In recent years, with the rapid development of modern industry, the micro holes have received more attention in the manufacture of high-end equipment in aerospace, automobile and medical industries , such as aero-engine turbines , combustion chambers , diesel fuel injectors  etc. At present, micro-holes can be machined by adopting various micro-machining methods, such as abrasive water jet drilling, laser drilling, electrochemical drilling, and micro-electrical discharge machining (micro-EDM)[5, 6]. Among them, micro-EDM has an obvious advantage in the fabrication of micro-holes on difficult-to-machine conductive materials with different hardnesses, especially titanium alloy [7, 8]. However, a continuing problem during the micro-EDM process is that a large amount of machining debris in the inter-electrode gap cannot be evacuated timely, resulting in severe tool electrode wear and decreasing the machining accuracy of micro-holes .
Researchers have been developing various methods to improve debris evacuation and increase the machining accuracy. Wang et al.  improve debris evacuation and decreased the tool electrode wear by using a helical tool electrode. Jacob et al.  successfully fabricated a double helical grooved tool electrode and found that the double helical grooved tool electrode performed better in debris removal than the single helical grooved tool electrode. Moreover, Kumar et al.  fabricated a series of micro-inclined slots on the surface of a solid cylindrical tool electrode. The results showed that the proposed tool electrode can effectively evacuate the debris existing in the inter-electrode gap and hence reduce the taper of a blind micro-hole . In another study, Kumar et al.  drilled a series of inclined through-holes in the solid cylindrical electrode. Their experimental results indicated that the debris could be evacuated via the through-holes, which eliminated the formation of secondary sparking and resulted in a lower value of the taper angle . Except for shaped electrode geometries, other methods are also used to improve the debris evacuation, i.e. ultrasonic vibration, planetary motion of tool electrode plus ultrasonic vibration and magnetic field assistance. Hirao et al.  applied ultrasonic vibration assistance to the tool electrode, and the debris in the machining zone was removed under the condition of high-frequency vibration. Zhao et al.  drilled micro-holes by using a notched tool electrode assisted by ultrasonic vibrations during the micro-EDM process. Their results indicated that both debris evacuation and machining accuracy were effectively improved. Yu et al.  proposed a new approach of the combination of ultrasonic vibrations and planetary motion of tool electrode in micro-EDM and found that the process of micro-EDM drilling had a tremendous improvement in the process performance, which was benefit from the debris evacuation timely. In addition, Bains et al.  introduced magnetic field assistance in EDM and promoted the flow of debris from the machining zone. Baseri et al.  used a constant magnetic field in EDM process and added TiO2 powder to the dielectric. Their experimental results showed that debris was easy to evacuate from the machining zone under the action of a magnetic field force and the powder. Although there are many methods favorable to increasing the debris evacuation during the micro-EDM process, the fabrication of a shaped tool electrode and multi-field assistance are complex and difficult to control. So, tool electrode wear is still inevitable.
The actual machined micro-hole depth is difficult to equal its expected depth due to the tool electrode wear. Therefore, micro-hole depth error is one of the important indicators of machine accuracy. Various tool electrode wear compensation methods have been proposed. Muralidhara et al.  developed an in-situ axial tool wear and machining depth measurement system to investigate the axial wear ratio variations with machining depth, and realized tool electrode wear compensation. A maximum machining depth error of 6% was observed using the proposed tool electrode wear compensation method. Ganesh et al.  proposed a new tool wear compensation method that integrated the micro-EDM machine to an image processing module and computer-controlled tool wear prediction algorithm. Deviation of less than 1% between the actual depth and the expected depth of the micro-hole can be obtained by the proposed novel method. Aligiri et al.  developed a real-time material removal volume estimator based on a theoretical electro-thermal model. The experimental and estimated results were found to be in satisfactory agreement with an average error less than 14.3% for titanium alloy workpeice material. Moreover, Nirala et al.  applied a modified volume removal per discharge approach in real-time. This approach realized tool electrode wear compensation by estimating real-time volume removal from the workpiece. The expected depth of the micro-hole in a brass workpeice material were successfully fabricated with an error of less than 4%. Although these tool electrode compensation methods can effectively improve the machining accuracy of micro-holes, these compensation systems are very complicated.
In our study, a novel machining method of tool electrode spiral motion feed mode combined with FRAC is proposed to improve the machining accuracy of micro-holes, with the processing parameters optimized by means of response-surface experiments. The effects of tool electrode feed mode and compensation method on the machining accuracy of micro-holes were investigated. Besides, the effects of voltage, current, frequency and tool speed on the depth error and machining time of micro-holes are also studied. Finally, high accuracy and high efficiency micro-holes are fabricated by using the optimum processing parameters.