Date post: | 17-Jul-2015 |
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Engineering |
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The nontraditional processes that use mechanical
energy other than a sharp cutting tool.
Types:
(1) ultrasonic machining.
(2) water jet processes.
(3) abrasive jet process.
Ultrasonic machining is a non-traditional
mechanical means of uniform stock removal,
applicable to both conductive and nonconductive
materials. Particularly suited for very hard and
brittle materials such as graphite, glass, carbide, and
ceramics.
It uses medium to high frequency vibrations (in the
area of 20KHz) through a designed or formed tool
having the shape of the cavity to be machined,
combined with a fine abrasive slurry to produce
accurate holes or cavities of regular or irregular
shapes. Vibration of the tool top is obtained through
a transducer.
The two processes described in this section remove
material by means of:
1) high-velocity streams of water.
2)a combination of water and abrasives
A water jet cutter is an industrial tool capable of
cutting a wide variety of materials using a very
high-pressure jet of water.
Pure water jets use the beam of water exiting the
orifice to cut soft material like candy bars, and thin
soft wood.
They use the technology of high-pressure water
being forced through a small hole (typically called
the "orifice" or "jewel") to concentrate an extreme
amount of energy in a small area. The restriction of
the tiny orifice create high-velocity beam, much like
putting your finger over the end of a garden hose.
In Abrasive Jet Machining abrasive particles are
made to impinge on the work material at a high
velocity. The jet of abrasive particles is carried by
carrier gas or air.
The high velocity stream of abrasive is generated
by converting the pressure energy of the carrier gas
or water to its kinetic energy and hence high
velocity jet.
The high velocity abrasive particles remove the
material.
Abrasive jet machining also known as micro-
abrasive blasting, is a mechanical energy based
unconventional machining process used to remove
unwanted material from a given workpiece.
The process makes use of an abrasive jet with high
velocity, to remove material and provide smooth
surface finish to hard metallic workpieces. It is
similar to water jet machining.
Material is removed by fine abrasive particles,
usually about 0.001 inch (0.025 mm) in diameter,
driven by a high velocity fluid stream;
common gases are air or inert gases. Pressures for
the gas range from 25 to 130 psi (170–900 kpa) and
speeds can be as high as 300 m/s.
• AFM can
Polish and deburr parts internally.
Through holes.
Intersecting holes.
Calibrate fuel injection nozzles to a specific flow
rate.
Produce surface finish as good as 0.05 μm deburr
holes as small as 0.2 mm radius edges from 0.025
mm to 1.5 mm.
1)One-way AFM.
One-way flow AFM processing pushes abrasive
media through the work piece in only one direction,
allowing the media to exit freely from the part.
2)Two-way AFM.
The typical two-way flow AFM process uses two
vertically opposed cylinders to extrude an abrasive
media back and forth through or around passages
formed by the workpiece and tooling.
ECM is used to machine workpieces by
electrolytically dissolving the metal. The process is
used in aerospace engineering and the automotive,
medical equipment, micro system and power supply
industries. Almost all kinds of metal can be electro-
chemically machined, even high-alloyed nickel- or
titanium-based ones, as well as hardened materials.
Material is depleted from anode workpiece and
transported to a cathode tool in an electrolyte bath.
Electrolyte flows rapidly between the two poles to
carry off depleted material, so it does not plate onto
tool.
Electrode materials: cupper, brass, or stainless steel.
Tool has inverse shape of part.
Tool size and shape must allow for the gap.
Electrochemical deburring (ECD) is an adaptation
of ECM designed to remove burrs or to round sharp
corners on metal work parts by anodic dissolution.
It is a highly productive, precision technology for
deburring especially at the intersections of internal
passages, or other difficult to access features of a
workpiece.
The electrode tool is designed to focus the metal
removal action on the burr. Portions of the tool not
being used for machining are insulated. The
electrolyte flows through the hole to carry away the
burr particles.
Electrochemical Grinding, is a variation of ECM
(Electrochemical Machining) that combines
electrolytic activity with the physical removal of
material by means of charged grinding wheels.
Electrochemical Grinding (ECG) can produce burr
free and stress free parts without heat or other
metallurgical damage .
Material removal processes based on thermal energy
are characterized by very high local temperatures—
hot enough to remove material by fusion or
vaporization. Because of the high temperatures,
these processes cause physical and metallurgical
damage to the new work surface.
Electrical discharge machining:
EDM is the thermal erosion process in which metal
is removed by a series of recurring electrical
discharges between a cutting tool acting as an
electrode and a conductive workpiece, in the
presence of a dielectric fluid.
1) A preshaped or formed electrode (tool),usually
made from graphite or copper, is shaped to the form
of the cavity it is to reproduce.
The formed electrode is fed vertically down and the
reverse shape of the electrode is eroded (burned)
into the solid workpiece.
2.Electric Discharge Wire Cutting
Wire EDM machining (Electrical DischargeMachining) is an electro thermal productionprocess in which a thin single-strand metalwire in conjunction with de-ionized waterallows the wire to cut through metal by the useof heat from electrical sparks.
Electron beam machining is a thermalnontraditional process, uses electrical energy togenerate thermal energy for removing material.A pulsating stream of high-speed electronsproduced by a generator is focused byelectrostatic and electromagnetic fields toconcentrate energy on a very small area ofwork.
Uses a high-velocity stream of electrons capable to
remove any material.
EB gun accelerates a continuous stream of electrons
to about 75% of light speed.
Kinetic energy of electrons is converted to thermal
energy of extremely high density which vaporizes
material in a very localized area.
Beam focused through electromagnetic lens.
Laser beam machining is an unconventional
machining process in which a laser is directed
towards the work piece for machining. Since the
rays of a laser beam are monochromatic and parallel
it can be focused to a very small diameter and can
produce energy as high as 100 MW of energy for a
square millimeter of area.
Light amplification by stimulated emission of
radiation.
A laser converts electrical energy into a highly
coherent light beam with the following properties:
– Monochromatic (theoretically, single wave length)
– Highly collimated (light rays are almost perfectly
parallel)
These properties allow laser light to be focused,
using optical lenses, onto a very small spot with
resulting high power densities.
The intense heat from an electric arc can be used to
melt virtually any metal for the purpose of welding
or cutting. Most arc-cutting processes use the heat
generated by an arc between an electrode and a
metallic work part to melt a kerfs that separates the
part. The most common arc-cutting processes are
(1) plasma arc cutting.
(2) air carbon arc cutting.
Plasma cutting is a process that is used to cut steel
and other metals of different thicknesses (or
sometimes other materials) using a plasma torch.
In this process, a gas (oxygen, air, inert and others
dependant on material) is blown at high speed out of
a nozzle; at the same time an electrical arc is formed
through that gas from the nozzle to the surface being
cut, turning some of that gas to plasma. The plasma
is hot enough to melt the metal being cut .
Air carbon arc cutting, previously known as air
arc cutting, is an arc cutting process where metal is
cut and melted by the heat of a carbon arc.
In this process, an intense arc is used to form a
molten pool on a metal workpiece, and compressed
air is used for blowing the molten metal thoroughly
from the metal surface. It is important that the metal
is only cut along the airflow direction. Moreover,
appropriate arc length has to be maintained to
completely remove the molten metal.
Oxy-fuel welding (commonly called oxyacetylene
welding, oxy welding, or gas welding) and oxy-
fuel cutting are processes that use fuel gases and
oxygen to weld and cut metals, respectively.
Use heat of combustion of fuel gases combined withexothermic reaction of metal with oxygen
Popularly known as flame cutting
Cutting torch delivers a mixture of fuel gas andoxygen and directs a stream of oxygen to cuttingregion.
Fuel:
– Acetylene (C2H2)
– Propylene (C3H6)
– Propane (C3H8)
Chemical machining is a nontraditional process inwhich material is removed by means of a strongchemical etchant. Applications as an industrialprocess began shortly after world war II in theaircraft industry.
Types
(1) chemical milling,
(2) chemical blanking,
(3) chemical engraving,
(4) photochemical machining
Chemical milling is the process of removing large
amounts of metal by means of chemical etching. All
metal types and shapes are candidates for chemical
milling, especially contoured parts which are not
easily machined.
Clean - to insure uniform etching.
Apply maskant - a maskant (chemically resistant toetchant) to portions of work surface not to beetched.
Materials: neoprene, polyvinylchloride,polyethylene, and other polymers.
Selectively remove maskant.
Etch - part is immersed in etchant which chemicallyattacks those portions not masked.
Remove maskant and clean.
Uses chemical erosion to cut very thin sheet metal
parts - down to 0.025 mm (0.001 in) thick and/or for
intricate cutting patterns
Conventional punch and die does not work because
stamping forces damage the thin sheet metal, or
tooling or both.
Maskant methods are either photo resist or screen
resist
PCM is a process of selectively removing material
by using a chemical action. The process begins with
cleaning the material of all debris, grease, and other
any contaminants; proper metal preparation is
essential to the quality of the final product.
Once it is fully cleaned, it is then coated with aphotosensitive film and etchant resistant, used forcutting the metal, reducing the surfacecontamination and oxidation. Dry film or a liquidresist is applied to both sides of the work piece toallow etching of both sides simultaneously. Thephotosensitive coating is then exposed to a UVlight, which transfers the photo tool image onto thecoated material. Then the material is developed.During this process any material not protected bythe resist is eroded.