• Practically all metals, which are not used in cast form are reduced to
some standard shapes for subsequent processing.
• Forming is the process in which the desired size and shape are obtained
through the plastic deformation of a material.
• Stress induced during the process are grater than the yeld strength, but
less then the fracture strength of the material.
• Sometimes continuous casting methods are also used to cast the liquid
metal into slabs, billets or blooms
• These shapes are further processed through hot rolling, forging or
extrusion, to produce materials in standard form such as plates, sheets,
rods, tubes and structural sections.
• Large group of manufacturing processes in which plastic deformation is
used to change the shape of metal workpieces.
• Sheet metal forming is a grouping of many complementary processes that
are used to form sheet metal parts.
• One or more of these processes is used to take a flat sheet of ductile
metal, and mechanically apply deformation forces that alter the shape of
• Before deciding on the processes, one should determine whether a
particular sheet metal can be formed into the desired shape without
• The sheet metal operations done on a press may be grouped into two
categories, cutting (shearing) operations and forming operations
Metal forming is mainly categorized into two parts
I. Bulk metal forming
I. Sheet metal forming
2. Punching & blanking
3. Stretch forming
4. Deep drawing
• These processes involve large amount of plastic deformation. The cross-
section of workpiece changes without volume change. The ratio cross-
section area/volume is small.
• For most operations, hot or warm working conditions are preferred
although some operations are carried out at room temperature.
• Cold working < 0.4 Tm
• Warm working 0.4 – 0.6 Tm
• Hot working > 0.6 Tm
Bulk metal forming
Sheet metal forming
• In sheet metal working operations, the cross-section of workpiece does
not change—the material is only subjected to shape changes.
• The ratio cross-section area/volume is very high.
• Sheet metal working operations are performed on thin (less than 6 mm)
sheets, strips or coils of metal .
• Rolling is the most extensively used metal forming process and its share
is roughly 90%.
• The material to be rolled is drawn by means of friction into the two
revolving roll gap.
• The compressive forces applied by the rolls reduce the thickness of
the material or changes its cross sectional area.
• The geometry of the product depend on the contour of the roll gap.
• In rolling the crystals get elongated in the rolling direction.
• In cold rolling crystal more or less retain the elongated shape but in
hot rolling they start reforming after coming out from the deformation
• In the deformation zone the thickness of the strip gets reduced and it
elongates. This increases the linear speed of the at the exit.
• Thus there exist a neutral point where roll speed and strip speeds are
equal. At this point the direction of the friction reverses.
• Roll materials are cast iron, cast steel and forged steel because of high
strength and wear resistance requirements.
• Hot rolls are generally rough so that they can bite the work, and cold
rolls are grind and polished for good finish.
• Roll torque, power etc. increase with increase in roll work contact
length or roll radius.
• When the angle of contact α exceeds the friction angle λ the rolls
cannot draw fresh strip
Pressure during rolling
Typical pressure variation along the contact length in flat rolling.The
peak pressure is located at the neutral point. The area beneath the curve,
represents roll force.
• It is a process of giving two stages reduction with the help of three
• In three high rolling mills the work piece is rolled on both the
forward and return passes. So that thickness is reduced at each pass.
(c). four high mills
• In four high mills, bigger rolls are provided for backup and power
is given to the smaller rolls.
• The top and the bottom rolls rotate in opposite direction as do the
two middle rolls.
• A four high rolling mill is used for the hot rolling of armor and
other plates as well as cold rolling of plates, sheets and strips.
(d). Cluster mill
• It is a special type of four high rolling mill in which each of the two
working rolls is backup by two or more of the larger backup rolls for
rolling hard in materials.
• It may be necessary to employ work rolls of a very small diameter but of
• In such cases adequate of the working rolls can be obtained by using a
(e). Tandem mill
• It is a set of two or three stands of roll set in parallel alignment.
• So that a continuous pass may be made through each one successively
with change the direction of material.
(f). Planetary Rolling mill
• This mill consists of a pair of heavy backing rolls surrounded by a
large number of small planetary rolls.
• The main feature of this mill is that it hot reduces a slab to coiled strip
in a single pass.
• The operation requires feed rolls to introduce the slab into the mill,
and a pair of planishing rolls on the exit to improve the surface finish.
• A wide variety of defects are possible in metal rolling
• Surface defects commonly occur due to impurities in the
material, scale, rust, or dirt.
• Most serious internal defects are caused by improper material
distribution in the final product.
• Defects such as edge cracks, center cracks, spread, crocodile
and wavy edges, are all common with this method of metal
• Extrusion is a compression process in which the heated billet of metal
is forced to flow through a die opening to produce a desired cross-
sectional shape, thus forming an elongated part of uniform cross
• The pressure is applied hydraulically or mechanically.
2. Extrusion Process
Types of extrusion process
1. Direct extrusion (forward extrusion)
2. Indirect extrusion, ( backward extrusion or reverse extrusion),
3. Impact Extrusion
4. Hydrostatic Extrusion
Direct extrusion (forward extrusion)
• A metal billet is loaded into a container, and a ram compresses the
material, forcing it to flow through one or more openings in a die at the
opposite end of the container.
• As the ram approaches the die, a small portion of the billet remains
that cannot be forced through the die opening.
• This extra portion, called the butt, is separated from the product by
cutting it .
• One of the problems in direct extrusion is the significant friction that
exists between the work surface and the walls of the container as the
billet is forced to slide toward the die opening.
• This friction causes a substantial increase in the ram force required in
Indirect extrusion, (backward extrusion)
• the die is mounted to the ram rather than at the opposite end of the
• As the ram move, the metal is forced to flow through the clearance in
a direction opposite to the motion of the ram.
• Since the billet is not forced to move relative to the container, there is no
friction at the container walls.
• the ram force is therefore lower than in direct extrusion.
• There are practical limitations on the length of the extruded part that can
be made by this method.
• Support of the ram becomes a problem as work length increases.
Depends on : strength of material ,friction between billet and
chamber & die surfaces
Process variables: temperature, velocity
Extrusion ratio, R,= A0/Af
F = A0k ln(A0/Af)
The extrusion constant, k, is determined experimentally, depends on
• As the name suggests, the punch impacts the work part rather than
simply applying pressure to it.
• Impacting can be carried out as forward extrusion, backward extrusion,
or combinations of these.
• Impact extrusion is usually done cold on a variety of metals.
• It is used to make individual components.
• Products made by this process include toothpaste tubes and battery
cases, very thin walls are possible on impact extruded parts.
• This problem can be addressed by surrounding the billet with fluid
inside the container and pressurizing the fluid by the forward motion of
• there is no friction inside the container, and friction at the die opening is
reduced. Consequently, ram force is significantly lower than in direct
• Hydrostatic pressure on the work increases the material’s ductility.
• Accordingly, this process can be used on metals that would be too
brittle for conventional extrusion operations.
• Ductile metals can also be hydrostatically extruded, and high
reduction ratios are possible on these materials.
• Problem- formation of the billet.
Defects In Extruded Products
(a) Center burst: It is an internal crack due to tensile stresses along the
(b) Piping: it is the formation of a sink hole in the end of the billet with
(c) Surface cracking: cracks develop at the surface. factors are high friction
and surface chilling of high temperature billets in hot extrusion.
• The cross section of solid rod, wire, or tubing is reduced or changed in
shape by pulling it through a die .
• Drawing is usually performed in round sections at room temperature,
thus it is classified as a cold working process
1. Wire drawing
2. Tube drawing
3. Deep drawing
Wire and Rod Drawing Process
• Drawing of wire is a metal working process used for the reduction of the
cross-section of the rod.
• During drawing the volume remains the same and hence there is increased in
the length of the drawn wire or rod.
• The difference between drawing and extrusion is that in extrusion the
material is pushed through a die, whereas in drawing it is pulled through it.
• The die angle influences the drawing force and the quality of the drawn
• several dies are used in succession (tandem) for small diameter wire
• Drawing force increases as reduction increases.
• Maximum ideal (no friction) theoretical reduction in cross
sectional area per pass is 63%.
• There is an optimum die angle for minimum force for a certain
reduction in diameter.
• The die angle influences the drawing force and the quality of the
F = YavgAf ln(Ao/Af )
Friction & Redundant Work
F = YavgAf [(1+μ/α)ln(Ao/Af )+2α/3]
α is die angle in radians is die angle,
Yavg is yield strength of the material.
• Tube drawing is a process to resize a tube by shrinking a large diameter
tube into a smaller one, by drawing the tube through a die .
• This process produces high-quality tubing with precise dimensions, good
surface finish, and the added strength of cold working.
• There are five types of tube drawing:
• 1.tube sinking, 2.mandrel drawing,
3.stationary mandrel, 4.moving mandrel, and
5. floating mandrel.
• A mandrel is used in many of the types to prevent buckling or
wrinkling in the workpiece.
• Bending is a forming operation in which a sheet metal is subjected to
bending stress thereby a flat straight sheet is made into a curved sheet.
• The sheet gets plastically deformed without change in thickness. Die and
punch are used for bending.
• During bending of a strip, the material outward of the neutral axis is
subjected to tensile stress. Material inside is subjected to compressive
• Bend radius R is the radius of curvature of the bent sheet inside the
Types of Sheet metal Bending
• performed with a V-shaped die
• For low production
• Performed on a press brake
• V-dies are simple and inexpensive
Edge bending –
• Performed with a wiping die
• For high production
• Pressure pad required
• Dies are more complicated and costly
Spring back in Bending
• increase in included angle of bent part relative to included angle of
forming tool after tool is removed
• Reason for spring back:
When bending pressure is removed, elastic energy remains in bent
part, causing it to recover partially toward its original shape
The force required for bending a sheet of thickness t, length L, to a radius R
is given by:
F = ((YLt2)/(2*(R+0.5t))*tan( α/2)
The maximum bend force is given by:
Fmax = kUtsLt2 / W
• Uts is ultimate tensile strength of the material,
• W is die opening width
• Y yield stresss of metal
• k takes values between 1.2 to 1.33 for v-die bending and
0.3 to 0.4 for wiping.
2. Blanking and punching
• Blanking and punching are shearing processes in which a punch
and die are used to modify webs.
• The tooling and processes are the same between the two, only the
terminology is different.
• The job is held by job holders to prevent distortion.
• A clearance is provided between the punch and die.
• Force is given on the job by the punching tool.
• A finite portion of metal is removed from the sheet.
• In blanking the punched out piece is used and called a blank.
• A small clearance, typically 10-20% of the material thickness, exists
between the punch and die.
• Clearance provided on the punch and shear on the die.
• In piercing the punched out piece is scrap.
• The most common punch shapes (circle, square, rectangle, etc.) or
• Secondary finishing performed to attain smoother edges.
• Clearance provided on the die and shear provided on the punch.
Depending on the shape of the metal removed or
process, there are some special types of punching
In notching, material is removed from the side of a sheet material.
Lancing process makes partway through the metal without producing
• It is an operation to cut a coiled sheet metal lengthwise to produce
• By using a small punch and die assembly the process of creating a
profile is called nibbling.
Trimming process refers to the removal of excess material in a flange or
Punching a close arrangement of a large number of holes in a single
4. Deep drawing
• Deep drawing of sheet metal is performed with a punch and die.
• The punch is the desired shape of the base of the part, once drawn.
• The die cavity matches the punch and is a little wider to allow for its
passage, as well as clearance.
• The sheet metal work piece, called a blank, is placed over the die
• A blank holder, that surrounds the punch, applies pressure to the entire
surface of the blank and holding it.
• The punch travels towards the blank.
• After contacting the work, the punch forces the sheet metal into the die
cavity, forming its shape.
Common Sheet Metal defects in drawn parts:
1.Wrinkling in the flange,
2.Wrinkling in the wall,
(a)Wrinkling in the flange:
Due to compressive buckling. Low compressive force.
(b) Wrinkling in the wall:
If and when the wrinkled flange is drawn into the cup, these ridges
appear in the vertical wall.
Due to high tensile stresses that cause thinning and failure of the metal
at this location.
Caused by anisotropy in the sheet metal drawing.
If the material is perfectly isotropic, earing doesn’t form.
(e) Surface scratches:
Due to the punch and die are not smooth or if lubrication is not
Metal forming process in which an axially symmetric part is gradually
shaped over a rotating mandrel using a rounded tool or roller
Conventional spinning: (1) setup at start of process; (2) during spinning;
and (3) completion of process