Machining operations

Machining
A material removal process in which a sharp cutting tool is used to
mechanically cut away material so that the desired part geometry
remains.
Most common application: to shape metal parts
Machining is the most versatile and accurate of all manufacturing
processes in its capability to produce a diversity of part geometries
and geometric features.
Casting can also produce a variety of shapes, but it lacks the
precision and accuracy of machining.
Different machining operations can be performed on different
machines with some of the operation being performed in more than
one machine.
 Machining operations
Turning operation
Turning utilizes a single cutting tool to create a surface of revolution.
The cylindrical workpiece is rotated around its axis while a cutting
tool is fed parallel to the axis of rotation.
As the cutting tool is engaged into the workpiece, a new surface of
revolution is generated by removing a layer of material whose
thickness is equal to the depth of the tool engagement.
A typical machine tool that generates the necessary motions for
carrying out this operation is an engine lathe.
The machine tool provides a primary motion to the workpiece in
revolutions per minute and a secondary motion to the cutting tool in
millimeters per revolution?.
 Machining operations
The combined motion that generates the surface is the vector
addition of these two motions.
For most practical applications, the feed motion is much
smaller than the primary motion and the cutting speed is
determined by the primary motion alone.
The cutting geometry in turning operation is presented in the
figure below;
 Machining operations
In turning operation one needs to consider some essential
relationship before start machining for the sake of setting
different parameters.
These relationships apply to single point tools with small
corner radius or when the depth of cut is very large as compared
to the corner radius.
Cutting speed
The cutting speed is determined by the rotational speed of the
spindle, N, given in rev/min, and the workpiece initial and final
diameters, Di and Do, respectively
 Machining operations
Feed speed
The average feed motion advances the tool per revolution
along a specified direction.
The feed, f , is given in mm/rev and the feed speed, vf, is
related to the feed by;

 Radial depth of cut

The radial depth of cut describes the thickness of material
removed from the workpiece and is given by;
 Machining operations
This material is removed in the form of a chip (swarf) which
flows upward on the tool rake face.
The uncut chip thickness is measured normal to the cutting
edge and is given by:
 Machining operations
Material removal rate
The material removal rate, Zw, is given as the product of
cutting speed and uncut chip area;

 Cutting time
The time required for turning a length, lw, in the feed direction
is given by;
 Machining operations
Specific cutting power
The specific cutting power, also referred to as specific cutting
pressure, is the net power required to remove a unit volume of the
material in a unit time.
It is related to the cutting force in the direction of cutting
speed, Fc, and to the material removal rate by;
 Machining operations
Then the power required to remove material is given by the
product of material removal rate and specific cutting power for
the workpiece material, us;
 Machining operations
Example 1
A cylindrical stainless steel rod with length L=150mm,
diameter D0 = 12mm is being reduced in diameter to Df
=11mm by turning on a lathe.
The spindle rotates at 400 rpm, and the tool is travelling at
an axial speed of υ=200mm/min.
Calculate
a.The cutting speed (maximum and minimum)
b.Material removal rate (MRR)
c.The cutting time
d.The power required if the unit power is estimated to 4w.s/
cubic (mm).
 Machining operations
Example 2
The part shown below will be turned in two machining steps.
In the first step a length of (50 + 50) =100mm will be reduced
from Ø100mm to Ø80mm and in the second step a length of 50
mm will be reduced from Ø80 mm to Ø60mm.
Calculate the required total machining time T with the
following cutting conditions:
Cutting speed (V) =80 m/min
Feed is f=0.8 mm/rev,
Depth of cut = 3 mm per pass
Machining operations
 Machining operations
Milling operation
In milling, material is removed from the workpiece by a
rotating cutter head that may have more that one active cutting
edge.
The types of milling operations that are most common in
machining are peripheral or profiling milling and end milling as
shown below;
 Machining operations
Peripheral milling uses the cutting edges on the periphery of
the tool.
The machined surface is parallel to the axis of rotation of the
cutter and the engagement into the workpiece is in the radial
direction of the cutter.
Peripheral milling is more appropriately called edge trimming
because the tool diameter is usually small and the axial
engagement encompasses the entire thickness of the workpiece.
End milling is similar to peripheral milling, except that the axial
engagement may be less than the thickness of the part and a slot
is obtained.
The machine tool most commonly used is a vertical milling
machine.
 Machining operations
The machine tool provides the primary motion to the spindle (to
which the cutter is held) and feed motions to the machine table (to
which the workpiece is held).
CNC routers capable of providing higher spindle speeds and feed
rates, more flexibility, and larger workspace than a typical milling
machine are commonly used in high production facilities.
End milling and trimming operations are further classified into up
(or conventional) milling and down (or climb) milling, depending on
how the cutting edge approaches the workpiece.
In up milling, the forces gradually increase from zero at beginning of
tool engagement to a maximum when the cutting edge is about to
leave the workpiece.
Forces drop to zero again when the cutting edge leaves the
workpiece.
 Machining operations
Figure below shows a schematic of the cutting geometry for
one cutting edge in up milling.
The tool path is torchoidal and is generated from the
combination of rotational (spindle) and translational
(feed)motions.
 Machining operations
The basic expressions describing this motion are given here.
The cutting speed is given as a function of the spindle speed,
N, and tool diameter, D, by the relationship;

The feed speed, vf, and the feed per revolution, f , are related
by;
 Machining operations
The feed per tooth, af, which defines the translation of the
workpiece between the engagement of successive cutting edges,
is expressed as a function of the feed speed(vf), the spindle
speed( N), and the number of cutting edges on the cutterhead, T.

The total engagement angle, φx, is given as a function of the

tool diameter and the radial depth of cut, ae;
 Machining operations
The instantaneous uncut chip thickness, ac, measured normal
to the cutting path varies continuously with engagement angle, φi,
and is maximum at the exit angle φx.
The lead angle is 90◦ for peripheral and end milling
The maximum uncut chip thickness, amax, is calculated from
the triangle ABC as;

For small depths of cut as compared to tool diameter, the

maximum chip thickness can also approximated by;
 Machining operations
The instantaneous uncut chip thickness, ac, at engagement
angle φi, and the average chip thickness, aavg, are given by:

The length of chip being cut is approximated by

 Machining operations
The material removal rate, Zw, in end milling is given by the
products of the radial depth of cut (ae), axial depth of cut (ap),
and feed speed.
In edge trimming and profiling, ap represents the thickness of
the workpiece.

The time required for milling a length (Lw), is the time taken by
the tool to traverse the length of the workpiece and any additional
distance required to completely clear the tool off the workpiece.
 Machining operations
The machining time is given by:

where Le is the distance required to clear the workpiece by the

tool and is given by;
 Machining operations
Example 3
An edge trimming operation of particleboard uses a 19.0mm
cutterhead with one major cutting edge in a down-milling configuration.
The spindle speed is 5,000 rpm and the feed rate is 1.27 m/min.
The radial depth of cut is 1 mm, the length of workpiece is 500mm and
its thickness is 19mm.
The specific cutting power for particleboard is 65N/mm2.
Determine:
Total engagement angle
Maximum chip thickness
Material removal rate
Time to finish one edge
 Machining operations
Drilling operation
Drilling is the most common material removal operation in metals
and composites machining. It is used for making holes required for
assembly.
Drilling is done on conventional upright drilling machines, milling
machines, and various specialized machines.
In drilling on a vertical drill press, the spindle provides the primary
rotational motion to the drill bit and the feed into the workpiece is
provided through the spindle axis.
The most common drill bit is a two flute twist drill depicted in Figure
below.
A two flute twist drill has two major cutting edges forming the drill
point angle.
 Machining operations

Each one of the major cutting edge acts like a single point
cutting tool as shown in figure b. The lead angle for the cutting
edge is half of the drill point angle.
 Machining operations
The flute provides a way for the chip to clear the cutting zone and
for coolant to be supplied to the cutting tip.
For a drill of diameter D, which is being rotated by N revolutions per
minute, the cutting speed is given by;

The drill bit is feed into the workpiece with a feed per revolution, f .
The feed speed, vf, is related to the feed per revolution by;

The feed per tooth is related to the feed per revolution and the
number of flutes, T, by;
 Machining operations
The width of the chip is related to the tool diameter and half
the drill point angle, κ (also known as the lead angle) by;

The uncut chip thickness is given by;

The material removal rate, Zw, is given by;

 Machining operations
The time required to drill a through hole in a workpiece of
thickness, Lh, is calculated as the time required for the drill point
to traverse the thickness and clear the drill cone.
An additional distance, Le, is required to clear the drill cone,
which is given by;

The time to drill through the workpiece is given by;

 Machining operations
Example 4
A two-flute twist drill is used to drill a hole in a particleboard
that is 19mm thick. The drill diameter is 5mm, the drill point
angle is 90◦, and the spindle speed is 500 rpm and the feed rate
is 0.5mm/s. Determine:
a.The cutting speed
b.Maximum chip thickness
c.Material removal rate
d.The time required for drilling one hole
 Furnaces
A furnace is ‘an enclosed structure for intense heating by fire,
especially of metals.
It is an equipment to melt metals for casting or heat materials
for change of shape (rolling, forging etc.) or change of properties
(heat treatment).
Different furnaces are employed for melting and re-melting
ferrous and nonferrous materials.
A furnace contains a high temperature zone or region
surrounded by a refractory wall structure which withstands high
temperatures and being insulating minimizes heat losses to
the surroundings.
Since flue gases from the fuel come in direct contact
with the materials, the type of fuel chosen is important.
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For example, some materials will not tolerate sulphur in the fuel.
Solid fuels generate particulate matter, which will interfere the
materials placed inside the furnace. For this reason:
Most furnaces use liquid fuel, gaseous fuel or electricity as
energy input.
Induction and arc furnaces use electricity to melt steel and
cast iron.
Melting furnaces for nonferrous materials use fuel oil.
Oil-fired furnaces mostly use furnace oil, especially for
reheating and heat treatment of materials.
Light diesel oil (LDO) is used in furnaces where sulphur is
undesirable.
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Selection of furnace
Selecting furnace to install requires to consider some preliminary factors
to insure its suitability when in use. One need to consider the following
factors;
Considerations of initial cost and cost of its operation.
Relative average cost of repair and maintenance.
Availability and relative cost of various fuels in the particular locality.
Melting efficiency, in particular speed of melting.
Composition and melting temperature of the metal.
Characteristics of an Efficient Furnace
Furnace should be designed so that in a given time, as much of the
charge as possible is melted with least possible fuel and labor used.
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To achieve this end, the following parameters can be
considered.
Determination of the quantity of heat to be imparted to the
material or charge.
Liberation of sufficient heat within the furnace to heat the
stock and overcome all heat losses.
Transfer of available part of that heat from the furnace gases
to the surface of the heating stock.
Equalization of the temperature within the stock.
Reduction of heat losses from the furnace to the minimum
possible extent.
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Major parts of furnace
The following figure points out some major parts of furnace
followed by their description.
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Steel shell
Steel shell constructed for supporting and casing.
Refractory chamber
Refractory chamber constructed of insulating materials to retain
heat at high operating temperatures.
Hearth
Hearth to support or carry the steel, which consists of refractory
materials supported by a steel structure, part of which is water cooled.
Burners
Burners that use liquid or gaseous fuels to raise and maintain the
temperature in the chamber. Coal or electricity can be used in
reheating furnaces.
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Chimney
Chimney to remove combustion exhaust gases from the chamber.
Charging and discharging doors
Charging and discharging doors through which the chamber is loaded
and unloaded.
Loading and unloading equipment
Loading and unloading equipment include roller tables, conveyors,
charging machines and furnace pushers.
Heat Transfer in Furnaces
In simple terms, heat is transferred to the stock by:
Radiation
It takes place from the flame, hot combustion products and the furnace
walls and roof.
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Convection
Convection is mainly due to the movement of hot gases over
the stock surface.
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At high temperatures, the dominant mode of heat transfer is
wall radiation.
Heat transfer by gas radiation is dependent on the gas
composition (mainly the carbon dioxide and water vapors
concentrations), the temperature and the geometry of the furnace.
Classification of furnaces
There are different criteria applied to the classification of
industrial furnaces. Most frequently they are divided by the
following characteristics:
According to the technological purpose:
Melting
Intended for melting materials (blast furnaces, cupola
furnaces, glass melting bath),
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Heating
Intended for heating the material before rolling, forging,
pressing, etc. (forge furnaces, rolling mill furnaces),
Heat treatment
Intended for heat treatment such as hardening, annealing,
tempering,
Burning
Intended for firing products(kilns for firing refractory
ceramic materials, lime kilns),
Drying
To remove moisture from the material(drying of moulds
and cores in foundries),
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Distillation
A new product is formed from the charge by distillation(coke
oven batteries).
According to the heat source:
Flame
Heat energy is obtained by combustion of solid, liquid or gaseous
fuels,
Electrical
Thermal energy is generated from electricity (arc furnaces,
resistance furnaces, plasma furnaces, induction furnaces, electron
furnaces).
No external source
They use internal chemical energy of the processed metal and its
additives.
 Furnaces
According to the shape of working space (chamber)
Shaft
The entire volume is filled with charge, the counter-current
principle is mostly applied here,
Tank
Only part of the working chamber is filled with charge,
Continuous
Either horizontal (pusher-type kilns) or vertical (tower
furnaces), the charge moves from the charging window to the
withdrawal window,
Rotary-hearth furnace
Charge moves together with the hearth, which has a
circular ring shape,
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According to the way of using the waste heat from flue
gases
Recuperative
The use of a recuperator for preheating the combustion air or
gas,
Regenerative
The use of a regenerator,
Note
The above classification of furnaces is, of course, not
complete.
Furnaces can also be divided according to additional criteria,
such as by the dependence of power on time (steady and non-
steady), by mode of charge transport (walking-beam, roller,
pneumatic, etc.)
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Melting furnace
These furnaces are intended for melting materials especially
for producing pig iron from oxides. These furnaces are discussed
here under;
Blast furnace
Blast furnace is the largest melting unit in which pig iron is
acquired from iron oxides through the reduction processes.
An iron-bearing charge (iron ore, sinter, pellets, etc.) is charged
into a blast furnace, plus slag-forming additives and coke.
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Oxidising and reducing agents
An oxidising agent is substance which oxidises something else. In
the above example, the iron(III) oxide is the oxidising agent.
A reducing agent reduces something else. In the equation, the
carbon monoxide is the reducing agent.
Oxidising agents give oxygen to another substance.
Reducing agents remove oxygen from another substance.
To burn the fuel, the preheated air (blast) is blown into the furnace
from Cowper stoves, sometimes enriched with oxygen.
The main product of the process is pig iron; by-products are blast
furnace slag and blast furnace gas.
Tuyeres bring to blast furnaces also the substitute fuels that can be
gaseous, liquid or powder.
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 Furnaces
The working space (chamber) of the furnace consists of a high
shaft of circular cross-section, which is lined with refractory
material.
The furnace stands on a concrete base; the bottom part is
disposed below ground. The upper cylindrical part is the throat
and downwardly, the furnace gets wider and this part is called the
shaft.
The widest part of the furnace is the belly and the conical
tapered part is the saddle.
The lower part is called the hearth, may be of cylindrical or
slightly conical shape, and it is closed by the bottom from below.
Tuyeres blowing the heated air are situated in the upper part of
the hearth.
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Pig iron and slag are discharged through the tap hole, which is
located 0.5 - 2m above the hearth. The number of tap holes depends
on the richness of the charge.
The furnace lining and shell must be protected from the effects of
high temperatures by cooling. Coolers are placed between the steel
shell and furnace lining.
Between the bottom and concrete base, cast iron slabs are laid
with embedded steel tubes that carry the cooling air.
Blast furnaces are built with a thin-walled or thick-walled lining.
Material of the lining is chosen according to the main causes of wear
of the brickwork at a given point of the furnace.
Fireclay materials are used for the upper and middle part of the
shaft, while fireclays of the finest grade, high alumina fireclay and
mullite materials are used for the lower part, belly and saddle.
 Furnaces
Cupola furnaces
Cupola furnace is a shaft furnace for the production of cast
iron from pig iron.
It uses coke as a fuel and fluorite and limestone are added for
the formation of an easily meltable slag.
The bottom of the cupola furnace has a circular opening which
is closed by a door.
At the bottom, there is the tap hole for cast iron; the tap hole
for slag is placed above.
In terms of the temperature progression in the charge, cupola
furnaces can be divided to preheating, melting, superheating and
hearth zones.
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 Furnaces
In the preheating zone, all materials are in the solid state. Gradually
they sink down and warm up.
At a temperature of 800°C, limestone decomposes and coke loses
moisture and volatile matter. In the melting zone, slag is formed by
melting the metal-bearing charge.
Coke is still in the solid state. Drops of molten metal and slag flow
down, where they pass through the superheating zone and their
temperature rises.
They are mainly heated by the hot coke. In the hearth zone, coke is
not burning any more. The temperature of the filling layer is
downwardly declining.
According to the nature of the slag, furnaces are lined with:
acidic linings – semi-hard fireclay, ramming materials with
ceramic binder,
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basic linings – magnesite,
neutral materials based on Al2O3, Zr2O3, SiC – in water-
cooled cupola furnaces.
Resistance furnace
An electric furnace in which the heat is developed by the
passage of current through a suitable internal resistance that may
be the charge itself, a resistor embedded in the charge, or a
resistor surrounding the charge.
It is an electric furnace in which heat is generated by
conductors that offer resistance to the passage of a current
through them.
Resistance furnaces are widely used in heat treatment, for
heating prior to pressure shaping, and to dry or melt materials.
 Furnaces
Resistance furnaces are used extensively because of their
numerous advantages. Any temperature up to 3000°C can be
obtained in the furnace chamber.
Articles can be uniformly heated either by appropriately locating
the heating elements along the walls of the furnace chamber or by
means of forced circulation of the furnace atmosphere.
Resistance furnaces are readily mechanized and automated,
thus alleviating the work of personnel and facilitating the inclusion
of such furnaces in automatic transfer lines.
A resistance furnace may be well sealed, in which case the
heating is carried out in a vacuum, or it may contain either a
gaseous medium that prevents oxidation or a special atmosphere
for chemical case hardening, for example, for carburization or
Nitriding.
 Furnaces
Most resistance furnaces are of the indirect type. In indirect-
heat resistance furnaces, electric energy is converted into heat
when a current flows through the heating elements.
The heat is transmitted to the articles to be heated by radiation,
convection, or conduction.
Such a furnace consists of a working chamber formed by a
lining composed of a layer of firebrick that supports both the
articles to be heated and the heating elements and that is
insulated from a metal casing by a layer of heat insulation.
The parts and mechanisms that operate in the chamber, as
well as the heating elements, are made of heat-resistant steels,
refractory steels, or other refractory materials.
Furnaces
 Furnaces
Continuous furnaces are used to heat large lots of identical
parts. In such furnaces, articles move continuously from one end
to the other.
As compared with other types of furnaces, the output of
continuous furnaces is higher, the heating of articles is more
uniform, and the power consumption is lower.
In both batch-type and continuous resistance furnaces with a
working temperature of up to 700°C, forced circulation of the
furnace gases is widely used.
The gases are circulated by fans or blowers that either are
installed in the furnace or are located together with the heating
elements outside the furnace in electric heaters.
 Furnaces
In direct-heat resistance furnaces, an article—for example, a
rod or a tube—is heated directly by a current that passes through
it.
Direct heating makes it possible to concentrate a large amount
of power in the article and provides for very rapid heating, on a
scale of seconds or fractions of a minute.
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 Electric induction furnace
The electric induction furnace is a type of melting furnace that
uses electric currents to melt metal.
Induction furnaces are ideal for melting and alloying a wide
variety of metals with minimum melt losses, however, little
refining of the metal is possible.
Principle of induction furnace
The principle of induction furnace is the induction heating.
Induction heating is a form of non-contact heating for
conductive materials.
The principle of induction heating is mainly based on two well-
known physical phenomena:
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Electromagnetic induction
The energy transfer to the object to be heated occurs by
means of electromagnetic induction.
Any electrically conductive material placed in a variable
magnetic field is the site of induced electric currents, called eddy
currents, which will eventually lead to joule heating.
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 Joule heating
Joule heating, also known as ohmic heating and resistive
heating, is the process by which the passage of an electric
current through a conductor releases heat.
The heat produced is proportional to the square of the current
multiplied by the electrical resistance of the wire.
 Furnaces
Induction heating relies on the unique characteristics of radio
frequency (RF) energy. (That portion of the electromagnetic spectrum
below infrared and microwave energy).
Since heat is transferred to the product via electromagnetic waves,
the part never comes into direct contact with any flame.
The inductor itself does not get hot and there is no product
contamination. Induction heating is a rapid, clean, and non-polluting
heating.
The induction coil is cool to the touch; the heat that builds up in the
coil is constantly cooled with circulating water.
Features of induction furnace
a) An electric induction furnace requires an electric coil to produce
the charge. This heating coil is eventually replaced.
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b) The crucible in which the metal is placed is made of stronger
materials that can resist the required heat, and the electric coil itself
cooled by a water system so that it doesnt overheat or melt.
c) The induction furnace can range in size, from a small furnace
about a kilogram in weight to a much larger furnaces made to mass
produce clean metal for many different applications.
d) The one major drawback to induction furnace usage in a foundry
is the lack of refining capacity.
Construction of induction furnace
There are many different designs for the electric induction furnace,
but they all center around a basic idea.
The electrical coil is placed around or inside of the crucible, which
holds the metal to be melted. Often this crucible is divided into two
different parts.
 Furnaces
The lower section holds the melt in its purest form, the metal
as the manufacturers desire it, while the upper section is used to
remove the slag, or the contaminants that rise to the surface of
the melt.
Crucibles may also be equipped with strong lids to lessen how
much air has access to the melting metal until it is poured out,
making a purer melt.
 Welding operations
Welding is a metal-joining process in which
coalescence is obtained by heat and pressure.
It may also be defined as a metallurgical bond
accomplished by the attracting forces between atoms.
Before these atoms can be bonded together, absorbed
vapors and oxides on contacting surfaces must be
overcome.
The number one enemy to welding is oxidation, and,
consequently, many welding processes are performed in
a controlled environment or shielded by an inert
atmosphere.
 Welding operations
If force is applied between two smooth metal surfaces
to be joined, some crystals will crush through the
surfaces and be in contact.
As more and more pressure is applied, these areas
spread out and other contacts are made.
The brittle oxide layer is broken and fragmented as the
metal is deformed plastically.
Coalescence is obtained when the boundaries
between the two surfaces are mainly crystalline planes.
This process, is known as cold welding.
 Welding operations
The breaking through or elimination of surface oxide
layers happens when a weld is made.
If temperature is added to pressure the welding of two
surfaces will be facilitated, and coalescence is obtained
in the same manner as cold-pressure welding.
As temperature is increased the ductility of the base
metal is increased and atomic diffusion progresses more
rapidly.
Nonmetallic materials on interfacial surfaces are
softened, permitting them to be removed or broken up by
plastic flow of the base materials.
 Welding operations
Hot-pressure welds are more efficient but not
necessarily stronger if the atom-to-atom bond is the
same.
Welding provides a permanent joint but it normally
affects the metallurgy of the components.
It is therefore usually accompanied by post weld heat
treatment for most of the critical components.
The welding is widely used as a fabrication and
repairing process in industries.
Most of the metals and alloys can be welded by one
type of welding process or the other. However, some are
easier to weld than others.
 Welding operations
To compare this ease in welding term ‘weldability’ is
often used.
The weldability may be defined as property of a metal
which indicates the ease with which it can be welded with
other similar or dissimilar metals.
Weldability of a material depends upon various factors
like the metallurgical changes that occur due to;
 welding,
 changes in hardness in and around the weld,
 gas evolution and absorption,
 extent of oxidation, and
 the effect on cracking tendency of the joint.
 Welding operations
Welding processes
Many welding processes have been developed. They
differ widely in the manner that heat is applied and in the
equipment used.
The principal processes are explained as follows;
 Arc welding
These processes use a welding power supply to
create and maintain an electric arc between an electrode
and the base material to melt metals at the welding point.
 Welding operations
They can use either direct (DC) or alternating (AC)
current, and consumable or non-consumable electrodes.
The welding region is sometimes protected by some
type of inert or semi-inert gas, known as a shielding gas,
and filler material is sometimes used as well.
 Welding operations
In arc welding, the voltage is directly related to the length
of the arc, and the current is related to the amount of heat
input.
Constant current power supplies are most often used for
manual welding processes such as gas tungsten arc welding
and shielded metal arc welding, because they maintain a
relatively constant current even as the voltage varies.
This is important because in manual welding, it can be
difficult to hold the electrode perfectly steady, and as a result,
the arc length and thus voltage tend to fluctuate.
Constant voltage power supplies hold the voltage constant
and vary the current, and as a result, are most often used for
automated welding processes.
 Welding operations
 Shielded metal arc welding (SMAW)
One of the most common types of arc welding is
shielded metal arc welding (SMAW), which is also known
as manual metal arc welding (MMA) or stick welding.
Electric current is used to strike an arc between the
base material and consumable electrode rod, which is
made of steel.
 Welding operations
The rod is also covered with a flux that protects the
weld area from oxidation and contamination by
producing CO2 gas during the welding process.
The electrode core itself acts as filler material, making
a separate filler unnecessary.
The process is versatile and can be performed with
relatively inexpensive equipment, making it well suited to
shop jobs and field work.
An operator can become reasonably proficient with a
modest amount of training and can achieve mastery with
experience.
 Welding operations
Weld duration are rather slow, since the consumable
electrodes must be frequently replaced and because slag, the
residue from the flux, must be chipped away after welding.
Furthermore, the process is generally limited to welding
ferrous materials, though speciality electrodes have made
possible the welding of cast iron, nickel, aluminium, copper,
and other metals.
 Gas metal arc welding (GMAW)
Gas metal arc welding (GMAW), also known as metal inert
gas or MIG welding, is a semi-automatic or automatic
process that uses a continuous wire feed as an electrode and
an inert or semi-inert gas mixture to protect the weld from
contamination.
 Welding operations
As with SMAW, reasonable operator proficiency can be
achieved with modest training.
Since the electrode is continuous, welding speeds are
greater for GMAW than for SMAW.
 Welding operations
Also, the smaller arc size compared to the SMAW process
makes it easier to make out-of-position welds (e.g., overhead
joints, as would be welded underneath a structure).
The equipment required to perform the GMAW process is
more complex and expensive than that required for SMAW,
and requires a more complex setup procedure.
Therefore, GMAW is less portable and versatile, and due to
the use of a separate shielding gas, is not particularly suitable
for outdoor work.
However, owing to the higher average rate at which welds
can be completed, GMAW is well suited to production
welding.
 Welding operations
The process can be applied to a wide variety of metals,
both ferrous and non-ferrous.
A related process, flux-cored arc welding (FCAW),
uses similar equipment but uses wire consisting of a
steel electrode surrounding a powder fill material.
 Welding operations
 Gas tungsten arc welding
Gas tungsten arc welding (GTAW), is a manual welding
process that uses a non-consumable tungsten electrode,
an inert or semi-inert gas mixture, and a separate filler
material.
 Welding operations
Especially useful for welding thin materials, this method
is characterized by a stable arc and high quality welds.
The method requires significant operator skill and can
only be accomplished at relatively low speeds.
GTAW can be used on nearly all weldable metals, though
it is most often applied to stainless steel and light metals.
It is often used when quality welds are extremely
important, such as in bicycle, aircraft and naval
applications.
A related process, plasma arc welding, also uses a
tungsten electrode but uses plasma gas to make the arc.
 Welding operations
The arc is more concentrated than the GTAW arc,
making transverse control more critical and thus
generally restricting the technique to a mechanized
process.
 Welding operations
Because of its stable current, the method can be used
on a wider range of material thicknesses than the GTAW
process, and furthermore, it is much faster.
It can be applied to all of the same materials as GTAW
except magnesium, and automated welding of stainless
steel is one important application of the process.
Submerged arc welding (SAW)
Submerged arc welding (SAW) is a high-productivity
welding method in which the arc is struck beneath a
covering layer of flux.
This increases arc quality, since contaminants in the
atmosphere are blocked by the flux.
 Welding operations
The slag that forms on the weld generally comes off
by itself, and combined with the use of a continuous wire
feed, the weld deposition rate is high.
 Welding operations
Working conditions are much improved over other arc
welding processes, since the flux hides the arc and almost no
smoke is produced.
The process is commonly used in industry, especially for
large products and in the manufacture of welded pressure
vessels.
Other arc welding processes include;
 atomic hydrogen welding,
 carbon arc welding,
 electroslag welding,
 electrogas welding, and
 stud arc welding.
 Welding operations
 Gas welding
Gas welding includes all the processes in which
gases are used in combination to obtain a hot flame.
Those commonly used are acetylene, natural gas,
and hydrogen in combination with oxygen.
Oxyhydrogen welding was the first gas process to
be commercially developed.
The maximum temperature developed by this
process is 3600 F (1980 C).
Hydrogen is produced by either the electrolysis of
water or passing steam over coke.
 Welding operations
The most used combination is the oxyacetylene
process, which has a flame temperature of 6300 F (3500
C).
 Welding operations
An oxyacetylene weld is produced by heating with a
flame obtained from the combustion of oxygen and
acetylene with or without the use of a filler metal.
Most often the joint is heated to a state of fusion and
as a rule no pressure is used. Oxygen is produced by
both electrolysis and liquification of air.
Electrolysis separates water into hydrogen and oxygen
by passing an electric current through it.
Most commercial oxygen is made by liquefying air and
separating the oxygen from the nitrogen. It is stored in
steel cylinders at a pressure of 2000 psi (14 MPa).
 Welding operations
Acetylene gas (C2H2) is obtained by dropping lumps of
calcium carbide in water.
The gas bubbles through the water, and any precipitate is
slaked lime.
The reaction that takes place in an acetylene generator is
CaC2 + 2H2O = Ca(OH)2 + C2H2.
Principle
Gas welding is a most important type of welding process. It
is done by burning of fuel gases with the help of oxygen which
forms a concentrated flame of high temperature.
This flame directly strikes the weld area and melts the weld
surface and filler material.
 Welding operations
The melted part of welding plates diffused in one
another and create a weld joint after cooling.
This welding method can be used to join most of
common metals used in daily life.
Equipment:
 Welding operations
Welding Torch:
Welding torches are most important part of gas welding.
Both the fuel gas and oxygen at suitable pressure fed
through hoses to the welding torch.
There are valves for each gas which control the flow of
gases inside the torch. Both gases mixed there and form a
flammable mixture.
These gases ignite to burn at the nozzle. The fire flame
flow through nozzle and strikes at welding plates.
The nozzle thickness depends on the size of the welding
plates and material to be welded.
 Welding operations
Oxygen Cylinder:
For proper burning of fuel, appropriate amount of
oxygen required. This oxygen supplied by a oxygen
cylinder. A black line is used to indicate oxygen cylinder.
Fuel Gas Cylinder:
Gas cylinder is filled either by oxy acetylene gas,
hydrogen gas, natural gas or other flammable gas. The
fuel gas selection depends on the welding material.
Mostly oxy acetylene gas is used for all general
purpose welding. Normally these cylinders have Maroon
line to indicate it.
 Welding operations
Pressure regulator:
Both oxygen and fuel gases are filled in the cylinder at
high pressure.
These gases cannot be used at this high pressure for
welding work, so a pressure regulator is used between
flow.
It supplies oxygen at pressure about 70 – 130 KN / M2
and gas at 7 – 103 KN / M2 to the welding torch.
Goggles and Gloves:
These are used for safety purpose of welder. It
protects eyes and hand from radiation and flame of fire.
 Welding operations
Application:
 It is used to join thin metal plates.
 It can be used to join both ferrous and non-ferrous
metals.
 Gas welding mostly used in fabrication of sheet metal.
 It is widely used in automobile and aircraft industries
Advantages:
 It is easy to operate and does not required high skill
operator.
 Equipment cost is low compare to other welding
processes like MIG, TIG etc.
 Welding operations
 It can be used at site.
 Equipment’s are more portable than other type of
welding.
 It can also be used as gas cutting.
Disadvantages:
 It provides low surface finish. This process needs a
finishing operation after welding.
 Gas welding have large heat affected zone which can
cause change in mechanical properties of parent
material.
 Welding operations
 Higher safety issue due to naked flame of high
temperature.
 It is Suitable only for soft and thin sheets.
 Slow metal joining rate.
 No shielding area which causes more welding defects
Welding defects includes;
 Porosity
 Spatter
 Slag inclusions
 Incomplete fusion
 Undercut
 Brazing
Brazing is the process of joining metal by heating
the base metal to a temperature above 800°F and
adding a nonferrous filler metal that melts below the
base metal’s temperature.
Brazing offers important advantages over some
other metal-joining processes such as oxy-gas
welding.
It does not affect the heat treatment of the original
metal as much as welding, does not warp the metal
as much, and allows you to join dissimilar metals.
 Brazing
Equipment
Brazing requires three basic items:
 a heat source,
 filler metal, and
 flux.
Heat source
The source of heat depends on the type and amount of
brazing required.
If you were doing production work and the pieces were small
enough you could put them into a furnace and braze them all at
once.
 Brazing
Alternatively, you could mount individual torches in
groups for assembly line work, to braze an individual item.
Filler Metals
Brazing filler metals are nonferrous metals or alloys with
a melting temperature below the base metal, but above
800°F.
They must have the ability to wet and bond with the base
metal, be stable, and not be excessively volatile.
The most commonly used filler metals for brazing are
the silver-based alloys available in rod, wire, powder, and
preformed form.
Brazing
 Brazing
Fluxes
Brazing requires flux to stop any oxides or similar
contaminants from forming during the process.
The flux also increases both the flow of the filler metal
and its ability to stick to the base metal.
Flux helps form a strong joint by bringing the filler metal
into immediate contact with the adjoining base metals and
permitting the filler to penetrate the pores of the metal.
Flux is available in powder, liquid, and paste form. You
can apply the powdered form of flux by dipping the heated
end of the brazing rod into the container, for flux to stick.
 Brazing
Alternatively, you can heat the base metal slightly
and sprinkle the powdered flux over the joint, allowing
the flux to partly melt and stick.
Sometimes you may find it desirable to mix the
powdered flux with distilled water to form a paste.
The most common type of flux for brazing is borax
or a mixture of borax with other chemicals.
Some commercial fluxes contain small amounts of
phosphorus and halogen salts of iodine, bromine,
fluorine, chlorine, or astatine.
 Brazing
Joint Design
Brazing has three basic joint designs: lap, butt, and
scarf.
 Lap Joints
The lap joint is one of the strongest and most
frequently used joint in brazing, especially in pipe work.
Its primary disadvantage is the increased thickness
of the final product.
For maximum strength, the overlap should be at
least three times the thickness of the metal.
 Brazing
A 0.001-inch to 0.003-inch clearance between joint
members provides the greatest strength with a silver-
based filler metal.
 Brazing
Butt Joints
The size of a butt joint is limited to the thinnest
section, so maximum joint strength is impossible.
However you can maximize the available butt joint
strength by maintaining a clearance of 0.001 to 0.003
of an inch in the finished braze.
 Brazing
The edges of the joint must be perfectly square to
maintain that uniform clearance between all parts of
the joint.
Butt joints are usually used where it is undesirable
to have double thickness.
 Scarf Joints
When double metal thickness is objectionable but
you still need more strength, the scarf joint is a good
choice.
A scarf joint provides an increased bond area
without increasing the thickness of the joint.
 Brazing
The amount of bond area depends on the angle the
scarf is cut; usually, an area two to three times the
butt joint area is desirable.
A 30° scarf angle gives a bond area twice that of a
90° butt joint, and a 19½° scarf angle increases the
bond area three times.
 Brazing
Brazing Procedures
The procedure for brazing is very similar to braze
welding and oxyacetylene welding.
You must clean the metal mechanically, chemically,
or with a combination of both to ensure;
 good bonding,
 fit the two pieces properly, and
 support them to prevent voids in the joint or
accidental movement during your brazing and
cooling operations.
 Brazing
 Surface Preparation
The metal surfaces must be clean for capillary
action to take place.
When necessary and practical, you can chemically
clean the surface by dipping it in acid then remove the
acid by washing the surface with warm water.
You can use steel wool, a file or abrasive paper for
mechanical cleaning, but do not use an emery wheel
or emery cloth; abrasive particles or oil might become
embedded in the metal.
 Brazing
 Work Support
If the joint moves during the brazing process, the
finished bond will be weak and subject to failure.
Therefore mount the work in position on firebricks or
other suitable means of support and if necessary clamp
it.
 Fluxing
Flux application varies depending on the form of flux
you are using and the type of metal you are brazing, but
the flux must be suitable for the job.
Refer to the previously described material on fluxes
and always refer to the manufacturer’s information.
 Brazing
 Brazing
The next step is to heat the parts to the correct
brazing temperature.
Use a neutral flame; it gives the best results under
normal conditions.
A reducing flame produces an exceptionally neat-
looking joint, but you sacrifice strength;.
An oxidizing flame produces a strong joint but you
get a rough-looking surface.
Watch the behavior of the flux as you heat it to
determine the temperature of the joint.
 Brazing
First, the flux dries out as the moisture (water) boils
off at 212°F. Then it turns milky in color and starts to
bubble at about 600°F.
Finally it turns into a clear liquid at about 1100°F,
just short of brazing temperature.
When the flux appears clear, it is time to start
adding the filler metal with the heat of the joint, not
the flame, melting the filler metal.
If you have properly aligned the parts and applied
the temperature, the filler metal will spread over the
metal surface and into the joint by capillary attraction.
 Brazing
For good bonding, ensure the filler metal penetrates
the complete thickness of the metal.