SHEET METAL PROCESSES

RAMCO INSTITUTE OF TECHNOLOGY
Mr.M.LAKSHMANAN
Assistant Professor (Senior Grade)
Department of Mechanical Engineering
RIT - M.Lakshmanan, AP (S.G)/MECH 1

UNIT IV
SHEET METAL PROCESSES

Syllabus
Sheet metal characteristics – shearing,
bending and drawing operations – Stretch
forming operations – Formability of sheet
metal – Test methods –special forming
processes-Working principle and
applications – Hydro forming – Rubber pad
forming – Metal spinning– Introduction of
Explosive forming, magnetic pulse forming,
peen forming, Super plastic forming – Micro
forming.

Introduction
Sheet Metal
Introduction
◼ Sheet metal is a metal formed into thin and flat pieces. It is
one of the fundamental forms used in metalworking, and
can be cut and bent into a variety of different shapes.
◼ Countless everyday objects are constructed by this
material. Thicknesses can vary significantly, although
extremely thin sheets are considered as foil or leaf, and
sheets thicker than 6 mm (0.25 in) are considered as plate.

Sheet Metal Processing
▪ The raw material for sheet metal manufacturing
processes is the output of the rolling process.
▪ Typically, sheets of metal are sold as flat,
rectangular sheets of standard size.
▪ If the sheets are thin and very long, they may be
in the form of rolls.
Therefore the first step in any sheet metal process
is to cut the correct shape and sized blank from
larger sheet.

Sheet Metal Working
▪ Performing Cutting and forming operations on
relatively thin sheets of metal
▪ Thickness of sheet metal = 0.4 mm to 6 mm
▪ Thickness of plate stock > 6 mm
▪ Operations usually performed as cold working

Sheet Metal Forming
Sheet metal forming is a process that
materials undergo permanent deformation by
cold forming to produce a variety of complex
three dimensional shapes.
The process is carried out in the plane of sheet
by tensile forces with high ratio of surface
area to thickness.

Sheet and plate metal parts for consumer and
industrial products such as
➢ Automobiles and trucks
➢ Airplanes
➢ Railway cars and locomotives
➢ Farm and construction equipment
➢ Small and large appliances
➢ Office furniture
➢ Computers and office equipment

Advantages of sheet Metal
• High strength
• Good dimensional accuracy
• Good surface finish
• Relatively low cost
• Economical mass production for large
quantities

Applications of sheet metals
• Aircraft Bodies
• Automobiles bodies
• Domestic purposes
• Beverage cans

Sheet Metal operations
Introduction
◼ Sheet metal formingis a grouping of many
complementary processes that are used to form sheet
metal parts.
◼ One or more of these processes is used to take a flat sheet of
ductile metal, and mechanically apply deformation forces
that alter the shape of the material. Before deciding on the
processes, one should determine whether a particular sheet
metal can be formed into the desired shape without failure.
◼ The sheet metal operations done on a press may be grouped
into two categories, cutting (shearing) operations and
forming operations.

◼ The art of sheet metal lies in the making of different
shapes by adopting different operations. The major types
of operations are given below
❑ Shearing (Cutting)
❑ Bending
❑ Drawing
❑ Squeezing

◼ Shearing
❑ Cutting to separate large sheets; or cut part
perimeters or make holes in sheets
◼ Bending
❑ Straining sheet around a straight axis
◼ Drawing
❑ Forming of sheet into convex or concave shapes
◼ Squeezing
❑ Forming of sheet by gripping and pressing firmly –
Coining & Embossing

Sheet Metal Characteristics
• Sheet metal is characterized by high ratio of
surface area to thickness.
• Forming is generally carried out in tensile
forces
• Decrease thickness should be avoided as far as
possible as they can lead to necking and failure.
• The major factors that contribute significantly
include elongation, anisotropy, grain size,
residual stresses, spring back, and
wrinkling.

Metal characteristics affecting sheet metal
processing
• Strength
• Elongation
• Anisotropy
• Grain size
• Spring back
• wrinkling
• Residual stresses
• Surface condition of the sheets

1. Strength:
The strength of a sheet metal is the property of
resistance to external loads or stresses while not
causing the structural damage.
❖ Tensile Strength
Its ability of a metal to resist being pulled apart by
opposing forces acting in a straight line.
❖ Shear Strength
Its ability of a material to resist being fractured by
opposing forces acting in a straight line but not in
the same plane.
❖ Compressive Strength
Its ability of a material to withstand to pressures
acting on a given plane.

2. Elongation:
It’s the capability of the sheet metal to stretch without necking
and failure. A specimen subjected to tension undergoes
uniform elongation. When the load is exceeded, the ultimate
tensile strength the specimen begins to neck.
A test to measure the ductility of a material. When a material is
tested for tensile strength it elongates a certain amount
before fracture takes place.
Materials like low carbon steels exhibit a behavior called yield
point elongation, exhibiting upper yield and lower yield points.
Higher elongation leads to Lueder’s band (strain marks)
To avoid this reduce yield point elongation by reducing the
thickness of sheet by 0.5% to 1.5% by cold rolling, known as
Temper rolling.

LUEDER’S BAND or STRAIN MARKS

3. Elasticity:
It’s the ability of material to return to its original size,
shape and dimensions after being deformed. Also
the property of regaining the original dimensions
upon removal of the external load is known as
elasticity.
4. Modulus of Elasticity:
It’s the ratio of the internal stress to the strain
produced. It expresses the stiffness of a material.

5. Ductility
It’s the capacity of a material to be drawn or stretched
under tension loading and permanently deformed
without rupture or fracture.
6. Malleability:
It’s the property of a metal to be deformed or
compressed permanently without rupture or
fracture.
7. Plasticity:
It’s the ability of a metal to be deformed extensively
without rupture.

8. Toughness:
It’s a combination of high strength and medium
ductility. It’s a ability of material or metal to resist
the fracture after the damage has begun.
9. Hardness:
Its ability of a material to resist penetration and wear
by another material.
10. Brittleness:
It’s the property of breaking the material without
visible permanent deformation. It’s the reverse of
toughness.

11. Corrosion Resistance:
It’s the resistance to eating away or wearing by the
atmosphere, moisture or other agents such as acid.
12. Anisotropy:
It is the property of being directionally dependent,
which implies different properties in different
directions, as opposed to isotropy. Its acquired
during the thermo mechanical processing.
❖ Crystallographic anisotropy (Grain orientation)
❖ Mechanical fibering (Alignment of impurities,
inclusions and voids)

13. Springback:
The tendency of the metal that tries to resume its
original position causing a decrease in bend
angle is known as springback.
It varies from 0.50 to 50 for steel. Greater
springback is caused by a larger bend radius.
It depends on the following factors:
• Material type
• Thickness
• Hardness
• Bend Radius

14. Wrinkling:
One of the primary defects that occurs in deep
drawing operations is the wrinkling of sheet
metal material, generally in the wall or flange of
the part.
The flange of the blank undergoes radial drawing
stress and tangential compressive stress during
the stamping process.

15. Grain size:
It determines the surface roughness on a stretched
sheet metal. It affects both mechanical properties
and surface appearance. Smaller grain size will
be stronger metal.
16. Residual Stress:
It can develop in sheet metal forming due to non
uniform deformation that take place. When
disturbed such as removing a portion of it, the
part may distort.
Tensile residual stresses can lead to stress
corrosion cracking of the part unless it is
properly relieved.

Sheet Metal Processes
Processes involving shear stress
• Blanking
• Piercing
• Shaving
• Notching
• Punching

• Processes involving compressive stresses
– Coining
– Sizing
– Ironing
– Stamping
• Processes involving tensile stresses
– Stretch forming
• Processes involving both tensile and compressive
stresses
– Spinning
– Drawing
– Bending, forming

Shearing Process
Shearing is the process of cutting sheet metal
strip. The shearing action takes place in a
sheet metal.
Cutting processes are those in which a piece of
sheet metal is separated by applying a great
enough force to caused the material to fail.
The most common cutting processes are
performed by applying a shearing force, and
are therefore sometimes referred to as
shearing processes.

Shearing Process

Cutting Processes:
• Shearing - Separating material into two parts
• Blanking - Removing material to use for parts
▪ Conventional blanking
▪ Fine blanking
• Punching - Removing material as scrap
▪ Piercing
▪ Slotting
▪ Perforating
▪ Notching
▪ Nibbling
▪ Lancing
▪ Slitting
▪ Parting
▪ Cutoff
▪ Trimming
▪ Shaving
▪ Dinking RIT - M.Lakshmanan, AP (S.G)/MECH 31

Shearing
• Sheet thickness: 0.005-0.25 inches
• Tolerance: ±0.1 inches (±0.005 inches feasible)
• Surface finish: 250-1000 μin (125-2000 μin feasible)

Blanking
During which a metal work piece is removed
from the primary metal strip or sheet when it
is punched.
Blanking is a cutting process in which a piece
of sheet metal is removed from a larger piece
of stock by applying a great enough shearing
force. In this process, the piece removed,
called the blank, is not scrap but rather the
desired part.

Fine blanking
Fine blanking is a specialized type of blanking
in which the blank is sheared from the sheet
stock by applying 3 separate forces.
This technique produces a part with better
flatness, a smoother edge with minimal burrs,
and tolerances as tight as ±0.0003.
As a result, high quality parts can be blanked
that do not require any secondary operations.

Punching
Punching is a cutting process in which material is
removed from a piece of sheet metal by applying a
great enough shearing force.
Punching is very similar to blanking except that the
removed material, called the slug, is scrap and
leaves behind the desired internal feature in the
sheet, such as a hole or slot.
Punching can be used to produce holes and cutouts
of various shapes and sizes. The most common
punched holes are simple geometric shapes (circle,
square, rectangle, etc.)

Punching

Piercing
The typical punching operation, in which a
cylindrical punch pierces a hole into the sheet.

Slotting
A punching operation that forms rectangular
holes in the sheet. Sometimes described as
piercing despite the different shape.

Perforating
Punching a close arrangement of a large number
of holes in a single operation.

Notching
Punching the edge of a sheet, forming a notch in
the shape of a portion of the punch.

Nibbling
Punching a series of small overlapping slits or
holes along a path to cutout a larger contoured
shape. This eliminates the need for a custom
punch and die but will require secondary
operations to improve the accuracy and finish of
the feature.

Lancing
Creating a partial cut in the sheet, so that no
material is removed. The material is left attached
to be bent and form a shape, such as a tab, vent, or
louver.

Slitting
Cutting straight lines in the sheet. No scrap
material is produced.

Parting
Separating a part from the remaining sheet, by
punching away the material between parts.

Cutoff
Separating a part from the remaining sheet,
without producing any scrap. The punch will
produce a cut line that may be straight, angled, or
curved.

Trimming
Punching away excess material from the
perimeter of a part, such as trimming the flange
from a drawn cup.

Shaving
Shearing away minimal material from the edges
of a feature or part, using a small die clearance.
Used to improve accuracy or finish. Tolerances of
±0.001 inches are possible.

Dinking
A specialized form of piercing used for punching
soft metals. A hollow punch, called a dinking die,
with beveled, sharpened edges presses the sheet
into a block of wood or soft metal.

BENDING

BENDING
Bending of sheet metal is a common and vital
process in manufacturing industry.
Sheet metal bending is the plastic
deformation of the work over an axis, creating
a change in the part's geometry. Similar to
other metal forming processes, bending
changes the shape of the work piece, while the
volume of material will remain the same.
In addition to creating a desired geometric
form, bending is also used to impart strength
and stiffness to sheet metal.

Bending Processes
1. ‘V’ Bending:
One of the most common types of sheet metal
manufacturing processes is V bending. The V shaped
punch forces the work into the V shaped die and hence
bends it. This type of process can bend both very acute
and very obtuse angles, also anything in between,
including 90 degrees.

2. Edge bending:
Edge bending is another very common sheet metal process and
is performed with a wiping die.
Edge bending gives a good mechanical advantage when forming
a bend. However, angles greater than 90 degrees will require
more complex equipment, capable of some horizontal force
delivery.
The punch then applies force to the cantilever beam section,
causing the work to bend over the edge of the die.

3. Rotary bending
Rotary bending forms the work by a similar mechanism
as edge bending. However, rotary bending uses a
different design than the wiping die.
A cylinder, with the desired angle cut out, serves as the
punch. The cylinder can rotate about one axis and is
securely constrained in all other degrees of motion by
its attachment to the saddle.
The sheet metal is placed cantilevered over the edge of
the lower die, similar to the setup in edge bending.
Unlike in edge bending, with rotary bending, there is no
pressure pad. Force is transmitted to the punch causing
it to close with the work.

The groove on the cylinder is dimensioned to
create the correctly angled bend. The groove
can be less than or greater than 90 degrees
allowing for a range of acute and obtuse
bends.

4. Air bending
Air bending is a simple method of creating a bend
without the need for lower die geometry. The sheet
metal is supported by two surfaces a certain distance
apart. A punch exerts force at the correct spot,
bending the sheet metal between the two surfaces.

5. ‘U’ and Channel Bending
Punch and die are manufactured with certain
geometries, in order to perform specific bends.
Channel bending uses a shaped punch and die to
form a sheet metal channel. U bend is made with a U
shaped punch of the correct curvature.

6. Offset Bending:
Many bending operations have been developed to
produce offsets and form the sheet metal for a
variety of different functions.

7. Roll Bending:
This process uses a three rollers set to bend a sheet
by adjusting the distance between rolls. This process
can utilize to form various curvatures.

Roll bending provides a technique that is useful for
relatively thick work. Although sheets of various
sizes and thicknesses may be used, this is a major
manufacturing process for the metal bending of large
pieces of plate.
Roll bending uses three rolls to feed and bend the
plate to the desired curvature.
The arrangement of the rolls determines the exact
bend of the work. Different curves are obtained by
controlling the distance and angle between the rolls.
A moveable roll provides the ability to control the
curve.

8. Four Slide machine Bending:
This process uses three movable and one stationary
slide to bent a work sheets as shown in figure. This is
used to bent small work pieces.

9. Beading:
It is a process in which the periphery of the sheet
metal is bent into the cavity of a die. It increases
moment of inertia of the section and stiffness. It also
eliminates exposed sharp edges.

Sheet metal beading processes produce a bead with
a single die. In a process called wiring, the metal's
edge is bent over a wire.

10. Flanging:
It is a process of bending the edges of the metal sheet
at perpendicular to the length. It can be further divided
according the shape like straight flange, stretch flange,
joggled flange, shrink flange etc. as shown in figure.

Flanging

11. Dimpling:
It is a process in which first a hole in made into the
sheet metal and then it is expanded into a flange
using punch die system.

12.Hemming:
It is an operation in which the edge of the sheet is
folded over itself. This process increases stiffness of
the part and eliminate sharp edges.

Bending Terminology

• Neutral Axis:
It is an imaginary axis which does not undergo
any stress during bending.
• Bend Allowance:
The length of the neutral axis in the bend zone
is known as bend allowance.
• Bend Angle:
The angle form by the bend area at the center
of bend is known as bend angle.
• Bend Radius:
Distance between bend center and neutral axis
is known as bend radius. It is denoted by r.

• Springback:
When load is removed, the sheet metal shows
some elastic recovery and tends to achieve its
original position. This phenomenon is called
springback. It will increase the final bend
radius and decrease the bend angle after
spingback.

Bend Allowance (BA)

Where,
• Lf = flat length of the sheet
• BA = bend allowance
• BD = bend deduction
• R = inside bend radius
• K = K-Factor, which is t / T
• T = material thickness
• t = distance from inside face to the neutral line
• A = bend angle in degrees (the angle through
which the material is bent)

Bend Deduction (BD)
The bend deduction BD is defined as the
difference between the sum of the flange lengths
(from the edge to the apex) and the initial flat
length.(OSSB-Outside Set Back)

The bend deduction (BD) is twice the outside
setback minus the bend allowance.
BD is calculated using the following formula,
where A is the angle in radians (=degrees*π/180)
K -Factor to the Bend Allowance;

Drawing/ Deep Drawing Operations
Deep drawing is a manufacturing process that
is used extensively in the forming of sheet
metal into cup or box like structures. Pots and
pans for cooking, containers, sinks, automobile
parts, such as panels and gas tanks, are among
a few of the items manufactured by sheet
metal deep drawing. This process is
sometimes called drawing.
For the primary sheet metal deep drawing
process the part will have a flat base and
straight sides.

Deep drawing of sheet metal is performed with a
punch and die. The punch is the desired shape of the
base of the part, once drawn. The die cavity matches
the punch and is a little wider to allow for its
passage, as well as clearance. This setup is similar to
sheet metal cutting operations.

The punch travels towards the blank. After
contacting the work, the punch forces the sheet
metal into the die cavity, forming its shape.

Equipment for sheet metal deep drawing processes
would involve a double action, one for the blank
holder and one for the punch. Both mechanical and
hydraulic presses are used in manufacturing industry.
Typically the hydraulic press can control the
blankholder and punch actions separately, but the
mechanical press is faster.
Punch and die materials, for the deep drawing of sheet
metal, are usually tool steels and iron. Parts are
usually drawn at speeds of 4 to 12 inches per second.

Drawing Ratio
Measurement of the amount of drawing performed
on a sheet metal blank can be quantified. This can be
done with the drawing ratio. The higher the drawing
ratio, the more extreme the amount of deep drawing.
Due to the geometry, forces, metal flow and material
properties of the work, there is a limit to the amount
of deep drawing that can be performed on a sheet
metal blank in a single operation.
Db is the diameter of the blank and
Dp is the diameter of the punch.
DR = Db/Dp

Reduction
Another way to express drawing ratio is the reduction
(r). Reduction is measured using the same variables
as drawing ratio. Reduction can be calculated by
r = (Db - Dp)/(Db)
Db and Dp being blank and punch diameters
respectively.
Percentage of reduction:
r = (Db - Dp)/(Db) X 100%
In this case the reduction should be 50% or
under.

Redrawing Sheet Metal
If required percent reduction of sheet metal is over
50%, the part must be formed in multiple operations.
Redrawing is the subsequent deep drawing of a work
that has already undergone a deep drawing process. By
using more than one operation, a greater magnitude of
deep drawing can be accomplished.
Initial reduction is usually 35% to 45%.
First redraw is commonly performed at a 20% to 30%
reduction.
Second redraw can typically range from 13% to 16%
reduction.

Redrawing

Reverse Redrawing
Containers or shells that are too difficult to draw in
one operation are generally redrawn.
In reverse redrawing, the metal is subjected to
bending in the direction opposite to its original
bending configuration.
This reversal in bending results in strain softening.
This operation requires lower forces than direct
redrawing and the material behaves in a more
ductile manner.

Reverse Redrawing

Stretch Forming
Stretch forming is a metal forming process in which a
piece of sheet metal is stretched and bent
simultaneously over a die in order to form large
contoured parts.
Stretch forming is performed on a stretch press, in
which a piece of sheet metal is securely gripped
along its edges by gripping jaws.
The gripping jaws are each attached to a carriage
that is pulled by pneumatic or hydraulic force to
stretch the sheet.

Types of Stretch Forming Process
❖Simple stretch forming
❖Tangential stretch forming
❖Stretch forming according to Cyril-Bath
❖Multi-sided stretch forming

Simple stretch forming

Tangential stretch forming

Cyril-Bath Process

Multi-sided stretch forming

Formability of Sheet Metal
Formability is the ability of a
given metal workpiece to undergo plastic
deformation without being damaged.
The plastic deformation capacity
of metallic materials, however, is limited to a
certain extent, at which point, the material could
experience tearing or fracture (breakage).
Processes affected by the formability of a material
include: deep drawing, cup drawing, bending etc.
involve extensive tensile deformation.

Formability = f(f1,f2)
Where,
f1= Material Variables
f2= Process variables
Formability Test Methods:
Simple uniaxial tensile test is not much useful for
the formability of sheet metals. Its due to the
biaxial or triaxial nature of stresses acting on the
sheet metal during forming operations.
1. Formability test for bulk deformation
2. Formability test for elastic –Plastic deformation
3. Simulative test for forming operation
4. Full scale forming tests

1. Formability test for bulk deformation
• Stress –strain characteristics under actual
working conditions
• Process Economic Analysis
• Full scale experiments

2. Formability test for elastic –Plastic
deformation
❖ Test methods based on tensile test:
– Tensile test for stretch forming operations:
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 = A*ɛn
Where,
f = Stress
ɛ = Strain
A and n are Constants (Values from 0.22 to 0.24)
–Tensile Test for Drawing Operations:
The sheet metal is deformed on 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 00 to 450
and 900.
Mean value of Radius, rm = ¼(r0+2r45+r90)1/2
Good drawability rm varies from 1.0 to 1.7.RIT - M.Lakshmanan, AP (S.G)/MECH 100

❖ Simulative Drawing tests
This test conducted in various cup forming
operations such as
1. Erichsen Test
2. Olsen Test
3. Swift Test
4. Fukui Test

Erichsen Test
• The standard specimen of 90mm wide is rigidly
clamped against a die having 27mm diameter
opening. A spherical punch of 20mm diameter is
moved against the sheet metal.
• The cup height at the fracture point is a measure of
the stretching ability.
• The maximum load point is measured during the
tests. It is hard to obtain repeatable data, because the
friction affects the results.

Olsen Test
The size of the standard specimen and rest are taken
same as mentioned in Erichsen test. But the die
opening size of 50mm diameter is used. This test is
also carried out for assessing the stretchability.

Swift Test
In this test, flat bottomed cups of uniform diameter
are formed from a series of metallic sheet blanks.
These metallic blanks are of different diameters. This
process is continued until the fracture occurs in all
cups.
Limiting Draw ratio, LDR = Blank Diameter
Punch Diameter

Fukui Test
The sheet is both drawn and stretched over a cup of
conical shape. So, the strechability and drawability
can be assessed. Both die and punch are in the form
of conical shape. The cup depth is measured at
maximum load which is referred as formability
index.

• The Fukui conical cup value is determined by the
ratio of the diameter of the base of the conical cup
formed and the diameter of the original specimen.

4. Full scale Forming Test
Forming Limit Diagram is obtained to describe the
different strain conditions and their
combinations with load to failure of sheet metal.
The strain distribution is assessed from the
surface.

Forming Limit Diagram (FLD)
A forming limit diagram, also known as a forming limit
curve, is used in sheet metal forming for predicting
forming behaviour of sheet metal. The diagram
attempts to provide a graphical description of
material failure tests, such as a punched dome test.
The mechanical test is performed by placing a circular
mark on the workpiece prior to deformation, and
then measuring the post-deformation ellipse that is
generated from the action on this circle.
Actual strain on the sheet metal
ɛ = (l-d)/d

Where,
l= Length of major or minor axes
d= Corresponding concentric circle
Max surface strain, ɛ1 = Length of major axis
Min Surface strain, ɛ2 = Length of Minor axis

Keeler – Goodwin Forming Limit Diagram

Special Forming Processes
❖Hydroforming
❖Rubber Pad Forming
❖Metal Spinning
❖Explosive Forming
❖Magnetic Pulse Forming
❖Peen Forming
❖Superplastic Forming

Hydroforming
Hydroforming is a drawing process. This process is
carried out by two ways.
➢Hydro-Mechanical Forming
➢Electro-Hydraulic Forming

Hydro-Mechanical Forming

The punch is connected to the lower die. The
required shape of inner configuration is made on
the punch. A Diaphragm or seal is used for making
perfect sealing between top and bottom die. The
pressure forming chamber is filled with a
hydraulic fluid. Then the blank is correctly
positioned over the top or lower die.
The required shape of the blank is obtained only by
drawing rather than by bending.

Electro-Hydraulic Forming

Electro-Hydraulic Forming (EHF)
Electrohydraulic forming is a type of metal
forming in which an electric arc discharge in
liquid is used to convert electrical
energy to mechanical energy and change the
shape of the workpiece.
A capacitor bank delivers a pulse of high current
across two electrodes, which are positioned a
short distance apart while submerged in a fluid
(water or oil).
The electric arc discharge rapidly vaporizes the
surrounding fluid creating a shock wave.
The workpiece, which is kept in contact with the
fluid, is deformed into an evacuated die.

Advantages of EHF:
• A single-step process
• Fine details and sharp lines can be easily formed
• Forming of negative and positive shapes
• Only a single one-sided die is required
• Enables extremely deep forming (much more than is
possible with conventional stamping)
• Even distribution and higher strength of thin material
• Extremely fast
• Equipment has small footprint
• No need for a press – the forming chamber is a self-balanced
system
• Allows forming of parts up to a few square meters in size

Rubber Pad Forming (RPF)
RPF is a metalworking process where sheet metal is
pressed between a die and a rubber block, made
of polyurethane. Under pressure, the rubber and
sheet metal are driven into the die and conform to its
shape, forming the part.
Rubber pad forming is a deep drawing technique that is
ideally suited for the production of small and
medium-sized series.
Deep drawing makes it possible to deform sheet metal
in two directions, which offers great benefits in terms
of function integration, weight reduction, cleanability.

Advantages:
• Short time to market through simple tools
• Low tooling costs
• Excellent for low and medium numbers
• With the use of rubber, a polished or sharpened surface
remains undamaged
• Suitable for steel, stainless steel, aluminum etc. up to a
thickness of about 4mm
Disadvantages:
• For very large numbers too laborable so too expensive.
• Somewhat less freedom in form compared to the regular
deep drawing process
• In most cases not suitable for sheet thicknesses greater
than 4mm.

Metal Spinning
Metal spinning, also known as spin
forming or spinning or metal turning most
commonly, is a metalworking process by which a disc
or tube of metal is rotated at high speed and formed
into an axially symmetric part. Spinning can be
performed by hand or by a CNC lathe.

1. Shear Spinning
Shear spinning is a process related to conventional
spinning and is also known as flow turning or spin
forging.
In a conventional spinning operation the work is
essentially formed by bending. There is usually not much
change in the thickness of the sheet metal.
The diameter of the work in conventional spinning must
be large enough to account for the size of the final part.
Shear spinning involves forming the work over the
mandrel, causing metal flow within the work.
This metal flow will act to reduce the thickness of the
work as it is formed. The initial diameter of the work can
be smaller in shear spinning.

2. Tube Spinning
Tube spinning is performed on cylindrical parts. Tube
spinning is similar to shear spinning in that metal flow
occurs within the work.
This metal flow acts to reduce the thickness of the metal.
While using tube spinning to reduce the thickness of the
tube, the tube's length will be increased.
This manufacturing process can be performed externally
with the tube over a mandrel or internally with the tube
enclosed by a die.

Explosive Forming
Explosive forming is a metalworking
technique in which an explosive charge is
used instead of a punch or press.
It can be used on materials for which a press
setup would be prohibitively large or require
an unreasonably high pressure, and is
generally much cheaper than building a large
enough and sufficiently high-pressure press;
on the other hand, it is unavoidably an
individual job production process, producing
one product at a time and with a long setup
time. RIT - M.Lakshmanan, AP (S.G)/MECH 136

Explosive Forming
It is used for blanking, cutting, expanding, coining,
embossing, flanging, drawing operations etc,.
Explosives used can be high energy chemicals such
as TNT, RDX and Dynamite or Gaseous mixture or
propellants.
These chemicals are used in various forms such as rod,
sheet, liquid, stick etc,.
According to the placement of explosives the
operations can be divided into the following
categories:
❖Unconfined or Stand-off Technique
❖Confined System or Contact TechniqueRIT - M.Lakshmanan, AP (S.G)/MECH 137

Unconfined or Stand-off Technique

An explosive forming process commonly used for the production
of large parts is called a standoff system. Typically the mold
and work piece are submerged in water. The sheet metal is
secured over the mold by a ring clamp. Air is drawn out,
creating a vacuum in the die cavity.
An explosive is placed between the die cavity and the work, a
certain distance from the work. This distance is called the
standoff distance. Standoff distance depends on the size of the
work, for larger parts it is usually about half the diameter of
the blank.
The explosive itself is also deeply submersed in water. Upon
detonation, the shock wave travels through the water and
delivers great energy to the work, forming it to the die cavity
near instantaneously. This high energy rate forming process
can be used to form big thick plates.

Confined System or Contact Technique
This is usually used for relatively smaller parts than
the standoff system. All of the energy is directed
into a closed container, the walls of which contain
the die cavity.
The energy from the canned explosive forces the
sheet metal into the walls of the mold, forming the
part.
Safety is always a consideration when manufacturing
by explosive forming, particularly with the confined
system, where die failure is a significant concern.

Explosive Forming: Unconfined and Confined

Magnetic Pulse Forming/ Electromagnetic Forming
The magnetic pulse forming process which uses
opposing magnetic fields to force a sheet of metal
onto a mandrel or other form.
First, an extremely large current discharge is directed
through a coil which creates a magnetic field.
Capacitor banks are used to store charge for larger
discharges. In the nearby sheet of metal, an opposing
magnetic field is induced which causes the metal
sheet to be pushed into a form of some shape.
The method generates pressures up to 50 Kpsi creating
velocities up to 900 fps. The process production rate
can climb to 3 parts a second.

A capacitor bank is charged up and a large electrical
surge is sent through the coil. The current creates a
magnetic field. When a conductive material disrupts
a magnetic field it produces a current in that
material, this is called an eddy current.
Due to the close proximity of the conductive sheet
metal to the coil, the coil's magnetic field is
disrupted and eddy currents are generated in the
work piece.
These currents in the sheet metal produce their own
magnetic field that opposes the original magnetic
field of the coil. The opposing forces push these
fields apart and form the work.

Advantages:
• Improved formability (the amount of stretch available
without tearing)
• Wrinkling can be greatly suppressed
• Forming can be combined with joining and assembling
with dissimilar components including glass, plastic,
composites and other metals.
• Close tolerances are possible as springback can be
significantly reduced.
• Single sided dies are sufficient which can reduce tooling
costs.
Disadvantages:
• Non conductive materials cannot be formed directly,
but can be formed using a conductive drive plate
• The high voltages and currents involved require careful
safety considerations.RIT - M.Lakshmanan, AP (S.G)/MECH 148

Peen Forming
It is also called as shot peening. The free forming obtain
where the stream of small steel balls are forced together
again the metal surface, when the metal forming
process restricted to fulfillment of fairly specialized
function.
The peen forming is used to form of various irregular
shapes on the aluminium sheet plate. That requires
large plate and small control in the process.
The peen forming process not requires any die and
forming press. The part to be made by the sheet metal
placed on block or it suspended from support and it has
blasting together with shot, small steel balls. During the
operation blanks are clamped over simple form blocks.

The ball forced by compressed air or rotating blade. The
ball is having high velocity directly imping the sheet
metal to the form of block. There is repeated force by
sheet metal get the require form block shape.
The numerous small balls is having diameter of 2.5 mm
size cast- steel ball blast again the metal surface. The
ball discharged from the rotating wheel or by air blast
from nozzle.
The balls travelling speed of 60 m/s. The residual stress
are induced to the compressive surface which improve
the fatigue strength of sheet metal.

Advantages of peen forming:
• Tooling cost will be low
• Require no maintenance cost of tooling
• The compound curvatures are easily produced
• It is a die less forming, So that require
minimum lead time
• That process permit rework and design
changes for improve the fitness of sheet metal
• Formed product improve the fatigue strength
and stress corrosion resistance

Disadvantage of peen forming process:
• Exhibited increase the resistance to flexural
bending fatigue
• The surface of metal compressed together,
that prevent stress corrosion cracking.
Application of peen forming:
• Provide smoothing and complex curvature
of aircraft wings
• From of large tubular shapes
• Military air craft

Superplastic Forming
Superplastic forming (SPF) is an industrial process used
for creating precise and complex components out of
certain types of materials called superplastic materials.
To begin with, the material is heated up to
promote superplasticity.
For titanium alloys:
E.g. Ti 6Al 4V and some stainless steels this is around
900 °C (1,650 °F) and
For aluminium alloys:
E.g. AA5083 it is between 450–520 °C. In this state the
material becomes soft so processes that are usually used
on plastics can be applied, such as: thermoforming, blow
forming, and vacuum forming. Inert gas pressure is
applied on the superplastic sheet forcing it into a female
die.

During the super plastic forming process the
elevated temperature will occur the sheet metal
stress is very low.
When the forming process the sheet metal and
tooling are heated together and then high
pressure of gas is applied. That case, the sheet
metals are plastically deformed into desire shape
of die cavity.

Advantage of superplastic forming:
• Lower strength require the tooling
• The most complex can be formed
• No residual stress induced during the parts
of formed
• Tooling cost low
• High rate of forming

Materials:
The superplastic forming process suitable for
some certain material only. That having very
fine grain structure of less than 10 to 15 µm
size.
The material of SFP:
• Iron based high carbon alloy
• Incol 100
• 7475- T6 Aluminium alloy
• Incoloy 718
• T1- 6A1- 4v Titanium alloy

Application:
• Fresh coater panels
• Water closet
• Cooler outlet ducts
• Heat- exchanger ducts
• Electric vehicle aluminum battery tray

Micro forming
It involves forming of parts and features with
dimensions below 1mm. It’s a recent area of research in
the wide field of metal forming technologies which is
expanding the limits for applying metal forming
towards micro technology.
The major sheet metal processes for micro forming are
shearing, blanking, bending, stamping, deep drawing,
hydro forming, stretch forming, super plastic forming,
spinning, explosive forming , etc.

Similar to conventional sheet metal, the mechanical
properties of the materials such as elasticity, Plasticity, stress
strain relations, work hardening, temperature effect,
anisotropy, grain size and residual stress involve in analysing
the deformation of micro forming products.
Micro Forming Products:
• Micro gears and micro shafts
• Connector pin
• IC Chip lead frame and IC Sockets
• Miniature Fasteners
• Micro springs for micro switches
• Micro knifes for micro surgery
• Hard disc drives
• Sensors

SHEET METAL PROCESSES

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