1.

LATHE
1.1 Introduction
Lathe is considered as one of the oldest machine tools and is widely used in industries. It is called as mother of machine tools. It is said
that the first screw cutting lathe was developed by an Englishman named Henry Maudslay in the year 1797. Modern high speed, heavy duty
lathes are developed based on this machine.

The primary task of a lathe is to generate cylindrical workpieces. The process of machining a workpiece to the required shape and size
by moving the cutting tool either parallel or perpendicular to the axis of rotation of the workpiece is known as turning. In this process, excess
unwanted metal is removed, Material will remove in the forms of chips from a piece of work by mounting the same rigidity on the machine
spindle and revolving at the required speed and the cutting tool is fed against the work piece.

Chuck

Work

Cutting tool Direction of feed

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1.2 Main parts of a lathe
 Every individual part performs an important task in a lathe. Some important parts of a lathe are listed below

1. Bed
2. Headstock
3. Spindle
4. Tailstock
5. Carriage
a. Saddle
b. Apron
c. Cross-slide
d. Compound rest
e. Compound slide
f. Tool post
6. Feed mechanism
7. Leadscrew
8. Feed rod
9. Thread cutting mechanism
1.2.1 Bed
Bed is mounted on the legs of the lathe which are bolted to the floor. It forms the base of the machine. It is made of cast iron and its top
surface is machined accurately and precisely. Headstock of the lathe is located at the extreme left of the bed and the tailstock at the right
extreme. Carriage is positioned in between the headstock and tailstock and slides on the bed guideways.

The top of the bed has flat or ‘V’ shaped guideways. The tailstock and the carriage slides on these guideways. Inverted ‘V’ shaped
guideways are useful in better guide and accurate alignment of saddle and tailstock.
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1.2.2 Headstock
Headstock is mounted permanently on the inner guideways at the left hand side of the leg bed. The headstock houses a hollow
spindle and the mechanism for driving the spindle at multiple speeds. The headstock will have any of the following arrangements for
driving and altering the spindle speeds

(i) Back gear drive

(ii) All gear drive

1.2.3 Spindle
The spindle rotates on two large bearings housed on the headstock casting. A hole extends through the spindle so that a long bar
stock may be passed through the hole. The front end of the spindle is threaded on which chucks, faceplate, driving plate and catch plate are
screwed. The front end of the hole is tapered to receive live center which supports the work. On the other side of the spindle, a gear known
as a spindle gear is fitted. Through this gear, tumbler gears and a main gear train, the power is transmitted to the gear on the leadscrew.

Hole Spindle Threaded end

Taper sleeve

Live centre

Fig 1.3 Spindle

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1.2.4 Tailstock
Tailstock is located on the inner guideways at the right side of the bed opposite to the headstock. The body of the tailstock is bored
and houses the tailstock spindle or ram. The spindle moves front and back inside the hole. The spindle has a taper hole to receive the dead
center or shanks of tools like drill or reamer. If the tailstock handwheel is rotated in the clockwise direction, the spindle advances. The
spindle will be withdrawn inside the hole, if the handwheel is rotated in anti-clockwise direction.

To remove the dead center or any other tool from the spindle, the handwheel is rotated in anticlockwise direction further. The
movement of the spindle inside the hole may be locked by operating the spindle clamp located on top of the tailstock. In order to hold
workpieces of different lengths, the tailstock can be locked at any desired position on the lathe bed. Tailstock clamping bolts and clamping
pates are used for this purpose.

Tailstock is designed to function as two units-the base and the body. The base of the tailstock is clamped to the bed. The body is
placed on the base and can be made to slide sidewards-perpendicular to the bed guideways up to a certain distance.

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 Clamp Bush

Spindle Barrel Screw

Dead centre

Handwheel

Set over screw

Tailstock clamping bolt

Bed

Fig 1.4 Tailstock

The uses of tailstock

1. It supports the other end of the long workpiece when it is machined between centers.
2. It is useful in holding tools like drills, reamers and taps when performing drilling, reaming and tapping.
3. The dead center is off set by a small distance from the axis of the lathe to turn tapers by set over method.
4. It is useful in setting the cutting tool at correct height aligning the cutting edge with lathe axis.

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1.2.5 Carriage
Carriage is located between the headstock and tailstock on the lathe bed guideways. It can be moved along the bed either towards
or away from the headstock. It has several parts to support, move and control the cutting tool. The parts of the carriage are : a) saddle
b) apron
c) cross-slide
d) compound rest
e) compound slide
f) tool post

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Saddle:
It is an “H” shaped casting. It connects the pair of bed guideways like a bridge. It fits over the bed and slides along the bed between
headstock and tailstock. The saddle or the entire carriage can be moved by providing hand feed or automatic feed.
Cross slide:
Cross-slide is situated on the saddle and slides on the dovetail guideways at right angles to the bed guideways. It carries compound
rest, compound slide and tool post. Cross slide handwheel is rotated to move it at right angles to the lathe axis. It can also be power driven.
The cross slide hand wheel is graduated on its rim to enable to give known amount of feed as accurate as 0.05mm.

Compound rest:
Compound rest is a part which connects cross slide and compound slide. It is mounted on the cross-slide by tongue and groove joint.
It has a circular base on which angular graduations are marked. The compound rest can be swiveled to the required angle while turning
tapers. A top slide known as compound slide is attached to the compound rest by dove tail joint. The tool post is situated on the compound
slide.

Tool post:
This is located on top of the compound slide. It is used to hold the tools rigidly. Tools are selected according to the type of operation
and mounted on the tool post and adjusted to a convenient working position. There are different types of tool posts and they are:

1. Single screw tool post

2. Four bolt tool post
3. Four way tool post
4. Open side tool post

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1.2.7 Leadscrew
The leadscrew is a long threaded shaft used as master screw. It is brought into operation during thread cutting to move the carriage
to a calculated distance. Mostly leadscrews are Acme threaded.

The leadscrew is held by two bearings on the face of the bed. A gear is attached to the lead screw and it is called as gear on
leadscrew. A half nut lever is provided in the apron to engage half nuts with the leadscrew.

Leadscrew is used to move the carriage towards and away from the headstock during thread cutting. The direction of carriage
movement depends upon the direction of rotation of the leadscrew.
1.2.8 Feed rod
Feed rod is placed parallel to the leadscrew on the front side of the bed. It is a long shaft which has a keyway along its length. The
power is transmitted from the spindle to the feed rod through tumbler gears and a gear train. It is useful in providing feed movement to
the carriage except for thread cutting and to move cross-slide. A worm mounted on the feed rod enables the power feed movements.

1.3 Spindle mechanism

The spindle is located in the headstock and it receives the driving power from the motor. The spindle speed should be changed to
suit different machining conditions like type of material to be cut, the diameter and the length of the work, type of operation, the type of
cutting tool material used, the type of finish desired and the capacity of the machine.

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 1.4 Types of lathe

Various designs and constructions of lathe have been developed to suit different machining conditions and usage. The following
are the different types of lathe
1. Speed lathe
a. Woodworking lathe
b. Centering lathe
c. Polishing lathe
d. Metal spinning lathe
2. Engine lathe
a. Belt driven lathe
b. Individual motor driven lathe
c. Gear head lathe
3. Bench lathe
4. Tool room lathe
5. Semi automatic lathea. Capstan lathe
b. Turret lathe
6. Automatic lathe/CNC
7. Special purpose lathe

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1.4.1 Speed lathe
Spindle of a speed lathe operates at very high speeds (approximately at a range of 1200 to 3600 rpm) and so it is named so. It
consists of a headstock, a tailstock, a bed and a toolslide only. Parts like leadscrew, feed rod and apron are not found in this type of lathe.

1. Centering lathes are used for drilling center holes.

2. The woodworking lathes are meant for working on wooden planks.
3. Metal spinning lathes are useful in making tumblers and vessels from sheet metal.
4. Polishing of vessels is carried out in polishing lathe.

1.4.2 Engine lathe or center lathe

Engine lathes are named so because the early lathes were driven by steam engines. As the turning operations are performed by
holding the workpiece between two centers, it is also known as centre lathe. Engine lathes are widely used in industries. It consists of parts
like headstock, tailstock and carriage. Parts like leadscrew and feed rod which are useful in providing automatic feed are also found in this
type of lathe.

1.4.3 Bench lathe

Bench lathe is a small lathe generally mounted on a bench. It consists of all the parts of a engine lathe. It is used for small works
like machining tiny and precise parts and parts of measuring instruments.
1.4.4 Tool room lathe
A tool room lathe has similar features of an engine lathe but is accurately built and has wide range of spindle speeds to perform
precise operations and different feeds. It is costlier than a centre lathe. This is mainly used for precision works like manufacturing tools,
dies, jigs, fixtures and gauges.

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1.4.5 Semi automatic lathe
Turret and Capstan lathes are known as semi-automatic lathes. These lathes are used for production work where large quantities of
identical workpieces are manufactured. They are called semi-automatic lathes as some of the tasks are performed by the operators and the
rest by the machines themselves.

A semi skilled operator can do this at low cost and at shorter time. So, the cost of production is reduced. There are two tool posts in
the machine namely four way tool post and rear tool post. Four tools can be mounted on the four way tool post and parting tool is mounted
on the rear tool post. The tailstock of an engine lathe is replaced by a hexagonal turret. As many tools may be fitted on the six sides of the
turret, different types of operations can be performed on a workpiece without resetting of tools.

Turret

Saddle

Bed

Handwheel

Fig 1.12 Toolhead of a turret lathe

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1.4.6 Automatic lathe
Automatic lathes are operated with complete automatic control. They are high speed, mass production lathes. An operator can look
after more than one automatic lathe at a time.
1.4.7 Special purpose lathe
Special purpose lathes are used for special purposes and for jobs, which cannot be accommodated and conveniently machined on
a standard lathe. Wheel lathe, ‘T’ lathe, duplicating lathe are some examples of special purpose lathe.

1.5 Size of a lathe( Specification)

The size of a lathe is specified by the following points

1. The length of the bed

2. Maximum distance between live and dead centers.
3. The height of centers from the bed
4. The swing diameter
The swing diameter over bed - It refers to the largest diameter of the work that will be rotated without touching the bed
The swing diameter over carriage - It is the largest diameter of the work that will revolve over the saddle.
5. The bore diameter of the spindle
6. The width of the bed
7. The type of the bed
8. Pitch value of the lead screw
9. Horse power of the motor

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 10. Number and range of spindle speeds
11. Number of feeds
12. Floor space required
13. The type of the machine
1.6 Work holding devices used in a lathe
The work holding devices are used to hold and rotate the workpieces along with the spindle. Different work holding devices are
used according to the shape, length, diameter and weight of the workpiece and the location of turning on the work. They are

1. Chucks
2. Face plate
3. Driving plate
4. Carriers
5. Mandrels
6. Rests

1.6.1 Chucks
Workpieces of short length, large diameter and irregular shapes, which can not be mounted between centres, are held quickly and
rigidly in chuck. There are different types of chucks namely, Three jaw universal chuck, Four jaw independent chuck, Magnetic chuck,
Collet chuck and Combination chuck.

Three jaw self-centering chuck

The three jaws fitted in the three slots may be made to slide at the same time by an equal amount by rotating any one of the three
pinions by a chuck key. This type of chuck is suitable for holding and rotating regular shaped workpieces like round or hexagonal rods
about the axis of the lathe. Workpieces of irregular shapes cannot be held by this chuck.

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 Body Key hole

Jaws

Jaw
Fig 1.14 Three jaw chuck

The work is held quickly and easily as the three jaws move at the same time.
Four jaw independent chuck
There are four jaws in this chuck. Each jaw is moved independently by rotating a screw with the help of a chuck key. A
particular jaw may be moved according to the shape of the work. Hence this type of chuck can hold woks of irregular shapes. But it
requires more time to set the work aligned with the lathe axis. Experienced turners can set the work about the axis quickly. Concentric
circles are inscribed on the face of the chuck to enable quick centering of the workpiece. A four jaw chuck is illustrated in Fig 1.15

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 Key hole Concentric circular lines

Jaws
Jaw

Body

1.6.2 Face plate

Faceplate is used to hold large, heavy and irregular shaped workpieces which can not be conveniently held between centres. It is a
circular disc bored out and threaded to fit to the nose of the lathe spindle. It is provided with radial plain and ‘T’ – slots for holding the
work by bolts and clamps.

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 Fig 1.18 Face plate

1.6.3 Driving plate

The driving plate is used to drive a workpiece when it is held between centers. It is a circular disc screwed to the nose of the lathe
spindle. It is provided with small bolts or pins on its face. Workpieces fitted inside straight tail carriers are held and rotated by driving
plates.

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 Fig 1.19 Driving plate

1.6.5 Carrier
When a workpiece is held and machined between centers, carriers are useful in transmitting the driving force of the spindle to the
work by means of driving plates and catch plates. The work is held inside the eye of the carrier and tightened by a screw. Carriers are of
two types and they are :

1. Straight tail carrier 2. Bent tail carrier

Straight tail carrier is used to drive the work by means of the pin provided in the driving plate. The tail of the bent tail carrier fits
into the slot of the catch plate to drive the work.

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 Screw

Eye

Tail

Bent tail carrier

Straight tail carrier Fig 1.21 Carriers

1.6.8 Rests
A rest is a mechanical device to support a long slender workpiece when it is turned between centres or by a chuck. It is placed at
some intermediate point to prevent the workpiece from bending due to its own weight and vibrations setup due to the cutting force. There
are two different types of rests

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 1. Steady rest
2. Follower rest

Follower rest
It consists of a ‘C’ like casting having two adjustable jaws to support the workpiece. The rest is bolted to the back end of the carriage.
During machining, it supports the work and moves with the carriage. So, it follows the tool to give continuous support to the work to be
able to machine along the entire length of the work.

In order to reduce friction between the work and the jaws, proper lubricant should be used. Fig 1.29 shows a follower rest.

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 Jaw

Work

Carriage

Fig 1.29 Follower rest

1.7 Cutting speed, feed and depth of cut

1.7.1 Cutting speed

The cutting speed is the distance travelled by a point on the outer surface of the work in one minute. It is expressed in meters per
minute.

πdn
Cutting speed = ——— m/min.
1,000

Where ‘d’ - is the diameter of the work in mm.

‘n’ - is the r.p.m. of the work.

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1.7.2 Feed
The feed of a cutting tool in a lathe work is the distance the tool advances for each revolution of the work. Feed is expressed in
millimeters per revolution.

1.7.3 Depth of cut

The depth of cut is the perpendicular distance measured from the machined surface to the uncut surface of the workpiece. It is
expressed in millimeters. In a lathe, the depth of cut is expressed as follows

d1 – d2
Depth of cut = ————
2
Where ‘d1’ - diameter of the work surface before machining
‘d2’ - diameter of the machined surface

1.8 Tools used in a lathe

Tools used in a lathe are classified as follows
A. According to the construction, the lathe tools are classified into three types
1. Solid tool
2. Brazed tipped tool
3. Tool bit and tool holders
B.According to the operation to be performed, the cutting tools are classified as
1. Turning tool
2. Thread cutting tool
3. Facing tool

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 4. Forming tool
5. Parting tool
6. Grooving tool
7. Boring tool
8. Internal thread cutting tool
9. Knurling tool
C. According to the direction of feed movement, the following tools are used
1. Right hand tool
2. Left hand tool

Solid tool

Tipped tool

Tool holder and tool bit

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 Fig 1.30 Types of tools

1.9 Operations performed in a lathe

Various operations are performed in a lathe other than plain turning. They are

1. Facing
2. Turning
a. Straight turning
b. Step turning
3. Chamfering
4. Grooving
5. Forming
6. Knurling
7. Undercutting
8. Eccentric turning
9. Taper turning
10.Thread cutting
11. Drilling
12. Reaming
13. Boring
14. Tapping

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1.9.1 Facing
Facing is the operation of machining the ends of a piece of work to produce flat surface square with the axis. The operation involves
feeding the tool perpendicular to the axis of rotation of the work. Facing operation is illustrated in Fig. 1.31

Work

Direction
of feed
Facing tool

Fig 1.31 Facing

1.9.2 Turning
Turning in a lathe is to remove excess material from the workpiece to produce a cylindrical surface of required shape and size.
Straight turning operation is illustrated in Fig. 1.32

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 workpieces.

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 Work

Turning tool

Fig 1.32 Straight turning

Step turning
Step turning is the process of turning different surfaces having different diameters. The work is held between centres and the tool
is moved parallel to the axis of the lathe. It is also called shoulder turning.

1.9.3 Chamfering
Chamfering is the operation of bevelling the extreme end of the workpiece. The form tool used for taper turning may be used for
this purpose. Chamfering is an essential operation after thread cutting so that the nut may pass freely on the threaded workpiece.

Work

Form tool

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 Fig 1.33 Chamfering

1.9.4 Grooving
Grooving is the process of cutting a narrow goove on the cylindrical surface of the workpiece. It is often done at end of a thread or
adjacent to a shoulder to leave a small margin. The groove may be square, radial or bevelled in shape. Different types of grooves are shown
in Fig. 1.34
Work

Grooving tool

Fig 1.34 Grooving

1.9.5 Forming

Forming is a process of turning a convex, concave or any irregular shape. For turning a small length formed surface, a forming tool
having cutting edges conforming to the shape required is fed straight into the work.
1.9.6 Knurling
Knurling is the process of embossing a diamond shaped pattern on the surface of the workpiece. The knurling tool holder has one
or two hardened steel rollers with edges of required pattern. The tool holder is pressed against the rotating work. The rollers emboss the
required pattern. The tool holder is fed automatically to the required length.

Knurls are available in coarse, medium and fine pitches. The patterns may be straight, inclined or diamond shaped. Fig. 1.35 shows
the operation of knurling.

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 Work Knurling roll

Tool holder

Fig 1.35 Knurling

The purpose of knurling is

1. to provide an effective gripping surface
2. to provide better appearance to the work
3. to slightly increase the diameter of the work

1.9.7 Undercutting
Undercutting is done
(i) at the end of a hole
(ii) near the shoulder of stepped cylindrical surfaces
(iii) at the end of the threaded portion in bolts

It is a process of enlarging the diameter if done internally and reducing the diameter if done externally over a short length. It is
useful mainly to make fits perfect. Boring tools and parting tools are used for this operation.

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 Fig. 1.36 shows the operation of undercutting.
Work

Parting tool

Fig 1.36 Undercutting

1.9.8 Eccentric turning

If a cylindrical workpiece has two separate axes of rotating, one being out of center to the other, the workpiece is termed as eccentric
and turning of different surfaces of the workpiece is known as eccentric turning. Eccentric turning is shown in Fig. 1.37. The distance
between the axes is known as offset. Eccentric turning may also be done on some special machines. If the offset distance is more, the work
is held by means of special centers. If the offset between the centers is small, two sets of centers are marked on the faces of the work. The
work is held and rotated between each set of centers to machine the eccentric surfaces.

Offset distance

Fig 1.37 Eccentric turning

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1.9.9 Taper turning
Taper
A taper may be defined as a uniform increase or decrease in diameter of a piece of work measured along its length.

Taper turning methods

1. Form tool method
2. Compound rest method
3. Tailstock setover method
4. Taper turning attachment method
5. Combined feed method
(i) Form tool method
A broad nose tool is ground to the required length and angle. It is set on the work by providing feed to the cross-slide. When the
tool is fed into the work at right angles to the lathe axis, a tapered surface is generated.

This method is limited to turn short lengths of taper only. The length of the taper is shorter than the length of the cutting edge.
Less feed is given as the entire cutting edge will be in contact with the work.

Taper turning by form tool method is illustrated in Fig. 1. 38

Work

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 Form tool

Fig 1.38 Taper turning by form tool method

(ii) Compound rest method

The compound rest of the lathe is attached to a circular base graduated in degrees, which may be swiveled and clamped at any
desired angle. The angle of taper is calculated using the formula

D - d tan Ø = ----------
2l

Where D – Larger diameter

d – Smaller diameter l – Length of the taper Ø - Half taper angle
The compound rest is swiveled to the angle calculated as above and
clamped. Feed is given to the compund slide to generate the

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 required taper. Taper turning by compound rest method is
illustrated in Fig. 1.39

Work

Compund rest Fig 1.39 Taper turning by compound rest method

(iii) Tailstock setover method

Turning taper by the setover method is done by shifting the axis of rotation of the workpiece at an angle to the lathe axis and feeding
the tool parallel to the lathe axis. The construction of tailstock is designed to have two parts namely the base and the body. The base is
fitted on the bed guideways and the body having the dead centre can be moved at cross to shift the lathe axis.

The amount of setover - s, can be calculated as follows

D - d s = L x --------
2l
where s - Amount of setover D – Larger diameter

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 d – Smaller diameter L - Length of the work
l – Length of the taper

The dead centre is suitably shifted from its original position to the calculated distance. The work is held between centres and
longitudinal feed is given by the carriage to generate the taper.
The advantage of this method is that the taper can be turned to the entire length of the work. Taper threads can also be cut by this
method.

The amount of setover being limited, this method is suitable for turning small tapers (approx. upto 8°). Internal tapers cannot be
done by this method.

Taper turning by tailstock setover method is illustrated in Fig. 1.40

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 Dead centre

Live centre

Work

Fig 1.40 Taper turning by tailstock setover method

(iv) Taper attachment method

The taper attachment consists of a bracket which is attached to the rear end of the lathe bed. It supports a guide bar pivoted at the
centre. The bar having graduation in degrees may be swiveled on either side of the zero graduation and set at the desired angle to the lathe
axis. A guide block is mounted on the guide bar and slides on it. The cross slide is made free from its screw by removing the binder screw.
The rear end of the cross slide is tightened with the guide block by means of a bolt. When the longitudinal feed is engaged, the tool mounted
on the cross slide will follow the angular path as the guide block will slide on the guide bar set at an angle of the lathe axis. The depth of
cut is provided by the compound slide which is set parallel to the cross-slide.

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 The advantage of this method is that long tapers can be machined. As power feed can be employed, the work is completed at a
shorter time. The disadvantage of this method is that internal tapers cannot be machined. Taper turning by taper attachment method is
illustrated in Fig. 1. 41

Guide block Guiding bar

Bracket

Binder screw

Fig 1.41 Taper turning by taper attachment method

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(v) Combined feed method
Feed is given to the tool by the carriage and the cross-slide at the same time to move the tool at resultant direction to turn tapers.

1.9.10 Thread cutting

Thread cutting is one of the most important operations performed in a lathe. The process of thread cutting is to produce a helical
groove on a cylindrical surface by feeding the tool longitudinally.

1. The job is revolved between centers or by a chuck. The longitudinal feed should be equal to the pitch of the thread to be cut
per revolution of the work piece.
2. The carriage should be moved longitudinally obtaining feed through the lead screw of the lathe.
3. A definite ratio between the longitudinal feed and rotation of the headstock spindle should be found out.
4. A proper thread cutting tool is selected according to the shape of the thread. It is mounted on the tool post with its cutting
edge at the lathe axis and perpendicular to the axis of the work.
5. The position of the tumbler gears are adjusted according to the type of the thread(right hand or left hand).
6. Suitable spindle speed is selected
7. Half nut lever is engaged at the right point as indicated by the thread chasing dial.
8. Depth of cut is set suitably to allow the tool to make a light cut on the work.
9. When the cut is made for the required length, the half nut lever is disengaged. The carriage is brought back to its original
position and the above procedure is repeated until the required depth of the thread is achieved.
10. After the process of thread cutting is over, the thread is checked by suitable gauges.

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 Spindle
gear Spindle Work

Tool

Carriage
Gear
on
the Leadscrew

leadscrew

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