Qinhe Zhang

Effect of Electrode Size on the Performances of Micro-EDM

Understanding the effect of processing parameters on the tool electrode wear during micro-electrical discharge machining (micro-EDM) is helpful to predict and compensate the electrode wear, so as to improve the machining precision. In this paper, experiments are carried out and the influences of tool electrode diameter on the micro-EDM process are discussed based on the skin effect and area effect. It is demonstrated that the machining speed, tool wear, and taper rate are different with the increase of tool electrode diameter. Due to the skin effect and area effect, larger electrode diameter results in higher material removal rate along with higher tool wear rate. The electrode material removal increment is more than the workpiece material removal increment with the increase of tool electrode diameter, which leads to the increase of relative tool wear ratio. Discharge energy is concentrated on the tool surface which enhances the possibility of discharge on the side face and the corner of the tool electrode during the micro-EDM, especially when drilling with a larger tool electrode. As a result, a tool electrode with larger diameter results in a higher taper rate.

Effects of grain size of AISI 304 on the machining performances in micro electrical discharge machining

The material removal process of micro electrical discharge machining is based on the instantaneous ultra-high temperature generated by a series of repetitive discharge pulses. Due to the size effects, the polycrystal cannot be considered as continuous and homogeneous material when machining is in micron scale, and the effects of material microstructure should not be neglected. In this article, the thermoelectric characteristics of grain and grain boundary are discussed, and the influence of grain size on the machining performances in micro electrical discharge machining is researched. Two kinds of austenitic stainless steels (AISI 304) which are different in grain size are chosen as the workpieces in experiments. It is verified by both theory models and experimental results that the smaller the grain size, the higher the material removal rate, under the same discharge conditions. Both thermal conductivity and melting point of the grain boundary are lower than those of the grain because of the grain boundary segregation. The effective thermal conductivity and local effective melting point of polycrystalline materials vary with their grain sizes since the grain boundary volume fractions change. As a consequence, the material removal rate of micro electrical discharge machining has direct relationship with grain size of the workpiece.

Effects of flushing on electrical discharge machining and electro-arc machining:

Both electrical discharge machining and electro-arc machining are non-contact machining processes, which use high temperatures produced by discharges to erode material. The gap between the tool and the workpiece is usually filled with liquid dielectric medium. Moreover, liquid flushing is commonly used in the machining processes of electrical discharge machining and electro-arc machining. Much research has been conducted to study the effects of flushing on these processes. However, there has been little study on the mechanism of the flushing procedure for these machining processes. In this article, electrical discharge machining and electro-arc machining experiments adopting different polarities and flow rates were designed and implemented. The plasma tunnel and crater morphology were studied. It was found that the plasma tunnel was lengthened, compressed and broken by flushing in the electro-arc machining experiment; however, the electrical discharge machining experiment did not produce this result. Similarly, tailing discharge craters and polarity effects caused by flushing were observed in the electro-arc machining experiment, while no such performances caused by flushing were found in the electrical discharge machining experiment. The flushing in electro-arc machining was able to reduce the material removal rate, tool wear rate and surface roughness simultaneously, while the effects of flushing in electrical discharge machining were inconspicuous. Moreover, the characteristics of the plasma tunnels of the electro-arc machining experiment were similar to those of arcs, which were different from those of the electrical discharge machining experiment. These results help to verify that the electro-arc machining process adopts arcs, while the electrical discharge machining process adopts sparks, during their respective material removal processes.

Scale effects and a method for similarity evaluation in micro electrical discharge machining

Electrical discharge machining(EDM) is a promising non-traditional micro machining technology that offers a vast array of applications in the manufacturing industry. However, scale effects occur when machining at the micro-scale, which can make it difficult to predict and optimize the machining performances of micro EDM. A new concept of “scale effects” in micro EDM is proposed, the scale effects can reveal the difference in machining performances between micro EDM and conventional macro EDM. Similarity theory is presented to evaluate the scale effects in micro EDM. Single factor experiments are conducted and the experimental results are analyzed by discussing the similarity difference and similarity precision. The results show that the output results of scale effects in micro EDM do not change linearly with discharge parameters. The values of similarity precision of machining time significantly increase when scaling-down the capacitance or open-circuit voltage. It is indicated that the lower the scale of the discharge parameter, the greater the deviation of non-geometrical similarity degree over geometrical similarity degree, which means that the micro EDM system with lower discharge energy experiences more scale effects. The largest similarity difference is 5.34 while the largest similarity precision can be as high as 114.03. It is suggested that the similarity precision is more effective in reflecting the scale effects and their fluctuation than similarity difference. Consequently, similarity theory is suitable for evaluating the scale effects in micro EDM. This proposed research offers engineering values for optimizing the machining parameters and improving the machining performances of micro EDM.