edges, spirit levels, frame levels etc.
1. Table surface parallel to arbor rising towards overarm.
2. Table surface parallel to longitudinal movement.
3. Transverse movement parallel with spindle axis.
4. T-slot parallel with longitudinal table movement.
5. Cutter spindle axial slip and eccentricity of external diameter.