Uncinventional Machining Process Unit 3

7/31/2019 Uncinventional Machining Process Unit 3

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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:

Uncinventional Machining Process Unit 3

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