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Development of Multi-Spark EDM
Development of Multi-Spark EDM Masanori Kunieda (Z), Hideyuki Muto Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan Received on January 3,2000
Abstract This paper describes the Multi-spark EDM method which was newly developed to obtain higher removal rates and lower energy consumption compared with conventional EDM. The basic circuit of Multi-spark EDM comprises a pulse generator, a first tool electrode, the workpiece, and a second tool electrode that are connected serially in the order as listed here. Accordingly, for each generator pulse, one discharge occurs in the gap between the first tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and the workpiece. To balance the removal rates in both gaps, the polarity of the pulse generator is changed adaptively to equalize the gap voltages measured at both gaps. Keywords: Die-sinking EDM, Multi-spark, Discharge point
INTRODUCTION In the conventional electrical discharge machining (EDM) process, there is only one discharge point for each pulse. According to this fact, the material removal rate of EDM is normally much lower than that of electro-chemical machining (ECM) in which the machining current flows uniformly over the whole working surface at the same time. To set up multiple discharge points for each pulse, Mohri et al. [I] divided a tool electrode into multiple electrodes, each of which is electrically insulated and connected to the pulse generator through a resistor. In this case, after a discharge occurs in the gap between one of the divided electrodes and the workpiece, the gap voltages at other electrodes are maintained at the open circuit voltage level for a certain period until the surface electric charge over these electrodes is redistributed or another discharge occurs. Hence, finally discharge can occur at different electrodes simultaneously. Suzuki et al. [2] and Kubota et al. [3] developed a twin electrode discharge system for the discharge dressing of metal bonded grinding wheels. The discharge circuit is formed by connecting the pulse generator to one of the two twin electrodes, the grinding wheel, and the other twin electrode serially. In this system, for each pulse, two discharge points can be obtained simultaneously at both the gaps between the twin electrodes and the grinding wheel using only one pulse generator. Hence, there is no necessity to insulate the grinding wheel from the machine body or attach a slip-ring brush [3]. This paper utilizes the same circuit configuration as the twin electrode discharge system. The aim of this paper, however, is to obtain higher removal rates and lower energy consumption in the normal applications of EDM processes compared with conventional EDM. 1
PRINCIPLE Figure 1 shows the principle of Multi-spark EDM newly developed. The basic circuit of Multi-spark EDM is comprised of a conventional pulse generator, a workpiece, and two tool electrodes, both of which are fixed onto the quill of the same EDM machine. The pulse generator, the first tool electrode, the workpiece, and the second tool
electrode are connected serially in the order as listed here. Accordingly, for each generator pulse, one discharge occurs in the gap between one tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and workpiece. However, since the polarity of the workpiece with respect to the tool electrode at one gap is reverse to that at the other gap, removal rates are different for the two gaps. To balance the removal rates in both gaps, the polarity of the pulse generator is changed adaptively to equalize the gap voltages measured at both gaps. Advantages of the Multi-spark EDM are as follows: 0 The removal rate and energy efficiency are higher than those of conventional EDM in which there is only one discharge point for each pulse. Figure 2(a) shows an example of the basic discharge circuit of conventional EDM. Since the gap voltage is generally around 20V, the discharge current in this circuit is 8A. In Multi-spark EDM shown in Figure 2(b), the total voltage drop through the two discharge gaps is two times larger than that in conventional EDM because discharge occurs in both gaps serially at the same time. To compare the power efficiencies between conventional and Multi-spark EDMs under the same discharge current, the resistance in the circuit of Multi-spark EDM Pulse generator
r-L--1
Circuit for polarity change I
I
2
Annals of the ClRP Vol. 49/1/2000
I
Workpiece
I
Figure 1 : Principle of Multi-spark EDM.
119
is reduced to 7.5 C2 . In this situation, the power consumption in both types of EDM is equally IOOVX 8A=800W. However, the power generated in the gaps of Multi-spark EDM is 20V X 8A X 2(gaps)=320W in total, which is two times greater than that in conventional EDM. This means that higher material removal rate can be obtained in Multi-spark EDM compared to in conventional EDM. Defining the power efficiency as the ratio of the power generated in the discharge gaps to the total power consumption in the discharge circuit, the power efficiency of Multi-spark EDM is 320W/800W=40%, which is two times higher than that in conventional EDM. In the conventional EDMing of high-electric-resistivity materials, such as electrically conductive ceramics and monocrystalline silicon, the voltage drop inside the materials is so great that the decreased discharge current brings about a lower material removal rate[4]. In this respect, the Multi-spark EDMing of highelectric-resistivity materials is advantageous because the voltage drop inside the materials can be minimized by placing both electrodes as close as possible to each other. CONTROL SYSTEM 3.1 Polarity control Figure 3 shows the block.diagram of the control system for Multi-spark EDM. To balance the removal rates in both gaps between the left and right electrodes and workpiece, the polarity of the pulse generator is changed adaptively. The unbalance is detected by measuring the gap voltages at the left and right gaps. The measured voltages of the left
3
and right electrodes u and u,, respectively, are converted to absolute values u, I and I u, I , and they are subsequently smoothed. Then the difference between the smoothed gap voltages < I u, 1 > - < I u, I > is transmitted to the circuit for polarity change. The circuit for polarity change generates a chain of discharge pulses which is comprised of eight pulses whose polarity combination set is varied depending on the difference between the smoothed gap voltages as shown in Table 1. The pulse is plus when the discharge current flows clockwise in the circuit, that is, from the right electrode through the workpiece to the left electrode. Figure 4(a) shows the equivalent circuit of Multi-spark EDM when the discharge gap length on the left electrode gL is shorter than that on the right electrode gP The ratio between the open voltages in the left and right gaps uoL and u, respectively, are determined according to the following relationships:
/
Here, E is the voltage of the power source. Figure 4(b) shows the measured waveforms of gap voltages. It was found that the open voltage in the left gap uOLis lower than that in the right gap uoP This result demonstrates that the polarity change control mentioned above is appropriate for the purpose of equalizing both gap lengths.
+ t
I
I " , ,
I
n i t r-
I
0110 nrnq
".".
,
O0O0
I
I
Workpiece
0
0
" I , ,
I
,
1010 10011000 ~
,
Workpiece
(a) Conventional EDM
(b) Multi-spark EDM
I
Figure 2: Principle of power saving.
0
-
Table 1: Variation of polarity combination set depending on difference between both gap voltages.
min. { < l u ,
Servo feed controller
.)
< l u , I>-
Z axis Quill
A #
I
Tool electrode L
+
Circuit for polarity change
Pulse generator
-
I
Tool electrode R
Differential Convert to absolute value
r L L - -- l J W orkpiece
Convert to absolute value
. ~-
IU L I
Smoothing circuit
IuR I
Smoothing circuit
0
Figure 3: Block diagram of control system for Multi-spark EDM
120
I>, < l u , I>}
1 selector Signal
3.2 Servo feed control In conventional EDM, the gap voltage averaged in a certain period of time is compared with the reference voltage which is set by the operator beforehand. If the measured gap voltage is smaller than the reference voltage, the tool electrode is retracted in order to avoid short circuit. The adoption of this conventional servo feed control does not work well in Multi-spark EDM, because there are two discharge gaps serially connected in the discharge circuit. In the case a short circuit occurs at the left hand side of the discharge gaps, for example, the conventional servo feed control cannot recognize the short circuit as long as a normal discharge is occurring at the right hand side. In this case, the right hand side gap continues to expand due to the removal of the working surfaces. As a result, the servo lets the quill go further down. In this way, short circuit cannot be avoided. To solve the above problem, a new servo feed system was developed. As shown in Figure 3, the values of the smoothed gap voltages < I u, I > and < I uR I > are compared, and only the smaller signal is selected and conveyed to the servo feed controller. Consequently, the new servo system controls the tool electrode feed based on the gap voltage of the narrower gap. 4 MACHINING CHARACTERISTICS 4.1 Experimental method The machining characteristics of Multi-spark EDM shown in Figure 5(a) were compared with those of conventional EDM. The types of conventional EDM tested were as follows: single positive tool electrode (Figure 5(b)), single negative tool electrode (Figure 5(c)), and double positive tool electrodes (Figure 5(d)). Each tool electrode was made of copper and measured 4mm in diameter. In the circuit of Multi-spark EDM, a high voltage power source whose no-load voltage is 280V was connected in parallel with the normal power source which supplies most of the discharge current. Since the no-load voltage is about two times higher than that of the normal power source, the open gap voltage at each gap in Multi-spark EDM is almost equal to that in conventional EDM. Since the impedance in the high voltage power source is very high, its power consumption can be ignored. The resistor of the normal power source in Multi-spark EDM, 26.8 Q , was determined in order to obtain more or less the same discharge current as conventional EDM. Other conditions are: discharge duration of 15O/~s,duty factor of 20%, and workpiece of bearing steel SUJ2. 4.2 Material removal rate The results of the experiments are shown in Table 2. The material removal of Multi-spark EDM per pulse is the greatest even though the discharge current was slightly lower than that in conventional EDM types. Theoretically, since, for each pulse, one discharge occurs with positive
tool electrode polarity and the other discharge occurs with negative tool electrode polarity simultaneously, the material removal of Multi-spark EDM per pulse should coincide with the sum of both the material removals of the single positive tool electrode type and single negative tool electrode type per pulse in conventional EDM. In reality, however, the removal of Multi-spark EDM per pulse was 1.30 times greater than that of the single positive tool electrode type, which is smaller than the theoretical value of 1.40 times. The removal of the double positive tool electrodes per pulse was almost equal to that of the single positive tool electrode. This is because there is only one discharge point even if there are two tool electrodes in conventional EDM. 4.3 Power efficiency The discharge power can be obtained from the measured discharge current i, and discharge voltage u,. In the case of Multi-spark EDM, since the discharge voltage averaged at each discharge gap was 17.4V, the total discharge power is 17.4VX3.18AX 2(gaps)=l105W. The total power consumption of the EDM pulse generator can be obtained by multiplying the discharge current by the noload voltage of the power source 120V. Power efficiency defined as the ratio of the discharge power to the total power consumption is highest in Multi-spark EDM. 4.4 Tool wear ratio The tool wear ratio of Multi-spark EDM was higher than that of the single positive tool electrode type in conventional EDM. It is, however, very interesting to find that the ratio was lower than the arithmetic mean of the tool wear ratios of the single positive and single negative types. 4.5 Surface roughness There is no significant difference in surface roughness between Multi-spark EDM and conventional EDM. The surface roughness of Multi-spark EDM is better than any other type of conventional EDM because the discharge power per discharge gap was lowest under the conditions used in the present experiment.
(%",*l _-I.
I
Tool electrode
-
(a) Epuivalent circuit of Multi-spark €DM
Time (b) Measured waveforms of gap voltages
Figure 4: Relationship between open voltage and gap length in both discharge gaps.
30 S2
1120v
30 Q
I 120v
I
Circuit for
(a) Multi-spark
(b) Single positive tool electrode
(c) Single negative tool electrode
(d) Double positive tool electrodes
Fiaure 5: ExDerimental method.
121
Multi-spark Material removal per pulse (mm3) (Ratio) icnarge currem i e (A) Discharge voltage ue
I
Power efficiencv (%) Tool wear ratio (%) Surface roughness R, (fim)
2.92X10-6 (1.05)
3.3
3.2
63.0
65.7
61.6
396.0
396.0
384.0
I n.
n
(17.4 per eacn gap) 110.5 (55.3 per each gap)
i e x UeW) AA/\
1.11 X10-6 (0.40)
3.18
M
.nt;.-.n
Conventional EDM Single Double negative positive
2.79X10-6 (1)
3.64X10-6 (1.30)
Discharge power power
Single positive
I
381.6
I
29.0 5.2 8.5
I
15.9 0.3
I
9.5
16.6 16 11.5
I
16.0 1.7 8.5
I Figure 7: Sample machined with planetary motion.
and discharge current of 35.5A. No interface was visible between sections eroded by each block.
CONCLUSIONS The Multi-spark EDM was newly developed to obtain higher removal rates and lower energy consumption compared with conventional EDM. The obtained results are as follows: 0 In Multi-spark EDM, one discharge occurs in the gap between one tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and workpiece. 0 To balance the Temoval rates in both gaps, the polarity of the pulse generator was changed adaptively to equalize the gap voltages measured at both gaps. 0 The removal rate and energy efficiency of Multi-spark EDM are considerably higher than those of conventional EDM. 0 The tool wear ratio of Multi-spark EDM was lower than the arithmetic mean of the tool wear ratios of the single positive tool electrode and single negative tool electrode of conventional EDM. 0 There is no significant difference in surface roughness between Multi-spark EDM and conventional EDM.
6
(a) Unsuccessful method
(b) Successful method
Figure 6 : Method of dividing tool electrode for Multi-spark EDM with planetary motion.
MACHINING WITH PLANETARY MOTION To promote the flushing of debris particles generated in the discharge gap, or to reduce the wear of the edges and sides of the frontal surface of the tool electrode, sometimes a planetary motion on the X-Y plane is superimposed on the tool feed in the Z-axis direction. This planetary motion is especially necessary when only one cavity is to be machined by the Multi-spark EDM, because the generation of a partition mark between the two sub-cavities, each of which is formed by each divided tool electrode, is undesirable. The application of planetary motion to Multispark EDM, however, needs special consideration on the method of dividing the tool electrode. Assuming that the tool electrode is divided as shown in Figure 6(a), when the X coordinate of the center of the pair electrodes is plus during planetary motion in the XY plane, the gap length over the lateral surface of the right electrode is much narrower than that of the left electrode. Even though the gap length of the right electrode satisfies the condition that electric breakdown can occur, the gap length of the left electrode absolutely does not. This means Multispark EDM is impossible using the electrode shown in Figure 6(a). To solve this problem, the tool electrode shown in Figure 6(b) was devised. One tool electrode was divided into four blocks, and the diagonal blocks were connected to each other to make two pairs of diagonal blocks. Then discharge pulse was applied between the pairs. Figure 7 shows the result of machining a carbon steel S50C (AISI 1049) by a depth of 250 M m under the pulse conditions: s, discharge interval of 40 IL s discharge duration of 100 i~ 5
122
ACKNOWLEDGEMENT The research presented in this paper was funded by the Ministry of Education, Science and Culture of Japan (Grant-in-aid for Exploratory Research, Project No. 11875032). The authors would like to express their thanks to Sodick Co., Ltd. for their kind support of this research. REFERENCES [ l ] Mohri, N., Saito, N., Takawashi, T., Kobayashi, K., 1985, Mirror-like Finishing by EDM, Proc. of the 25th MTDR Conf.: 329-336. [2] Suzuki, K., Mohri, N., Uematsu, T., Nakagawa, T., 1985 ED Truing Method with Twin Electrodes, Preprint of Autumn Meeting of JSPE: 575-578 (in Japanese). [3] Kubota, M., Tamura, Y., Okita, T., 1989, Electrocontact Discharge Dressing of Metal Bonded Diamond Grinding Wheels Using Twin Electrode System, Proc. Of ISEM 9: 22-25. [4] Saeki, T., Kunieda, M., Ueki, M., Satoh, Y., 1996, Influence of Joule Heating on EDM Processes of HighElectric-ResistivityMaterals, ASME HTD-Vol.336/FEDVO1.240: 95-103.
Abstract This paper describes the Multi-spark EDM method which was newly developed to obtain higher removal rates and lower energy consumption compared with conventional EDM. The basic circuit of Multi-spark EDM comprises a pulse generator, a first tool electrode, the workpiece, and a second tool electrode that are connected serially in the order as listed here. Accordingly, for each generator pulse, one discharge occurs in the gap between the first tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and the workpiece. To balance the removal rates in both gaps, the polarity of the pulse generator is changed adaptively to equalize the gap voltages measured at both gaps. Keywords: Die-sinking EDM, Multi-spark, Discharge point
INTRODUCTION In the conventional electrical discharge machining (EDM) process, there is only one discharge point for each pulse. According to this fact, the material removal rate of EDM is normally much lower than that of electro-chemical machining (ECM) in which the machining current flows uniformly over the whole working surface at the same time. To set up multiple discharge points for each pulse, Mohri et al. [I] divided a tool electrode into multiple electrodes, each of which is electrically insulated and connected to the pulse generator through a resistor. In this case, after a discharge occurs in the gap between one of the divided electrodes and the workpiece, the gap voltages at other electrodes are maintained at the open circuit voltage level for a certain period until the surface electric charge over these electrodes is redistributed or another discharge occurs. Hence, finally discharge can occur at different electrodes simultaneously. Suzuki et al. [2] and Kubota et al. [3] developed a twin electrode discharge system for the discharge dressing of metal bonded grinding wheels. The discharge circuit is formed by connecting the pulse generator to one of the two twin electrodes, the grinding wheel, and the other twin electrode serially. In this system, for each pulse, two discharge points can be obtained simultaneously at both the gaps between the twin electrodes and the grinding wheel using only one pulse generator. Hence, there is no necessity to insulate the grinding wheel from the machine body or attach a slip-ring brush [3]. This paper utilizes the same circuit configuration as the twin electrode discharge system. The aim of this paper, however, is to obtain higher removal rates and lower energy consumption in the normal applications of EDM processes compared with conventional EDM. 1
PRINCIPLE Figure 1 shows the principle of Multi-spark EDM newly developed. The basic circuit of Multi-spark EDM is comprised of a conventional pulse generator, a workpiece, and two tool electrodes, both of which are fixed onto the quill of the same EDM machine. The pulse generator, the first tool electrode, the workpiece, and the second tool
electrode are connected serially in the order as listed here. Accordingly, for each generator pulse, one discharge occurs in the gap between one tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and workpiece. However, since the polarity of the workpiece with respect to the tool electrode at one gap is reverse to that at the other gap, removal rates are different for the two gaps. To balance the removal rates in both gaps, the polarity of the pulse generator is changed adaptively to equalize the gap voltages measured at both gaps. Advantages of the Multi-spark EDM are as follows: 0 The removal rate and energy efficiency are higher than those of conventional EDM in which there is only one discharge point for each pulse. Figure 2(a) shows an example of the basic discharge circuit of conventional EDM. Since the gap voltage is generally around 20V, the discharge current in this circuit is 8A. In Multi-spark EDM shown in Figure 2(b), the total voltage drop through the two discharge gaps is two times larger than that in conventional EDM because discharge occurs in both gaps serially at the same time. To compare the power efficiencies between conventional and Multi-spark EDMs under the same discharge current, the resistance in the circuit of Multi-spark EDM Pulse generator
r-L--1
Circuit for polarity change I
I
2
Annals of the ClRP Vol. 49/1/2000
I
Workpiece
I
Figure 1 : Principle of Multi-spark EDM.
119
is reduced to 7.5 C2 . In this situation, the power consumption in both types of EDM is equally IOOVX 8A=800W. However, the power generated in the gaps of Multi-spark EDM is 20V X 8A X 2(gaps)=320W in total, which is two times greater than that in conventional EDM. This means that higher material removal rate can be obtained in Multi-spark EDM compared to in conventional EDM. Defining the power efficiency as the ratio of the power generated in the discharge gaps to the total power consumption in the discharge circuit, the power efficiency of Multi-spark EDM is 320W/800W=40%, which is two times higher than that in conventional EDM. In the conventional EDMing of high-electric-resistivity materials, such as electrically conductive ceramics and monocrystalline silicon, the voltage drop inside the materials is so great that the decreased discharge current brings about a lower material removal rate[4]. In this respect, the Multi-spark EDMing of highelectric-resistivity materials is advantageous because the voltage drop inside the materials can be minimized by placing both electrodes as close as possible to each other. CONTROL SYSTEM 3.1 Polarity control Figure 3 shows the block.diagram of the control system for Multi-spark EDM. To balance the removal rates in both gaps between the left and right electrodes and workpiece, the polarity of the pulse generator is changed adaptively. The unbalance is detected by measuring the gap voltages at the left and right gaps. The measured voltages of the left
3
and right electrodes u and u,, respectively, are converted to absolute values u, I and I u, I , and they are subsequently smoothed. Then the difference between the smoothed gap voltages < I u, 1 > - < I u, I > is transmitted to the circuit for polarity change. The circuit for polarity change generates a chain of discharge pulses which is comprised of eight pulses whose polarity combination set is varied depending on the difference between the smoothed gap voltages as shown in Table 1. The pulse is plus when the discharge current flows clockwise in the circuit, that is, from the right electrode through the workpiece to the left electrode. Figure 4(a) shows the equivalent circuit of Multi-spark EDM when the discharge gap length on the left electrode gL is shorter than that on the right electrode gP The ratio between the open voltages in the left and right gaps uoL and u, respectively, are determined according to the following relationships:
/
Here, E is the voltage of the power source. Figure 4(b) shows the measured waveforms of gap voltages. It was found that the open voltage in the left gap uOLis lower than that in the right gap uoP This result demonstrates that the polarity change control mentioned above is appropriate for the purpose of equalizing both gap lengths.
+ t
I
I " , ,
I
n i t r-
I
0110 nrnq
".".
,
O0O0
I
I
Workpiece
0
0
" I , ,
I
,
1010 10011000 ~
,
Workpiece
(a) Conventional EDM
(b) Multi-spark EDM
I
Figure 2: Principle of power saving.
0
-
Table 1: Variation of polarity combination set depending on difference between both gap voltages.
min. { < l u ,
Servo feed controller
.)
< l u , I>-
Z axis Quill
A #
I
Tool electrode L
+
Circuit for polarity change
Pulse generator
-
I
Tool electrode R
Differential Convert to absolute value
r L L - -- l J W orkpiece
Convert to absolute value
. ~-
IU L I
Smoothing circuit
IuR I
Smoothing circuit
0
Figure 3: Block diagram of control system for Multi-spark EDM
120
I>, < l u , I>}
1 selector Signal
3.2 Servo feed control In conventional EDM, the gap voltage averaged in a certain period of time is compared with the reference voltage which is set by the operator beforehand. If the measured gap voltage is smaller than the reference voltage, the tool electrode is retracted in order to avoid short circuit. The adoption of this conventional servo feed control does not work well in Multi-spark EDM, because there are two discharge gaps serially connected in the discharge circuit. In the case a short circuit occurs at the left hand side of the discharge gaps, for example, the conventional servo feed control cannot recognize the short circuit as long as a normal discharge is occurring at the right hand side. In this case, the right hand side gap continues to expand due to the removal of the working surfaces. As a result, the servo lets the quill go further down. In this way, short circuit cannot be avoided. To solve the above problem, a new servo feed system was developed. As shown in Figure 3, the values of the smoothed gap voltages < I u, I > and < I uR I > are compared, and only the smaller signal is selected and conveyed to the servo feed controller. Consequently, the new servo system controls the tool electrode feed based on the gap voltage of the narrower gap. 4 MACHINING CHARACTERISTICS 4.1 Experimental method The machining characteristics of Multi-spark EDM shown in Figure 5(a) were compared with those of conventional EDM. The types of conventional EDM tested were as follows: single positive tool electrode (Figure 5(b)), single negative tool electrode (Figure 5(c)), and double positive tool electrodes (Figure 5(d)). Each tool electrode was made of copper and measured 4mm in diameter. In the circuit of Multi-spark EDM, a high voltage power source whose no-load voltage is 280V was connected in parallel with the normal power source which supplies most of the discharge current. Since the no-load voltage is about two times higher than that of the normal power source, the open gap voltage at each gap in Multi-spark EDM is almost equal to that in conventional EDM. Since the impedance in the high voltage power source is very high, its power consumption can be ignored. The resistor of the normal power source in Multi-spark EDM, 26.8 Q , was determined in order to obtain more or less the same discharge current as conventional EDM. Other conditions are: discharge duration of 15O/~s,duty factor of 20%, and workpiece of bearing steel SUJ2. 4.2 Material removal rate The results of the experiments are shown in Table 2. The material removal of Multi-spark EDM per pulse is the greatest even though the discharge current was slightly lower than that in conventional EDM types. Theoretically, since, for each pulse, one discharge occurs with positive
tool electrode polarity and the other discharge occurs with negative tool electrode polarity simultaneously, the material removal of Multi-spark EDM per pulse should coincide with the sum of both the material removals of the single positive tool electrode type and single negative tool electrode type per pulse in conventional EDM. In reality, however, the removal of Multi-spark EDM per pulse was 1.30 times greater than that of the single positive tool electrode type, which is smaller than the theoretical value of 1.40 times. The removal of the double positive tool electrodes per pulse was almost equal to that of the single positive tool electrode. This is because there is only one discharge point even if there are two tool electrodes in conventional EDM. 4.3 Power efficiency The discharge power can be obtained from the measured discharge current i, and discharge voltage u,. In the case of Multi-spark EDM, since the discharge voltage averaged at each discharge gap was 17.4V, the total discharge power is 17.4VX3.18AX 2(gaps)=l105W. The total power consumption of the EDM pulse generator can be obtained by multiplying the discharge current by the noload voltage of the power source 120V. Power efficiency defined as the ratio of the discharge power to the total power consumption is highest in Multi-spark EDM. 4.4 Tool wear ratio The tool wear ratio of Multi-spark EDM was higher than that of the single positive tool electrode type in conventional EDM. It is, however, very interesting to find that the ratio was lower than the arithmetic mean of the tool wear ratios of the single positive and single negative types. 4.5 Surface roughness There is no significant difference in surface roughness between Multi-spark EDM and conventional EDM. The surface roughness of Multi-spark EDM is better than any other type of conventional EDM because the discharge power per discharge gap was lowest under the conditions used in the present experiment.
(%",*l _-I.
I
Tool electrode
-
(a) Epuivalent circuit of Multi-spark €DM
Time (b) Measured waveforms of gap voltages
Figure 4: Relationship between open voltage and gap length in both discharge gaps.
30 S2
1120v
30 Q
I 120v
I
Circuit for
(a) Multi-spark
(b) Single positive tool electrode
(c) Single negative tool electrode
(d) Double positive tool electrodes
Fiaure 5: ExDerimental method.
121
Multi-spark Material removal per pulse (mm3) (Ratio) icnarge currem i e (A) Discharge voltage ue
I
Power efficiencv (%) Tool wear ratio (%) Surface roughness R, (fim)
2.92X10-6 (1.05)
3.3
3.2
63.0
65.7
61.6
396.0
396.0
384.0
I n.
n
(17.4 per eacn gap) 110.5 (55.3 per each gap)
i e x UeW) AA/\
1.11 X10-6 (0.40)
3.18
M
.nt;.-.n
Conventional EDM Single Double negative positive
2.79X10-6 (1)
3.64X10-6 (1.30)
Discharge power power
Single positive
I
381.6
I
29.0 5.2 8.5
I
15.9 0.3
I
9.5
16.6 16 11.5
I
16.0 1.7 8.5
I Figure 7: Sample machined with planetary motion.
and discharge current of 35.5A. No interface was visible between sections eroded by each block.
CONCLUSIONS The Multi-spark EDM was newly developed to obtain higher removal rates and lower energy consumption compared with conventional EDM. The obtained results are as follows: 0 In Multi-spark EDM, one discharge occurs in the gap between one tool electrode and the workpiece, and another discharge occurs at the same time in the gap between the other tool electrode and workpiece. 0 To balance the Temoval rates in both gaps, the polarity of the pulse generator was changed adaptively to equalize the gap voltages measured at both gaps. 0 The removal rate and energy efficiency of Multi-spark EDM are considerably higher than those of conventional EDM. 0 The tool wear ratio of Multi-spark EDM was lower than the arithmetic mean of the tool wear ratios of the single positive tool electrode and single negative tool electrode of conventional EDM. 0 There is no significant difference in surface roughness between Multi-spark EDM and conventional EDM.
6
(a) Unsuccessful method
(b) Successful method
Figure 6 : Method of dividing tool electrode for Multi-spark EDM with planetary motion.
MACHINING WITH PLANETARY MOTION To promote the flushing of debris particles generated in the discharge gap, or to reduce the wear of the edges and sides of the frontal surface of the tool electrode, sometimes a planetary motion on the X-Y plane is superimposed on the tool feed in the Z-axis direction. This planetary motion is especially necessary when only one cavity is to be machined by the Multi-spark EDM, because the generation of a partition mark between the two sub-cavities, each of which is formed by each divided tool electrode, is undesirable. The application of planetary motion to Multispark EDM, however, needs special consideration on the method of dividing the tool electrode. Assuming that the tool electrode is divided as shown in Figure 6(a), when the X coordinate of the center of the pair electrodes is plus during planetary motion in the XY plane, the gap length over the lateral surface of the right electrode is much narrower than that of the left electrode. Even though the gap length of the right electrode satisfies the condition that electric breakdown can occur, the gap length of the left electrode absolutely does not. This means Multispark EDM is impossible using the electrode shown in Figure 6(a). To solve this problem, the tool electrode shown in Figure 6(b) was devised. One tool electrode was divided into four blocks, and the diagonal blocks were connected to each other to make two pairs of diagonal blocks. Then discharge pulse was applied between the pairs. Figure 7 shows the result of machining a carbon steel S50C (AISI 1049) by a depth of 250 M m under the pulse conditions: s, discharge interval of 40 IL s discharge duration of 100 i~ 5
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ACKNOWLEDGEMENT The research presented in this paper was funded by the Ministry of Education, Science and Culture of Japan (Grant-in-aid for Exploratory Research, Project No. 11875032). The authors would like to express their thanks to Sodick Co., Ltd. for their kind support of this research. REFERENCES [ l ] Mohri, N., Saito, N., Takawashi, T., Kobayashi, K., 1985, Mirror-like Finishing by EDM, Proc. of the 25th MTDR Conf.: 329-336. [2] Suzuki, K., Mohri, N., Uematsu, T., Nakagawa, T., 1985 ED Truing Method with Twin Electrodes, Preprint of Autumn Meeting of JSPE: 575-578 (in Japanese). [3] Kubota, M., Tamura, Y., Okita, T., 1989, Electrocontact Discharge Dressing of Metal Bonded Diamond Grinding Wheels Using Twin Electrode System, Proc. Of ISEM 9: 22-25. [4] Saeki, T., Kunieda, M., Ueki, M., Satoh, Y., 1996, Influence of Joule Heating on EDM Processes of HighElectric-ResistivityMaterals, ASME HTD-Vol.336/FEDVO1.240: 95-103.