Unit 4-SHEET METAL PROCESSES

Introduction
The working of metal thickness from 3mm
to 5mm with hand tools and simple
machines into various forms is known as
Sheet metal work.
These are formed by cutting,forming and
joining processes.
Sheet components are less expensive, lighter
in weight and easily replaceable.
Example: Funnels, bends, boxes, pipes, covers
etc.

The basic sheet metal hand tools used in sheet
metal work are as follows:
1.Measuring tools 2.Staight edge
3.Steel square 4.Scriber
5.Divider 6.Trammel points
7.Punches 8.Chisel
9.Hammers 10.Snips or shears
11.Pliers 12.Stakes
13.Groovers 14.Rivet set
15.Soldering iron

Sheet Metal Characteristics
1.Roll forming
2.Stretch forming
3.Drawing
4.Stamping
5.Rubber forming
6.Spinning
7.Super plastic forming
8.Peen forming
9.Explosive forming
10.Magnetic pulse forming

1.Roll forming:
 Long parts with constant complex cross
sections, good surface finish, high
production rates, high tooling costs are
produced.
2.Stretch forming:
Large parts with shallow contours
suitable for low-quantity production, high
labor costs, tooling and equipment costs
depend on part size are produced.

3.Drawing:
Shallow or deep parts with relatively
simple shapes, high production rates, high
tooling and equipment costs are produced.
4.Stamping:
 It includes a variety of operations, such as
punching, blanking, embossing, bending,
flanging and coining, simple or complex
shapes formed at high production rates,
tooling and equipment costs can be high, but
labour cost is low.

5.Rubber forming:
It includes drawing and embossing of
simple or complex shapes, sheet surface
protected by rubber membranes, flexibility
of operation, low tooling costs.
6.Spinning:
Small or large axisymmetric parts, good
surface finish, low tooling costs, but labor
costs can be high unless operations are
automated.

7.Super plastic forming:
Complex shapes, fine detail and close
tolerances, forming times are long, hence
production rates are low.
So, parts are not suitable for high
temperature use.
8.Peen forming:
Shallow contours on large sheets,
flexibility of operation, equipment costs
can be high, process is also used for
straightening parts.

9. Explosive forming:
Very large sheets with relatively complex
shapes, although usually axisymmetric, low
tooling costs, but high labor cost, suitable
for low-quantity production, long cycle
times are produced.
10.Magnetic pulse forming:
Shallow forming, bulging and embossing
operations on relatively low-strength
sheets, most suitable for tubular shapes, high
production rates, require special tooling.

SHEARING OPERATION:
The process of cutting a straight line across a
strip, sheet or bar is known as shearing
process.
The broad classification of sheet metal
operations are under the following two
categories:
(i) Cutting operations
(ii) Forming operations

CUTTING OPERATIONS:
In cutting operations, the workpiece is stressed
beyond its ultimate strength and cut- off into
two pieces.
The common cutting operations are discussed
below:
(i)Blanking:
Blanking is the operation of cutting a flat
shape from the sheet metal.
The metal that is punched out is called as
“blank” and the metal that is left out is called
as “scrap”.

(ii) Punching:
Punching is the operation to produce circular
holes on a sheet metal by a punch and a die.
The metal punched out is removed as waste.
When removing metal from the metal, if the
pierced metal is the final product the
operation is called as “Punching”
(iii) Piercing:
Piercing is an operation to produce holes of
any desired shape.
Piercing produces a raised hole rather than a
cut hole.

(iv) Power shearing:
This operation is carried out on power
shearing machines where in the stock
material (plate or sheet metal) is cut between
the two cutting blades in the form of dies.
The lower cutting blade is fixed and the
upper cutting blade is movable and it is set at
slight angle to the edge of the stationary blade.
(v) Cutting off:
In this operation a piece is removed from a
strip by cutting along a single line.

(vi) Parting:
It is the operation through which the metal
is cut simultaneously along two parallel
lines or contours or any other two lines which
can balance each other to neutralize side thrust.
(vii) Notching:
It is the operation through which metal
pieces are cut from the edge of a sheet , strip
or blank.

(viii) Trimming:
It is the operation used for removing excess
metal, irregular outlines and waved edges,
etc., from the walls of drawn shells or the
surfaces of swaged and cast parts.
(ix) Shaving:
It is also similar to trimming operation but
here the amount of metal removal is usually
about 10% of the thickness of the blank.

(x) Perforating:
In this operation, multiple holes which are very
small and close together are cut in flat work
material.

(xi) Slitting:
 It is the operation of making an unfinished cut
through a limited length only.
(xii) Lancing:
 Lancing consists of cutting the sheet metal through
a small length and bending this small cut portion
downwards.

FORMING OPERATIONS
(a)Bending:
Bending is the operation of forming the metal
between a suitably shaped punch and a
forming block.
The included angle on the tool is usually
smaller than that to be produced to allow for
the ‘spring back’ of the metal after forming.
Spring back is a term which denotes the
property of sheet metal to partially fall back
from its bent position when the punch retards
after completing the operation.

(b) Drawing:
 Drawing operation consists of a punch forcing a
sheet metal blank to flow plastically into the
clearance available between the punch and die
surfaces so as to acquire top shape, a cylindrical
shape or a box shape.

(c) Squeezing:
In squeezing operation, the metal is caused to
flow to all portions of a die cavity under the
action of compressive force.
(d) Embossing:
It is the process of producing required
shapes on sheet metal blanks by means of
punches and dies.

(e) Nibbling:
Nibbling is an operation of cutting any shape
sheet metal without special tools.
It is done on a nibbling machine.
The required shape of profile is connected in
the form of tracer or templates in nibbling
machine.

BENDING OPERATIONS
Bending is the operation of forming the metal
between a suitably shaped punch and a
forming block.
The included angle on the tool is usually
smaller than that to be produced to allow for the
‘spring back’ of the metal after forming.
Spring back is a term which denotes the property
of sheet metal to partially fall back from its
bent position when the punch retards after
completing the operation.

The metal is stressed beyond its elastic limit.
The line in which there is no change in length
is called neutral line.
The layers above the neutral line are stressed
by tension .
But the layers below this neutral line are
stressed by compression.

Types of Bending operations:
1. Angle Bending:
In this case, the metal is bent at an angle to
each edge.
It is denoted by ‘ϴ’.
If ϴ < 90º on side, it is called as single
bending.
If ϴ = 90º , this is called as vertical or
straight bending.
If ϴ > 90º but in two places on the same work,
it is called as double bending.

2. Roll bending:
If the metal is bent in the form of rolled edge
at the edges of the work, it is called as roll
bending.

3. Roll forming:
If the edges are formed to a desired shape or
any impression in the form of bend is made
on the sheet metal, it is called as roll forming
operations.

4.Seaming:
The process of providing lock between the
two edges of the different work metal is
called as seaming.
To perform this, the edges of sheet metal to
be locked are bent in opposite direction to
each other.
Then they are inserted with each other and
pressed or tightened to make complete lock.

5.Bending edge of a sheet using wiping-die:
Wiping die bending, also known as edge
bending, is performed by holding the sheet
between a pad and die then sliding the
wiping flange across the face pushing and
bending the sheet metal which protrudes from
the pad and die.
The flange is driven by an upper shoe and
the die is supported by a lower shoe.
A spring between the pad and upper shoe
grabs the metal before the flange hits it and
holds the workpiece down during the bending
process.

DRAWING OPERATIONS
Drawing operation consists of a punch
forcing a sheet metal blank to flow
plastically into the clearance available
between the punch and die surfaces and die
surfaces so as to acquire an upshape, a
cylindrical shape or a box shape.
Types of Drawing operations:
1. Deep drawing
2. Shallow or box drawing

1.Deep drawing:
In this case, the length of the part to be
drawn is deeper than its width.
2. Shallow or box drawing:
In this case , the length of the part to be
drawn is lesser than its width.
Example: Seamless pots, tubs, cans and
covers etc.

STRETCH FORMING
OPERATIONS
Stretching is the process of stressing the work
blank beyond its elastic limit by moving a form
block towards the blank or sheet metal.
The form block has projections of exact size
required on the blank which is in the form of
depressions on the same blank.
Stretching is mainly done for straightening a
part to obtain a straight axis and uniform cross-
section.
During stretching the blank, the spring back
occurs after completing the stretching process.

Spring back is defined as the movement of the
metal to resume its original position causing
a decrease in bend angle after the applied force
is withdrawn.
So, this spring back has to be considered to
obtain exact shape and size of the blank
after the stretching process.
Spring back always depends on material
type, thickness of the blank, hardness of the
blank and bend radius.

Generally large bend radius produces
greater spring back on the blank.
But, this spring back can be avoided by
(i) Over stretching using V-type blocks, and
(ii) By coining the metal slightly at the corners
of the blank to remove elastic stresses called
corner setting.

Methods of Stretch Forming:
1. Form-block method:
In this method, the two ends of the blank or
sheet metal is tightly held by an adjustable
gripper.
These grippers are fixed but adjustable.
Then, the form block is moved towards the
blank to make the required shape.
In this case, the form-block is operated by
hydraulic cylinder.
When the form-block moves towards the
blank, the hydraulic fluid inside cylinder gets
compressed and delivered through the outlet
valve.

The movement of the form always depends
the hydraulic fluid pressure inside cylinder.
The fluid is entered the cylinder when the
form-block moves away from the blank after
completing stretching process.
Force exerted on the piston is calculated as
F=Π d2 p
4
Where, d= Diameter of the piston
p= Hydraulic fluid pressure.

Stretching the blank can also be done by fixing
the form block stationary and moving the
grippers towards the form-block.
It is performed by holding the blank ends in
movable grippers.

2.Mating-die method:
In this method, the blank is held in movable
grippers.
The blank is placed between the lower and
upper die.
The lower die is kept stationary and the
upper die is movable one which is operated
by hydraulic or pneumatic cylinders.
First, the movable grippers are moved
towards the lower die on which only elastic
deformation takes place.

Next, the upper die is moved towards the
blank.
When the upper die touches the blank, only
elastic change takes place.
Due to continuous stretching of the blank by
the upper die, plastic flow of sheet metal
takes place between lower and upper dies.
When the upper die edges reach the top surface
of the blank, the stretching process is
completed.

Materials for die and form block:
Wood, Masonite, zinc alloys and cast iron.
Advantages:
Blanks can be stretched in a single operation.
No need of any heat treatments.
Spring back is reduced or eliminated.
Direct bending is not introduced.
Tooling costs are low.
More suitable for low volume production.

Disadvantages:
1.Blank thickness should be uniform
throughout the length.
2. Sudden changes in contour surfaces cannot
be stretched.
3. Maintenance cost of hydraulic cylinders is
high.
4. The process requires high quality form-
blocks.

Limitations:
1. Uneven thickness of blank cannot be
stretched .
2. Stretching of blank to the required shape of
contour is limited.
Applications:
1. Production of aircraft wing and fuselage
parts.
2. Production of contoured panels for truck
trailer and bus bodies in automobile industry.

FORMABILITY OF SHEET METAL
Generally, the sheet metal is specified in terms
of gauge and mechanical properties.
The behaviour of sheet metal may be
altered in actual working conditions.
The reason behind this alteration of its
behaviour is mainly material variables and
process variables.
So, we can simply say the formality of sheet
metal is a function of material variables and
process variables.

Formability = f(f1,f2)
Where, f1 = Material variables and f2 = Process
variables
Material variables are nothing but the basic
variables that governing ductility of material.
Process variables are applied stress system,
interface friction, lubrication at the interface,
die-design, production shape etc.
A few universal laws exist which govern the
material properties or variables.

Law 1: Process of fracturing:
The ductility of the same material is lower if
the section size is larger.
Law 2: Law of geometrical similitude:
This law is applicable to fabrication of sheet and
strip expressed by few statements.
1. Blanks are geometrically similar in all
aspects with respect to another blank such as
dimension, thickness, width, length etc. These
geometrically similar blanks should be fabricated
by using similar tools.

2. Unit strains at corresponding locations are
identical for geometrically similar blanks.
3. The forces required to form any required
shape on geometrically similar blanks are
directly proportional to the square of the
thickness.
4. The consumption of work for forming the
required shape is also proportional to the cube
of its thickness for geometrically similar blanks.

TEST METHODS
1. Formability tests for bulk deformation.
2. Formability test for elastic-plastic
deformation
3. Simulative tests for forming operation
4. Full scale forming tests.

1. Formability Tests for bulk Deformation:
Bulk deformation refers the deformation of
sheet metal due to both elastic and plastic
deformation.
Sometimes, the elastic deformation may be
ignored.
Elastic deformation is always less than 0.2.
Plastic deformation
This ratio is valid only for forging, extrusion
and rolling process.

This test includes the following parameters.
1. Stress-strain characteristics under actual
working conditions.
2. Process-economic analysis.
3. Full scale experiments.
2.Formability Tests for Elastic-Plastic
Deformation:
To predict the forming behavior of sheet metal,
the following three tests are carried out.
1.Test-methods based on tensile test.
2. Simulative drawing tests, and
3. Full scale forming tests.

1.Test-methods based on tensile test:
General metal forming operations are
stretching and drawing.
Tensile test is carried out seperately for
stretching and drawing operations.
(i) Tensile test for stretch forming operation:
Fracture of sheet metal is predicted by local
thinning.
At the same time, failure is avoided.
The important property of work-hardening is
predicted in terms of stress-strain.

f = Aԑn
Where, f = stress
ԑ = Strain
A and n are constants.
For higher value of n, the strain distribution
will be more uniform and possibility to obtain
deep pressing.
If friction between the blank and tool is high,
the critical zone will be restricted.
Generally n varies from 0.22 to 0.24.

(ii) Tensile test for drawing operations:
In this test, the sheet metal is deformed in the
lower punch by thinning under bi-axial stresses.
Then the average value of sheet metal radius is
determined by orienting the axis of the metal
flow at 0º at 45º and 90º.
Mean value of radius, rm = 1/4 r0 + 2r45+ r90
Higher value of rm indiacates good drawability
which varies from 1.0 to 1.7.

2. Simulative test:
This test is conducted in various cup forming
operations.
They are
1. Erichson test
2. Olsen test
3. Surift test
4. Fukui test

(i) Erichsen test:
First the standard specimen of 90mm wide is
clamped rigidly against a die having 27mm
diameter opening.
A spherical punch of 20mm diameter is moved
against the sheet metal.
The movement of the punch continues until the
fracture starts.
At fracture the bulge forms.
The depth of bulge gives Erichsen number.
This value is very small in assessing drawability.
This test is mainly carried in assessing
stretchability of sheet metal.

(ii) Olsen test:
This test slightly varies from Erichsen test.
The size of the standard specimen and rest
are taken same as mentioned in Erichsen test.
But the die opening size of 50mm daimeter is
used.
Here, the specimen is clamped lightly.
The bulge and fracture are formed due to
stretching only when the punch presses the
sheet metal.

(iii) Swift test:
Flat bottom med cups of uniform diameter are
formed from a series of metallic sheet blanks.
But these metallic blanks are different diameters.
This process is continued until a blank size is
found which all cups fracture.
Then, the limiting drawing ratio is defined as
LDR = Blank diameter
Punch diameter
This test is carried out for assessing drawability.
The main disadvantage of this test is edge
wrinkling due to lightly clamped blanks.

(iv) Fukui test:
In this test, both stretchability and drawability
can be assessed.
Here, both the die and punch are in the form of
conical shape.
The fixed diameter blank is drawn between die
and punch.
The holding pressure on blank is gradually
increased until the edge wrinkling is completely
eliminated.
Then the cup depth is measured at maximum
load referred as formability index.
This index is used to assess both the drawability
and stretchability.

3. Full scale forming test:
In this test, the forming limit diagram is
obtained to describe the different strain
conditions and their combinations with load to
failure of sheet metal.
Usually, the strain distribution is assessed
from the surface.
But, the magnitude of strain is determined by
impregnating the sheet metal with a grid
pattern or concentric circle followed by pressing.
During pressing of sheet metal, concentric
circles are stretched into elliptical shape.

Actual strain on the sheet metal is determined as
ԑ = (l-d) / d
Where, l = Length of major or minor axes
d = Corresponding concentric circle
diameter
Maximum surface strain, ԑ1 = Length of
major axis
Minimum surface strain, ԑ2 = Length of
minor axis.

The graph is plotted between major and
minor surface strains.
In this graph, the left hand side states ԑ2 in –ve
which indicates the combined tension and
compression of deep drawing operations.
The right hand side refers the minor surface
strain ԑ2 in +ve which indicates the bi-axial
tension of stretching operations.

Applications of forming limit diagram:
1.The new set of tools is easy, hard or
impossible to work can be easily determined.
2. Good materials used in forming operations
are identified.
3. Location of source of trouble is also easy
from a reference pressing by the designer.

Special forming processes:
 Generally, forming process is done by
pressing the form tool over the blank to
obtain the required shape.
The form tool is actuated by hydraulic
cylinder using hydraulic fluid.
In the case of mating die method, sheet
metal is placed over the lower die and
its ends are fixed on movable grippers.

Then, the upper die is moved towards
the blank.
If the female or upper die is actuated by
any other means except hydraulic fluid
contained in the cylinder in forming
process called special forming process.
Example: Explosive forming, metal
spinning, hydro forming etc.

Types of special forming process:
There are various types special
forming process as follows:
1. Hydro forming
2. Rubber pad forming
3. Metal spinning
4. Explosive forming
5. Magnetic pulse forming
6. Peen forming
7. Super plastic forming

1. Hydro forming Process:
 Hydro forming is a drawing
process.
 It is forming process is carried
out in two ways.
They are
(i) Hydro mechanical forming and
(ii) Electro hydraulic forming

(i) Hydro mechanical forming:
 In this type of forming process, the
punch is connected to the lower die
called male die.
 The required shape of inner
configuration is made on the punch.
 A rubber diaphragm or seal is used for
making perfect sealing between the male
and female die.
 This seal is placed across the bottom of
the pressure forming chamber.
 The pressure – forming chamber is filled
with a hydraulic fluid.

 Then, the blank is correctly positioned over
male die or lower die.
 Now, the pressure forming chamber called
dome is lowered over the blank in such a way
that the dome is made to just contact with
the blank.
 After this, hydraulic pressure is applied over
the blank.
 Simultaneously, the punch is pushed into the
blank.
 The pressure applied by the hydraulic fluid
is increased continuously.
 Due to this, the blank metal flows around
the punch to form the required shape.

 The inverted shape of the punch is made on the
blank. (Convex shape of the punch is made as
concave shape on the blank and vice versa).
 After forming the required shape, the chamber
pressure is released.
 Then the chamber is raised from the blank.
 Finally, the blank is stripped out from the
punch.
 In this case, the required shape of the blank is
obtained only by drawing rather than by
bending.
 And also, the blank metal is displaced due to
plastic flow instead of stretching.

Advantages:
 Thinning of metal, spot stresses and
spring back are drastically reduced or
completely eliminated.
 It is used for mass production because
work performed per operation is high.
 Tool changing can be done rapidly.
 Complicated contours can also be
made.

 Sharp corners are also possible.
 All types of sheet metals can be
handled.
 Due to uniform flow metal between
punch and pressure chamber, the
mechanical and physical
properties are improved.
 Tolerance of 0.005 mm/mm are
possible practically.

(ii) Electro hydraulic forming process:
 The working principle of metal
forming process is same as that of
hydro mechanical forming process.
 But, the applied pressure over the
blank differs because the pressure inside
the pressure forming chamber is
produced by electrical means.
 The arrangement of this electro hydraulic
forming system is shown in figure.

 When the supply is given to electrical
circuit, a high energy is discharged
through a bank of capacitor to the
hydraulic fluid contained pin the
chamber.
 The discharged energy in the chamber is
in the form of shock waves and
pressure.
 This mechanical energy is used for
metal forming operations in the same
manner as mentioned in hydro
mechanical forming operations.

Advantages:
 The pressure inside the chamber
is high due to combined shock
wave and fluid pressure.
 Time required per operation is
low when compared to hydro
mechanical forming operations.

Disadvantages:
 Energy losses occur between
electrical components to hydraulic
fluid.
 Due to shock waves drag force and
lift force is created and finally it
results stagnation pressure in the
fluid.
 Stagnation properties refer to the
properties at zero velocity.

2. Rubber pad forming Process:
 Rubber pad forming process is also called
as mar form process.
 This process is mainly used for bending
and stretching or drawing operations.
 This process is also preferred for
quantities of different shaped machine
parts needed at regular intervals.
 For this, the number of different form
blocks called punches is arranged at
regular intervals along the pressing bed
called rubber pad.

 The pad is made of rubber or polyurethane.
 This rubber pad is placed in a ram of a press.
 The force applied on the blank by hydraulic
cylinder through the ram and rubber pad.
 First, the blank is placed over the punch
called male die.
 Then, the upper platen called female part is
moved to just touch the top surface of the
work.
 After this, the force is applied and gradually
increased on the blank through the rubber pad.

 The blank holder ring is used to distribute
uniform pressure throughout the blank.
 Thus, the required shape is formed on the
sheet metal between male and female parts.
 The retainers are placed on both sides of the
rubber pad.
 The function of retainer is to apply
essential hydrostatic pressure on the blank
and prevents sideward motion.
 Then, the rubber pad is released by moving
the ram upwards.
 Now, the completed shell is stripped out
from the punch.

Advantages:
 Process is more economical.
 Tooling cost is less.
 Many required shapes can be performed in
one rubber pad itself.
 There is no need of lubricants.
 No thinning metal blank takes place.
 Tool setting time is less.
 Deeper shells can be drawn.
 Parts produced are wrinkle – free, shrink
flanges and improved shallow shapes.

Limitations:
 Rubber pads will wear out rapidly.
 Sharp corners cannot be made accurately.
Applications:
 Production of flanged cylindrical and
rectangular cups.
 Production of spherical domes.
 Production of parallel and tapered wall
shells.
 Production of unsymmetrical shape
components.

3. Rubber hydro forming Process:
 In this process, the forced applied on
the blank through a pressurized
liquid behind the rubber pad.
 This force is used to form the sheet
metal into the required shape.
 Here, the rubber pad acts as a seal
between pressure forming chamber
and blank.
 Due to application of hydrostatic
pressure over the blank is formed
into required shape.

 In hydro forming process, the hydraulic
pressure energy is directly applied over
the surface of the blank.
 In the case of rubber pad forming
process, the pressure is applied over the
surface of the blank by the rubber pad
which is operated by hydraulic ram.
 But in this case, the force is applied over
the blank in the form of hydraulic
pressure but through the rubber pad.

Advantages:
 Hammering action of hydraulic
fluid over the blank is avoided.
Disadvantages:
 Pressure loss occurs between
hydraulic fluid to rubber pad.

4. Metal Spinning Process:
 The process of forming seamless
metal parts from a circular sheet
metal or from a tube length on a
lathe is called as spinning process.
 Only symmetrical shapes can be
produced from metal spinning
process.

Methods of spinning process:
There are two methods used
to produce sheet metal parts.
(i) Manual spinning
(ii) Power spinning

(i) Manual spinning:
First, the circular blank is centered on a
lathe which is placed against a form block.
The form block is mounted on the head
stock of the spinning lathe.
The blank is tightly held between form
block and tail stock spindle.
The required contour surface is made on
the form block.
The pressure is applied by the roller type
forming tool which is placed on the tool
post of the spinning lathe.

The required shape is gradually formed by
continuous application of pressure by the
roller.
During spinning process, some stretching and
thinning of material take place.
Metal spinning can be done both in cold and
hot states.
Heat generation due to friction between
spinning tool or roller type forming and blank
can also be used to retain the plastic state of
sheet metal.

Spinning speed various with size, design, type
of metal and thickness of sheet metal.
Aluminum copper, bras and stainless steel
can also be spun in spinning process.
This process is mainly suitable for producing
conical shape parts and suitable for low
volume production.
Components produced in this process do not
require any trimming or beading operations.

For producing more complex shapes,
segmental chucks made from cast
aluminium, magnesium alloys or hard wood
reinforced with clod rolled steel sheets are
used.
The lubricants of grease, linseed oil, and
bees wax are used while using bead and
tallow between form tool and blanks during
spinning process.

Advantages:
 The parts not be drawn by drawing
operations can be easily spun.
 Heat generated due to friction is
used to retain the sheet metal in
the plastic state.
 The process is more economical for
low volume production.

Disadvantages:
 Thinning takes place during
spinning process.
 More complex shapes require
segmental chucks. Finally, it leads
to increase in cost.
 Accuracy and quality of finished
products mainly depend on the skill
of the operator.

(ii) Power spinning process:
The quality of finished products mainly
depends on the skill and experience of
the operator.
The accuracy of the finished products is
also less in manual spinning process.
Even though segmental chucks are used
for making complex shapes, the metal
thickness and contour shapes are
restricted in manual spinning.

In order to ensure skill of the operator and
reduced machining time, spinning
machines are referred as power spinning.
In power spinning process, the action of
form tool is controlled either by any
tracer mechanism or by Numerical
Controls(NC).
Now a days, computer controlled
numerical machines are used to change
the shape of contour whenever the existing
program needs to be changed.

HMT lathe is semiautomatic spinning
and flow turning machine with a
hydraulic copying attachment.
The spinning of sheet metal is done
automatically but the loading and
unloading of sheet metal are done
manually.
This HMT lathe also consists of head
stock, tail stock and slides mounted on
the bed.

Tail stock is operated hydraulically.
The blank is held between the
tailstock and the form block.
Then, the roller is pressed over the
blank to obtain the required shape.
In one pass or in successive passes
of roller, the required shape is
formed.

Advantages of power spinning:
 Time taken to spin the sheet metal
is greatly reduced.
 Thickness more than 1 mm can be
handled easily.
 Both accuracy and quality can be
maintained.
 It is suitable for high volume
production.

Applications of metal spinning process:
Production of ash trays, flower
pots, lamp shades, missile and
radar units, jet plane components
tanks, air conditioning units and
heating plants.

5. Explosive forming process:
 Explosive forming process is used
for blanking, cutting, expanding,
coining, embossing, flanging,
powder compacting, drawing and
sizing operations etc.
 Explosives are used in various
forms such as rod, sheet granules,
liquid, stick etc.

 According to the placement of
explosive, the operations can be
divided into two categories.
(i) Stand – off operations
(ii) Contact operations

(i) Stand – off operations
 In this case, the explosive charge
is located at some distance away
from the blank and it energy is
transmitted through some fluid
medium such as water.
 This technique is used to form
blanks into various shape except
welding, hardening, compacting,
cutting process.

(ii) Contact operations
 In this case, the explosive charge is
directly located over the blank.
 This operation is mainly used for
welding, hardening, compacting and
cutting process.
 So, the sheet metal is formed in
stand-off operation method.
 In this process, the forming of sheet
metal is done by generating pressure
wave in a fluid.

 The pressure wave is generated by
detonating the enough quantity of
explosives.
 The blank being formed is placed
against the female die.
 The female die has the required
configurations.
 This entire set up of female die and
blank is placed inside the work tank.
 The work tank contains water to
receive vibrations in the form of
pressure wave.

The applied pressure by this process may vary
from several hundred to thousands of kg/cm2 with
several hundred m/s displacement velocity.
At any given distance from the explosive charge
center, the high intensity portion of the pressure
pulse generated by the detonating explosive can be
approximately represented by expression.
P = Pm e-t/ϴ  (1)
Where, P is the pressure as a function of time.
Pm is peak pressure at that distance
t is the time after arrival of pressure front at
the blank surface.
ϴ is time constant characteristic of the
charge weight, type of explosive, and distance from the
charge.

 One same conduit is filled with charge or
explosives which are also placed inside the
work tank.
 The work tank should be perfectly
insulated to avoid heat transfer from system
to surrounding.
 Now, the explosive charge is ignited by the
detonator.
 Due to this, a high-pressure energy is
released in the form of waves and pulse
(Vibration) on the water contained in the
work tank.
 This pressure waves are applied over the
blank to obtain the required shape of female
die.

The time constant represents the time that it
takes the pressure to fall to 50% of its peak
value, and it defines the limit of the straight line
portion of a pressure- time curve.
Equation (1) is applicable only along the
straight-line portion of the curve or out to a
point where P is equal to about 30% of Pm.

Advantages:
1.Less capital investment
2. Presses are not required
3. Only one die is enough to form the sheet
metal.
4. Required shapes of components are formed in
one stroke.
5. Ultimate and yield strength of sheet metals
are improved.
6. Large and complex shapes can also be
handled.

Disadvantages:
1. Highly trained operators are needed.
2. Noisy operation.
Applications:
This process is mainly used for producing
aerospace components.

MAGNETIC PULSE FORMING
The required shape of sheet metal is obtained
by specially designed magnetic coil.
The basic principle is that discharging of a
capacitor through a coil over a period of
microseconds, on the blank to obtain the
required shape.
During this, the magnetic flux densities of the
order of hundreds of kilogauss can be
produced.

The basic circuit consists an energy storage
capacitor, a switch, a coil and a power source.
The current through the coil produces a
high intensity magnetic field between the coil
and the work piece.
Due to the eddy current in the blank, the
magnetic field is restricted over the surface
of blank.
Due to the interaction of the eddy current on
the blank, the applied magnetic field creates
an inward force on the workpiece.

The repelling force between work piece and
the coil is high.
Since, the coil is placed rigidity.
So, the blank is repelled and forced against a
coil.
Properly designed coils are used for
compression and certain regions is shaped by
utilizing massive conducting structure as
flux concentrator device and not connecting it
directly to the basic coil.
This process is most suitable for copper,
aluminium, Silver and gold.

Advantages:
1. This process is carried out with uniform rate
of forming.
2. It is also better process than convention
process for certain materials.
3. The surface finish of the process is excellent.
4. Time of operation is less as compared to
conventional process.

Disadvantages:
1. Non-Conducting materials are not processed
without aid of conducting materials.
2. It is limited for sheet metal forming not an
forming bulk material.
Applications:
1. Both compression and expansion of circular
bar can be carried out.
2. Producing bulging of tube, shrinkage of tube,
attaching tubes at end fitting without leaking
are possible.

3. Forming a torque joints, forging of structural
joints between tubes and fitting are easily formed.
4. It is used for instrument gear assembly,
embossing and sizing of cups etc.,

PEEN FORMING
Peen forming is a process of well-established
surface cleaning.
In this process, a stream of metal shots is
blasted against the surface of the blank to be
made into required shape.
This technique is also known as free-forming
technique.
A stream of small steel ball is suddenly
forced with very high velocity against the
surface of the blank.

This process is used to form various
irregular contour surfaces of aluminium
sheet and plates.
The length of the contour of the blank to be
formed may be larger.
According to the direct shape of contour, the
sheet metal or blank is clamped over form
blocks during peening operation.

Advantages:
1. Complex contours can be produced easily.
2. Peening is also used as salvage operations for
correcting bent or disorted parts.
3. This process does not require any die and
punch.
Disadvantages:
1. Requires longer time for forming.
2. Requires additional devices for forcing out
metal shots.

Applications:
1. This process is used in producing specific
portions on crankshafts, connecting rods
and gears.
2. It is used for producing honeycomb panels
like aircraft wings and large tubular shapes.

SUPER-PLASTIC FORMING PROCESS
Super plastic forming (SPF) is a valuable tool
for the fabrication of complex parts used in
the aircraft and automobile industries.
Super plastic forming (SPF) of sheet metal has
been used to produce very complex shapes
and integrated structures that are often
lighter and stronger than the assemblies they
replace.
Super-plasticity in metals is defined by very
high tensile elongations, ranging from two
hundred to several thousand percent.

Super-plasticity is the ability of certain
materials to undergo extreme elongation at
the proper temperature and strain rate.
The process typically conducted at high
temperature and under controlled strain
rate, can give a ten-fold increase in elongation
compared to conventional room temperature
processes.
Components are formed by applying gas
pressure between one or more sheets and a
die surface, causing the sheets to stretch and
fill the die cavity.

The evolution of pressures must be closely
controlled during the process since the alloys of
interest only exhibits Super plastic behaviour for
certain temperature dependent range of strain
rates.
Specific alloys of titanium, stainless steel, and
aluminum are commercially available with
the fine-grained microstructure and strain rate
sensitivity of flow stress that are necessary for
Super plastic deformation.

Process:
SPF can produce parts that are impossible to form
using conventional techniques.
During the SPF process, the material is heated to
the SPF temperature within a Sealed die.
Inert gas pressure is then applied, at a controlled
rate forcing the material to take the shape of the die
pattern.
The flow stress of the material during deformation
increases rapidly with increasing strain rate.
Super plastic alloys can be stretched at higher
temperatures by several times of their initial
length without breaking.

Some of the materials developed for super
plastic forming are:
1. Bismuth-tin (200% elongation)
2. Zinc- aluminium
3. Titanium (Ti-6Al-V)
4. Aluminium (2004, 2419, 7475)
5. Aluminium- lithium alloys (2090, 2091, 8090)

Super- Plastic Forming-Process:
This method consists in hot forming up to 1000º C
super plastic alloy by using an inert gas
pressured up to 50 bars.
Combined with diffusion bonding, this process
allows honeycomb structures made of several
sheets in a single operation.
Loading:
The blank is loaded in the form die.
The hot press heats the die and the blank to the
material super-plastic temperature.

Forming:
Once the temperature is reached, it is accurately
controlled, while the gas pressure slowly
inflates the blank.
The gas keeps inflating the part to fit the die.
The material at the super-plastic temperature can
allow up to 500% elongation.

Release:
At the end of the forming cycle, the part
perfectly conforms to the die, even in its
smallest details.

Advantages of SPF process:
Super plastic forming technology offers the
potential to reduce the weight and cost of
automotive structural components for advance
vehicle applications.
The main advantages of this process are:
1.It is a one step process.
2.The process can be used to form complex
components in shapes that are very near the final
dimensions.
3.Higher material elongations.

4.Eliminations of unnecessary joints and rivets.
5.Reduction of subsequent machining.
6.Minimizes the amount of scrap produced.
Applications:
The process is increasingly being applied in the
aerospace industry as a way of manufacturing
very complex geometries.
(i) In automotive body panels.
(ii) In forming of aircraft frames and skins.
(iii)Diaphragm forming of plastics.
(iv) Complex shape parts- window frames, seat
structures.

MICRO FORMING IN SHEET METAL
PROCESSES
It is well known that the sheet metal thickness is
between 0.4 and 6 mm but while micro-sheet
forming usually handles the sheet metals of
which the thickness is less than 0.3 mm.
Therefore, it is called as thin strips or coils.
The major sheet processes in micro sheet forming
are shearing, cutting, bending, unbending,
stretching, compressing, stress relaxation etc.,

Similar to conventional sheet metal, the
mechanical properties of the materials such as
elasticity, plasticity, stress strain relations,
strain rate, work hardening, temperature
effect, anisotropy, grain size and residual stress
involve in analyzing the deformation of micro-
forming products.
The effects of grains sizes, orientations and
grain boundary properties are more
significant in micro-sheet forming while
considering the effects of overall stress-strain
relationships, sheared-section qualities, spring
back phenomenon, stress relaxation, etc.

Generally, the micro forming processes are used to
make parts of the following:
 Cellular Telephones
 IC Lead frames
 Electronics
 Healthcare
 Miniature Fasteners
 Hard Disc Drivers
 National Security and Defense
Automobiles
 Sensors

Sheet metal components are mainly used in
various applications such as vehicles, aircrafts,
electronic products, medical implants and
packaging for consuming goods, car panels, aircrafts
skins, cans for food and drinks and frames of: TV,
computer screens, monitors and displays, etc.
Especially, micro-formed components are used in
high precision applications such as electrical
connectors and lead frames, micro-meshes for
masks and optical devices, micro springs for micro
switches, micro-cups for electron guns and micro-
packaging, micro laminates for micro-motor and
fluidic devices, micro gears for micro mechanical
devices, casings for micro-device assembly, micro
knives for surgery etc.,

Unit 4-SHEET METAL PROCESSES

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Unit 4-SHEET METAL PROCESSES