Lathe

Machining is an essential process of
finishing by which jobs are produced to the
desired dimensions and surface finish by
gradually removing the excess material
from the work piece in the form of chips
with the help of cutting tool moved past
the work surface.
Machining

Machining operation done in lathe
Straight turning
Grooving
Chamfering
•Parting off
•Facing
•Knurling

Turning ..
 Excess Material is removed to
reduce Diameter
Cutting Tool: Turning Tool
a depth of cut of 1 mm will
reduce diameter by 2 mm

Facing
Flat Surface/Reduce length

Facing ..
Machine end of job To Reduce
Length of Job
Feed: in direction perpendicular to
work-piece axis
Length of Tool Travel = radius of
work-piece

Grooving
Produces a Groove on
workpiece
Shape of tool  shape of
groove
Also called Form Turning

Parting
Cutting workpiece into Two
Similar to grooving
Parting Tool
Hogging – tool rides over – at slow feed
Coolant use

Chamfering
 Beveling sharp machined edges
 Similar to form turning
 Chamfering tool – 45°
 Avoid Sharp Edges
 Make Assembly Easier

Lathe Accessories
 Lathe centers, chucks, faceplates
 Mandrels, steady and follower rests
15

Lathe Centers
Work to be turned between centers must have
center hole drilled in each end
Provides bearing surface
Support during cutting
Most common have
solid Morse taper shank
60º centers, steel with carbide tips
Care to adjust and lubricate occasionally
16

For holding cylindrical stockFor holding cylindrical stock
centered.centered.
For facing/center drilling the end ofFor facing/center drilling the end of
your aluminum stockyour aluminum stock
Four-Jaw Chuck
This is independent chuck generally has
four jaws, which are adjusted
individually on the chuck face by means
of adjusting screws
Three jaw chuck

 Thin jobs can be held by means of
magnetic chucks.
Collet Chuck
Magnetic Chuck
Collet chuck is used toCollet chuck is used to
hold small workpieceshold small workpieces
Thin jobs can be heldThin jobs can be held
by means of magneticby means of magnetic
chucks.chucks.

Used to hold round, square, hexagonal, and
irregularly shaped workpieces
Has four jaws
Each can be adjusted independently by chuck
wrench
Jaws can be reversed to hold work by inside
diameter
20
Four-Jaw Independent Chuck

Collet Chucks
Most accurate chuck
Used for high-precision work
Spring collets available to hold round, square,
or hexagon-shaped workpieces
Each collet has range of only few thousandths
of an inch over or under size stamped on collet
21

Magnetic Chucks
Used to hold iron or steel parts that are too thin
or may be damaged if held in conventional
chuck
Used only for light cuts and for special grinding
applications
22

Steadyrest
Used to support long work held in chuck or
between lathe centers
Located on and aligned by ways of the lathe
Positioned at any point along lathe bed
Three jaws tipped with plastic, bronze or rollers
may be adjusted to support any work diameter with
steadyrest capacity
24

Mandrel
Holds internally machined workpiece between
centers so further machining operations are
concentric with bore
Several types, but most common
Plain mandrel
Expanding mandrel
Gang mandrel
Stub mandrel
27

Direct & Indirect speeds in Lathe
BACKGEAR
As its name implies, "backgear" is a gear mounted
at the back of the headstock that allows the
spindle to rotate slowly.
 Screw-cutting requires slow speeds, typically
between 25 to 50 r.p.m.
 To tackle heavy-duty drilling, big-hole boring

Disadvantages
Some power is lost due to friction of gears
All -geared lathes are costlier than the belt driven lathe
owing to more complicated gear and lever mechanism.
In case of overloading the machine, for having no
arrangement of belt slipping, there is little possibility to
prevent damage to parts

Operating condition in a lathe
Cutting speed
Feed
Depth of cut
1-Cutting speed
In a lathe, for the turning operation, cutting speed is the
peripheral speed of the work piece past the cutting tool.
Expressed in meters/minute.
Cutting speed= m/min
Where D= diameter of the work piece in mm.
N=rpm of the work

Cutting Speed
The Peripheral Speed of Work-piece past the Cutting
Tool
=Cutting Speed
D – Diameter (mm)
N – Revolutions per Minute (rpm)
m/min
1000
ND
v
π
=

Feed
so– the distance the tool advances for every rotation
of workpiece (mm/rev)

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 as mm/revolution.

Depth of Cut
perpendicular distance between machined surface
and uncut surface of the Work-piece
t = (D1 – D2)/2 (mm)

Machining time
The time required to machine a component is called
MACHINING TIME.
Machining time depends upon
Size of the work piece
Amount of material removed and
The operating condition(speed, feed, depth of cut)
Consider the feed and speed(f×N) the feed rate in
mm/min.

It gives the distance that the tool moves (f×N) in mm
is one minute.
Hence , for a distance L , the time required for one
complete cut, “t” in minute is given by
t= minute
Alternative
If “f” is the feed of the job per revolution expressed in
mm/revolution and L is the length of the job in mm ,
then number of revolutions of the job required for a
complete cut will be

If the rpm of the work is N , time taken to revolve the
job through number of revolution for a complete
cut will be:
There fore time taken for a complete cut
t= minute
Problem -1
A mild steel rod having 50 mm diameter and 500 mm length is to be turned
on a lathe. Determine the machining time to reduce the rod to 45 mm in one
pass when cutting speed is 30 m/min and a feed of 0.7 mm/rev is used.
Solution
Given data: D = 50 mm, Lj = 500 mm
v = 30 m/min, f = 0.7 mm/rev
Substituting the values of v and D in
V = ΠDN/1000 M/min
Required spindle speed as: N = 191 rpm

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.
D1= larger diameter of taper in mm
D2= smaller diameter of taper in mm
L= length of tapered part in mm
2α= full taper angle
α=angle of taper or half taper angle

Taper Turning
Taper:
L
DD
2
tan 21 −
=α

The amount of taper in a work piece is usually specified
by the ratio of the difference in diameters of the taper
to its length.
This is termed as the CONICITY and it is designated by
the letter K.
L
DD
K 21 −
=

Taper Turning..
MethodsMethods
 Form Tool
 Swiveling Compound Rest
 Taper Turning Attachment
 Simultaneous Longitudinal and
Cross Feeds
Tailstock set over

Taper turning using a form tool

Shape of the tool is remain same as the shape of the
component to be produced.
Accuracy of taper produce depends on accuracy of
taper present on tool
Width of tool must be greater than or equal to the
length of workpiece to be taper turned.
Maximum length of component which can be taper
turned is 20 mm only
Only external taper turning is possible.

Limitation-
This method is limited only for short length
taper. Because the metal is removed by the entire
cutting edge, and only 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 piece.

Taper turning by swiveling the compound rest

The compound rest has a circular base graduated in
degrees, which can be swiveled at any angle.
While turning a taper, the base of compound rest is
swiveled through an angle equal to the half taper
angle. The tool is then fed by hand.
Once the compound rest is set at the desired half
taper angle, rotation of the compound slide screw will
cause the tool to be fed at that angle and generate a
corresponding taper.

This method is limited to turn a short but steep taper
owing to the limited movement of the cross slide.
But a small taper may also be turned. The compound
rest may be swiveled at 45 degree on turn a steep taper.
The movement of the tool in this method being purely
controlled by hand, this gives a low production
capacity and poorer surface finish. The setting of the
compound rest is done by swiveling the rest at the half
taper angle, if this is already known. If the diameter of
the small and large end and length of taper are known,
the half taper angle can be calculated.

1. calculate the compound rest angle for turning short taper
of 1:15.
sol.
Let α be the angle at which the compound rest will be set
tan α =
152
1
2 ×
=
−
l
dD
.91.1
30
1
tan 01
Ans=





=∴ −
α

3. Calculate the compound rest angle to turn a short
taper of 1mm per 12mm.
Sol. Given taper : 1 mm per 12mm
04167.0
122
1
2
tan =
×
=
−
=
l
dD
α
Ans0
386.2=∴α

Determine the angle at which the compound rest would be swiveled for
cutting a taper on a work piece having a length of 150 mm and outside
diameter 80 mm. The smallest diameter on the tapered end of the rod
should be 50 mm and the required length of the tapered portion is 80 mm.
Solution
Given data: D1 = 80 mm, D2 = 50 mm, Lj = 80 mm (with usual notations)
tan α = (80-50) / 2×80 or α = 10.620
The compound rest should be swiveled at 10.62o

Taper turning by a taper attachment

The principle of turning taper by a taper attachment
is to 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 centers 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
centers. The bar having graduations in degrees may
be swiveled on either side of the zero graduation and
is set at the desired angle with the lathe axis.

When the taper turning attachment is used, the cross
slide is first made free from the lead screw by
removing the binder screw. The rear end the cross
slide is then 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 gear bar at
an angle to the lathe axis.
The required depth of cut is given by the compound
slide which is placed at right angles to the lathe axis.

The guide bar must be set at half taper angle and the
taper on the work must be converted in degrees. The
maximum angle through which the guide bar may be
swiveled is 10 degree to 12 degree on either side of the
centre line.
If the diameters D,d and the length L of the taper are
specified, the angle of swiveling the guide bar can be
determined from equation
tanα=

The advantage 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 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
Taper turning by combining feeds 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.

AB
BCsin α =
From fig.
In Δ le ABC ,
For small angles sin α ≈ tan α ;
BC = amount of set over and
AB = overall length of work
piece
set over , S = BC = AB tan α
S = L tan α = L (BC/l) = L(D-d)/2l

The length of work is 300 mm, the amount of taper is
1 : 25. Find the tail stock set over required
50
1
252
1
tan =
×
=α
set over, S = L tanα
mm6
50
1
300 =×=

Q. A component having 400 mm length in which
central 150 mm length of component can be taper
turned to an angle of 4 Degree. Which of the Taper
turning Method is used for this ?
Ans: Compound and taper attachment method
offsets
s
==
=
28
400
4tan
Maximum offset will produced on tailstock is 10 mm
The length of taper part is 150 mm but by form tool
method we can produce taper up to 20 mm length only

Thread cutting
There are a large number of thread forms that can be
machined in lathe such as Whitworth, Acme, ISO
metric, etc.
Thread cutting can be considered as turning only
since the path to be travelled by the cutting tool is
helical

Right-hand thread
Helical ridge of uniform cross
section onto which nut is
threaded in clockwise direction
When cut on lathe, tool
advanced from right to left
Left-hand thread
Helical ridge of uniform
cross section onto which nut
is threaded in
counterclockwise direction
When cut on lathe, tool
advanced from left to right

summary
Lathe is the most important and common machine
tool found in practically all machine shops.
A large variety of lathes have been developed to cater
for different processing requirements.
A lathe consists of a bed, headstock, tailstock and a
carriage as major components along with a few other
items that provide the necessary support and
motions.

summary
A variety of chucks such as universal 3-jaw, independent
4-jaw, and faceplate, are used to locate and support work
pieces in a lathe for common machining applications.
There are a variety of tools available depending upon the
type of surface that needs to be generated.
There are a large variety of operations such as turning,
facing, knurling, contouring, etc. that can be carried out in
a lathe. In fact practically all types of surfaces can be
generated in a lathe.

summary
Taper turning is a special type of operation that requires
the tool to be moved in two different direction
simultaneously to generate the surface.
For this purpose, a variety of methods are used in a lathe
such as compound slide, tailstock offset or a special
attachment.
Precision threads can be cut in a lathe using the lead
screw and special methods

There are various special attachments such as milling
attachment, grinding attachment, etc. have been
developed that enhance the range of surfaces that can
be generated in a lathe.
Machining time for different operations can be
estimated using the cutting process parameters and
the geometry of the part
summary

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