Response Surface Modeling of Electric Discharge Machining ... · experimental data. The table 1...
Transcript of Response Surface Modeling of Electric Discharge Machining ... · experimental data. The table 1...
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: [email protected]
2Department of Mechanical Engineering, Roever Engineering College,Perambalur –621212,Tamilnadu, India.E-mail: [email protected]
3Department of Mechanical Engineering, SaranathanCollege of Engineering, Tiruchirappalli – 620012, Tamilnadu, India. E-mail: [email protected]
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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