5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

150-1

Response Surface Modeling of Electric Discharge Machining Process

Parameters for EN 24 Low Alloy Steel.

N. Annamalai1+, V. Sivaramakrishnan2, N.Baskar3

1+Department of Mechanical Engineering, Mookambigai College of Engineering, Pudukkottai – 622502, Tamilnadu, India.E-mail: nannamalai1966@gmail.com

2Department of Mechanical Engineering, Roever Engineering College,Perambalur –621212,Tamilnadu, India.E-mail: vsmp1967@yahoo.com

3Department of Mechanical Engineering, SaranathanCollege of Engineering, Tiruchirappalli – 620012, Tamilnadu, India. E-mail: baskarnaresh@yahoo.co.in

ABSTRACT

Electric Discharge machining is used to produce complex shapes that would be difficult to produce in

conventional machine tools and also good surface finish can be obtained in EDM. The work material EN24 is

machined by using copper as electrode. The EN24 contains nickel, chromium and molybdenum and it is used in

automobile and aircraft transmission components .For this reason, the EN24 is experimentally investigated with the

machining parameters for achieving maximum MRR and minimum electrode wear rate and surface roughness. The

RSM is also used to identify the machining parameter responses on MRR, EWR and SR. The input parameters are

peak current, pulse on time and pulse off time. The experimental design is done using Box Behnken design of RSM.

Regression equations are formulated based on the experimental results. The effects of input parameters are analyzed

on MRR, EWR and SR. Keywords: EDM, Response surface methodology, Material removal rate, Regression analysis

1. Introduction Electric Spark erosion is the methodology used to

control the metal removal process in Electrical

Discharge Machining. Very tiny amount of material is

removed from the work piece. It is immersed in

dielectric fluid and the pulse generator creates electric

spark between the work piece and electrode tool. Heat

resistant steels, super alloys, carbides, heat treated tool

steels, composites and ceramics which are difficult to

machine can be machined to attain geometrically

complex shape in EDM for which the process

parameters must be optimized. Optimum process

parameters are to be followed while machining in EDM

to obtain maximum Material Removal Rate (MRR),

minimum Electrode Wear Rate (EWR) and Surface

Roughness (SR). In the heat treated condition EN24 is

capable of developing high strength. Its toughness and

retaining good fatigue strength is commendable.

Previously few researchers have considered

optimizing material removal rate, tool wear and overcut

with Taguchi methodology but not surface roughness

[7]. So this work considers MRR and surface

roughness. MRR is more influenced by duty factor and

peak current while machining AISI 4140 grade alloy

steel in EDM [1]. Pulse on time, duty cycle, peak

current and concentration of the silicon powder added

into the dielectric fluid of EDM are used as process

parameters to study the process performance in terms of

material removal rate and surface roughness [1].The

debris evacuation efficiency and low work piece

conductivity posed a challenge of low material removal

rate and using design of experiments peak current, duty

ratio, gap voltage and pulse duration were studied and

analysis of variance was conducted to achieve higher

MRR [8].

Matrix nano composite of Al 7075 reinforced with 0.5

in weight%SiC nano particles is machined in electrical

discharge machining with copper electrode using face-

centered central composite design of response surface

methodology and mathematical model was developed

for MRR, EWR and SR.The experimental values fitted

with a 95% confidence interval [9].The effects of

discharge current, pulse ontime, duty factor and open

discharge voltage were used to analyze the performance

characteristics of material removal rate, electrode wear

rate and surface roughness in the EDM process of

Al2O3+TiC mixed ceramic[10].

Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

150-2

2. Methodology Box Behnken Design of Response Surface

Methodology is used to model the equation.

Experimental investigation is done through design of

experiments and the parameter influence and interaction

effect on peak current, pulse on time and pulse off time

are examined.

2.1 Work Piece and Electrode Material EN 24 is the work material which is a low alloy

medium carbon steel used for large size parts which

requires high strength and toughness. The electrode

material is copper.

2.2 Characteristics This nickel-chromium-molybdenum alloy possesses

increased ductility and toughness and much deeper

harden ability. EN 24 is ideal for all highly stressed

parts in the most severe conditions because of its high

fatigue strength. It has good wear resistance and used in

both elevated and low temperature environments.

Typical applications include aircraft landing gear,

power transmission gears and shafts and other structural

parts, high strength machine parts, heavy-duty shafting,

high tensile bolts and studs, gears, axle shafts,

crankshafts, boring bars and down-hole drilling

components.

2.3 Experimental Setup Elektra Puls SE35 ZNC Electric Discharge machine

manufactured by Electronica Machine Tools is the

machine used to carry out this experiment. The input

parameters considered for process optimization are Ton,

Pulse on time, Ip,Peak current and Toff, Pulse off time.

Weight of work piece and electrode is measured using

electronic weighing scale before and after machining to

measure material removal rate and electrode wear rate.

Mathematical models are developed on the basis of

experimental data. The table 1 shows the EDM input

parameters and their levels.

Table 1: Input parameters and their level

Param

eter

Descriptio

n Unit

Level

1

Level

2

Level

3

Ip

Peak

current amp 3 6 10

Ton

Pulse on

time µsec 20 50 100

Toff

Pulse off

time µsec 150 200 250

The other standard settings are work time potentiometer

Tw for the sparking amplitude, antiarc sensitivity pot

duty cycle, gap control potentiometer and sensitivity pot

to vary the speed of Z axis. The die electric used is

clean kerosene. Through pressure flushing kerosene

under pressure of 0.80 kg/cm2 is admitted in the vicinity

of the spark area and the debris is carried away. The

response surface methodology is analyzed to maximize

MRR and to minimize SR.

2.4 Measurement Procedure An electronic weighing scale is used to measure the

weight of work piece before and after trial. The digital

timer is used to measure the period of trial in minutes.

SURFCORDER, a surface roughness measuring

instrument is used to measure the surface roughness Ra

in terms of µM.

2.4.1Measurement of MRR

( )xt

xMRR

ww fi

8.7

1000−=

min

3mm

Where Wi = weight of work piece in grams

before trial

Wf= weight of work piece in grams after

trial

t = period of trial in minutes

7.8 = Density of steel in gms/cc

2.4.2Measurement of Surface Finish The surface finish is measured on a surface test

recorder, SURFCORDER of Kosaka Laboratory

Ltd,Japan.

Stroke length: 4 mm

Stylus Speed: 2 mm/sec

Cut – off value: 0.8 mm

Arithmatic mean of Surface Roughness Ra is recorded in

terms of µM.

3. Mathematical Modeling

Mathematical models are developed on the basis of

experimental data. The experimental planning is done

based on Design of Experiments. The Box Behnken

design was used to find the quadratic response surfaces

to construct the second order polynomial models.

Design of Experiments (DOE) is a method used to

obtain useful information about a process by conducting

only minimum number of experiments. Each controllable variable (Ton, Ip, Toff) can be set

on EDM machine at three consecutive levels and hence

the design consisting of 17 experiments based on box

behnken design was generated as shown in fig-1

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

150-3

As per the box behnken design of response surface

methodology for 17 runs the various input parameters

and the results obtained are shown in table 2.

Regression analysis for MRR indicates that the

individual and higher order effects of variablessuch as

Ip, Toff and higher order effect of tonand Ip have

significant contributions in MRR model since these P-

values are less than 0.05.

Regression analysis for Electrode Wear Rate

indicates that the individual and higher order effects of

variablessuch as Ton,Ip and higher order effect of Tonand

Ip and interactive terms of Tonand Ip have significant

contributions in Electrode Wear Rate model since these

P-values are less than 0.05. Regression analysis for Surface Roughness

indicates that the individual and higher order effects of

variablessuch as Ton,Ip and higher order effect of Tonand

Ip and interactive terms of Tonand Ip have significant

contributions in Surface Roughness model since these

P-values are less than 0.05.

Figure -1 Work piece after experiment

Theequationforcalculatingthe approximateMRRis

T

ITTITT

ITTIT

off

ponoffpoffon

ponoffpon

xE

xxExxExxE

xxExxxMRR

2

22

00454043.1

15895.000498260.200376239.100570741.4

00391488.2065230.084774.2032718.005567.13

−−

−−−−−−−

−++++−=

Theequation for calculating the approximate Electrode Wear Rate is

T

ITTITT

ITTIT

off

ponoffpoffon

ponoffpon

xE

xxExxExxE

xxExxxEWR

2

22

00554016.6

054038.000512159.500467736.100509994.4

00338117.1018249.082753.0003-6.26448E-02708.3

−−

−−−−+−+

−+++−=

The equation for calculating the approximate Surface Roughness is

T

ITTITT

ITTIT

off

ponoffpoffon

ponoffpon

xE

xxExxExxE

xxExExxSR

2

22

00553903.2

12854.000440737.300491892.300548665.2

00363971.400381413.986190.1027995.018238.5

−−

−−−−+−+

−+−+++−=

4. Results and Discussions Pulse on time,Peak current and Pulse off time are

the three parameters which are used as conrolling

parameters.Its effect of Material removal rate,Electrode

wear rate and Surface roughness are discussed.

4.1.1Effect of Peak Current on MRR : The fig-2 shown below indicates that MRR is 0.323

mm3/min when the peak curent is 3 amp.It increases to

4.931 mm3/min when the peak current is 6 amps.It

reaches 5.222 mm3/min when the peak current is

increased to 10 amps.The pulse off time is kept constant

as 200 µsec.

Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

150-4

Figure -2 Effect of Peak Current on MRR

4.1.2 Effect of Pulse on time on MRR: The MRR increases from 0.2585 to 5.2222

mm3/min when the pulse on time increases from 20

µsec to 100 µsec. Fig-3 shown below indicates the

change in MRR when the peak current is kept constant

at 6.0 amp.

Figure -3 Effect of Pulse on Timeon MRR

4.1.3Effect of Pulse off time on MRR: There is no much changes on MRR when the pulse

off time increases from 150 µsec to 250 µsec. Fig-

4shown below indicates the change in MRR when the

pulse on time is kept constant at 60 µsec.

Table 2: Plan of Experiments and Output Responses

Standard

order Run

PULSE ON

TIME

(µsec)

PEAK

CURRENT

(amp)

PULSE

OFF TIME

(µsec)

MRR

(mm3/min)

EWR

(mm3/min)

SR

(µM)

1 6 20 3 200 0.322996 0.176922 1.49

2 5 100 3 200 0.311954 0.136699 1.775

3 11 20 10 200 3.619058 1.585879 3.604

4 16 100 10 200 5.222205 2.288382 6.678

5 2 20 6 150 4.105849 1.799192 3.529

6 3 100 6 150 4.342257 1.902787 5.159

7 15 20 6 250 2.952335 0.64686 3.962

8 1 100 6 250 2.957442 1.295958 5.718

9 13 50 3 150 0.329195 0.180318 1.675

10 8 50 10 150 5.133338 1.68708 5.35

11 9 50 3 250 0.258504 0.141596 1.716

12 17 50 10 250 3.74595 1.641484 5.707

13 10 50 6 200 4.203447 1.84196 4.771

14 14 50 6 200 4.930966 1.08038 4.792

15 12 50 6 200 4.060337 1.334436 4.845

16 4 50 6 200 4.102564 1.797753 5.098

17 7 50 6 200 4.56246 1.99928 5.331

Figure -4 Effect of Pulse Off Time on MRR

4.2.1 Effect of Peak Current on EWR

The fig-5 shown below indicates that EWR is

0.1769 mm3/min when the peak curent is 3 amp.It

increases to 1.798 mm3/min when the peak current is 6

amps and reaches 2.288 mm3/min when the peak current

is increased to 10 amps .The Pulse Off Time is kept

constant as 200 µsec.

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

150-5

Figure -5 Effect of Peak Current on EWR

4.2.2 Effect of Pulse on time on EWR The EWR increases from 0.1367 to 2.2884 mm

3/min

when the pulse on time increases from 20 µsec to 100

µsec.The fig 6 shown below indicates the change in

EWR when the peak current is kept constant at 6.0 amp.

Figure -6 Effect of Pulse on Time on EWR

4.2.3 Effects on Pulse off time on EWR: There is no much changes on EWR when the pulse

off time increases from 150 µsec to 250 µsec.The fig-7

shown below indicates the change in EWR when the

pulse on time is kept constant at 60 µsec.

Figure -7 Effect of Pulse off Time on EWR

4.3.1 Effect ofPeak Current on SR The surface roughness is 1.49 µM when the

peak current is 3 amps, increases to 5.098 µM when the

peak current is 6 amps and finally reaches 6.678 µM

when the peak current is 10 amps.The fig 8 below

indicates the change in Surface Roughness while

keeping pulse off time in 200 µsec as constant.

Figure -8 Effect of Peak Current on Surface

Roughness

4.3.2 Effect of Pulse on time on SR The surface roughness increases from 1.49 µM to

6.678 µM when the pulse on time increases from 20

µsec to 100 µsec.The fig 9 shown below indicates the

change in surface roughness while keeping peak current

as constant in 6.0 amp.

Figure -9 Effect of Pulse on Time on Surface

Roughness

4.3.3Effect of Pulse off timeon SR The fig 10 shown below indicates that there is no

impact or change on surface roughness when the pulse

off time increases from 150 µsec to 250 µsec .

Figure -10 Effect of Pulse off Time on Surface

Roughness

Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

150-6

5. Conclusions The following conclusions are derived based on the

results and discussions done on machining EN24 in

EDM .

1. The effect on MRR improves when peak

current and pulse on time is increased whereas

there is no much impact when pulse off time is

increased.

2. The EWR increases when peak current and

pulse on time is increasedwhereas there is no

much change when pulse off time is increased.

3. The surface roughness increases when peak

current and pulse on time is increased whereas

there is no much impact when pulse off time is

increased.

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