# Principles and Characteristics of Different EDM Processes in Machining Tool and Die Steels
## 4 Research in EDM of Tool Steel
### Combined and Hybrid Processes of Tool Steel  
Many studies which combined two or more methods to improve the performance of the EDM process were proposed. For example, [PERSON] and [PERSON] [141] studied the machining of M2 grade high-speed steel with EDM using an ultrasonic-assisted cryogenically cooled copper electrode (UACEDM). The study considered discharge current, pulse-on time, duty cycle, and gap voltage as process variables and MRR, TWR, and SR as performance parameters. A comparison of EDM, EDM with cryogenic cooling of the electrodes (CEDM), and UACEDM was presented. The authors reported that the UACEDM process significantly reduced TWR and SR compared to the conventional EDM process, while the MRR was close in the two processes. The surface machined by UACEDM had an abundance of cracks; the density of cracks increased with increasing discharge current. It was found that the surface integrity was better in the UACEDM process than in conventional EDM. [PERSON] et al. [127] developed a control strategy for gap conditions in near-dry EDM milling by using a high-speed piezoelectric actuator. The detailed structure of a spindle system that retracts the electrode at high bandwidth was also presented. The authors reported that the proposed controller improved the MMR by about 30% compared to the conventional controller. It also improved the electrode retraction efficiency. [PERSON] et al. [206] used micro-reversible EDM to develop a new micromachining  
Figure 14: Micro-tool electrodes and optical section views of machined micro-holes using very short pulses and open gap voltages of (**a**) 60 V, (**b**) 75 V, and (**c**) 90 V [185]. method for the fabrication of micro-metal structures, which combined micro-EDM deposition with the micro-EDM selective removal process. The authors reported that it was easy to achieve reversible machining from the micro-EDM deposition process to the micro-EDM selective removal process by controlling the process condition. They also fabricated micro-structures with fine surface quality and high shape precision by using the micro-reversible EDM method. Figure 15 shows an example of selective removal with micro-EDM drilling. [PERSON] et al. [157] developed a versatile process for EDM using a magnetic force-assisted standard EDM machine. The effects of magnetic force on EDM machining characteristics were explored. Moreover, the study adopted an L18 orthogonal array based on the Taguchi method to conduct a series of experiments, and the experimental data were statistically evaluated using analysis of variance. [PERSON] et al. [162] presented hybrid EDM process finish and polish machining in dielectric water with a ductile carbon-polymer composite electrode. It was found that a fine graphite grade and positive polarity current impulses improved the workpiece surface finish. [PERSON] et al. [213] proposed an ultrasonic vibration EDM (UEDM) in gas. The study developed a theoretical model to estimate the MRR for tool steel material. The authors reported that the UEDM improved the MRR. They found that the most important outcomes of this technique were lower pollution and TWR. [PERSON] et al. [165] also proposed a hybrid EDM to automate the polishing process of tool steel cavities. The proposed method involved a dry EDM process with a thermoplastic composite electrode. Earlier studies also used a combination of two processes to improve EDM performance, such as [PERSON] and [PERSON] [212], who studied the effect of grinding and EDM on the response parameters, and [PERSON] and [PERSON] [135], who compared the machining characteristics in vibratory, rotary, and vibro-rotary EDM, and studied the effect of ultrasonic vibration and rotation combination on the performance of the EDM process. In other studies [214; 215], photolithography technology and EDM were combined, when processing other classes of steel, to enhance the precision in geometry and position, as well as the throughput of micro-EDM.