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Lesson

Electro Chemical

Machining

Instructional Objectives

(i) Identify electro-chemical machining (ECM) as a particular type of

non-tradition processes (ii) Describe the basic working principle of ECM process

(iii) Draw schematically the basics of ECM

(iv) Draw the tool potential drop (v) Describe material removal mechanism in ECM

(vi) Identify the process parameters in ECM

(vii) Develop models for material removal rate in ECM (viii) Analyse the dynamics of ECM process

(ix) Identify different modules of ECM equipment

(x) List four application of ECM (xi) Draw schematics of four such ECM applications

1. Introduction

Electrochemical Machining (ECM) is a non-traditional machining (NTM) process

belonging to Electrochemical category. ECM is opposite of electrochemical or

galvanic coating or deposition process. Thus ECM can be thought of a controlled

anodic dissolution at atomic level of the work piece that is electrically conductive by a

shaped tool due to flow of high current at relatively low potential difference through

an electrolyte which is quite often water based neutral salt solution.

Fig. 1 schematically shows the basic principle of ECM.

In ECM, the workpiece is connected to the positive terminal of a low voltage high

current DC generator or power source. The tool is shaped and shape of the tool is

transferred to the workpiece. The tool is connected to the negative terminal.

Machining takes place due to anodic dissolution at atomic level of the work material

due to electrochemical reaction. A gap between the tool and the workpiece is

required to be maintained to proceed with steady state machining.

W

O

R

K

T

O

O

L

W

O

R

K

T

O

O

L

electrolyte

Initial stage of

ECM

Steady state of

ECM

Fig. 1 Schematic principle of Electro Chemical Machining (ECM)

2. Process

During ECM, there will be reactions occurring at the electrodes i.e. at the anode or

workpiece and at the cathode or the tool along with within the electrolyte.

Let us take an example of machining of low carbon steel which is primarily a ferrous

alloy mainly containing iron. For electrochemical machining of steel, generally a

neutral salt solution of sodium chloride (NaCl) is taken as the electrolyte. The

electrolyte and water undergoes ionic dissociation as shown below as potential

difference is applied

NaCl ↔ Na

• Cl

H 2 O ↔ H

• (OH)

Anodic

overvoltage

Anode potential

Activation over potential

ohmic potential

concentration potential

concentration potential

Ohmic drop

activation overpotential

anode cathode

cathodic potential

cathodic overpotential

Voltage

Voltage

Fig. 3 Total potential drop in ECM cell

3. Equipment

The electrochemical machining system has the following modules:

• Power supply
• Electrolyte filtration and delivery system
• Tool feed system
• Working tank

Fig. 4 schematically shows an electrochemical drilling unit.

Flow control

valve

Pressure

relief valve

Pump

Filters

sludge

centrifuge

Spent electrolyte

Tool

Flow meter

Pressure

gauge

Constant feed to

the tool

Low voltage

high current

power supply

+ve

-ve

PS

O M

M O

P

Fig. 4 Schematic diagram of an electrochemical drilling unit

4. Modelling 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 Faraday’s 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

of the atomic weigh and valency. Thust

ν

α α

A m ECE

Thus

QA

m

where F = Faraday’s constant

= 96500 coulombs

F ν

ItA ∴ m =

ρ Fρν

IA

t

m ∴ MRR = =

where I = current

ρ= density of the material

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 A 1 ,

A 2 , ………….., An with valency during electrochemical dissolution as ν 1 , ν2, …………, νn.

The weight percentages of different elements are α 1 , α 2 , ………….., αn (in decimal

fraction)

Now for passing a current of I for a time t, the mass of material dissolved for any

element ‘i’ is given by

mi= Γa ρα i

where Γa is the total volume of alloy dissolved. Each element present in the alloy

takes a certain amount of charge to dissolve.

i

i i i F

QA

m

i

i i i A

Fm Q

i

a i i i A

F

Q

The total charge passed

rh

V

A

F

x

x

for a given potential difference and alloy

h

c

h

F r

A V

dt

dh

x

x = =

where c = constant

F r

A V

x

x

i

i i

A

F r

V

c

h

c

dt

dh ∴ =

hdh = cdt

At t = 0, h = ho and at t = t 1 and h = h 1

h 1

ho

t

0

hdh c dt

h h 2 ct

2 o

2 ∴ 1 − =

That is the tool – workpiece gap under zero feed condition grows gradually following

a parabolic curve as shown in Fig. 6

h

t

ho

Fig. 6 Variation of tool-workpiece gap under zero feed condition

As

h

c

dt

dh

Thus dissolution would gradually decrease with increase in gap as the potential drop

across the electrolyte would increase

Now generally in ECM a feed (f) is given to the tool

f h

c

dt

dh ∴ = −

Now if the feed rate is high as compared to rate of dissolution, then after sometime

the gap would diminish and may even lead to short circuiting. Under steady state

condition the gap is uniform i.e. the approach of the tool is compensated by

dissolution of the work material. Thus with respect to the tool, the workpiece is not

moving

Thus f

h

c 0 dt

dh = = =

h

c ∴ f =

or h* = steady state gap = c/f

Now under practical ECM condition it is not possible to set exactly the value of h* as

the initial gap. Thus it is required to be analysed if the initial gap value would have

any effect on progress of the process

Now (^) f

h

c

dt

dh = −

Now

c

hf

h*

h h' = =

And c

f t

h*

ft t'

2

= =

dt

dh . f

dt

dh . f /c

f/c

dt'

dh' 2

Thus f h

c

dt

dh = −

f h'c

cf f h'h*

c

dt'

dh' ⇒ f = − = −

h'

1 h' f dt'

dh' f

h '

1 h'

dt'

dh' − ⇒ =

dh ' 1 h'

h' dt ' −

Now integrating between t’ = 0 to t’ = t’ when h’ changes from ho’ to h 1 ’

dh ' 1 h'

h' dt'

t '

0

h 1 '

ho'

∫ + ∫ (^ − )

h 1 '

ho'

h 1 '

ho'

d 1 h ' 1 h'

d 1 h' t'

h 1

h 1 t' h h ln ' 1

' ' o 1

' o −

now for different value of ho’, h 1 ’ seems to approach 1 as shown in Fig. 7

as ECM leads to atomic level dissolution, the surface finish is excellent with almost

stress free machined surface and without any thermal damage.

ECM is used for

• Die sinking
• Profiling and contouring
• Trepanning
• Grinding
• Drilling
• Micro-machining

Die sinking

3D profiling

Work

Tool

Fig. 8 Different applications of Electro Chemical Machining

drilling

(drilling)

work

tool

trepanning

tool

work

Fig. 9 Drilling and Trepanning by ECM

7. 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 NaNO 3

Temperature 20

o C – 50

o C

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 μm

Answers

Answers to Quiz Test

1 – (b) 2 – (b)

3 – (c)

4 – (a)

Solution to Prob. 1

F ν

AI

t

m MRR m

. = = =

ρ ρν

Γ F

AI

t

m MRR

.

∴ = = =

MRR = 600 mm

3 /min = 600/60 mm

3 /s = 10 mm

3 /s = 10x

• cc/s

96500 x 7. 8 x 2

56 xI 10 x 10

3 ∴ =

−

As AFe = 56

νFe = 2

F = 96500 coulomb

ρ = 7.8 gm/cc

96500 x 10 x 10 x 7. 8 x 2 I

− 3 ∴ =

I = 268.8 A Answer

Solution of Problem 2

Now,

i

i

alloy

Ti

Ti

Fe

Fe

Cr

Cr

Ni

Ni

8. 07 gm/ cc

Now

i

i i

A

F

I

t

m MRR

1. 05 x 3

1. 05 x 2

1. 2 x 2

1. 75 x 2 96500 x 8. 07 x

= 0.0356 cc/sec

= 2.14 cc/min

= 2140 mm

3 /min

1. 43 mm/min 1500

Area

MRR

∴ Rate ofdissolution= = = answer

Solution to Prob. 3

f

c h* =

where Fe Fe

Fe

F r

VA

c

96500 x 7. 8 x 10 x 50 x 2

V 2. 5 x 55. 85 C − 3

V − 2. 5

1347 x

V 2. 5

f

c h* 2

V 2. 5

∴ V = 8. 73 Volt. Answer

Version 2 ME, IIT Kharagpur

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