Variation of cutting forces has been investigated and correlated with the development of tool wear during dry machining of Ti-6Al-4V alloy at cutting speeds of 150 and 220 m/min, respectively. Both the average and maximum flank wear increased more significantly with volume of material removed at higher cutting speed. The tool failure mode is flank wear at both cutting speeds; however, the mechanisms for the excessive flank wear are different. The average width of flank wear reaches the criterion at cutting speed of 150 m/min after 57.2 cm³ of material has been removed as a result of the gradual recession of cutting edge due to crater wear, which led to significant rise of feed force only at the end of tool life. Both average width and maximum width of flank wear exceeds the criteria at cutting speed of 220 m/min after 20.8 cm³ of material has been removed because of severe plastic deformation of cutting edge at the nose radius, which resulted in dramatic increase in all three components of cutting forces at the end of tool life. The cutting edge at the nose radius was pushed up from the flank face in the chip flow direction due to the oversized built-up edge-induced compressive stress applied from the flank face.

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.