MANUFACTURING TECHNOLOGY

1. Sheet-Metal Forming Processes
COMPILED BY
M.BALASUBRAMANIAN

2. Metal Characteristics affecting Sheet
Metal Processing
• Elongation- Determines the capacity of the sheet metal to be
stretched without failure
• Anisotropy - Different behavior at different planar directions
• Grain Size - Grain size determines the mechanical properties
of the material
• Residual Stress - Since this being a cold working process,
residual stress may lead to distortion or
cracking.
• Spring Back - Elastic recovery of a plastically deformed
work piece
• Wrinkles - If the w/p is not clamped properly or
clearance is not provided, wrinkles may
occur

3. Basic Types of Sheet Metal
Processes
1. Cutting
– Shearing to separate large sheets
– Blanking to cut part perimeters out of sheet
metal
– Punching/ Piercing to make holes in sheet
metal
2. Bending
– Straining sheet around a straight axis
3. Drawing
– Forming of sheet into convex or concave
shapes

4. Various Sheet Metal Forming Process
• Roll Forming
• Stretch Forming
• Drawing
• Stamping
• Rubber Forming
• Spinning
• Super Plastic Forming
• Peen Forming
• Explosive Forming
• Magnetic Pulse Forming

5. Shearing
• Shearing is a process for cutting sheet metal to
size out of a larger stock such as roll stock.
• Shears are used as the preliminary step in
preparing stock for stamping processes, or smaller
blanks for presses
• Name the three phases in shearing
i) Plastic deformation
ii) Penetration
iii) Fracture

6. EMU
Shearing of sheet metal between two cutting edges:
(1) just before the punch contacts work;
(2) punch begins to push into work, causing plastic deformation; Shearing of
sheet metal between two cutting edges:
(3) punch compresses and penetrates into work causing a smooth cut
surface;
(4) fracture is initiated at the opposing cutting edges which separates the
sheet.
Sheet Metal Cutting - Shearing

7. EMU
Blanking and Punching
Blanking - sheet metal cutting to separate piece
(called a blank) from surrounding stock
Punching - similar to blanking except cut piece is
scrap, called a slug
(a) Blanking and (b) punching.

8. Punching and Piercing
• A slug (the material punched out) is produced in
punching operations but not in piercing work
• Piercing is “forming a hole in sheet metal with a
pointed punch with no metal fallout (slug).”
• In this case, a significant burr or deformed sharp
edge is created on the bottom side of the material
being pierced.
EMU
PIERCE
PUNCHES

9. Punching
• The punching process forces a steel punch, made
of hardened steel, into and through a workpiece.
• The punch diameter determines the size of the
hole created in the workpiece
• Punching is often the cheapest method for
creating holes in sheet metal in medium to high
production.
EMU

10. Shearing Operations
• Punching: Interior portion of the sheared sheet is
to be discarded.
• Blanking: Exterior portion of the shearing operation
is to be discarded.
• Perforating: punching a number of holes in a sheet
• Parting: shearing the sheet into two or more pieces
• Notching: removing pieces from the edges
• Lancing: A hole is partially cut in a sheet and the cut
material is bent without removing any material.
• Nibbling: Cutting irregular shapes using a small
punch. Sheet metal is guided through the m/c along
a guided path.

11. Punch and Die Sizes
• For a round blank of diameter Db:
– Blanking die diameter = Db
– Blanking punch diameter = Db - 2c
where c = clearance
• For a round hole of diameter Dh:
– Hole punch diameter = Dh
– Hole die diameter = Dh + 2c
where c = clearance
In blanking, you give the required diameter of the blank to
the DIE, while in punching, you give the required diameter
of the hole to the PUNCH

12. Die size determines blank size
Db;
Punch size determines hole
size Dh.;
c = clearance
Punch and Die Sizes
Purpose: allows slug or blank
to drop through die
• Typical values: 0.25 to 1.5
on each side
Angular Clearance

13. Example 1: The shear strength of a cold rolled steel is 300 MPa. Determine the
punch force required for making a blank of 120 mm diameter from a strip of 3
mm thickness ofthe above material. What is the maximum punch force required?
Theoretical punch force requires can be calculated from the shear strength.
F = Shear strength X perimeter of the blank X thickness of blank = 300 120X3 =
339.4 kN
Maximum punch force can be calculated using the equation: Fmax = 0.7 tL
Assuming = 2
Shear strength = 2X300 = 600 Mpa, Fmax = 474.8 kN
Example 2: A cold rolled steel sheet with a shear strength of 350 MPa and a
thickness of 3 mm is to be subjected to blanking operation. The diameter of the
blank to be obtained is 130 mm. What is the appropriate die and punch diameter
and punch force required for the operation?
The clearance for blanking operation can be taken to be 0.075.
Punch and die diameters for blanking operations are given by:
Dia of punch = Db-2c = 130 – 2X0.075 = 129.85 mm
Dia of die = Blank diameter = Db = 130 mm
Punch force = Lt = 350 Dt = 428.61 kN

14. SHEARING DIES
Inverted die is designed with the die
block fastened to the punch holder and
the punch fastened to the die shoe.
During the downward stroke of ram,
the blank is sheared from the strip. The
blank and shedder are forced back into
the die opening, which loads a
compression spring in the die opening .
At the same time the punch is forced
through the scrap strip and a spring
attached to the stripper is compressed
and loaded. On the upstroke of the
ram, the shedder pushes the blank out
of the die opening and the stripper
forces the scrap strip off the punch.
Compound die combines the principles of the
conventional and inverted dies in one station.
This type of die may produce a workpiece
which is pierced and blanked at one station
and in one operation. The blanking punch and
blanking die opening are mounted in an
inverted position. The blanking punch is
fastened to the die shoe and the blanking die
opening is fastened to the punch holder.

16. EMU
Metal on inside of neutral plane is compressed,
while metal on outside of neutral plane is
stretched
Sheet Metal Bending
•The material is stressed beyond
the yield strength but below the
ultimate tensile strength.
•The surface area of the material
does not change much.
•Bending usually refers to
deformation about one axis

17. Types of Sheet Metal Bending
• V-bending - performed with a V-shaped die
• Edge bending - performed with a wiping die
• For low production
• Performed on a press brake
• V-dies are simple and
inexpensive
• For high production
• Pressure pad required
• Dies are more complicated and
costly in edge bending

19. Roll Forming
• Cold roll forming
• Roll Bending

20. EMU
Large metal sheets and plates are formed into
curved sections using rolls
Roll Bending

21. Bend Allowance Formula
Where B = bend allowance;  = bend angle;
R= bend radius; t = stock thickness; and Kba is
factor to estimate stretching
• If R < 2t, Kba = 0.33
• If R  2t, Kba = 0.50
)(
360
2 tKRB ba



22. Calculate Bend Allowance
• Let’s start with a simple L bracket. The picture shows that the legs of the
bracket are 2” and 3”. The material thickness is 0.036”, the inside radius is
0.125”, and the angle of bend is 90 degrees. The K-factor is the percentage
of the material thickness where there is no stretching or compressing of
the material, for example, the neutral axis. For this simple L bracket, I will
use a K-factor of 0.42.
• Bend Allowance = Angle * (PI / 180) * (Radius + K-factor * Thickness).
• Plugging in our numbers, we have:
• Bend Allowance = 90 * (PI / 180) * (0.125 + 0.42 * 0.036) =
0.2200999813105009

23. EMU
Springback
Increase in included angle of bent part relative
to included angle of forming tool after tool is
removed
• Reason for springback:
– When bending pressure is removed, elastic energy
remains in bent part, causing it to recover partially
toward its original shape

24. NECKING
• During tensile elongation of ductile material,
beyond the ultimate tensile strength, localized
necking begins. At the ultimate point where
necking or instability is about to begin, we know
that the strain is equal to the strain hardening
exponent. Higher the strain hardening exponent
for a metal, higher the stress it can withstand
before necking. In isotropic materials, necking is
usually found to occur at an angle of 55 degrees
with reference to the tensile axis. Larger strain rate
sensitivity parameter leads to diffuse necking
(uniform ) rather than localized. This means larger
amounts of uniform elongation occurs before
fracture of the sheet metal.

25. EMBOSSING & IRONING
• Embossing is the operation of forming surface
details of figures, letters or designs on sheet metal
by displacement of the metal between two mating
dies
• Coining is the process where impressions on both
the faces can be different.
IRONING
Ironing is a process that is normally used to get
uniform wall thickness in deep drawings. A punch and
die pushes the part through a clearance that will act to
reduce the entire wall thickness to a value.

26. Drawing (shallow & Deep)
Sheet metal forming to make cup-shaped,
box-shaped, or other complex-curved,
hollow-shaped parts
• Sheet metal blank is positioned over die cavity
and then punch pushes metal into opening
• Products: beverage cans, shells, automobile
body panels, cups, etc.
• Also known as deep drawing (to distinguish it
from wire and bar drawing)

27. (a) Drawing of cup-shaped
part:
(1) before punch contacts work,
(2) near end of stroke;
(b) workpart: (1) starting blank,
(2) drawn part.
Drawing
c = 1.1 t

28. DEEP DRAWING
• Deep drawing is the manufacturing process of forming
sheet metal stock, called blanks, into geometrical or
irregular shapes that are more than half their
diameters in depth
• Common shapes for deep drawn products include
cylinders for aluminum cans, cups and baking pans.
Fire extinguishers and Kitchen sinks are also commonly
manufactured by the deep drawing method.
• Limiting Drawing Ratio (LDR) = (Do/Dp)
• Do – Max blank Dia and Dp – Punch Dia
Determines the largest blank which can be drawn.
Maximum value of LDR is 2.7

29. This process is used for forming large shallow shapes which
Cannot be strained beyond the elastic limit by drawing alone
Sheet metal is stretched and simultaneously bent to achieve
Shape change
Stretch forming: (1) start of process; (2) form die is pressed into the
work with force F, causing it to be stretched and bent over the form.
F = stretching force.
Stretch Forming (Form Block Method)

30. Types & Specification of a Press
• Screw Press
• Crank Press
• Hydraulic Press
• Press Stroke - Max distance travelled by ram from
top to bottom
• No. of Strokes/Min - Indicates speed at which operations
can be performed
• Press Capacity - Force exerted by the press
• Die Space - Space available to mount punch and die
• Shut Height - The distance from top of the bed to the bottom
of the ram with its stroke down and adjustment up.

31. Classification of presses.
Classified by one or a combination of characteristics, such as source of power, number of slides, type
of frame and construction, type of drive, and intended applications.
Classification on the basis of source of power.
• Manual Presses.
• Mechanical presses. These presses utilize flywheel energy which is transferred to the work piece
by gears, cranks, or eccentrics.
• Hydraulic Presses. These presses provide working force through the application of fluid pressure on
a piston
• Pneumatic Presses. These presses utilize air cylinders to exert the required force.
Classification on the basis of number of slides.
• Single Action Presses. A single action press has one reciprocation slide. It is the most widely used
press for operations like blanking, coining, embossing, and drawing.
• Double Action Presses. A double action press has two slides moving. It is more suitable for drawing
operations, especially deep drawing, than single action press. For this reason, its two slides are
generally referred to as outer blank holder slide and the inner draw slide. Widely used for deep
drawing operations and irregular shaped stampings.
• Triple Action Presses. A triple action press has three moving slides. Two slides (the blank holder
and the inner slide) move in the same direction as in a double – action press and the third or lower
slide moves upward through the fixed bed in a direction opposite to that of the other two slides.
This action allows reverse – drawing, forming or bending operations.

32. Classification on the basis of frame and construction.
• Arch – Frame Presses. These presses have their frame in the
shape of an arch. These are not common.
• Gap Frame Presses. These presses have a C-shaped frame. These
are most versatile and common in use, as they provide un –
obstructed access to the dies from three sides and their backs
are usually open for the ejection of stampings and / or scrap.
• Straight Side Presses. These presses are stronger since the heavy
loads can be taken in a vertical direction by the massive side
frame
• Horn Presses. These presses generally have a heavy shaft
projecting from the machine frame instead of the usual bed. This
press is used mainly on cylindrical parts involving punching,
riveting, embossing, and flanging edges.

34. Spinning
• Metal forming process in which an axially symmetric part
is gradually shaped over a rotating mandrel or form block
using a rounded tool or roller.
• An adjustable steady rest provides a movable fulcrum
pin,
Which can be positioned in any one of the holes.
• The forming tool gradually displaces the metal in steps to
confirm to the shape of the form block.

35. Conventional Spinning
Tube Spinning

36. Sheet Metal Tools
Marking and measuring tools
• Steel Rule - It is used to set out
dimensions.
• Try Square - Try square is used for
making and testing angles of 90degree
• Scriber – It used to scribe or mark lines
on metal work pieces.
• Divider - This is used for marking circles,
arcs, laying out perpendicular lines,
bisecting lines, etc
Cutting Tools
• Straight snip - They have straight jaws
and used for straight line cutting.
• Curved snip - They have curved blades
for making circular cuts.
Striking Tools
• Mallet

37. FORMABILITY OF SHEET METAL
• Formability may be defined as the ease with which material
may be forced into a permanent change of shape.
• The formability of a material depends on several factors. The
important one concerns with the properties of material like
yield strength, strain hardening rate, and ductility. These are
greatly temperature - dependent. As the temperature of
material is increased, the yield strength and rate of strain
hardening progressively reduce and ductility increases. The hot
working of metal, therefore, permits relatively very large
amount of deformation before cracking.

38. FORMABILITY TESTS
• Intrinsic Tests
Measures the basic characteristic properties of
materials related to formability.
Uniaxial tensile testing
Biaxial Stretch Testing
Hydraulic bulge testing
Hardness Test

39. Simulative Tests
Subjects the material to deformation that closely
resembles the deformation that occurs in a
particular forming operation.
Ball Punch Test and Bend Test
• Olsen cup test
Uses a 22.2mm dia hardened steel ball and a die of
25.4mm internal dia
• Erichsen cup test
20mm dia ball and Die 27mm
In both the test, the cup height at fracture is used as
measure of stretchability

40. Uniaxial tensile test
Biaxial tensile test
Hydraulic Bulge test Olsen or Erichsen cup test

41. Bulge tests results on steel sheets
Punch Test on specimen

43. n= strain hardening
exponent
R = Anisotropy
coefficient of the
material

44. STRAIN MEASUREMENT METHODS
After sheet metal is formed the marked circles
will deform into ellipses of different sizes.
Strain is calculated from the following
formula.
Major strain = (major axis length – original circle
dia ) X 100 / Original circle diameter
Minor strain = (minor axis length – original
circle dia ) X 100 / Original circle diameter

45. Formability Limit Diagram (FLD)
a) FLD for various sheet metals.
This Diagram indicate the limiting strains the sheet metal can
sustain over a range of major to minor strain ratios

46. HYDROFORMING
Hydroforming uses fluid pressure in place of the punch
in a conventional tool set to form the part into the
desired shape of the die
There are four main types of hydroforming:
· Hydroforming of tubes, usually at low pressure, is the
most widely used technology at present, with
hydroformed tubular parts offering improved integrity
and structural performance.
· Low pressure hydroforming simply re-shapes tubes,
producing a very good shape, but is not as useful if
better cross-section definition is required.
· High-pressure hydroforming, totally changes the tube
shape and alters the length to circumference ratio by up
to 50%. It gives very good tolerance control, being a
highly robust process.
· Panel hydroforming at high pressures is used in the
aerospace industry, and is expected to be used for
applications in the automotive industry in which
hydroforming is needed to get the right material flow.

47. Rubber Pad Forming (Guerin Process)
RUBBER-PAD FORMING, also known as flexible-die forming,
employs a rubber pad or a flexible diaphragm as one tool half and
only one solid tool as another half to form a part to final shape.
The rubber acts somewhat like hydraulic fluid in exerting nearly
equal pressure on all workpiece surfaces as it is pressed around
the form block.

48. RUBBER PAD FORMING
Advantages
• Only a single rigid tool half is required to form a part.
• One rubber pad or diaphragm takes the place of many different die shapes,
returning to its original shape when the pressure is released. Tools can be made
of low-cost and easy-to-machine materials.
• Different metals and thicknesses can be formed in the same tool. Parts with
excellent surface finish can be formed, because no tool marks are created. Set-up
time is considerably shorter, because no lining-up of tools is necessary.
Disadvantages are:
• The pad or diaphragm has a limited lifetime that depends on the severity of the
forming in combination with the pressure level.
• Lack of sufficient forming pressure results in parts with less sharpness or with
wrinkles, which may require subsequent hand work.
• The production rate is relatively slow, making the process suitable primarily for
prototype and low-volume production work.

49. HIGH ENERGY RATE FORMING
• This process is characterized by the
application of high pressure for a shorter
duration at high velocity.
• Velocity ranges between 10 to 230m/s
compared to 0.03 to 0.75m/s for mechanical
press forming
• Tooling cost is eliminated

50. Explosive forming
(High Energy Rate Forming (HERF):
• Explosives used are PETNPentaerythritol tetranitrate, C5H8N12O4)
RDX (Cyclotrimethylene trinitramine, C3H6N6O6)
TNT (Trinitrotoluene, C7H5N3O6)
Advantages of Explosion Forming
• Maintains precise tolerances.
• Eliminates costly welds.
• Controls smoothness of contours.
• Reduces tooling costs.
• Less expensive alternative to super-plastic forming.

51. Methods of Explosive Forming
• Standoff Method
In this method, the explosive charge is located at some
predetermined distance from the workpiece and the
energy is transmitted through an intervening medium
like air, oil, or water. Rapid conversion of explosive
charge into gas generates a shock wave. The pressure
of this wave is sufficient to form sheet metals
• Contact Method
In this method, the explosive charge is held in direct
contact with the workpiece while the detonation is
initiated. The detonation produces interface pressures
on the surface of the metal up to 35000 MPa.

52. Confined and Un-Confined system
• When explosive is detonated, it produces
large amount of gases, which when confined
produces large pressure. When size of the part
is large, un-confined system is preferred.
• In the un-confined system efficiency is less
• Confined system is largely used for small
tubular parts bulging
• Since it is a closed die operation die failure
may be resulted. Die erosion is also a problem.
Hence confined system is not preferred.

53. Magnetic-Pulse-Forming Process
(a) Schematic illustration of the magnetic-pulse-forming process.
The part is formed without physical contact with any object

54. Principle
• The current through the coil produces a high
intensity magnetic field between the coil and
the workpiece.
• This creates a repelling force between the
workpiece and coil.
• This repelling force, forces the workpiece
against the die

55. Advantages, Limitations & Applications
Advantages
• Uniform rate of forming
• Surface finish is good
• Operation time is less than conventional method
Limitations
• Non-Conducting materials cannot be processed
without the aid of conducting materials
• Limited to sheet metal forming
Applications
• Both expansion and compression of circular section can
be done
• Both bulging and compression of tubes at the end of
any joints possible
• Embossing

56. PEEN FORMING
• In this process a stream of metal shots is blasted against
the surface of the blank to be made to required shape.
Because it is a dieless process, shot peen forming reduces
material allowance from trimming and eliminates costly
development and manufacturing time to fabricate hard
dies
Advantages
• Complex contours can be produced easily
• Salvage operation for restoring distorted parts
• Does not require any punch or die
Limitation
• It requires longer time for forming the shape
• It requires additional devices for forcing the
metal shots
Applications
• Specific portions of connecting rod
• Aircraft panels
• Saddle shaped parts

57. SUPER PLASTIC FORMING
• The superplastic forming (SPF) operation is based on
the fact that some alloys can be slowly stretched well
beyond their normal limitations at elevated
temperatures.
• Superplasticity can be defined as the ability of certain
materials to undergo very high strains at specific temp.
and strain rate.
• If processed at correct temp. and strain rate, very high
tensile elongation from 200% to several thousand of the
normal elongation at room temp. can be achieved.
• First the alloy is heated to 1000 c by using a inert gas
with a pressure up to 50 bar.
• The pressure of the gas slowly inflates the blank, takes
the inside contour of the forming die.

58. Super plastic forming
Process Advantages
• Reduced weight for high fuel efficiency
• Improved structural performance
• Increased metal formability and part complexity
• Near net shape forming of complex shapes reduces part count
• Cost/weight savings
• Low-cost tooling
• Low environmental impacts - non-lead die,
• low noise
Materials used -
• Titanium alloys
• Aluminum alloys
• Bismuth-tin alloys
• Zinc-aluminum alloys
• Stainless steel
• Aluminum-lithium alloys

59. Electrohydraulic forming
• 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.

60. Advantages and Limitations
• Advantages of Electrohydraulic Forming process:
1) It is essential for forming of disc and the tube shaped
components.
2) A single die is only required for the process and hence
it is cost saving.
3) The rate of production is higher.
• Disadvantages:
1) It is suitable for mass production of small castings only.
2) If the impact velocity of metals is about 30 meter per
second, the metal cannot be used for the process.