INVESTIGATION%20IN%20ELECTRIC%20DISCHARGE%20MACHINING%20ON%20OHNS%20STEEL%20TO%20INCREASE%20THE%20SURFACE%20FINISH%20AND%20REDUCE%20THE%20HEAT%20AFFECTED%20ZONE%20BY%20VARIOUS%20MACHINING%20PARAMETERS

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INVESTIGATION IN ELECTRIC DISCHARGE MACHINING ON
OHNS STEEL TO INCREASE THE SURFACE FINISH AND
REDUCE THE HEAT AFFECTED ZONE BY VARIOUS
MACHINING PARAMETERS

• PRESENTED BY
• ALBERT.N

Under the guidance of
Mr.N.RAMANUJAM M.E (Ph.d)
(Associate Prof/Hod- Mech)
DEPARTMENT OF MECHANICAL ENGINEERING E.G.S.PILLAY
ENGINEERING COLLEGE NAGAPATTINAN
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INVESTIGATION IN ELECTRIC DISCHARGE MACHINING ON
OHNS STEEL
N.ALBERT
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ELECTRIC DISCHARGE MACHINING

• Electric discharge machining is a thermo-electric
  non-traditional machining process
• Material is removed from the work piece
  through localized melting and
  vaporization of material
• It is the reverse process of Electroplating

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SCHEMATIC OF AN ELECTRIC DISCHARGE MACHINING
(EDM) MACHINE TOOL 
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EDM DISCHARGE PHENOMENA

• Phases of Discharge
• The discharge process during EDM can be separated
  into three main phases 
• Preparation phase
• Discharge phase
• Interval phase

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EFFECT OF INPUT PARAMETERS

• The effect of various input parameters on
  material removal rate (MRR) and surface roughness
  (Ra) is discussed below
• Pulse-on time
• Pulse-off time
• Flushing Pressure
• Tool rotation

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THEORIES OF MATERIAL REMOVAL

• The removal of material in electrical discharge
  machining is based upon the erosion
  effect of electric sparks occurring between two
  electrodes
• 1. Electro-mechanical theory2.
  Thermo-mechanical theory3. Thermo-electric
  theory

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ELECTRO-MECHANICAL THEORY

• This theory suggests that abrasion of material
  particles takes place as a result of the
  concentrated electric field.
• The theory proposes that the electric field
  separates the material particles of the work
  piece as it exceeds the forces of cohesion in the
  lattice of the material.
• This theory neglects any thermal effects.

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THERMO-MECHANICAL THEORY

• This theory suggests that material removal in EDM
  operations is attributed to the melting of
  material caused by "flame jets".
• These so - called flame jets are formed as a
  result of various electrical effects of the
  discharge.
• However, this theory does not agree with
  experimental data and fails to give a reasonable
  explanation of the effect of spark erosion.

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THERMO-ELECTRIC THEORY

• This theory, best-supported by experimental
  evidence, suggests that metal removal in EDM
  operations takes place as a result of the
  generation of extremely high temperature
  generated by the high intensity of the discharge
  current
• Although well supported, this theory cannot be
  considered as definite and complete because of
  difficulties in interpretation.

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DIELECTRIC FLUID

• The EDM setup consists of a power supply whose
  one lead is connected to the work piece immersed
  in a tank having dielectric oil.
• The tank is connected to a pump, oil reservoir,
  and a filter system.
• The pump provides pressure for flushing the work
  area and moving the oil while the filter system
  removes and traps the debris in the oil

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DIELECTRIC FLUID
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THE MAIN FUNCTIONS OF THE DIELECTRIC FLUID

• To flush the eroded particles produced during
  machining, from the discharge gap and remove the
  particles from the oil to pass through a filter
  system.
• To provide insulation in the gap between the
  electrode and the work piece.
• To cool the section that was heated by the
  discharge machining.

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PROPERTIES OF DIELECTRIC FLUID

• Flash Point
• Dielectric Strength
• Viscosity
• Specific Gravity
• Color
• Odor

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EDM PERFORMANCE MEASURES

• A signif?cant number of papers have been focused
  on ways of yielding optimal EDM performance
  measures of high MRR, low tool wear rate (TWR)
  and satisfactory SQ.

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WORK MATERIAL

• The work material chosen for this experimental
  work is OHNS (Oil-Hardening Non-Shrinkable) die
  steel which is one of the most widely used die
  steel material for the manufacture of press
  tools cutting dies and punches for blanking,
  trimming, flanging and forming operations.
• The important characteristics responsible for the
  commercial popularity of this material are its
  ability to retain accurate dimensions at elevated
  temperatures, abrasion resistance, toughness and
  negligible deformation during the hardening
  process.
• It is commercially machined in the hardened
  state with EDM process using copper as the
  electrode material and kerosene as the dielectric
  medium.
• The chemical composition of this work material is
  given in Table. No literature on the machining
  aspects of this material using any other
  electrode material or dielectric medium besides
  copper and kerosene could be found.

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EXPERIMENTAL PROCEDURE

• The electrical discharge machine (model
  Electronica M2S) with Servo-head and positive
  polarity for the electrode was used to conduct
  the experiments.
• Commercial grade kerosene used as dielectrics
  and side flushing with a pressure of 15 lbs/in2
  was maintained for the whole experiment. The
  sparking voltage was fixed at 80 V and discharge
  current was varied from 4A to 15 A. In all,
  values of discharge current, namely, 15A, 12A,
  10A,6A were used for Brass electrode material,
  and 10, 7A, 6A, 5A, 4A were used for Copper
  electrode material All the experiments were
  conducted with positive polarity of the
  electrode.
• Besides discharge current, two other variables
  related to the spark pulse wave are on time and
  off- time which can be set independently. The
  diameter of crater formed during each spark is
  proportional to the applied current while its
  depth is proportional to the on- time.

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EXPERIMENTAL SETUP

• Two electrodes were machined to a cylindrical
  shape of 14 mm diameter and 50mm length
• A plate of 200 mm 60 mm size and thickness 18 mm
  of OHNS die steel was taken. It was subjected to
  a standard hardening cycle and it has a hardened
  at the range of 40 to 45 HRC
• After mounting the work piece and one of the
  electrodes on the machine, the depth of machining
  was set at 25mm.

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EXPERIMENTAL SETUP

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ELECTRODE MATERIALS

• Electrode material has a significant influence on
  important output parameters, such as, material
  removal rate, surface roughness and dimensional
  accuracy
• Copper can be easily machined to any shape,
  suffers less wear, has good thermal conductivity,
  and is economical.
• Brass is inexpensive and very easy to machine,
  but it has high electrode wear.

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COMPARISON OF COPPER AND BRASS ELECTRODE
PROPERTY UNIT MATERIAL MATERIAL
PROPERTY UNIT COPPER BRASS
Thermal conductivity W/m-K 391 159
Electrical resistivity Ohm-cm 1.67 4.7
Specific heat capacity J/g-deg C 0.385 0.38
Melting point Deg C 1083 990

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CHEMICAL COMPOSITIONS OF OHNS DIE STEEL

ELEMENT COMPOSITION,wt
Carbon 09 to 1.0
Silicon 0.3 to 0.5
Manganese 0.7 to 0.8
Chromium 0.5 to 0.6
Tungsten 0.5 to 0.6
Vanadium 1.0
Iron Balance
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MACHINING PARAMETERS
Sparking Voltage (V) V805
Discharge Current (A) 15,12,10,8,7,6,5,4
Servo Control Electro Mechanical
Polarity Normal (Electrode Positive
Dielectric fluid Commercial Grade Kerosene
Flushing side Flushing with Pressure
Work piece Material OHNS Die Steel (Hardened and Tempered)
Electrode Material Copper and Brass