During electric discharge machining process there are start and stop intervals known as ON time and OFF time. The machining is performed during the pulse ON time when in the gap between work piece and electrode the voltage is applied. Alternatively the time during which no voltage is applied is the pulse off time. A greater value of On time is selected to obtain a long duration of electric discharge to increase the material removal rate but also leads to poor surface finish. During the pulse off time re-ionization of dielectric occurs and it is also essential to avoid any short circuit leading to wire breakages.
2.1.2 Peak current and gap voltage
In WEDM, the most important machining parameter is the peak current. Measured in unit of amperage it is basically the amount of power used in WEDM. Peak current is the preset level reached by the increasing current during each pulse on-time. In both processes, die sinking and wire-EDM, the surface area of cut governs the maximum amount of amperage. In roughing operations and in cavities or details with large surface areas, a higher amperage is used.
The supply voltage to be placed on the gap is expressed as the gap voltage or open circuit voltage. The electric discharge energy is directly proportional to this value i.e. greater the gap voltage value greater is the electric discharge energy. Moreover gap voltage and peak current are also dependent and peak current increases with the increase in gap voltage. In some WEDM machines machining voltage is shown by both of these factors.
2.1.3 Servo voltage and servo feed rate:
Fig.2. Feed rate and gap size (Roger Kern, 2008)
For controlling the advances and retracts of the wire a servo voltage is used. Depending on the state of machining between the work piece and electrode the mean machining voltage is varied. The reference voltage for governing the advances and retracts of the wire is established using the servo voltage.
The wire advances when the mean machining voltage is greater than the set voltage and retracts if it is lower. A superior value of servo voltage will lead to widening of gap between electrode and work piece. The number of electric sparks also decreases thus steadying the electric discharge albeit the machining rate slows down. For a smaller value of servo voltage the mean gap gets attenuated which results in rise in number of electric sparks. It speeds up the process but may also result in wire breakage as it gets unstable.
The feed rate of table during machining is detailed by the servo feed rate. Normally this factor is selected automatically by the WEDM machines in accordance with the servo voltage but it can also be controlled manually. In such a case the machining table will have a constant speed with no remark of servo voltage.
Overall the feed rate can be altered by both servo voltage and servo feed rate.
2.1.4 Dielectric flow Rate
Electro discharge which occurs in the air can't be used for rough cut machining as it is not...