THEORETICAL AND EXPERIMENTAL RESEARCH CONCERNING EDM OF COMPLEX AND REVOLUTION-PROFILED SURFACES

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1 Nonconventional Technologies Review Romania, December, Romanian Association of Nonconventional Technologies THEORETICAL AND EXPERIMENTAL RESEARCH CONCERNING EDM OF COMPLEX AND REVOLUTION-PROFILED SURFACES Buidoș Traian 1, Ursu Mircea-Petru 2 and Crăciun Dan 1 University of Oradea, tbuidos@uoradea.ro 2 University of Oradea, mpursu@uoradea.ro University of Oradea, dancraciun28@yahoo.com ABSTRACT: this paper presents a synthesis of the theoretical and experimental research of the authors concerning the use of EDM in order to produce complex surfaces with revolution profiles. This research refers to the performances of the manufacturing by means of the working and productivity characteristics, dimensional accuracy, quality of the worked surfaces and the electrode wear characteristics. The experimental research has been carried out in the Nonconventional Technologies Laboratory (University of Oradea) with an ELER-01-GEP-50F ED machine, using a specially designed device to rotate the tool-electrode. KEY WORDS: EDM, rotary electrode, pause time, pulse time, surface quality, roughness 1 INTRODUCTION The necessity to manufacture parts with complex revolution-profiled surfaces of hard and extra-hard materials (highly-allied steel, metal carbide etc) has required the use of new manufacturing procedures, and one of them is electric discharge machining (EDM), which ensures the production of such profiles with adequate dimensional accuracy, surface quality and relatively high productivity. When EDM machines are used, often the part is fixed and the tool-electrode advances towards it, and the part surface is attained by means of shape copying [6, 8, 9]. Thus, the revolution-shaped surfaces cannot be attained on these machines without using specialized devices with adequate cinematic [, 5]. of this solution allows the manufacturing of complex interior or exterior surfaces, such as metal carbide rolling mills, spline shafts, gearwheels etc. Figure 1. Sketch of the manufacturing of revolution-shaped surfaces by EDM. A possible sketch for this kind of manufacturing is shown in figure 1 [4]. It is easy to see that both the part-electrode 2, clamped on support 1, and the toolelectrode must perform rotation motions. The use Figure 2. Device for manufacturing exterior revolution-shaped surfaces by EDM (1 = base plate; 2 = part-electrode axle; = belt wheel; 4 = belt; 5 = bearing; 6 = column; 7 = gearwheel; 8 = middle gearwheel; 9 = belt wheel; 10 = pinion; 11 = electric motor drive; 12 = motor body and upper bearings; 1 = machine work-head; 14 = upper body, orienting element; 15 = tool-electrode; 16 = tool-electrode axle; 17 = belt stretcher assembly; 18 = column; 19 = mobile peak assembly; 20 = peak; 21 = base plate; 22 = machine table. The experimental research, carried out by specialised companies (Charmilles and Agie of Switzerland, AEG Siemens of Germany, Ona of USA, Japax of Japan etc), shown that it is possible to attain high quality, accuracy and productivity, along with reduction of electrode wear, by means of a rotating motion of the electrode relatively to the work piece. These companies made such 21

2 manufacturing devices, but they fit only for the respective machines, and the rotating motion is transmitted by means of direct-drive clutches built into the machine work-head. In order to manufacture complex revolution-shaped surfaces with ELERtype EDM machines, it is necessary to build specialized devices, such as the one shown in figure 2 [4]. Also, this research continues at the University of Oradea, along with other prestigious Romanian universitary centers (București, Iași, Timișoara etc). The series production of the Romanian ELER-type electric discharge machines has begun in years 70 by company Electrotimiș (Timișoara), and now the Romanian-Italian company STIMEL (Timișoara) produces the STIMEM-type electric discharge machines with massive electrode and the STIMEFIL-type electric discharge machines with wire electrode. 2 DEVICES USED FOR THE MANUFACTURING OF COMPLEX SURFACES At the University of Oradea, Faculty of Managerial and Technological Engineering, several orbital and planetary movement devices were made, for horizontal and vertical threading, working of copper and graphite electrodes etc., in order to finalize some research contracts and endowment of the Nonconventional Technologies Laboratory. Some of these devices have been presented in other scientific papers [1, 2]. Figure. EDM for the rotation of the electrode. These devices must be able to perform the following actions: orienting and fixing of the tool-electrode; orienting and fixing of the part-electrode; transmission of the motion from the electric motor to the tool-electrode axle; transmission of the motion to the part-electrode axle; positioning of the part-electrode versus the toolelectrode. Such devices are shown in figure and figure 4. Figure 4. Device for electrode rotation (sketch and view); 1 electrode, 2 electric motor, fastening assembly, 4 worm gear mechanism, 5 auxiliary electronic device. The device shown in figure is fixed on the EDM machine workhead by means of a gripping body. The device shown in figure 4 is fixed on the EDM machine workhead by means of a flange fitted with holes at 120º each, which allows the setting of the perpendicularity of the electrode versus the part to be machined. The device is fitted with a 12V DC geared motor (2), with adjustable speed 1-60 rot/min. The motor can be fed directly from the ELER machine, which implies one speed, or from a separate adjustable power supply (5), which allows adjustable speed. The rotation of the electrode is attained by means of a worm-gear assembly, which also transfers the movement from horizontal to vertical. The gripping of the electrodes into the device is made by means of several 1-25mm elastic bushings, which are fastened with a screw nut. Figure 5 shows manufacturing possibilities for several profiled cavities, by means of electrode rotation (left) and shape copying (right), and figure 6 22

3 shows a section through the EDM manufactured cavities. Figure 5. Manufacturing possibilities for profiled cavities, by electrode rotation (left) and shape copying (right). Figure 6. Section through EDM machined profiled cavities. EXPERIMENTAL RESEARCH AND RESULTS High accuracy analytical balances, roughness meters and micrometers were used in order to check the final manufacturing factors (figure 8) productivity, surface quality, dimensional accuracy and electrode wear [7]. The experimental research was carried out in the Nonconventional Technologies Laboratory, Managerial and Technological Engineering Faculty, University of Oradea, by means of the ELER-01GEP-50F EDM machine of this laboratory. The device shown in figure 4 has been used for the rotating motion of the electrode, with speeds of 1-60 rot/min. Cr120 steel probes, HRC thermally treated, were used for the tests. The surfaces quality was measured by means of a Surtronic 25 roughness meter for flat surfaces, and with a Taylor-Hobson roughness meter for the profiled surfaces. The second meter can be connected to a computer and the measuring results can be visualized on the computer display and can be stored by means of an adequate software program. The software can transform the profiled shapes into linear shapes, thus the minimum and maximum values of the measured roughness can be determined. Figure 8. Final factors of the electrical erosion processing. Productivity is given by the quantity of the EDM removed material in time unit [mm/min] or [g/min], according to the measuring method. Electrode wear is similarly defined, with the difference that it refers to the tool-electrode and not to the worked part. In order to attain roughness values of µm, diamond pastes of 1-25 µm granulation (figure 7) were used after EDM. The surface quality refers to two aspects: the microgeometry of the surface itself and the properties of the superficial thermally influenced layer. The final factors shown in figure 8 depend of several parameters that are set by means of the ELER machine generator, such as current intensity, pulse time and pause time. Several values of the productivity and surface quality are attained by combining various value sets of these parameters, taking into account the work voltage polarity, the dielectric flow into the work place and the properties of the couple of tool and part materials [10]. Figure 7. Diamond pastes. 2

4 I {, 6,12,15,18, 24, 27, 0, 7, 40, 46, 50} [A] TI {2.5, 4, 6, 8,12, 24, 48, 95,190, 420, 900,1800} TP {2.5, 4, 6, 8,12, 24, 48, 95,190, 420, 900,1800} By combining the 12 values of the current intensity ( 50 A) and of the pulse and pause times ( µs), several values of productivity and worked surfaces quality are resulted (figure 11). [µs] [µs] By means of the device shown in figure, manufacturing tests have been carried out by two different methods. The part was EDM processed with an adequate shaped copper electrode, both by shape copying and by rotation of the electrode (figure 9), using the same operating parameters current intensity, pause time, pulse time, polarity, dielectric flow. Figure 11. Variation of current intensity I = f(prod,ra); I current intensity [A], PROD productivity [mm/min], RA roughness [µm]. When special quality of superfinished (lapped) surface is required, the lapping of the worked surfaces with diamond pastes (figure 7) is performed after EDM. These special surfaces are often met in the active parts of thermoplastic materials injection dies. Figure 12 shows the test part after EDM, with 1.8 µm roughness, and figure 1 shows the same test part after superfinishing, with µm roughness. Figure 9. EDM manufacturing by shape copying (left) and bu rotating electrode (right). Table 1 shows the roughness measuring results for different current values, at the same values for pause time and pulse time. Table 1. Roughness of the worked surface; pause time 6µs, pulse time 24µs. Current intensity [A] Roughness Ra shape copying Roughness Ra rotating electrode The measurements have been carried out by means of a Taylor-Hobson roughness meter for the concave surface of the worked part (figure 10). Figure 12. Test part after EDM. Figure 10. Measuring with Taylor-Hobson roughness meter. Figure 1. Test part after superfinishing. 24

5 4 CONCLUSIONS According to the experimental research concerning the EDM of the complex revolution-shaped surfaces, the following have been observed: The use of the devices shown in figure and figure 4, which allow the rotation of the toolelectrode with speed of 1-60 rot/min, leads to reduction of the electrode wear as the productivity and working precision increase and roughness decreases, compared to the shapecopying EDM. The working precision of complex surfaces by means of EDM depends on how the centering and setting of the tool-electrode versus the partelectrode were performed, and on the dimensional accuracy of the tool-electrode. In order to properly center the tool-electrode in the gripping device, it is required to check the angular misalignment of the electrode head and its adequate compensation by means of dial comparator with µm accuracy. In practice, in order to execute various active parts of dies, punches and shells, we may meet different situations concerning the choice of the optimization criterion, which may be the working productivity, the surface of the worked surface or the dimensional accuracy. Sometimes the surface quality is the most important, other times the dimensional precision is the most important. According to the chosen performance criterion, we will use various versions of the parameters values which comply with the requirements of the part to be made. When very high dimensional precision is needed, it becomes necessary to produce several finishing electrodes. It is obvious that we cannot attain high performances for all three final parameters simultaneously. The choice of one as the main parameters turns out to be disadvantageous for the other two. For example, the increase of productivity causes the decrease of the surface quality and dimensional accuracy. Special care must be taken in the case of productivity increase over 400 mm /min, because this requires current intensities that exceed 25 A, which yields a thermally influenced layer of greater thickness that will increase the superfinishing / lapping time, and sometimes it exfoliates, leading to greater manufacturing costs. This can be avoided by using the finishing generator of the ELER machine, by currents below 6A and by increasing the pause time versus the pulse time. For example, the pause time can be set to µs, and the pulse time can be set to 4-24 µs. When large complex surfaces are to be machined, in order to increase productivity and reduce electrode wear, it is recommended to use first classical machining means for coarse grinding, then thermal treatment, then EDM. 5 REFERECES 1. Buidoş Traian, Mihăilă Ioan, Device For Thread-Making By Means Of Electrical Erosion, pp.59 62, International Scientific Conference TMCR-2005, Chişinău, Republica Moldova (2005); 2. Buidoş Traian, Mihăilă Ioan, Ursu Mircea-Petru, Three-dimensional surface generation possibilities using massive electrode by means of ELER-01-GEP-50-F electric erosion machine, Revista de Tehnologii Neconvenţionale nr./2007, pp.7 10, pg.4, Editura PIM Iaşi, Romania (2007);. Che Chung Wang, Biing Hwa Yan, Blind-hole drilling of Al 2 O /6061Al composite using rotary electro-discharge machining, Journal of Materials Processing Technology, Volume 102, Issues 1-, 15 May 2000, Pages (2000); 4. Gavrilaș Ionel, Marinescu Nicolae, Prelucrări neconvenționale în construcția de mașini, vol.1, pp.24-27, Editura Tehnică București, Romania (1991); 5. Koshy Philip, V. K. Jain, and G. K. Lal, Experimental Investigations Into Electrical Discharge Machining With A Rotating Disk Electrode, Precision Engineering, Volume 15, Issue 1, January 199, Pages 6-15 (199); 6. Mihăilă Ioan, Tehnologii neconvenţionale, Ed. Imprimeriei de Vest Oradea, Romania (200); 7. Nanu Aurel, Tratat de tehnologii neconvenţionale, vol.i, Editura Augusta Timișoara, Romania (200); 8. Obaciu Gheorghe, Sisteme şi tehnologii pentru prelucrarea prin eroziune electrică, Universitatea din Brașov, Romania (2000); 9. Slătineanu Laurențiu, Tehnologii neconvenţionale în construcţia de maşini, Editura Tehnică INFO, Chişinău, Republica Moldova (2000); 10. ***, Handbook of ELER EDM machine 25