Mechanism design and process control of micro EDM for drilling spray holes of diesel injector nozzles

Machining quality of spray holes directly affects injection performance and combustion efficiency of diesel engines. With the high standards of less emission and fuel economy, the spray holes used in the case of high injection pressure require the characters of small diameter <Φ300 μm, thick wall >1 mm, multi-holes >4, micro-taper shape with K-factor 0–2 and multidimensional space position. Mechanical drilling is difficult to meet the machining requirements of the spray holes. Micro electro discharge machining (EDM) has the advantages of less cutting force, without burrs, and even finish machining after heat treatment, so it fits for machining micro holes on metal alloy materials. In this study, a micro EDM equipment was developed for drilling the spray holes. Key technologies were discussed including an electrode feed head, a workpiece positioning mechanism and process control methods. In order to machine micro-taper holes and improve processing efficiency, the electrode feed head was designed with the special multifunction modules of a novel taper-swinging mechanism (TSM), a piezoelectric (PZT) actuator for assisting high-frequency vibration, and a dual-clamps inchworm mechanism for wear compensation of wire tool electrode. The equipment can achieve the spatial positions of pitch angle, roll angle, focal distance, and reference points of nozzles. The particular attention was also given to the proposed process control methods to ensure high consistency accuracy in machining multi-nozzles. In addition, performance tests and applied experiments were carried out. The study results show that the equipment can meet the machining requirements of the spray holes with diameters Φ140–300 μm, taper-angles 0–1.3° (K-factor 0–2.3) with adjustment error <0.037°, space accuracy <0.2°, dimensional accuracy ±2 μm and machining-feed efficiency of 1.5 mm/min.