Institute of Technology
Department of Mechanical Engineering
Production Engineering II
• Sheet metal forming operation
• Applications of sheet metal
• Cutting operation
• Engineering analysis of sheet metal
Sheet metal forming
Sheet metal working includes cutting and forming
operations performed on relatively thin sheets of metal.
Typical sheet metal thicknesses are below 6mm (1/4in).
When the thickness exceeds about 6mm,the stock is
usually referred to as plate rather than sheet.
The sheet or plate stock used in sheet metalworking is
produced by rolling.
Automobile and truck
Ship building furnitureRailway Cars
Chemical industry;Drink & food industry.
Shower cabinets and othersWashing machines
Five basic sheet metal
• Cutting of sheet metal is accomplished by a shearing action
between two sharp cutting edges.
• The upper cutting edge (the punch) sweeps down past a
stationary lower cutting edge(the die).
Just before the punch
contact the work
Punch begins to push into
the work, causing plastic
Punch compresses and
penetrates into the work
and causing a smooth cut
Fracture is initiated at the
opposing cutting edges
that separate the sheet.
Symbols V and F
represents motion and
applied force ,
respectively, t = stock
thickness , c = clearance.
• Is a sheet metal cutting operation along a straight line
between two cutting edges.
• Shearing is typically used to cut large sheets into smaller
sections for subsequent press working operations.
• It is performed on a machine called a power shear, or squaring
• The upper blade of the power shears is often inclined (to reduce
• The shearing operation constitutes the first stage of any forming
process by producing either
The starting material (cutting out of a sheet) OR
Preparing an existing work piece e.g by punching a hole or a
series of holes before forming.
• Involves cutting of sheet metal along a closed outline in a
single step to separate the piece from the surrounding
stock, as shown in figure a the part that is cut out is the
desired product in the operation and is called the blank.
• Is similar to blanking except that the separated piece is
scrap, called the slug. the remaining stock is desired part.
the distinction is illustrated in figure b.
Engineering analysis of
• One of the important parameters in sheet metal cutting is
clearance between the punch and die
• The clearance c in a shearing operation is the distance
between the punch and die.
• Typical clearance in conventional press working range
between 4% and 8% of the sheet metal thickness t.
• Measured perpendicular to
the direction of the blade
• It affects the finish of the
cut(burr) and machine’s
• If the clearance is too small,
then the fracture line tend to
pass each other ,causing a
double burnishing and large
• If the clearance is too large ,the
metal becomes pinched between
the cutting edges and an
excessive burr results.
• The correct clearance depends on sheetmetal type and
• The recommended clearance can be calculated by the
C = Act
C=Clearance , mm;
Ac = Clearance allowance (percentage of the material
t=Stock thickness, mm.
• The clearance allowance is determined according to the
type of metal 4.5%, 6% or 7.5% of the material
• For convenience, metals are classified into three groups
with an associated allowance value for each group.
Clearance allowance for three sheet metal groups
• The calculated clearance values can be applied to
conventional blanking and hole-punching operations to
determine the proper punch and die sizes.
• The die opening must always be larger than the punch
• Because of the geometry of the sheared edge, the outer
dimension of the part cut out of sheet will be larger than the
• Punch and die sizes for a round blank of diameter Db are
Blanking punch diameter =Db-2c
Blanking die diameter =Db
• Punch and die sizes for a round hole of diameter Dh are
Hole punch diameter = Dh
Hole die diameter = Dh+2c
Purpose: allows slug
or blank to drop
Typical values: 0.25°
to 1.5° on each side
• Estimates of cutting force are important because this force
determines the size (tonnage) of the press needed. Cutting
force F in sheetmetal working can be determined by
Where S= Shear strength of the metal, Mpa
t = Sthock thickness, mm
L= length of the cut edge,mm
• In blanking ,punching ,slotting ,and similar operations , L
is the perimeter length of the blank or hole being cut.
A round disk of 150mm diameter is to be blanked from a
strip of 3.2mm cold rolled steel whose shear
strength=310MPa. Determine (a) the appropriate punch
and die diameters, and (b) blanking force.
Other sheet-metal cutting
Is a shearing operation in which blanks are
separated from a sheet-metal strip by cutting the
opposite sides of the part in sequence.
With each cut, anew part is produced.
Distinguish it from a conventional shearing operation.
• The cut edges are not necessarily straight
Involves cutting a sheet-metal strip by a punch
with two cutting edges that match the opposite
sides of the bank.
This might be required because the part outline
has an irregular shape.
Parting is less efficient than cutoff in the sense
that it results in some wasted materials.
Is the term sometimes used for a punching
operzation that cuts out an elongated or
Involves the simultaneous punching of a pattern
of holes in sheet metal.
The hole pattern is usually for decorative
purposes, or allow passage of light, gas, of fluid.
• Notching involves cutting out a portion of metal
From the side of the sheet or strip.
• Seminotching removes a portion of metal from
the interior of the sheet.
Seminotching might seem the same as punching
and slotting operation.
The difference is that the metal removed by
seminotching creats part of the blank outline,
while punching and slotting creates holes in the
Is a shearing operation used to blank sheet metal parts with
close tolerances and smooth, straight edges in one step, as
illustrated in figure (b).
Pressure pad applies holding force(Fh) in order to
The punch then descends with slower than the
normal velocity and smaller clearance to provide
the desired dimension and cut edge.
The process in usually for small stock thicknesses.
Bending in sheet metal work is defined as the straining of the
metal around a straight axis (figure a).
During bending the metal on the inside of the neutral plane is
compressed ,while the metal on the outside of the neutral plane is
stretched (figure b).
• The metal is plastically deformed so the bend
takes a permanent set upon removal of stresses
that caused it.
• Bending produces little or no change in the
thickness of the sheet metal.
V-Bending and Edge bending
Bending operations are performed by using punch
and die tooling .
The common bending methods are
1. V-Bending ,performed with a V-die.
2. Edge bending , performed with wiping die.
Engineering analysis of bending
The metal of thickness t is bend through an angle
called the bend angle α.
This results in a sheet metal part with an included
angle α’, where α + α’= 180°
R=Inside Bend radius
W= The bend is made over the width of the work piece W.
Kba= k factor ( the location of the neutral axis in the
• Calculated as the ratio of the neutral axis (measured from
the inside bend surface to the material thickness)
• Since neutral axis is the theoretical location at which the
material is neither compressed nor stretched .we use the
following recommended values of K.
Recommended value of Kba
The bend allowance (Ab) is the length of the neutral
axis between the bend lines, or the arc length of the
When the bending pressure removed at the end of deformation ,elastic
energy remains in the bent part ,causing it to recover particularly to its
original shape. This elastic recovery is called springback.
Figure: (1) during the operation ,the work is forced to take the radius Rt and
included angle α’t determined by the bending tool (punch in v-bending) (2) after
the punch is removed ,the work springs back to radius R and included angle α’.
Symbol: F=applied bending force.
• Spring back in bending show itself as a decrease
in bend angle and an increase in bend radius.
Sb= spring back
α’= Included angle of the sheet metal (after spring
α’t =Included angle of the bending tool
Compensation for springback
Can accomplished by several methods one of these
methods is over bending. Due to this elastic
recovery, it is necessary to over-bend the sheet a
precise amount to achieve the desired bend radius
and bend angle . This is called over bending.
F = Bending force
TS = Tensile strength of the sheet metal
W = Width of part in the direction of the bend
t= stock thickness
D=die opening dimension
Kbf= Is a constant for differences encountered in
actual bending process. Its value depends on
type of bending: For V-bending, Kbf=1.33; and
for edge bending, Kbf=0.33
Example: A sheet metal blank is to be bent as shown in figure
below. The metal has a modulus of elasticity=205(103
) MPa, yield
strength=275MPa, and tensile strength=450MPa. Determine (a) the
starting blank size and (b) the bending force if a V-die is used with
a die opening dimension = 25mm.
Drawing is a sheet metal forming operation used
to make cup shaped ,box-shaped ,or other
complex curved, hollow shaped parts.
It is performed by placing a piece of sheet metal
over a die cavity and pushing the metal into the
opening with a punch.
The blank must be held down flat against the die
by blank holder.
( a ) Drawing of cup
shaped part :
(1) start of operation
before punch contacts
(2) near end of stroke
Mechanics of drawing
A blank diameter Db is drawn into a die by means
of a punch of diameter Dp.
The punch and die must have a corner radii, given
by Rp and Rd .
If the punch and die were to have sharp corners
(Rp and Rd=0) a hole punching process wood be
accomplished rather than drawing operation.
The sides of the punch and die are separated by a
The clearance in drawing is about 10% grater than
the stock thickness.
Stages in deformation of the work
1) Punch makes initial
contact with work
4) Friction and
5) Final cup shape
Engineering analysis of drawing
Checking for feasibility of the operation
Drawing ration: The ratio of blank diameter Db
to punch diameter Dp.
Un upper limit on the drawing ration is a value
of 2.0 (DR≤2.0)
The value of reduction r should be
less than 0.50
Thickness to diameter ration
It is desirable the ratio to be
greater than 1%.
F = drawing force, N
t = original blank thickness
TS= tensile strength
Db and Dp = starting blank and punch diameter.
Constant 0.7 is correction factor to account for
Fh=holding force in drawing ,N
Y=yield strength of the sheet metal
t =starting stock thickness,mm
Rd=die corner radius, mm
The holding force (Fh) required can be calculated by using the