Download Modern Manufacturing Process(Non Conventional Machining)

Modern Manufacturing Processes
RUBIAT MUSTAK
Modern Machining / Unconventional Machining Process
• As the world advancing forth technically in the field of space research,
missile and nuclear industry; very complicated and precise component
having some special requirements are demanded by these industries.
• Some metals like hastalloy, nitralloy, nimonics etc , are such that they
can’t be machined by conventional methods but requied some special
technicques.
Hastalloy C-276
Element
Ni
Mo
Cr
Fe
W
Uses
Content
55 %
15 - 17 %
14.5 - 16.5 %
4-7%
3 - 4.5 %
1.
2.
3.
4.
5.
Pollution control
Chemical processing
Waste treatment
Marine engineering
Pulp and paper
production
Modern Machining / Unconventional Machining Process(cont..)
• The method are not limited by hardness, toughness, and brittleness of
material
• It can produce any intricate shape on any work piece.
• It has suitable control over the various physical parameters of the
processes.
• It is only complementing the conventional machining methods.
Classified into various groups
• Types of energy required to shape the material
• Mechanical
• Thermal
• Electro thermal
• Chemical and electro chemical
• Basic mechanism involve in the process
• Erosion
• Ionic dissolution
• Vaporization
Classification(cont..):
• Source of energy required for material removal
• Hydrostatic pressure
• High current density
• High voltage
• Ionized material
• Medium for transfer of these energy
• High velocity particles
• Electrolyte
• Electron
• Hot gases
Classification(cont..)
 In thermal and electro thermal method- heat energy is concentrated on a
small area of the work piece, to melt and vaporize the tiny bits of work
material. (EDM, LBM, PAM, EBM, IBM)
 In chemical and electro chemical method- the work piece material in
contact with a chemical solution is etched in a controlled manner.
(ECM, ECG, ECH and ECD)
 In mechanical method- the material is removed by mechanical erosion
of the work piece material. ( USM, AJM, and WJM)
Our concern:
1.
Electro Discharge Machining (EDM)
2.
Electro Chemical Machining (ECM)
3.
Laser Beam Machining (LBM)
4.
Ultrasonic Machining (USM)
5.
Plasma Arc Machining (PAM)
Electrochemical Machining (E.C.M.):
•Electrochemical machining (ECM) is an
electrolytic material removal process involving a
negatively charged shaped electrode (cathode), a
conductive fluid (electrolyte), and a conductive
workpiece (anode).
•ECM is an electrolytic process and its basis is the
phenomenon of electrolysis, whose laws were
established by Faraday in 1833.
Electrochemical Machining (E.C.M.)(cont..)
In the ECM process, the dc power source charges the workpiece positively and
charges the tool negatively. As the machine slowly brings the tool and
workpiece close together, perhaps to within 0.010 of an inch, the power and
electrolyte flow are turned on. Electrons flow across the narrow gap from
negative to positive, dissolving the workpiece into the shape as the tool
advances into it. The recirculating electrolytic fluid carries away the dissolved
material as a metal hydroxide.
Electrochemical Machining (E.C.M.)(cont..)
•In ECM one employs a cathode electrode shaped to provide the
complementary structure in an anode work piece.
•A highly conductive electrolyte stream separates the cutting tool from the work
piece, and metal removal is accomplished by passing a dc current of up to
100A/cm2 through the salt solution cell. As the cathode tool approaches the
anode work piece it erodes its complementary shape in it.
•Thus complex shapes may be made from a material such as soft copper and
used to produce negative duplicates of it. The process is also called
electrochemical sinking.
Electrochemical Machining (E.C.M.)(cont..)
Material Removal Rate (MRR):
According to Faraday’s first law of electrolysis, mass of ion liberated by
the substance,
M= ZIt
Where, I = current flowing through the electrolytic cell
t= time in sec
Z= constant known as the electro- chemical equivalent of the
substance.
Principle:
•Based on the principle of Faraday’s law of electrolysis.
Characteristic:
•
Small gap between tool and work-piece (≈ 0.5mm).
•
Wok-piece stationary, feed by tool.
•
Electrolyte: Aqueous solution of common salt, dilute acid.
•
Pressure of electrolyte ≈ 14 kg/cm2
•
Velocity of electrolyte = 30 to 60 m/sec.
•
Temperature of electrolyte = 25 to60°C .
•
Voltage of electrolyte = 5 to 15V
ECM: Advantages
•Components are not subject to either thermal or mechanical stress.
•There is no tool wear in ECM.
•Non-rigid and open work pieces can be machined easily as there is no contact
between the tool and work piece.
•Complex geometrical shapes can be machined repeatedly and accurately.
•ECM is a time saving process when compared with conventional machining
•During drilling, deep holes can be made or several holes at once.
•Fragile parts which cannot take more loads and also brittle material which tend
to develop cracks during machining can be machined easily in ECM
•Surface finishes of 25 µ in. can be achieved in ECM
ECM: Disadvantages
o Keeping the solution conductivity constant.
o More expensive than conventional machining.
o Need more area for installation.
o Electrolytes may destroy the equipment.
o Not environmentally friendly (sludge and other waste)
o High energy consumption.
o Chemical attack by electrolytes.
o The danger of a burn in the case of a short circuit between the positive and
negative leads.
o The danger of a fire damp explosion.
o Material has to be electrically conductive
ECM Applications
•The most common application of ECM is high accuracy duplication. Because
there is no tool wear, it can be used repeatedly with a high degree of accuracy.
• It is also used to make cavities and holes in various products.
• It is commonly used on thin walled, easily deformable and brittle material
because they would probably develop cracks with conventional machining.
Products
•The two most common products of ECM are turbine/compressor blades and
rifle barrels. Each of those parts require machining of extremely hard metals
with certain mechanical specifications
•Some of these mechanical characteristics achieved by ECM are:
*Stress free grooves.
*Any groove geometry.
*Any conductive metal can be machined.
*Repeatable accuracy of 0.0005”.
*High surface finish.
*Fast cycle time.
Function of Electrolyte in ECM:
•Completes the electric circuit between tool and work piece.
•Allows desirable machining to occur.
•Carries away products of reaction from the zone of machining.
•Carries away heat generated during chemical reactions.
Properties of Electrolyte For ECM:
•High electrical conductivity.
•Chemical stability.
•High specific heat and low viscosity.
Electrical Discharge Machining (EDM):
•Its 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 – ‘tool-electrode’ or ‘tool’ or ‘electrode’.
•Other electrode - workpiece-electrode or ‘workpiece’.
•As distance between the two electrodes is reduced, the current intensity becomes
greater than the strength of the dielectric (at least in some points) causing it to
break.
Electrical Discharge Machining (EDM)(cont..):
•It is a process of metal removal based on the principle of material removal
by an interrupted electric spark discharge between the electrode tool and
the work piece.
•In EDM, a potential difference is applied between the tool and workpiece.
•Essential - Both tool and work material are to be conductors.
•The tool and work material are immersed in a dielectric medium.
•Generally kerosene or deionised water is used as the dielectric medium.
•A gap is maintained between the tool and the workpiece
Electrical Discharge Machining (EDM)(cont..):
Electrical Discharge Machining (EDM)(cont..):
•Depending upon the applied potential difference (50 to 450 V) and the gap
between the tool and workpiece, an electric field would be established.
•Generally the tool is connected to the negative terminal (cathode) of the
generator and the workpiece is connected to positive terminal (anode).
•As the electric field is established between the tool and the job, the free electrons
on the tool are subjected to electrostatic forces.
•If the bonding energy of the electrons is less, electrons would be emitted from
the tool.
•As they gain velocity and energy, and start moving towards the job, there would
be collisions between the electrons and dielectric molecules.
•Such collision may result in ionization of the dielectric molecule.
Electrical Discharge Machining (EDM)(cont..):
•all of a sudden, a large number of electrons will flow from tool to job and
ions from job to tool.
•Such movement of electrons and ions can be visually seen as a spark.
•Thus the electrical energy is dissipated as the thermal energy of the spark.
•The kinetic energy of the electrons and ions on impact with the surface of
the job and tool respectively would be converted into thermal energy or heat
flux.
•Such intense localized heat flux leads to extreme instantaneous confined
rise in temperature which would be in excess of 10,000oC.
•Such localized extreme rise in temperature leads to material removal.
•Material removal occurs due to instant vaporization of the material as well as due
to melting.
•The molten metal is not removed completely but only partially.
Applications:
 Drilling of micro-holes, thread cutting, helical profile milling, rotary forming,
and curved hole drilling.
 Delicate work piece like copper parts can be produced by EDM.
 Can be applied to all electrically conducting metals and alloys irrespective of
their melting points, hardness, toughness, or brittleness.
 Other applications: deep, small-dia holes using tungsten wire as tool, narrow
slots, cooling holes in super alloy turbine blades, and various intricate shapes.
 EDM can be economically employed for extremely hardened work piece.
 Since there is no mechanical stress present (no physical contact), fragile and
slender work places can be machined without distortion.
 Hard and corrosion resistant surfaces, essentially needed for die making, can
be developed.
Principle:
When a potential difference is applied between two conductors immersed in a
dielectric fluid. The fluid will be ionized and a spark will occurs. If the
potential difference is maintained, the spark will develop into an arc.
Characteristic of EDM:
 A suitable gap between tool and work-piece called spark
maintained (0.01 to 0.5 mm)
gap must be
 More erosion on anode (+ve), so the work-piece is kept +ve.
 Dielectric fluid (Paraffin, kerosene, transformer oil etc.)
 D.C. voltage: 40-300V,Current: 0.5to400Amp, local temperature 10000°C
 A true replica of the tool surface will be produced on the work-piece.
 Dielectric fluid is circulated to maintain a certain temperature.
EDM Advantages:
Some of the advantages of EDM include machining of:
 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.
Disadvantages:
Some of the disadvantages of EDM include:
 The slow rate of material removal.
 For economic production, the surface finish specified should not be too fine.
 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.
 "Overcut" is formed.
 Excessive tool wear occurs during machining.
 Electrically non-conductive materials can be machined only with specific setup of the process
 Function of Dielectric Fluid:
 Serves as a conducting medium and convey the spark.
 Cools the work-piece and tool.
 Carries away the eroded metal.
 Properties of Dielectric Fluid:
 Should not evolve toxic vapours or gases.
 Must be inflammable.
 Chemically inert on tool and work-piece
 Tool material:
 Brass, copper or alloy of copper, cast iron used as cutting tool material.
Ultrasonic Machining (USM):
•In the process of Ultrasonic Machining, material is removed by micro-chipping
or erosion with abrasive particles.
•In USM process, the tool, made of softer material than that of the workpiece, is
oscillated by the Booster and Sonotrode at a frequency of about 20 kHz with an
amplitude of about 25.4 um (0.001 in).
• The tool forces the abrasive grits, in the gap between the tool and the
workpiece, to impact normally and successively on the work surface, thereby
machining the work surface.
•During one strike, the tool moves down from its most upper remote position
with a starting speed at zero, then it speeds up to finally reach the maximum
speed at the mean position.
•Then the tool slows down its speed and eventually reaches zero again at the
lowest position.
•When the grit size is close to the mean position, the tool hits the grit with its full
speed
Ultrasonic Machining (USM) (cont..)
Ultrasonic Machining (USM) (cont..)
•The smaller the grit size, the lesser the momentum it receives from the tool.
•Therefore, there is an effective speed zone for the tool and, correspondingly
there is an effective size range for the grits.
•In the machining process, the tool, at some point, impacts on the largest grits,
which are forced into the tool and work piece.
•As the tool continues to move downwards, the force acting on these grits
increases rapidly, therefore some of the grits may be fractured.
•As the tool moves further down, more grits with smaller sizes come in contact
with the tool, the force acting on each grit becomes less.
•Eventually, the tool comes to the end of its strike, the number of grits under
impact force from both the tool and the workpiece becomes maximum.
•Grits with size larger than the minimum gap will penetrate into the tool and
work surface to different extents according to their diameters and the hardness
of both surfaces
Advantages ultrasonic machining:
•Machining of any material regardless of conductivity.
•Precision machining of brittle hard materials.
•Does not produce electric, thermal or chemical defects at the surface.
•Can drill circular or non-circular holes in very hard materials.
•Less stress because of its non-thermal nature.
Disadvantages ultrasonic machining:
•Low material removal rate.
•Tool wears fast.
•Machining area and depth are quite restricted.
 Abrasives:
Selection on the type of work-piece material, hardness of work- piece, metal
removal rate, desired surface finish etc.
 Types:
Al2O3, Sic, Boron Carbide etc.
Water slurry with 30 to 60% by volume of abrasive.
 Applications:
1) For machining intricate shaped products.
2) Small holes.
3) Machining of hard and non-conductive material.
Plasma Arc Machining(PAM):
• When a flowing gas is heated to sufficiently high temperature of the
order of 16500oC to become partially ionized, it is known as “Plasma”.
• Heat for machining produced by high temperature plasma and also due
to direct electro bombardment.
• Metal removal rate can be increased
by increasing gas flow rate .
There are two type of plasma arc system
1.
2.
Transferred arc type
Non transferred are type
Laser Beam Machining (LBM):
 Laser is an electro magnetic radiation.
 For production of laser beam, generally ruby rod used in which
aluminum is the main ingredient
 Laser beam techniques involves
 Pumping of energy.
 Production of stimulated effect
Advantages:
• No direct contact between tool and work-piece
• Laser beam can be sent from longer distance
• Micro shaped hole can be laser drill.
Limitation:
• High initial cost
• Overall low efficiency
• Not able to drill too deep hole
 Applications:
1)
Small holes, suitable for fine and minute holes.
2)
Welding like non-conducting
protect) material.
3)
Cutting complex profiles on hard materials.
4)
Partial cutting etc.
and refractory
(high temperature