DESIGN AND VERIFICATION OF SOFTWARE FOR SIMULATION OF SELECTED QUALITY INDICATORS OF MACHINED SURFACE AFTER WEDM

Transcription

1 DESIGN AND VERIFICATION OF SOFTWARE FOR SIMULATION OF SELECTED QUALITY INDICATORS OF MACHINED SURFACE AFTER WEDM Slavomíra HAŠOVÁ1, Ľuboslav STRAKA1* Wire electrical discharge machining with tool electrode is currently one of progressively developing technology of machining electively conductive materials. In practise, it is often used for cutting of materials with high strength, for example, for manufacturing of shape-complicated parts, for manufacturing of moulds for pressure casting, cutting tolls, etc. Because of this advanced technology is mainly used for the final operation, the resulting machined surface is dependent on the reliability of mechanical construction of the particular electroerosion equipment, but also on the reliable software regulation of electroerosion process. The paper deals with the design of software for simulation of selected qualitative indicators of machined surface in a base of optimal base choice of technological parameter of wire electrical discharge process. ABSTRACT: KEY WORDS: Wire Electrical Discharge Machining WEDM), mathematical model, surface roughness, simulation; 1 INTRODUCTION Wire electrical discharge machining (WEDM) occupies an important position among the advanced machining methods. [11] A long term development of this technology built to a level from which it can successfully compete with much precision machining technology, such as grinding, polishing, lapping, finishing, etc. [1,26] This technology has thus become a final technology which addresses the important issues of tool-making practice. In practice, it is mainly used for the production of precision parts with complex shapes such as moulds for die casting, cutting tools, and others. With its development undoubtedly is related the development of hardware and software, whereby both segments are closely linked. In general, without quality and reliable software control of electroerosive processes, or quality and reliable machinery it cannot be produced the quality products. [2] Therefore, these companies are working closely with reputable computer companies that are developing for their equipment the software tools. Development of own software for electroerosion equipment undoubtedly requires verification. It means the verifying that the result of each partial section, which the software addresses, meets the requirements [3]. 1 Faculty of Manufacturing Technologies with a seat in Prešov, Department of Manufacturing Processes Operation, Štúrova 31, Prešov, Slovakia, luboslav.straka@uke.sk Verification involves the testing modules and software, and its aims to prove that each module fulfils a function in practice and it does not also perform any other undesirable functions. 2 MACHINED SURFACE AFTER WEDM QUALITY Because the electrical discharge machining is among thermal processes, the machined surface must be assessed for the surface roughness and geometrical accuracy rate, and in terms of changes in microhardness, and depth of heat-affected zone [4,5]. 2.1 Assessment of machined surface quality after WEDM The resulting machined surface quality, achieved by electroerosion machining, in terms of roughness parameters according to ISO 4287 [29], is comparable to the quality of surface of fine grinding (Fig. 1). [6,7] Grinded surfaces are often characterized by high gloss of machined surface. But in many cases these have higher values of the arithmetic mean deviation of the roughness profile Ra, or the maximum height of the roughness profile Rz, or root mean square deviation of profile Rq than matte eroded surfaces. 13

2 To measure individual parameters Ra, Rz, respectively Rq of machined surface after wire electrical discharge machining with coat tool electrode are often used contact roughness testers, in practice. The resulting value of the maximum height of roughness profile Rz of the finished surface after WEDM is in the range from 1.5 to 20.0 micrometres. The arithmetic mean deviation of profile Ra is greater than 3 to 5 times. Fig. 1. The relief of the material surface after wire electrical discharge machining with coated electrode (magnification 1000 ). Conversely, the structure changes do not affect the materials for which the structural change occurs due to high temperatures during long-term exposure. [16-19]. The total depth of heat-affected zone hhaz of eroded surface after WEDM in dependence if setting of the main process technological parameters is in the range of 15 to 70 m. This value can be decreased to the less than 10 m by after-treatment, for example by blast cleaning with glass beads. Fig. 2. Structural changes of the surface and closely subsurface layers in WEDM. 2.2 Assessment of heat-effecting impact on machined surface quality after WEDM Another, no less important qualitative indicator which characterizes the surface finish after WEDM is total depth of heat-affected zone influence in the place of cut. In general, the heat affecting of surface or subsurface layers of the machined surface undesirable concomitant of electroerosion technology. [8,9] The extent and nature of heataffecting the machined surface after WEDM is largely dependent on the setting of the main process and process parameters. [21] The heat-affected zone (HAZ) of machined surface after WEDM is in most cases characterized by three sub-indicators, namely: - structural changes of material in the place of electroerosion, - total depth of heat-affected zone (hhaz), - microhardness changes in the place of cut (HV). Because WEDM belongs to the thermal processes, it can be expected certain structural changes beneath the subsurface layer (Fig. 2). These structural changes are highly dependent on the structure of the starting (base material), and also by the heat treatment. The largest structural changes can be expected of the materials in which by the short effect of high temperatures results in a change in the structure. 14 Fig. 3. Microcracks formation due to impact of the fault setting the main and process technological parameters in WEDM. Action of a high temperature and with vigorous cooling using dielectric fluid occurs to structural changes and microhardness changes of surface and subsurface eroded area (Fig. 3). Change takes effect on the increase or decrease in microhardness in comparison with the hardness of the basic material. The magnitude of this change is again largely dependent on the setting of the main process and process parameters [12,13]. 2.3 Impact assessment of geometric accuracy on the quality of machined surface after WEDM Geometric and dimensional accuracy of the machined surface in the WEDM is mainly dependent on the accuracy and reliability of electrical erosion equipment, precision mounting of the workpiece, guide of the tool (Fig. 4), and thermal expansion, internal stress of the base

3 material of the workpiece, and not least by setting the main and process parameters of the technology. Fig. 4. Impact of wire tension on geometric accuracy of machined surface in WEDM. The geometrical and dimensional accuracy that occurs in the WEDM is very much affected by oscillation of the wire electrode. Vibration of the wire electrodes can be attributed to an electric discharge between the tool and the workpiece when the tension strength of wire electrode [14,15]. This derogation is in WEDM in the range of 5 to 20 µm, especially depending on the setting of the main and process technological parameters. 2.4 Impact assessment of the reliability of the electroerosion equipment for the quality of machined surface after WEDM Influence of the reliability of electroerosion equipment for quality machined surface after WEDM can be assessed from two perspectives: a. influence of the reliability of the constructional mechanical parts reliability of the constructional mechanical parts of electroerosion equipment significantly affects the quality of machined surface after WEDM. Typical examples are mistakes of geometric shape of eroded contours (Fig. 5) as a result of the faulty guide the wire electrode due to wear of parts of the guiding device, bearing play, transfers and so on. Although the modern electroerosion equipment for these errors are order of magnitude of microns and sporadically occur, it is necessary to count with them. Fig. 5. Typical mistakes of shape and position in WEDM due to incorrect guide of the wire electrode. b. influence of the reliability of the control part reliability of control parts of electroerosion equipment significantly affects the quality of machined surface after WEDM. Due to lack of software management functions of mechanical parts, it occurs to the typical mistakes, such as the overtravel tool electrode in the corners (Fig. 6), error of the tool path correction, interpolation errors, etc. Fig. 6. Typical defects of the shape and the course in WEDM due to incorrect control of the wire electrode. 3 TECHNOLOGICAL PARAMETERS INFLUENCING THE MACHINED SURFACE QUALITY AFTER WEDM Technological parameters (TP) for Wire electrical discharge machining with coated tool electrode is understood the parameters which enter into electroerosion processes. These can be divided into two main groups [20,22]. The first group represents the main technological parameters which directly, significantly affect the final quality of machined surface in terms of higher defined qualitative indicators. These include: - peak current I A, - pulse on-time ton μs, - pulse off-time toff μs, - voltage of discharge U V. The first group represents the process technological parameters which indirectly affects the final quality 15

4 of machined surface in terms of higher defined qualitative indicators. These include: In relation to the tool - wire tool diameter Ø mm ; - wire tool material; - distance the electrode from the workpiece (working gap) mm ; - wire feed rate vp m.min-1. In relation to the dielectrics- electrical conductivity S. cm-1 ; - chemical composition; - purity; - temperature C ; - flushing pressure Fw Pa ; In relation to the workpiece - workpiece dimensions mm ; - workpiece material (mechanical, physical and chemical properties); - heat-treatment of workpiece material; - workpiece material homogeneity. The resulting quality of the machined surface in the WEDM undoubtedly relates to the process parameters, i. e. internal parameters [23,24]. Process parameters in electrical discharge machining are understood the parameters that are directly dependent on the main technological and service performance and also significantly affect the 16 quality of machined surface. [25] These include the speed of wire electrode vf mm.min-1 and material removal rate MRR mm3.min-1. 4 DESIGN OF SOFTWARE SOLUTION FOR SIMULATION OF SELECTED QUALITY INDICATORS OF MACHINED SURFACE AFTER WEDM The focus of the proposed software solution for simulation of selected quality indicators machined surface after WEDM comprises computing core that is based on mathematical models detailed in references [17-22]. These mathematical models replicate the functional dependence of the main technological parameters of the experimental measurements. [27] The software is intended to enable simulation of the machined surface quality based on the optimal selection of the main technological parameters. [28] The following algorithm (Fig. 7) shows the functional structure of the proposed simulation software. The focus of the software is a module based on the input data, i. e. main technological and process parameters, and it simulates the final quality of machined surface after WEDM in terms of surface roughness parameters, geometric accuracy, and the total depth of the heat-affected zone.

5 Fig. 7. The structure of the application software CNC TP Simul WireEDM. 4.1 Proposed application simulation software 1. Program start-the running the simulation software in the operational program XP and higher. 2. Input data (Fig. 8): machining material => wire electrode diameter => workpiece thickness => main technological parameters. 5 VERIFICATION OF THE PROPOSED SIMULATION SOFTWARE Due to the objective assessment of the quality of the proposed software solution for simulation of selected indicators of machined surface after WEDM was carried out verification on several types of related products. In all cases a cutting tool, and used the material X210Cr12 EN (STN ). [30] For verification was used electroerosive equipment AGIECUT 200D by Swiss company Agie [31] which is shown in the following fig. 10. Fig. 8. Entering the input data in the proposed simulation software. 3. Simulation of selected quality indicators of machined surface after WEDM: calculation of values of selected qualitative indicators using mathematical models, protocol of simulation of selected qualitative parameters of machined surface after WEDM: 1 roughness parameters of machined surface, 2 parameters of total depth of the HAZ, 3 parameters of maximum geometric tolerances of machined surface. 4. Graphical representation of simulation results allows graphically to show the results of simulated values (Fig. 9). 5. Simulation protocol pressing 6. The archiving the data of simulation protocol 7. Program stop. The legend: 1 - table (tank with dielectric fluid) 2 - the supporting construction 3 - wire electrode transport system 4 - control panel 5 - equipment for regulation of dielectrics Fig. 10. The basic structure of electroerosion equipment AGIECUT 200D. 5.1 Conditions of the experimental verification of simulation results: In relation to the workpiece: - workpiece material EN X210Cr12, - workpiece thickness 100 mm. In relation to the tool: - wire electrode diameter 0.25 mm, - wire electrode material Ms63, - wire feed rate vp = 6 m.min-1, - wire electrode tension Fw = 8 N. In relation to the dielectrics: - electrical conductivity 50 S. cm-1, Fig. 9. Graphical representation of simulated values - temperature 25 C, and simulation protocol - flushing pressure p = 2kPa. 17

6 The aim of the experimental verification of the quality of the proposed software solution was to verify the simulation results of one of the indicators of quality machined surface after WEDM based on the input parameters on the workpiece, tool and dielectric fluid to verify the simulation results of one of the indicators of quality machined surface after WEDM. Verified was the qualitative parameter roughness of machined surface Ra (Fig. 11). surface roughness Ra and the values established by the proposed simulation software for the production of cutting tools made of EN X210Cr12 through technology WEDM By comparing the simulated and experimentally measured values of the surface roughness Ra was found that the deviation is Ra 0,03 0, 02 between m with accuracy 0.01 m surface roughness tester Mitutoyo Surftest SJ The benefits achieved by proposed simulation software: a) benefits for the user of EDM technology: achieving higher quality products, reducing operating costs, reducing the risk of spoilage. shortening the production times, increasing economic efficiency and competitiveness of the company operating the WEDM technology. Fig. 11. Verification of machined surface roughness parameters Ra in the manufacture of cutting tools using technology WEDM. The Table 1 shows the comparison between experimental values obtained by measured equipment of qualitative parameter of machined b) the benefits to the customer: closer communication between the customer and manufacturer. more exact specification of requirements for product quality, optimum choice between quality and price. Table 1. Comparison of the obtained and simulated values of surface roughness Ra Surface roughness parameter Ra m Controlled part The value obtained by experimental measurements of the cutting Average tool No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 value Part Part 2 Part Part Part Part 6 Value determined by simulation software The difference of values implementation, and verification in practice. The main part of the proposed simulation software are Product quality and production efficiency is the aim mathematical models that allow to simulate on input of every manufacturing process. Product quality in data quality parameters for WEDM machined technology WEDM significantly affect the surface roughness parameters in terms of the total reliability of the software management depth of heat-affected zones and the maximum electroerosion processes using modern control geometric deviations of machined surface. The systems. The paper aimed to describe the procedure verification was carried out in real conditions on design software for the simulation of selected samples made of tool steel EN X210Cr12 in the quality indicators machined surface after WEDM manufacture of cutting tools. That the proposed based on the input technological and process simulation software will become the universal parameters. It describes in detail the design, software for simulation of selected quality indicators machined surface after WEDM, it would 18 6 CONCLUSIONS

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