Dos métodos de evaluación: criterios y teoría del programa*
3. Evaluación de la teoría del programa
3.6. Métodos y técnicas
5.1 1.1 Senro mechanism
The first type, the cavity type with vertical axis only under servo-control is best for illustration as shown in figure 5 .9. Positive polarity indicates that the electrode is positive and also negative polarity can be used. If the ram is allowed to move forward
unchecked there would be a direct contact between the electrode and the work piece causing an electrical short circuit. This is prevented by a servo mechanism in which the potential is monitored and compared with a reference. If the potential is greater than the reference ram advances. If it is less, the ram retracts. The movement of the ram may be accomplished either by a direct drive servo motor or by hydraulic cylinder.
During the machining, the distance between electrode and work piece increases. Then the potential goes up and the ram advances until the potential matches the reference. By this, servo mechanism maintains a constant gap. Machining continues until the pre set depth is reached. At this point, the spark stops and in most machines the electrode retracts from the work piece.
de power _ source
T
5. 11.2 Power supply
Servo
control Tool feed
Tool (negative electrode)
Figure 5.9 Schematic diagram ofEDM
Power supply is an important part of the EDM system. It transforms alternating
current (ac) from mains supply into direct current (dc) pulses which are required to produce spark discharges for machining. Sensing the potential between the electrode and the work piece is an additional function of the EDM power supply, because a direct relationship exists between the potential voltage and the electrode-work piece gap. This signal is used to control the servo system, enabling it to maintain a constant
gap. On the other hand, the power supply helps to facilitates the selection of the optimum parameter for a wide range of cutting conditions by controlling pulse voltage, current, pulse frequency and pulse duration etc . . Finally the EDM power supply helps to terminates power if an over voltage, over current or dc arc occur as a result of a short circuit between the electrode and the work piece it then alerts the operator.
5.11.3 Dielectric system
The EDM dielectric system consists of the dielectric fluid, delivery devices, pumps and fitters. Requisite properties of the fluid are high viscosity and high electric resistance. Hydro-carbon fluids such as Kerosene etc., are most commonly used. There are three functions of the dielectric fluid. It acts as an insulator between the electrode and work piece, as a coolant to draw away the heat generated by the spark and as a flushing medium to remove the metal by products from the cutting gap. As shown in figure 5 . 1 0 there are methods available for flushing the dielectric fluid through the cutting zone. Any of these flushing techniques can be performed exactly as illustrated in figure 5 . 1 0 with or without the work piece submerged in a tank. of dielectric fluid. Whenever
flammable dielectric fluids such as kerosene, lamp oil etc. are being used, submersion of the work piece is recommended to reduce the chances of accidental fires.
, , , , ' ''' I " , ' I '
SUCTION THROUGH ELECTRODE
1
;UCTION THROUGH WORKPIECE
, I " , " , , "' , , ,
PRESSURE THROUGH aECTRODE
PRESSURE THROUGH WORKPIECE
JET FLUSHING
Figure 5 . 1 0 Various flushing techniques
5. 12
Principle of OperationExperimental evidence suggests that metal removal in EDM operation take place as a result of the generation of extremely high temperatures generated by the high intensity of the discharge current. The discharge mechanism can be explained by division of the
whole process in to phases and stages. A model representation of the electrical discharge for all three phases and stages are shown in figure 5. 1 1 a to 5. 1 1 i.
1�
" I ( i : . \ . � . � . , ; . .<!'��
� � 1/ (1 .� !)4 :� , . ,. i . .. .. \ � . � � . . .... 1� • "' � ; :# 11' �The tool electrode is marked with (+ )ve sign and the work piece is marked with (-)ve sign. Variation in voltage and current over the duration of the discharge is shown schematically on the screen of oscilloscope.
Phase 1 : Preparatory phase
Formation of a high conductivity channel due to ionisation of the dielectric medium due to the powerful electric field, which lasts only a very limited time.
Stage 1 : Application of a voltage to machining site by the spark generator used.
Stage 2: Polarisation and orientation of the molecules and ions of the dielectric medium
Stage 3 : Negative and positive streamers organise and a current flow begins to occur when polarisation reaches a certain point.
Phase2: Formation of a heavy current similar to an avalanche of electrons due to the discharge. Ejection and erosion of metals in the work piece begins.
Stagel : Resistance of the dielectric medium drops to very low value. Current intensity rises to a very high value and voltage drops.
The
ionised channel consists of positive ions, free electrons, vapour metal of both electrodes, gases due to chemical decomposition of dielectric.Stage 2: Discharge channel is ionised by the flow of high intensity current. Formation of strong magnetic field occurs which may attracts the ions towards the axis of the discharge channel.
Stage 3 : As a result of compressed current beam due to ions, channel is greater than
1 OOOOoC. A certain volume of electrodes, metal melts and evaporation occurs.
Phase 3 : Ejection of eroded metal in the work piece may continue further even after the end of discharge.
Stage 1 : Ejection and evaporation of metal leaves a specific mark on the edges of the crater both in anode and cathode.
Stage 2: Vapour bubbles collapses.
Stage 3 : Cycle completes as soon as the energy of the discharge is over.