Introduction

Lathe is an oldest machine tool. The entire machine tools are developed from the lathe, therefore it is also known as mother of machine tools. A number of cutting operations can be performed on lathe with or without some attachments.On lathe, a rotational motion is provided to the job and translational motion is provided to the cutting tool.Lathe machines can be classified on the basis of speed and purposes of applications.

Classification of Lathes

According to the construction and design lathe can be classified as follows:

Bench Lathe: It is small in size and mounted on a separate table. It has all the attachments, which a larger lathe has. It is used to perform a precise work.
Speed Lathe: This may be bench type or legs supported lathe. It has no gear box, carriage, and lead screw. Therefore, tool is actuated and fed by hand. This lathe is used for wood turning,polishing, and spinning purposes.
Engine Lathe: This is most widely used lathe. In early days,during the development phase of the lathe, this lathe was driven by steam engine, therefore named as engine lathe. Nowadays all the engine lathes have separate engines or electric motors. Various speeds are achieved using cone pulley and gears.
Tool Room Lathe: This is very similar to engine lathe but equipped with some extra attachments for more accurate and precise works. The usual attachments are taper turning attachment, follower rest, collets, chucks, etc.
Capstan and Turret Lathes: This is semi-automatic type lathe and a wide range of operations can be performed on them. It can hold a large number of cutting tools compared to engine lathe.

Functions and Components of a Lathe

A lathe is a versatile machine tool that primarily functions by rotating a workpiece on its axis while a cutting tool removes material to create a desired shape.The key components of a lathe include:

Bed: Supports all major components.
Headstock: Holds the jaws for the workpiece,supplies power to the jaws and has various drive speed.
Tailstock: Support the other end of the workpiece.
Carriage: Slides along the ways and consists of the cross-slide,tool post,apron.

1. Cross Slide: The cross slide is mounted on the carriage.The main role of it is to move the cutting tool 90° to the workpiece.
2. Apron: The apron contains the control and the gear that allow you to move the carriage and cross slide.
3. Tool Post: The tool post support and secure the cutting tool or tool holder.

Importance in Modern Manufacturing

The lathe machine remains a fundamental tool in manufacturing for several reasons:

Precision Machining: Lathes are renowned for their ability to produce highly accurate and symmetrical parts, making them indispensable in industries where precision is critical.
Versatility: From simple operations like turning and facing to more complex tasks like threading and profiling, lathes can handle a wide range of machining operations.
Customization: Modern CNC (Computer Numerical Control) lathes have revolutionized customization, allowing manufacturers to create unique and complex components with ease.
Mass Production: Lathes are also essential for mass production, enabling the rapid and consistent production of standardized components.
Education and Training: Lathes play a significant role in training future machinists and engineers, helping them understand the fundamentals of machining.

Specifications of Lathe

Lathe machine can be specified by following dimensions (Figure 1):

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Figure 1

Height of centre over bed (A).
Maximum swing over bed (B).
Maximum swing over carriage (C).
Maximum swing in gap (D).
Maximum length of work (E).

Constructional Detail of Lathe

A lathe machine consists of a number of components. These components perform various functions, for example, facilitate variation in speed, hold the cutting tool, rigidly hold the job, provide end support to the job,automatic movement of tool, etc. A lathe with nomenclature of various parts is shown in Figure 2.

Bed: All the fixed and moving parts of lathe are mounted on bed. It is made of cast iron in single piece, it may be in two or three pieces for large size lathe which are bolted together. It has v-ways for collection of chips produced during machining.

Head Stock:Head stock is housing of cone pulleys, back gear, main spindle, live centre, and feed reverse livers. It provides driving mechanism to the job and tool post, carriage, apron, etc.

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Figure 2

Figure 3

Tail Stock: The function of tail stock is to support the job at the end. It slides over the bed. It may have dead centre or live centre for point support to the job as per requirement. A tailstock is shown in Figure 3 For tapping, drilling, or boring, a tape or drill/boring tool may be used in place of dead centre. The dead centre moves forward or backward with sleeve by rotating the hand wheel manually.

Carriage and Tool Post: It provides support to the tool post, cross slide, compound rest, apron, etc. The function of tool post is to hold cutting tool rigidly; tool post moves in both axial and transverse directions on compound rest. The function of swivel plate is to give angular direction to the tool post whereas the function of cross slide is to give the linear motion to the tool by rotating the attached hand wheel. Apron is a hanging part in front of the carriage. It is housing of gear trains and clutches. It gives automatic forward and reverse motion to the tool.

Legs: The legs provide rigid support to the entire machine tool.Both the legs are firmly secured to the floor by means of foundation bolts in order to prevent vibrations in the machine.

Chucks:The function of the chuck is to hold the job.There may be three or four-jaw chuck as shown in Figure 4 In three-jaw chuck,all the jaws move inwards or outwards simultaneously and there is no problem of centring hence it is also known as universal chuck.Whereas in four-jaw chuck each jaw moves independently.It may accommodate irregular shape of job but there is problem of centring which is to be done manually.A magnetic chuck is also used to hold the job which works on the principle of electromagnetism.

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Figure 4

Power Transmission System in Lathe Machine

Head stock spindle drive system may include stepped or cone pulley drive or all geared head drive. In stepped pulley, the number of speed equals to number of steps in pulley. In all geared head drive, total nine various speed can be obtained.

Stepped Pulley (Cone Pulley) Drive: V-belt is used to transmit the power from driver shaft to spindle shaft. In four-stepped pulley drive, four different speeds of the head stock can be attained. Spindle speeds are varied in arithmetic progression (Figure 5).

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Figure 5

Let driver shaft rotates at the speed of N rotation per minute (rpm) and the stepped diameters of the pulley are `D_1`,`D_2`,`D_3` and `D_4`.Driven shaft has pulley of same steps diameters but in
reverse order as shown in Figure 5. We know the speed is inversely proportional to the diameter, therefore,

`frac{N_1}N`=`frac{D_4}{D_1}`;`frac{N_2}N`=`frac{D_3}{D_2}`:`frac{N_3}N`=`frac{D_2}{D_3}`;`frac{N_4}N`=`frac{D_1}{D_4}`

where N is speed of driver shaft and `N_1`,`N_2`,`N_3` and `N_4` are speeds of spindle shaft.

Here `D_1` < `D_2` < `D_3` < `D_4`.

All Geared Head Drive

This drive comprises of nine gears on three shafts. By operating two levers attached with two cluster gears on pulley shaft and head stock main spindle,respectively, nine speeds can be obtained. Three gears 2–4–6 are fixed on intermediate shaft. Spur gear, 10 is fixed on head stock spindle to transmit power to the feed shaft and lead screw. The constructional detail of all geared head drive is shown in Figure 6.

Figure 6

The gear combinations for nine different speeds are given below:

`frac{T_1}{T_2}`×`frac{T_2}{T_7}`;`frac{T_1}{T_2}`×`frac{T_4}{T_8}`;`frac{T_1}{T_2}`×`frac{T_6}{T_9}`

`frac{T_3}{T_4}`×`frac{T_2}{T_7}`;`frac{T_3}{T_4}`×`frac{T_4}{T_8}`;`frac{T_3}{T_4}`×`frac{T_6}{T_9}`

`frac{T_5}{T_6}`×`frac{T_2}{T_7}`;`frac{T_5}{T_6}`×`frac{T_4}{T_8}`;`frac{T_5}{T_6}`×`frac{T_6}{T_9}`

where `T_1`,`T_2`,`T_3`,`T_4`,`T_5`,`T_6`,`T_7`,`T_8` and `T_9` are number of teeth on gear 1, 2, 3, 4, 5, 6, 7, 8 and 9 respectively.

Cutting Tools Used in Lathe

A number of cutting operations are performed on a lathe machine. Therefore, various cutting tools are used in lathe such as left-hand and right-hand turning tools, facing tools, threading tools, parting-off tool, etc., as shown in Figure 7.

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Figure 7

Types of Operations on Lathe Machine

Following are the various types of operations performed on the lathe machines.

Turning

Turning is a metal removal process in which job is given rotational motion while the cutting tool is given linear (feed and depth of cut) motion.Different types of turning operations are mentioned as follows:

Straight Turning

It is the operation of producing a cylindrical surface of a job by removing excess material. In this operation, the job rotates and tool is fed longitudinally by giving the desired depth of cut (Figure 8).

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Figure 8

Face Turning or Facing: Face turning is also known as facing operation. It is the operation of making the ends of a job to produce a square surface with axis of operation or to make a desired length of the job. In this operation job rotates and the tool advances in perpendicular direction to the axis of the job rotation (Figure 9).

Figure 9

Shoulder Turning

If a job is turned with different diameters, the steps for one diameter to the other so formed, the surface is known as shoulder turning. There are several types of shoulder turning such as square, radius, bevelled, etc., as shown in Figure 10. It is also known as step turning.

Figure 10

Eccentric Turning

When a job having more than one axis of rotation, each axis may be parallel with each other but never coincides; turning of different cylindrical surfaces of the job is known as eccentric turning. In Figure 11, the job is first turned through centres `C_1`-`C_1` and then through centres `C_2`-`C_2`.

Taper Turning

Taper turning is an operation in which taper cylindrical surface, i.e.,cone type surface is produced as shown in Figure 11.

Figure 11

Figure 12

Taper on a cylindrical surface of a job can be produced by the following methods:

Taper turning by swivelling compound rest: Job rotates on lathe axis and tool moves on angular path. It can be applied from any angle `0^circ`-`90^circ` for short length of taper up to 150 mm (approximate) `tanleft(alpharight)`=`frac{D_1-D_S}{2l}`.It is used for shorter length and steeper angle. Here,`D_1` and `D_S` are larger and shorter diameters, l is length of the job, and is angle of taper.
Taper turning by off-setting the tailstock: Job rotates at an angle to the lathe axis and the tool travels longitudinally to the lathe axis. Any angle 0° – 8°, long job of smaller diameter can be turned by this method. It is also used for internal taper turning.
Taper turning attachment: Job rotates on lathe axis and tool moves in guided angular path. Long jobs of steeper angle of taper (0° – 12°) can be done by this attachment. Guide rail is set as per angle of taper. It is applied for mass production.
Taper tuning by a form tool: Job rotates on lathe axis and tool moves crosswise direction,perpendicular to the lathe axis. Very small length of taper and any angle 0° – 90°.Tool itself designed as per requirements. It is used for mass production for Chamfering on bolts, nuts, bushes, etc.
Taper turning by combination fed: Job rotates on lathe axis and tool travels on resultant path, for any length and any angle. Taper angle is to be determined by trial and error method.It is applied by hand feeds by combined feeding of tool (axial and perpendicular) for taper turning.

Parting-off (Grooving)

It is the operation of cutting-off/grooving a bar after it has been machined to the required shape and size. In this operation, the job is held on a chuck, rotates to the turning speed and the parting-off tool is fed into the job very slowly until the tool reaches to the centre of the job. The parting-off operation is shown in Figure 13.

Figure 13

Knurling

Knurling is the process of embossing, producing a roughened surface on a smooth surface of a cylindrical job to provide effective gripping, for example, thimble and ratchet of micrometer and plug gauge handle. Knurling tools (single, two, or three sets of rollers) are held rigidly on tool post, pressed against the rotating (one third speed of the turning) surface of a job, leaving exact facsimile of the tool on the surface of the job as shown in Figure 14.

Figure 14

Thread Cutting

For thread cutting on the lathe, there is definite relationship between the speeds of the job and tool. The relationship is obtained by gear ratio selection which depends on the pitch of the job, pitch of the lead screw, number of start of thread on the job. Every machine is supplied with a spur gear box (a set of 23 gears) having teeth from 20 to 120 with an interval of 5 and a special gear or transfer gear is of 127 teeth for cutting metric thread. Two 20 teeth spurs are available. Lead screw has single start thread. The simple process of thread cutting on lathe is shown in Figure 15.

Figure 15

Steps for Thread Cutting on Lathe

Hold the job on machine and turn up to major diameter of the thread.
Choose suitable thread cutting tool.
Select slower speed of the lathe spindle.
Calculate the change gear ratio based on the following formula:

Change gear ratio=pitch of the job×No. of startPitch of the lead screw

Fix the calculated change gear ratio to the head stock spindle, intermediate shaft, and lead screw shaft.
Choose suitable depth of cut. Three or four cuts are necessary to complete the thread.
Arrange job and tool proper position and give desired depth of cut.
Engage half nut with respect to chasing dial according to odd/even threads.
Allow the movement of the tool up to the portions of the job necessary for thread cutting
then lifting the tool from the job.
Disengage the half nut, move the carriage to the right side up to the position from where
second cut will start. Allowing the second depth of cut again engages the half nut with
respect to chasing dial.

Drilling

The operation of producing a circular hole by removing metal by rotating the cutting edges of a drill is known as drilling. But on lathe drill is static and only feed motion is given through the movement of tail stock and rotating motion is given to the job. Metal is removed by shearing and extrusion. Drilled hole will be slightly oversized than the drill used due to the non-alignment of the drill and vibration of the spindle. For producing accurate hole, the drill bit should be chosen slightly undersize and subsequent reaming or boring operation is essential after drilling.Drill moves up to the length of the hole required as shown in Figure 16.

Figure 16

Drilling on lathe is very easy. Drill bit is held in tail stock in place of dead centre and moved in forward direction applying pressure at the end of the rotating job.

Tapping

Tapping is an operation for producing internal thread. A hole of minor diameter is produced in
the job by holding the drill tool in tail stock and applying pressure on the rotating job in chuck.
After drilling the hole, tap is hold in tail stock and inserted in drilled hole of the rotating job as
shown in Figure 17.

Figure 17

Reaming

The operation of finishing and sizing a previous drilled hole using a multi-edges straight cutting
tool named as reamer is known as reaming. Very small amount of material (0.4 mm) removal
is possible by this operation. Reaming operation on lathe is very similar to drilling on lathe as
shown in Figure 18.

Figure 18

Boring

The operation of enlarging and finishing a previous drilled hole throughout its length by means
of an adjustable single edge cutting tool (named as boring tool) is known as boring. Boring on
lathe is also very similar to drilling but this process is used to enlarge the drilled hole as shown
in Figure 19.

Figure 19

Spinning

Spinning is a process to produce a circular homogeneous pot or household utensil. In this operation, the sheet metal job is held between a former attached with headstock spindle and the tail stock centre and rotates at high speed with the former. The long round nose forming tool fixed rigidly on special tool post presses the job on the periphery of the former as shown in Figure 20. Thus, the job is deformed exactly in the shape of former and the operation is known as spinning. It is chipless machining process.

Figure 20

Conclusion

The lathe machine is not as simple as it was and it is today a complex machine that helps produce various produces. It is not surprising that it is a foundation of industries, in a wide field of application such as aerospace and automotive, electronic technologies of medicine, electronics and healthcare. The skilled hands of the lathe operators mold the world and propel, innovation still serves as a testimony of the multifaceted importance skill and technology has brought to human society.

Mastering the Art of Lathe in Mechanical Engineering: A Comprehensive Guide