Unit 3-METAL FORMING PROCESSES

UNIT-III
METAL FORMING
PROCESSES

INTRODUCTION:
A product is produced by shaping the metal
into the required shape and size.
In mechanical working method, “no
machining process” is carried out but it is
used to achieve optimum mechanical
properties in the metal.
By applying the force, the metal is
plastically deformed into required shape.

This deformation takes place when the
stress caused in the metal reaches the “yield
point”.
The mechanical working reduces the
cavities present in the metal and also used to
remove the impurities.
The wastage of material removal is
negligible or too small and production is
high when compared to other manufacturing
processes.

HOT WORKING AND COLD WORKING
OF METALS:
The metal forming or working processes are
mainly classified into hot working and cold
working processes.
The above division is on the basis of
working temperature.

HOT WORKING OF METALS:
The mechanical working of a metal above
the recrystallization temperature but
below the melting point is known as hot
working.
The recrystallization temperature of metal
will be about 30 to 40% of its melting
temperature.

Unit 3-METAL FORMING PROCESSES

Semi Hot Working (Warm Working):
The drawbacks of both cold working and hot
working processes are eliminated by this
method of working.
In this method the metal are deformed under
the conditions of temperature and strain
rate.
The Ductile, Tolerance, Yield Strength
factors should be considered for the selection
of temperature and strain rate.

Advantages of Hot Working:
Force requirement is less when compared
to cold working process.
Toughness, ductility and resistance can be
improved.
Quick and economical process.
Porosity is eliminated and density of the
metal is increased.
Suitable for all Metals.

Disadvantages of Hot Working:
Surface finish may be poor.
Close tolerances and automation cannot
be achieved due to high working
temperature.
Tooling and handling cost are high.
Sheets and wires cannot be produced.

Types of Hot working Process:
1. Hot forging
a) Hammer forging
b) Drop forging
c) Upset forging
d) Press forging
e) Roll forging
2. Hot forging
3. Hot extrusion

4. Drawing
5. Swaging
6. Hot spinning.

Cold Working:
The plastic deformation of a metal to the
required shape being performed below the
recrystallization temperature is known as
Cold working process.
The recrystallization temperature is about
melting temperature but generally cold
working is carried out only at room
temperature.

Classification of Cold Working Process:
1.Drawing
a) Blank drawing
b) Wire drawing
c) Tube drawing
d) Embossing

2. Squeezing
a) Coining
b) Sizing
c) Swaging
d) Knurling
e) Extrusion

3. Bending
a) Plate bending
b) Roll forming
c) Angle bending
d) Seaming

Materials used for Cold Working Processes:
1. Low and medium carbon steel.
2. Copper and light alloys.
3. Materials such as Al, Mg, Titanium.

Advantages of Cold Working:
Widely applied as a forming process for
steel.
Better surface finish is being obtained.
Provides higher dimensional accuracy
Thin material can be obtained
Suitable for mass production

Limitations:
The surface finish may be poor.
Close tolerances cannot be achieved.
Stress formation in the metal during cold
working is higher.

COMPARISON BETWEEN Hot and
Cold Working

Forging Processes:
Forging is the process of mechanical
working of metals.
Desired shape is obtained by the application
of a compressive force.
In Hot forging, the metal is heated above
the recrystallization temperature.
Then, it is compressed and squeezed to the
required shape by using hammer or press
tool.

Classification of Forging:
1.Smith forging (or) open die forging
a) Hand forging
b) Power forging
2. Closed die or impression forging
a) Drop forging
b) Press forging
c) Upset forging
3. Roll forging

Open Die Forging:
In this process, the forging is done in a
heated work at the proper temperature by
placing on flat surface of anvil through
hammering the metal piece.
Hammering is done by giving repeated
blows manually using a hammer.
Heavy forging weighing upto 25,000 Kg are
produced.

The forging is very simple and flexible.
It is very useful for producing simple
shapes such as ‘U’ bolts, chisels,
Rectangular, circular and hexagonal shapes.
1. Hand Forging:
The metal is heated and placed over the
anvil by using tongs.
One side of the former is held on the parts
to be forged while the other side is struck
with a sledge by a helper.

The repeated blows are given by a sledge
hammer to obtain the metal into required
shape.
2. Power Forging:
Power hammer or power presses are used.
Machines, which work on forging by blow
are called hammers where working by
pressure are called presses.
A sudden falling weight which strikes on
the metal makes into required shape.

The compressive force is used to shape the
metal.
Power forging is used in mass production.

Closed Die Forging:
1.Drop Forging:
Impression dies called closed dies are used.
The upper die is fitted on the ram and the
lower die is fitted on the anvil.
Both the dies have impressions.
Two Rollers are fixed on the board when
both rolls rotate opposite to each other.
It drives the board upward and lifting the
ram.

When the rolls are released, the ram will
fall down and produce a working stroke.
Single blow of press makes small and simple
parts and large complicated shapes are
made by number of steps.
Applications:
Used for making spanner, automobile
parts, and machine parts.

2.Press Forging:
Press forging is done in a press.
Operated by either mechanically or
hydraulically.
It is a closed die forging operation.
Slow squeezing rather than heavy blows.
Anvil to fix the lower die and the upper die
is fixed in the ram.
Ram is allowed to move slowly and presses
the metal slowly with high pressure.

Finished component may be automatically
removed by providing ejectors in the die set.
The capacity ranges from 50x10³ to 80x10³ kg
and speeds vary from 34 to 40 strokes per
minute.
Applications:
1. Spanner
2. Connecting rod
3. Machine components

3.Upset Forging:
It is used to form the head of bolt and river
or pins.
The head may be square hexagonal or
hemispherical.
The machine is having a die set which
consists of a fixed die and movable punch.
The heated metal bar is held inside the solid
die and the force is given to the punch.

The punch will squeeze the heated metal to
the shape of the die cavity.
4.Roll Forging:
A heated metal bar is passed between two
rolls.
Roll forging is done by an impression-die
forging operation.
The roll forging machine consists of two
horizontal rolls.

A piece of heated stock is passed between
rolls.
As the rolls rotate, the heated metal is
squeezed.
Used for reducing the cross section of a bar
and producing a taper end.
Most important use of this process is the
preparation of performed blanks.

Typical Forging operations
1.Upsetting:
This process is called as hot heading in
which the metal is heated at one end and it
is rest on the anvil and force is applied on
the other end.
This force will increase the cross sectional
area and decrease the length.
This operation of increasing cross
sectional area is known as upsetting.

The equipment used in this operation is called
upsetter.
The operation is performed with the help of a
die and a punch.

2.Drawing Down:
This process is also known as drawing out in
which the length of the metal increases and
the cross-sectional area decreases.
The metal is heated to the required length,
then the bar is placed on the anvil to draw the
bar by using fuller and hammer.
It is exactly a reverse process to that of
upsetting operation.

3.Punching:
Punching is a main forging operation used for
producing hole in metal plate by using a tool
known as punch.
The metal plate is placed over the hollow
cylindrical die and punch is placed above it at
required location where hole is being made.
For punching a hole, the metal job must be at near
welding heat and the punch is driven part way
through the job with hammer blows.
The work is then turned over and the hole is
completed from the other side.
The above said practice is adopted for thicker jobs.

4. Bending:
Bending is a very commonly used forging
operation in forging shop to give a turn to a metal
rod or plate.
It is accompanied by spreading of the metal in the
inside of the bend and narrowing at outside.
The simplest method of bending a piece of metal
in hand forging is to support it on the anvil (over
the horn) and to strike its free end with a hammer.
 When bent, the metal of the work piece thins out
round bend causing weakness.
This can be overcome by upsetting the bar prior to
bending.

5.Cutting:
The removal of excess metal from the work
or for making pieces from a bar stock is
known as cutting process.
A pair of blade is used to cut the metal for
the required shape.

6.Forge Welding:
Joining the two work pieces by forging
operation is called forge welding.
The work pieces are heated and cleaned
thoroughly before going to the weld and force
is applied by hammer blows to join together.
There are three distinct types of joints,
1.Butt weld
2.Scarf weld
3.‘V’ weld

7.Piercing:
Making a blind or through holes with the
help of a punch in the metal is known as
piercing.
Piercing is also performed to produce hollow
regions in forgings using single acting
auxiliary equipment.
Piercing forces depends on the punches cross
sectional area and tip geometry, strength and
friction of the material.

8.Swaging:
Reducing or changing the cross sectional area
of the metal is known as Swaging operation.

9.Flattering:
It is used to flat the stock and that the stock is
fitted properly in the closed die.
10.Fullering:
Reducing the stock and increasing the length
of the work piece by applying pressure is
known as fullering.

11.Edging:
 Edging is used to shape the ends of the bars and
to gather metal.
 The metal flow is confined in the horizontal
direction but it is free to flow laterally to fill the
die.

Rolling of Metals
 Rolling is the most rapid method of forming metal
into desired shapes by plastic deformation
through compressive stresses using two or more
than two rolls.
 It is one of the most widely used of all the metal
working processes.
 The main objective of rolling is to convert larger
sections such as ingots into smaller sections
which can be used either directly in as rolled state
or as stock for working through other processes.
 Significant improvement is accomplished in rolled
parts in their various mechanical properties such as
toughness, ductility, strength and shock
resistance.

 The crystals in parts are elongated in the direction of rolling,
and they start to reform after leaving the zone of stress.
 Hot rolling process is being widely used in the production of
large number of useful products such as rails, sheets, structural
sections, plates etc.

Types of Rolling:
Flat Strip Rolling
According to number of Rolls
Two high rolling mills
Three high rolling mill
Four high rolling mill
Multi rolling mill
Universal rolling

Shape rolling operations
Ring rolling
Thread rolling

Flat Strip Rolling:
In rolling plates and sheets with high width-
to-thickness ratios, the width of the material
remains essentially constant during rolling
process.
For square section, the width increases
considerably in the roll gap.
The increased width in rolling is called
spreading.
For calculating the rolling force, the width is
taken as average width.

The spreading can be prevented by using
vertical rolls.

The schematic diagram of flat strip rolling is shown
below,

The thick of the strip h0 is reduced to hf by a
pair of rotating mills.
The velocity of the strip increases from V0 to
Vf .
 Since, the surface speed of the roll is constant
in the roll gap L.

The frictional forces which are acting on the flat
strip is also given in figure below,

The roll force and power requirement for the
rolling are given by the figure below,

1.Roll force:
F= Lw Yavg
Where, L = Roll strip contact length
w = Width of the strip
Yavg= Average true stress
2.Power per Roll:
Power= 2ΠFLN/6000 kW
Where, F= Force in Newton
N= Speed of the roll in rpm

3.Roll strip contact length (L):
L= R(h0-hf)
Where, h0-hf = Difference between initial and
final thickness
R= Roll radius

Two-High Rolling Mill
A two-high rolling mill has two horizontal
rolls revolving at the same speed but in
opposite direction.
The rolls are supported on bearings housed
in sturdy upright side frames called stands.
The space between the rolls can be adjusted
by raising or 1owering the upper roll.
Their direction of rotation is fixed and
cannot be reversed.

The reduction in the thickness of work is
achieved by feeding from one direction
only.
However, there is another type of two-high
rolling mill, which incorporates a drive
mechanism that can reverse the direction of
rotation of the rolls.
In a two-high reversing rolling mill, there is
continuous rolling of the workpiece
through back-and-forth passes between the
rolls.

Three-High Rolling Mills
It consists of three parallel rolls, arranged one
above the other.
 The directions of rotation of the upper and
lower rolls are the same but the intermediate roll
rotates in a direction opposite to both of these.
This type of rolling mill is used for rolling of two
continuous passes in a rolling sequence without
reversing the drives.
This results in a higher rate of production than
the two-high rolling mill.

Four-High Rolling Mill:
 Practically, it consists of four horizontal rolls, the
two middle rolls are smaller in size than the top
and bottom rolls .
 The smaller size rolls are known as working rolls
which concentrate the total rolling pressure over the
work piece.
 The larger diameter rolls are called back-up rolls
and their main function is to prevent the deflection
of the smaller rolls.
 The common products of these mills are hot or cold
rolled plates and sheets.

MULTIPLE ROLL MILLS
 In multiple rolls, the work rolls are
supported by back up rolls.
 The work rolls are driven by driving rolls.
 In four high rolling mills the diameter of the
back up rolls cannot be increased to 2 to 3
times that the work rolls.
 In multi Roll mills the back up rolls itself bend
for the support.
 There are 12 to 20 rolls used to manufacture
the strips of thickness 0.001 mm.

UNIVERSAL ROLLING MILL:
 The metal is reduced by both horizontal and
vertical rolls called universal rolling mill.
 The edges of the bar is smoothened by
vertical rolls and the vertical rolls are
mounted either one side or both side of the
horizontal mill.
 In this type of mills, two or three or four
horizontal mill are arranged in a row.
 It is used to make sheets plates, slab and
beams.

SHAPE ROLLING OPERATIONS:
 In shape rolling process the various shapes
can be produced.
 Example: Straight and long structural
shapes, solid bars, I-beams, channels,
railroad rails.
 The various stages in shape rolling process
are given below.
 The types of shape rolling process are
 Ring rolling
 Thread rolling

1. Ring rolling:
 In ring rolling process, a thick ring is
expanded into a large diameter ring with a
reduced cross section.
 First, the ring is placed in between the two
rolls and one of the roll is driven and the
ring thickness is reduced by bringing the
rolls closer together as they rotate.
 The reduction in thickness of the ring is
compensated by an increase in the ring’s
diameter.

The figure shows procedure for producing a
seamless ring for a tapered roller bearing.

 The ring rolling process has the
advantages of short production times,
close tolerances, material savings.
 It can be carried out at room
temperature depends upon the size,
strength and ductility of the material.

2. Thread rolling:
 It is a cold forming process by which straight
or tapered threads are formed.
 The threads are formed on the rod or wire with
each stroke of a pair of flat reciprocating dies.
 It has the advantages of generating threads
without any loss of material.
 The surface finish is very good and the fatigue
life high.
 The thread rolling is the high advantage
method compare with other thread making
methods.

 Rolled threads have a grain-flow pattern that
improves the strength of the thread.
 The threads are rolled on metals in the soft
conditions and threaded fastness such as bolts,
are made by this process.
 The lubricant is very important in thread
rolling process to minimize the defects.
 Spur and helical gears can be produced by
cold rolling process similar to thread rolling.

DEFECTS IN ROLLED PARTS
There are two types of defects which can
occur in rolled products.
1. Surface defects.
2. Internal structural defects.

1. SURFACE DEFECTS
 It includes scale, rust, scratches, cracks
and pits.
 It is due to the impurities and inclusions
in the original cast material.

2. INTERNAL STRUCTURAL DEPICTS:
It includes the following defects:
(i) Wavy edges
(ii) Zipper cracks
(iii) Edge cracks
(iv) Folds
(v) Alligatoring
(vi) Laminations

The defects wavy edges and zipper cracks
occurs due to bending of rolls.
Rolls act as straight beams.
If the material flow is continuous,
maintains the continuity and strains within
the material.
There are compressive strain on the edges
and tensile strain at the centre.

Because of the edges are restrained from
expanding freely in the longitudinal direction
wavy edges on the sheet will be produced,
the zipper cracks occur due to poor material
ductility at the rolling temperature.
To avoid this, a ‘camber’ to be provided to
the rolls i.e. diameter of rolls is made
slightly larger at the centre than edges.

3.Other defects in Rolling:
(i) In homogeneous deformation of elements
across the width:
It is due to the decrease in thickness for the
elements near the centre will mainly be
converted into increase in length and near
the edges.
The decrease in thickness is converted into
lateral spread.

(ii)In homogeneous deformation in the
thickness section:
It is due to rolling in which the reduction
in height is converted into increase in
length and the thickness of the sheet does
not undergo the same lateral deformation in
the direction of rolling.
(iii) Folds:
It is created during plate rolling, if the
reduction per pass is very small.

(iv)Lamination:
Due to incomplete welding of pipe and
blowholes during the rolling process, the
internal defects such as fissures are created.

Principle of Wire Drawing
The diameter less than 16 mm has drawn in
the form of wire coil.
Initially, the point of the wire is sized.
So, it is freely enter into the die.
This sized point coming out of the die orifice
is fixed on the pliers or carriage which
pulls the rod through all zones of the die
orifice which will reduce the diameter of
the rod.

 For making fine wire, the rod is passed through
the number of dies.
 Finally the wire is connected to the power reel to
get the wire coil.

Principle of Rod Drawing
In rod drawing the rod which is to be drawn
should be straight and the maximum length of
the rod drawn is depending upon the carriage
movement.
The drawing speed varies rod to rod
depending upon the size of the rod.
The process consists of placing the hot drawn
bar through a die of which the bore size
confirms to the finished size of product.

Tube Drawing
In tube drawing, cylinders and tubes which
are made by extrusion process is finished
by drawing process.
Tube drawing is classified into:
Tube sinking
Tube drawing with plug
Tube drawing with mandrel

 In tube sinking process, only the outer diameter of the
tube is reduced.
 For reducing the inner diameter of the tube, the tube, the
other two processes .i.e., tube drawing with plug or Tube
drawing with mandrels is used.

 In tube mandrel, the mandrel is placed in the tube
and the pull is given to the tube which will reduce
the inner diameter of the tube.

 In plug drawing, both internal and external surfaces of
the tube are controlled and the dimensional accuracy is
good compared to the other two methods.
 In this process plug is fixed or floating.
 The friction obtained in fixed plug is more than floating
plug and drawing load is high in fixed plug and less in
floating plug.

Principles of Extrusion
 The heated metal is compressed and forced
through a suitable shaped die.
 The force requirement for the cold extrusion
process is high.
 The extruded products may be either solid or
hollow.
 Metals such as steel and nickel rods are extruded
in hot conditions.
 The most extruded parts are door trim, hardware
items and aircrafts parts.

The major extrusion defects are:
Surface cracks
Internal cracks
 Types of Extrusion
1.Hot Extrusion
 Forward (or) Direct extrusion
 Backward (or) Indirect extrusion
2.Cold extrusion (or) Impact extrusion

Hot Extrusion:
Metals such as lead, copper, aluminium and
magnesium are having low yield strength
and extrusion temperature so they are hot
extruded.
Hot extrusion is hydraulically operated.
The size are rated from 250 to 5500 tones.
Presses are used in hot extrusion process.
The steel is most extruded at high
temperature.

The hot extrusion process has the following
characteristics:
1. No fracture of metal is due to tensile stress.
2. Extrusion size depends upon the size of the
cylinder.
3. Hollow shapes can also be extruded.

Forward or Direct extrusion:
The heated billet metal is placed in a press
which is operated by the ram and a
cylinder.
The heated billet is pushed by the ram and
with the application of ram pressure the
metal first plastically fills the die.
Then, it is forced out through the die
opening and finally cut at the die face.

Indirect or Backward extrusion:
The extruded part is forced through the hollow
ram.
The ram is operated by a horizontal hydraulic
drive.
The working principle of this process is that, the
heated metal is placed in the die and the
force is applied by the power operated hollow
ram.
So, the extruded metal is passed through the
hollow ram which requires less force than
direct extrusion.

Cold Extrusion or Impact extrusion:
The working principle of this process is that
the work material is placed between the
die and ram.
The punch is connected with the ram.
When the sudden impact is given to the
ram, the metal flows plastically in the
upward direction, metals such as
aluminium and tin are extruded in an
impact extrusion.

The various items of daily use such as tubes for
shaving creams, tooth pastes and paints,
condenser cans and thin walled products are
impact extruded.
The metal flows up along the punch forming a
cup shaped component.
When the punch moves up, the compressed air is
used to separate the component from the punch.
The production rate is fairly high giving 60
components per minute.
Main advantage is, it speeds up product
uniformity and lowers scrap yield.

Unit 3-METAL FORMING PROCESSES

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Unit 3-METAL FORMING PROCESSES