SHEET METAL FORMING

Other Cutting Operations

• Cutoff • Perforating
– Each cut is a new part – punching of a pattern
• Parting • Notching
– 2 cutting edges – edge portion removed
– Less efficient • Trimming,Shaving
• scrap and Fine Blanking
• Slotting – operations to clean up
– punching a whole and smooth out edges
 Sheet Metal Forming
• AGENDA
➢Introduction
➢Sheet Metal characteristics
➢Shearing
➢Piercing and Blanking
➢Die types
➢Machines
 1.0 Introduction

✓ Sheet metal is simply metal formed into thin and flat pieces.
✓ Sheet metal is essentially metal pressed into sheets. These sheets are used at various places.
These sheets can be bent, cut and molded into any shape for use anywhere.
✓ Sheet metal is generally produced in sheets by reducing the thickness of work piece by
compressive forces applied through a set of rolls.

Sheets
 Sheet Metal Forming
How to differentiate a sheet and a plate ?

• If thickness is less than 6 mm (1/ 4 inches) then

it is regarded as sheet.
• If thickness is greater than 6 mm (1/ 4 inches)
then it is regarded as plate.
 Applications of sheet metals
• Aircraft Bodies

• Automobiles bodies

• Utensils used for domestic purposes

• Beverage cans
 Sheet Metal Operations

• Shearing
• Blanking
• Bending
• Stretch forming
• Deep drawing
• Redrawing etc
 2.0 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.
 Elongation
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. The two
pieces are placed together and the amount of
extension is measured against marks made before
starting the test and is expressed as a percentage of
the original gauge length.
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 or 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
 ANISOTROPY
• They may be two types i.e. crystallographic
anisotropy (grain orientation) and mechanical
fibering (alignment of impurities, voids, inclusions).
The anisotropy present in plane of the sheet is called
planar anisotropy, and the anisotropy present in
thickness direction is called normal anisotropy.

R = εw / εt

normal anisotropy
 Anisotropy
• Planar anisotropy R’ is given by
R’ = (R0 + 2R45 + R90) / 4
where, subscripts 0, 45, 90 refer to angular
orientation of the specimen with respect to rolling
direction of sheet.
• Grain size : coarser the grain rougher the surface finish
Generally ASTM grain size of No. 7 is preferred for
general sheet metal forming.
 Residual stresses :

• Residual stresses can develop in sheet

metal forming due to non uniform
deformation that take place.
• When disturbed such as by 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.
 Spring back
• Because sheet metal generally are thin and are subjected to
relatively small strains during forming, sheet metal parts
are likely to experience considerable springback.
• This effect is particularly significantly in bending and
other forming operations where the bend radius to
thickness ratio is high, such as in automotive body parts.
Spring back (wiping die)
 Wrinkling

• Compressive stresses are formed in plane of the

sheet results in wrinkling (buckling).
The tendency of wrinkling increases with
Unsupported length of sheet metal,
Decreasing thickness,
Non uniformity in thickness,
Lubricants trapped can also contribute to wrinkling.
 SHEARING
• Shearing is a process for cutting sheet
metal to size out of a larger stock such as
roll stock.
Shearing Machine
 Shearing
• Material thickness ranges from 0.125 mm to
6.35 mm (0.005 to 0.250 in). The dimensional
tolerance ranges from ±0.125 mm to ±1.5 mm
(±0.005 to ±0.060 in).
• The shearing process produces a shear edge
burr, which can be minimized to less than 10%
of the material thickness.
• The major variables that affect the shearing are
punch force, speed of the punch, lubrication,
The hardness of punch and die materials, the
corner radii of the punch and the clearance
between die and punch.
Paper cutter slicing sheet of paper

Variables affecting shearing

Shear force
• Shear force is basically the product of shear
strength of sheet metal and the cross sectional
area being sheared
Fmax = 0.7 * UTS * t * L
Where, UTS – ultimate tensile strength
t – Thickness
L – Total length of sheared edge
(For round hole of diameter D, L = π * D)

• maximum force can be obtained only when

maximum speed is derived or vice versa.
 Engineering Analysis (cont.)
• Cutting Forces (F)
–F=S*t*L
• S - shear strength
• t - thickness
• L - length of cutting edge
– F = 0.7TS * t *L
• TS - Ultimate tensile strength
– Max F is used to determine press for
operation
 HARDNESS, BURR, RADIUS ON
CUTTING SURFACES
• There should be optimum hardness, more than
required hardness would make it brittle, and
lesser would not withstand higher force .
• Burr are small projection that are formed on
outer surface of a formed component. These
are undesirable as they increas operaetions,
cost, time.
• Small radius should be given to cutting surfaces
to impart strength, it should be optimum
 Clearance between Punch and Die or
Clearance between two cutting blades
CLEARANCE = THE MEASURED SPACE
BETWEEN THE MATING MEMBERS OF A DIE
SET ( C )
C/2 P C/2

d1

D D
d
 Engineering Analysis
• Clearance (C)
– Distance between punch and die
– 4%-8% of sheet thickness
– Small Clearance
• double burnishing
• large cutting force
– Large Clearance
• Sheet becomes pinched
• excessive burr
 CLEARANCE
CLEARANCE = C = d – d1

therefore CLEARANCE PER SIDE = C= C / 2

ALSO CLEARANCE PER SIDE C / 2 IS GIVEN

BY
c /2 = 0.01* s* sqr (Tb)
S=material thickness Tb= shear stress
Generally clearance is of 5 to 10% of material
thickness.
 Clearance – its effects

• Greater clearance – rough edges, formation of

burr, material is pulled rather being cut
• Smaller clearance – though has good cut edges
– forces required is more – also tool withdrawal
is difficult .
• Tool and die materials: generally steels are
used, ex:- Die steel, HSS for most common
operations.
• For higher production carbides are employed.

Scraps : amount of scrap can be a high as upto

30% of original sheet. This can be reduced by
proper nesting.
 Shearing Operations

• Die cutting
• Slitting
• Nibbling
• Steel rules
 DIE CUTTING
• It is combination of following operations
• Perforating.
• Parting or shearing a sheet into two.
• Notching or removing pieces in edges.
• Lancing or leaving a tab with removing any
material
 SLITTING
• Shearing operation –using circular blades
• These blades follow straight line or circular path or
curved path – depending upon requirement.
 NIBBLING
• Nibbler (tool) move straight up and moves down
rapidly.
• Sheet metal is fed in gap between two cutting
tools and produces overlapping holes
• Suitable small runs, several intrcate shapes can
be produced.
 PIERCING
• It is a shearing operation.
• Creates open hole in sheet metal by
separating the interior section.
• Removed metal is discarded as scrap

slug

PART
 BLANKING:

• It is also a shearing operation.

• It enlarges earlier pierced hole.
• Removed metal is desired one.

PART

SLUG
• Very smooth and square edges can be obtained
by fine blanking.
 DIE TYPES
• Open type
• Compound types
• Progressive types
• Transfer type
 Dies and Presses
for Sheet Metal Processes
• Dies
– Components of Dies (picture)
– Types
• Simple- single operation with a single stroke
• Compound- two operations with a single stroke
• Combination- two operations at two stations
• Progressive- two or more operations at two or
more stations with each press stroke, creates what
is called a strip development
Open type
Compound type
Progressive type
Transfer type
 Dies and Presses
for Sheet Metal Processes
• Presses
– Definition- In sheet metal working it is a
machine tool with a bed and powered ram
that can be driven toward and away from the
frame to perform various cutting and forming
operations
 Dies and Presses
for Sheet Metal Processes
•Solid- one piece construction,up to 1000 tons
•Adjustable bed-accommodates different die sizes
•Open Back-tilted for easy removal of stampings
•Press Brake-wide bed for use of various dies
•Turret-suited for sequence of punching and
notching
Straight Sided Frame-high tonnage presses (4000),
greater structural stability
Power Systems
• Hydraulic-driven by a piston/cylinder system
• Mechanical-eccentric,crankshaft,knuckle joint
CRANK PRESS
FRICTION PRESS
HYDRAULIC PRESS
 BENDING
• Most commonly used metal forming process.
• Used – form parts such as flanges curls
seams and corrugation
• To impart stiffness by increasing moment of
inertia
• Here straight length is transformed into
curved length
Bending Processes

ME 4210: Manufacturing Processes and Engineering 4

Prof. J.S. Colton
 Terminology used in bending
• Bend allowance – length of neutral axis in bend
area.
• Used to determine blank length and bend part.
Lb = α (R = kt)
Where α = bend angle in radians
R = bend radius
k = constant
t= thickness of sheet
For ideal case k = 0.5
 Terminology used in bending
MINIMUM BEND RADIUS
As load is applied compression – inner fiber,
tension –outer fiber.
Theoretically strains are equal eo = et
But due to shifting of neutral axis length of
bend is smaller in outer surface hence
there is difference in strains.
eo = e t = 1 / ((2R /t)+1).
 MINIMUM BEND RADIUS
• As R/ t increases tensile strain decreases
increases in outer fiber and material may
crack a certain strain
• The radius R @ which crack appears on
outer surface is called minimum bend radius
• It usually expressed in 2t, 3t, 4t……….
• If R / t approaches 0 then it can completely
bendable ex: paper. This characteristic is
called bend ability.
 Bendability

ME 4210: Manufacturing Processes and Engineering 25

Prof. J.S. Colton
 Factors affecting bend ability
• Bend ability can be increased by increasing
tensile stresses
• It can be increased by increasing temperature
• It can be increased by increasing pressure.
• It can be increased by increasing
compressive stresses in the plane of the
sheet and minimizing tensile stresses in outer
stresses.
 Bend length L
• As the length increases outer fiber change
from uniaxial stresses to biaxial stresses.
• Reason for this: L tends to become smaller
due to stretching of outer fiber.
• Biaxial stretching tends to reduce ductility,
increases chances of failure.
• As L increases minimum bend radius also
increases
 Edge condition
• Edges - being rough bend ability
decreases.
• Removal of cold work regions such as
shaving, machining, or heat treating
greatly improves the resistance to edge
cracking.
 Spring back
• Plastic deformation is followed by elastic
recovery upon removal of load this
recovery is SPRING BACK.
• This is shown in figure NEXT SLIDE
• Can be observed in short strip metal.
• It can occur in any cross section.
Spring back (wiping die)

ME 4210: Manufacturing Processes and Engineering 24

Prof. J.S. Colton
Bending Operations

ME 4210: Manufacturing Processes and Engineering 5

Prof. J.S. Colton
 Common bending operations
PRESS BRAKE FORMING
✓Sheet metal can be bent using simple fixtures,
and presses.
✓Press brake is usually machine that is used.
✓This M/C uses mechanical or hydraulic press,
usually suitable for short runs and can be
automated.
✓Die materials can of wood, steels ,carbides .
 Press Brake

ME 4210: Manufacturing Processes and Engineering 6

Prof. J.S. Colton
 AIR BENDING
• Here only one die is used
• Bending is carried out with a pair of rolls, the
larger one is made of polyurethane.
• Upper roll pushes into flexible lower roll.

ROLL BENDING
•Here three rolls is used, adjusting distance
between three rolls produces various
curvatures.
•Here short pieces can also be bent.
 ROLL-bending

ME 4210: Manufacturing Processes and Engineering 8

Prof. J.S. Colton
 Air Bending

ME 4210: Manufacturing Processes and Engineering 9

Prof. J.S. Colton
ROLL BENDING

ME 4210: Manufacturing Processes and Engineering 10

Prof. J.S. Colton
 Common bending operations
• BEADING – here sheet is bent into cavity
of the die, improve stiffness, imparts
moment of inertia of edges.
• FLANGING – here sheet edges are bent
@ 90 deg. This causes hoops stress, if
excessive bent then wrinkling chances are
more – leads to cracking
 STRETCH FORMING
• SHEETS ARE CLAMPED AROUND ITS
EDGES AND STRECHTED OVER A DIE.
• Can be moved upwards downwards
sideways depending upon requirement.
• Primarily used to make aircraft wing skin,
automobile door panels and window
frames.
• Although used for low volume prod. It is
versatile and economical.
Stretch Forming

ME 4210: Manufacturing Processes and Engineering 13

Prof. J.S. Colton
Stretch Forming

ME 4210: Manufacturing Processes and Engineering 14

Prof. J.S. Colton
 STRECTH FORMING
• In most operation blank is rectangular
sheet, clamped along narrow edges and
stretched length wise.
• Controlling amount of stretch is important
to avoid tearing.
• This process cannot produce parts with
sharp edges
• Dies- zinc alloys, hard plastics, wood.
 DEEP DRAWING
• Used to shaping flat sheets into cup
shaped articles.
• This is done by placing blank of
appropriate shaped die and pressed into
with punch.
Deep Drawing

ME 4210: Manufacturing Processes and Engineering 19

Prof. J.S. Colton
 Cracks
• It is regarded as the ultimate defect.
• Development of cracks destroys its
structural integrity.
 Buckling or wrinkling
• Wrinkling of the edges results in buckling of
the sheet due to high circumferential
compressive stresses.
• If a blank diameter is too high punch load
will also rise which may exceed critical
buckling value. this may lead to failure of
sheet .
• To prevent this it is necessary to have
sufficient hold pressure to suppress the
buckling.
 Wrinkling

• Compressive stresses are formed in plane of the

sheet results in wrinkling (buckling).
The tendency of wrinkling increases with
Unsupported length of sheet metal,
Decreasing thickness,
Non uniformity in thickness,
Lubricants trapped can also contribute to wrinkling.
 SURFACE DEFECTS
• SINCE SHEET METAL IS CHARACTERISED
BY HIGH SURFACE AREA – SURFACE IS
PRONE TO DEFECTS.
• Susceptible to surface blemishes, peeling of
surface also known as orange peeling – it is
mostly occurs in sheets having large grain size.
• This can be corrects by using sheets having
smaller grain size.
 Stretcher marks or worms
• This is regarded another serious defect.
• This defect is characterized by flame like patterns or
depressions on the surface.
• These depressions first appear along planes of shear
stresses, they continue growing as they join – they give
depression like surface or rough surface.
• This directly related to yield point elongation.
• Main difficulty – it appears in regions where strain is less
than yield point
• The remedy is to give sheet metal a small cold reduction,
temper rolling, cold works can reduce the defect.
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
 EARING
• Here directional properties are essential.
• This usually occurs in deep drawing
processes.
• It is formation of wavy edges on a top of a
drawn cup – necessitates extensive
trimming.
• It is related to planar isotropy, can be co-
related as R =R0 + R90 -2 * R45
• Subscripts represents degree of orientation
of fibers.
 BURR
• This defects is usually seen in shearing and
blanking operations
• These are real productivity killers.
• They not only increase time but cost to deburr.
• Deburring operations add no value to process.
• This is related to clearance between cutting
edges
• This can be reduced by decreasing the cutting
clearances but note that clearance can be
decreased without increasing cutting forces.
 What is Sheet Metal?

• 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
 Sheet Metal Operations: -

➢ Bending ➢ perforating
➢ Shearing ➢ Nibbling
➢ Blanking ➢ Embossing
➢ Punching ➢ Shaving
➢ Trimming ➢ Cutoff
➢ Parting ➢ dinking
➢ Slitting ➢ Coining
➢ Lancing ➢ Deep drawing
➢ Notching ➢ Stretch forming
➢ Roll forming
Bending

• 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.
Press Brake machine
Bending types
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“.
• 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 Operations

Straight flanging Stretch flanging Shrink flanging

Hemming Seaming Curling

 Bending Operations

Channel bending U-bending air-bending

Offset-bending Corrugating Tube forming

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 example
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.
Cutoff
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.
Deep Drawing Sequence
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.
1. Simple die
2. Compound die
3. Combination die
4. 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