LATHE AND LATHE OPERATIONS

LATHE

INTRODUCTION
The lathe is father of all machine tools; in early days it was equipped with a fixed tool rest and was used for woodworking. In operation, the lathe holds the job between two rigid supports called centres or by some chuck or face plate screwed to the nose or and of the spindle.
Function of lathe
The main function of lathe is to remove metal from a piece of work to give it the required shape and size. This is accomplished by holding the work securely and rigidly on the machine and then turning it against cutting tool, which will remove, metal from the work in the form of chips.
Types of lathes
Lathes of various designs and constructions have been developed to suit the various conditions of metal machining. But all of them employ the same fundamental principle of operation and perform the same function.  The lathes are classified as follow.
1. Speed lathe

• Wood working
• Centering
• Polishing
• Spinning

2.Engin lathe

• Belt drive
• Individual motor drive
• Gear head lathe

3.Bench lathe
4.Tool room lathe
5.Capstan and turret lathe
6.Special purpose

• Wheel lathe
• Gap bed lathe
• T-lathe
• Duplicating lathe

7.Automatic lathe

The speed lathe
➢ The speed lathe, in construction and operation, is the simplest of all types of lathes. It consists of a bed, a headstock, and a tailstock and tool post mounted on an adjustable slide. There is no feed box, lead screw or conventional type carriage. The tool is mounted on the adjustable slide and is fed into work purely by hand control. This characteristic of the lathe enables the designer to give high spindle speeds, which is usually, range from 1200 to 3600 r.p.m. As the tool is controlled by hand, the depth of cut and thickness of chip is very small.
➢ Light cut and high speed necessitate the use of this type of machine where cutting force is minimum such as in wood working, spinning, centering, polishing, etc.

The engine lathe or center lathe:

This lathe is most important member of lathe family and is most widely used. Similar to the speed lathe, the engine lathe has got all the basic parts, e.g. bed, headstock, and tailstock. But the headstock of an engine lathe is much more robust in construction and it contains additional mechanism of driving the lathe spindle at multiple speeds.The engine lathe that can feed the cutting tool both in cross and longitudinal direction with reference to the lathe axis with help of a carriage feed rod and lead screw.

The bench lathe:
This is a small lathe usually mounted on bench. It has practically all the parts of an engine lathe or speed lathe and it performs almost all the operations, its only difference being in size this is used for small and precision work.

The tool room lathe:
A tool room lathe having features similar to an engine lathe is much more accurately built and has a wider range of spindle speeds ranging from a very low to a quite high speed up to 2500 r.p.m.This lathe is mainly used for precision work on tools, dies, and gauges and in machining work where accuracy is needed.

Numerically Controlled Lathes:
In these lathes the path of the tools to produce the desired shape, along with other auxiliary functions like speed/feed changes, turret indexing, tail stock positioning, coolant supply, etc., are controlled by pre-programmed numerical data input in the form of punched tape to an electronic control system.  One of the latest developments is the computer control of NC machines which is popularly called the computer numerical control (CNC).  With the concept of numerical control, the configuration of lathes has undergone radical changes with features like slant bed design infinitely variable seed headstock, antifriction lead screws, auto-tool changing, etc.  Numerically controlled lathes are available in various versions. The control system employed on NC lathes enable straight cut and continuous path control.

The cutting tools on a CN C turning center are usually mounted in a device called a turret.  The turret is then mounted on a heavy cross slide and carriage similar to that of a conventional lathe.  The turret can be indexed very rapidly to automatically change to the next tool.
A number of smaller CNC lathes have also come to market in recent years to compete with the turret lathes and automatic screw machines.

The caption and turret lathe:
These lathes are development of the engine lathe and are used for production work. The distinguishing feature of this type of lathe is that 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 proper sequence.

Special purpose lathe:
As the name implies, they are used for special purpose and for job, which cannot be accommodated or conveniently machined on a standard lathe. The gap bed lathe, in which a section of the bed adjacent to the headstock is recoverable, is used to swing extra large diameter pieces.
The duplicating lathe is one for duplicating the shape of a flat or round template on to the work piece. The missile lathe, which has a very large swing for accommodating long missile component for very large diameter, is the most modern and latest in lathe design.

Automatic lathe:
These are high speed, heavy duty; mass production lathes which complete automatic control. Once the tools are set and the machine is started it performs automatically all the operations to finish the job.

The size of lathe:

The size of lathe is expressed or specified by following items and illustrated in fig.

1. The height of the centers measured from the lathe bed.
2.The swing diameter over bed.
3.The length between centers.
4.The swing diameter over carriage.
5.The maximum bar diameter.
6.The length of bed.

And width of bed, depth of the bed, depth and width of gap if it is a gap bed lathe, swing over gap, spindle nose diameter, center taper Morse number and range of spindle speeds, number of feeds, number of and range of metric and English threads that may be cut, pitch value of lead screw, power input and floor area required.

Description and functions of lathe parts
Following are the principal parts:
1 Bed.
2.Headstock.
3.Tailstock.
4.Carriage.
5.Feed mechanism.
6.Screw cutting mechanism.

The bed

The lathe bed forms the base of the machine. The headstock and the tailstock are located at either end of the bed and carriage rests over the lathe bed and slides on it.
The bed should be seasoned naturally to avoid distortion or warp that may develop when it is cooled after the bed is cast.
The guide ways of  the lathe may be flat and inverted- v having included angle of 90.
The bed material should have high compressive strength, should be wear resistance and absorb vibration cast iron alloyed with nickel and chromium forms a good material suitable for lathe.

The headstock 
The headstock is secured permanently on the inner ways as the left hand end of the lathe bed, and it provides mechanical means of rotating the work at multiple speeds. It comprises essentially a hollow spindle and mechanism for driving and alternating the spindle speed. All parts are housed within the head stock casting.
The spindle of headstock is made of carbon or nickel-chrome steel.

The back gear
The back gear is an additional feature of a belt driven lathe and is used to obtain wider range of spindle speeds, for the number of speeds obtained from “direct speeds” is limited to number of steps only. When the back gear is engaged, the spindle is speed is reduced considerably. So it is also used when it is necessary to have a slow speed of the spindle that cannot other wise be obtained by direct speed. A slow speed is necessary in the following cases.
1. In turning jobs of large diameter within the available cutting speed of the material
2. In turning jobs tough or hard material when the material is hard it becomes necessary to apply greater cutting force by the tool to shear out the metal. This increase in cutting force will require greater turning torque necessitating slower spindle speed.
3.In operations like thread cutting, reaming, e.t.c.
4.In taking deep cut as rough turning.

Tail stock or loose head stock

Tail stock is located at the inner ways at the right hand end of the bed. This has two main uses:
1) It supports the other end of the work when it is being machined between centers, and
2) It holds a tool for performing operations such as drilling, reaming, taping, e.t.c.
To accommodate different length of work, the body of the tailstock can be adjusted along the ways chiefly by sliding it to the desired position where it can be clamped by bolts and plates. The upper casting of the body can be moved toward or away from the operator by means of the adjusting screws to offset the tail stock for taper turning and to realign the tailstock centered for straight turning. The body is bored to act as barrel which carries the tail stock spindle that moves in and out of the barrel by means of the screw when the tail stock handle is turned. The front of the spindle has taper hole into which the dead center or other tool fits.After the adjustment is made, the spindle is clamped in the position by tightening the locking bolt on spilt lug.

Carriage
The carriage of a lathe has several parts that serve to support, move and control the cutting tool. It consists of the following parts:
1. Saddle
2. Cross-slide
3. Compound slide or compound rest
4. Tool post and
5. Apron

Saddle
The saddle is an H-shaped casting that fits over the bed and slides along the ways. It carries the cross slide and tool post. Some means are generally provided for locking the saddle to prevent any movement when surfacing operations are carried out.

The cross slide 
The cross-slide comprises a casting machined on the under aside for attachment to the saddle and carries locations on the upper face the tool post or compound rest. The crosspiece of the saddle is mechanized with a dovetail way, at right angles to the center axis of the lathe, which serves to guide the cross-slide itself.

The compound rest 
The compound rest or compound slide is a mounted on the top of the cross-slide and has a circular base graduated in degrees. It is used for obtaining angular cuts and short taper as well as convenient positioning of the tool to work. By loosening two setscrews, which fit in a v- grove around the compound-rest base, the rest slide may be swiveled to any angle within circle. There is no power feed to the compound rest and it is hand operated. The compound rest handle is also equipped with a micrometer dial to assist in determining the depth of the cut. After necessary setting the compound slide is locked solid with its base.

The tool post
This is located on the top of the compound rest to hold the tool enable it to be adjusted to a convenient working position. Following are the common tool post:
1. Single screw tool post.
2. Four bolt tool post.
3. Open side tool post.
4. Four way tool post.

The apron
The apron is fastened to the saddle and hangs over the front of the bed. It contains gears, clutches, and levers for operating the carriage by hand and power feeds. The e apron also contains function clutches for automatic feeds. In addition, there is a split nut which engages, when required with the lead screw, when cutting either internal or external threads. The lay out of the apron includes an inter locking device which prevents the simultaneous engagement of the feed shaft and the lead screw. The apron handle wheel can turned to move the carriage back and forth longitudinally by hand. The complementary motion to this is obtained by the cross- feed handle, which moves the cross- slide back and forth across the saddle. The handle wheel is connected via a pinion meshing with a rack fitted to the lathe bed Usually a chasing dial or thread cutting dial is fitted either to the side or top of the apron and consists of a graduated dial. It has entirely independent drive provided by a worm wheel, which is a constant mesh with the lead screw.

Feed mechanism
The movement of the tool relative to the work is termed as “feed”. A lathe tool may have three types of feed-longitudinal, cross, and angular. When the tool moves parallel to the lathe axis, the movement is termed as longitudinal feed and is effected by the movement of carriage. When the tool moves at right angles to the lathe axis with the help of the cross slide the movement is termed as cross feed, while the movement of the tool by compound slide when it is swiveled at an angle to the lathe axis is termed as angular feed. Cross and longitudinal feed is both hand and power operated, but angular feed is only hand operated.
The feed mechanism has different units through which motion is transmitted from the head stock spindle to the carriage. Following are the units:
1. End of bed gearing
2. Feed gearbox
3. Feed rod and lead screw
4. Apron mechanism

End of bed bearing
The gearing serves the purpose of transmitting the drive to the lead screw and feed shaft, either direct or through a gearbox. In modern lathes, tumbler gear mechanism or bevel gear feed reversing mechanism is incorporated to reverse the direction of feed.

Tumbler gear mechanism
Tumbler gear mechanism is used to give the desired direction of movement to the lathe carriage, via lead screw or the feed shaft. The tumbler gearing comprise of two pinions mounted on a bracket. The bracket is pivoted about the 1st stud shaft. The design provides three positions of bracket: forward, neutral, and reverse. With the forward position, only one gear train, and the lathe carriage is moved towards the headstock. With the introduced only to reverse the direction of rotation, and the carriage is neutral position, the spindle is disengaged from the lead screw is disengaged from the lead screw or feed shaft gearbox.

Bevel gear feed reversing mechanism
The tumbler gear mechanism being a non-rigid construction cannot be used in a modern heavy-duty lathe. The clutch-operated bevel gear feed reversing mechanism incorporated below the headstock or in apron provides sufficient rigidity in construction.

Feed gearbox
The feed gearbox or quick-change gearbox is fitted directly below the head stock assembly. Power from the lathe spindle is transmitted through gears to the quick-change gearbox. This gear box contains a number of different sizes of gear which provides a means to change the rate of the feed, and the ratio between the revolutions of the head stock spindle and the movement of the carriage for thread cutting by altering the speed of motion of feed rod or lead screw.
The arrangement which are employed in feed gear boxes to obtain multiple speeds and different rates of feed are:
1. Sliding gear mechanism
2. Sliding clutch mechanism
3. Gear cone and tumbler gear mechanism
4. Sliding key mechanism
5. Combination of any two or more of the above

Feed rod
The feed rod is a long shaft that has the key way extending from the feed box across and in front of the bed. The power is transmitted from the lathe spindle to the apron gears through a feed rod via large number of gears. The feed rod is used to move the carriage or cross-slide for turning, boring, facing and all other operations except thread cutting.

Lead screw
The lead screw is a long threaded shaft used as a master screw, and is brought into operation only when threads have to be cut. In all other times the lead screw is disengaged from the gear box and remains stationary, but this ma be used to provide motion for turning, boring, etc. in lathes that are not equipped with a feed rod.

Apron Mechanism
The apron mechanism is used for transforming rotary motion of the feed rod and the lead screw into feed motion of the carriage. The mechanism also ensures that when the half nut is engaged with the lead screw the worm drops down disconnecting the feed motion. This arrangement is called foolproof arrangement and saves the machine from any damage.

Thread cutting mechanism
The rotation of the lead screw is used to transverse the tool along the work to produce screw thread.  The half nut mechanism makes the carriage to engage or disengage the lead screw. It comprises a pair of half nuts capable of moving in or out of mesh with the lead screw. The two halves of the nut are connected in the cam slots in circular disc by two pins . When the disc is rotated by hand lever attached to it, the pins being guided in the cam slots serve to open or close the split nuts and thus engages or disengages with the lead screw. The half nuts slide with in the guide or frame. Closing the half nuts causes the carriage to move a fixed distance for each revolution of the spindle. The direction in which it moves depends up on the position of the feed reverse lever on the headstock. The split nut is used only for thread cutting and never for any other operation.

WORK HOLDING DEVICES

Lathe accessories and attachments

Lathe accessories include centers, catch plates and carriers, chucks, collets, face plates, angle plates, mandrels and rests. They are used either for holding and supporting the work or for holding the tool.
Attachments are additional equipment used for specific purposes. They include stops, ball turning rests, thread chasing dials, and taper turning, milling, grinding, gear cutting, turret, cutter, relieving and crank pin turning attachments.

Lathe centre

The most common method of holding the work in lathe is between the two centres-live and dead centres. They are made of very hard materials to resist deflection and wear. The center is usually 60degrees for general-purpose work and 75 degrees for heavy work.
The ordinary centre is the type used for most general work. The tipped centre, the point consists of a hard alloy tip brazed into an ordinary steel shank. This is more expensive type of center, one that will give excellent service against wear and strain. The ball centre is used to minimize wear and
strain on the ordinary center except that little less than half of the centre has been ground away. This construction facilitates facing of the bar ends with out removal of centre.
The insert type of centre is used for reasons of economy as only high-speed steel “insert” can be replaced instead of replacing the whole centre.

The rotating or Frictionless centre is always used in tail stock center for supporting heavy work revolving at high-speed. An ordinary insert type centers revolves freely on the ball bearing fitted in a housing having a taper shank corresponding to the taper of the spindle. The ball and roller bearings reduce friction and take up end thrust and allow the center to revolve with the work for a long period without developing any appreciable heat. The pipe center is used for supporting the open end of pipes, shells, e.t.c. for thread cutting or turning in the lathe.
To reduce the friction at the dead center point tallow, tallow and graphite or graphitized oil may be used.

Carriers and catch plates

Carriers and catch plates are used to drive a work piece when it is held between two centers. Carriers or driving dogs are attached to the end of the work piece by asset screw, and catch plates are either screwed or bolted to the nose of the head stock spindle. A projecting pin from the catch plate or carrier fits into the slot provided in either of them. This imparts a positive drive between the lathe spindle and work piece.

Chucks
A chuck is one of the most important devices for holding and rotating a work piece in lathe. Work pieces of short length, and large diameter or of irregular shape, which cannot be conveniently mounted between centers are held quickly and rigidly in a chuck.
The different types of chucks are:
1. Four jaw independent chuck
2. Three jaw universal chuck
3. Air or hydraulic operated chuck
4. Magnetic chuck
5. Collet chuck
6. Combination chuck
7. Drill chuck

Four jaw independent chuck
This chuck has four jaws, which may be made to slide within the slots provided in the body of the chuck for gripping different sizes of the work piece. Each jaw may be moved independently by rotating the screw which meshes with the work teeth cut on the under side of the jaw. This type of chuck is particularly used in the setting up of heavy and irregular shaped articles. The diameter of the body specifies the size of the chuck.

Universal self-centering chuck
In the three jaw universal chuck all the jaws may be made to slide simultaneously by an equal amount within the slots provided on the body by rotating any one of the three pinions which meshes with the teeth cut on the under side of the scroll disc.
The chuck is suitable for holding round, or hexagonal, and other similar shaped work piece and the job is centered automatically and quickly

Combination chuck
As the name implies, a combination chuck may be used a both self-centering and independent chuck to take advantage of both the types.

Magnetic chuck
The chuck is used for holding a very thin work piece made of magnetic material, which cannot be held in an ordinary chuck. It also used where any distortion of the work piece due to the pressure of the jaws is undesirable.

Collet chuck
Collet chuck are used for holding bar stock in production work where quick setting and accurate centering is needed. The inside bore of the collet may be cylindrical, hexagonal square, etc. dependin0 on the shape of the work that will pass through it .the out side surface of the collet which is tapered fits in the taper hole on the body of the chuck.

Air or hydraulic operated chuck
This type of the chuck is used in mass production work for its fast and effective gripping capacity. The mechanism incorporates a hydraulic or air cylinder mounted at the back end of the head stock spindle and rotates with it.

Drill chuck
A drill chuck is sometimes used in a lathe for holding straight shank drill, reamer or tap for drilling, reaming, taping operations. The chuck may be held either in headstock or tailstock spindle it has self-centering jaws, which may be operated by rotating a key.

Faceplates:
A faceplate consists of a circular disk bored out and threaded to fit the nose of the lathe spindle. Faceplates are used for holding the work pieces that cannot be conventionally held between centers or by chucks.

Angle plates:

This is a cast iron plate having two faces machined to make them absolutely at right angles to each other. Holes and slots are provided on both faces so that it may be clamped on a faceplate and can hold the work piece on other face by bolts and clamps. Angle plates are used in conjunction with a faceplate when the holding surface of the work piece should be kept horizontal.

Mandrels
The lathe dog and the catch plate rotate the mandrel and it drives the work by friction. To secure the mandrel in the work, it is driven by copper or lead hammer or pressed by special mandrel press. Different types of mandrels are employed according to specific requirements.

Pain mandrel
The plain mandrel is most commonly used in workshop and finds wide application where a large number of identical pieces having standard size holes are required to be mounted on it.

Step mandrel
A step mandrel having steps of different diameters may be employed to drive different work pieces having different sizes of holes without replacing the mandrel each time.

Collar mandrel
A collar mandrel having solid collar is used for turning workpieces having holes of large diameter, usually above 100 mm. This construction reduces weight and fits better than a solid mandrel of equal size.

Screw mandrel
A screw mandrel is threaded at one end with a collar. Work pieces having internal threads are screwed on to it against the collar for machining. The size and type of thread used on the mandrel depends on the internal thread of the work piece.

Cone mandrel
This type of mandrel is suitable for holding work pieces having different hole diameters by placing the work piece on two cones and tightening the nut. Forcing the cone too much tightly up on the work piece may spoil its outer edge.

Gang mandrel This has a fixed collar at one end and a movable collar at the threaded end, which may be adjusted, to this position by a nut. The mandrel is used to hole a set of hallow work pieces between two collars by tightening the nut.

Expansion mandrel  this construction enables an expansion mandrel to grip various work pieces with different hole diameters within a limit that cannot otherwise be held in an ordinary mandrel. This has proved its use in repairing workshops. An expansion mandrel with a particular sleeve can hold work pieces of varying hole diameters ranging from 0.5 to 2 mm. This range can be increased with different sizes of the sleeve.

Rests          
A rest is a mechanical device, which support along slender work piece, which is turned between centers or by a clutch, at some intermediate point to prevent bending of the work piece due to its own weight and vibrations set up due to cutting force that acts on it.

Steady rest

A steady rest consists of a cast iron base, which may be made to slide on the lathe bed ways and clamped at any desired position here a support is necessary. this is so designed that the upper portion is hinged at one end which facilitates setting and removal of the work piece without disturbing the position of the steady rest. it is also used to support the free end of a long work piece for drilling, boring, tapping operations etc.when support from the tailstock end cannot be given. The carriage cannot be fed to the full length of the work when the steady rest is used.

FOLLOWER REST:

A follower rest consists of a ″ C like casting having two adjustable jaws, which support the work piece.  The rest is bolted to the back end of the carriage and moves with it.  The follower rest prevents the job from springing away when the cut is made and is used in finish turning operations or where the entire length of the work piece is required to be turned without disturbing the setting

LATHE OPERATIONS
In order to perform different machining operations in a lathe, the work piece may be supported and driven by any one of the following methods:
1) Held between centers and driven by caries and catch plates.
2) Held on a mandrel, which is supported between centers and driven by, carries and catch plates
3) Held and driven by chuck with the other end supported on the tailstock center
4) Held and driven by a chuck or a faceplate or an angle plate.
The above methods of holding the work may be broadly classified under two headings:
1) Work piece held between centers
2)   Work piece held by a chuck or any other fixture.
Operations which are performed in a lathe either by holding the work piece between centers or by a chuck are:
1) Straight turning                8) Taper turning
2)   Shoulder turning             9) Eccentric turning
3) Shoulder turning            10) Polishing
4) Chamfering                    11) Grooving
5) Thread cutting                12) Spinning
6) Facing                             13) Spring winding
7) Knurling                          14) Forming.

Operation which are performed by holding the work by a chuck or a faceplate or an angle plate are:
1) Drilling     
2) Reaming
3) Boring
4)   Counter boring
5)   Taper boring
6)   Internal thread cutting
7)   Tapping
8)   Undercutting
9)   Parting-off

Operations, which are performed by using special attachments, are:
1) Grinding
2) Milling

CENTERING:
Where the work is required to be turned between centers or between a chuck and a center, conical shaped holes must be provided at the ends of the work piece to provide bearing surface for lathe centers.

TURNING:
Turning in a lathe is to remove excess material from the workpiece to prodice a cone-shaped or a cylindrical surface.

STRAIGHT TURNING:
     The work is turned straight when it is made to rotate about the lathe axis, and the tool is fed parallel to the lathe axis.  The straight turning produces a cylindrical surface by removing excess metal from the work piece. There are two kinds of cuts in a machine shop work.
1)Roughing cut or rough turning
2)Finishing cut or finish turning.

ROUGH TURNING:
     The rough turning is the process of removal of excess material from the work piece in a minimum time by applying high rate of feed and heavy depth of cut. The depth of cut fro roughing operations in average machine shop work is form 2 to 5mm and the rate of feed is from 0.3 to 1.5mm per revolution of the work.

FINISHING TURNING:
     The finish turning operation requires high cutting speed, small feed and a very small depth of cut of to generate a smooth surface.  In finish turning operation the depth of cut ranges from 0.5 to 1mm and feed from 0.1to 0.3mm per revolution of the work piece

SHOULDER TURNING:
     When a work piece having different diameters turned the surface forming the step from one diameter of the other is called the shoulder and machining this part of the work piece is called shoulder turning. There are four kinds of the shoulder:
1) Square shoulder
2) Angular or beveled shoulder
3) Radius shoulder
4)   Under cut shoulder.
                                                                                                            
TAPERS AND TAPER TURNING:
   A taper may be defined as a uniform increase or decrease in diameter of a piece of work measured along its length.  In a lathe taper turning means to produce a conical surface by gradual reduction in diameter from a cylindrical work piece

TAPER ELEMENTS:
     A tapered piece may be designated the follow symbols
          D   = Large diameter of taper in mm
          d   = Small diameter of taper in mm
          L   = Length of tapered part in mm
          2α = full taper angle.
           α  = Angle of taper or half taper angle.
     The amount of taper in a work piece is usually specified by the ratio of the difference in diameter of the taper to its length.  This is termed as the coincity and it’s designated by the letter K.

K   =   D - d
            l

STANDARD TAPERS:
     Machine and tools having inside or outside taper are standardized to facility interchangeability of parts. Tapered surfaces which follow standard dimensions are called standard tapers. Standard tapers adapted by the Indian Standard Institution for various tools and machine parts like drills, reamers, milling cutter shanks, arbors, lathe centers, etc. are Morse tapers. Morse tapers are available in seven sizes numbered: 0, 1, 2, 3, 4, 5, and 6. The No. 0(zero) Morse taper is the smallest while No. 6 is the largest in size.
     The non-uniformity of the angle of the taper for different Morse taper sizes is its greatest disadvantage.
Metric tapers are sometimes used as standard tapers. Metric tapers are made in seven sizes and designated by the number 4, 6, 80, 100, 120, 160, and 200. The taper number stands for the large diameter of the taper in mm. The advantage is that all metric tapers have the same angle of the taper. 

TAPER TURNING METHODS:
     A taper may be turned in a lathe by feeding the tool at an angle to the axis of rotation of the work piece. The angle formed by the path of the tool with the axis of work piece should correspond to the half taper angle.
     While turning taper, it is essential that the tool cutting edge should be accurately on the centre line of the work piece, otherwise correct taper will not be obtained. A taper may be turned by any one of the following methods:
1.By a broad nose form tool.
2.By setting over the tail stock center.
3.By swiveling the compound rest.
4.By a taper turning attachment.
5.By combining longitudinal and cross feed in a special lathe.

Taper Turning by a form tool:

A broad nose tool having straight cutting edge is set on to the work at half taper angle, and is fed straight into the work to generate a tapered surface. The half angle of taper will correspond to 90 minus side cutting edge angle of the tool. In this method the tool angle should be properly checked before use.  This method is limited to turn short length of taper only. 

This is due to the reason that the metal is removed by the entire cutting edge, and any increase in the length of the taper will necessitate the use of a wider cutting edge.

This will require excessive cutting pressure, which may distort the work due to vibration and spoil the work surface.
The amount of set over required to machine a particular taper may be calculated as:

                    L x  D - d
Stover =  ------------------
                   2L

          D - d
=       ---------
         2

          D - d
=       ---------
         L

Being termed as the iconicity or amount of taper, the above formula  may be written in the   following from:

                  Entire length of work x conicity
Setover  ---------------------------------------------
                                    2

Taper Turning by swivelling the compound rest:

This method employs the principle of turning taper by rotating the work piece on the lathe axis and feeding the tool at an angle to the axis of rotation of the work piece.  The tool mounted on the compound rest is attached to a circular base, graduated in degree, which may be swiveled and clamped at any desired angle. This method is limited to turn a short taper owing to limited movement of the compound rest.
The movement of the tool in this method purely controlled hand, this gives a low production capacity and poorer surface finish.
Swiveling the rest at the half taper angle. If the diameter of the small and large end does the setting of compound rest and lengths of taper are known, the half taper angle can be calculated from the equation.   

               D - d
Tanα =   ----------------
               2l

And is set at the at the desired angle with the lathe axis. 
       If the diameters D and d and the length l of the

Taper turning by taper attachment:

The principle of turning taper by a taper attachment is guide the tool in a straight path set at an angle to the axis of rotation of the work piece, while the work is being revolved between centre or by a chuck aligned to the lathe axis. A taper turning attachment consists essentially of a bracket or frame, which is attached to the rear end of the lathe bed and supports a guide bar pivoted at the centre. The bar having graduations in degrees may be swiveled on either side of the zero graduation taper are specified, the angle of swiveling the guide bar can be determined from the equation:

               D - d
Tanα =   ----------------
               2l

The advantages of using a taper turning attachment are:                   
                     
       1.The alignment of live and dead centers being not disturbed, both straight and taper turning may be performed on a work piece in one setting without much loss of time. 
       2.once the taper is set, any length of a piece of work may be turned taper with in its limit.    
       3.very steep taper on a long work piece may be turned, which cannot be done by any other method.
       4.Accurate taper on a large number of work pieces may be turned.
       5.Internal tapers can be turned with ease.       

Taper turning by combining feeds:

     This is a more specialized method of turning taper. In certain lathes both longitudinal and cross feeds may be engaged simultaneously causing the tool to follow a diagonal path which is the resultant of the magnitude of the two feeds. Varying the rate of feeds by change gears provided inside the apron may change the direction of the resultant.

ECCENTRIC TURNING
         If a cylindrical work piece has two separate axis of rotation one being out of centre to the other, the work piece is termed eccentric and turning of different surfaces of the wok piece is known as eccentric turning. The shaft is first mounted on its true centre and the part forming the journal is turned. The job is then remounted on the offset centre and the eccentric surfaces are machined.

CHAMFERING
         Chamfering is the operation of beveling the extreme end of a work piece this done to remove the burrs, to protect the end of the work piece from being damaged and to have a better look. The operation may be preformed after knurling, rough turning, boring, drilling or thread cutting. Chamfering is an essential operation after thread cutting so that the nut may pass freely on the threaded work piece.  

THREAD CUTTING:
Thread cutting is the most important operation performed in lathe.The principle of thread cutting is to produce a helical groove on a cylindrical or conical surface by feeding the tool longitudinally when job is revolved between centers or by a chuck. The longitudinal feed should be equal to the pitch of thread to be cut per revolution of the work piece. The lead screw of the lathe, through which the saddle receives its traversing motion, has a definite pitch. A definite ratio between the longitudinal feed and rotation of the head stock spindle should therefore be found out so that the relative speeds of rotation of the work and the lead screw will result in the cutting of a screw of the desired pitch. This is affected by change gears arranged between the spindle and lead screw or by the change gear mechanism or feed box used in a modern lathe where it provides a wider range of feed and the speed ratio can be easily and quickly changed.

Calculation for change - wheels: 
         To calculate the wheels required for cutting a screw of certain pitch it is necessary to know how the ratio is obtained, and exactly where the driving and driven wheels are to be placed. Suppose the pitch of a lead screw is 12 mm and it is required to cut a screw of 3 mm pitch, then the lathe spindle must rotate 4 times the speed of the lead screw that is

Spindle turn                    4
----------------------- =   ------------
Lead screw turn              1

Driver teeth                     1
---------------------- = ------------
Drive teeth                       4

since a small gear rotates faster than a larger One with which it is  connected

Hence,

Drive teeth                lead screw turn
------------------------   =   ----------------------------
Driven teeth                spindle turn

        Pitch of the screw to be cut
=       ---------------------------------------------
     pitch of the lead screw

Thread Cutting operation:

In a thread cutting operation the first step is to remove the excess material from the work piece to make its diameter equal to the major diameter of the screw thread. Change gears of correct size are then fitted to the end of the bed between the spindle and the lead screw.  The shape or form of the thread depend s on the shape of the cutting tool to be used. In a metric thread, the included angle of the cutting edge should be ground exactly 60o.  The top of the tool nose should be set at the same height as the centre of the work piece.  A thread tool gauge is usually used against the turned surface to check the cutting tool so that each face of the tool may be equally inclined to the center line of the work piece.
The speed of the spindle is reduced by one half to one-fourth of the speed required for turning according to the type of the material being machined, and the half-nut is then engaged.  The depth of cut which usually varies from 0.05 to 0.2 mm is applied by advancing the tool perpendicular at to the axis of the work or at an angle equal to one-half of the angle of the thread, and 30o in the case of metric thread, by swiveling the compound rest.

Facing
Facing is the operation of machining the ends of a piece of work to produce a flat surface square with the axis. This is also used to cut the work to the required length.  The operation involves feeding the tool perpendicular to the axis of rotation of the work piece.

KNURLING
Knurling is the process of embossing a diamond shaped pattern on the surface of a work piece.  The purpose of knurling is to provide an effective gripping surface on a work piece to prevent it from slipping when operated by hand.  The tool is held rigidly on the tool post and the rollers are pressed against the revolving work piece to squeeze the metal against the multiple cutting edges, producing depressions in a regular pattern on the surface of the work piece.

FILING
Filing is the finishing operation performed after turning.  This is done in a lathe to remove burrs, shape corners, and feed marks on a work piece and also to bring it to the size by removing very small amount of metal.  The speed is usually twice that of turning.  The file should b slowly moved forward so that the work may pass 2 to 3 revolutions during the cutting stroke.  During the return stroke the pressure is relived but an end wise feeding movement is given, overlapping the previous cut.

POLISHING
Polishing is performed after filing g to improve the surface quality of the work piece.  Polishing with successively finer grades of emery clothe after filing results in very smooth, bright surface.  The lathe is run at high speeds form 1500 to 1800m per min, and oil is used on the emery cloth.

GROOVING
Grooving is the process of reducing the diameter of a work piece over a very narrow surface. It is often done at the end of a thread or adjacent to a shoulder to leave a small margin.

SPINNING
Spinning is the process of forming a thin sheet of metal by revolving the job at high speed and pressing it against a “former” attached to the headstock spindle.

SPRING WINDING
Spring winding is the process of a making a coiled spring by passing a wire around a mandrel which is revolved on a chuck or between centers.  A small hole is provided on a steel bar which is supported on the tool post and the wire is allowed to pass through it.
The diameter of the mandrel should be less than the desired spring diameter as all springs expand in diameter after they are taken out of the mandrel. In order to wind the spring of the required pitch, the lathe is geared similar to the thread cutting operation.

FORMING

Forming is the process of turning convex, or concave or of any irregular shape. Form turning may be accomplished by the following methods:  
                 1.using a forming tool                    
                 2.combining cross-land longitudinal feed.
                 3.tracing or coping a template.

DRILLING
Drilling is the operation of producing a cylindrical hole in a work piece by the rotating cutting edge of a cutter known as drill.

REAMING:
Reaming is a process of finishing and sizing a hole, which has been drilled or bored. The tool used is called reamer, which has multiple cutting edges. The reamer is held on tailstock spindle, either direct or through a drill chucks and is held stationary while the work is revolved at a very low speed. The feed varies from 0.5 to 2mm per revolution.
Boring
Boring is the operation of enlarging and truing a hole produced by drilling, punching, casting or forging.
1. The work is revolved in a chuck or a faceplate and the tool, which is fitted to the tool post, is fed in to the work.
2. The work is clamped on the carriage and a boring bar holding the tool is supported between the centers and made to revolve.

Counter boring
It is the operation of enlarging a hole through a certain distance from one end instead of enlarging the whole-drilled surface. It is similar to a shoulder work in external turning. The operation is similar to boring and a plain boring tool or a counter boring may be used.

Internal thread cutting
The principle of cutting internal threads is similar to that of an external thread, the only difference being in the tool used. The tool is similar to a boring tool with cutting edges ground to the shape conforming to the type of thread to be cut. The hole is first bored to the root diameter of the thread. For cutting metric thread, the compound slide is swiveled 30 degrees towards the headstock. The tool is fixed on the tool post ort on the boring bar after setting it at right angles to the lathe axis. Using a thread gauge.

Tapping
Tapping is the operation of cutting internal threads of small diameter using a multi point cutting tool called the tap.
In a lathe, the work is mounted chuck or on a faceplate and revolved at a very low speed. A tap of required size held on a special fixture is mounted on the tail stock spindle. The axis of the tap should be coinciding exactly with the axis of the work the tap will automatically feed into the work with help of the special fixture.

Under cutting
Under cutting is similar to a grooving operation when performed inside a hole. It is the process of boring a groove or a large hole at a fixed distance from the end of a hole this is similar to boring operation. Except that a square nose parting tool is used. Under cutting is done at the end of an internal thread or a counter bore to provide clearance for the tool or any mating part.

Parting off
Parting off is the operation of cutting a work piece after it has been machined to the desired size and shape.

MILLING:      Milling is the operation of removing metal by feeding the or against a rotating cutter having multiple cutting edges. It is performed in a lathe by any one of the two methods:
1. For cutting keyways or grooves, the work is supported on the crosslide by a special attachment and fed against a rotating milling cutter held by a chuck. The depth of cut is given by vertical adjustment of the work provided by the attachment.
2. The work may be supported between centers and held stationary. The attachment mounted on the carriage drives the cutter from an individual motor. The feeding movement of the cutter is arranged in the attachment. Rotating the work by a fixed amount and machining it against the utter may cut a number of grooves on the periphery of the work. A gear wheel may be cut on a lathe by fixing a universal dividing head at the rear end of the headstock spindle. This permits dividing the periphery of the work by an equal amount.

GRINDING: 
     Grinding is the operation of removing metal in the form of minute chips by feeding the work against a rotating abrasive wheel known as the grinding wheel. Both internal and external surfaces of a work piece may be ground by using a special attachment mounted on the cross slide. For grinding external surface, the work may be revolved between centers or on a chuck. For internal grinding the work must be revolved on a chuck or faceplate. The carriage does the feeding and the depth of cut is provided by the cross slide. Grinding is performed in a lathe for finishing a job, sharpening a cutter, or sizing a work piece after it has been hardened.