uncinventional machining process unit 3
TRANSCRIPT
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UNIT III - ELECTRO CHEMICAL PROCESS
3.1 CHEMICAL MACHINING PROCESSES (CHM) 3.1.1 Fundamentals
Chemical machining is one of the non-conventional machining processes where material
is removed by bringing it in contact of a strong chemical enchant. There are different chemical
machining methods base on this like chemical milling, chemical blanking, photochemical
machining, etc.
3.1.2 Working Principle of CHM
The main working principle of chemical machining is chemical etching. The part of the
workpiece whose material is to be removed, is brought into the contact of chemical called
enchant. The metal is removed by the chemical attack of enchant. The method of making contactof metal with the enchant is masking. The portion of workpiece where no material is to be
removed, is mashed before chemical etching.
3.1.3 Process Details of CHM
Following steps are normally followed in the process of CHM :
Cleaning
The first step of the process is a cleaning of workpiece, this is required to ensure that
material will be removed uniformly from the surfaces to be processed.
Masking
Masking is similar to masking action is any machining operation. This is the action of
selecting material that is to be removed and another that is not to be removed. The material
which is not to be removed is applied with a protective coating called maskant. This is made of a
materials are neoprene, polyvinylchloride, polyethylene or any other polymer. Thinkers of
maskent are maintained upto 0.125 mm. The portion of workpiece having no application of
maskent is etched during the process of etching.
EtchingIn this step the material is finally removed. The workpiece is immersed in the enchant
where the material of workpiece having no protective coating is removed by the chemical action
of enchant. Enchant is selected depending on the workpiece material and rate of material
removal; and surface finish required. There is a necessity to ensure that maskant and enchant
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should be chemically in active. Common enchants are H 2SO 4, FeCL 3, HNO 3. Selection of
enchant also affects MRR. As in CHM process, MRR is indicated as penetration rates (mm/min).
Demasking
After the process is completed demasking is done. Demasking is an act of removing
maskent after machining.
3.1.4 Application of CHM
The application and working of CHM process are indicated in Figure 1, various
applications of CHM are discussed below.
Chemical Milling
It is widely used in aircraft industry. It is the preparation of complicated geometry on the
workpiece using CHM process.
Fig.1 Application and Working of CHM
Chemical Blanking
In this application cutting is done on sheet metal workpieces. Metal blanks can be cut
from very thin sheet metal, this cutting may not be possible by conventional methods.
Photochemical MachiningIt is used in metal working when close (tight) tolerances and intricate patterns are to be
made. This is used to produce intricate circuit designs on semiconductor wafers.
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3.1.5 Advantages of CHM
Advantages of CHM process are listed below:
(a) Low tooling cost.
(b) Multiple machining can be done on a workpiece simultaneously.
(c) No application of force so on risk of damage to delicate or low strength workpiece.
(d) Complicated shapes/patterns can be machined.
(e) Machining of hard and brittle material is possible.
3.1.6 Disadvantages and Limitations of CHM
(a) Slower process, very low MRR so high cost of operation.
(b) Small thickness of metal can be removed.
(c) Sharp corners cannot be prepared.
(d) Requires skilled operators.
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3.2 Electrochemical Machining (ECM)
Electrochemical machining (ECM) process uses electrical energy in combination with
chemical energy to remove the material of workpiece. This works on the principle of reverse of electroplating.
3.2.1 Working Principle of ECM
Electrochemical machining removes material of electrically conductor workpiece. The
workpiece is made anode of the setup and material is removed by anodic dissolution. Tool is
made cathode and kept in close proximity to the workpiece and current is passed through the
circuit. Both electrodes are immersed into the electrolyte solution. The working principle and
process details are shown in the Figure . This works on the basis of Faradays law of electrolysis.
The cavity machined is the mirror image of the tool. MRR in this process can easily be
calculated according to Faradays law.
3.2.2 Process Details
Process details of ECM are shown in Figure.2 and described as below :
Fig.2 Working Principle and Process Details of ECM
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Workpiece
Workpiece is made anode, electrolyte is pumped between workpiece and the tool.
Material of workpiece is removed by anodic dissolution. Only electrically conducting materials
can be processed by ECM.
Tool
A specially designed and shaped tool is used for ECM, which forms cathode in the ECM
setup. The tool is usually made of copper, brass, stainless steel, and it is a mirror image of the
desired machined cavity. Proper allowances are given in the tool size to get the dimensional
accuracy of the machined surface.
Power Supply
DC power source should be used to supply the current. Tool is connected with the
negative terminal and workpiece with the positive terminal of the power source. Power supply
supplies low voltage (3 to 4 volts) and high current to the circuit.
Electrolyte
Water is used as base of electrolyte in ECM. Normally water soluble NaCl and NaNO3
are used as electrolyte. Electrolyte facilitates are carrier of dissolved workpiece material. It is
recycled by a pump after filtration.
Tool Feed Mechanism
Servo motor is used to feed the tool to the machining zone. It is necessary to maintain aconstant gap between the workpiece and tool so tool feed rate is kept accordingly while
machining.
In addition to the above whole process is carried out in a tank filled with electrolyte. The tank is
made of transparent plastic which should be non-reactive to the electrolyte. Connecting wires are
required to connect electrodes to the power supply.
3.2.3 Process characteristics
As shown in Fig., ECM relies mainly on the ECD phase that occurs by the movement of
ions between the cathodic tool and the anodic workpiece.
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Fig.3 ECM process components
Process Parameters
Power Supply
Type direct current
Voltage 2 to 35 V
Current 50 to 40,000 A
Current density 0.1 A/mm 2 to 5 A/mm 2
Electrolyte
Material NaCl and NaNO3
Temperature 20oC 50
oC
Flow rate 20 lpm per 100 A current
Pressure 0.5 to 20 bar
Dilution 100 g/l to 500 g/l
Working gap 0.1 mm to 2 mm
Overcut 0.2 mm to 3 mm
Feed rate 0.5 mm/min to 15 mm/min
Electrode material Copper, brass, bronze
Surface roughness, Ra
0.2 to 1.5 m
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3.2.4 Mechanics of material removal rate
Material removal rate (MRR) is an important characteristic to evaluate efficiency of a
non-traditional machining process.
In ECM, material removal takes place due to atomic dissolution of work material.
Electrochemical dissolution is governed by Faradays laws.
The first law states that the amount of electrochemical dissolution or deposition is
proportional to amount of charge passed through the electrochemical cell, which may be
expressed as:
M Q ,
where m = mass of material dissolved or deposited
Q = amount of charge passed
The second law states that the amount of material deposited or dissolved further depends
on Electrochemical Equivalence (ECE) of the material that is again the ratio atomic weigh and
valency. Thus
The engineering materials are quite often alloys rather than element consisting of
different elements in a given proportion.
Let us assume there are n elements in an alloy. The atomic weights are given as A1, A
2,
.., An
with valency during electrochemical dissolution as 1,
2, ,
n. The weight
percentages of different elements are 1,
2, ..,
n(in decimal fraction)
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Now for passing a current of I for a time t, the mass of material dissolved for any element
i is given by
where a
is the total volume of alloy dissolved. Each element present in the alloy takes a
certain amount of charge to dissolve.
The total charge passed
3.2.5 Tool Design
The design of a suitable tool for a desired workpiece shape, and dimension forms a major
problem. The workpiece shape is expected to be greater than the tool size by an oversize. In
determining
the geometry of the tool to be used under steady-state conditions, many variables should be
specified in advance such as the required shape of the surface to be machined, tool feed rate, gap
voltage, electrochemical machinability of the work material, electrolyte conductivity, and anodic
and cathodic polarization voltages. With computer integrated manufacturing, cathodes are
produced at a lower cost and greater accuracy. Computer-aided design (CAD) systems are used
first to design a cathodic tool. This design is programmed for CNC production by milling and
turning. After ECM, the coordinate measuring machine inspects the workpiece produced by this
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tool and sends data back to the CAD computer- aided manufacturing (CAM) unit for analysis of
the results.
Iterations of the cathodic tool are made so that the optimum tool design is selected. The
material used for ECM tools should be electrically conductive and easily machinable to the
required geometry. The various materials used for this purpose include copper, brass, stainless
steel, titanium, and copper tungsten. Tool insulation controls the side electrolyzing current and
hence the amount of oversize. Spraying or dipping is generally the simplest method of applying
insulation. Durable insulation should
ensure a high electrical receptivity, uniformity, smoothness, heat resistance, chemical resistance
to the electrolyte, low water absorption, and resistance against wear in the machine guides and
fixtures. Teflon, urethane, phenol, epoxy, and powder coatings are commonly used for tool
insulation.3.2.6 Accuracy of ECM
A small gap width represents a high degree of process accuracy. As can be seen in Fig.4,
the accuracy of machined parts depends on the current density, which is affected by
1. Material equivalent and gap voltage
2. Feed rate and gap phenomena including passivation
3. Electrolyte properties including rate, pH, temperature, concentration, pressure, type,
and velocity
For high process accuracy, machining conditions leading to narrow machining gaps are
recommended. These include use of
1. A high feed rate
2. High-conductivity electrolytes
3. Passivating electrolytes, such as NaNO3, that have a low throwing power
4. Tool insulation that limits the side-machining action
3.2.7 Surface finish
According to Konig and Lindelauf (1973), considerable variations in surface finish occur
due to the workpiece characteristics and machining conditions. Crystallographic irregularities,
such as voids, dislocation and grain boundaries, differing crystal structures and orientation, and
locally different alloy compositions produce an irregular distribution of current density, thus
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leaving the microscopic peaks and valleys that form the surface roughness. The mechanism of
surface formation can be understood using Fig.4, which shows the effect of machining feed rate
Fig.4 Parameters affecting ECM accuracy, surface quality, and productivity
on the local gap width for an alloy containing two elements X and Y. Accordingly, due to the
difference in their machining rates and their corresponding gap width, the generated maximum
peak-to-valley surface roughness Rt decreases at higher feed rates, and thus better surfaces are
expected at these higher rates.
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Fig.5 Surface roughness generation in ECM
The improvement of surface quality, at higher machining currents, shown in Fig.5 was
related to the formation of a loose salt layer, which results in a more even distribution of the
current density and hence a better surface finish. More fine grained and homogenous structures
produce a better surface quality. The roughness obtained with the larger grain size of theannealed carbon steel (Fig.5), was possibly due to the reduced number of grain boundaries
present on such a surface.
3.2.8 Applications of ECM Process
There are large numbers of applications of ECMs some other related machining and
finishing processes as described below:
(a) Electrochemical Grinding: This can also be named as electrochemical debrruing.
This is used for anodic dissolution of burrs or roughness a surface to make it smooth. Any
conducting material can be machined by this process. The quality of finish largely depends on
the quality of finish of the tool.
(b) This is applied in internal finishing of surgical needles and also for their sharpening.
(c) Machining of hard, brittle, heat resistant materials without any problem.
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(d) Drilling of small and deeper holes with very good quality of internal surface finish.
(e) Machining of cavities and holes of complicated and irregular shapes.
(f) It is used for making inclined and blind holes and finishing of conventionally
machined surfaces.
3.2.9 Advantages of ECM Process
Following are the advantages of ECM process:
(a) Machining of hard and brittle material is possible with good quality of surface finish
and dimensional accuracy.
(b) Complex shapes can also be easily machined.
(c) There is almost negligible tool wear so cost of tool making is only one time
investment for mass production.
(d) There is no application of force, no direct contact between tool and work and no
application of heat so there is no scope of mechanical and thermal residual stresses in the
workpiece.
(e) Very close tolerances can be obtained.
3.2.10 Disadvantages and Limitations of ECM
There are some disadvantages and limitations of ECM process as listed below:
(a) All electricity non-conducting materials cannot be machined.
(b) Total material and workpiece material should be chemically stable with the electrolyte
solution.
(c) Designing and making tool is difficult but its life is long so recommended only formass production.
(d) Accurate feed rate of tool is required to be maintained.
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3.3 Electrochemical Grinding3.3.1 Introduction
Electrochemical grinding (ECG) utilizes a negatively charged abrasive grinding wheel,
electrolyte solution, and a positively charged workpiece, as shown in Fig. 6. The process is,therefore, similar to ECM except that the cathode is a specially constructed grinding wheel
instead of a cathodic shaped tool like the contour to be machined by ECM. The insulating
abrasive material (diamond or aluminum oxide) of the grinding wheel is set in a conductive
bonding material. In ECG, the nonconducting abrasive particles act as a spacer between the
wheel conductive bond and the anodic workpiece. Depending on the grain size of these particles,
a constant interelectrode gap (0.025 mm or less) through which the electrolyte is flushed can be
maintained.
Fig.6 Surface ECG
The abrasives continuously remove the machining products from the working area. In the
machining system shown in Fig.7, the wheel is a rotating cathodic tool with abrasive particles
(60 320 grit number) on its periphery. Electrolyte flow, usually NaNO3, is provided for ECD.
The wheel rotates at a surface speed of 20 to 35 m/s, while current ratings are from 50 to 300 A.
3.3.2 Material removal rate
When a gap voltage of 4 to 40 V is applied between the cathodic grinding wheel and the
anodic workpiece, a current density of about 120 to 240 A/cm 2 is created. The current density
depends on the material being machined, the gap width, and the applied voltage. Material is
mainly removed by ECD, while the MA of the abrasive grits accounts for an additional 5 to 10
percent of the total material removal.
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Fig.7 ECG machining system components
Removal rates by ECG are 4 times faster than by conventional grinding, and ECG always
produces burr-free parts that are unstressed. The volumetric removal rate (VRR) is typically
1600 mm3/min. McGeough (1988) and Brown (1998) claimed that to obtain the maximum
removal rate, the grinding area should be as large as possible to draw greater machining current,
which affects the ECD phase.
3.3.3 Accuracy and surface quality
Traditional grinding removes metal by abrasion, leaving tolerances of about 0.003 mm
and creating heat and stresses that make grinding thin stock very difficult. In ECG however a
production tolerance of 0.025 mm is easily obtainable. Under special circumstances a tolerance
of 0.008 mm can be achieved. The ability to hold closer tolerances depends upon the current,
electrolyte flow, feed rate, and metallurgy of the workpiece itself. Accuracies achieved are
usually about 0.125 mm. A final cut is usually done mostly by the grinding action to produce a
good surface finish and closer dimensional tolerances. It is recommended that lower voltages be
used for closer tolerances, reduced overcut, sharp edges, and a bright surface finish. ECG can
grind thin material of 1.02 mm, which normally warp by the heat and pressure of the
conventional grinding thus making closer tolerances difficult to achieve. In ECG there is little
contact between the wheel and workpiece, which eliminates the tendency of the workpiece to
warp as it might with orthodox grinding (Brown, 1998).
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The main drawback of ECG is the loss of accuracy when the inside corners are ground.
Because of the electric field effect, radii better than 0.25 to 0.375 mm can seldom be achieved.
The reason for this problem is that the point of highest pressure of the electrolyte is the wheel
corner. However, high-speed grinding benefits both inside and outside corners.
3.3.4 Applications
The ECG process is particularly effective for
1. Machining parts made from difficult-to-cut materials, such as sintered carbides, creep-
resisting (Inconel, Nimonic) alloys, titanium alloys, and metallic composites.
2. Applications similar to milling, grinding, cutting off, sawing, and tool and cutter
sharpening.
3. Production of tungsten carbide cutting tools, fragile parts, and thin walled tubes.4. Removal of fatigue cracks from steel structures under seawater. In such an application
holes about 25 mm in diameter, in steel 12 to 25 mm thick, have been produced by ECG at the
ends of fatigue
cracks to stop further development of the cracks and to enable the removal of specimens for
metallurgical inspection.
5. Producing specimens for metal fatigue and tensile tests.
6. Machining of carbides and a variety of high-strength alloys. The process is not adapted
to cavity sinking, and therefore it is unsuitable for the die-making industry.
3.3.5 Advantages and disadvantages
Advantages
_ Absence of work hardening
_ Elimination of grinding burrs
_ Absence of distortion of thin fragile or thermosensitive parts
_ Good surface quality
_ Production of narrow tolerances
_ Longer grinding wheel life
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Disadvantages
_ Higher capital cost than conventional machines
_ Process limited to electrically conductive materials
_ Corrosive nature of electrolyte
_ Requires disposal and filtering of electrolyte
3.4. Electrochemical Honing
3.4.1 Introduction
Electrochemical honing (ECH) combines the high removal characteristics of ECD and
MA of conventional honing. The process has much higher removal rates than either conventional
honing or internal cylindrical grinding. In ECH the cathodic tool is similar to the conventional
honing tool, with several rows of small holes to enable the electrolyte to be introduced directly to
the interelectrode gap. The electrolyte provides electrons through the ionization process, acts as a
coolant, and flushes away chips that are sheared off by MA and metal sludge that results from
ECD action. The majority of material is removed by the ECD phase, while the abrading stones
remove enough metal to generate a round, straight, geometrically true cylinder.
During machining, the MA removes the surface oxides that are formed on the work
surface by the dissolution process. The removal of such oxides enhances further the ECD phase
as it presents a fresh surface for further electrolytic dissolution. Sodium nitrate solution (240 g/L)is used instead of the more corrosive sodium chloride (120g/L) or acid electrolytes. An
electrolyte temperature of 38C, pressure of 1000 kPa, and flow rate of 95 L/min can be used.
ECH employs dc current at a gap voltage of 6 to 30 V, which ensures a current density of 465
A/cm 2 . Improper electrolyte distribution in the machining gap may lead to geometrical errors in
the produced bore.
3.4.2 Process characteristics
The machining system shown in Fig.8 employs a reciprocating abrasive stone (with
metallic bond) carried on a spindle, which is made cathodic and separated from the workpiece by
a rapidly flowing electrolyte. In such an arrangement, the abrasive stones are used to maintain
the gap size of 0.076 to 0.250 mm and, moreover, depassivate the machining surface due to the
ECD phase occurring through the bond. A different tooling system (Fig.9) can be used where the
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cathodic tool carries nonconductive honing sticks that are responsible for the MA. The machine
spindle that rotates and reciprocates is responsible for the ECD process. The material removal
rate for ECH is 3 to 5 times faster than that of conventional honing and 4 times faster than that of
internal cylindrical grinding. Tolerances in the range of 0.003 mm are achievable, while surface
roughnesses in the range of 0.2 to 0.8 m Ra are possible. To control the surface roughness, MA
is allowed to continue for a few seconds after the current has been turned off. Such a method
leaves a light compressive residual stress in the surface. The surface finish generated by the ECH
process is the conventional cross-hatched cut surface that is accepted and used for sealing and
load-bearing surfaces. However, for stress-free surfaces and geometrically accurate bores, the
last few seconds of MAaction should be allowed for the pure ECD process.
Fig.8 ECH schematic
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Fig.9 ECH machining system components.
3.4.3 Applications
As a result of the rotating and reciprocating honing motions, the process markedly
reduces the errors in roundness through the rotary motion. Moreover, through tool reciprocation
both taper and waviness errors are also reduced as shown in Fig.10. Because of the light stone
pressure used, heat distortion is avoided. The presence of the ECD phase introduces no stresses
and automatically deburrs the part. ECH can be used for hard and conductive materials that are
susceptible to heat and distortion. The process can tackle pinion gears of high-alloy steel as well
as holes in cast tool steel components. Hone forming (HF) is an application that combines the
honing and electro deposition processes. It is used to simultaneously abrade the work surface and
deposit metal. In some of its basic principles the method is the reversal of ECH.
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Fig.10 ECH effects on bore errors.
According to the Metals Handbook (1989), this method is used in case of salvaging parts that
became out-of-tolerance and reconditioning worn surfaces by metal deposition and abrasion of
the new deposited layers.
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3.5 Electrochemical Deburring
When machining metal components, it is necessary to cross-drill holes to interconnect
bores. Hydraulic valve bodies are a typical example where many drilled passages are used todirect the fluid flow. The intersection of these bores creates burrs, which must be removed (Fig.
11) to avoid the possibility of them breaking off and severely damaging the system.
Fig.11 Burrs formed at intersections of holes
Figure 12 shows conventionally cut parts that require deburring. Manual removal of burrs
is tedious and time-consuming. In the 1970s the thermal energy method (TEM) was introduced
to remove burrs in hard-to-reach places.
Fig.12 Different components that require deburring
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In this method, burrs are hit by 2760C blast of heat for milliseconds, which burns them
away, leaving everything else including threads, dimensions, surface finish, and the physical
properties of the part intact. Parts subjected to TEM should be cleaned of oil and metal chips to
avoid the formation of carbon smut or the vaporization of chips.
Burrs can be removed using several other methods including vibratory and barrel
finishing, tumbling, water blasting, and the applicationof ultrasound and abrasive slurry.
Abrasive flow machining (AFM) provides a reliable and accurate method of deburring for the
aerospace and medical industries. AFM can reach inaccessible areas and machine multiple holes,
slots, or edges in one operation. It was originally devised in the 1950s for deburring of hydraulic
valve spools and bodies and polishing of extrusion dies. The drawbacks of these methods include
lack of reliability, low metal removal rates, and contamination of surfaces with grit.
Fig.13 Hole deburring.
In electrochemical deburring (ECDB), the anodic part to be deburred is placed in a
fixture, which positions the cathodic electrode in close proximity to the burrs. The electrolyte is
then directed, under pressure, to the gap between the cathodic deburring tool and the burr. On the
application of the machining current, the burr dissolves forming a controlled radius. Since the
gap between the burr and the electrode is minimal, burrs are removed at high current densities.
ECDB, therefore, changes the dimensions of the part by removing burrs leaving a controlled
radius. Figure 8 shows a typical EC hole deburring arrangement. ECDB can be applied to gears,
spline shafts, milled components, drilled holes, and punched blanks. The process is particularly
efficient for hydraulic system components such as spools, and sleeves of fluid distributors.
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3.5.1 Mechanism of Deburring
Faradays laws of electrolysis dictate how the metal is removed by ECDB. The deburring
speed may be as high as 400 to 500 mm/min. ECDB using a rotating and feeding tool electrode
(Fig.9) enhances the deburring process by creating turbulent flow in the inter electrode gap. The
spindle rotation is reversed to increase the electrolyte turbulence. Normal cycle times for
deburring reported by Brown (1998) are between 30 to 45 s after which the spindle is retracted
and the part is removed. In simple deburring when the tool is placed over the workpiece, a burr
height of 0.5 mm can be removed to a radius of 0.05 to 0.2 mm leaving a maximum surface
roughness of 2 to 4 m.
Fig.14 Electrochemical deburring using a rotating tool.
When burrs are removed from intersections of passages in housing, the electrolyte is
directed and maintained under a pressure of 0.3 to 0.5 MPa using a special tool. That tool has as
many working areas as practical so that several intersections are deburred at a time. Proper toolinsulation guarantees the flow of current in areas nearby the burr. The deburring tool should also
have a similar contour of the work part thus leaving a 0.1 to 0.3 mm inter electrode gap.
Moreover the tool tip should overlap the machined area by 1.5 to 2 mm in order to produce a
proper radius. The choice of the electrolyte plays an important role in the deburring process.
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Table.1 Different electrolyte and the operating conditions for ECDB
Table 1 presents different electrolytes and the operating conditions for ECDB of some
materials. ECDB power units supply a maximum current of 50 A. However, power units having500 Aare used to remove burrs generated by turning and facing operations on large forged parts.
Fig.10 shows an EC deburring application.
Fig.15 EC Deburring Applications
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3.5.2 Advantages
_ Elimination of costly hand deburring
_ Increase of product quality and reliability
_ Ensures the removal of burrs at the required accuracy, uniformity,proper radius, and
clean edge
_ Reduced personnel and labor cost
_ Can be automated for higher productivity
3.6 Simple Problems on Material Removal Rate
1. In electrochemical machining of pure iron a material removal rate of 600 mm3 /min is
required. Estimate current requirement.
Solution:
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2. Composition of a Nickel superalloy is as follows: Ni = 70.0%, Cr = 20.0%, Fe = 5.0% and
rest Titanium. Calculate rate of dissolution if the area of the tool is 1500 mm2
and a current
of 2000 A is being passed through the cell. Assume dissolution to take place at lowest
valency of the elements.
Solution: