1.0 INTRODUCTION TO MANUFACTURING PROCESSES
CHAPTER 1 : INTRODUCTION TO MANUFACTURING
PROCESSES
1.1 INTRODUCTION
The word manufacture is derived from two Latin words manus (hand) and factus (make); the
combination means “made by hand”. Made by hand” accurately described the fabrication
methods that were used when the English word “manufacture” was first coined around 1567
A.D. Most modern manufacturing operations are accomplished by mechanized and
automated equipment that is supervised by human workers as shown in Figure 1.1.
Figure 1.1 Modern manufacturing.
.
This subject provides strong fundamental concepts and techniques related to processes and
technology. Basically, manufacturing processes course covers main topics: conventional
machining processes, casting processes, sheet metal processes. bulk deformation
processes, joining processes, polymer processing, non-conventional processes, powders
processing and manufacturing costs.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. Define the manufacturing process;
2. Identify the type of materials that mostly used in manufacturing;
3. Recognize the classification of the manufacturing process.
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1.2 DEFINITION OF MANUFACTURING PROCESSES
1. Manufacturing processes is application of physical and chemical processes to alter
the geometry, properties, and/or appearance of a starting material to make parts or
products.
2. Manufacturing also includes assembly or joining of products.
3. It almost always carried out as a sequence of operations (Figure 1.2).
Figure 1.2 Manufacturing as a technical process.
4. Transformation of materials into items of greater value by means of one or more
processing and/or assembly operations.
5. Manufacturing adds value to the material by changing its shape or properties, or by
combining it with other materials (Figure 1.3).
Figure 1.3 Manufacturing as an economic process.
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1.3 MATERIALS IN MANUFACTURING
1. Most engineering materials can be classified into one of three basic categories
(Figure 1.4):
a. Metals.
b. Ceramics.
c. Polymers.
2. Their chemistries are different.
3. Their mechanical and physical properties are dissimilar.
4. These differences affect the manufacturing processes that can be used to
produce products from them.
5. Nonhomogeneous mixtures of the other three basic types rather than a unique
category.
Figure 1.4 Three basic material types plus composites.
1.3.1 METALS
1. Usually alloys, which are composed of two or more elements, at least one of which
is metallic.
2. Two basic groups of metals:
a. Ferrous metals - based on iron, comprises about 75% of metal tonnage in the
world:
Steel = Fe-C alloy (0.02 to 2.11% C).
Cast iron = Fe-C alloy (2% to 4% C).
b. Nonferrous metals - all other metallic elements and their alloys: aluminum,
copper, magnesium, nickel, silver, tin, titanium, etc.
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1.3.2 CERAMICS
1. Compounds containing metallic (or semi-metallic) and nonmetallic elements.
2. Typical nonmetallic elements are oxygen, nitrogen, and carbon.
3. For processing, ceramics divide into:
a. Crystalline ceramics – includes:
i. Traditional ceramics, such as clay (hydrous aluminum silicates).
ii. Modern ceramics, such as alumina (Al2O3).
b. Glasses – mostly based on silica (SiO2).
1.3.3 POLYMERS
1. Compound formed of repeating structural units called mers, whose atoms share
electrons to form very large molecules.
2. Three categories:
a. Thermoplastic polymers - can be subjected to multiple heating and cooling
cycles without altering molecular structure.
b. Thermosetting polymers - molecules chemically transform (cure) into a rigid
structure – cannot be reheated.
c. Elastomers - shows significant elastic behavior.
1.3.4 COMPOSITES
1. Composite material consisting of two or more phases that are processed
separately and then bonded together to achieve properties superior to its
constituents.
2. The phase of composite contains homogeneous mass of material, such as grains
of identical unit cell structure in a solid metal.
3. Usual structure of composite consists of particles or fibers of one phase mixed in
a second phase.
4. Properties depend on components, physical shapes of components, and the way
they are combined to form the final material.
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1.4 CLASSIFICATION OF MANUFACTURING PROCESSES
1. Two basic types of manufacturing processes:
a. Processing operations - transform a work material from one state of completion to a
more advanced state.
b. Assembly operations - join two or more components to create a new entity.
2. Figure 1.5 shows the diagram of manufacturing process classification.
Figure 1.5 The classification of manufacturing process.
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1.4.1 PROCESSING OPERATIONS
1. Processing operations are the process that changes the geometry, properties, or
appearance of the starting material as shown in Figure 1.6.
2. The process will alter a material’s shape, physical properties, or appearance to add
value.
3. One of the main processing operations is shaping operations, that alter the geometry of
the starting work material.
4. Four Categories of Shaping Processes:
a. Material removal processes - starting material is a ductile or brittle solid. Topics will
be covered in Conventional Machining and Non-conventional Machining.
b. Solidification processes - starting material is a heated liquid or semifluid. Topics will
be covered in Casting Processes and Polymer Processes.
c. Deformation processes - starting material is a ductile solid (commonly metal). Topics
will be covered in Sheet Metal Processes and Bulk Deformation processes.
d. Particulate processing - starting material consists of powders. Topics will be covered
in Powder Processing.
Figure 1.6 Shaping process using machining.
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1.4.2 ASSEMBLY OPERATIONS
1. Two or more separate parts are joined to form a new entity. Topics will be covered in
Joining Processes. Figure 1.7 shows the welding works to joint the steel pipe.
2. Types of assembly operations:
a. Joining processes – create a permanent joint such as welding, brazing, soldering,
and adhesive bonding.
b. Mechanical assembly – fastening by mechanical methods such as threaded
fasteners (screws, bolts and nuts); press fitting, expansion fits.
Figure 1.7 Welding works.
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SUMMARY
In this chapter we studied that:
1. Manufacturing processes is application of physical and chemical processes to alter
the geometry, properties, and/or appearance of a starting material to make parts or
products;
2. The materials that mostly used in manufacturing processes are metals, ceramics and
polymers;
3. Two types of basic manufacturing process include processing operations and joining
operations.
REFERENCES
rd
1. Groover Mikell, Fundamental of Modern Manufacturing, 3 Edition, 2007.
th
2. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5 edition,
Prentice Hall, 2005.
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2.0 CASTING PROCESSES
CHAPTER 2 : CASTING PROCESSES
2.1 INTRODUCTION
Figure 2.1 (a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast, high purity
magnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die
casting process.
Casting process is among the old process known to mankind, which is discovered since
4000 B.C when man first knew the technique of melting of metals. Various casting processes
have been developed over a long period of time, each with its own characteristics and
applications, to meet the specific engineering and service requirement. From that, many
parts and components are made by casting such as carburetors, engine blocks, crankshafts,
automotive components, agricultural and railroad equipment, pipes and plumbing fixtures,
power tools, gun barrels, frying pans and very large components for hydraulic turbines.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. define the casting process;
2. describe the type of casting;
3. Apply the right casting process to produce the right product.
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2.2 GENERAL ASPECTS OF CASTING PROCESS
1. Casting is a solidification process in which molten metal is poured into a mould and
allowed to cool.
2. Normally, the materials that used in casting processes is metal. Because a pure
metal has a clearly defined melting (or freezing) point, it solidifies at a constant
temperature.
3. The casting process basically involves:
a) pouring molten metal into a mould patterned after the part to be
manufactured;
b) allowing it to solidify;
c) removing the part from the mould.
4. Major factors affecting casting processes:
a) properties of mould such as shape, geometric, process etc.
b) properties of metal such as fluidity, solidification, surface etc.
5. Figure 2.2 shows example of casting product.
Figure 2.2 Example of casting product.
Think
Another process that almost same as casting processes
Injection Moulding - Molten polymer injected into a mould while casting is a molten
metal poured into a mould.
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2.3 PROPERTIES OF MOULD
1. Basically, properties of mould controls the properties of end casting products.
2. Several mould properties should be considered before implementing casting
processes such as design, size, product, cost, weight etc.
3. The major categories moulds are as follows:
a) Expendable moulds.
b) Permanent moulds.
c) Composite moulds.
2.4 EXPENDABLE-MOULD CASTING PROCESSES
1. In expendable-mould casting, as the molten metal solidified, the mould that used in
casting process is broken up to remove the casting.
2. The major categories of expendable-mould casting are sand, shell mould, plaster
mould, ceramic mould, evaporative pattern, and investment casting.
3. Generally these casting moulds are mixed with various binders (bonding agents) for
improved properties.
2.4.1 SAND CASTING
1. Sand casting is a casting technique that applied in almost any metal cast.
2. Most sand-casting operations use silica sand as mould material.
3. Sand casting applied with no limit to size, shape, weight and low tooling cost.
4. Sand is inexpensive and is suitable as mould material because of its high-
temperature characteristics and high melting point.
5. Good permeability of moulds and cores allows gases and steam evolved during the
casting to escape easily.
6. Figure 3.3 shows the outline of production steps in a typical sand-casting operation.
Think
What is permeability?
Permeability means; allow escape of gas, function of sand practical size,
bonding agent and moisture.
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Figure 2.3 Production steps in a typical sand-casting operation.
EXERCISE 3.1
Describe the process of sand casting.
2.4.2 SHELL MOULDING
1. Shell-moulding process also called the dump-box technique.
2. This process can produce the high quality of the finished casting.
3. It can reduce cleaning, machining, and other finishing costs significantly.
4. Complex shapes can be produced with less labor.
5. The process can be automated fairly easily.
6. Fig. 3.4 shows the schematic illustration of the shell moulding process.
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Figure 2.4 Illustration of the shell moulding process.
2.4.3 PLASTER-MOULD CASTING
1. This process also known as precision casting because of the high dimensional
accuracy and good surface finish obtained.
2. In the plaster-moulding process, the mould is made of plaster of paris (gypsum or
calcium sulfate).
3. Talc and silica flour may be added to improve strength and to control the time
required for the plaster.
4. Plaster moulds have very low permeability where gases evolved during solidification
of the metal cannot escape.
5. Because plaster moulds have lower thermal conductivity than others, the castings
cool slowly, and form uniform grain structure.
2.4.4 CERAMIC-MOULD CASTING
1. The ceramic-mould casting process (also called cope-and-drag investment casting)
is similar to the plaster-mould process.
2. It uses refractory mould materials suitable for high-temperature applications.
3. Fig. 3.5 shows the sequence of operations in making a ceramic mould.
4. The high-temperature resistance of the refractory moulding materials allows these
moulds to be used for casting ferrous and other high-temperature alloys, stainless
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steels, and tool steels.
Figure 2.5 Sequence of operations in making a ceramic mould.
Think
How to perform casting process for ceramic materials?
Processing of ceramic casting normally started with ceramic powders poured into a
liquid medium to obtain fluidity reactions. The powder and solution are then cast into
a mould. This process called slip casting.
.
2.4.5 EVAPORATIVE-PATTERN CASTING (LOST-FOAM PROCESS)
1. Evaporative-pattern casting is sometimes referred to as the expendable-pattern
casting processes or the expendable mould-expendable pattern processes.
2. It is unique in that a mould and a pattern must be produced for every casting,
whereas the patterns in the processes described thus far are reusable.
3. The evaporative-pattern casting process uses a polystyrene pattern, which
evaporates upon contact with molten metal to form a cavity for the casting; this
process is also known as lost-foam casting and falls under the trade name Full-Mould
process.
4. Fig. 3.6 shows the schematic illustration of the expendable-pattern casting process,
also known as lost-foam or evaporative–pattern casting.
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Figure 2.6 Schematic illustration of the expendable pattern casting process.
5. The evaporative-pattern process has a number of advantages over other casting
methods:
a) Simple process because there are no parting lines, cores, or riser systems.
b) Flexibility design because polystyrene can be processed easily into patterns
in complex shapes, various sizes, and fine surface detail.
c) Inexpensive flasks.
d) The casting requires minimal finishing and cleaning operations.
e) The process can be automated and is economical for long production runs.
6. Some of the limitations of the evaporative-pattern casting consist of:
a) High cost to produce the die used for expanding the polystyrene beads.
b) The need of many tooling preparations.
2.4.6 INVESTMENT CASTING
1. The investment-casting process, also called the lost-wax process.
2. A variation of the investment-casting process is ceramic-shell casting.
3. Fig. 3.7 shows the schematic illustration of the investment casting (lost-wax) process.
4. The evaporative-pattern process has a number of advantages over other casting
methods:
a) Can produce fine and detail products.
b) It is suitable for casting high-melting-point alloys.
c) Good surface finishes.
d) Close dimensional tolerances.
e) Economical because of few or no finishing operations.
5. However, the mould materials and labor involved increase the cost of investment-
casting.
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Figure 2.7 Schematic illustration of investment casting, (lost-wax process).
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2.5 PERMANENT-MOULD CASTING
1. In permanent-mould casting, (also called hard-mould casting), the moulds are made
from metals that maintain their strength at high temperatures.
2. This process has certain advantages over other casting processes such as it can be
run automated for large production runs.
3. This process is used mostly for aluminum, magnesium, copper alloys, and gray iron
because of their generally lower melting points, although steels also can be cast
using graphite or heat-resistant metal moulds.
4. Some of the examples of permanent-mould casting are vacuum casting, slush
casting, pressure casting, die casting and centrifugal casting.
Example 2.2:
What is the difference between expandable-mould casting and permanent-mould
casting?
Solution:
1. In expendable-mould casting, the mould that used in casting process is broken up
to remove the casting while permanent mould casting still maintiain the use of its
mould.
2. Compare to the expandable-mould casting, the mould that used permanent-mould
casting can be used several times to produce high volume casting productions.
2.2.1 VACUUM CASTING
1. Vacuum casting is a casting process with molten metal absorbed into a mould with
vacuum actions.
2. Fig 3.8. shows the schematic illustration of the vacuum-casting process.
3. Vacuum casting is an alternative to investment, shell-mould, and sand casting and is
suitable particularly for complex shapes with uniform properties.
4. The process can be automated, and production costs are similar to those for sand
casting.
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Figure 2.8 Schematic illustration of the vacuum-casting process. Note that the mould has a
bottom gate. (a) Before and (b) after immersion of the mould into the molten metal.
2.5.2 SLUSH CASTING
1. Slush casting involves with hollow castings with thin walls that made by permanent-
mould casting.
2. This process is suitable for small production runs and generally is used for making
ornamental and decorative objects and toys from low melting-point metals such as
zinc, tin, and lead alloys.
3. The molten metal is poured into the metal mould. After the desired thickness of
solidified skin is obtained, the mould is inverted (or slung) and the remaining liquid
metal is poured out.
4. The mould halves then are opened, and the casting is removed.
2.5.3 PRESSURE CASTING
1. It is also one of the vacuum casting technique.
2. The molten metal flows into the mould cavity by gravity. In pressure casting (also
called pressure pouring or low-pressure casting), the molten metal is forced upward
by gas pressure into a graphite or metal mould as shown in Figure 3.9.
3. The pressure is maintained until the metal has solidified completely in the mould.
4. The molten metal also may be forced upward by a vacuum, which also removes
dissolved gases and produces a casting with lower porosity.
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Figure 3.9 (a) The bottom-pressure casting process utilizes graphite moulds for the
production of steel railroad wheels. Source: The Griffin Wheel Division of Amsted Industries
Incorporated. (b) Gravity-pouring method of casting a railroad wheel. Note that the pouring
basin also serves as a riser. Railroad wheels can also be manufactured by forging.
Think
Why does casting processes can produce the smallest cast parts?
Casting processes can produce the smallest cast parts because of :
most of the car parts made from metal and casting processes is suitable with metal
processing;
high pressures involved in some casting processes such as vacuum casting, pressure
casting and die casting which is suitable to produce small product with intricate shape.
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2.5.4 DIE CASTING
1. The die casting process is a further example of permanent-mould casting.
2. Typical parts made by die casting are motor housings, engine blocks, business-
machine and appliance components, hand tools, and toys.
3. In this process, the molten metal is forced into the die cavity at high pressures.
4. Die casting has the capability for rapid production of strong, high-quality parts with
complex shapes, especially with aluminum, brass, magnesium, and zinc.
5. These machines are large compared to the size of the casting, because high forces
are required to keep the two halves of the dies closed under pressure.
6. It also produces good dimensional accuracy and surface details, so that parts require
little or no subsequent machining or finishing operations (net-shape forming).
7. There are two basic types of die-casting machines:
a) Hot-chamber die-casting.
b) Cold-chamber die-casting.
2.5.5 HOT-CHAMBER DIE-CASTING
1. In hot chamber die-casting, a piston used to trap a molten metal and forced into die
cavity at certain pressure and heated at elevated temperature.
2. Fig. 3.10 shows the schematic illustration of the hot-chamber die-casting process.
3. Low melting point alloys such as zink, magnesium, lead and tin commonly cast using
this process.
Fig 3.10 Schematic illustration of the hot-chamber die-casting process.
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2.5.6 COLD-CHAMBER DIE-CASTING
1. Fig. 3.11 shows the schematic illustration of the cold-chamber die-casting process.
2. In cold-chamber die-casting, molten metal is poured into the injection cylinder at high
pressure without increase the heating temperature.
3. High melting point alloys such as aluminum, magnesium and copper commonly cast
using this process.
Fig 3.11 Schematic illustration of the cold-chamber die-casting process.
2.5.7 DIE-CASTING DIES
1. Both die-casting dies may be single cavity, multiple cavity (with several identical
cavities), combination cavity (with several different cavities), or unit dies (simple small
dies that can be combined in two or more units in a master holding die).
2. Fig. 3.12 shows the various types of cavities in a die-casting die.
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Figure 3.12 Various types of cavities in a die-casting die.
Example 2.3:
Describe the differences between hot-chamber and cold-chamber die casting
processes respectively.
Solution:
Hot-chamber die casting Cold-chamber die casting
Implementing high temperature. Use normal temperature.
Suitable for low melting point alloys such Suitable for high melting point alloys such
as zink, magnesium, lead and tin. as aluminum, magnesium and copper.
2.5.8 CENTRIFUGAL CASTING
1. Centrifugal-casting process utilizes inertial forces (caused by rotation) to distribute
the molten metal into the mould.
2. There are three types of centrifugal casting:
a) True centrifugal casting.
b) Semi-centrifugal casting.
c) Centrifuging.
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2.5.9 TRUE CENTRIFUGAL CASTING
1. In true centrifugal casting, hollow cylindrical parts (such as pipes, gun barrels,
bushings, engine-cylinder liners, bearing rings with or without flanges, and streetlamp
posts) are produced.
2. Castings with good quality, dimensional accuracy, and external surface detail are
produced by this process.
3. Figure 3.13 (a) shows the schematic illustration of the centrifugal-casting process.
4. Figure 3.13 (b) shows the side view of the machine.
Figure 2.13 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners,
and similarly shaped parts can be cast with this process.
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2.5.10 SEMI-CENTRIFUGAL CASTING
1. An example of semi-centrifugal casting is shown in Figure 3.14.
2. This method is used to cast parts with rotational symmetry, such as a wheel with
spokes.
Figure 2.14 Schematic illustration of the semicentrifugal casting process.
2.5.11 CENTRIFUGING (CENTRIFUGE CASTING)
1. In centrifuging (also called centrifuge casting), mould cavities of any shape are
placed at a certain distance from the axis of rotation.
2. The molten metal is poured from the center and is forced into the mould by
centrifugal forces (Figure 2.15).
3. The properties of the castings can vary by distance from the axis of rotation, as in
true centrifugal casting.
Figure 2.15 Schematic illustration of casting by centrifuging. The moulds are placed at
the periphery of the machine, and the molten metal is forced into the moulds by centrifugal
force.
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2.6 COMPOSITE MOULDS CASTING
1. Composite moulds are made of two or more different materials (such as sand,
graphite, and metal) combining the advantages of each material.
2. Generally composite moulds are use in casting complex shape such as turbine,
impellers etc as shown in Figure 3.16.
3. Composite moulds increase the strength of the mould, improve the dimensional
accuracy and surface finish of castings, and can help reduce overall costs and
processing time.
Figure 2.16 (a) Schematic illustration of a semipermanent composite mould.
(b) A composite mould used in casting an aluminum-alloy torque converter. This part
was previously cast in an all-plaster mould.
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Table 2.2 Advantages and Limitations of Casting Processes.
Process Advantages Limitations
Sand a) Almost any metal cast. a) Some finishing required.
b) No limit to part size, b) Relatively coarse surface finish.
shape or weight. c) Wide tolerance.
c) Low tooling cost.
Shell Mould a) Good dimensional a) Part size limited.
accuracy and surface b) Expensive patterns and
finish. equipment.
b) High production rate.
Evaporative a) Most metals cast with no a) Patterns have low strength.
Pattern limit size. b) Can be costly for low quantities.
b) Complex part shape.
Plaster a) Intricate part shapes. a) Limited to nonferrous metal.
Mould b) Close tolerance parts. b) Limited part size and volume of
c) Good surface finish. production.
d) Low porosity. c) Long mould making time.
Ceramic a) Intricate part shapes. a) Limited part size.
Mould b) Close tolerance parts.
c) Good surface finish.
Investment a) Intricate part shape. a) Limited part size.
b) Excellent surface finish b) Expensive patterns, moulds, and
and accuracy. labor.
c) Almost any metal cast.
Permanent a) Good surface finish and a) High mould cost.
mould dimensional accuracy. b) Limited part shape and
b) Low porosity. complexity.
c) High production rate. c) Not suitable for high melting
point metals.
Die a) Excellent dimensional a) High die cost.
accuracy and surface b) Limited part size.
finish. c) Generally limited to nonferrous
b) High production rate. metal.
d) Long lead time.
Centrifugal a) Large cylindrical or a) Expensive equipment.
tubular parts with good b) Limited part shape.
quality.
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Example 2.4:
Which of the casting processes would be most suitable for making small metal toys?
Why?
Solution:
Small toys, such as metal cars, are produced in large quantities so that the mould
cost is spread over many parts. To produce the intricate shapes needed at large
quantities reduces the options to investment casting and die casting. Among these
two, die casting is the logical choice as a reason of higher production rate.
EXERCISE 2.2
If you need only a few units of a particular casting, which process would
you use? Why ?
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SUMMARY
In this chapter we studied that:
1. casting process principally involves pouring molted metal into mould cavity and
letting it solidify into the shape of the cavity;
2. this process can produce a wide variety of product including complex shape. Casting
processes generally are classified as expendable-mould casting, permanent-mould
casting and composite mould casting;
3. the most common expendable-mold processes are sand, shell mould, plaster mould,
ceramic mould, evaporative pattern, and investment casting;
4. common permanent-mold processes include slush casting, pressure casting, die
casting, and centrifugal casting.
REFERENCES
th
1. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5 edition,
Prentice Hall, 2005.
2. Steve F.Krar Technology of Machine Tools, , Mc Graw Hill, 2005.
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CHAPTER 3 : SHEET METAL PROCESSES
3.1 INTRODUCTION
The term press working or press forming is used commonly in industry to describe
general sheet-forming operations, because they typically are performed on presses
using a set of dies. Low-carbon steel is the most commonly used sheet metal because of
its low cost and generally good strength and formability characteristics. Fig. 4.1 shows
examples of sheet-metal parts (a) Stamped parts. (b) Parts produced by spinning. Table
3.1 shows the general characteristics of sheet-metal forming processes.
Figure 3.1 (a) Stamped parts. (b) Parts produced by spinning.
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Table 3.1 The general characteristics of sheet-metal forming processes.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. Define the sheet metal processes such as shearing, bending and forming etc;
2. Identify the equipment used for sheet-metal prosesses such as punch, die, slug, etc.
3. Identify specific material properties use in sheet-metal prosesses;
4. Design consideration in sheet sheet-metal prosesses.
5. Analyze the economics factor of sheet-metal prosesses.
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3.2 SHEARING PROCESSES
1. Before a sheet-metal part is made, a blank of suitable dimensions first is removed
from a large sheet (usually from a coil) by shearing.
2. Fig 4.2 – 4.4 show schematic illustration of shearing with a punch and die, indicating
some of the process variables. Characteristic features of a punched hole and the
slug.
Figure 3.2 Punch and die.
Figure 3.3 Punched hole.
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Figure 3.4 The slug.
3. The rough fracture surfaces are due to these cracks; the smooth and shiny burnished
surfaces on the hole and the slug are from the contact and rubbing of the sheared
edge against the walls of the punch and die, respectively.
4. The major processing parameters in shearing are:
a. the shape of the punch and die;
b. the speed of punching;
c. lubrication;
d. the clearance, c, between the punch and the die.
5. Fig. 3.5 (a) shows effect of the clearance, c, between punch and die on the
deformation zone in shearing.
6. As the clearance increases, the material tends to be pulled into the die rather than be
sheared. In practice, clearances usually range between 2 and 10% of the thickness
of the sheet.
7. Fig. 3.5 (b) shows micro-hardness (HV) contours for a 6.4-mm (0.25-in) thick AISI
1020 hot-rolled steel in the sheared region.
Figure 3.5 (a) Effect of the clearance. (b) Micro-hardness.
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3.2.1. PUNCH FORCE
1. The force required to punch is basically the product of the shear strength of the sheet
metal and the total area being sheared along the periphery.
2. The maximum punch force, F, can be estimated from the equation
F = 0.7TL(UTS)
Where T is the sheet thickness, L is the total length sheared (such as the perimeter of a
hole), and UTS is the ultimate tensile strength of the material.
3. Friction between the punch and the work piece can, however, increase punch force
significantly.
4. Furthermore, in addition to the punch force, a force is required to strip the punch from
the sheet during its return stroke.
Example 3.1
Calculation of punch force
Estimate the force required for punching a 25-mm diameter hole through a 3.2-mm thick
annealed titanium-alloy Ti-6Al-4V sheet at room temperature. Ultimate tensile strength of
the titanium-alloy Ti-6Al-4V is 100 MPa.
Solution:
The force is estimated from equation, where the UTS for this alloy is found to be 1000
MPa. Thus,
F = 0.7(3.2)(π)(25)(100 X 10 )
6
= ………………..
3.2.2 SHEARING OPERATIONS
1. The most common shearing operations are punching—where the sheared slug is
scrap or may be used for some other purpose—and blanking—where the slug is the
part to be used and the rest is scrap.
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2. Fig. 3.6 (a) Punching (piercing) and blanking, while (b) shows the examples of
various die-cutting operations on sheet metal.
Figure 3.6 (a) Punching and blanking. (b) Various die-cutting.
a. Die Cutting
1. This is a shearing operation that consists of the following basic processes:
a. Perforating: punching a number of holes in a sheet
b. Parting: shearing the sheet into two or more pieces
c. Notching: removing pieces (or various shapes) from the edges
d. Lancing: leaving a tab without removing any material
b. Fine blanking
1. Very smooth and square edges can be produced by fine blanking.
2. Fig. 3.7 (a) shows the comparison of sheared edges produced by conventional (left)
and by fine-blanking (right) techniques and (b) shows schematic illustration of one
setup for fine blanking.
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Figure 3.7 (a) Conventional and fine blanking. (b) Setup of fine blanking.
c. Slitting
1. In slitting, the blades follow either a straight line, a circular path, or a curved path.
2. Fig. 4.8 shows the slitting with rotary knives.
3. This process is similar to opening cans.
Figure 4.8 The slitting with rotary knives.
d. Steel rules
1. Soft metals (as well as paper, leather, and rubber) can be blanked with a steel-rule
die.
2. The die is pressed against the sheet which rests on the flat surface, and it shears the
sheet along the shape of the steel rule.
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e. Nibbling
1. In nibbling, a machine called a nibbler moves a small straight punch up and down
rapidly into a die.
2. A sheet is fed through the gap and many overlapping holes are made.
3. Using manual or automatic control, sheets can be cut along any desired path.
f. Scrap in shearing
1. Scrap can be a significant factor in manufacturing cost, and it can be reduced
substantially by efficient arrangement of the shapes on the sheet to be cut (nesting).
2. Computer-aided design techniques have been developed to minimize the scrap from
shearing operations.
3.2.3 TAILOR-WELDED BLANKS
1. An important variation from these conditions involves laser-beam butt welding of
two or more pieces of sheet metal with different shapes and thicknesses.
2. Because of the small thicknesses involved, the proper alignment of the sheets
prior to welding is important.
3. The welded assembly subsequently is formed into a final shape.
4. The result of this technique is
a. reduction in scrap;
b. elimination of the need for subsequent spot welding (i.e., in the making of the
car body);
c. better control of dimensions;
d. improved productivity.
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Example 3.2 Tailor welded sheet metal for automotive applications
An example of the use of tailor-welded sheet metals in automobile bodies is shown in
Figure 3.8. Note that five different pieces are blanked first, which includes cutting by
laser beams. Four of these pieces are 1 mm thick, and one is 0.8 mm thick. These
pieces are laser butt-welded and then stamped into the final shape. In this manner, the
blanks can be tailored for a particular application, not only as to shape and thickness but
also by using different quality sheets—with or without coatings.
Laser-welding techniques are highly developed; as a consequence, weld joints are very
strong and reliable. The growing trend toward welding and forming sheet-metal pieces
makes possible significant flexibility in the product design, structural stiffness, formability,
and crash behavior of an automobile. It also makes possible the use of different
materials in one component, weight savings, and cost reduction in materials, scrap,
equipment, assembly, and labor.
There are increasing applications for this type of production in automotive companies.
The various components shown in Fig. 3.9 utilize the advantages outlined above. For
example, note in Fig.3.9 (b) that the strength and stiffness required for the support of the
shock absorber are achieved by welding a round piece onto the surface of the large
sheet. The sheet thickness in such components is varied (depending on its location and
on its contribution to such characteristics as stiffness and strength) and, thereby, makes
possible significant weight savings without loss of structural strength and stiffness.
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Figure 3.8 Five different pieces of blanking.
Figure 3.9 The various components automotive.
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3.2.4 CHARACTERISTICS AND TYPE OF SHEARING DIES
1. Clearance
1. Because the formability of the sheared part can be influenced by the quality of its
sheared edges, clearance control is important.
2. The appropriate clearance depends on:
a. the type of material and its temper;
b. the thickness and size of the blank;
c. its proximity to the edges of other sheared edges or the edges of the original
blank.
3. Fig 3.10 shows the schematic illustrations of the shaving process where (a)
Shaving a sheared edge; and (b) Shearing and shaving combined in one stroke.
4. As a general guideline, (a) clearances for soft materials are less than those for
harder grades; (b) the thicker the sheet, the larger the clearance must be; and (c)
as the ratio of hole diameter to sheet thickness decreases, clearances should be
larger.
Figure 3.10 (a) Sheared edge, (b) Shearing and shaving combined in one stroke.
2. Punch and Die
1. Both the surfaces of the punch and of the die are flat.
2. Because the entire thickness is sheared at the same time, the punch force
increases rapidly during shearing.
3. The location of the regions being sheared at any particular instant can be
controlled by beveling the punch and die surfaces.
4. Fig 3.11 shows the examples of the use of shear angles on punches and dies.
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Figure 3.11 The examples of the use of shear angles on punches and dies.
3. Compound dies
1. Several operations on the same sheet may be performed in one stroke at one
station with a compound die.
2. Such combined operations usually are limited to relatively simple shapes,
because (a) the process is somewhat slow and (b) the dies rapidly become much
more expensive to produce than those for individual shearing operations,
especially for complex dies.
3. Fig 3.12 shows the schematic illustrations: (a) before and (b) after blanking a
common washer in a compound die. Note the separate movements of the die (for
blanking) and the punch (for punching the hole in the washer); (c) Schematic
illustration of making a washer in a progressive die. (d) Forming of the top piece
of an aerosol spray can in a progressive die. Note that the part is attached to the
strip until the last operation is completed.
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Figure 3.12 (a) Before blanking, (b) After blanking, (c) Progressive die (d) Strip
4. Progressive dies
1. Parts requiring multiple operations to produce can be made at high production
rates in progressive dies.
2. The sheet metal is fed through as a coil strip, and a different operation (such as
punching, blanking, and notching) is performed at the same station of the
machine with each stroke of a series of punches.
5. Transfer dies
1. In a transfer-die setup, the sheet metal undergoes different operations at different
stations of the machine which are arranged along a straight line or a circular path.
2. After each step in a station, the part is transferred to the next station for further
operations.
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Figure 3.13 Transfer die.
6. Tool and die materials
1. Tool and die materials for shearing generally are tool steels and (for high
production rates) carbides.
2. Lubrication is important for reducing tool and die wear, thus improving edge
quality.
3.2.5 MISCELLANEOUS METHODS OF CUTTING SHEET METAL
1. There are several other methods of cutting sheets and, particularly, plates:
a. Laser-beam cutting is an important process typically used with
computer-controlled equipment to cut a variety of shapes consistently, in
various thicknesses, and without the use of any dies.
b. Water-jet cutting is a cutting process that is effective on many metallic as
well as nonmetallic materials.
c. Cutting with a band saw; this method is a chip-removal process.
d. Friction sawing involves a disk or blade which rubs against the sheet or
plate at high surface speeds.
e. Flame cutting is another common method, particularly for thick plates; it
is used widely in ship building and on heavy structural components.
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EXERCISE 3.1
Describe the cutting process that takes place when a pair of scissors cuts through
aluminum foil.
EXERCISE 3.2
Explain why cupping tests may not predict well the formability of sheet metals in actual
forming processes.
3.3 SHEET METAL BENDING
1. Bending is one of the most common industrial forming operations. Common
bending operations shown in Figure 3.14 and Figure 3.15.
Figure 3.14 Common bending operations.
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Figure 3.15 Examples of various bending operations.
3.3.1 CONSIDERATIONS IN BENDING
a. Springback
1. Because all materials have a finite modulus of elasticity, plastic deformation
always is followed by some elastic recovery when the load is removed.
2. In bending, this recovery is called springback, which can be observed easily
by bending and then releasing a piece of sheet metal or wire.
3. Fig. 3.16 shows the springback in bending. The part tends to recover
elastically after bending, and its bend radius becomes larger. Under certain
conditions, it is possible for the final bend angle to be smaller than the original
angle (negative springback).
4. Fig. 3.17 shows the methods of reducing or eliminating springback in bending
operations.
5. Springback in forming operations usually is compensated for by overbending
the part.
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Figure 3.16 Springback in bending.
Figure 3.17 Methods of reducing or eliminating springback in bending operations.
b. Bending radius
1. The radius at which a crack first appears at the outer fibers of a sheet being bent is
referred to as the minimum bend radius.
2. Another significant factor in radius forming is the amount, shape, and hardness of
inclusions present in the sheet metal and the amount of cold working that the edges
undergo during shearing.
3. Note that the bend radius is measured to the inner surface of the bent part.
4. Fig 3.18 (a) and (b) shows the effect of elongated inclusions (stringers) on cracking
as a function of the direction of bending with respect to the original rolling direction of
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the sheet; (c) Cracks on the outer surface of an aluminum strip bent to an angle of
90°.
Figure 3.18 (a) Cracks (b) No cracks (c) Cracks at an angle of 90
o
c. Bending allowance
1. The bend allowance is the length of the neutral axis in the bend and is used
to determine the length of the blank for a part to be bent.
d. Bending force
1. The bending force for sheets and plates can be estimated by assuming that
the process is one of the simple bending of a rectangular beam.
e. Anisotrophy
1. Anisotropy of the sheet is another important factor in bendability.
2. Cold rolling results in anisotropy by preferred orientation or by mechanical
fibering due to the alignment of any impurities, inclusions, and voids that may
be present.
3.3.2 PRESS BRAKE FORMING
1. Sheets or narrow strips that are 7 m or even longer usually are bent in a
press brake.
2. Fig 3.19 (a) through (e) shows the schematic illustrations of various bending
operations in a press brake, and (f) shows the schematic illustration of a
press brake.
3. The process can be automated easily for low-cost, high-production runs.
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4. Die materials for press brakes may range from hardwood (for low-strength
materials and small-production runs) to carbides for strong and abrasive
sheet materials and also are chosen to improve die life.
Figure 3.19 (a) through (e) shows the schematic illustrations of various bending
operations in a press brake, and (f) shows the schematic illustration of a press brake.
a. Beading
1. In beading, the periphery of the sheet metal is bent into the cavity of a die.
2. The bead imparts stiffness to the part by increasing the moment of inertia
of that section.
3. Also, beads improve the appearance of the part and eliminate exposed
sharp edges that can be hazardous.
4. Fig 3.20 (a) shows bead forming with a single die (b) though (d) bead
forming with two dies in a press brake.
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Figure 3.20 (a) Bead forming with a single die. (b) Though (d) Bead forming
b. Flanging
1. This is a process of bending the edges of sheet metals, usually to 90°.
2. In shrink flanging, the flange is subjected to compressive hoop stresses
which, if excessive, can cause the flange periphery to wrinkle.
3. The wrinkling tendency increases with decreasing radius of curvature of
the flange.
4. In stretch flanging, the flange periphery is subjected to tensile stresses
that, if excessive, can lead to cracking along the periphery.
5. Fig 3.21 shows various flanging operations. (a) Flanges on flat sheet. (b)
Dimpling. (c) The piercing of sheet metal to form a flange. In this
operation, a hole does not have to be pre-punched before the punch
descends. Note, however, the rough edges along the circumference of the
flange. (d) The flanging of a tube. Note the thinning of the edges of the
flange.
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Figure 3.21 (a) Flanges on flat sheet (b) Dimpling (c) Piercing (d) Flanging
3.3.3 TUBE BENDING AND FORMING
1. Bending and forming tubes and other hollow sections requires special
tooling because of the tendency for buckling and folding, as one notes
when trying to bend a piece of copper tubing or even a plastic soda straw.
2. Forming tubes and tubular shapes such as exhaust pipes, fuel filler tubes,
and exhaust manifolds also can be done using internal fluid pressure
(replacing the polyurethane plug) with the ends of the tubes sealed by
mechanical means
3. Fig 3.22 shows the methods of bending tubes. Internal mandrels or filling
of tubes with particulate materials such as sand are often necessary to
prevent collapse of the tubes during bending.
4. Fig 3.23 (a) shows the the bulging of a tubular part with a flexible plug.
Water pitchers can be made by this method (b) Production of fittings for
plumbing by expanding tubular blanks under internal pressure.
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