Sheet Metal Forming Guide

Sheet Metal Working, Temperature and sheet metal forming,
Applications Sheet Metal Parts, Categories of sheet metal
processes, Shearing, stages in shearing action, Punch and Die
Sizes, Sheet Metal Bending
UNIT – IV
Sheet metal operations – Part -1
Dr. Loveleen Kumar Bhagi
Associate Professor
School of Mechanical Engineering
Lovely Professional University1
MEC323
PRIMARY MANUFACTURING

Forming
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MEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

Sheet Metal Working
Cutting and forming operations performed on
relatively thin sheets of metal.
Typical sheet-metal thicknesses are between 0.4 mm
and 6 mm.
Above this thickness is plate and below this
thickness is called foil.
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ability of sheet metal to
undergo shape change
without failure by necking
or tearing.

Sheet Metal Working
The sheet or plate stock used in sheet metalworking
is produced by flat rolling.
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Material in sheet metal forming processes
• The most commonly used sheet metal is low carbon steel
(0.06%–0.15% C) because of its low cost and generally good
strength and formability characteristics.
• Aluminum is the most common material for such sheet-metal
applications as beverage cans, packaging, kitchen utensils, and
applications where corrosion resistance is a concern. The
common metallic materials for aircraft and aerospace
applications are aluminum and titanium.
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Temperature and sheet metal forming
Sheet-metal processing is usually performed at room
temperature (cold working). The exceptions are when
the sheet is thick, the metal is brittle, or the
deformation is significant. These are usually cases of
warm working rather than hot working.
warm working performed in order to increase formability and
decrease forming loads on machinery. Typical materials in
worm stamping operations are titanium alloys and various high-
strength steels.05-10-2020
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Applications Sheet Metal Parts
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
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Advantages of Sheet Metal Parts
• High strength
• Good dimensional accuracy
• Good surface finish
• Relatively low cost
• Economical mass production for large quantities
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Categories of sheet metal processes
The two major categories of sheet-metal processes
are
(1) Cutting,
(2) Deformation
Cutting is used to separate large sheets into smaller
pieces, to cut out part perimeters, and to make holes
in parts.
Deformation is used to form sheet-metal parts into
their required shapes.
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Cutting
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shearing
mechanism
involving the cutting in the number of processes like
Punching
Blanking
Notching
Nibbling
shaving.
(achieved through)

Cutting
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shearing
mechanism
involving the cutting in the number of processes like
Punching
Blanking
Notching
Nibbling
shaving.
(achieved through)

Deformation
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Compression, tension and
the combination of the
tension and compression
both
Compression
(achieved by the application)
sheet metal is subjected to the compressive
stresses with the application of the force
Ironing and coining

Deformation
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both
Tension
sheet metal is subjected to the tensile stresses
with the application of the force
stretch forming

Deformation
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both
Compression + Tension
bending, drawing,
embossing

Sheet Metal Processes » Stresses
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Sheet Metal Working » Shearing
Cutting of sheet metal is accomplished by a shearing action
between two sharp cutting edges.
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At the top of the cut surface is a region called the Rollover.
This corresponds to the depression made by the punch in the
work prior to cutting. It is where initial plastic deformation
occurred in the work.
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Then another portion, Just below the rollover which rubs with
the surface of the punch, this is called Burnishing zone. This
results from penetration of the punch into the sheet before
fracture began. The smooth and shiny burnished surfaces on
the hole and the slug are from the contact and rubbing of the
sheared edge against the walls of the punch and die.
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Beneath the burnish is the Fractured Zone, a relatively rough
surface of the cut edge where continued downward movement
of the punch caused fracture of the metal.
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Finally, at the bottom of the edge is a Burr, a sharp corner on
the edge caused by elongation of the metal during final
separation of the two pieces.
Burr height increases with increasing clearance and ductility
of the sheet metal.
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So, if we sum up the deformation zone and the burnishing
zone, sum of both these is called PENETRATION.
Penetration is the depth by which punch must penetrate
the sheet before fractures meet. Depending upon the
thickness and depending upon the hardness of the
material, the different amount of the penetration is
required.
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» Penetration of Cutting Edge

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» Penetration of Cutting Edge
with the increasing entry of the punch into
the sheet metal the compressive strain as
well as the stresses will keep on increasing
and stage will arrive when the cracks will
nucleate both the sides and their growth
will start.

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» Penetration (% of Sheet Thickness)
Softer the metal, higher the penetration
Similarly, for increase the thickness, greater the % of the
penetration is required.
So, Hard metal requires less penetration and the thick
metals require more penetration

» Penetration (% of Sheet Thickness)
So, if we sum up the deformation zone and the burnishing
zone, sum of both these is called PENETRATION.
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» Quality of the Edge
The ratio of the shining (burnished) area to the rough area on
the sheared edge decreases with increasing clearance and
sheet thickness.
The quality of sheared edge is also affected by punch speed;
greater the punch speed better the edge quality.
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» STAGES IN SHEARING ACTION
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In phase I, during which the work material is compressed across and
slightly deformed between the punch and die, the stress and
deformation in the material do not exceed the elastic limit. This phase
is known as the ELASTIC PHASE.

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In Phase II, at this point in the operation, the material has been obviously deformed at
the rim, between the cutting edges of the punch and die. This concentration of outside
forces causes plastic deformation at the rim of the material. At the end of this phase, the
stress in the work material close to the cutting edges reaches a value corresponding to
the material shear strength, but the material resists fracture. This phase is called the
plastic phase.

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During Phase III, the strain in the work material reaches the fracture limit, and
micro-cracks appear which turn into macro-cracks, followed by separation of
the parts of the workpiece. The cracks in the material start at the cutting edge
of the punch on the upper side of the work material, also at the die edge on
the lower side of the material; the cracks propagate along the slip planes until
complete separation of the part from the sheet occurs.

The first phase is elastic
deformation (Step 1)
Plastic deformation (Step 2)
The third phase is the
shearing deformation phase
(Step 3),
Fourth phase, namely fracture
separation, will begin and
Fracture (Step 4)
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» Angular Clearance
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Purpose: allows blank to drop through die
Typical values: 0.25 to 1.5 on each side

» Punch and Die Sizes
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– Blanking (Die) (Blank diameter is controlled)
For a round blank of diameter, Db
– Diameter of die = blank diameter ( Db)
– Diameter of punch = Db - 2c
where c = clearance
– Punching (Punch) (Hole diameter is controlled)
- For a round hole of diameter, Dh
– Diameter of punch = hole diameter (Dh)
– Diameter of die = Dh + 2c
– Diameter of punch = hole diameter (Dh) = Diameter of die − 2c
c = radial clearance; 2c = Diametral Clearance

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In case of Punching operation
Exact hole Size
Purpose is to
Diameter of punch = hole diameter (Dh)
Clearance provided on die Ddie= Dh + 2c
So

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In case of Blanking operation
Exact Blank Size
Purpose is to
Diameter of Die = Blank diameter (Db)
Clearance provided on
Punch
DP= Ddie − 2c
So

» Clearance
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Typical clearances in conventional press working range between 6%
and 18% of the sheet-metal thickness t.
For Hard metal – 6 to 10% of t
For Soft metal – 10 to 18% of t
 t.mmc 00320)(in
 t.mmc 00640)(in2

» Cutting Forces (Punch Load)
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Important for determining press size (tonnage)(Punch load requirement)
F =τ×t ×L
Where,
τ = shear strength of the metal;
t = stock thickness,
L = length of cut edge (contact length) = πD
L is basically perimeter of blank or hole being cut
The above formula is based on fact that entire
punch face is engaged in cutting.
If angled punched is used, cutting force will
reduce.

» Cutting Forces (Punch Load)
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For rectangular punch
Perimeter = 2(a+b)
F =τ×t×2(a+b)
Blank
a
b
Work done = Punch force × punch travel
Punch travel = (% of penetration )× (material thickness)

» Numerical Problem 01
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A metal disc of 20 mm diameter is to be punched from a sheet of
2mm thickness. The punch and the die clearance is 3%. The
required punch diameter is
Blanking operation
Ddie= Dblank=Dpunch+2c
Dpunch = Ddie – 2c
Dpunch = 120202
100
3
220 . mm.8819

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A round disk of 150 mm diameter is to be blanked from a strip of
3.2-mm, half-hard cold rolled steel whose shear strength = 310
MPa. Determine (a) the appropriate punch and die diameters, and
(b) blanking force. Given: clearance allowance for half-hard cold-
rolled steel is Ac = 0.075.

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Determine the die and punch sizes for blanking a circular disc of 20
mm diameter from C20 steel sheet whose thickness is 1.5mm.
Shear strength of C20 steel is 294MPa.
 t.mmc 00320)(in
2945100320)(in  ..mmc
mm..mmc 10008230)(in 
Since, it is blanking operation,
Die size (Ddie) = blank size (Db) = 20 mm
DP= Ddie − 2c
Punch Size = blank size (Db) − 2C = 20−2×0.10 = 19.8 mm

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Diameter of punch = hole diameter (Dh)
Ddie= Dh + 2c

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A diameter 25 mm hole is pierced in a t = 2.5 mm thick steel sheet
having a shear strength τ = 350MPa. If the diametric clearance is
given by the expression C = 0.0064t√τ . Find the die diameter (in
mm), punch diameter (in mm) and punch force (in KN)
respectively.
2c = Diametral Clearance = C = 0.0064t√τ
2c = 0.0064×2.5×√350 = 0.2993 = 0.3 (approx.)
In case of Punching operation
die Ddie= Dh + 2c Ddie= DP + 2c
Clearance provided on

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respectively.
2c = 0.0064×2.5×√350 = 0.2993 = 0.3 (approx.)
Dh = DP = 25 mm
Ddie= DP + 2c
Ddie= DP + 2c = 25+0.3=25.3 mm

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respectively.
2c = 0.0064×2.5×√350 = 0.2993 = 0.3 (approx.)
Dh = DP = 25 mm
Ddie= DP + 2c = 25+0.3=25.3 mm
Punch force (F) =τ×t ×L
F =τ×t ×πD F =τ×t ×πDP
F =350×2.5×3.14(25) = 68687.5 N or 68.7 kN

»Difference b/w Punching & Piercing
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PiercingPunching
Bur
….
……..
Punching tool
Piercing tool
Slug is cut and bur is
minimum
No slug is cut, only bur

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TiN coated tool steel punches
To reduce punch wear
To increase punch life
To increase dimensional accuracy of holes

»Other miscellaneous cutting operations
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This is an operation in which
punch is formed without
removing the blank part.
No metal removal takes place.
The blank remain attached in
bent form.
Lancing

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is a piercing tooling that involves
punching a large number of
closely spaced holes.
Perforating
These holes may be round or some other shape. Perforating sheet
metal will allow for the passage of light or fluid material through the
sheet. It can often serve in ventilation and filtration of fluid
substances. Perforated sheet metal is also used in structure and
machine construction, to reduce weight and for cosmetic appearance.

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Notching is a sheet metal cutting
process that involves the removal
of material from a work piece,
starting at the edge and cutting
inward. The objective of
notching is to create a sheet
metal part with a desired profile.
Notching

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refers specifically to the
punching of rectangular or
elongated holes.
Slotting

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Slitting is a shearing process in
which the sheet metal is cut by
circular blades. Slitting can be
performed in a straight line or on
a curved path.
No Scrap material is produced.
Slitting

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

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is a finishing operation where a small
amount of metal is sheared away from an
already blanked part.
Its main purpose is to obtain better
dimensional accuracy,
but secondary purposes include squaring the
edge and smoothing the edge.
Shaving

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The trimming operation is the last operation
performed, because it cuts away excess or
unwanted irregular features from the walls
of drawn sheets.
Trimming

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Difference between Trimming and Shaving
trim you remove a few mm/cm or lesser right
from the ends. Cut as close to the skin as possible

Sheet Metal Working »Sheet Metal Bending
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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 some cases bending may produce a small change in sheet
thickness. For most operations, however, bending will produce
essentially no change in the thickness of the sheet metal.

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Why bending?
In addition to creating a desired geometric form, bending is
also used to impart strength and stiffness to sheet metal, to
change a part's moment of inertia, for cosmetic appearance
and to eliminate sharp edges

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Bending Processes
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.

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Bending Processes »
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.
Edge bending - with a wiping die

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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.
Rotary bending

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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.
Rotary bending

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Air bending is the most common type of 3 Types Of
Bending used in sheet metal shops today.
In this process the workpiece comes in contact with the outside
edges of the die, as well as the punch tip. The punch is then
forced past the top of the die into the v-opening without coming
into contact with the bottom of the v. The v opening is typically
deeper than the angle which is sought in the work piece.
Air BendingMiscellaneous Bending Processes »

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Air bending is the most common type of 3 Types Of
Bending used in sheet metal shops today.
https://www.machinemfg.com/press-brake-
bending-basics/#Air_bending

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In this process the workpiece comes in contact with the outside
edges of the die, as well as the punch tip. The punch is then
forced past the top of the die into the v-opening without coming
into contact with the bottom of the v. The v opening is typically
deeper than the angle which is sought in the work piece.
https://www.machinemfg.com/press-brake-bending-
basics/#Air_bending

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Advantages of air bending process
 In air bending different bend angles can be produced by adjusting the
punch travel alone into the die, without the need for tool changes,
and this makes the technique more flexible than closed die bending.
 Since the punch does not force the sheet against the die, the force
necessary to form the parts is small and hence much less press
tonnage is required compared to closed die bending.
 Air bending can allow a degree of over bending necessary to
compensate for spring back..

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Limitations of air bending process
A precise control of the punch stroke is necessary to obtain the
desired bend angle.
Spring back in an air bending process is large in the absence of
bottoming.

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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. A U bend is made with a U
shaped punch of the correct curvature.
Channel Bending &
U Bending
Miscellaneous Bending Processes »

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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. A U bend is made with a U
shaped punch of the correct curvature.
Channel Bending &
U Bending

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Many bending operations have been developed to produce offsets
and form the sheet metal for a variety of different functions.
Offset BendingMiscellaneous Bending Processes »
Horizontal
Offset Bending
Spring Up
Offset Bending

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Offset tooling allows two V bends to be formed closer together
than regular stamping Dies would allow. This creates a Z shaped
profile in the sheet metal. The two basic forms of offset tooling
are ‘Spring Up’ and ‘Horizontal’.
Horizontal
Offset Bending
Spring Up
Offset Bending

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Offset dies can perform two close bends in one stroke
increasing production up to 500% compared with single bends
but the tonnage requirement can range from 3 to 15 times that
of a simple 90 bend. So, Offset bending is usually restricted to
relatively light-gage metal (3.2 mm or less).
Offset dies are also more expensive.
Offset bends can also be made in two strokes with standard V-
dies.

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Corrugating BendingMiscellaneous Bending Processes »

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Corrugated
Oxford dictionaries
(of a material or surface) shaped into a series of parallel ridges
and grooves so as to give added rigidity and strength.
Cambridge dictionary
(especially of sheets of iron or cardboard) having parallel rows of
folds that look like a series of waves when seen from the edge:
The roof is made from sheets of corrugated iron.

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Corrugating is a type of bending process in which a symmetrical
bend is produced across the width of sheet metal and at a regular
interval along its entire length.

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A variety of shapes are used for corrugating, but they all have the
same purpose, to increase the rigidity of the sheet metal and
increase its resistance to bending moments.
This is accomplished by a work hardening of the metal and a
change in the sheet's moment of inertia, caused by the bend's
geometry.
Corrugated sheet metal is very useful in structural applications
and is widely used in the construction industry.

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• Sheet metal of different sizes can be bent an innumerable amount
of ways, at different locations, to achieve desired part
geometries.
• Edge bending operations are commonly used in industrial sheet
metal processing and involve bending a section of the metal that
is small relative to the part.
• These sections are located at the edges. Edge bending is used to
eliminate sharp edges, to provide geometric surfaces for
purposes such as joining, to protect the part, to increase stiffness
and for cosmetic appearance.
Edge BendingMiscellaneous Bending Processes »

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Flanging is a bending operation in which the edge of a sheet metal
is bent at a 90° angle to form a rim or flange.
It is often used to strengthen or stiffen sheet metal.
The flange can be straight, or it can involve stretching or shrinking.
Edge Bending » FlangingMiscellaneous Bending Processes »
Straight flanging Stretch flanging Shrink flanging

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In stretch flanging the curvature of the
bending line is concave and the metal
is circumferentially stretched, i.e., A >
B. The flange undergoes thinning in
stretch flanging.
In shrink flanging the curvature of the
bending line is convex and the material
is circumferentially compressed, i.e., A
< B. The material undergoes thickening
in shrink flanging.

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In stretch flanging the curvature of the
bending line is concave and the metal
is circumferentially stretched, i.e., A >
B. The flange undergoes thinning in
stretch flanging.
In shrink flanging the curvature of the
bending line is convex and the material
is circumferentially compressed, i.e., A
< B. The material undergoes thickening
in shrink flanging.

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The term hemming has its origins in fabric making where the edge of
cloth is folded back on itself and then stitched shut. In sheet metal
hemming means to fold the metal back on itself. When working with
a Brake Press hems are always created in a two step process:
Edge Bending » Hemming
and seaming
consist of a sharp knife die set and
a flat die set.Remove the burred edge and
Improve the appearance of the
edge.
The hem also adds slightly to the
rigidity of the edge
Improves wear resistance.

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The term hemming has its origins in fabric making where the edge of
cloth is folded back on itself and then stitched shut. In sheet metal
hemming means to fold the metal back on itself. When working with
a Brake Press hems are always created in a two step process:
and seaming
Create a bend with Acute Angle
Tooling in the metal, 30° is
preferable but 45° will work for
some circumstances.
Place the acute bend under a
flattening bar and apply enough
pressure to finish closing the
bend.
Press brakes, also known as
Brake Presses or just brakes
are machines used to bend
sheet metal using tooling
known as dies and punches.

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Hems are three types
a) Single hem
b) Double hem
and seaming

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Seaming is a sheet metal joining process.
Seaming involves bending the edges of two parts over on each other.
As the bends are locked together, each bend helps resist the
deformation of the other bend, providing a well fortified joint
structure.
Seaming has been employed
to create watertight or airtight joints
between sheet metal parts.
and seaming
provided with
defensive works as
protection against
attack.

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Roll bending 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.
Roll bendingMiscellaneous Bending Processes »

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Roll bendingMiscellaneous Bending Processes »

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