Study On Micro Hole Drilling With High Aspect Ratio By Micro EDM

With the development of industries and technologies, the application of micromachining increases quickly. Micro EDM can produce micro parts and features, but there are also problems when it is used to drill micro holes with high aspect ratios. The machining technology and equipment development for micro holes have become the key technology for manufacturing products with micro structures.This paper aims at providing new thoughts and methods to the key difficulties mentioned above. A high precision horizontal micro EDM lathe and a set of computer control system were developed. The control system is based on NI LabVIEW and has the functions of discharge state detection and control, manual control, contact sensing (surface detection), data acquisition and preservation, etc.In micro EDM, the electrode wear is large. When a micro hole with high aspect ratio is drilled, the viscous resistance of dielectric in the narrow discharge gap increases with the increase of the hole depth, causing bubbles and debris generated difficult to escape from the gap, leading to frequent occurrences of abnormal discharges and resulting in extra electrode wear. To solve the problem of machining micro-holes with high aspect ratio, a new method that with planetary movement of electrode and ultrasonic vibration on the workpiece was proposed. A through hole with an average diameter of 120μm is successfully drilled in stainless steel 304 with a thickness of 3.5mm. The aspect ratio reaches 29.In micro EDM, although there is a discharge gap between the electrode and the workpiece, there is no mechanical contact between them, the process reaction force is not zero in the actual EDM process. The force includes the electrostatic and electromagnetic force, the pressure of bubbles generated by evaporation and dissociation of the dielectric liquid, the force due to flushing of the dielectric and the gravity of the electrode. In deep hole machining, due to the large electrode ware, the electrode has to be prepared long to compensate the wear length. It is found that the discharge gap varies with the electrode length. Three different theoretical models, that the electrode is only acted by a radial force, the electrode is acted by a radial force and consider the influence of the rotation speed of the electrode and the electrode is acted by a axial force and consider the influence of the rotation speed of the electrode, are developed and used to estimate the reaction force and the bending deformation of the electrode. Experiments are carried out and the results indicate that the theoretical values agree with the measured values. Finally the finite element analysis software ANSYS is used to simulate the deflection of the electrode in the three different models. The simulation results also agree with the real machining results and verify the correction of the theoretical models.Due to the existence and influence of the process reaction force, when micro EDM is used to drilling micro holes, especially high aspect ratio micro holes, the length of the electrode must be taken into account.