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Файл:Materials Processing Technology Part 2. Coursebook
.pdf
delicate body tissues. The atomization of molten materials produces a certain
amount of dust and fumes. The use of breathing masks, fitted with suitable filters,
is strongly recommended, where equipment cannot be isolated.
Unit 4
Ultrasonic Machining
4.1 Components and applications
Ultrasonic machining is the removal of hard and brittle materials using an
axially oscillating tool at ultrasonic frequencies. During that oscillation, the
abrasive slurry is continuously fed into the machining zone between a soft tool
(brass or steel) and the workpiece. The abrasive particles are, therefore,
hammered into the workpiece surface and cause chipping of fine particles from it.
The oscillating tool imposes a static pressure on the abrasive grains and feeds
down as the material is removed to form the required tool shape.
The machining system is composed mainly from the magnetostrictor,
concentrator, tool, and slurry feeding arrangement. The magnetostrictor is
energized at the ultrasonic frequency and produces small-amplitude vibrations.
USM should be applied for shallow cavities cut in hard and brittle
materials.
The drilling and coring process ensures high removal rates, lower tool
pressures for delicate parts, improved deep hole drilling, less breakout or through
holes, and no core seizing during core drilling.
The process allows the uninterrupted drilling of small-diameter holes,
while conventional drilling necessitates a tool retraction, which increases the
machining time.
During the ultrasonic sinking and contour machining the material
removal is difficult when the machined depth exceeds 5 to 7 mm or when the
active section of the tool becomes important. Under such conditions the removal
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of the abrasive grits at the interface becomes difficult and hence the material
removal process is impossible.
Ultrasonic polishing occurs by vibrating a brittle tool material such as
graphite or glass into the workpiece at an ultrasonic frequency and relatively low
vibration amplitude.
Micro-ultrasonic machining is a method that utilizes workpiece vibration.
Vibrating the workpiece allows for freer tool system design because it does not
include the set of transducer, horn, and cone. By means of such method,
microholes of 5 μm diameter on quartz, glass, and silicon have been produced
using tungsten carbide alloy microtools.
4.2 Advantages and limitations
The process accuracy is measured through the overcut (oversize) produced
during drilling of holes. The oversize measures the difference between the hole
diameter, measured at the top surface, and the tool diameter. The side gap
between the tool and the machined hole is necessary to enable the abrasives to
flow to the machining zone under the oscillating tool. Hence the grain size of the
abrasives represents the main factor, which affects the overcut produced. The
overcut is considered to be about two to four times greater than the mean grain
size when machining glass and tungsten carbide. However, the magnitude of the
overcut depends on many other process variables including the type of workpiece
material and the method of tool feed. In general USM accuracy levels are limited
to ±0.05 mm.
The overcut is usually greater at the entry side than at the exit one due to
the cumulative abrasion effect of the fresh and sharp grain particles. The conicity
can be reduced by: direct injection of the abrasive slurry into the machining zone;
the use of tools having negatively tapering walls; the use of high static pressure
that produces finer abrasives, which in turn reduces the amount of tool wear and
the resulting conicity; the use of wear-resistant tool materials; the use of an
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undersized tool in the first cut and a final tool of the required size, which will cut
faster and reduce the conicity.
The out-of-roundness arises by the lateral vibrations of the tool. Such
vibrations may arise due to the out of perpendicularity of the tool face and the
tool centerline and when the acoustic parts of the machine are misaligned.
The surface finish is closely related to the machining rate in USM. The
larger the grit size, the faster the cutting but the coarser the surface finish.
However, other factors such as tool surface, amplitude of tool vibration, and
material being machined also affect the surface finish. The larger the grit, the
smoother becomes the produced surface. The amplitude of tool oscillation has a
smaller effect on the surface finish. As the amplitude is raised the individual
grains are pressed further into the workpiece surface thus causing deeper craters
and hence a rougher surface finish.
Smoother surfaces can also be obtained when the viscosity of the liquid
carrier of the abrasive slurry is reduced. It is evident that the surface irregularities of
the sidewall surfaces of the cavities are considerably larger than those of the bottom.
Unit 5
Lasers
5.1 Principle of laser working
Laser is the abbreviation of light amplification by stimulated emission of
radiation. Most lasers are actually oscillators (generators or sources of light) and
not amplifiers (devices for increasing the strength of a signal). However, nearly
all lasers have the following in common:
Lasing medium. This can be a solid, liquid, gas, or semiconductor material,
which can be pumped to a higher energy state.
Lasers are based on a simple principle of atomic behavior. Normally,
nearly all atoms, ions, or molecules (depending on the particular laser) of the
lasing medium are at their lowest energy level or ‘ground state’. To produce laser
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action, the energy-pumping device must achieve population inversion through
driving the majority of particles to the upper energy level. Sometimes dropping to
the ‘ground state’ the excited particle emits a single photon of light. This is called
«spontaneous emission», not exactly useful, although causing the glow of a neon
sign or the phosphor coating of a fluorescent lamp.
Laser drilling is performed by the following three methods:
Direct (percussion) drilling, which produces the required hole size by the
application of one or more focused laser pulses. The hole size is determined by
the input power and the focusing adjustments. While most holes are drilled with a
single laser pulse of high energy, it is more efficient to drill deep holes with
multiple pulses of low energy.
Drill and ream consists of drilling a pilot hole and then changing the laser
parameters, by increasing the spot size to open out the hole to the required
diameter the true size.
Trepan drilling is used for large holes by either moving the part or rotating
the laser beam system of delivery or lens. Because of the low energy of each
pulse, the recast layer and out-of-cylindricity are reduced. The process, however,
provides easier control of the hole size by rotating the diameter of the beam.
5.2 Electron beam machining
The heated cathode emits electrons. Just after the cathode, there is an
annular bias grid. A high negative bias is applied to this grid, so that the
electrons generated by this cathode do not diverge and approach the next
element, the annular anode, in the form of a beam. The annular anode now
attracts the electron beam and gradually gets accelerated. As they leave the
anode section, the electrons may achieve a velocity as high as half the velocity
of light. The nature of biasing just after the cathode controls the flow of
electrons and the biased grid is used as a switch to operate the electron beam
gun in pulsed mode. After the anode, the electron beam passes through a series
of magnetic lenses and apertures. The magnetic lenses shape the beam and try
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to reduce the divergence. Apertures on the other hand allow only the
convergent electrons to pass and capture the divergent low energy electrons
from the fringes. This way, the aperture and the magnetic lenses improve the
quality of the electron beam.
Then the electron beam passes through the final section of the
electromagnetic lens and deflection coil. The electromagnetic lens focuses the
electron beam to a desired spot. The deflection coil can maneuver the electron
beam, though by small amount to improve shape of the machined holes.
Generally in between the electron beam gun and the work piece, which is also
under vacuum, there would be a series of slotted rotating discs. Such discs
allow the electron beam to pass and machine materials but helpfully prevent
metal fumes and vapor generated during machining to reach the gun. Thus it is
essential to synchronize the motion of the rotating disc and pulsing of the
electron beam. Electron beam guns are also provided with illumination facility
and a telescope for alignment of the beam with the work piece.
One of the major requirements of electron beam machining operation of
electron beam gun is maintenance of desired vacuum. Maintenance of suitable
vacuum is essential so that electrons do not loose their energy, and a
significant life of the cathode cartridge is obtained.
Unit 6
Using automation system and robotics in machine tools
6.1 Automation in industry
Automation is the system of manufacture performing certain tasks,
previously done by people, only by machines. The sequences of operations are
controlled automatically.
The term automation is also used to describe manufacturing systems in
which automatic devices can operate independently of human control. Such
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devices as automatic pilots, automatic telephone equipment and automated
control systems are used to perform various operations much faster and better
than could be done by people.
Automated manufacturing had several steps in its development.
Mechanization was the first step necessary in the development of automation.
The simplification of work made it possible to design and build machines that
resembled the motions of the worker. These specialized machines were motorized
and they had better production efficiency
The feedback principle is used in all automatic control mechanisms when
machines have ability to correct themselves. The feedback principle has been
used for centuries. An outstanding early example is the centrifugal «flyball»
governor, invented in 1788 by James Watt to control the speed of the steam
engine.
Using feedback devices, machines can start, stop, speed up, slow down,
count, inspect, test, compare, and measure. These operations are commonly
applied to a wide variety of production operations.
Computers have greatly facilitated the use of feedback in manufacturing
processes. Computers gave rise to the development of numerically controlled
machines.
More recently, the introduction of microprocessors and computers have
made possible the development of computer-aided design and computer-aided
manufacture technologies. When using these systems a designer draws a part and
indicates its dimensions with the help of a mouse, light pen, or other input device.
After the drawing has been completed the computer automatically gives the
instructions that direct a machining centre to machine the part.
6.2.1 Robots in manufacturing
Throughout history, inventors have produced a variety of seemingly
magical mechanical devices capable of quite life-like actions. These devices were
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not programmable; they were simply performing a set of operations. Different
scientists and engineers have applied many advances in this field since that time.
Present day robot is a very complex structure. A metal or plastic frame
serves for a skeleton, and a variety of actuators provide muscle power. However,
the new humanoids are not just bodies; they are also sophisticated sensing
machines with cameras, microphones, even specific sensors that imitate the sense
of touch. These robots handle tasks that are difficult, dangerous or boring to
human beings.
The most common manufacturing robot is the robotic arm. It typically
consists of seven metal segments. Tiny motors or actuators put them into
operation when a special computer gives them certain instructions. An industrial
robotic arm with six joints closely resembles a human arm – it has the equivalent
of a shoulder, an elbow and a wrist. This type of robot has six degrees of
freedom, i.e. it can turn in six different ways. A human arm, by comparison, has
seven degrees of freedom. Your arm moves your hand from place to place.
Similarly, the robotic arm moves an end effector from place to place. You can
supply robotic arms with all sorts of end effectors, which will perform a certain
task, for example it will grasp and carry different objects. Robotic hands often
have built-in pressure sensors that tell the computer how hard the robot is
gripping a particular object. That is why the robot does not drop or break
whatever it is carrying. Robots do their work more efficiently than human beings
because they are so precise. They always drill in the exactly the same place, and
they always tighten bolts with the same amount of force, no matter how many
hours they have been in operation.
Different companies all over the world are trying to improve robots.
6.2.2 Robots in manufacturing
Robots are ideal for doing jobs that require repetitive, precise and fast
movements. Robots are good at doing the same thing without asking for a safe
working environment, salary, breaks, food and sleep, without getting bored or
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tired, without making mistakes. Factories are so highly automated that most
human workers carry out only supervising and maintaining the robots.
People keep finding new uses for robots: making and packing drugs and
foods, soldering tiny wires to semiconductor chips, inserting integrated circuits
onto printed circuit boards used in electronics, working in radioactive ‘hot zones’,
exploring space, etc.
A robot is a machine that gathers information about its environment
(senses) and uses that information (thinks) to follow instructions to do work
(acts).
Imitating humans, robots also sense magnetic fields and ultrasonic waves.
Robotic light sensors work by creating or changing an electric signal when light
falls on them. When navigating, the robot sends out a beam of infrared light,
which bounces off objects and returns to a light sensor of the robot. However,
making 3D images requires large amounts of computer memory.
A mechanical device for producing motion is known as an actuator. A
single robot is supplied with dozens of actuators, each chosen to do a specific
task. Electric motors are actuators that produce motion from electricity by the
electromagnetic effect. Their high speed and a small turning power make a
gearbox necessary. Special stepper motors turning in precise «steps» are ideal for
adjusting position. A servomotor is used for turning only 90° to the right or left.
Solenoids are electric motors for producing linear or in-and-out motion.
Solenoids are used in switches turning things off and on.
Today most robots are used in manufacturing operations. The applications
of robots can be divided into three categories:
1) Material handling;
2) Processing operations;
3) Assembly and inspection.
Material-handling is the transfer of material and loading and unloading of
machines. Material-transfer applications require the robot to move materials or
work parts from one to another. Many of these tasks are relatively simple: robots
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pick up parts from one conveyor and place them on another. Other transfer
operations are more complex, such as placing parts in an arrangement that can be
calculated by the robot. Machine loading and unloading operations utilize a robot
to load and unload parts. This requires the robot to be equipped with a gripper
that can grasp parts. Usually the gripper must be designed specifically for the
particular part geometry.
In robotic processing operations, the robot manipulates a tool to perform a
process on the work part. Examples of such applications include spot welding,
continuous arc welding and spray painting. Spot welding of automobile bodies is
one of the most common applications of industrial robots. The robot positions a
spot welder against the automobile panels and frames to join them. Arc welding is
a continuous process in which robot moves the welding rod along the welding
seam. Spray painting is the manipulation of a spray-painting gun over the surface
of the object to be coated. Other operations in this category include grinding and
polishing in which a rotating spindle serves as the robot’s tool.
The third application area of industrial robots is assembly and inspection.
The use of robots in assembly is expected to increase because of the high cost of
manual labor. But the design of the product is an important aspect of robotic
assembly. Assembly methods that are satisfactory for humans are not always
suitable for robots. Screws and nuts are widely used for fastening in manual
assembly, but the same operations are extremely difficult for a one-armed robot.
Assembly operations have traditionally been performed manually, either at
single assembly workstations or on assembly lines with multiple stations. Owing
to the high labor content and high cost of manual labor, greater attention has been
given in recent years to the use of automation for assembly work. Assembly
operations can be automated using production line principles if the quantities are
large, the product is small, and the design is simple (e.g., mechanical pencils,
pens, and cigarette lighters). For products that do not satisfy these conditions,
manual assembly is generally required.
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Unit 7
Mathematical modeling and intelligent control in technological
processes
7.1 The use of neural networks for electron beam welding
The process of welding the titanium alloys, which occurs during the
manufacture of parts for aviation industry, is characterized by stochasticity,
irregularity, nonlinearity, and non-stationarity. All of those affect the quality of
welding. Traditional controlling cannot be used for these features. Obviously, in
these conditions, methods and tools of artificial intelligence should be involved in
the process. Effectiveness may be achieved by the combination of classical
methods and approaches based on artificial intelligence.
Therefore, the development of algorithms and control of electron beam
welding is a necessary and urgent task.
Artificial neural network does not perform a calculation, it transforms the
input signal into the output signal according to its topology and the values of the
coefficients of inter-neuronal communication. In the operation of the neural
network there are two main modes: training and work.
Artificial Intelligence based on artificial neural networks proved to be a pertinent
tool to predict characteristics and structural attributes from the knowledge of processing
parameters. The flexibility along with robustness of this methodology permits to consider
its implementation as on-line control tool.
A neural network is a mathematical model processing system, which is capable to
relate input to output parameters and learn from the data set through iteration, without
requiring a prior knowledge on the relationships between the process variables. This is a
system of simple processing elements (or neurons) typically organized in layers (i.e.,
input layers, hidden layers and output layers). Each neuron receives multiple inputs in
proportion to their connection weights and generates a single output which may be
propagated to several other neurons.
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