MANUFACTURING PROCESSES II PROF.DR.

MUSTAFA GÜNAY

1. FORMING METHODS

1.1. Introduction
The forming of metals with plastic deformation extends to the earliest known times in history.
It is known that the method of forging began to be used together with the creation of human
beings. Processes such as rolling and wire drawing are commonly used in the middle ages and
are known shaping or forming methods. Despite the fact that most of the plastic deformation
processes cannot be changed, major changes and improvements have been made in detail and
equipment. During the industrial revolution, manual operations were started with machines.
Today, computer-controlled, automated systems have become quite common. The main
forming methods are:
• Forging
• Rolling
• Extrusion
• Bar and wire drawing, pipe manufacturing
• Sheet forming methods

1.1.1. Basics of Plastic Forming

Plastic forming is the shaping process in which the volume of metals in the solid state does not
change. It is the phenomenon that a solid metal can be shaped without deteriorating continuity,
that is, without breaking or leaving. Plastic shaping can be done in cold, warm and hot
environments. In these operations, three basic forces are applied: tension, compression and
shear.

Shear
σ = P/Ao
Compression  = (L – Lo)/Lo
Tension γ=a/b

Volume before deformation (Vo) = Volume after deformation (V)

If plastic forming is done at high temperature, it will be covered with void and pore formed
during solidification of the material. Instead of coarse casting structure, a fine-grained
homogeneous structure arises. Properties such as yield strength, tensile strength, fatigue
strength, fracture toughness, ductility, and impact strength are improved. If plastic forming is
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

made cold, the strength of the material can be increased by making use of the work-hardening
that occurs. The plastic forming process can be understood with the best drawing curve. If the
cross-sectional area of the drawing sample is assumed to be constant (unchanged), an
engineering stress-strain graph is obtained.

Fig.1.1. Graph of tensile test

Material properties obtained from the tensile test

Materials can be handled in two ways, according to their behavior at the yield point; a) Materials
with specified yield point, b) Materials not specified yield point.

a) Materials with specified yield point: Stress value which causes 0.2% permanent (plastic)
deformation in these materials is called yield strength and is symbolized as σ0.2. In the diagram,
the greatest stress is called the ultimate tensile strength and is symbolized as σ. The slope of the
linear part in the elastic region of the diagram gives the modulus of elasticity (E). It determines
the ductility of the material, the percent elongation (δ) and the reduction of area (ψ). It is not
possible to obtain the value of the section narrowing from the diagram.

After the test, the area of the fracture cross-section is measured and is obtained by dividing the
difference between the initial cross-sectional area and the fracture cross-sectional area into the
initial area. The value of the break extension can be obtained from the diagram or it can be
obtained by reassembling the broken pieces together to determine the final length of the
measure and then the difference between this value and the first measure size as the first split
of the paint.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.2. Engineering stress-strain graph

It is called uniform extension, which is formed by the maximum stress, and is represented by
the permanent unit shape change (the elastic part from the total shape change at that point). The
deformation energy that the material consumes until it is broken during the test is called static
toughness (toughness) and is indicated by up. This value is equal to the area under the σ - ε
diagram. The energy that the material only requires to flow in the elastic zone is called the
resilience. This value is equal to the area under the elastic region in the σ - ε diagram.

Cold Forming:
A material subjected to cold deformation with tensile loads causes strain hardening (Fig. 1.3a).
It will be seen that the yield strength increases when subjected to a recoil load on a material that
has undergone a deformation cure. The tensile strength is also increased but not as fast as the
yield strength. The yield strength / tensile strength ratio approaches 1 (Figure 1.3b).

The resistance of the material increases when any metal material is subjected to loading beyond
the elastic limit. In the strain-strain diagram given above, the material has an elastic limit of 1
point. If the material is loaded above this limit and the load is lifted, plastic deformation occurs
in the material. When the material is reloaded, it appears that the elastic limit of the material
has reached the point 2 from point 1. Thus, the plastic deformation increases the strength of the
material. This process is referred to as strain hardening in the literature.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

u
→1
y

 = K n

a) Strain hardening

K: Material constant,
n: Strain hardening exponent
n coefficient changes from 0.2 to
0.5.
b) Influence of cold forming on stress-strain graph

Figure 1.3. Strain hardening

During cold forming, a number of changes occur in the microstructure of the material under the
influence of force. Manufacture of metallic machinery parts by cold plastic forming; cold
forming, recuperation and recrystallization (Figure 1.4). The amount of cold working in
metallic materials is limited. Because after a certain cold deformation the ductility of the
material goes down. Beyond that, deformation to be done causes cracking and breakage in the
material.
Tempering at T> 0.5 Tm is carried out to remove the adverse effects (permanent stresses) in the
cold-formed machine parts. This process is called recrystallization annealing. Until these
temperature values are reached, increasing dislocations between temperatures of 0.3Tm <T
<0.5Tm result in the formation of lower grain boundaries and regular lower grades. This
process, which is considered as recovery if sufficient time has been recognized, increases the
ductility by removing some of the excess hardening which occurs due to deformation in the
materials. In order to be recrystallized, a certain cold shape and temperature are needed. The
less the plastic cold forming, the higher the recrystallization temperature. During cold
deformation, the high dislocation density formed in the internal structure of the material forms
the repulsive force necessary for recrystallization. 1-10% of the energy consumed during cold
deformation is stored as cold deformation energy. This energy is released when the material is
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

annealed. The annealing temperature decreases with increasing deformation rate. Because the
stored energy increases with increasing deformation.

Figure 1.4. Change of mechanical properties and microstructure by cold forming

Advantages of cold forming:

1. High precision and closer tolerances,
2. Better surface quality,
3. Increase in resilience,
4. The better mechanical properties obtained by orienting the granules,
5. No heating of the piece is required.

Disadvantages of cold forming:

1. If higher force and / or energy is needed,
2. It is necessary to clean the surface of the part before processing and not to include layers
such as oxide,
3. Limited application of shaping due to low ductility

Hot Forming:
If the plastic deformation is made at a temperature above the recrystallization temperature
(T>0.5Tm), the hot plastic deformation is made. If the speed and temperature of deformation
are chosen properly during the process, a grain structure is obtained with a small and high
strength structure. The energy consumed for the shape change by hot shaping is less than the
energy consumed for cold shaping. The hot shaping has a high ability to change shape without
cracking in the material. Generally, it is done in the air environment and due to the oxidation
coming to the water, there is a considerable loss of metal.
Depending on the deformation temperature (T), the relationship between stress and deformation
also varies. At low temperatures (T <0.3Tm); While the effect of the rate of deformation on the
plastic stress remains insignificant, the deformation hardening (reinforcement) is effected on
the plastic strain. At high temperatures (T> 0.5Tm); Plastic strain is very sensitive to the rate of
deformation, while deformation hardening is negligible.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

The strain stress (breakdown strength) can be found by means of real stress-strain curves
(Figure 1.5). Here, a given strain rate (T = constant) indicates that the strain stress is a function
of the strain rate (deformation) base. With increasing temperature and deformation rate, plastic
tensile value of the material is also increasing a little.
Here; C = Strength coefficient, m = Strain rate represents the sensitivity probability. C can be
found when the value of the deformation rate is equal to 1 and m is equal to the slope of the
curve. C and m vary with temperature. Along with the increase in temperature, the rate of
deformation increases while the strength coefficient and strain voltage decrease.

Figure 1.5. Strain rate in forming

Advantages of hot forming:
1. A large shape change can be given at a time,
2. The need for lower force and energy,
3. The fact that the materials have higher formability,
4. Does not increase the resistance of the part and shows more suitable properties for
subsequent operations.

Disadvantages of hot forming:

1. Low dimensional accuracy,
2. If a higher total energy (due to heating of the part) is needed,
3. Poor surface quality,
4. Shorter mold life.

The most important advantage of semi-hot (warm) forming compared to hot forming is energy
saving. The most important advantage with respect to cold working is that the total deformation
rate to be achieved is higher and at the same time the product can be obtained with strengths
close to the cold worked material strength. In the warm process, there is no recrystallization in
the material, but dynamic recovery occurs. The effect of deformation hardening in warm
treatment is less than in cold treatment.

Advantages of warm forming:

1. The need for lower force and energy than cold forming,
2. To reduce the need for intermediate conditioning (recrystallization) during cold forming,
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

3. Very fast consolidation of materials (such as Hadfield Steel) is facilitated by machining

and shaping,
4. Providing simplicity when more complex parts are formed.
1.1.2. Mechanisms of Plastic Forming
Plastic deformation of metallic materials takes place by four different mechanisms.
1. Shear mechanism
2. Twinning mechanism
3. Creeping mechanism
4. Shifting of grain boundaries

The mechanism of shear and twinning in plastic deformation has a bigger share.

1.1.3. Friction and Lubrication in Forming Process

In the forming process, the molds must firstly contact on it in order to force deformation by
applying pressure to the part. When we push a cylinder between two flat molds, the height of
the cylinder decreases, but the volume is fixed in the plastic deformation, so the sectional area
is trying to grow. In this case, it is necessary to overcome the frictional force generated at the
interface so that the material can spread between the molds. This makes shape change difficult
and increases the applied stress. The cause of the barrel seen during the cylinder stacking is the
large shear stresses that occur on the mold surfaces and arise from friction.

The variable defining the friction is the friction coefficient and is expressed as:

Where μ is the friction coefficient at the interface, P is the normal force, and F is the friction
force. In cases where the coefficient of friction is large, the friction force and tension that are
preventing the material from spreading during deformation are also increasing. Due to friction,
larger normal forces have to be applied during shaping. Secondary stresses resulting from
friction lead to tearing and even breakage in the material. Also, when high shear stresses and
normal stresses are used in forming process, the molds undergo early damage. For these reasons
it is aimed to reduce the effect of friction by lubricating in the majority of the forming
operations. If the shear stress on the interface reaches very high values, for example when the
material exceeds the shear strength of the material, the material adheres to the mold surface and
the layer below it is cut and the deformation continues. This is called "Adhesion" or "Adhesion
Friction" in metal forming processes.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

In this case, assuming that the material has half the normal stiffness of the cutting strength (τ =
0.5 σ) and that the normal pressure is equal to the yield strength of the material (p = σ), the
coefficient of friction μ can vary from 0 to 0.5 and that the ideal friction- It is understood that
in case of adhesion friction, the coefficient of friction will increase to 0.5 order. In addition to
reducing the forming force, the mold wear is reduced, better surface qualities are provided, the
molds are cooled during the hot forming operations and the mold sticking of the workpiece is
prevented. In the choice of lubricant:
1. The type of forming process,
2. Process temperature,
3. The type of material being shaped,
4. Chemical properties,
5. The ease of application,
6. Cost elements should be considered.
If no lubricant is used, the coefficient reaches values of 0.1 in cold, 0.2 in warm and 0.40-0.50
in hot.

Table 1.1. Classification of plastic forming methods.

Process Schematic Diagram Stress occurring in the main part
during forming

Rolling Biaxial compression

Forging
Triaxial compression

Extrusion
Triaxial compression

Spinning Biaxial shear and

triaxial compression
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Rubbing Triaxial compression

Deep drawing Uniaxial tension and compression

Wire and tube drawing

Biaxial tension and compression

Stretching Biaxial tension

Bending Biaxial tension and compression

Cutting Uniaxial tension, compression and

shear

1.1.Forging

Forging is a method of imparting desired shape to metal by providing plastic deformation by

hand or by local impact and compression forces applied by power hammers, presses or other
special forging machines. Parts of various sizes and shapes, from a simple bolt to a turbine rotor
or a single wing aircraft wing, can be produced with this method. Forged products by land, air
and sea transport, agriculture in machinery and tools, construction and road machines in
missiles and rockets, the arms industry, turbines, engines and used in a variety of machines,
which is particularly important in terms of safety, constitute shocks and stress-resistant critical
components.

Forging is classified in different ways according to different criteria. According to the working
principles of the machines used, two grubs are separated as ramming or pressing forging.
Forging with a rammer is carried out with the bumps applied to the surface of the plastic forming
workpiece. In presses, the plastic deformation of the material is carried out under static
compressive forces and at relatively lower speeds. A more common type of classification for
forging is based on the path followed by the forging process or the characteristics of the mold
used. Open die forging (free forging) is done with flat dies or simple shaped dies. This method
is usually few in number but is applied in beating large pieces. The open die forging method is
also used in preforming operations required for closed die forging. In the closed die forging
process, the material is beaten between the dies having the traces and voids of the desired part
shape. This method fills all the details of the die of the material with the high pressure applied
to the complex shaped pieces, producing with very small dimensional tolerances.

Most of the forging process, improve the ability of the material to deform plastically and is
done in order to reduce the force to be applied hot. some metal part, which is made of small
pieces is produced by the cold forging. High strength and toughness are achieved by forging.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

1.2.1. Forging Machines

Forging is carried out by hand or machine, as highlighted before. Manual forging heated to a
forge piece is shaped forging hammer on an anvil. In the meantime, some auxiliary tools are
used. In the Industrial Age began with forging machine use and spread over time. Forging
machines can be classified as follows.

Rams: ram from first principles, the upper mold forming a hammer or ram called a weight from
a given height onto the workpiece placed on the lower die is to reduce as the free or compressed.
Here the lower mold is attached to a base mass called anvil. The rams are separated from each
other by some sort:
• Mechanical moving rams
• Airlifted and free fall rams
• Steam or pneumatic rams
• Counter-impact (horizontal or vertical) rams

Mechanically movable rams are separated in some types according to the mechanism of
movement between them, such as wooden, crankshaft, spring, belt. In the wooden rams, which
are a type of rams of this type, the hammer is connected to a vertical shaft made of wood as
shown in Figure 1.11a. In these types of rams, steel pieces up to several kilograms in weight
can be forgeable. In the rams with airlifted and free fall, is raised to a height of 0.8-2 m with air
pressure and the hammer weighing 200-4500 kg is allowed to fall freely. In pressurized rams,
in addition to gravity force, air or hydraulic pressure is applied. As can be seen from Figure
1.11b, the hammer and upper die are connected to a piston in this type of ram.

Figure 1.11. Schematic diagrams of ram devices

In counter-impact rams, the hammers strike the workpiece at their final speed and the entire
energy of the hammers is consumed by the workpiece. Here, there is no energy loss (10-30%)
as in other devices.

Forging Press:
Presses are press machines which apply energy to the workpiece at a lower speed than the rams
and are given the desired shape to the workpiece by the pressure exerted by the press head.
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Usually the part is printed once in each mold cavity. During plastic shaping, no shaking, no
noise, no energy loss. Forging presses are divided into two main groups as mechanical and
hydraulic presses according to their working principle. The most common types of mechanical
presses are eccentric presses (Fig 1.13a).

In hydraulic presses (Figure 1.13b), water, emulsions and mineral oils are used as liquids. The
pressure applied by the liquid can increase to 300 kg / cm². In this type of presses, the speed
and pressure application period can be controlled as desired or even changed during the process.
These properties are always stable in mechanical presses. The most important feature of
hydraulic presses is that the movement is applied in the form of pushing in the direction of
impact.

a) b)
Figure 1.13. Schematic diagrams of mechanical and hydraulic presses.

Apart from these, screw presses (friction presses) are used for forging precision parts such as
turbine blades.

Upset forging machines: These are generally horizontal presses (Figure 1.17) and are used in
the production of a large number of symmetrical parts. As the machine may be able to perform
double-impact forging tasks, often the reciprocal dies serve the task of holding the bar to be
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

forged firmly. In the latter case, another die performs stacking (heading and edge making).
Bolts, nails and some small gear can be produced in series with this method.

Figure 1.17. Upset forging machine applications

Forging Roller: In the process of forming the preforms for the closed die-casting, in the
preforming processes of the thin and long pieces, the forging rolls are utilized in the preforming
processes (tapering) of the parts having increasingly investigated sections. Only a part of the
roller surfaces is used in these devices. In other parts, the material moves without changing
shape between rolls. The workpiece is taken back from the side of the roll stand.

Figure 1.18. Schematic diagram of the forging rolling devices

Radial forging machine: In these devices used to reduce the section of the bar or pipe, the
starting material is shaped by pounding between two or four dies rotating in the form of rod or
tube (stepped miller, gun and gun barrel etc.). The hammers that support them with the dies are
placed in a rotating shaft and rotate freely. The rotating shaft moves in a cylindrical inner body
containing a plurality of symmetrically disposed rolls. Hammers (2 or 4 in circumference)
approach each other when they come into contact with the rollers, then move away from the
effect of centrifugal force. Thus, the forging process is performed.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

a) Schematic representation of the

b) Manufacturing of tubular part by radial forging
radial forging machine

1.2.2. Classification of Forging Operations

A. Open Die Forging

Generally, it is a method used in forming small parts and large parts in volume and it is also
used to improve mechanical properties at the same time. Since the shaping is carried out in flat
and simple shaped dies, such forging processes are used in rough formwork which does not
require size and shape precision. When machining is needed after shaping, soft materials are
usually shaped by this method. This is because the machining of the hard materials after the
forming process is quite difficult. The surface quality is worse than that of the materials by
closed die forging, and therefore it is necessary to find processing parts in the produced parts.

The dimensions of workpieces cannot be precisely determined during free-forging. The

shrinkage of the workpiece (1-1.3% depending on the annealing temperature) and the quality
of the contact surfaces of the tools used for the forging affect the dimensional change. Since
reliable temperature control cannot be performed because it is necessary to work quickly to
avoid losing the workpiece temperature, it is very difficult to determine the size difference in
open die forging and requires great experience.

Manufacturing of parts with open die forging requires a great deal of mastery and experience.
Given the large increases in workers' wages, it is possible to achieve the result that
manufacturing parts by free forging is disadvantageous both in terms of quality and cost. For
this reason, free forging or open die forging is used as an auxiliary process in production. For
example, the piece that is to be struck in the die is applied to a great extent for the preforming
and milling of the structures and for the increase of the toughness of the material. Free forging
is done with anvils, hammers, rams, etc.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

With this method, the following shaping operations can be easily performed such as extension,
expansion, upset, splitting, punching, cutting, step forming, bending etc.

Figure 1.20. Schematic views of various applications.

B. Closed Die Forging

In order to get the desired shape of the material, it is necessary to control the flow of the material
to all the cavities by beating or pressing in the previously produced die cavities. It is used in the
production of numerous and complex parts. Surface quality and dimensional accuracy are better
than open die forging. Due to this feature, the reduction of the machining shares both avoids
unnecessary material consumption and shortens the labor and time required for chip removal.

There are various types of closed die forging, such as burry forging, burrless forging and
stamping. With the shape change by pressing, the material tends to flow more and more because
the speed of deformation is small. However, since the recesses and protrusions of the die do not
create a collision effect, they do not fill as well as they do in shaping. In general, the excessively
recessed and protruding parts are formed in the rammer, the surface is flat and the symmetrical
parts are formed in the press. If the volume of the material placed in the mold is equal to the
volume of the die, the forging finish material fills the die completely. If the material volume is
larger, the excess part is overflowed. These overflowing parts are called burrs. In order to fill
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

the die well with the die, the material is always put too much material and the burrs are removed
by means of chip removal methods.

Figure 1.25. Burry forging and burrless forging applications

Manufacturing of coins, medals and small reliefs by means of closed die casting is usually
carried out by cold forming. In this method is called as stamping, 5-6 times the force of the
yield strength is applied to the material. Oil is not used during forging.

In the forging operations, the dies should be made from materials having high abrasion
resistance, sufficient hardness strength and toughness properties to withstand dynamic forces,
which may exhibit dimensional stability during forging. In addition to these, it is necessary to
bear resistance to heat exchanges, high workability and good workability. Factors such as the
material of the workpiece, the method of forging, the number of pieces, the shape and size of
the mold, and the material price must be taken into consideration in the selection of the mold
material. Cast iron and steel are used in the production of press dies without dynamic and
sudden forcing. The ram molds are made from forged steel.

1.3. Rolling

Rolling is the process of shaping metallic materials by passing them through rollers, which are
called rollers and rotate about their axes. Rolling is the most widely used method of plastic
forming because of its speed and continuity of production, and ease of processing and product
control. The characteristics of the rolling operation can be briefly summarized as follows:
1. The cross section of the material is continuously reduced,
2. Generally, longitudinal extension occurs; in products such as lathes and profiles, there is
a tendency to increase in width even in very small quantities, and there are considerable width
increases in flat products such as sheet metal and the like.
3. Since production is fast and continuous, operation has high capacity.
4. Measurement control is easy. Therefore, the most commonly used plastic forming method.
About 95% of all materials with plastic deformation are formed by rolling.
5. The rolls rotate in opposite directions relative to each other, applying radial pressure to
the material and therefore compressive stress. The surface shear stresses caused by friction
between the material and the rollers provide the deformation and movement of the material.
6. Each pass between the rolls of the materials is called pass.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.26. Rolling: (a) Simple roller set and (b) Rolling mill

Rolling can be classified as hot and cold rolling according to the process temperature. As ingot
and billet castings are rolled, large sections are narrowing, in other words rolling is done as a
hot process. When the casting structure is degraded by hot rolling at the temperatures above the
recrystallization temperature of the material, smaller cross-sectional products are obtained.
From steel ingots, hot-rolled products of large cross-section such as slabs, blooms and logs can
be obtained, as well as small-section products such as sheets, sheets, bar pipes, rails and profiles.
Cold rolling is also important and is used to produce very small sections such as sheet, foil, thin
rod and wire. Cold rolling leads to an increase in the rolling forces and strength required for the
operation, in contrast to providing a smooth surface, defect-free size and high strength.

1.3.1. Rolling Products

The rolling operation of all metallic materials starts with ingots by continuous casting methods.
Rolled products are named with special terms according to section, size and shape. The first
intermediate products obtained by rolling steel ingot are bloom and billet, and the first
intermediate product with rectangular section is slab. End mill products are also divided into
flat and long mill products (Figure 1.27). Slabs are rolled and flat products such as sheet, sheet
and tape of smaller cross-section are converted to shape.

1.3.2. Screw Thread and Ring Rolling

The mass-forming method in which cylindrical parts are rolled between two dies to form threads
(Figure 1.28). It is an important commercial method for serial production of bolts and screws.
It is applied by cold working on the thread rolling machines. Advantages according to
machining:
• Higher production rates,
• Better material handling,
• It provides stronger teeth and higher fatigue resistance due to cold hardening.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.27. Rolling products

Figure 1.28. Screw thread with flat plates in rolling.

It is a method of forming a small diameter and thick-walled ring rolled in the form of a thinner,
larger diameter ring (Figure 1.29). As the thick-walled ring is compressed, the deformed metal
extends so as to allow the diameter of the ring to grow. For larger rings, the hot working method
is applied and for the smaller rings the cold working method is applied. Ball and roller bearing
guides, steel wheels for railway carriages and rings for pipes, pressure vessels and rotating
machines are manufactured using this method. They have advantages such as material saving,
ideal grain orientation, and strength increase by cold hardening.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.29. Ring rolling

1.3.3. Rolling Machine

A rolling machine consists of rollers, bearings, rolling mills and motors connected to the shafts
used to rotate the rollers. Since the forces required for rolling are usually too great, very robust
looms and very large engines are needed to provide the necessary power. For this reason, the
construction of a modern rolling mill with many rolling mills requires a large investment,
experienced engineering design and construction knowledge.

Rollers are placed on top of each other in a rolling mill. There are arrangements where the
horizontal and vertical positions of the rolls can be adjusted in the roller shaft. By adjusting in
the vertical direction, the clearance between rolls is changed and the height of the rolled material
is controlled. The adjustment in the horizontal direction determines the position of the rolls
relative to each other. The rolls are the most important part of the milling machine because it
determines the geometry of the produced part.

Rolling mills/rollers:
A rolling roll consists of the body part where the rolling process is performed, the journal part
bearing the body and the journal part rotating in the bearing, and the grip parts connected to the
rotating shafts of the rollers (Figure 1.30a). The diameter of the rolls is between 5-10 cm and
150 cm. In general, the length (L) and the diameter (D) of the body part are in the range of 2.2-
2.7 (L/D).
According to the shape of the product to be rolled, the body is divided into two parts as flat
surface (straight) and various profiled (calibrated) (Fig. 1.30b). In the rolling of the flat
products, cylindrical rolls with cylindrical bodies are used, and when the profiles are rolled,
calibrated rolls with cavities in various shapes according to the desired profile are used in the
body.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.30. a) Parts of a roll, b) A set of calibrated rollers.

Rolls are made of steel and cast iron. Steel rolls are divided into forging and casting groups and
made of carbon steel and alloy steel. Cast iron rolls are made of white cast iron, gray cast iron
and spherical graphite cast iron. Generally, in rudders the fracture toughness is high so that the
surface hardness is high and the inner parts are not broken by bending deformation in order to
prevent wear. Rolling mills are named according to the arrangement of the rolls. Depending on
the position and the number of rolls, the roller configuration may be different (Figure 1.31).

One-way double roller: The simplest and most widely used roller layout (Figure 1.31a).
Reversible roller: A reversible twin table is called a reversible twin table when the direction of
rotation of the rollers is changed so that the material can be extruded without having to roll over
the rolls after the material has been rolled in a uni-directional twin table. In other words, the
material can be rolled both forward and backward. The distance between rolls is reduced in the
case of back-rolling (Figure 1.31b).
Triple roller arrangement: Three rollers are placed on top of each other at various intervals.
The upper and lower rollers are rotated with the motor power, while the middle roller rotates
with the effect of the resulting friction force (Figure 1.31c).
Quadruple roller: It consists of small rollers in contact with the material and large rollers that
support them. The purpose of using a small diameter roller is to reduce the power required for
rolling. Large diameter rollers and small diameter rollers need to be supported. With small
diameter rollers, very thin sheets can be easily rolled up to the smallest tolerances (Figure
1.31d).
Twelve roller: Small diameter mill rolls are supported by two medium and large roll mills
(Figure 1.31e).
Planet type roller: There are a lot of small roller mills rotating like a planet around two large
diameter support rollers. With the rolling mills used in the hot rolling of flat metallic materials,
30-40% deformation is achieved in single pass, while this ratio increases up to 90% with planet
rolling mills (Figure 1.31f).
Series tandem: Consists of high-speed and continuous rollers for mass production. Since the
cross-section reduction in each case will be different, the speed of material advance will be
different at every moment. The speeds of the rolls are adjusted to be equal to the speed of the
previous rolled material. The speed of the material and rolls reaches the maximum in the final
run. (Figure 1.31g).
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.31. Roller arrangements

Rolling profiles: When two rollers working together on the roller come face to face, the shapes
that will give the desired profiles are opened. These gaps between the rolls are calibrated, and
the gap between the two rolls is called the caliber gap. Caliber range is parallel to the axis of
the roller or roll axis a angle made by 60 ° small open caliber, 60 ° big if closed caliber is called.
The height of the caliber is more than the width of the part to be rolled. The name of the profile
paso that comes up when two opposing rollers come together.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

1.3.4. Rolling Defects

1) Defects caused by compression forces
• The emergence of products of varying thicknesses after the elastic deformation of the rolls:
As a result of the bending of the rolls, pressing on the edges causes tensile stresses in the
middle part.
• Wavy edge formation in the sheet metal: The thickness at the edges is lower than in the
middle, causing too much elongation at the center but undulating spreading edges.

b)

Me
Ke

Ke
na

na
ez
rk
r

r
a)

• Creation of cracks in the middle and edges of sheets: If the middle part is too long, but the
material is not ductile enough, the center of the sheet cracks (c).
• Crocodile cracking: The shape change is not homogeneous and starts to occur due to a
nipple initially present in the ingot (d).

c) d)

2) Defects caused by friction forces

• Rounding of the two ends of the sheet: The reduction in thickness from the edge to the
edge causes the sheet to stretch in the middle of the sheet. The head and the end of the sheet
are rounded (e).
• Separation of the sheet into two parts: Pressing in the middle due to friction, pulling
tension in the edges. If the material is not ductile, these tensile stresses cause the sheet to
rupture more than the edges and eventually rupture (f).

e) f)

1.4. Extrusion

Extrusion is the process of compressing a cylindrical metal block (billet) in a receiver (sleeve)
with a large force and passing it through a matrix to reduce the cross-sectional area (Figure
1.35). It is one of the most important and most widely used plastic forming methods of light
metal industry (Al, Cu, Mg etc.) and it usually produces straight and long products such as rods,
pipes and strips.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.35. Extrusion process

A variety of more complex shaped profiles can be produced by extrusion than materials with
high deformability such as aluminum. Since large forces are required in the extrusion process,
the process is usually carried out at high temperatures where metallic materials exhibit the best
plastic deformation characteristics. In addition, cold extrusion can be applied to some metals
which do not require great force. The greatest advantage of the extrusion process is that the
probability of cracking is very small despite the large deformation rates provided by a single
operation. This is the machine and die/mold design in operation. The metal block is under the
influence of large compression stresses between the piston-block and the receiver, reducing
crack formation.

1.4.1. Extrusion types

The extrusion process is done by two main methods, direct (direct) and indirect (indirect). In
addition, hydrostatic and impact extrusion methods have been developed.

In direct extrusion method, the die and the sleeve are fixed (Figure 1.37). The plastic
deformation is accomplished by the compression of the metal block with a piston or a presser
shaft moving towards the die and the flow of metal through the die cavity. In the indirect
extrusion, the die is pushed by the punch into the hot metal in the sleeve, and the shape is
changed (Figure 1.38). The most important difference between these two methods is that the
friction between the metal block and the sleeve in the direct extrusion is not present in the
indirect extruder and therefore requires less force. However, due to the presence of the hole in
the piston section, the area of use is limited as it limits the force to be applied.
In hydrostatic extrusion, the principle is as if it were directly extruded. The most important
difference is the presence of a fluid layer between the sleeve and the billet in the hydrostatic
extruder, which, as seen in Figure 1.39, removes friction forces.

Figure 1.37. Direct extrusion

MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.38. Indirect extrusion

Figure 1.39. Hydrostatic extrusion

Impact extrusion method is generally used for making short and hollow parts (drug tube,
toothpaste tube etc.). This process is like indirect extrusion and cold extrusion joining and can
usually be done in high speed mechanical presses. The process is mostly applied to cold metals
such as lead, tin, aluminum and copper as a cold process.

Figure 1.40. Impact extrusion

1.4.2. Extrusion Equipment

Extrusion presses are generally hydraulic presses. They are classified as horizontal presses and
vertical presses according to the movement style of the piston. Horizontal presses are generally
used in commercial rod, pipe and profile production. They have a load application capacity of
1,500-5,000 tons. In this type of press, the lower part of the metal block cools earlier than the
upper part because there is more contact with the sleeve. In this case, the deformation does not
occur homogeneously. Vertical presses are generally 300-2000 tons capacity. This type of press
is used in the production of pipes with very thin wall thickness. Centering between the piston,
sleeve and die is easy. The metal cooling is at the same speed at every point and therefore higher
dimensional precision is achieved. Production speed is high.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Dies and other extrusion tool materials used in extrusion, high stresses, thermal shocks and
oxidation. The extrusion presses are designed in such a way that the die and tooling materials
can be easily replaced and replaced. Extrusion dies are made from high alloy steels. The hive
is usually made up of two parts and is protected by a tightly fitted shirt. The piston is protected
by a hot metal pressing plate.

There are two types of extrusion dies (Figure 1.42). In flat surface dies, the metal enters the die
through cutting itself and there is a dead metal zone in the entrance area of the die. Dies with
conical inserts can be used with good lubrication. Shrinkage of the die angle increases the
homogeneity of the deformation and reduces the extrusion pressure. However, after a certain
die angle, friction on the die surface may increase too much. The optimum die half angle (α) is
between 45° and 60°.

Figure 1.42. Flat surface and b) Conical entrance dies.

1.4.3. Metal flow in extrusion

Metal flow during extrusion is often complicated and some precautions must be taken to prevent
the formation of faults and cracks. At the direct extrusion, the material at the surface of the
billet and near the surface is subjected to deformation at very high rates as the material in the
center of the sleeve or in the center of the billet undergoes a slight deformation or plastic
deformation. In addition to this, the friction between the forward moving billet and the barrel
makes surface flow difficult. As a result, the flow model is occurred in Fig. 1.43. If the metal
on the surface shows excessive cooling, deformation is much more difficult and cracks can
form on the surfaces of the products. Therefore, it is necessary to show the maximum attention
and control of the process such as die design, lubrication, extrusion speed and temperature
control for the production of quality products.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.43. Model showing plastic flow of metal in direct extrusion

a) Homogeneous material flow is seen. There is no friction between the metal wedge and the
receiver. If such material flow is present, the effect of oiling is super.
b) The friction of such material flow is very high. The metal enters into the mold with high
shear stress values. This can lead to defects in the product.
c) There is an extrusion process with high shear stresses. It is an indication that it is very high
friction and therefore delays the flow of material through the die. While the middle of the metal
wedge flows easily, the outer parts hardly flow. As a result, the dead metal zone becomes large.
The flow is not homogeneous and causes defects in the urine.

1.4.4. Factors Affecting Extrusion Force

The main factors that influence the extrusion force are:
• Extrusion type
• Extrusion ratio
• Deformation temperature
• Deformation rate
• Friction forces
• Billet structure

The ratio of the extrusion force to the cross section of the metal block is called the extrusion
pressure. Figure 1.44 shows the change in extrusion pressure versus piston motion in direct and
indirect extrusion. The first increase in this diagram is sudden and very high. The reason for
this is the strong pressure that occurs when the material/block is filling the extrusion mold.
When the pressure reaches the maximum value, the metal starts to come out of the die. In the
direct extrusion, the friction between the billet and the sleeve is reduced and the extrusion
pressure drops as the sleeve portion of the material becomes smaller and smaller.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.44. Pressure change in extrusion

Since there is no friction between the billet and the sleeve in the indirect extrusion, the pressure
remains unchanged constant by the action of the stamp. When the material is reduced in
pressure, the pressure is increased in the same way in direct and indirect extrusion. The reason
for the increase in pressure is that the remaining material is harder to deform and harder to
extrude because it is small in size.

Extrusion Ratio: The ratio of the cross-sectional area (Ao) of the billet to the cross-sectional
area (As) of the extrusion product is defined as: R = Ao / As.
Deformation Temperature: It is usually carried out at elevated temperature in order to reduce
the force applied to the forming by extrusion, taking advantage of the increase in the
deformation ability of the materials with increasing temperature and hence the decrease in
deformation resistance. However, the process temperature should not be at a level that can cause
to soften materials of die and sleeve, to form oxidation, to hot tearing.
Deformation Rate: A 10-fold increase in deformation rate increases the extrusion pressure by
about 50%. The rate of deformation is important in terms of the quality and temperature of the
material produced. Direct extrusion: the product temperature continuously rises from the
beginning of the process to the end due to the friction between the billet and the sleeve. In
indirect extrusion, the output temperature of the product is lower than the direct method and is
constant throughout the entire process. Therefore, the maximum deformation rate that can be
applied in the indirect method is higher than in the direct method.

Friction Forces: Effective frictional forces in extrusion processes are generally.

• In direct extrusion, without friction between the ticket and the barrel,
• Without rubbing between the mold and metal,
• In the pipe production, without friction between the mandrel surface and the metal
The friction between the die and the metal depends on the shape of the extrusion die. The
extrusion pressure increases as the die angle increases in conical insert dies. The length of the
cylindrical part of the die, which is the contact product with the extrusion product, is another
factor affecting the extrusion pressure.
Billet Structure: The castings of billets are modified by homogenization heat treatment before
the extrusion process to ensure maximum operability during extrusion. For this purpose,
optimization of the homogenization parameters of the designed macro and microstructures
(grain structure, amount of matrix element soluble in the matrix, amount of alloy element in
precipitate form, type of second phases and dispersion parameters-size, shape, distance between
particles and volume ratio).
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

1.5. Bar and Wire Drawing

Metal drawing is a manufacturing process that forms metal work stock by reducing its cross
section. A drawing force is applied to the material from the exit end of the matrix and the exit
cross-section of the matrix is obtained (Figure 1.45).

Figure 1.45. A round bar or wire drawing operation

During the wire drawing process, the plastic deformation is provided by the pressing forces
applied to the material of the die. With the withdrawal operations performed after the
succession, the cross section of the material can be continuously reduced. Depending on the
diameter of the product obtained, the process is called bar or wire drawing. Pulling is usually
done at room temperature, but the large deformation rates applied during the process cause the
material temperature to increase during the process.

1.5.1. Bar and Wire Drawing Processes

Bar and wire drawing are in principle the same, only the used devices are different. The rods
can be pulled straight, although they can be wound as wire coils or bobbins. When the bar is
pulled, the bar is inserted to the core or die and connected between the jaws of the drawing
vehicle. The drawing vehicle can be moved by a hydraulic or mechanical system. While the
raw materials of steel wires are wire rod, the raw materials of non-ferrous wires are extruded
rod. Drawing operation is divided into two as lubrication dry and wet. Wet drawing is only
applied on thin wires of 0,5 mm. Grease or soap dust is used for dry drawing, while oil is used
for wet drawing. Thicknesses of less than 5 mm are obtained only by extrusion, whereas those
of greater than 5 mm are obtained by hot-rolling.

Wire production is not possible with hot extrusion or hot extrusion, as the increase in surface
to volume ratio of wires with finer wires results in rapid cooling of the material during hot
processing and formation of undesirable oxide layers on the surface. The rust is removed
mechanically or chemically. In the mechanical method, the oxide on the surface of the twisted
wire on two bobbin perpendicular to each other is cracked. Then, the wire is cleaned by passing
it through a metal brush.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.46. Schematic diagram of bar drawing.

Another mechanical method is spraying metal balls. Oxidation by chemical process is carried
out in sulfuric acid (H2SO4) and hydrochloric acid (HCl) baths. The wire rods is immersed in
acid baths. Depending on the thickness of the rust layer and the degree of acidity, it is kept in
the bath for a certain period of time. If the wire is too stayed in bath, the hydrogen in the acid
will penetrate the steel and make the steel fragile. Wire rods in order to prevent corrosion again;
coating, drying and tempering processes.
Wires in precision sizes and uniform cross-section can only be produced by cold wire drawing.
In order to avoid the occurrence of surface defects on wire and to reduce core/die wear, the
surface-cleaned and tipped wire rod is passed through the die and is drawn to the drawing block
(Figure 1.47).

Figure 1.47. Drawing die section view

Die Materials:
• Alloy tool steels: for example, 145 V12, 140 Cr V1, X130W5, 145 Cr6, X210CrW12
steels; drawing of rods and profiles
• Cast steels: G-X270CrV15, G-X250cRWV26; Hot drawing process
• Hard metals (Wolfram carbide and cobalt based); drawing of wire
• Diamond: Drawing of very thin wires between 5 μm and 1.5 mm in diameter

The material must be ductile in order to apply large deformation rates when drawing the wire.
For example, a wire of 0.1 mm diameter can be drawn unbroken by continuous wire drawing
with a 5 mm diameter mild steel wire rod. In continuous wire drawing, a few cores are passed
behind the wires. 15-20% of thin wires and 20-50% of thick wires are applied at each step. The
production rate is high in wire drawing processes and the drawing speed is 1,500 m / min in
modern wire drawing devices.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

1.5.2. Factors Affecting Wire Drawing

Die angle (α): If you want to make easily drawing and to reduce the friction conditions in the
drawing process, you should have the "optimum die angle". This angle varies between 6 and150.

Reduced section: The maximum section reduction per pass is 63%. However, more than 45%
of the oil may degrade and the surface of the part may deteriorate.
Wire drawing speed: In consecutive drawing process, the wire cross section is reduced, the wire
length and speed increase proportionally. For this reason, the circumferential speed of wrapping
rollers must be increased according to the output speed from each die. This happens when each
roller is rotated by a separate motor.
Temperature effect: The metal flow at high temperature depends on ε’.
Lubrication: It is necessary to use "special oil" to increase the life of the die, reduce the drawing
force, and ensure that the surface of the product is smooth.

1.5.2. Defects in Bar and Wire Drawing Processes

1. Generation of sergeant marking (>>) cracks in the center of the product: It is the cause (h /
L) ratio. As this rate increases, homogeneity of plastic deformation is lose. In the middle there
is a hydrostatic tensile stress called secondary drawing stresses. This causes sergeant-marked
(>>) cracks to appear.

2. Lubrication is not good: The surfaces of cold drawn bars and wires may crack. In addition,
during defective lubrication, scratches may be formed in the longitudinal direction of the
resulting product due to compression between a hard particulate material and the die.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

3. Improper surface preparation: Color discoloration may occur in cold drawn products.
4. Inhomogeneous deformation: Causes residual stresses to be created in cold drawn wires and
rods or in pipes. These stresses cause cracks in the time-lapse corrosion cracking and in the
event of some chip removal from the surface.
5. Defects originating from raw materials in bars and wire rods: If there are imperfections such
as layering, casting cavities in the material, their reflection on the product is inevitable.

1.6. Pipe Manufacturing

There are two types of pipes according to the manufacturing method. Welded pipe
manufacturing based on the principle of bending and welding of metallic materials such as
pipes, sheets or bands, or seamless pipe manufacturing methods based on the principle of
extrusion of a metal block or pipe forming by special rolling methods. Welded pipes are usually
made of steel, in low pressure networks, seamless pipes can be made of steel and all non-ferrous
metals and are used in high pressure places such as boilers, steam and hydraulic circuits.

1.6.1. Welded pipe manufacturing

Welded pipes can be produced with longitudinal welded or spiral welded forehead welds and
electric arc welding methods. Welded steel pipes are grouped as small (d <170 mm), medium
(170 mm <d <400 mm) and large (d> 400 mm) diameter. In the production of small- and
medium-diameter pipes, the sheet strip or strip is bent through a bell-shaped funnel and the
edges are welded to the forehead (Figure 1.50).

Figure 1.50. Welded pipe manufacturing

In the manufacture of large-diameter pipes, the forming is progressively carried out in

sequential rolls and combined with the electric arc welding unit placed at the end of the rolling
group using a filler material. Special welding methods such as special shielded electrodes or
submerged arc welding are applied to large diameter thick pipes. The quality of the welds in
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

the pipes is determined by water pressure, pressure testing and non-destructive methods such
as ultrasonic inspection or radiography.

1.6.2. Seamless pipe manufacturing

Metal blocks are usually made of seamless pipes in extrusion and special rolling mills. In the
method of manufacturing by extrusion, it is carried out by attaching mallets which have
movement mechanisms in various sizes and shapes on the piston. First, the metal blot will
accumulate. Then the shaping is carried out by movement of the malleus and piston. A hole is
previously formed in the central portion of the metal block used. By the action of the piston and
the spar, the hole is widened and plastered. The hole material may undergo oxidation during
annealing.

In the case of the seamless pipe manufacturing method with special rolls, pipes are
manufactured by hot forming with the help of two rollers and a spindle which are shown in Fig.
1.51 and whose axes are slightly inclined with respect to each other. This method is more
economical than the seamless pipe manufacturing method by extrusion. With this method, the
desired meat thickness and diameter are given to the pipes which have more meat thickness.

Figure 1.51. a) Seamless pipe

production by Mannesmann
method,
b) Rolling and change of pipe
diameter and meat thickness.
c) Expansion of pipe diameter
by conical roller and
d) Forming by rolling and
machining inner and outer
pipe surfaces

1.6.3. Pipe drawing

In addition to the above methods, the pipe can be manufactured by pulling as shown in Figure
1.52 with the help of a die and a spade by a method similar to rod or wire drawing. Drawing
process are often applied to the pipes produced by hot working with extrusion or special rolling
methods. This cold process provides more accurate dimensional tolerances, smoother surface
and better mechanical properties. In addition, steel pipes with a small diameter or a thin wall
thickness that cannot be achieved by hot drawing with drawing can be made. As can be seen
from figure, there are four main types of pipe drawing methods. These; Hollow drawing,
drawing with fixed mandrel, drawing with tapered mandrel, drawing with moving mandrel.
Since any piece controlling the inner section is not used for hollow drawing, the wall thickness
of the drawn pipe may not be homogeneous and the deformation rates provided by this method
are low and it is generally used in the production of small pipes.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.52. Pipe drawing methods

The inner diameter is controlled by the fixed arbor/mandrel which is placed in the middle of
the core and the wall thickness is decreased. The mandrel may be cylindrical or conical. With
this method, pipes can be manufactured with more precise dimensions compared to the hollow
method. The pipe can be of the floating type and the section can be narrowed at a higher ratio.
In the same diameter pipe manufacturing, the operation can be performed with lower force than
the fixed mandrel system. In the drawing with punch method, there are no problems caused by
friction when drawing with the mandrel, and drawing process is done on a long bar. The punch
diameter determines the pipe inner diameter. After the drawing operation is completed, the
punch is pulled out. As the length of the product increases, the precise control of the inner
diameter becomes difficult. This is why floating mandrel is used. The inside diameter provides
the floating mandrel, while the exit of the die shapes the outer surface of the tube.

1.7. Sheet Metal Forming

Sheet forming method has an important place among manufacturing methods because it is
possible to produce large amounts of sheet/plate by rolling method. Sheet metal forming are
used intensively in the conversion of metals into sheet products and / or plates. Especially, in
this method utilized in the white goods and automotive sector, 0.4-6 mm thickness flat metal
products sheet, thickness> 6 mm products are called plates.

Among forming methods of metal materials, the fastest development has been seen in the
methods of forming sheet metals. Sheet metal forming methods are classified as follows:
• Cutting
• Bending and folding
• Spinning
• Stretching
• Deep drawing
In methods other than cutting/shearing, the material undergoes stretching and compression
under the effect of stretching and shrinking, resulting in the final shape. In these processes, the
molds usually consist of two parts. The first one has a protruding shape and is called a punch
or male die. The second one has a recessed shape and is called a die, a female die, or a core.
The punch is usually attached to the moving part of the press, and the die is fixed. Auxiliary
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

tools such as a compression die or a ring can be used to prevent creasing during forming. In
recent years, a rubber mold method has been developed to reduce die cost. In this method, the
profile of the product shape is given to the punch and thick and hard rubber is used instead of
the die. The punch pushes the sheet metal into the rubber part. With the reaction force of the
jammed rubber, it takes the shape of sheet metal.

1.7.1. Cutting
Cutting is used in processes such as slitting, separating, perimeter cutting, drilling, splitting,
and burr cutting to final dimensions. Cutting, in other words, shearing is the separation of the
sheet metal by one movable two cutting edges (dies). The production of the material from the
tape which is applied between the male (cutting) punch and the female cutting plate in the dies
is performed by applying the force with the help of the press. When the cut part is examined, it
can be seen that the cutting or punching process takes place in four stages (Figure 1.53).

Figure 1.53. 1) Contact to sheet metal of punch, 2) compressing of sheet by plastic deformation
of punch, 3) penetration of the punch by some amount from surface, 4) separation of sheet metal
by breaking at the mutual cutting edges

1.7.1.1. Important Points in Cutting / Punching Die

This process is called cutting if the part coming out from the die is used or punching if not.

Cutting gap (die clearance): The distance between the punch and the female die is called the
one-sided cutting gap. In particular, the punch is more difficult to cut and do not cut properly if
no cutting gap is provided in the dies. The cutting gap must be equal on all sides along the
cutting edges. Thus, unwanted burrs do not occur and the life of the die is extended.

If the holes on a part are basic, the punch size is kept constant and the female die is machined
as large as the cutting gap. If the outer dimensions of the piece are essential, the female die size
should be kept constant and should be made as small as the male punch clearance.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Clearance (c) value can be calculated or can be obtained by using different schedules. The value
to be calculated is the gap value on one side, and the result is multiplied by two when the die is
designed.

t≤3 mm (thin sheet);

c = x t   c= Clearance for one side (mm)
t= Sheet thickness (mm)
τ= Shear strength (kg/mm2)
x= Coefficient (0,005-0,01 mm)
t3 mm (thick sheet);
c = (1,5  x  t − 0,015)  
Cutting gap; a) The type and quality of the material to be cut, b) The thickness of the material
to be cut, c) Punch dimensions and shape, d) Sensitivity of the die.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

If the value of c is too small, it causes double If the value of c is too large, the sheet metal
curling and larger forces, the breaking lines is bent between the cutting edges and
go from one to the other. excessive burr occurs.

Angular clearance: The amount of inclination given to the female die base from the cutting
plane. The single sided clearance is recommended between α = 1/4°-2°. An angular or straight
clearance is provided to prevent squeezing the female part of the cut-off part. In general, it is
difficult to give an angular clearance, so it is drilled from the back of hole with a larger drill to
the cutting plane. In other forms, the female die is given a space with an end milling cutter
behind it. Nowadays, angular clearance can be given in the wire EDM.

Calculation of shear and stripping force: Shear force in punching-cutting dies; The thickness
of the sheet to be cut, the shear strength of the sheet, and the total circumferential length to be
cut. Stripping force is considered equal to 1/5 of this value.

Fk =   t  L (kg) L = total circumferential length, mm

t = Sheet thickness, mm
τ= Shear strength, kg/mm2
If the shear strength of the material is not known, shear force is calculated using the following
equation.
Fk= 0.7(TS) L TS= Tensile strength (MPa)

1.8.2. Bending and Folding

Bending is a process in which a portion of a sheet metal is made to provide another leveling
transition at a certain angle with the plane in which it is located. Folding is the process of
performing an infinite number of bends to bring them to a closed or open curve.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

During the bending process, the outer surface of the material is stretched (tension) and the inner
surface is pressed (compression) (Figure 1.55). For a given material thickness, as the bending
radius decreases, the unit deformation in the outer surface increases. Elongation at the outer
surface during bending leads to reduction in thickness due to volume stability. The smaller the
bending radius, the more the material thickness is examined. The amount of elongation at the
outer surface should not exceed the amount of elongation of the material homogeneously
(deformation rate of necking) in the tensile test. Otherwise, thinning and cracking may occur in
the material due to local necking.

Figure 1.55. Stresses during bending

V-Bending
• V die is used.
• Lower costs
• Low production quantities

Edge Bending
• The pressure chamber, the die
and the punch are performed in
triplicate.
• Dies are more complex and
expensive

Bending share: During bending, there is some elongation of the sheet metal. It is important to
determine the extent of the elongation in terms of knowing the length of the product. The length
of the starting sheet should be determined taking into account the amount of elongation that
will occur during the process.

Ab = 2  ( R + K ba  t )
360
Ab = bending amount; Α = bending angle; R = bending radius; T = sheet thickness; Kba =
constant due to sheet thickness and bending radius (R < 2t, Kba = 0.33; R ≥ 2t, Kba = 0.50)

Bending force: The force required for bending is calculated by the following equation.
F = bending force, TS = tensile strength of sheet metal
K bf TS  w  t 2 W = sheet width in the direction of bending axis,
Fb =
D t = sheet thickness, D = die cavity size (V- bükme için
Kbf = 1.33; Kenar bükme için, Kbf = 0.33)
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Springback: It is an elastic part of the sheet forming operation. When the bending moment
used to make the angle αb 'is released, it is back-spun as far as the sheet (Δα) angle. The yield
strength of the springback sheet (σAK) is a function of the strain-hardening exponent (n) and the
front extension (E). Due to the special use of technology and materials, springback can be
reduced or balanced. Other bending operations performed by bending are shown below.

a) Doubling, b) Locking, c) Folding

Figure 1.56. (1) during bending, the part is forced to receive the internal angle α and the
bending radius Rb, (2) after the punch is lifted, the part is spring back to radius R and α '.

1.8.3. Spinning

Deep parts with circular symmetry can be obtained by pressing the flat metal sheet onto a
rotating die in the form of a part to be produced. As can be seen from Figure 1.56, the die is
compressed towards the axis of symmetry of the sheet metal to be shaped. Then, as the die
rotates, the metallic sheet is pressed onto the die with the help of a mandrel so as to take the
shape of the die. By means of spinning, metallic kitchenware such as hollow cylinders, cone
sphere, bells, plates, pots, car falter reflectors, side covers of liquid tanks and some parts
belonging to the aircraft body can be produced. Steel sheets up to 3 mm thick, aluminum sheets
up to 6 mm thick and up to 2 m long can be spinnable with human power. When more force is
needed, force is applied with mechanical systems. Spinning process; A) conventional spinning,
b) slip spinning and c) tube spinning.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

a)

b)

c)
Figure 1.56. Spinning application

1.8.4. Stretch Forming

In the stretch forming process, the metallic sheet is bonded along two sides or along its round.
Then the forming die moves towards the sheet, allowing the material to stretch and take the
shape of the die. In the meantime, the material yield strength (σY) usually plastically deforms
by 2-4% (Figure 1.57). The process is four stages: 1. Loading, 2. Pre-stretching, 3. Extension,
4. Releasing

Figure 1.57. 1) Process start; 2) Application of Fdie force to sheet for stretching and bending.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

In this method, the Fdie force can be determined by balancing the vertical force components
while the stretching force is calculated by the following equation.
F= L t σak F = Stretching force
L = Sheet length perpendicular to stretch
T = Instantaneous block thickness
σak = Yield stress of the material
The necking phenomenon is a limiting factor in the stretching process. Due to the part geometry,
only the uniaxial tensile stress should not exceed the homogeneous deformation rate of the
necking point in the tensile test. If the part geometry leads to biaxial tensile stresses, the necking
phenomenon is delayed and in this case it is possible to further thin the thickness of the sheet
homogeneously. In biaxial stretching, the strain limit is determined by the shaping boundary
diagrams. In this method, the "springback" phenomenon is prevented because of the
homogeneous distribution of stresses. Parts with large radius of curvature can be produced
easily and economically by stretching.

Materials to be shaped by this method must have high ductility and have high n (strain
hardening exponent) and high m (deformation rate sensitivity exponent) values in order to
distribute strain and stress. Thus, the strain is prevented from concentrating in certain regions
and plastic deformation can be achieved at higher ratios by distributing to adjacent zones. The
stretch forming ratio (GBO) is obtained by the stretching depth (hG), the stretching width (1G).
As the sheet thickness increases, the GBO ratio increases and the material can be formed
without cracking.

Figure 1.58. Schematic representation of stretch forming ratio

1.8.5. Deep Drawing

Deep drawing can be defined as a process of obtaining a three-dimensional deep container from
a flat metallic sheet material. Base of product is straight due to the punch used in the deep
drawing process. But, spherical or more complex base shapes can be produced by deep drawing
or by stretch forming in succession. During the cylindrical deep drawing process shown in
Figure 1.59, the circular punch forces are first transferred to the lower section of the punch and
part and then to the section that will form the container wall. The pot-ring/blank holder force
prevents the wrinkle-crease problem due to tangential compression stress in the forming zone.
As the punch moves downward, the sheet flows under the blank holder, through the draw-ring
radius, and is drawn toward the die gap. During the process "sheet thickness" remains almost
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

constant. The steel with diameter Db is pulled deep into the die cavity with punch a diameter
of Dp. It is necessary to corner round in both the punch and the die (Rp, Rd). The space between
the punch and die is denoted by c (1.1 t), compressive force F and blank holder force Fh. In the
deep drawing process, the suitability tests for drawing must be carried out. These:

Deep Drawing Ratio: Deep drawing ratio (DDR) is defined as the ratio of the starting part
diameter (Db) to the punch diameter (Dp). DÇO = Db / Dp
In this method, the main purpose is to obtain as deep a container as possible. The blank diameter
cannot be increased unlimitedly to increase depth. The maximum blank diameter that can be
used is determined by the deep drawing ratio limit (DDRL2). It explains to what diameter the
material can be pulled just before the start of the crack.
Reduction Ratio: It is calculated by RO = (Db-Dp) / Db equation, and is desired to be less than
0.5 mm.
Thickness / Diameter Ratio: It is calculated with KÇO = t / Db equation and it is desired to be
larger than 1%.

Figure 1.59.
a) Deep drawing of
tubular part:
(1) Process start
(2) Process finish
b) Part:
(1) Blank
(2) Product

The deep drawing ratio depends on the properties of theoretical DDRL and the operating
conditions under ideal conditions. The deep drawing process can be repeated on the same piece
successively to obtain deep containers. Forms outside cylindrical tubes; square or rectangular
boxes (as in houses), stepped tubes, cones, spherical base tubes, irregular curved forms
(automobile bodies) are formed by drawing.

Stresses and Forces in Deep Drawing:

At the deep drawing of the cylindrical containers, the part under the blank holder is subjected
to "radial tensile stress" and "tangential compressive stress". At the same time, the blank holder
force must be applied the minimum value and the vertical compressive shaped to prevent
wrinkling on the flange. During deep drawing, four distinct zones occur (Figure 1.62):
1) Forming zone: This is the part of the blank holder pressed against the part. The blank holder
forces is prevent to be form the first "wrinkle".
2) Bending zone: The part is forced to bend over the pull ring radius.
3) Force bearing zone: The walls of part are shaped.
4) Strength zone: Forming is start with this zone
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.62. Stress zones created by drawing

Friction phenomenon in Deep Drawing:

• sheet / oil / pot ring; sheet / oil / pull ring; sheet / oil / pull ring radius
It should be as little as possible between them. The friction between the punch and the wall
thickness of the sheet must be as high as possible to form very high punch forces. The system
must provide the following stable conditions:
Fp ≤ Ft = a. σÇ .U. t
Here; a: Tear factor (It is affected by the friction between the part thickness / oil / punch). If
the tear factor is large, larger tearing force is generated and then tearing occurs on the part wall.
σÇ: tensile strength of sheet, U: punch circumference, t: sheet thickness, Ft: Tear force

Defects encountered during deep drawing are shown as follows:

Figure 1.64. Drawing defects; a) Wrinkle in the flange, b) Wrinkle on the sheet metal,
c) Tearing, d) Earing, e) Surface scratches

1.8.6. Hydro-Mechanical Forming

This method is investigated in two groups as tube hydroforming and hydro-mechanical
drawing.
1- Tube Hydroforming: An advantageous forming process in active environments. Shallow
and semi-finished products, such as circular tubes, extrusion parts, are brought into complete
finished product by this method. Parts with complex surface geometry are shaped by means of
pressurized fluids and simultaneously the aid of axial forces. Water-based fluids are generally
used, whereas oil and elastomer-based materials are suitable for the pressure medium.
MANUFACTURING PROCESSES II PROF.DR.MUSTAFA GÜNAY

Figure 1.65.Tube Hydroforming

2- Hydro-mechanical Deep Drawing: In this method, the hydraulic pressure is applied from
the opposite side during the deep drawing of the symmetrical round metal sheet metal part.
Reverse pressure increases the friction between the sheet metal surface and the punch. Thus,
the tear factor increases as the coefficient increases, which means that the ability to draw deeper
sheets is increased. In addition, it is possible to produce parts that we want to be even thinner
without wrinkling in a single operation. Hydro-mechanical deep drawing, only according to the
mechanical deep drawing process:
• Provides higher tearing factor, which allows you to do deeper sheet forming.
• We can easily obtain the thin walled part that we want to study progressively.

Figure 1.66. Hydro-mechanical deep drawing.