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Electrical discharge machining (EDM) is one of the most widely used non-conventional methods to machine electrically conductive materials in the manufacturing industry because of its strong capability in machining difficult-to-cut materials irrespective of their strength and hardness. Electrical discharge drilling (EDD) is an important variant of EDM. Due to the limitation of conventional drilling processes, special holes, particular those with high aspect ratios on hard-to-cut materials, can only be drilled by EDD. Extensive research has been carried out to improve the efficiency and quality of the EDD process by using different approaches, such as assisted EDD and powder-mixed EDM drilling aiming to improve the material removal rate (MRR), tool wear rate (TWR), surface quality and accuracy. This paper provides a comprehensive review of the EDD process. Different methods were compared; the advantages and disadvantages of each process were summarised; state-of-the-art technologies and the latest development were introduced, and research trends and new directions were presented.

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Exotic materials such as titanium offer superior characteristics that, paradoxically, make them hard-to-cut by conventional machining. As a solution, electric discharge machining (EDM) stands out as a non-conventional process able to cut complex profiles from hard-to-cut materials, delivering dimensional accuracy and a superior surface. However, EDM is embodied in CNC machines with a reduced axis and machining envelope, which constrains design freedom in terms of size and shape. To overcome these CNC constraints, traditional machining using six-axis industrial robots have become a prominent research field, and some applications have achieved cost efficiency, an improved envelope, and high flexibility. However, due to the lack of stiffness and strength of the robot arm, accuracy, material rate removal, and surface finishing are not comparable to CNC machining. Therefore, the present study investigates the design of a novel WEDM combined with six-axis robotic machining to overcome the limitations of traditional robotic machining and enhance EDM applications. This study extends the work of a conference paper to confirm potential outcomes, quantifying and ranking undesired interactions to map technical problems and applying the TRIZ approach to trigger solutions. Finally, an effective robotic end-effector design is proposed to free EDM from CNC and deliver robotic machining as a flexible and accurate machining system for exotic materials.

Exotic materials such as hardened steel and tungsten carbide tool steel have unique resistance and properties that make them hard-to-cut. Thus, research to find better ways to process such materials requires an innovative approach and new ideas. Therefore, the present study investigates the design of a novel WDEM combined with robotic machining to overcome limitations of traditional robotic machining. Wire EDM (WEDM) stands out as a non-traditional machining process able to cut complex profiles of hard-to-cut materials, achieving high dimensional accuracy and superior surface finishing. Unfortunately, WEDM is designed in rigid bed-based CNC machines which restrict design freedom in terms of size, shape and features due to machining envelope constraint. On the other hand, traditional machining processes such as drilling and milling using six-axis industrial robots have been investigated and some applications have successfully delivered cost efficiency, improved envelope and high flexibility. However, due to the structure and strength of the robot arm, accuracy, repeatability and finishing are not comparable to CNC machining outcomes. These researches are also restricted by the power of the robot arm holding the machining tool. This paper explores, identifies and selects suitable configurations and define research actions that must be taken to achieve a highly flexible, accurate machining system for exotic materials.

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