Research on electrical discharge machining with stepped electrodes considering the effects of dielectric fluid field
Creators: name: Mao, Xuanyu
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401406
subject: Machining
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)

description: Electrical Discharge Machining (EDM) is one of the most widely used non-conventional methods to machine electrically conductive materials in manufacturing industry owing to its strong capability in machining difficult-to-cut materials irrespective of their hardness; Electrical discharge hole making is an important variant of EDM. Due to the limitation of conventional drilling processes, special-shaped holes particular those with high aspect ratio drilled on hard-to-cut materials can only be made by EDM or electrical discharge drilling (EDD). However, because of the accumulation of debris in the small discharging gap, the efficiency of EDM hole making processes is low and the quality of drilled holes is poor especially when drilling deep blind holes. The material removal rate (MRR) can be dramatically lower than that in conventional drilling processes and the material structure of hole surfaces can be changed due to the large amount of heat generated by the sparks in the EDM process. 

In order to increase the machining efficiency, extensive research including the finding of optimal electrical parameters has been conducted by carrying out a large number of experiments which are time consuming and error prone. The setting of electrical parameters is dependent on the electrode material, workpiece material, dielectric and surface finishing requirements. Since the low material removal rate is mainly caused by the poor debris removal capability, there is a pressing need to develop new approaches to improve the efficiency of expelling debris from the discharging gap. 

The current body of knowledge shows that the shapes of electrodes have significant influences on EDM process. Geometrical characteristics of the electrode plays a critical role in EDM performance. In order to improve EDM efficiency in machining blind holes, a new strategy is developed in this research by applying electrodes of different geometric profiles.

To investigate the correlation between the electrode shape and material removal efficiency in EDM processes, the effects of different types of electrodes on dielectric flow were simulated by using the Computational Fluid Dynamic (CFD) software. Then, a series of EDM holes drilling experiments were conducted by using different electrodes under different machining conditions to validate the predicted results. The MRR, electrode wear rate (EWR) and surface roughness (SR) were used to evaluate the machining performance. The quality of the machined surface (surface topography) was analysed based on the optical microscope images and Scanning Electron Microscope (SEM) images. The results show that, compared with conventional cylindrical electrodes, the maximum effective cutting depth of two types of newly developed electrodes increased by 13.793% and 132.184%, respectively, and the MRR of the new stepped electrodes was significantly higher after the machining depth reached 2 mm and 4 mm for electrodes with the working length of 2 mm and 4 mm, respectively. The surface roughness was identical for holes machined by all types of electrodes.

Flushing is another commonly used strategy in EDM hole making processes to enhance debris removal capability. It not only removes eroded materials from the inter-electrode gap, but also contributes to a number of other effects that can have significant impact on the efficiency and outcome of the machining process.  The flushing can be typically applied both externally and internally through the electrodes. In EDM machining of blind holes, injection flushing is more efficient because the removal of debris becomes more and more difficult with the increase in hole depth. By applying injection flushing, the debris can be directly flushed away from the machining zone, and the chance of short circuit and secondary discharge can be significantly reduced. However, although flushing can improve the machining performance and stability of EDM, it has been found that particular flushing methods can have negative impacts on the machining efficiency of EDM under certain conditions. Although the separate application of stepped electrodes and injection flushing in EDM hole making process can result in increased MRR, when the two were used together, the combined effects may lead to low machining efficiency.

In this study, the working mechanisms of stepped electrodes and injection flushing in EDM drilling of blind holes have been thoroughly investigated. The effects of flushing combined with the new types of electrodes were investigated. A series of experiments were conducted using the cylindrical electrode and two types of stepped electrodes. Experimental results show that the addition of internal flushing affects the formation of plasma channels, and the vortex caused by the flow of dielectric fluid in the machining area results in the concentration of debris which leads to abnormal discharges. When stepped electrode and injection flushing were coupled, the maximum effective machining depth can be increased, but the material removal rate decreased, and the tool wear rate increased significantly. The type of electrodes has little effect on SR and surface topography; however, better surface topography (lower SR value) can be found when no injection flushing was applied. Furthermore, the dielectric flow behaviours were simulated when the stepped electrodes were used in EDM blind holes drilling process based on the commercial CFD software Ansys Fluent. The comparative simulations with different electrodes, different dielectric flow velocities and different particle residence times were simulated. The simulation outcomes showed good agreement with the experimental results. 

This project investigates the effects of stepped electrode and injection flushing in EDM blind hole machining and thoroughly studies their mechanisms, which will have a profound impact on the future use of stepped electrode in manufacturing industry.