Non Conventional Machining Methods
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Transcript of Non Conventional Machining Methods
NON CONVENTIONAL MACHINING METHODS
Conventional machining sufficed the requirement of the industries over the decades. But
new exotic work materials as well as innovative geometric design of products and
components were putting lot of pressure on capabilities of conventional machining
processes to manufacture the components with desired tolerances economically. This led
to the development and establishment of non conventional machining processes in the
industry as efficient and economic alternatives to conventional ones. With development
in the non conventional machining processes, currently there are often the first choice and
not an alternative to conventional processes for certain technical requirements. It is based
on unconventional machining techniques using Laser beam, Electron beam, Electric arc
etc. Conventional machining involves the direct contact of tool and work -piece, whereas
unconventional machining does not require the direct contact of tool and work piece.
Conventional machining has many disadvantages like tool wear which are not present in
Non-conventional machining.
Non conventional machining methods utilises :-
Electrical energy
Thermal energy
Chemical energy [1]
The classification of the machining processes is based upon the type of energy used, the
mechanism of metal removal in the process, the source of the intermediate energy
required for material removal and the medium for transfer of those energies.[2]
TABLE-1
TYPES OF
ENERGY
BASIC
MECHANISM
OF METAL
REMOVAL
TRANSFER
MEDIA
ENERGY SOURCE PROCESSES
Mechanical Erosion shear High velocity
of particles,
physical
contact
Pneumatic/hydraulic
pressure
Abrasive Jet
Machining,
Ultrasonic
Machining
Chemical Chemical
ablation
Reactive
environment
Corrosive agent Chemical
Machining
Electro
Chemical
Ion
displacement
Electrolyte High current Electro
Chemical
Machining,
Electro
Chemical
Grinding
Thermo-
Electric
Fusion
vaporization
Hot gases
Electron
radiation
Ionized material
high voltage
Amplified light
IBM, PAM,
EDM, EBM,
LBM
IBM-Ion beam machining, PAM-Plasma Arc Machining, EDM-Electrical Discharge
Electro Chemical Machining, EBM-Electron Beam machining, LBM-Laser Beam
Machining.
Process Selection—In order to make use of the non traditional machining processes
efficiently, it is necessary that the exact nature of the machining problem must be known.
The points which should be looked into before the selection of these processes are;-
Physical parameters
Properties of the work material and the shape to be machined
Process capability or machining characteristics
Economic considerations
The applications of the non traditional machining processes are also influenced by the
workpiece shape end size to be produced, viz. holes, through holes, surfacing, through
cutting and special applications.
The process capability or machining characteristics can be analyzed with respect to:-
o Metal removal rate obtained
o Tolerance maintained
o Surface finish obtained
o Depth of surface damage
o Power required for machining
The economics of the various processes are analyzed by considering:-
Capital cost
Tooling cost
Consumed power cost
Metal removal rate efficiency
Wear of tooling[3]
ABRASIVE JET MACHINING
Abrasive jet machining (AJM), also known as abrasive micro-blasting, pencil
blasting and micro-abrasive blasting,[4] is an abrasive blasting machining process that
uses abrasives propelled by a high velocity gas to erode material from the workpiece.[5]
The filtered gas, supplied under a pressure of 2 to 8 kgf/cm2 to the mixing chamber
containing the abrasive powder and vibrating at 50 Hz entrains the abrasive particles and
is then passed into connecting hose. This abrasive and gas mixture emerge from a small
nozzle mounted on a fixture at a high velocity ranging from 150-300m/min. To control
the size and shape of the cut either the workpiece or the nozzle is moved by cams,
pantographs or other suitable mechanisms. Air and nitrogen are the most widely used gas
in AJM.[6]
EQUIPMENT
AJM machines are usually self-contained bench-top units. First it compresses the gas and
then mixes it with the abrasive in a mixing chamber. The gas passes through a
convergent-divergent nozzle before entering the mixing chamber, and then exits through
a convergent nozzle. The nozzle can be hand held or mounted in a fixture for automatic
operation.[5]
Nozzles must be highly resistant to abrasion and are typically made of tungsten carbide or
synthetic sapphire. For average material removal, tungsten carbide nozzles have a useful
life of 12 to 30 hours, and sapphire nozzles last about 300 hours. The distance of the
nozzle from the workpiece affects the size of the machined area and the rate of material
removal.[7]
The abrasives generally employed are Aluminium Oxide , Silicon Carbide, Glass powder
or specially prepared Sodium bicarbonate. The average particle sizes vary from 10 to 50
microns. Larger sizes are used for rapid removal rate while smaller sizes are used for
good surface finish and precision work.
The metal removal rate depends upon
the diameter of nozzle
composition of abrasive-gas mixture
jet pressure
hardness of abrasive particles and that of work material
particle size
velocity of jet
distance of work piece from the jet[8]
ADVANTAGES AND DISADVANTAGES
The main advantages are
its flexibility
low heat production
ability to machine hard and brittle materials.
Its flexibility owes from its ability to use hoses to transport the gas and abrasive to any
part of the workpiece.
One of the main disadvantages is its slow material removal rate; for this reason it is
usually used as a finishing process. Another disadvantage is that the process produces a
tapered cut.
APPLICATION
Common uses include cutting heat-sensitive, brittle, thin, or hard materials. Specifically it
is used to cut intricate shapes or form specific edge shapes.[9]
ULTRA SONIC MACHINING
Ultrasonic machining, also known as ultrasonic impact grinding, is a machining
operation in which an abrasive slurry freely flows between the workpiece and a vibrating
tool. It differs from most other machining operations because very little heat is produced.
The tool never contacts the workpiece and as a result the grinding pressure is rarely more
than 2 pounds, which makes this operation perfect for machining extremely hard and
brittle materials, such as glass, sapphire, ruby, diamond, and ceramics.
Abrasives contained in a slurry are driven at high velocity against the work by a tool
vibrating at low amplitude (.003in) and high frequency (20-100khz).The tool oscillates in
a direction perpendicular to the workpiece surface and is fed slowly into the workpiece so
that the shape of the tool is formed in the part
The action of the abrasives impinging against the work surface performs the cutting
Tool materials - soft steel, stainless steel
Abrasive materials - boron nitride, boron carbide, aluminum oxide, silicon carbide and
diamond
The vibration amplitude should be set approximately equal to the grit size, and the gap
size should be maintained at about two times the grit size
The ratio of work material to tool material removed during the cutting process ranges
from ~100:1 for cutting glass down to ~1:1 for cutting tool steel
Workpiece materials: hard and brittle such as ceramics, glass and carbides; successfully
used on certain metals such as stainless steel and titanium
Shapes obtained by USM include non round holes, holes along a curved axis and coining
operation, in which an image pattern on the tool is imparted to a flat work surface. [10]
The liquid to produce abrasive slurry should have the following characteristics:-
Good welding characteristic
Low viscosity
High thermal conductivity
Anti corrosive property
Approximately having equal density with abrasive
Low cost[11]
The surface finish of ultrasonic machining depends upon the hardness of the
workpiece/tool and the average diameter of the abrasive grain used. Up close, this
process simply utilizes the plastic deformation of metal for the tool and the brittleness of
the workpiece.
Machine time-- Machine time depends upon the frequency at which the tool is vibrating,
the grain size and hardness (which must be equal or greater than the hardness of the
workpiece), and the viscosity of the slurry fluid. Common grain materials used are silicon
carbide and boron carbide, because of their hardness. The less viscous the slurry fluid, the
faster it can carry away used abrasive.[10]
APPLICATIONS
Introducing round holes
Performing machining operations like drilling, grinding, profiling etc.
In machining glass, ceramic, tungsten
Cutting threads
CHEMICAL MACHINING
Chemical machining is the stock removal process for the production of the desired shapes
and dimensions through selective or overall removal of material by controlled chemical
attack with acids or alkalis.
Steps of chemical machining
Chemical machining process has several steps for producing machine parts. These are
given below:
1 Workpiece preparation
2. Coating with masking material
3. Scribing of the mask
4. Etching
5. Cleaning masking material
Maskants
Masking material which is called maskant is used to protect workpiece surface from
chemical etchant. Polymer or rubber based materials are generally used for masking
procedure. The selected maskant material should have following properties [12].
1. Tough enough to withstand handling
2. Well adhering to the workpiece surface
3. Easy scribing
4. Inert to the chemical reagent used
5. Able to withstand the heat used during chemical machining
6. Easy and inexpensive removal after chemical machining etching
Workpiece material Masking material
Aluminium and alloys Polymer, Butyl rubber, neoprene
Iron based alloys Polymer, Polyvinyl chloride,Polyetilien butyl rubber
Nickel Neoprene
Magnesium Polymer
Copper and alloys Polymer
Titanium Polymer
Silicon Polymer
ADVANTAGES
Easy weight reduction
No effect of workpiece materials properties such as hardness
Simultaneous material removal operation
No burr formation
No stress introduction to the workpiece
Low capital cost of equipment
Easy and quick design changes
Requirement of less skilled worker
Low tooling costs
The good surface quality
Using decorative part production
DISADVANTAGES
Difficult to get sharp corner
Difficult to chemically machine thick material (limit is
depended on workpiece material, but the thickness should be
around maximum 10 mm)
Scribing accuracy is very limited, causes less dimensional
accuracy
Etchants are very dangerous for workers
Etchant disposals are very expensive[12][13]
ELECTRO CHEMICAL MATCHING
Electrochemical machining (ECM) is a method of removing metal by an
electrochemical process. It is normally used for mass production and is used for working
extremely hard materials or materials that are difficult to machine using conventional
methods. ECM is often characterized as "reverse electroplating," in that it removes
material instead of adding it.It is similar in concept to electrical discharge machining
(EDM) in that a high current is passed between an electrode and the part, through an
electrolytic material removal process having a negatively charged electrode (cathode), a
conductive fluid (electrolyte), and a conductive workpiece (anode); however, in ECM
there is no tool wear. The ECM cutting tool is guided along the desired path close to the
work but without touching the piece. Unlike EDM, however, no sparks are created. High
metal removal rates are possible with ECM, with no thermal or mechanical stresses being
transferred to the part, and mirror surface finishes can be achieved.
In the ECM process, a cathode (tool) is advanced into an anode (workpiece). The
pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate
is the same as the rate of liquefaction of the material. The area between the tool and the
workpiece varies within 80-800 micrometers (.003 in. and .030 in.) As electrons cross the
gap, material on the workpiece is dissolved, as the tool forms the desired shape. The
electrolytic fluid carries away the metal hydroxide formed in the process.[14] [15]
ADVANTAGES AND DISADVANTAGES
Because the tool does not contact the workpiece, its advantage over conventional
machining is that
there is no need to use expensive alloys to make the tool tougher than the
workpiece.
There is less tool wear in ECM, and
less heat and stress are produced in processing that could damage the part. Fewer
passes are typically needed, and the tool can be repeatedly used.
Disadvantages are the high tooling costs of ECM, and that up to 40,000 amps of current
must be applied to the workpiece. The saline electrolyte also poses the risk of corrosion
to tool, workpiece and equipment.
APPLICATIONS
Some of the very basic Applications of ECM are listed below:
It can be used for Die-Sinking operations.
Drilling a jet engine turbine blade.
Multiple Hole drilling.
Steam turbine blades can be machined within close limits.[15]
ELECTRO CHEMICAL GRINDING
Electro chemical grinding is one of the latest methods of grinding. This method is
introduced in early 1970’s. In this process a grinding wheel in which an insulating
abrasive is set in a conducting bonding material is employed. The D.C power required
here is 5-15V. So for obtaining a voltage between 5 to 15V a step down transformer
is used. The metal bonded abrasive wheel (or) the grinding wheel acts as a cathode.
The work piece acts as anode. Very small distance is maintained between the anode
and cathode.
An electrolyte is allowed to pass through the gap between the electrodes. The
insulating abrasive particles get spread over the surface of the wheel. The height of
the abrasive particles over the wheel in the gap between the electrodes indicates
effective gap between the electrodes. The electrolysis can take place effectively in
between this gap only. The current densities used are 2A/cm2 to 3A/cm2.
electrolyte that can be employed here should satisfy the following properties.
1) High electrical conductivity
2) Low viscosity and high specific heat chemical stability.
3) Resistance to formation of passive film on work surface.
4) Non-corrosive and non-toxic in nature
5) Readily available and inexpensive
The electrolyte should perform many functions like
1) These should complete the electrical circuit between tool and work piece.
2) Electrolyte must allow all the desirable machining processes to occur.
3) It should function as a coolant by carrying away the heat generated during the
chemical reactions.
4) Electrolyte should be effective in carrying away the products obtained by reactions
in machining zone.
Removal of metal in ECG
Most of metal removal is done by electrochemical action. But some of the metal is
also removed by the contact of abrasive particles to work piece. The abrasive particles
have two main functions in electro chemical grinding.
1) To find the effective gap between the anode and cathode
2) To remove any passive layer formed over the work piece.
ADVANTAGES
1) Metal removal rate is very high.
2) Though the machine requires very high investment increased metal removal rate
and less abrasive consumption acts as more than compensate for extra capital cost. On
large scale production the cost per piece gets highly reduced.
3) Less risk of thermal damage as the heat generated is very low.
4) No presence of burrs on the finished surface.
5) High surface finish and no grinding scratches are present on the finish surface.
6) Pressure over the wheel due to work gets minimized.
APPLICATION
1) This process is extensively used for grinding carbide tools. Electro chemical
grinding provides a savage of 75% in wheel cost and 50% in labor cost
2) Electro chemical grinding is also used for grinding fragile (or) very hard and tough
materials.
DISADVANTAGES
Electrochemical grinding loses accuracy when grinding inside corners, due to the
effects of the electric field. [16]
ION BEAM MACHINING PROCESS
Focused ion beam machining is carried out on any type of work material. In ion beam
machining process the stream of ions of a particular inert gas like Argon or Neon is
accelerated in a medium of vacuum. The high energy of this impact is sent to the work
material. This powerful beam removes atom from the work material by transferring the
energy and the momentum. Clusters of atom are removed due to this process from the
work material.
Ion beam machining uses the principle called as sputtering. Sputtering is a technique
where the stream of ions is made to bombard the work surface. The energy in form of
kinetic energy is transferred to the surface.
The apparatus is a filament that acts like a cathode. A suitable anode is placed near the
cathode. Poles of magnet are mounted and the argon gas is made to pass inside the
vacuum chamber.
High vacuum conditions are necessary for this process. Extraction grids are used to
remove argon from the ion source.
ION BEAM SOURCE
Most widespread are instruments using Liquid-metal ion sources (LMIS),
especially gallium ion sources. Ion sources based on elemental gold and iridium are also
available. In a Gallium LMIS, gallium metal is placed in contact with a tungsten needle
and heated. Gallium wets the tungsten, and a huge electric field (greater than 108 volts per
centimeter) causes ionization and field emission of the gallium atoms.
Source ions are then accelerated to an energy of 5-50 keV (kiloelectronvolts), and
focused onto the sample by electrostatic lenses. LMIs produce high current density ion
beams with very small energy spread.[17]
ADVANTAGES AND DISADVANTAGES
ADVANTAGES
The process is almost universal
No chemical reagents or etching
Etching rates are easily controlled
DISADVANTAGE
Expensive method
Etching rates are slow
There is a possibility of some thermal or radiation damage
APPLICATIONS
Applied in micro machining components
For etching typical materials like glass, alumina, quartz etc.[18]
PLASMA ARC MACHINING
Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to
melt and displace material in its path. Called PAM, this is a method of cutting metal with
a plasma-arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of
high-temperature ionized gas called plasma that cuts by melting and removing material
from the workpiece. Temperatures in the plasma zone range from 20,000° to 50,000° F
(11,000° to 28,000° C).
It is used as an alternative to oxyfuel-gas cutting, employing an electric arc at very
high temperatures to melt and vaporize the metal. [19]
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, an inert
gas (in some units, compressed air) 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 sufficiently hot to melt the metal being
cut and moves sufficiently fast to blow molten metal away from the cut. Plasma is an
effective means of cutting thin and thick materials alike. Hand-held torches can usually
cut up to 2 in (48 mm) thick steel plate, and stronger computer-controlled torches can cut
steel up to 6 inches (150 mm) thick. Since plasma cutters produce a very hot and very
localized "cone" to cut with, they are extremely useful for cutting sheet metal in curved
or angled shapes
STARTING METHODS
Plasma cutters use a number of methods to start the arc. In some units, the arc is created
by putting the torch in contact with the work piece. Some cutters use a high voltage, high
frequency circuit to start the arc. This method has a number of disadvantages, including
risk of electrocution, difficulty of repair, spark gap maintenance, and the large amount of
radio frequency emissions. Plasma cutters working near sensitive electronics, such as
CNC hardware or computers, start the pilot arc by other means. The nozzle and electrode
are in contact. The nozzle is the cathode, and the electrode is the anode. When the plasma
gas begins to flow, the nozzle is blown forward. A third, less common method is
capacitive discharge into the primary circuit via a silicon controlled rectifier.
COSTS
Plasma torches were once quite expensive. For this reason they were usually only found
in professional welding shops and very well-stocked private garages and shops. However,
modern plasma torches are becoming cheaper, and now are within the price range of
many hobbyists. Older units may be very heavy, but still portable, while some newer
ones with inverter technology weigh only a little, yet equal or exceed the capacities of
older ones. [20]
ADVANTAGES
Extremely effective on any metal
No contact between tool and workpiece
DISADVANTAGES
Metallurgical change on the surface
Safety precautions are necessary and this adds to the cost[21]
ELECTRICAL DISCHARGE MACHINING
Electric discharge machining (EDM), sometimes colloquially also referred to as spark
machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing
process whereby a desired shape is obtained using electrical discharges (sparks). Material
is removed from the workpiece by a series of rapidly recurring current discharges
between two electrodes, separated by a dielectric liquid and subject to an electric voltage.
One of the electrodes is called the tool-electrode, or simply the ‘tool’ or ‘electrode’, while
the other is called the workpiece-electrode, or ‘workpiece’.
When the distance between the two electrodes is reduced, the intensity of the electric
field in the volume between the electrodes becomes greater than the strength of the
dielectric (at least in some point(s)), which breaks, allowing current to flow between the
two electrodes. This phenomenon is the same as the breakdown of a capacitor
(condenser) (see also breakdown voltage). As a result, material is removed from both the
electrodes. Once the current flow stops (or it is stopped - depending on the type of
generator), new liquid dielectric is usually conveyed into the inter-electrode volume
enabling the solid particles (debris) to be carried away and the insulating proprieties of
the dielectric to be restored. Adding new liquid dielectric in the inter-electrode volume is
commonly referred to as flushing. Also, after a current flow, a difference of potential
between the two electrodes is restored to what it was before the breakdown, so that a new
liquid dielectric breakdown can occur.
Electrical discharge machining is a machining method primarily used for hard metals or
those that would be very difficult to machine with traditional techniques. EDM typically
works with materials that are electrically conductive, although methods for machining
insulating ceramics with EDM have also been proposed. EDM can cut intricate contours
or cavities in pre-hardened steel without the need for heat treatment to soften and re-
harden them. This method can be used with any other metal or metal alloy such as
titanium, hastelloy, kovar, and inconel. Also, applications of this process to shape
polycrystalline diamond tools have been reported.
ADVANTAGES
Complex shapes that would otherwise be difficult to produce with conventional
cutting tools
Extremely hard material to very close tolerances
Very small work pieces where conventional cutting tools may damage the part
from excess cutting tool pressure.
There is no direct contact between tool and work piece. Therefore delicate
sections and weak materials can be machined without any distortion.
A good surface finish can be obtained.
Very fine holes can be easily drilled.
DISADVANTAGE
The slow rate of material removal.
The additional time and cost used for creating electrodes for ram/sinker EDM.
Reproducing sharp corners on the workpiece is difficult due to electrode wear.
Specific power consumption is very high.
Power consumption is high.
"Overcut" is formed.
Excessive tool wear occurs during machining.
Electrically non-conductive materials can be machined only with specific set-up of the
process.[22]
LASER BEAM MACHINING
The word laser is an acronym for Light Amplification by the Stimulated Emission of
Radiation.
The three most important attributes of laser light are:
It is coherent i.e. all photons that make up the beam are in phase with each other.
It is collimated, because photons that diverge from the parallel are lost through the
chamber walls a very parallel beam is issued.
It is monochromatic, literally one colour, that is of one wavelength. Different media
used to stimulate the photons generate different wavelengths, but each type of laser has
a specific wavelength (e.g. CO2 is 10.6 mM). The purity of the medium used is of
paramount importance.
PROCEDURE
The workpiece rests on a sacrificial table (minimal point contact, when heavily pitted by
laser overshoot is simply thrown away, hence the name). Workholding is minimal due
to absence of cutting forces and when used is mainly for location.
· The focal point of the laser is focused onto the surface of the workpiece. The
follower takes into account any variation in height of the workpiece.
· The material vapourises instantly, producing a kerf in the material.
· The machine axes move to generate the correct profile. The speed of cutting is such
that the Heat Affected Zone (HAZ) is minimal - compared to flame cutting.
· A gas assist jet clears the molten metal that has not vapourised (as in oxy-fuel
cutting). Note: the gas assist gas may be one of two types, inert and exothermic. Inert
gasses commonly used are Nitrogen and Argon. Exothermic gasses, Air or pure
Oxygen. Inert gasses help keep oxidisation to a minimum, cool the cutting zone and
prevent flammable materials burning. Exothermic gasses cause a reaction that
improves cutting performance.
· Welding is broadly similar except for the omission of the gas assist jet. In this case
the column of molten metal needs to remain in place until after the beam has passed,
to allow solidification.
· A significant advantage of laser welding is that filler rods need not be used and
two dissimilar metals can be welded. The two pieces to be welded are butted together (to
a close tolerance) the laser beam passes along the intersection, melting both sides and
‘stirring’ the metals together. Very accurate welds with good structural integrity (due in
part to the small HAZ) can be made with a laser beam. EBM provides very high drilling
rates when small holes with large aspect ratio are to be drilled. Moreover it can machine
almost any material irrespective of their mechanical properties. As it applies no
mechanical cutting force, work holding and fixturing cost is very less. Further for the
same reason fragile and brittle materials can also be processed. The heat affected zone in
EBM is rather less due to shorter pulses. EBM can provide holes of any shape by
combining beam deflection using electromagnetic coils and the CNC table
with high accuracy.
Though heat affected zone is rather less in EBM but recast layer
vacuum. However this can be reduced to some extent using vacuum load
significant amount of non-productive pump down period for attaining desired
applicable for any equipment using vacuum system. Moreover in EBM there is
high capital cost of the equipment and necessary regular maintenance
However, EBM has its own share of limitations. The primary limitations are the
high initial capital cost
high maintenance cost
not very efficient process
not suitable for heat sensitive material[23][24]
ELECTRON BEAM MACHINING
A high velocity stream of electrons is focused on the workpiece surface to remove
material by melting and vaporization. The production of free electrons is obtained from
thermo electronic cathodes wherein metal are heated to the temperature at which the
electrons acquire sufficient speed for escaping the space around the cathode.
Electron beam gun accelerates a stream of electrons to ~3/4 c and focused through an
electromagnetic lens. Kinetic energy of beam converted to thermal energy of extremely
high density, melting or vaporizing material in a very localized area
EBM must be carried out in a vacuum
ADVANTAGES
can drill holes or cut slots
can cut any known material, metal or non metal
no cutting tool wear
distortion free machining
DISADVANTAGES
high equipment cost
employment of high skill labour
only small cuts can be made
APPLICATIONS
To drill fine gas orifices
To produce metering holes in injector nozzles
To scribe thin films
To remove small broken taps from holes