Micro-EDM milling is an effective machining process for three-dimension (3D) micro-cavity of high hardness materials. Because of the sharp wear of the electrode, fix-length compensation method is proposed, thus results in a cone-shaped electrode. This compensation method can produce a slot with stable depth and conic cross-section. Because of layer by layer strategy in micro structure machining, the challenge for fix-length compensation method is to keep the whole process repeating continuously. This paper gives a double layers’ machining strategy in order to maintain the repeated machining for layer units. A concept of equivalent plane is proposed in order to construct a curved surface that can use fix-length compensation method. Then a simplified triangle model is presented in order to get corresponding algorithm for the compensation length of the second slot. Experiment results showed that the error of the compensation length was within 2 μm. A simulation about the influence of the curved surface's fluctuation on the profile of the machined slot bottom of next layer was performed with Matlab. Simulation results showed that the influence was limited. An experiment of two layers’ machining with proposed strategy and algorithm was conducted. The evenness of the machined surface with depth of 66 μm was within 4 μm. Therefore, fix-length compensation method can be used to machining 3D cavity.
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