# Principles and Characteristics of Different EDM Processes in Machining Tool and Die Steels
## EDM Process Details
### EDM Process Parameters  
The EDM process parameters which drive this process are divided into two types, namely, electrical and non-electrical parameters. The major electrical parameters are discharge voltage, peak current, pulse duration and interval (pulse-on and pulse-off times), electrode gap, polarity, and pulse wave form. Important non-electrical parameters include flushing of the dielectric fluid, workpiece rotation, electrode rotation, etc. These parameters are discussed below. The discharge or machining voltage is the average voltage across the spark gap during machining. The discharge voltage directly influences the regulation of the size spark gap and overcut [49, 50, 51, 52]. Normally, electrode and workpiece materials of high electrical conductivity use low voltage. In contrast, higher voltage is considered with materials of low conductivity. This parameter has a direct effect on the material removal rate (MRR), tool wear rate (TWR), and machining accuracy [53, 54, 55, 56, 57]. An increase in current increases MRR and TWR and adversely affects the accuracy. These characteristics of the parameter current and its responses opened the door for electrodes of high wear resistance that can be used in high-current conditions [47]. The pulse-on time is the time during which the discharge is applied. The amount of energy generated during pulse-on time has a direct influence on the MRR [54, 55, 56, 57, 58, 59, 60]. Accordingly, increasing the discharge energy by applying longer pulse-on time also increases the MRR [61]. The pulse-off time is the time during which there is no discharge. During this time, the debris as a result of sparking and erosion is flushed out of the gap between the workpiece and the electrode [62]. Flushing improves the ionization conditions and avoids the formation of an insulating layer; thus, proper selection of pulse-off time provides stable machining [63, 64, 65]. A shorter period of this time increases MRR as long as enough flushing of debris takes place. Otherwise, it may result in unsuitable conditions during the next pulse-on time period [66]. Furthermore, a pulse wave has many forms such as rectangular and trapezoidal waves or even a composite geometric form. It is demonstrated in the literature that, among standard forms, the trapezoidal wave form generator minimizes the electrode wear [67]. Recently, other generators were developed such as a typical one which initially facilitates the main pulse by producing a high-voltage pulse with a low current of narrower duration before the main pulse [63]. The polarity in EDM depends on many factors, including electrode and workpiece materials, current density, and pulse length [68, 69]. Either the electrode or the workpiece has a positive charge polarity and the other has a negative charge polarity. Negative electrode polarity is recommended for high-precision machining when the MRR is high. In the wire EDM process, the electrode \"wire\" usually has a negative polarity to keep machining rate high, and, since the wire wear keeps on moving continuously during arcing, its wear rate is less. The electrode and workpiece are located at a small predetermined distance called the \"discharge gap\" which is controlled by the discharge gap servo [70]. A discharge gap on the order of 0.005 mm and 0.1 mm is usually maintained. Finishing or high-precision machining requires a relatively low voltage in the gap on the order of 50 V and 300 V, as too high a voltage reduces machining precision. Non-electrical parameters include flushing of the dielectric fluid, workpiece rotation, and electrode rotation. The function of the dielectric fluid is to provide insulation against premature discharging, reduce the temperature during machining, and clean away the debris from the machining area [71]. Good dielectric fluids should have characteristics such as high dielectric strength, flushing ability, fast recovery after breakdown, etc. [72].