Introduction :
➠ The lathe is the oldest machine tool and came into existence from the early tree lathe which was then an important device for rotating and machining a workpiece held between two trees.
➠ For machining purpose, tools were used.
➠ After further development, a wood strip known as lath was used to support the rope and hence the machine is called as lathe.
➠ Actually, lathe was the first machine tool which came into being as a useful machine for metal cutting.
➠ Now-a-days, large number of modern machine tools are found, still the lathe machines are used in all modern tool rooms, repair shops and training workshops.
Working Principle of the Lathe :
➠ Lathe is a machine tool which holds the workpiece securely between the two rigid and strong supports, called as centres or in a chuck or face plate, while the workpiece revolves.➠ The cutting tool is rigidly held and supported in a tool post and is fed against the revolving workpiece.
➠ While the workpiece revolves about its own axis, the tool is made to move in parallel or at an inclination with the axis of a material to be cut.
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| Working principle of the lathe |
➠ The material from the workpiece is removed in the form of chips.
➠ Fig. shows the working principle of the lathe machine.
➠ Also, to cut the material properly, the tool material should be harder than the workpiece material.
Types of Lathe :
➠ To suit the various conditions of metal machining, lathes of various designs and constructions have been developed.
➠ But, all of them follow the same fundamental operating principle and perform the same function.
➠ The type of laths which are generally used are as follows :
1. Bench lathe
2. Speed lathe
3. Engine lathe
➪ Belt drive lathe
➪ Individual motor drive lathe
➪ Gear head lathe
➪ T-lathe
4. Tool room lathe
5. Capstan and Turret lathe
6. Automatic lathe
7. Special purpose lathe
➪ Wheel lathe
➪ Gap bed lathe
➪ Duplicating lathe
Now,,,,,
( 1 ) Bench lathe :
➠ It is a very small lathe and usually mounted on a bench or cabinet.
➠ It is very accurate, hence used for small and precision work.
➠ It is provided with all the attachments, which a larger lathe carries and performs almost all the operations which a larger lathe can do.
( 2 ) Speed lathe :
➠ The speed lathe is the simplest of all types of lathe.
➠ They are named speed lathe because; their spindle rotates at very high speed ranging from 1200 to 3600 r.p.m.
➠ Speed lathe consists of a bed, headstock, tailstock and a tool-post mounted on an adjustable slide.
➠ There is no provision for gear box, carriage and lead screw.
➠ These lathes are generally employed for wood turning, polishing, centring and metal spinning.
( 3 ) Engine lathe or Centre lathe :
➠ This type of laths is the most widely used and it is an important type of lathe.
➠ The term engine is associated with the lathe owing to the fact that, early lathes were driven by steam engines.
➠ It consists of all the similar parts as that of the speed lathe, but its construction is relatively more robust.
➠ As per the headstock design and methods of transmitting power to the machine, engine lathes are classified.
➠ A lathe which receives its power from an over-head line shaft is known as belt-driven lathe.
➠ A lathe that receives its power from an individual motor which is integral with the machine is known as motor driven lathe.
➠ Similarly, a lathe that receives its power from a constant speed motor and all speed changes, are obtained by shifting various gears which are located in the headstock, is known as gear-head lathe.
( 4 ) Tool room lathe :
➠ A tool room lathe having features similar to an engine lathe.
➠ It is built more accurately and has a wide range of spindle speeds ranging from a very low to high speed i.e. upto 2500 r.p.m.
➠ Tool room lathe is equipped with a chuck, taper turning attachment, thread chasing dial, relieving attachment, pump for coolant, etc.
➠ It is used for precision work on tools, dies, gauges and in machining work, where accuracy is required.
( 5 ) Capstan and Turret lathe :
➠ These lathes are newer development of the engine lathe and are used in mass production.
➠ In this type of lathe, the tailstock of an engine lathe is replaced by a hexagonal turret, on the face of which multiple tools may be fitted and fed into the work in a proper sequence.
➠ Hence, without resetting of workpiece and tools, several different operations can be done on the workpiece.
➠ Due to this, in a minimum time, number of identical components can be produced.
( 6 ) Automatic lathe :
➠ Automatic lathes are high speed, heavy duty and mass production lathes.
➠ These lathes help in enhancing the quality and quantity of production.
➠ The working movements of the complete manufacturing process for a job are done automatically.
➠ After the job is complete, without attention of an operator, the machine will continue to repeat the cycles and produce identical parts.
➠ Number of attachments such as work holding magazine, cross drilling attachment, polygon turning attachments are available.
( 7 ) Special purpose lathe :
➠ As the name indicates, these machines are used for special purposes and for the components which cannot be conveniently machined on a standard lathe.
➠ The lathe which is used for finishing the journals and turning the thread on railroad car and locomotive wheels is known as wheel lathe.
➠ In gap bed lathe a section of the bed adjacent to headstock is recoverable and is used to swing extra large diameter pieces.
➠ For duplicating the shape of a flat or round template on the workpiece, duplicating lathe is used.
➠ The lathe used for machining of rotors for jet engines is known as T-lathe.
➠ In T-lathe, the axis of the lathe bed is at right angles to the axis of the headstock spindle.
Size of Lathe :
In order to specify the size of a lathe, following specifications should be included :
1. Height of the centres : It is measured from the lathe bed.
2. The length between centres : It is the maximum length of the workpiece that can be mounted between centres.
3. The length of bed : It includes the length of headstock and tailstock.
4. The swing diameter over bed : It is the largest diameter of the workpiece that revolves without touching the bed.
5. Swing diameter over carriage : It is the largest diameter of the workpiece that can revolve over the lathe saddle. It is always less than swing diameter over the bed.
6. Maximum bar diameter : It is the maximum diameter of bar stock that passes through the hole of the headstock spindle.
7. Spindle speed.
8. Spindle nose diameter.
9. Lead screw pitch.
10. Motor horse power and R.P.M.
Various Parts of the Lathe :
Fig. shows the basic parts of a geared head lathe machine.
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| Lathe parts |
( 1 ) Bed
( 2 ) Headstock
( 3 ) Tailstock
( 4 ) Carriage
( 5 ) Feed mechanism
( 6 ) Thread cutting mechanism
Lathe Accessories :
The devices which are employed for holding and supporting the workpiece and the tool, on the lathe are called as accessories. They include the following devices :
( 1 ) Centres
( 2 ) Chucks
( 3 ) Carriers or dogs
( 4 ) Face plate
( 5 ) Catch plate
( 6 ) Angle plate
( 7 ) Mandrels
( 8 ) Rests
Lathe Operations :
The operations performed on a lathe are divided into two groups :
( A ) Standard or common operations : It includes,
1. Straight turning
2. Step turning
3. Eccentric turning
4. Taper turning
5. Facing
6. Drilling
7. Reaming
8. Boring
9. Knurling
10. Thread cutting
11. Parting off
12. Chamfering
13. Grooving
14. Forming
( B ) Special operations : It includes,
1. Grinding
2. Milling
3. Duplicating
4. Spinning
5. Spherical and elliptical cutting
6. Relieving
7. Tapping
8. Internal thread cutting
Now,,,,,
( 1 ) Straight Turning :
➠ This operation is performed for producing a cylindrical surface by removing the excess material from the workpiece.
➠ The cutting tool is held in the tool post and fed into the rotating work parallel to the lathe axis.
➠ It may be rough turning or finish turning.
➠ Rough turning is done by taking heavy depth of cut and giving high feed rate at low cutting speed.
➠ Finish turning is done by taking small depth of cut and giving small feed at high cutting speed.
( 2 ) Step Turning :
➠ Step turning is also called as shoulder turning.
➠ When workpiece of different diameters are turned, the surface formed from one diameter to the other is called as shoulder.
➠ The machining of this part of the workpiece is known as shoulder turning.
➠ There are four types of the shoulder,
- Square shoulder
- Radius shoulder
- Angle shoulder
- Under cut shoulder
( 3 ) Eccentric Turning :
➠ If a cylindrical workpiece has two separate axis of rotation one being out of centre to the other, then the workpiece is called as eccentric.
➠ The turning of different surfaces of the workpiece is known as eccentric turning.
➠ For example, crank shaft turning on a lathe. For eccentric turning, the workpiece is first mounted on its true centre and turned, then it is remounted on the offset centre and the eccentric surfaces are machined.
( 4 ) Facing :
➠ It is the operation of machining the ends of a workpiece to produce a flat surface with the axis.
➠ It involves feeding of the tool perpendicular to the axis of rotation of the workpiece.
➠ The tool used for facing is properly ground and mounted in a tool holder of the tool post.
( 5 ) Drilling :
➠ For drilling, the workpiece is held in a suitable device such as face plate, chuck and the drill is held in the sleeve or barrel of the tailstock.
➠ Dead centre is removed and drill chuck or sleeve is inserted in its place.
➠ Then, the drill is fed by rotating the hand wheel of the tailstock.
➠ First a shorter length is drilled, by using smaller and shorter drill, followed by producing the required diameter by using the correct drill size.
➠ The already drilled hole acts as a guide for the latter drill.
( 6 ) Boring :
➠ It is an operation which is employed for machining internal surfaces, hence also called as internal turning.
➠ Boring is done to enlarge the already drilled hole and bring them to the exact required size.
➠ Generally, a single point solid boring tool is used for this purpose, which may be either rough or finish.
➠ Sometimes, for machining long holes, boring bars are also used.
➠ These bars are cylindrical in shape and carry a slot to accommodate the tool bit.
( 7 ) Reaming :
➠ Reaming is a finishing operation because a very small amount of material is removed during the operation.
➠ It is performed when a very high grade of surface finish and dimensional accuracy is required.
➠ For performing reaming a multi-teeth tool is used, which is called as reamer.
➠During the operation, the workpiece is held in a chuck or face plate and the reamer shank is fitted in a sleeve or inserted in the tapered hole of the tailstock spindle.
( 8 ) Knurling :
➠ Outer surfaces of some components such as dumbels, handles, measuring instruments and tools, gauges, etc. are generally provided with rolled impressions on them.
➠ These depressions are provided for better grip as compared to smooth surface.
➠ These indentations are known as knurls and the corresponding surface is known as knurled surface.
➠ The operation performed for producing this knurled surface is called as knurling.
The tool used for knurling is termed as knurling tool, which consists of a straight shank fitted with one or two knurling wheels at its front.
➠ These knurling wheels are made up of hardened tool steel and carry teeth on its outer surface.
➠ Three types of knurlings are mostly used i.e. straight, diagonal and diamond.
( 9 ) Parting off :
➠ It is an operation employed for cutting away a desired length from the bar stock hence, also called as cutting off.
➠ Tools used for this purpose is known as parting tool which have a longer point as they are required to cut from the outside surface right upto the centre of the job.
➠ During the operation, the workpiece is held in chuck at one end and supported at the dead centre on the other end.
➠ Cross feed is given to the tool by hand and workpiece rotated at a relatively high speed.
( 10 ) Chamfering :
➠ It is an operation of bevelling the extreme end of a workpiece.
➠ Chamfering is done to remove the burrs, to protect the end of the workpiece from being damaged and to have better appearance.
➠ Generally, chamfering is performed at the end of all the operations.
( 11 ) Grooving :
➠ It is an operation through which a groove of approximately same width is produced on the job as that of the cutting edge of the tool.
➠ For grooving, the reduction in diameter is effected by cross feed of the tool.
➠ Longitudinal feed is rarely used because width of the groove is similar to the cutting edge of the tool.
➠ Grooving is also called as necking.
( 12 ) Forming :
➠ Some components having neither cylindrical nor tapered surfaces are said to have shaped or formed surfaces.
➠ For machining such shapes, special tools are used which are called as form tools.
➠ In this, shape and size of the cutting edge of a tool corresponds to the shape and size of the surface to be produced.
➠ Different types of shapes produced by using various form tools.
( 13 ) Tapping :
➠ It is an operation through which internal threads are cut on the drilled hole with the help of tap.
➠ For tapping, the workpiece is held in the chuck and rotate at slow speed.
➠ The tap is fixed in the tailstock and then it is fed to the rotating workpiece.
➠ Hence, threads are cut on the internal diameter of workpiece.
( 14 ) Spinning :
➠ Spinning is the pressure forming of metal on a rotating chuck, former or die.
➠ For metal spinning, a spinning lathe is used.
➠ The set up of a machine is similar to the centre lathe.
➠ The machine consists of a bed, headstock, former, follower block and tailstock.
➠ In the headstock of the machine, a hard wooden or metal form block in the shape of required part is fixed.
➠ After clamping, the blank is rotated and a metallic tool is moved back and forth on the blank so that no thinning takes place any where on the blank.
➠ Usually, spinning process is used for making cup shaped articles which are symmetrical such as pressure vessels, refinery equipments, tanks, etc.
( 15 ) Spring Winding :
➠ It is an operation from which the coiled springs are manufactured.
➠ In this operation, the mandrel is held in the chuck.
➠ The spring wire is wound around this rotating mandrel.
➠ Then it is taken out from the mandrel and it is in shape of coiled spring.
Taper Turning :
➠ A large variety of articles, used in engineering practice, are found to have conical shapes or flat.
➠ Flat components having a gradual reduction in its width or thickness along their length, such components are called as tapered components.
➠ A uniform increase or decrease in diameter of workpiece measured along its length is known as taper, in case of conical components.
➠ In a lathe, taper turning means to produce conical surface by gradual reduction in diameter from a cylindrical workpiece.
➠ This tapering of a part is used in almost all machine spindles which have taper holes and receive taper shank of various tools and work holding devices.
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| Elements of taper |
D = Large taper diameter in mm,
d = Small taper diameter in mm,
L = Length of tapered part in mm,
𝛼 = Taper angle or half taper angle,
2𝛼 = Full taper angle.
➠ The amount of taper in a workpiece is specified by the ratio of the difference in diameters of the taper to its length, which is called as conicity and denoted by K, i.e.
K = D-d / L
➠ In fig., EF is drawn parallel to the axis and in the △ AFE,
AF = D-d / 2 and EF = L
➠ Consider △ AFE,
tan 𝛼 = D-d / 2L = K / 2 or K = 2 tan 𝛼
Taper turning methods :
➠ A taper may be turned on the lathe by feeding the tool at an angle to the rotation axis of the workpiece.
➠ While taper turning, it is essential that the cutting edge of the tool should be set accurately on the centre line of the workpiece, otherwise correct taper will not be obtained.
➠ Various methods are used for turning taper on a workpiece, some of them are as follows :
- Using tailstock set-over method
- By swivelling the compound rest
- Using taper turning attachment
- Using a form or broad nose tool
Lathe Attachments :
➠ Attachment is additional equipment used for increasing production and efficiency.
➠ The most commonly used attachments are as follows :
1. Stops
2. Grinding attachment
3. Milling attachment
4. Taper turning attachment
5. Copying attachment
6. Relieving attachment
Now,,,,,
( 1 ) Stops :
➠ Stops are used in conjunction with both i.e. the carriage as well as cross-slide.
➠ It is used for a quick and accurate positioning of the carriage and cross-slide.
➠ When similar operation is to be done repeatedly, the use of stops effects a considerable saving in time and gives more accuracy.
( 2 ) Grinding Attachment :
➠ It is also called as tool post grinder.
➠ Typical form of this attachment which consists of a bracket, a grinding wheel and a separate motor.
➠ The grinding wheel is driven separately by this motor.
➠ The workpiece is held between chuck and the grinding wheel is fed against the work piece.
➠ The attachment shown is used only for external grinding.
( 3 ) Milling Attachment :
➠ Milling attachment consists of a vertical pillar, to which an individual motor is attached and device, to mount a cutter on it. Base of the pillar is fastened rigidly to the saddle.
➠ The unit carrying the motor and the cutter can be moved vertically up and down by using a screw and the hand wheel provided at the top of the pillar.
➠ The workpiece is held stationary between the centres and the rotating cutter is moved longitudinally along it by operating the saddle.
➠ Depth of cut can be adjusted by moving the unit along the pillar by using a hand wheel.
( 4 ) Taper Turning Attachment :
➠ In this method, the tool is guided in a straight path set at an angle to the rotation axis of the workpiece and the workpiece is revolved between the centres.
➠ Fig. shows the taper turning attachment which consists of a frame or bracket.
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| Taper turning attachment |
➠ A guide bar is having graduations in degrees and can be swivelled on either side of the zero graduation.
➠ During the operation, the cross-slide is first made free from the lead screw by removing the binder screw.
➠ The rear end of the cross-slide is then tightened with the guide block by using a bolt.
➠ As the guide block will slide on the guide bar set at an angle to the lathe axis, the tool mounted on the cross-slide will follow the angular path.
➠ The guide bar must be set at half the taper angle and the taper on the workpiece is converted in degrees.
➠ The maximum angle through which the guide bar may be swivelled is in the range of 10ᣞ to 12ᣞ on either side of the centre line.
➠ The half angle of taper is calculated by,
tan 𝛼 = D-d / 2L
( 5 ) Copying Attachment :
➠ Copying attachment is also called as tracer attachment.
➠ It consists of an auxiliary slide, which is fitted on to the regular cross-slide of the lathe.
➠ At the rear of this slide, a cylinder and piston is provided, which is operated by hydraulic pressure.
➠ A bent arm is attachment to it and the free end of this arm carries a tracer.
➠ One end of the tracer moves along the profile of the template fitted at the backside of the lathe, of which the shape is to be copied.
➠ The other end operates an air nozzle which in turn, through the air pressure, regulates the oil supply and hence the hydraulic pressure.
( 6 ) Relieving Attachment :
➠ Relieving is generally done on most of the multiple point tools such as milling cutters, taps and reamers, etc.
➠ This attachment consists of an auxiliary slide mounted on the cross-slide in place of the compound rest.
➠ The tool post, carrying the tool is mounted on the auxiliary slide.
Cutting Speed, Feed and Depth of Cut :
Cutting speed :
➠ The cutting speed of a tool is the speed at which the metal is removed from the workpiece with the help of tool.
➠ In a lathe, it is the peripheral speed of the workpiece past the cutting tool which is expressed in m/min.
Cutting speed = ㄫDN / 1000 m/min.
where, D = Workpiece diameter in mm,
N = Workpiece speed in r.p.m.
Feed :
➠ In a lathe, the feed of a cutting tool is the distance tool advances for each revolution of the workpiece.
➠ Feed is generally expressed in mm/rev.
➠ Increase in feed reduces the cutting time, but reduces the tool life.
➠ The amount of feed depends on the following factors :
- Size of workpiece
- Shape of workpiece
- Strength of workpiece
- Holding method of workpiece
- Rigidity of the machine
- Depth of cut
- Shape of the tool
- Power available
Depth of cut :
➠ It is the perpendicular distance measured from the machined surface to the uncut surface of the workpiece.
➠ In a lathe, depth of cut is expressed as,
Depth of cut = D1 - D2 / 2
Where, D1 = Diameter of the workpiece before machining,
D2 = Diameter of the machined workpiece.
Machining Time :
➠ The machining time in lathe work can be calculated for a particular operation, if the speed of the workpiece and feed length of the workpiece is known.
Let, f = Feed of the workpiece in mm/revolution,
L = Length of the workpiece in mm,
N = Speed of the workpiece in r.p.m.,
T = Machining time in minutes.
Machining time is given by,
T = L / f * N , minute
Alignment Test on Lathe 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 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, etc. and some practical tests.
➠ The instruments used for these alignment tests are dial gauges, test mandrels, straight edges, spirit levels, frame levels etc.
1. Levelling of the lathe machine
2. True running of locating cylinder of main spindle
3. True running of headstock centre
4. Parallelism of the main spindle to saddle movement
5. True running of tapered socket in main spindle
6. Parallelism of tailstock guide ways with the movement of carriage
7. Alignment of the centres in vertical plane
8. parallelism of tailstock sleeve to saddle movement
9. Axial slip of main spindle
10. Pitch accuracy of lead screw
11. Axial slip of lead screw
Now,,,,,
( 1 ) Levelling of Lathe machine :
➠ The machine should be installed perfectly in horizontal and vertical direction.
➠ The level of lathe machine bed is generally tested by spirit level in both the longitudinal and transverse direction.
➠ For testing in transverse direction, the spirit level is kept at bridge piece. The reading should be taken simultaneously in each direction.
➠ The permissible error in level testing is 0.02 per 1000 mm.
( 2 ) True running of locating cylinder of main spindle :
➠ Locating cylinder is used to locate the chuck or face plate.
➠ The dial indicator is fixed to the carriage of lathe machine and its pointer should touch the locating surface.
➠ The spindle is then rotated on its axis and the dial indicator should not given any deflection in reading.
➠ The deflection in reading shows that the spindle is not running truely about its axis.
( 3 ) True running of headstock or live centre :
➠ As the workpiece has to rotate with headstock centre it should run truely with spindle axis.
➠ if it is not runs truely the eccentricity will occur while turning the workpiece.
( 4 ) Parallelism of the main spindle to saddle movement :
➠ The proportionate size mandrel and dial indicator is used to carry out the test.
➠ The dial indicator is mounted on the saddle and the pointer of the dial indicator should touch the surface of the mandrel.
➠ The deflection in the reading shows that the axis of spindle is not parallel to bed in horizontal axis and hence tapered surface is produced while machining.
➠ Also, deflection in the reading in vertical axis from bed will produce a hyperbolic surface.
( 5 ) True running of tapered socket in main spindle :
➠ The test is carried out using a test mandrel and the dial indicator.
➠ The mandrel is fitted into the tapered socket and dial indicator is mounted on the fixed member of the lathe machine.
➠ The readings at the two extreme end of the mandrel are taken.
➠ If the axis of tapered socket is not coincide with the axis of main spindle, the eccentric and tapered workpiece are produced.
( 6 ) Parallelism of tailstock guide ways and movement of carriage :
➠ This has to be checked in both vertical and horizontal plane.
➠ The block is mounted on the tailstock guide ways.
➠ The plunger of the dial indicator should touch the vertical and horizontal surface of the block.
➠ The dial indicators are mounted on the carriage and the carriage is moved along the block.
➠ It results in the tapered surface while turning the job held in headstock and tailstock.
( 7 ) Alignment of both the centres in vertical plane :
➠ The axis of the headstock and tailstock should be coincide with each other. If it is not then the job held between the two centres will not be parallel to the carriage movement.
➠ For the test, mandrel is held between both the centres and dial gauge is mounted on the carriage.
➠ The plunger of the dial gauge should be pressed against the mandrel in vertical plane.
➠ The permissible error is 0.03 mm.
( 8 ) Parallelism of tailstock sleeve to saddle movement :
➠ If the tailstock sleeve is not parallel to saddle movement, the height of the tailstock centre will vary along the length.
➠ Due to this the workpiece held may get tilted and results in faulty machining.
➠ The carriage is then moved along the length of the sleeve and deflections in the readings are noted down.
➠ The permissible error is upto 0.04 mm.
( 9 ) Axial slip of main spindle :
➠ This test is carried out to check the spindle movement which follows the same pattern.
➠ The dial gauge is mounted on the bed and its plunger is pressed against the face of the spindle.
➠ The spindle is then rotated and deflection in the reading is noted down.
➠ The permissible error is upto 0.01 mm.
( 10 ) Pitch accuracy of lead screw :
➠ The pitch of lead screw should be uniform throughout its length for the accuracy of threads.
➠ Generally, progressive and periodic errors in the pitch of the lead screw should be tested.
➠ In this test, fix the positive stop on the lathe bed.
➠ The length bars and slip gauges are located against the stop.
➠ The dial gauge is mounted on the saddle and its plunger is pressed against the calculated length of slip gauges. This initial reading is noted down.
➠ The reading of dial gauge against the stop is taken if it is same as before there is no error.
( 11 ) Axial slip of lead screw :
➠ The lead screw axis must be exactly perpendicular to the thrust face and collers of the lead screw.
➠ If they are not perfectly square, the cyclic endwise movement is setup which is similar to axial slip.
➠ In this test, the ball is fitted in the end of lead screw.
➠ The plunger of the dial gauge is pressed against the ball and then the lead screw is rotated.
➠ Hence, deviation in the reading noted down and the permissible error is 0.015 mm.
