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A study on influence of polarity on the machining characteristics of Sinker EDM

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A study on influence of polarity on the machining characteristics of Sinker EDM Powered By Docstoc
					  International Journal of Advanced Research OF ADVANCED RESEARCH IN
  INTERNATIONAL JOURNAL in Engineering and Technology (IJARET), ISSN
  0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME
             ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)                                                        IJARET
Volume 4, Issue 3, April 2013, pp. 158-162
© IAEME: www.iaeme.com/ijaret.asp
Journal Impact Factor (2013): 5.8376 (Calculated by GISI)
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      A STUDY ON INFLUENCE OF POLARITY ON THE MACHINING
                CHARACTERISTICS OF SINKER EDM

                     A. Parshuramulu1, K. Buschaiah2* and P. Laxminarayana3
                             1
                                 Asst. Professor, University College of Technology,
                                  OsmaniaUniversity, Hyderabad, A.P. – 500007
                     2*
                          Scientist, Department of Mechanical Engineering, University
                                 College of Engineering, Osmania University,
                                           Hyderabad, A.P. –500007.
                             3
                              Professor, Department of Mechanical Engineering,
                           University College of Engineering, Osmania University,
                                         Hyderabad, A.P. –500007.

   ABSTRACT

            Electrical discharge machining (EDM) has been recognized as an efficient production
  method for precision machining of electrically conducting hardened materials. Electrical Discharge
  Machining is a machining method primarily used for hard metals or those that would be impossible
  to machine with traditional techniques. One critical limitation, however, is that EDM only works with
  materials that are electrically conductive. Sometimes referred to as spark machining or spark eroding,
  EDM is a nontraditional method of removing material by a series of rapidly recurring electric arcing
  discharges between an electrode (the cutting tool) and the work piece, in the presence of an energetic
  electric field. The most important study of this paper is the effect of the polarity on the machining
  tool / work piece using electrical discharge machining to the material removal rate, electrode wear
  and surface roughness, to determine the optimum condition, and to determine the most significant
  factor.
           In this paper an elaborative methodology is suggested to choose option between electrode and
  work piece as terminal positive or terminal negative for different categories of tools and work pieces.
  The wrong polarity can have significant implications on wear, and stability. A set of experiments are
  conducted and the results are represented numerically and graphically.
           The most important output parameters are material removal rate, electrode wear and surface
  roughness. From the obtained data one can easily determine the optimum parameters and most
  significant factors related to material removal rate (MRR), electrode wear and surface finish very
  easily.

  Index Terms: Material removal rate (MRR), Straight and reverse polarity, Surface roughness.

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME

I. INTRODUCTION

         Electrical Discharge Machining (or EDM) is a machining method primarily used for hard
metals or those that would be impossible to machine with traditional techniques. One critical
limitation, however, is that EDM only works with materials that are electrically conductive. EDM can
cut small or odd-shaped angles, intricate contours or cavities in pre-hardened steel without the need
for heat treatment to soften and re-harden them as well as exotic metals such as titanium, hastelloy,
and inconel.
         The control parameters optimization for individual machining characteristic is concerned with
separately maximize the material removal rate, separately minimize the tool wear ratio and separately
obtained a good surface finish. There are many input parameters which can be varied in the EDM
process which have different effects on the EDM machining characteristics. Sometimes referred to as
spark machining or spark eroding, EDM is a nontraditional method of removing material by a series
of rapidly recurring electric arcing discharges between an electrode (the cutting tool) and the work
piece, in the presence of an energetic electric field. The EDM cutting tool is guided along the desired.
path very close to the work but it does not touch the piece [1]. Consecutive sparks produce a series of
micro-craters on the work piece and remove material along the cutting path by melting and
vaporization. The particles are washed away by the continuously flushing dielectric fluid. It is also
important to note that a similar micro-crater is formed on the surface of the electrode, the debris from
which must also be flushed away [2].

II. THEORY

         Experimental research generally targets regression analysis of process parameters and
modeling to optimize the process characteristics. This involves maximization of machining rate and
minimization of tool wear and surface roughness. This also helps in the development of adaptive
control systems. The advances in computer applications in manufacturing processes and their control
has led to the development of artificial intelligence approaches in the form of expert systems, neural
networks and fuzzy logic towards optimization and other control systems like prevention of wire
rupture. However most of the experimental research has a simplistic approach and tries the variation
of dielectric (hydrocarbons and water based) and electrode materials (Copper, tungsten, graphite etc.),
method of gap flushing (tool rotation, vibration or oscillation, magnetic and ultrasonic field’s
application) and studies of surface integrity (hardness, residual stress, defects like micro cracks
alloying with electrode material). The other type of popular research area is hybridization of EDM
with another assisting process for combining the beneficial features of both processes. In the case of
EDM the assistance of Electro Chemical Machining, Ultrasonic Machining and magnetic field have
been reported. In spite of being so extensively researched there are considerable grey areas in the
literature on the EDM process and the associated theories and mechanism [3].

III. STRAIGHT AND REVERSE POLARITY

        For a better understanding of spark erosion mechanism, and the final surface characteristics
which includes morphological, metallurgical and textural features it is always necessary to study the
effect of polarity which is defined as reverse polarity (electrode positive) and straight polarity
(electrode negative) [4].

IV. DESCRIPTION OF EXPERIMENTAL SETUP

        An experimental study was carried out on CREATOR CR-6C (SY CNC PC-60) Electric
discharge machine with hydrocarbon oil (ED-30 oil) as the dielectric fluid. The selected work piece
material is stainless steel in the form of rectangular shapes of 20mm×20mm×5mm. A cylindrical copper

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME

tool with a diameter of 12mm was used as an electrode which was finish ground before experimental
study and was mounted axially in line with work pieces. 12 specimens of stainless steel have been taken
for experiment. Those have been made as rectangular shape of 20mm×20mm×5mm pieces. By varying
the current (6, 8,10,12,14 & 16 Amps) i.e., 6 specimens each for both Straight and Reverse polarity
machining is carried out keeping time of observation as 20 minutes. The specimens are weighed before
and after the machining, the difference of both is considered to arrive at the Material Removal Rate
(MRR). Surface roughness for 12 specimens of stainless steel has been measured by using stylus type
instrument. The sampling length taken was 5mm. Surface morphology was also studied in depth using
SEM photographs on all the 12 specimens (six specimens in straight polarity and six specimens in
reverse polarity)

V. RESULTS AND DISCUSSIONS
A. Material Removal Rate:
    The effect of polarity was interesting whereby negative polarity produced shallow and small
craters whereas positive polarity led to larger craters.

B. Morphology and integrity of EDM surfaces:
    The physical and metallurgical studies of EDM surfaces show some interesting aspects. The spark
eroded surfaces are matty in appearances owing to the overlapping spark craters. Ideally these craters
are expected to be spherical as a result of melting from the spark energies with the assumptions of a
point source of heat with dissipation radially there from. In practice the evacuation of molten metal
from spark craters is not complete and owing to variables energies in the spark trains the crater sizes
also differ, leading to the formation of a randomly varying surface morphology (Fig.1). Erosion in
molten form is also evident from spherical debris trapped in the resolidified residual layer. This apart
the eroded surfaces exhibit typical EDM characteristics of micro cracks and gas pockets. The
absorption of carbon from the pyrolysis of hydrocarbon dielectric leads to the formation of hard
carbides, which form a hard surface layer, which is not etchable by the conventional etchant and
appears and so names as “White Layer” [5], [10], [11].




                                                                             (b)
                       (a)

                  Fig. 1. SEM photographs after EDM of steel surfaces (a) Straight
                                 polarity and (b) Reverse polarity

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME

         Electrode positive setup is superior in the form of high erosion of work material. This in
machining terminology is equivalent to high machining rates. This is due to higher liberation of spark
energy at cathode which absorbs ions higher mass through the plasma channel dimensions compared
to electrons absorbed at anode which are of negligible mass and result in large and expanded channel
dimensions due to mutual repulsion leading to low energy concentration. Next in importance is the
effect of pulse current on erosion rate. But this is a well established fact. The pulse energy is being a
product of pulse voltage, current and on time, naturally any increase in these variables results in
higher erosion rates both at work and electrodes surfaces. Of all these the effect of pulse current is
higher [6].

                         (A)                                               (B)




                        Fig. 2. EDAX photographs after EDM of steel surfaces
                                 (a) Straight polarity and (b) Reverse polarity.


         There is evidence of inter-electrode mass transfer whereby the anodic electrode material gets
diffused on to the cathodic work surface. EDXA analysis shows such a mass transfer where electrode
is anode (Fig.2). One can anticipate higher roughness to be associated with higher erosion rates and is
certainly evident in most of the results. The only exception being the pulse times. The factor, which
promotes spark erosion, also promotes surface roughness owing to larger size of spark craters.
Consequently the effects of polarity, pulse current and pulse voltage are similar on erosion rates and
roughness. But in the case of pulse on time the roughness is reduced though erosion rate increase
significantly. This phenomenon is attributed once again to expansion of spark channel, which not only
reduces the energy concentration but also results in higher diameter of spark craters. Though the
effect of both pulse current and on time are similar in increasing the size of spark craters , leading to
higher erosion rates, their effect on the geometry of the spark craters is different. Increase in current
for the same pulse on time results in deeper craters owing to higher energy concentration. On the
other hand the increase in pulse on time promotes plasma channel expansion thus leading to larger
diameter of spark craters.

VI. CONCLUSIONS
        Summarizing the main features of the present experimental work, the following conclusions
were drawn:
1. It is evident from the results obtained that, the polarity and current setting have dominant effect
    on erosion rates of steel.


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME

2. Pulse current has direct effect on both MRR and surface roughness, which are increasing with
   increase in the former for both the polarities.
3. Electrode positive and higher currents produce higher erosion rates compared to electrode
   negative at higher currents for steel.
4. MRR obtained is in the range of 5.94 – 24.46 mm3/min in case of reverse polarity, whereas in
   case of straight polarity the range obtained is 0.035 – 0.120 mm3/min for same peak pulse current
   and time of machining.
5. Surface roughness obtained is in the range of 1.72 – 5.92 µm in case of straight polarity, whereas
   in case of reverse polarity the range obtained is 7.36 – 12.64µm for same peak pulse current and
   time of machining.
6. It is inferred that higher current promote deeper craters and higher roughness.

     Finally the following concluding remarks are put forth about the practical significance of these
studies. For improving erosion rates reverse polarity and higher pulse currents are advisable to
electrode positive. For improving the surface finish slower erosion is advisable which is possible with
straight polarity only.

REFERENCES

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