Sheet Metal Working & Process

Compiled & Edited
By
Sivaraman Velmurugan
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 A piece of metal whose thickness is between 0.006(0.15 mm)
and 0.25 inches(6.35 mm).
 Anything thinner is referred to as a foil and thicker is considered
as a plate.
 Sheet thickness is generally measured in gauge. Greater the
gauge number, thinner the sheet of metal.
 Sheet metal can be cut, bent and stretched into nearly any
shape.
 Generally two types of operations are performed- forming and
cutting.
What is Sheet Metal?
2/17/2016 Compiled & Edited by SIVARAMAN VELMURUGAN 2


 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 forming is a grouping of many complementary
processes that are used to form sheet metal parts.
What is Sheet Metal?


 The most common sheet metal
used in automotive to make
bodies is steel. It is reasonably
cheap and easy to press into
shape to make body parts.
 The next best is aluminum. It is
lighter but harder to bend into
tight shapes without cracking.
It is also harder to weld in mass
production.
What metals are used in automotive?

 There are three types of steel
used in the automotive body and
only two are commonly used.
 The first type which is used by
Volvo™ is boron steel which is
stronger than the other two types
of steel.
 The two types commonly used
are:
 Mild and low-carbon steel
 Higher carbon steel
Steel and it’s types..


 Boron steel is developed using boron as an
alloying element in developing Ultra High-
strength Steel (UHSS).
 Once bent, it can’t be straightened and it
requires replacement if damaged.
 Boron steel are also sensitive to heat and it
weakens when it’s heated rapidly.
 Because of its sensitivity to heat, it can’t be
galvanized. Therefore, corrosion protection
is crucial and essential after welding.
More on Boron Steel…


 Also called plain-carbon steel, is the
most common form of steel because of
its price is low while it provides material
properties that are acceptable for
many applications, more so than iron.
 Contains approximately 0.05-0.3%
carbon making it malleable and
ductile.
More on Mild and Low-Carbon Steel…


 Mild steel has a relatively low
tensile strength, but its cheap
and malleable; surface hardness
can be increased by carburizing.
Note:
Carburizing is a heat treatment process in
which iron or steel absorbs carbon liberated
when the metal is heated in the presence of
carbon bearing material, such as charcoal
or carbon monoxide with the intent of
making the metal harder.

 Carbon steels which can successfully
undergo heat treatment have a content in
the range of 0.3-1.7% by weight.
 Medium carbon steel :- approximately 0.3-
0.59% carbon content. (Balances ductility
and strength and has good wear resistance).
 High-carbon steel :- 0.6-0.99% carbon
content. (Very strong).
 Ultra-high-carbon steel :- 1.0-2.0% carbon
content. (Steels that can be tempered to
great hardness).

 6111 aluminum and 2008 aluminum alloy are
extensively used for external automotive body
panels, with 5083 and 5754 used for inner
body panels.
 Hoods have been manufactured from 2036,
6016, and 6111 alloys.
 Truck and trailer body panels have used 5456
aluminum.
 Automobile frames often use 5182 aluminum
or 5754 aluminum formed sheets, 6061 or 6063
extrusions
Aluminum and it’s types…

 2000 series(2008,2036) – alloyed with copper, can be
precipitation hardened to strengths, comparable to
steel. Formerly referred as duralumin, they were
once the most common aerospace alloys, but were
susceptible to stress corrosion cracking and are
increasingly replaced by 7000 series in new designs.
 5000 series(5083,5754,5456,5182) – alloyed with
magnesium.
 6000 series(6111,6016,6061,6063) – alloyed with
magnesium and silicon, are easy to machine, and
can be precipitation hardened, but not to the high
strengths that 2000 and 7000 can reach.
Aluminum and it’s types…


Sheet Metal Working & Process
Sheet Metal Forming Process
Large group of manufacturing
processes in which plastic deformation is
used to change the shape of metal
workpieces.
 The tool, usually called a die, applies
stresses that exceed the yield
strength of the metal
 The metal takes a shape determined
by the geometry of the die.


Stresses in Metal Forming
 Stresses to plastically deform the
metal are usually compressive
 Examples: rolling, forging, extrusion
 However, some forming processes
 Stretch the metal (tensile stresses)
 Others bend the metal (tensile and
compressive)
 Still others apply shear stresses
(shear spinning)


Material Properties in Metal Forming
 Desirable material properties:
 Low yield strength
 High ductility
 These properties are affected by
temperature:
 Ductility increases and yield strength
decreases when work temperature is raised
 Other factors:
 Strain rate and friction


Process Classification
 Bulk Deformation Process
 Rolling
 Forging
 Extrusion
 Wire and bar drawing
 Sheet Metalworking
 Bending
 Deep drawing
 Cutting
 Miscellaneous processes


Bulk Deformation Process
Characterized by significant deformations and massive shape
changes "Bulk" refers to workparts with relatively low surface
area - to - volume ratios. Starting work shapes include cylindrical
billets and rectangular bars
 Rolling - compression process to reduce the thickness of a
slab by a pair of rolls.
 Forging - compression process performing between a set of
opposing dies.
 Extrusion - compression process squeezing metal flow a die
opening.
 Drawing - pulling a wire or bar through a die opening.


Rolling Forging
Extrusion


Sheet metalworking
Forming and related operations performed on metal sheets, strips,
and coils. High surface area-to-volume ratio of starting metal, which
distinguishes these from bulk deformation.
Often called pressworking because presses perform these
operations. Parts are called stampings : Usual tooling: punch and
die. Forming on metal sheets, strips, and coils. The process is
normally a cold working process using a set of punch and die.
 Bending - straining of a metal sheet to form an angle bend.
 Drawing - forming a sheet into a hollow or concave shape.
 Shearing - not a forming process but a cutting process.




Material Behavior in Metal Forming
 Plastic region of stress-strain curve is
primary interest because material is
plastically deformed.
 In plastic region, metal's behavior is
expressed by the flow curve : σ =Kϵn
where K = strength coefficient; and
n = strain hardening exponent.
 Flow curve based on true stress and
true strain.


Flow Stress
 For most metals at room
temperature, strength increases
when deformed due to strain
hardening.
 Flow stress = instantaneous
value of stress required to
continue deforming the
material : Yf =Kϵn where Yf =
flow stress, that is, the yield
strength as a function of strain.


Average Flow Stress
 Determined by integrating the flow
curve equation between zero and
the final strain value defining the
range of interest
where Ȳf = average flow stress; and  =
maximum strain during deformation
process
n
K
Y
n
f


1
_



Stress-Strain Relationship
 Average flow stress in relation to
 Flow stress Yf
 Yield strength Y
Temperature in Metal Forming
For any metal, K and n in the flow curve
depend on temperature
 Both strength (K) and strain hardening
(n) are reduced at higher temperatures.
 In addition, ductility is increased at
higher temperatures.


Temperature in Metal Forming
 Any deformation operation
can be accomplished with
lower forces and power at
elevated temperature
 Three temperature ranges
in metal forming:
 Cold working
 Warm working
 Hot working


Cold Working
 Performed at room temperature
or slightly above,
 Many cold forming processes are
important mass production
operations,
 Minimum or no machining usually
required,
 These operations are near net
shape or net shape processes.


Advantages of Cold Forming
 Better accuracy, closer tolerances
 Better surface finish
 Strain hardening increases
strength and hardness
 Grain flow during deformation
can cause desirable directional
properties in product
 No heating of work required


Disadvantages of Cold Forming
 Higher forces and power required for
deformation
 Surfaces of starting work must be free of
scale and dirt
 Ductility and strain hardening limit the
amount of forming that can be done
 In some cases, metal must be
annealed before further deformation
can be accomplished
 In other cases, metal is simply not
ductile enough to be cold worked


Warm Working
 Performed at temperatures above room temperature but below
recrystallization temperature
 Dividing line between cold working and warm working often
expressed in terms of melting point:
 0.3Tm, where Tm = melting point (absolute temperature) for metal


Advantages and Disadvantages of Warm Working
 Advantages
 Lower forces and power than in cold working
 More intricate work geometries possible
 Need for annealing may be reduced or eliminated
 Disadvantage
 Workpiece must be heated


Hot Working
 Deformation at temperatures above the recrystallization
temperature
 Recrystallization temperature = about one-half of melting point on
absolute scale
 In practice, hot working usually performed somewhat above 0.5Tm
 Metal continues to soften as temperature increases above 0.5Tm,
enhancing advantage of hot working above this level


Why Hot Working
Capability for substantial plastic deformation - far more than is
possible with cold working or warm working
Why?
 Strength coefficient (K) is substantially less than at room temperature
 Strain hardening exponent (n) is zero (theoretically)
 Ductility is significantly increased


Advantages of Hot Working
 Workpart shape can be significantly altered
 Lower forces and power required
 Metals that usually fracture in cold working can be hot formed
 Strength properties of product are generally isotropic
 No strengthening of part occurs from work hardening
 Advantageous in cases when part is to be subsequently processed
by cold forming


Disadvantages of Hot Working
 Lower dimensional accuracy
 Higher total energy required, which is the sum of
 The thermal energy needed to heat the workpiece
 Energy to deform the metal
 Work surface oxidation (scale)
 Thus, poorer surface finish
 Shorter tool life
 Dies and rolls in bulk deformation


Isothermal Forming- A Type of Hot Forming
When highly alloyed metals such as Ti and Nickel alloys are heated
to hot temp and bring in contact with cold tooling, the heat
radiates from the metal to tooling. This result in high residual stresses
and temp variation over metal and hence irregular material flow
occurs during forming, causing cracks.
In order to avoid this problem, both metal and tooling are heated
to same temp. However, this causes reduction in tooling life.
** Mostly, Forging is performed through this process


Strain Rate Sensitivity
 Theoretically, a metal in hot working behaves like a perfectly
plastic material, with strain hardening exponent n = 0
 The metal should continue to flow at the same flow stress, once that
stress is reached
 However, an additional phenomenon occurs during deformation,
especially at elevated temperatures:
 Strain rate sensitivity


What is Strain Rate?
 Strain rate in forming is directly related to speed of deformation v
 Deformation speed v = velocity of the ram or other movement of
the equipment
 Strain rate is defined:
 Where = true strain rate; and h = instantaneous height of
workpiece being deformed
h
v

.




Evaluation of Strain Rate
 In most practical operations, valuation of strain rate is
complicated by
 Workpart geometry
 Variations in strain rate in different regions of the part
 Strain rate can reach 1000 s-1 or more for some metal forming
operations.


Effect of Strain Rate on Flow Stress
 Flow stress is a function of temperature
 At hot working temperatures, flow stress also depends on strain
rate
 As strain rate increases, resistance to deformation increases
 This is the effect known as strain-rate sensitivity


Strain Rate Sensitivity
 (a) Effect of strain rate on flow stress at
an elevated work temperature
 (b) Same relationship plotted on log-log
coordinates
Strain Rate Sensitivity Equation
where C = strength constant (analogous but not
equal to strength coefficient in flow curve equation),
and m = strain-rate sensitivity exponent
m
f CY ε=


Effect of Temperature on Flow Stress
 The constant C, indicated by the intersection
of each plot with the vertical dashed line at
strain rate = 1.0, decreases
 And m (slope of each plot) increases with
increasing temperature


Observations about Strain Rate Sensitivity
Increasing temperature decreases C and increases m
 At room temperature, effect of strain rate is almost negligible
 Flow curve alone is a good representation of material
behavior
 As temperature increases
 Strain rate becomes increasingly important in determining flow
stress


Friction in Metal Forming
If the coefficient of friction becomes too large, a condition known
as STICKING occurs. Sticking in metal working is the tendency for the
two surfaces in relative motion to adhere to each other rather than
slide.
When Sticking Occurs?
The friction stress between the surfaces becomes higher than the shear flow stress
of the metal thus causing the material to deform by a shear process beneath the
surface rather than slip at the surface. Sticking is a prominent problem in forming
operations, especially rolling.


Friction in Metal Forming
 In most metal forming processes, friction is undesirable:
 Metal flow is reduced
 Forces and power are increased
 Tools wear faster
 Friction and tool wear are more severe in hot working


Lubrication in Metal Forming
 Metalworking lubricants are applied to tool-work interface in
many forming operations to reduce harmful effects of friction
 Benefits:-
 Reduced sticking, forces, power, tool wear
 Better surface finish
 Removes heat from the tooling


Considerations in Choosing a Lubricant
 Type of forming process (rolling, forging, sheet metal drawing,
etc.)
 Hot working or cold working
 Work material
 Chemical reactivity with tool and work metals
 Ease of application
 Cost


 Bending
 Shearing
 Blanking
 Punching
 Trimming
 Parting
 Slitting
 Lancing
 Notching
 Perforating
 Nibbling
 Embossing
 Shaving
 Cutoff
 Dinking
 Coining
 Deep drawing
 Stretch forming
 Roll forming


 Bending is a metal forming process in which a force is applied to
a piece of sheet metal, causing it to bend at an angle and form
the desired shape.
Bending


Press Brake Machine

Two common bending methods are:
 V-Bending
 Edge bending
 V-Bending - The sheet metal blank is
bent between a V-shaped punch
and die.
 Air bending - If the punch does not
force the sheet to the bottom of the
die cavity, leaving space or air
underneath, it is called “air bending”.
Bending Types


 Edge (or) Wipe Bending - Wipe
bending requires the sheet to be
held against the wipe die by a
pressure pad. The punch then presses
against the edge of the sheet that
extends beyond the die and pad.
The sheet will bend against the radius
of the edge of the wipe die.
Bending Types


Bending Operations


Shearing
 Shearing is defined as separating
material into two parts.
 It utilizes shearing force to cut sheet
metal.


Blanking
 A piece of sheet metal is removed
from a larger piece of stock.
 This removed piece is not scrap, it is
the useful part.


Fine Blanking
 A second force is applied
underneath the sheet, directly
opposite the punch, by a "cushion".
 This technique produces a part with
better flatness and smoother edges.


Punching Operations

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


Blanking & Punching examples

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

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

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


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.

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

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


Nibbling
 Punching a series of small overlapping slits or holes along a path
to cut-out a larger contoured shape.


Embossing
 Certain designs are embossed on the sheet metal.
 Punch and die are of the same contour but in opposite
direction.


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 ±0.025 mm are possible.


Cuttoff
 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.


Dinking
 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.


Coining
 Similar to embossing with the difference that similar or different
impressions are obtained on both the sides of the sheet metal.


Deep Drawing
 Deep drawing is a metal forming process in which sheet metal is
stretched into the desired shape.
 A tool pushes downward on the sheet metal, forcing it into a die
cavity in the shape of the desired part.


Process overview in deep drawing


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 bent parts.


Roll Forming
 Roll forming is a continuous bending
operation in which a long strip of sheet
metal is passed through sets of rolls
mounted on consecutive stands, each
set performing only an incremental
part of the bend, until the desired
cross-section profile is obtained.
 Roll forming is ideal for producing
constant-profile parts with long lengths
and in large quantities.


Dies
 Made up of tool steel and used to cut or shape material.
 Simple die
 Compound die
 Combination die
 Progressive die


Simple Die
 Simple dies or single action dies perform single operation for
each stroke of the press slide.
 The operation may be one of the cutting or forming operations.


Compound Die
 In these dies, two or more operations may be performed at one
station.
 Such dies are considered as cutting tools since, only cutting
operations are carried out.


Combination Die
 In this die also , more than
one operation may be
performed at one station.
 It is different from
compound die in that in
this die, a cutting
operation is combined
with a bending or drawing
operation, due to that it is
called combination die.


Progressive Die
 A progressive has a series of operations.
 At each station, an operation is performed on a work piece
during a stroke of the press.


Progressive Die


Rolling Defects
 Wavy edges
 Result from concave roll bending and
 Thinner along its edges than at its center
 Cracks
 Result from poor material ductility
 Convex roll bending
 Severe conditions cause center split
 Alligatoring
 Defects in the original cast material
 Only surface of work is deformed


Forging defects
 Surface crack
 Excessive working at low temperatures
 High sulphur concentration
 Crack at flash
 More prevalent for thinner flash
 Penetrates to work
 Internal cracks
 Secondary tensile stresses
 Cold shuts
 Lubricant residue


Drawing Defects
 Wrinkling in the flange
 Occurs due to compressive buckling in
the circumferential direction (blank
holding force should be sufficient to
prevent buckling.
 Wrinkling in the wall
 Takes place when a wrinkled flange is
drawn into the cup or if the clearance is
very large, resulting in a large
suspended (unsupported) region.


Drawing Defects
 Tearing
 High tensile stresses that cause thinning
and failure of the metal in the cup wall.
 If the die has a sharp corner radius.
 Earring
 When the material is anisotropic
 Varying properties in different directions.
 Surface scratches
 If the punch and die are not smooth
 If the lubrication of the process is poor.


Defects in Extrusions
 Surface Cracking / Fir-tree cracking
 High friction or speed.
 Sticking of billet material on die land.
 Material sticks, pressure increases,
product stops and starts to move
again.
 produces circumferential cracks on
surface, similar to a bamboo
stem.(bambooing).


Defects in Extrusions
 Internal Cracking/ Chevron cracking
 Center of extrusion tends to develop
cracks of various shapes.
 Center-burst, and arrowhead
 Center cracking:
 Increases with increasing die
angle.
 Increases with impurities.
 Decreases with increasing R and
friction.


Mail me – sudhavel@yahoo.com
Questions & Comments



Sheet Metal Working & Process

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