Study On Distribution Of Discharge Point And Technology Of Multiple Cutting In Multi-media For WEDM

The high-speed wire cut electrical discharge machining(HS-WEDM)machine has the unique advantages of low processing cost,fast cutting speed and low electrode loss,which is widely used in the field of numerical control machining in China.The surface quality of the workpiece after cutting has been greatly improved by the application of multi-cutting technology.The use of atmosphere or water mist media in the second cutting and the subsequent multiple cutting can further improve the cutting efficiency and workpiece surface quality.So the adoption of multiple media provides a new method for improving machining quality in multiple cutting for wire cut electrical discharge machining(WEDM).However,as the atmosphere and water mist media are different from the liquid media in nature,there are great differences in the process of WEDM.And some existing research and theories are mostly limited to the processing in liquid,which are not applicable to the processing conditions of WEDM in atmosphere and water mist media.When the discharge points tend to concentrate at a certain position,the concentration of electrolytic corrosion product around it increases,which reduces the insulation strength of interelectrode processing media.Then the certain position is more essily to form discharge.Loop like this,there will be an unprocessed area and arc discharge on the cutting surface.And it even burns the workpiece.In order to achieve higher processing quality and cutting efficiency,the discharge points in WEDM should be evenly distributed as much as possible.Therefore,it is necessary to detect the position of discharge point for monitoring the distribution of discharge point during the WEDM cutting process.Meanwhile,the processing mechanism and process characteristics of multiple cutting in multi-media need to be deeply studied to establish the machining theory suitable for WEDM in atmosphere and water mist,which improves the machining quality and cutting efficiency.It is of great significance to expand the application scope and improve the technical level of WEDM.This thesis analyzed the processing mechanism and finishing characteristics of WEDM in atmosphere,water mist and liquid media deeply.The selection principle of each cutting parameter for multiple cutting in multi-media was proposed,in which the processing media for initial cutting is water-based working fluid,the processing media for the second cutting is steam water mist or ultrasonic water mist,the processing media for the third cutting is ultrasonic water mist or gas,and the processing media for the fourth cutting is gas.In order to further achieve ideal surface quality and processing accuracy,the initial cutting selects larger electrical standard parameters for high-speed and stable cutting to form a rough outline,and the second to the fourth cuttings are sequentially polished using precision machining standards and precision micro standards in WEDM.Compared with the multi-cutting processes using water-based working fluid during each cutting stage,not only the surface roughness but the cutting speed and material removal rate are improved by the multi-cutting technology in multi-media,which provides an effective way to improve the surface quality and cutting efficiency of workpiece for WEDM.The detection technology of discharge point position in WEDM was studied.According to the theoretical analysis,the potential difference method was determined to detect the discharge point position,and the detection system and device of discharge point position based on voltage acquisition board,data acquisition card and Lab VIEW platform were established.The method of combining area division and relative standard deviation was proposed to quantitatively evaluate the distribution of discharge point,and the corresponding evaluation system of discharge point distribution was established.Then the distribution of discharge point under different media and different experimental parameters can be detected online,and the uniformity of discharge point distribution in WEDM can be quantitatively analyzed through the above system,which provides a new way to study the processing mechanism of multiple cutting in atmosphere and water mist.The temperature field simulation of WEDM in atmosphere and the overlapping distribution model of electro-etching pit were studied.The heat transfer model of a single pulse discharge under the moving heat source in atmosphere was established.The temperature field of a single pulse discharge under the moving heat source was simulated and analyzed by using the finite element simulation software ANSYS.Based on the simulation results of temperature field,the electro-etching pit formed by single pulse discharge was obtained and its erosion volume model was formulated.Also,the overlapping degrees of electro-etching pit in uniform distribution were classified,and the solving models of the material removal amount and surface roughness in different types of electro-etching pit overlapping distribution were built.According to the distribution of discharge point in WEDM,the overlapping modeling of electro-etching pit in different regions on the cutting workpiece surface was carried out,and the correctness of the model was verified through experiments.It provides a theoretical basis for the study of continuous pulse discharge erosion mechanism in WEDM when the atmosphere media is used in multiple cutting.The on-line thickness identification technology and process characteristics of variable thickness workpiece processed by WEDM were studied.A new method for on-line identification of workpiece thickness based on detecting the position of discharge point was proposed.The workpiece thickness identification system and device for WEDM based on potential difference method were built,and the on-line identification of workpiece thickness was realized.The method of multiple cutting of variable thickness workpiece with variable parameters in multi-media was adopted,that is,when the variable thickness workpiece is first cut in liquid,different processing parameters are used at different thickness,and in the second cutting and the subsequent multiple cutting,different processing media and processing parameters are used at different thickness of the workpiece.This method improves the high cutting efficiency of the variable thickness workpiece in the process of WEDM and achieves the relatively consistent machining surface at different thicknesses,which avoids the adverse impact on the overall machining quality of variable thickness workpiece by using fixed parameters in WEDM.The processing characteristics of variable thickness workpiece in HS-WEDM were experimentally studied.The influence of various processing parameters on the surface roughness,cutting speed and slit width at different thickness of the workpiece during the initial cutting in liquid was obtained by the orthogonal experiment.Then the relevant influence mechanisms were analyzed and the regression models of each process index were determined.Based on the central composite experiment,the regression models of cutting speed,surface roughness,cutting depth and relative standard deviation during the second cutting in steam water mist was formed.The non-dominated sorting genetic algorithm-II was used to optimize the processing parameters of variable thickness workpiece in WEDM,which obtained the optimal parameters at different thickness of workpiece in the first and second cutting.Also,the weight of each process index during the second cutting in steam water mist was determined by the analytic hierarchy process.A comprehensive model of cutting speed,surface roughness,cutting depth and relative standard deviation was established through the weighted comprehensive evaluation method for HS-WEDM.