SPARK EROSION THE BASICS

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SPARK EROSIONTHE BASICS

• Anotronic Ltd.
• UNIT 3 HOLLINGDON DEPOT, STEWKLEY ROAD,
• SOULBURY, NR. LEIGHTON BUZZARD, BEDS, LU7 ODF.
• TELEPHONE [phone] FAX [phone]
• EMAIL sales_at_anotronic.com
• INTERNET http//www.anotronic.com

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SPARK EROSION

• Spark erosion is a modern machining technique
  which has distinct advantages over more
  conventional methods of machining metal. As a
  result of this it is a practice that is becoming
  more and more popular.

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HOW SPARK EROSION WORKS

• The principal of spark erosion is really quite
  simple, but unfortunately, like all things
  electrical, most of what happens is invisible
  therefore its function must be shown in
  diagrammatic form.

ELECTRODE
ERODED WORKPEICE
WORKPEICE
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HOW SPARK EROSION WORKS

• The workpiece (or material to be machined) and
  the electrode (tool) have a small gap between
  them and are connected to a D.C. supply. The gap
  is filled with a liquid such as a hydrocarbon oil
  which act as a dielectric (or insulator).
  Therefore the process takes place in a tank.

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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
POWER SUPPLY
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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
POWER SUPPLY
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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
POWER SUPPLY
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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
POWER SUPPLY
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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
POWER SUPPLY
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HOW SPARK EROSION WORKS

• The current is switched on and off intermittently
  and a series of sparks pass between the workpiece
  and the tool, similar to the action of the spark
  plug on a motor car.

SWITCH
Click the Switch to see what happens
POWER SUPPLY
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HOW SPARK EROSION WORKS
ELECTRODE
WORKPEICE BEFORE MACHINING
WORKPEICE AFTER MACHINING
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HOW SPARK EROSION WORKS
(TOOL) ELECTRODE

• It is this sparking action occurring many
  thousands of times per second, that erodes the
  workpiece to the desire shape. Obviously this
  cannot be done manually, so it is performed
  electronically by transistors.

COMPLETED WORKPEICE
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THE MACHINE TOOL

• The machine tool shown here gives a good
  indication of the type of equipment used in spark
  erosion.

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THE MACHINE TOOL

• The machine is of C-frame construction and
  consists of a column and a base.

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THE MACHINE TOOL
COLUMN

• The column is mounted on the base and the work
  head is attached to the column.

WORK HEAD
BASE
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THE MACHINE TOOL

• The table rests on the base and consists of two
  slides which can be adjusted at right angles in
  relation to each other, one longitudinally and
  the other laterally.

LATERAL SLIDE
LONGTITUDINAL SLIDE
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THE MACHINE TOOL

• A tank is mounted on the longitudinal slide,
  whilst in the head a kind of ram called the quill
  which can be moved up and down. The workpiece is
  fixed to the table in the tank and the electrode
  is fixed to the lower end of the quill or quill
  nose.

QUILL
TANK
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THE DIELECTRIC

• When the equipment is set up for machining, the
  dielectric is pumped into the tank and is
  constantly circulated and filtered throughout the
  machining process so as to remove particles of
  metal eroded from the workpiece and the
  electrode. This shows the dielectric system in
  diagrammatic form. The filter, of course, needs
  to be changed at regular intervals.

FILTER
PUMP
CLEAN DIELECTRIC
RESERVIOR
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THE POWER SUPPLY

• The power supply unit shown here houses the
  transformers and current rectifiers to make the
  A.C. mains supply into a D.C. square wave signal.
  This square wave can be adapted to suit any
  operational requirement by means of switching
  devices within the power supply unit.

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THE POWER SUPPLY

• The switching transistors which continuously
  'make and break', supply the intermittent pulse
  of high current to cause the spark.

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THE POWER SUPPLY

• The beauty of these switching devices, lies in
  the fact that there are no moving parts and they
  are therefore very reliable. The operation of
  these transistors is shown diagramatically here,
  the two metal parts C and E both conduct, whilst
  the material B behaves either as a conductor or
  an insulator.

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THE POWER SUPPLY

• If B does not have current applied, it acts as an
  insulator.

CLICK THE SWITCH
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THE POWER SUPPLY

• But if it has an electric current applied to it,
  the material conducts.

CLICK THE SWITCH
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THE POWER SUPPLY

• This current application and removal is by means
  of an electronic circuit known as a
  multivibrator, which causes the current to
  oscillate at a predetermined level.

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THE POWER SUPPLY

• There are two controls for setting the operation
  of the multivibrator, one for setting the opening
  time and one for the closing time. for example 10
  microseconds open and 100 microseconds closed.

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THE POWER SUPPLY

• Thus the multivibrator acts as an automatic
  switch, sending pulses of energy to B which
  transmits energy or blocks it to the same
  programme as the multivibrator. So that whenever
  B is conducting, a spark is taking place between
  the electrode and the workpiece.

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DISTANCE CONTROL AND REGULATION

• The distance between the electrode and the
  workpiece, the spark gap, is very important and
  though small, it must be set correctly to obtain
  the right results. As material is eroded from the
  workpiece the distance between the workpiece and
  electrode is constantly and automatically
  adjusted to maintain the correct gap.

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DISTANCE CONTROL AND REGULATION

• The regulator adjusts the gap so that this
  remains constant and also advances the electrode
  as the workpiece is eroded. The method of
  regulating the spark gap is by measuring the
  discharges between the electrode and the
  workpiece, thus enabling the spark gap to be
  adjusted automatically when necessary.

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DISTANCE CONTROL AND REGULATION

• The electrode is attached to the nose of the
  quill which runs in precision bearings in the
  head, and is driven via a recirculating ballscrew
  and a D.C. Servo motor. A regulator feeds a
  signal into a D.C. servo amplifier which turns
  the motor, either clockwise or anticlockwise,
  thus regulating the spark gap.

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DISTANCE CONTROL AND REGULATION

• The discharge between the electrode and the
  workpiece is constantly monitored and as
  variations occur they are compared with the known
  value for the particular operation.

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DISTANCE CONTROL AND REGULATION

• If the gap grows too large, the servo motor is
  energised and moves the electrode nearer to the
  workpiece

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DISTANCE CONTROL AND REGULATION

• If the gap grows too large, the servo motor is
  energised and moves the electrode nearer to the
  workpiece

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DISTANCE CONTROL AND REGULATION

• If the gap grows too large, the servo motor is
  energised and moves the electrode nearer to the
  workpiece

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DISTANCE CONTROL AND REGULATION

• If the gap is too small, the servo motor moves
  the electrode away from the workpiece.

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DISTANCE CONTROL AND REGULATION

• If the gap is too small, the servo motor moves
  the electrode away from the workpiece.

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DISTANCE CONTROL AND REGULATION
MEASUREMENT OF ACTUAL VALUE
TOO LARGE
TOO SMALL
COMPARASON WITH SET VALUE
CORRECT
DECISION
COMMAND
ADVANCE ELECTRODE
ERODE
RETRACT ELECTRODE
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DISTANCE CONTROL AND REGULATION

• Thus there is a constant feedback circuit the
  distance between the electrode and the workpiece
  is constantly measured, the result is converted
  into electrical energy and the spark gap is
  adjusted as necessary.

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TOOL WEAR AND EROSION RATE

• Spark erosion using a 'low' current gives a slow
  erosion (removal) rate of metal from the
  workpiece, whereas a 'high' current gives a much
  faster rate of removal there is a slightly
  higher rate of tool (electrode) wear when using
  copper electrodes to erode metal workpieces.

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TOOL WEAR AND EROSION RATE

• It has been found that graphite electrodes behave
  differently with these tools the wear declines
  up to a certain level and then remain more or
  less constant. Using short pulses of energy for
  eroding means an increase in the amount of wear
  on the electrode, whilst long pulses of energy
  result in lower electrode wear.

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TOOL WEAR AND EROSION RATE

• The surface roughness of the eroded metal varies
  depending on the rate of discharge and speed of
  erosion. The higher the discharge of energy, i.e.
  the larger the pulse, then the rougher the finish
  and likewise the smaller the pulse, then the
  smoother the finish.

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SPARK EROSION

• These are the basics of spark erosion (EDM,
  Electro Discharge Machining). Any electrically
  conductive material can be machined and standard
  Anotronic EDM machines can machine holes as small
  as 0.1mm and equally accurately large, heavy
  workpieces. Spark erosion is flexible in its
  applications and accurate in the extreme.