Indian Journal of Engineering & Materials Sciences Vol. 26, April 2019, pp. 142-148

Study of discharge effects by lateral electrodes in EDM

A-Cheng Wanga*, Lung Tsaia, Cheng-Yi Liua, Hsin-Min Leeb & Yan-Cherng Linc

aDepartment of Mechanical Engineering, Chien Hsin University of Science and Technology, Zhong-Li, Taoyuan 32097, Taiwan

bDepartment of Mechanical Engineering, Army Academy, Zhong-Li, Taoyuan 32093 Taiwan cDepartment of Mechanical Engineering, Nankai University of Technology, Caotun, Nantou 54243, Taiwan

Received: 8 July 2016

Lateral electrodes creating the side machining in electrical discharge machining (EDM) produce the plow grooves and the curved surfaces on the side wall of the blind hole, enhancing the value of the electrode in the machining process. However, lateral electrodes developed by strip spring or triangle copper block are not very stable in the lateral discharge machining. This study develops a new lateral feed electrode included gear and rack transmitted elements to obtain a good discharge effect in lateral EDM. In the process, two rotated gears have been utilized to deliver two horizontal racks, then these racks push copper plates to have horizontal deformation, grooves and curve surface would be machined during this stage. Moreover, copper plates would go back to the original position due to the recover forces of the springs after EDM. In the result, new lateral electrode performs good efficiency in the side discharge machining because of stable feeding mechanism of this electrode.

Keywords: Electrical discharge machining, Modified lateral electrode, Gear, Rack, Groove

1 Introduction The electrical discharge machining (EDM)

provides an alternative means in the precision machining fields by using high temperature during electrical discharging to remove the melting materials; with EDM it is suitable to process materials of high strength and high hardness due mainly to the creation of very small mechanical forces. The method of EDM can furthermore be money saving during processing as compared to conventional precision machining; and in the EDM processing, the design of electrodes has dominated the effects of discharging accuracy and efficiency. Therefore, how to design and produce high-quality electrodes has become the main subject in the research fields. José 1 utilized copper-tungsten electrode combined with the planetary EDM to perform the precision machining on the helix in the mold cavity. The results reveal, under the processing of the copper-tungsten electrode with negative polarity and planetary motions, that the method can create a high precision and smooth helix. Chen et al.2 applied semi-sintering electrode to low-carbon-steel EDM, during which it will produce huge electrode and then transform the so-called electrode-compound products onto the surface of work piece, thereby

fulfilling the demands of fast and efficient surface modification. Yan et al.3 designed, for the ceramic composite material with the metal matrix, an electrode which combines the EDM with ball burnishing to perform the electrical discharging in holes and burnishing on surfaces. In the study, Yan employed copper electrode at one end to perform EDM and then zirconia ball at the other end for the burnishing. By using this kind of processing, not only can the demands be carried out, but the recasting layers on the surfaces of holes can be removed. Shu et al.4 utilized carbon-silicon composite as the compound electrode. During processing, the rotational electrode combined with the abrasive in kerosene contributes to the actions of grinding and EDM simultaneously on the machining surfaces, so as to obtain a high surface roughness of work piece. However, EDM has some limitations during manufacturing, especially for the circular ring shape in the holes or grooves on the vertical walls, which will make it even more difficult to meet the demands. So far, the resulting methods have been no more than the use of electrolysis or electro-chemical machining5-8. Convenient as they are during processing, however, they are easily to cause environmental pollution.

From the above mentioned researches, the EDM can effectively process materials of high strength

—————— * Corresponding author (E-mail: acwang@uch.edu.tw)

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and high hardness; however, it is conventionally the up-and-down machining for the mold and hole. For plow grooves and curved surfaces on the side walls of the blind hole, on the other hand, it is difficult to execute the performance by the use of the general EDM; therefore, the development of lateral electrode EDM is desirable in response to those special shapes. Wang9-11 utilized EDM head to push an inclined plane, transforming the vertical motion of electrode into that in horizontal direction, such that horizontal micro holes can be carried out in micro EDM. However, this mechanism is not applicable to the manufacturing of inner rings or grooves in the tube surfaces. Consequently, in this study, an emphasis is made to characterize the lateral discharge effect on the elastic electrode and lateral feeding electrode.

2 Methods

2.1 Different types of lateral electrode

2.1.1 Lateral electrode with inclined plane elements In the experiment, Fig. 1 shows the first design

of the lateral-feeding electrode by inclined plane elements. When the electrode squeezes downwards, it will push the inclined 450 copper electrodes, giving rise to a motion laterally. During processing, because of the spring connected two blocks with two inclined planes, not only make the electrode plates match perfectly with the inclined planes but it will render them to be back to the original position as well, such that the discharged machining can process the inner walls of the tube easily. However, the quick spring back forces generating by two springs may be induced two blocks having a tilted motion during these blocks moving back to its original position, thus results the

electrodes not running smoothly in machining and making poor effects of the lateral-feeding EDM. On the basis of the drawbacks described above, a second lateral feeding electrode was designed by using the gear-and-rack transmitted elements in this study.

2.1.2 Lateral electrode with gear and rack transmitted elements The present design of the structure refers to the

lateral electrode discharge machining apparatus with gear-and-rack transmitted elements which comprises a bracket, gears, slides, electrodes and a copper rod with axial rack, as shown in Fig. 2, Among them, the bracket consists of the radial chute, gear pivots and slides located in the radial chute, each slide having a radial rack and a side discharge electrode, and the radial rack engaging with the gear system. Besides, the copper rod is embedded in the bracket and the axial rack of the copper rod engages with two gears in the radial chute. The backlash design existed in gear and rack, to avoid no smooth horizontal motion. Then, these gears drive two radial racks and electrodes to create the lateral motion when the axial rack of the copper rod pushes two gears to rotate. This novel mechanism design can enhance the boundary forces of sliders to increase the horizontal moving accuracy; and to provide stable discharge effect in the electrode discharge machining. Accordingly, the present design can easily generate the grooves or special holes on the inner tube or blind holes. The explosion diagram of mechanism is shown in Fig. 3. Figure 4 displays the design dimensions of lateral electrode part, which use in the discharge process of the general grooves. In addition, the simply diagram of moving lateral electrodes with downward Z-axis is shown in Fig. 5.

Fig. 1 — Structure of lateral-feeding electrode with inclined planes. Fig. 2 — Structure diagram of lateral electrode.

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3 Processes In this research, the machining work pieces of

SUS304 stainless steel are made by cutting out by using WEDM process. Die sinking discharging machine with four axes (including X, Y, Z, and a rotation axis C) is used to study the lateral discharging effect in this experiment. The machining polarity in EDM is decided by the polarity of the electrode; that is, the polarity is positive when the electrode is connected to positive pole. During the experimental operation, firstly by virtue of the four inclined 45o copper electrodes placed between two electrodes, when the upper electrode moves in downward direction, it will squeeze the inclined copper

electrodes to move laterally, leading to the EDM performance on the inside walls of holes or on sides of vertical surfaces. Moreover, by virtue of the fact that two rotated gears are utilized to deliver two horizontal racks, these racks will push copper plates to have horizontal deformation, and then grooves and curve surface can be machined during this stage. Table 1 shows the machining parameters of lateral EDM, such as the peak currents, pulse duration, off time, and the working voltages. The main purpose of the experiment is to investigate effects on the material removal rate (MRR), machining depth ratio (MDR), Surface roughness (SR), and electrode wear ratio (EWR) of the working parameters of EDM. The EWR and MDR are defined as the following equations.

𝐸𝑊𝑅 …(1)

𝑀𝐷𝑅 … (2)

4 Results and Discussion

4.1 Lateral feeding with inclined planes and gear -and- rack transmitted elements

Setting the parameters of peak currents, off time, gap voltage, open voltage, and rotating speed as constants, shown in Fig. 6 is the plot of material removal rates versus pulse durations for the two abovementioned lateral feeding methods. From the figure, it is clearly seen that, with the lateral feeding

Fig. 3 — Explosion diagram of lateral electrode mechanism withgear-and-rack-transmitted elements.

Fig. 4 — Diagram of lateral electrode part.

Fig. 5 — Diagram of moving lateral electrodes with downward Z-axis (a) motionless lateral electrodes with static Z-axis and (b)protruding lateral electrodes with downward Z-axis.

Table 1 — Setting values of EDM parameters.

Machining parameters Setting values Polarity +

Peak current(A) 3、6、9 Pulse duration(μs) 25~250 Off time(μs) 20~60 Gap voltage(V) 40 Open voltage(V) 100 Rotating speed(rpm) 200

Fig. 6 — MRR of lateral-feeding electrode under different pulse durations.

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by using the inclined planes, the material removal rate will increase as the pulse duration increases; however, the increasing will become less and close to a constant as the pulse duration of 125 μs is reached. On the other hand, by using the gear-and-rack transmitted elements, the material removal rate will increase sharply compared to that from the former method until the pulse duration of 175 μs arrives and yet increase smoothly after that value. Figure 7 shows the plot of relative electrode wear rates against pulse durations based on the same setting constant parameters mentioned above. From the figure, by using the inclined planes, the relative electrode wear rate has the tendency of decreasing from the beginning of 25 μs to 125 μs but after that increases slightly. However, when the gear-and-rack transmitted element is employed, though the tendency of decreasing is relatively parallel to the former during the same interval of duration, the relative electrode wear rate will approach to a horizontal line. Due to the fact that the novel mechanism of gear-and-rack transmitted structure can correctly constrain the motion of electrodes to enhance the moving accuracy.

Therefore, the less percentage of EWR is better to reduce the wear of the electrode. Figure 8 shows that currents and voltages are not very stable (especially open voltages in the machining) after large pulse duration in EDM when the electrode with inclined planes is applied. This is because free constrains of two bottom blocks in Fig. 1 will warpage during the spring back motions of two spring in EDM. However, gear-and-rack transmitted design can deform very smoothly, hence the stable currents and voltages are generated in the discharge method as shown in Fig. 9.

4.2 Effects of gear-and-rack-transmitted lateral feeding under different pulse durations

Figure 10 displays the values of MRR and EWR along the different pulse durations, in which whatever the values of the peak current, as the pulse duration progresses, there will result the greater value of MRR and less value of EWR. The main reason can lend itself to the following simple interpretation. When pulse duration progresses, the discharged column will grow bigger and therefore the values of MRR relatively higher; however, as it keeps growing, because of the less intensity of discharging energy, the slope of MRR

Fig. 9 — Current and voltage diagrams of lateral-feeding electrode with gear-and-rack mechanism.

Fig. 10 — MRR and EWR of lateral-feeding electrode under different pulse durations.

Fig. 7 — EWR of lateral-feeding electrode under different pulsedurations.

Fig. 8 — Current and voltage diagrams of lateral-feeding electrodewith Incline mechanism.

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will decrease, as shown in the right hand part of the figure. Considering the rank of priority sequence is 9A> 6A> 3A if only evaluating the item of MRR performance. But the 9A machining would result in the higher intensity of discharging to damage the electrode based on the higher percentage of EWR.

Figure 11 shows the SR versus the pulse durations for three different peak currents. As the peak current increases, there will result the deep craters and therefore the poor SR. As is shown in the figure, when the values of peak current and pulse duration are respectively, 9 A and 20 μs, the value of SR is 3.275 μm; however when the value of pulse duration reaches 100 μs, the value of SR will rise up to 3.887 μm. It is further revealed that when the values of peak current and pulse duration are 3 A and 20 μs, respectively, the value of SR is 1.632 μm; yet when the value of pulse duration reaches 100 μs, the value of SR will arrive at 1.899 μm. Therefore, although from this figure both the values of SR increase as the value of pulse duration increases, the value of SR will be lower as a result of the low peak current. It can further be seen from the three lines that the increase of SR is not significant as the pulse duration increases, which means the advantage in discharging precision of the gear-and-rack-transmitted method.

Figure 12 represents the plot of MDR versus pulse durations for three different values of peak currents. In general, the machining depth ratio (MDR) is a very important criterion to evaluate the precision of the lateral EDM, it will be more accurate if the MDR is closer to unity. In this experiment, the setting machining depth (DS) by the vertical cylinder electrode is designated as 0.3 mm. Evidently, as shown in the figure, the MDR, i.e., the ratio of actual machining depth (DM) to the setting machining depth, has the tendency of increasing as the values of pulse

durations increase, which is of the similar phenomena to that in the previous figure. It can further be seen, however, that the value of MDR is closer to unity when the peak currents are 3 A and 6 A at the value of 60μs of the pulse duration, and when the peak current is 9 A at the value of 40μs of the pulse duration; which means the precision of the lateral EDM at this points preferable to other situations. In addition, through this figure, with a view to obtaining the better machining precision, it is of importance to set up the proper setting machining depth. Based on the foregoing results and discussions in investigating the lateral EDM with different parameters, the peak current of 6 A value at the pulse duration of 60 μs is a better combination according to the higher MRR, less EWR, and consistency MDR performances.

4.3 Effects of gear-and-rack transmitted lateral-feeding under different off time

Figure 13 represents the plots of MRR and EWR against the off time. It demonstrates that, no matter what the values of the peak currents are, that the MRR decreases as the off time increases can clearly been seen; the reason resides mainly with the decreasing of

Fig. 11 — SR of lateral-feeding electrode with gear-and-rack-transmitted elements under different pulse durations.

Fig. 12 — MDR of lateral-feeding electrode with gear-and-rack-transmitted elements under different pulse durations.

Fig. 13 — MRR and EWR of lateral-feeding electrode with gear-and-rack-transmitted elements under different off times.

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the discharge intensity when the off time increases, thereby inducing the decrease of MRR and therefore the poor efficiency of machining. As to the EWR part, it can also been seen that, the more is increased the off time, the higher will be the EWR; this phenomenon can be interpreted as discharge machining time decreasing as off time increases, thus resulting in high EWR.

Figure 14 is the plot of SR along different off times for three values of peak currents. It is revealed that SR is decreased with increasing the off time in EDM, due to the fact that there will be more time to remove the burrs when off time increases, so as to obtain the better SR. As in Fig. 14, the three lines showing the decrease of SR are not significant as the off time increases, which demonstrates again the advantage in discharging precision of the gear-and-rack transmitted method.

The MDR versus the off time are plotted in Fig. 15. In this figure, MDR can clearly be seen decreasing as off time increases, which means the result of poor machining efficiency when off time increases. Moreover, from the present figure, MDR and off time

possessing the tendency of linear relationship can clearly been seen, such that the actual machining depth can readily be predicted during machining, and will be helpful for the demand of precision in lateral-feeding discharge machining.

4.4 Workpiece after EDM

Figure 16 shows the cross section of a deep hole after lateral-feeding electrodes performing the discharge machining by using the gear-and-rack transmitted method. The processing of the workpiece is executed at 9.5 cm from the left of the hole, in which the size of the hole in the radial direction is enlarged by 0.11 mm. Based on the data, the amount of enlargement of the hole is small enough to conclude that the better machining precision can be met by the present method. Figure 17 displays the design of the electrode plates in the experiment, on the left being the circular shape and on the right the rectangular shape. As shown in Fig. 18, there are the workpieces after EDM, where on the right is the ring-shaped hole and on the left the multiple rectangular holes. They demonstrate that by changing the shape of electrode plate, and with different machining

Fig. 14 — SR of lateral-feeding electrode with gear-and-rack-transmitted elements under different off times.

Fig. 15 — MDR of lateral-feeding electrode with gear-and-rack-transmitted elements under different off times.

Fig. 16 — Cross section of a deep hole after lateral-feeding electrodes.

Fig. 17 — Cross design of the electrode plates (a) circular shape electrode and (b) rectangular shape electrode.

Fig. 18 — Workpieces after EDM (a) ring-shaped hole in the stainless tube machining by circular shape electrode and (b) rectangular holes in the stainless tube machining by rectangular shape electrode.

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methods, not only can the circular shapes be carried out in the tube, but also the irregular shapes of holes can be fulfilled. Therefore, the lateral-feeding electrode discharging is appropriate to perform irregular shapes in the tube or in the blind holes.

5 Conclusions The design in this study of the lateral feeding

electrode with the gear-and-rack transmitted elements has provided the advantage in the problem of stability over the inclined-plane method. Moreover, the gear-and-rack transmitted mechanism was adopted to find out the suitable machining parameters, with a fixed 40 μs close off time, the peak current of 6 A, the pulse duration 60μs is a better combination according to the higher MRR and less EWR performances. However, as the experimental results of SR indicated that SR doesn’t have obviously change with increasing the pulse duration. Furthermore, in the case of MDR, it will be even closer to unity when under the conditions of low peak currents, high pulse durations, and appropriate off time. In view of off time, the higher the off time, the lower will be the value of the MDR, and it reveals the change in linear relationship with off time, such that the lateral-feeding depth can be predicated easily.

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