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Study of Machining Accuracy of Micro Punching Mold Using Micro-EDM
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 588 – 593
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Study of Machining Accuracy of Micro Punching Mold Using Micro-EDM Zhijie Zeng, Dongping Li, Zuyuan Yu*, Xiaolong Yang, Jianzhong Li, Renke Kang Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, China * Corresponding author. Tel.: +86-0411-84707231; E-mail address: [email protected]
Abstract As one of the micro-forming technologies, micro punching process produces workpieces in high efficiency and low cost. It has attracted the attention of researchers from all over the world in recent years. The current micro punching mold is prepared on-line by micro electrical discharge machining (EDM) with simple cross-sectional shape. During punching, the punching load is small due to the soft material and small circumference of workpiece. To form micro part with complex cross sectional shape using micro punching technology, three-dimensional micro-EDM milling and reversed micro EDM are used to generate micro die and punch on-line to provide a solution to alignment of die and punch on machine. In order to obtain high dimensional accuracy of micro punching mold, in this paper, dimensional errors including electrode, reversed electrode, micro die and micro punch caused during machining process are identified. The correlation between dimensional errors and the clearance of micro die and micro punch are analyzed. Based on preliminary experimental results and dimensional error control strategy, the final punching clearance of micro punch molds was machined within the required range. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Micro EDM, Dimensional accuracy, Micro-punching, Complex shaped parts
1. Introduction Micro parts are in great demand in industry area nowadays [1]. The fabrication of micro holes, particularly the non-regular shaped micro holes is urgent in many industrial fields. Various methods including micro EDM, laser machining and micro chemical machining have been developed to fabricate various micro holes on different substrates [2]. However, the micro-EDM and laser machining will generate recast layers, while the chemical machining has a low processing efficiency [3]. Conversely, micro punching technology is widely used in machining different features with the advantages of low cost, high efficiency and accuracy. Masuzawa et al. proposed a novel process for the fabricating of micro punching molds on-line to solve the problem of alignment of micro mold [4]. Chen et al. improved the efficiency and quality of micro punching molds by Vibration-EDM, and noncircular micro holes are fabricated by the molds [5, 6]. However, cross-sectional shapes of all these punched workpieces are convex
polygons. Micro punching molds with irregular crosssectional shape were reported. Restricted by the machining level of micro punching molds, punching pieces with are rarely fabricated. On the other hand, a micro punch is hold by high precision spindle, which large punching load may cause the damage of spindle. In previous study, Xiangwei Gong et al. proposed a machining method to fabricate micro molds with complex cross-sectional shape using micro EDM [7]. However, the dimensional accuracy of the molds needs to be improved. In order to improve the accuracy of punching molds, in this paper, dimensional errors including electrode, reversed electrode, micro die and micro punch caused by machining process are identified. The correlation between dimensional errors and the punching clearance are analyzed. Based on preliminary experimental results and dimensional error control strategy, the final punching clearance of micro punch molds was machined within the required range.
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.119
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2. Experimental equipment and analysis of machining accuracy of the punching molds 2.1. Experimental equipment Fig. 1 shows the structure of experimental equipment It consists of XYZ moving stages, micro punching module, wire electrical discharge grinding (WEDG) unit used to prepare micro electrode, a granite base, a set of computer, and control system.
ܨൌܧ
(1)
ܦൌ ܨെ ʹܥிି
(2)
ܤൌ ܦ ʹܥି
(3)
Where B --- Designed dimension of reverse electrode F --- Designed dimension of micro die D --- Designed dimension of micro punch E --- Designed dimension of blanking parts CB-D --- Designed discharge gap for machining micro punch CF-D --- Designed punching clearance
Fig. 2. Process of the micro mold fabrication and the micro punching process Fig. 1. Photograph of experimental equipment
2.2. Schematic of the Fabrication of micro molds and punching process Fig. 2 shows the process of the micro punching mold fabrication and the micro punching process. On the experimental equipment, a micro mold is fabricated using reversed EDM technique and electrical discharge milling. As shown in Fig. 2(a), a tool electrode is machined using WEDG unit on-machine. A micro die and a reversed electrode are generated using micro EDM milling technique as shown in Figs. 2(b) and 2(c), respectively. A micro punch installed on the micro punching module is fabricated using the reversed EDM technology shown in Fig. 2(d). During the reversed EDM, an ultrasonic vibrating unit is placed below the reversed electrode to flush the discharge gap between the reversed electrode and the micro punch. Due to the micro die and micro punch prepared on machine in the same coordinate system, the alignment of die and punch is subsequently achieved by adjusting their position. The punching process is finally carried out as shown in Fig. 2(e). 2.3. Theoretical dimension relationship of the molds The dimensions in micro mold and machining process include micro die, micro punch, tool electrode, reversed electrode, discharge gaps and punching clearance. Their relationship can be expressed by Eqs. (1), (2) and (3).
2.4. Analysis of actual dimension accuracy of the molds In the actual processing, there are many factors affecting the mold dimension accuracy. Fig. 3 shows the relationship of factors in the mold processing. These factors include the discharge gap, position error of moving stage, measurement error and error caused by mechanical structure deformation. The discharge gap varies within a certain range under a certain discharge conditions. The position error of moving stage includes the repeated position accuracy of moving stage and the overshoot of moving stage. The repeated position accuracy of moving stage in this study is within 1ȝm. The overshoot of moving stage can be minimized by adjusting PID properly. In this study, the position error of moving stage is included in the discharge gap. The machining time used to prepare the micro die, reversed electrode and micro punch lasts from several hours to dozens of hours. The thermal deformation of mechanical structure may occur during machining. The symmetricity of mechanical structure may cause different deformation values. In Fig. 3, the punch clearance is decided by the dimensions of micro die and micro punch. The micro die and the reversed electrode are generated by controlling electrodes moving along designed tool paths. Fig. 4 shows the processing of reversed electrode and micro die.
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Punching clearance
Dimension of die
Tool path
Electrode dimension
Dimension of punch
Structural deformation
Discharge gap
Discharge gap
Dimension of reversed electrode
Electrode dimension
Tool path
Structural deformation
Structural deformation
Discharge gap
Fig. 3. The relationship of the error of machined mold
ǻG --- Deviation of tool path ǻ1 --- Comprehensive deviation of micro die.
Actual machining path
It can be seen the comprehensive deviation of micro die consists of the deviation of discharge gap, deviation of tool electrode size and the deviation of tool path. Fig. 5 shows the relationship of the comprehensive deviation of micro punch and its related factors. Eqs. (11) to (13) describe this relationship. It is dominated by deviation of tool electrode, deviation of tool path, deviation of discharge of micro punch and deviation of discharge gap of reversed electrode.
Tool path Discharge gap Tool electrode
X
Y
Fig. 4. Schematic of the machining of 3D carve
The dimensions of processing micro die and the reversed electrode include those of micro die, the reversed electrode, tool electrode, tool path and discharge gap. The relationship of the dimension and the deviation of each item are shown as the following formulas. ܤൌ ܣ ܩ ʹܥି
(4)
ܨൌ ܣ ܩி ʹܥି
(5)
߂ ൌ כܣെ (ܣ6) ߂ ൌ כ ܥെ ܥ
(7)
ܥି ൌ ሺ ܤെ ܩ െ ܣሻȀʹ
(8)
ܥିி ൌ ሺ ܨെ ܩி െ ܣሻȀʹ
(9)
߂ଵ ൌ ߂ ʹ߂ ߂ீ where A --- Designed dimension of tool electrode GB--- Designed path of reversed electrode CA-B --- Designed discharge gap of reversed electrode GF--- Designed path of micro die CA-C --- Designed discharge gap of micro die ǻA--- Deviation of tool electrode A*--- Actual dimension of tool electrode ǻC --- Deviation of discharge gap C*--- Actual discharge gap C --- Theoretical discharge gap
(10)
ൌ െ ʹିୈ
(11)
ȟଵୈ ൌ ȟ ȟୋా ʹȟେఽషా െ ʹȟେాషీ
(12)
כ ȟେాషీ ൌ ିୈ െ ିୈ
(13)
where ȟୋా ---Deviation of tool path used for machining the reversed electrode ȟେాషీ --- Deviation of discharge gap of micro punch כ ିୈ --- Actual discharge gap of micro punch ȟେఽషా --- Deviation of discharge gap of reversed electrode
ȟଵୈ --- Comprehensive deviation of micro punch Deviation of reverse electrode
+ Deviation of tool electrode
+
Deviation of the punch
Ͳ
Gap deviation of the punch
+ Path deviation of reverse electrode
Fig. 5. Deviation influencing factors of micro punch
Fig. 6 shows the relationship between the deviation of punch clearance and its related factors. This relationship is described in Eqs. (14) and (15). ߂భ and ߂మ are deviations of
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tool electrode size, which are same under the same machining conditions. ȟୋా and ߂ீಷ are the same deviations of tool path. כ ܥିி ൌ ܥିி
߂భ െ ߂మ ߂ீಷ െ ߂ீಳ െ ߂ಲషಳ ߂ಲషಷ ߂ಳషವ ʹ
dimensions of micro features in X axis were measured using an optical microscope with a resolution of 1ȝm. Experimental results, mean values, expectation and variance are summarized in Figs. 7, 8 and 9.
(14)
ൌ ܥିி െ ߂ಲషಳ ߂ಲషಷ ߂ಳషವ ߂ವషಷ
591
ο െ οమ οீಷ െ οீಳ ൌ ߂ಲషಷ െ ߂ಲషಳ ߂ಳషವ భ ʹ
(15)
ൌ ߂ಲషಷ െ ߂ಲషಳ ߂ಳషವ
where ߂భ --- Deviation off tool electrode for micro die ߂మ --- Deviation of tool electrode for reversed electrode כ ܥିி --- Actual punching clearance ܥିி --- Designed punching clearance ߂ವషಷ --- Deviation of punching clearance
Fig. 7. Deviation in X entrance of reversed electrode
߂ಲషಷ --- Deviation of discharge gap of micro die ߂ீಷ --- Deviation of tool path used for machining the micro die. Gap deviation of female die
+
+
Ͳ Gap deviation of the punch
Gap deviation of reverse ecectrode
Ͳ
+
Path deviation of female die
Path deviation of reverse electrode
Punching gap deviation
+
Deviation of electrode 1
Ͳ
Deviation of electrode 2
Fig. 8. Deviation in X entrance of micro die
Fig. 6. Deviation influencing factors of the punching gap
It can be seen in Eq. (15) that the deviation of punch clearance consists of deviations of discharge gap of micro die, discharge gap of reversed electrode and discharge gap of micro punch. The discharge gap is dominated by discharge parameters, materials of workpiece and electrode, and the discharge conditions in the gap. Even under the same machining conditions, the discharge gap varies within a certain range. To minimize the deviation of punch clearance, it is necessary to stabilize the machining process to reduce the deviation of discharge gap. On the other hand, the deviation of punch clearance can also be reduced by taking the average values of deviations of discharge gap of micro die, discharge gap of reversed electrode and discharge gap of micro punch into the tool path generation. 2.5. Accuracy analysis and control strategy of micro mold To obtain the deviations of micro die, micro punch and reversed electrode used to machine the micro punch. Experiments of machining micro features with various crosssectional shapes are carried out. The material of tool electrode is tungsten. The material of micro die and micro punch is Cr12MoV and the material of reversed electrode is brass. The open voltage of RC circuit is 80V and the capacitator is 470pF. To analyse the dimensional accuracy easily, the entrance
Fig. 9. Deviation in X entrance of micro punch
These expectation values in X axis in Figs. 7, 8 and 9 are taken into Eq. (15) as discharge gaps, which are further compensated in tool path generation for the reversed electrode and micro die. 3. Experimental results and discussion 3.1. Fabricating of micro punching molds Fig. 10 shows the shape and dimension code of the workpiece. Tables 1 and 2 show machining parameters and designed dimensions of mold. Among them, codes 1 and 4 are in the X direction. In processing, the reversed electrode and the micro die are fabricated by the same tool electrode. The tool paths are
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generated based on the discharge gaps shown in Figs. 7, 8 and 9. The average discharge gap value of die in Table 1 which is from Fig. 4 is used in the tool path generation for micro die. The average deviation of reversed electrode in Fig. 7 is used in tool path generation for reversed electrode. Fig.11 shows the SEM of the machined mold. It can clearly be seen the good quality of micro mold with sharp edges.
6
(a)
4
1
5
2 3
Fig. 10. Complex workpiece shape and dimension code Table 1. Machining parameters of the molds
Die
Discharge conditions 80V/470pF
Discharge gap (ȝm) 4
punch
80V/470pF
21
Electrode(ȝm) 100
(b)
100
Fig. 11. SEM of (a) the entrance of die and (b) the end face of micro punch
Table 2. Feature sizes of micro punching mold with complex shape Dimension Code Pieces(ȝm) Die (ȝm) Punch (ȝm)
1 500 500 480
2 300 300 280
3 600 600 580
4 120 120 100
5 100 100 110
6 150 150 140
The dimensions of the machined mold are measured. Deviations are listed in Table 3. The diameter of the tool electrode is 99ȝm. As listed in Table 3, the dimension deviations of mold in X axis (dimension codes 1 and 4) are within 1ȝm. However, some dimension deviations in Y axis are larger than 1ȝm. This might be caused by the thermal deformation of the asymmetrical mechanical structure of equipment in Y axis. Similar process of compensating the deviations in X axis by adjusting the tool path in Y axis might be applied to reduce the dimensional deviations in Y axis (dimensional codes 2 and 3). Further investigation and corresponding compensation have to be done to meet the accuracy requirements.
Due to the serious wear of micro die and micro punch during punching process when the material of Cr12MoV is used, micro die and micro punch shown in Fig. 11 are not used in this study. Fig. 12 shows micro punching results of square micro die and micro mold with workpiece thickness of 80ȝm. Tungsten carbide is used as the material of micro die and micro mold in future study.
(a) Brass punched hole
(b) 3J21 punched hole
Table 3. Deviations of micro punching mold with complex shape Dimension Code
1
2
3
4
5
6
Die (ȝm)
0.2
2.9
1.7
0.6
0.5
-0.1
Punch(ȝm)
-0.3
3.5
4.5
0.4
0.3
-0.4
Punching clearance(ȝm)
0.5
-0.6
-2.8
0.2
-0.2
0.3
(c) Brass blanked part
(d) 3J21 blanked part
Fig. 12. SEM of the punching results of workpiece
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4. Summary To solve the problem of generate micro mold with complex cross-sectional shape on-line by micro EDM, this paper proposed an novel machining process to prepare the micro punching mold based on the reverse-EDM. The relationship between the deviations of micro die, micro punch and reversed electrode is described. Based on experimental results of deviations, tool paths were generated. The final results indicate that the dimension deviations in X axis have been controlled within 1ȝm. However, it was also found the dimension deviations in Y axis are larger than 1ȝm. It might be caused by the thermal deformation of the asymmetrical mechanical structure of equipment in Y axis. Further investigation is needed. 5. Acknowledgements This work was supported by National Science and Technology Major Projects of China (Grant No. 2013ZX04001091-1) References [1] Fu MW, Chan WL. A review on the state-of-the-art micro forming technologies. Int J Adv Manuf Tech 2013;67:2411–37. [2] Liu H, Yan B, Huang F, Qiu K. A study on the characterization of high nickel alloy micro-holes using micro-EDM and their applications. J Mater Process Technol 2005;169:418–26. [3] Chien W, Hou S. Investigating the recast layer formed during the laser trepan drilling of Inconel 718 using the Taguchi method. Int J Adv Manuf Tech 2007;33:308–16. [4] T. Masuzawa et al., A micro punching system using wire-EDM. Proceedings of ISEM, 1989, 9:86-89. [5] G.L. Chern et al., Study on vibration-EDM and mass punching of microholes. Journal of materials processing technology, 2006, 180: 151-160. [6] G.L. Chern et al., Punching of noncircular micro holes and development of micro-forming. Precision engineering, 2007, 31: 210-217. [7] Xiangwei Gong et al., Fabrication of Micro Punching Mold by MicroEDM. Proceedings of 11th International Conference on Micro Manufacturing, 2016.
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ScienceDirect Procedia CIRP 68 (2018) 588 – 593
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Study of Machining Accuracy of Micro Punching Mold Using Micro-EDM Zhijie Zeng, Dongping Li, Zuyuan Yu*, Xiaolong Yang, Jianzhong Li, Renke Kang Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, China * Corresponding author. Tel.: +86-0411-84707231; E-mail address: [email protected]
Abstract As one of the micro-forming technologies, micro punching process produces workpieces in high efficiency and low cost. It has attracted the attention of researchers from all over the world in recent years. The current micro punching mold is prepared on-line by micro electrical discharge machining (EDM) with simple cross-sectional shape. During punching, the punching load is small due to the soft material and small circumference of workpiece. To form micro part with complex cross sectional shape using micro punching technology, three-dimensional micro-EDM milling and reversed micro EDM are used to generate micro die and punch on-line to provide a solution to alignment of die and punch on machine. In order to obtain high dimensional accuracy of micro punching mold, in this paper, dimensional errors including electrode, reversed electrode, micro die and micro punch caused during machining process are identified. The correlation between dimensional errors and the clearance of micro die and micro punch are analyzed. Based on preliminary experimental results and dimensional error control strategy, the final punching clearance of micro punch molds was machined within the required range. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Micro EDM, Dimensional accuracy, Micro-punching, Complex shaped parts
1. Introduction Micro parts are in great demand in industry area nowadays [1]. The fabrication of micro holes, particularly the non-regular shaped micro holes is urgent in many industrial fields. Various methods including micro EDM, laser machining and micro chemical machining have been developed to fabricate various micro holes on different substrates [2]. However, the micro-EDM and laser machining will generate recast layers, while the chemical machining has a low processing efficiency [3]. Conversely, micro punching technology is widely used in machining different features with the advantages of low cost, high efficiency and accuracy. Masuzawa et al. proposed a novel process for the fabricating of micro punching molds on-line to solve the problem of alignment of micro mold [4]. Chen et al. improved the efficiency and quality of micro punching molds by Vibration-EDM, and noncircular micro holes are fabricated by the molds [5, 6]. However, cross-sectional shapes of all these punched workpieces are convex
polygons. Micro punching molds with irregular crosssectional shape were reported. Restricted by the machining level of micro punching molds, punching pieces with are rarely fabricated. On the other hand, a micro punch is hold by high precision spindle, which large punching load may cause the damage of spindle. In previous study, Xiangwei Gong et al. proposed a machining method to fabricate micro molds with complex cross-sectional shape using micro EDM [7]. However, the dimensional accuracy of the molds needs to be improved. In order to improve the accuracy of punching molds, in this paper, dimensional errors including electrode, reversed electrode, micro die and micro punch caused by machining process are identified. The correlation between dimensional errors and the punching clearance are analyzed. Based on preliminary experimental results and dimensional error control strategy, the final punching clearance of micro punch molds was machined within the required range.
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.119
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Zhijie Zeng et al. / Procedia CIRP 68 (2018) 588 – 593
2. Experimental equipment and analysis of machining accuracy of the punching molds 2.1. Experimental equipment Fig. 1 shows the structure of experimental equipment It consists of XYZ moving stages, micro punching module, wire electrical discharge grinding (WEDG) unit used to prepare micro electrode, a granite base, a set of computer, and control system.
ܨൌܧ
(1)
ܦൌ ܨെ ʹܥிି
(2)
ܤൌ ܦ ʹܥି
(3)
Where B --- Designed dimension of reverse electrode F --- Designed dimension of micro die D --- Designed dimension of micro punch E --- Designed dimension of blanking parts CB-D --- Designed discharge gap for machining micro punch CF-D --- Designed punching clearance
Fig. 2. Process of the micro mold fabrication and the micro punching process Fig. 1. Photograph of experimental equipment
2.2. Schematic of the Fabrication of micro molds and punching process Fig. 2 shows the process of the micro punching mold fabrication and the micro punching process. On the experimental equipment, a micro mold is fabricated using reversed EDM technique and electrical discharge milling. As shown in Fig. 2(a), a tool electrode is machined using WEDG unit on-machine. A micro die and a reversed electrode are generated using micro EDM milling technique as shown in Figs. 2(b) and 2(c), respectively. A micro punch installed on the micro punching module is fabricated using the reversed EDM technology shown in Fig. 2(d). During the reversed EDM, an ultrasonic vibrating unit is placed below the reversed electrode to flush the discharge gap between the reversed electrode and the micro punch. Due to the micro die and micro punch prepared on machine in the same coordinate system, the alignment of die and punch is subsequently achieved by adjusting their position. The punching process is finally carried out as shown in Fig. 2(e). 2.3. Theoretical dimension relationship of the molds The dimensions in micro mold and machining process include micro die, micro punch, tool electrode, reversed electrode, discharge gaps and punching clearance. Their relationship can be expressed by Eqs. (1), (2) and (3).
2.4. Analysis of actual dimension accuracy of the molds In the actual processing, there are many factors affecting the mold dimension accuracy. Fig. 3 shows the relationship of factors in the mold processing. These factors include the discharge gap, position error of moving stage, measurement error and error caused by mechanical structure deformation. The discharge gap varies within a certain range under a certain discharge conditions. The position error of moving stage includes the repeated position accuracy of moving stage and the overshoot of moving stage. The repeated position accuracy of moving stage in this study is within 1ȝm. The overshoot of moving stage can be minimized by adjusting PID properly. In this study, the position error of moving stage is included in the discharge gap. The machining time used to prepare the micro die, reversed electrode and micro punch lasts from several hours to dozens of hours. The thermal deformation of mechanical structure may occur during machining. The symmetricity of mechanical structure may cause different deformation values. In Fig. 3, the punch clearance is decided by the dimensions of micro die and micro punch. The micro die and the reversed electrode are generated by controlling electrodes moving along designed tool paths. Fig. 4 shows the processing of reversed electrode and micro die.
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Punching clearance
Dimension of die
Tool path
Electrode dimension
Dimension of punch
Structural deformation
Discharge gap
Discharge gap
Dimension of reversed electrode
Electrode dimension
Tool path
Structural deformation
Structural deformation
Discharge gap
Fig. 3. The relationship of the error of machined mold
ǻG --- Deviation of tool path ǻ1 --- Comprehensive deviation of micro die.
Actual machining path
It can be seen the comprehensive deviation of micro die consists of the deviation of discharge gap, deviation of tool electrode size and the deviation of tool path. Fig. 5 shows the relationship of the comprehensive deviation of micro punch and its related factors. Eqs. (11) to (13) describe this relationship. It is dominated by deviation of tool electrode, deviation of tool path, deviation of discharge of micro punch and deviation of discharge gap of reversed electrode.
Tool path Discharge gap Tool electrode
X
Y
Fig. 4. Schematic of the machining of 3D carve
The dimensions of processing micro die and the reversed electrode include those of micro die, the reversed electrode, tool electrode, tool path and discharge gap. The relationship of the dimension and the deviation of each item are shown as the following formulas. ܤൌ ܣ ܩ ʹܥି
(4)
ܨൌ ܣ ܩி ʹܥି
(5)
߂ ൌ כܣെ (ܣ6) ߂ ൌ כ ܥെ ܥ
(7)
ܥି ൌ ሺ ܤെ ܩ െ ܣሻȀʹ
(8)
ܥିி ൌ ሺ ܨെ ܩி െ ܣሻȀʹ
(9)
߂ଵ ൌ ߂ ʹ߂ ߂ீ where A --- Designed dimension of tool electrode GB--- Designed path of reversed electrode CA-B --- Designed discharge gap of reversed electrode GF--- Designed path of micro die CA-C --- Designed discharge gap of micro die ǻA--- Deviation of tool electrode A*--- Actual dimension of tool electrode ǻC --- Deviation of discharge gap C*--- Actual discharge gap C --- Theoretical discharge gap
(10)
ൌ െ ʹିୈ
(11)
ȟଵୈ ൌ ȟ ȟୋా ʹȟେఽషా െ ʹȟେాషీ
(12)
כ ȟେాషీ ൌ ିୈ െ ିୈ
(13)
where ȟୋా ---Deviation of tool path used for machining the reversed electrode ȟେాషీ --- Deviation of discharge gap of micro punch כ ିୈ --- Actual discharge gap of micro punch ȟେఽషా --- Deviation of discharge gap of reversed electrode
ȟଵୈ --- Comprehensive deviation of micro punch Deviation of reverse electrode
+ Deviation of tool electrode
+
Deviation of the punch
Ͳ
Gap deviation of the punch
+ Path deviation of reverse electrode
Fig. 5. Deviation influencing factors of micro punch
Fig. 6 shows the relationship between the deviation of punch clearance and its related factors. This relationship is described in Eqs. (14) and (15). ߂భ and ߂మ are deviations of
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tool electrode size, which are same under the same machining conditions. ȟୋా and ߂ீಷ are the same deviations of tool path. כ ܥିி ൌ ܥିி
߂భ െ ߂మ ߂ீಷ െ ߂ீಳ െ ߂ಲషಳ ߂ಲషಷ ߂ಳషವ ʹ
dimensions of micro features in X axis were measured using an optical microscope with a resolution of 1ȝm. Experimental results, mean values, expectation and variance are summarized in Figs. 7, 8 and 9.
(14)
ൌ ܥିி െ ߂ಲషಳ ߂ಲషಷ ߂ಳషವ ߂ವషಷ
591
ο െ οమ οீಷ െ οீಳ ൌ ߂ಲషಷ െ ߂ಲషಳ ߂ಳషವ భ ʹ
(15)
ൌ ߂ಲషಷ െ ߂ಲషಳ ߂ಳషವ
where ߂భ --- Deviation off tool electrode for micro die ߂మ --- Deviation of tool electrode for reversed electrode כ ܥିி --- Actual punching clearance ܥିி --- Designed punching clearance ߂ವషಷ --- Deviation of punching clearance
Fig. 7. Deviation in X entrance of reversed electrode
߂ಲషಷ --- Deviation of discharge gap of micro die ߂ீಷ --- Deviation of tool path used for machining the micro die. Gap deviation of female die
+
+
Ͳ Gap deviation of the punch
Gap deviation of reverse ecectrode
Ͳ
+
Path deviation of female die
Path deviation of reverse electrode
Punching gap deviation
+
Deviation of electrode 1
Ͳ
Deviation of electrode 2
Fig. 8. Deviation in X entrance of micro die
Fig. 6. Deviation influencing factors of the punching gap
It can be seen in Eq. (15) that the deviation of punch clearance consists of deviations of discharge gap of micro die, discharge gap of reversed electrode and discharge gap of micro punch. The discharge gap is dominated by discharge parameters, materials of workpiece and electrode, and the discharge conditions in the gap. Even under the same machining conditions, the discharge gap varies within a certain range. To minimize the deviation of punch clearance, it is necessary to stabilize the machining process to reduce the deviation of discharge gap. On the other hand, the deviation of punch clearance can also be reduced by taking the average values of deviations of discharge gap of micro die, discharge gap of reversed electrode and discharge gap of micro punch into the tool path generation. 2.5. Accuracy analysis and control strategy of micro mold To obtain the deviations of micro die, micro punch and reversed electrode used to machine the micro punch. Experiments of machining micro features with various crosssectional shapes are carried out. The material of tool electrode is tungsten. The material of micro die and micro punch is Cr12MoV and the material of reversed electrode is brass. The open voltage of RC circuit is 80V and the capacitator is 470pF. To analyse the dimensional accuracy easily, the entrance
Fig. 9. Deviation in X entrance of micro punch
These expectation values in X axis in Figs. 7, 8 and 9 are taken into Eq. (15) as discharge gaps, which are further compensated in tool path generation for the reversed electrode and micro die. 3. Experimental results and discussion 3.1. Fabricating of micro punching molds Fig. 10 shows the shape and dimension code of the workpiece. Tables 1 and 2 show machining parameters and designed dimensions of mold. Among them, codes 1 and 4 are in the X direction. In processing, the reversed electrode and the micro die are fabricated by the same tool electrode. The tool paths are
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generated based on the discharge gaps shown in Figs. 7, 8 and 9. The average discharge gap value of die in Table 1 which is from Fig. 4 is used in the tool path generation for micro die. The average deviation of reversed electrode in Fig. 7 is used in tool path generation for reversed electrode. Fig.11 shows the SEM of the machined mold. It can clearly be seen the good quality of micro mold with sharp edges.
6
(a)
4
1
5
2 3
Fig. 10. Complex workpiece shape and dimension code Table 1. Machining parameters of the molds
Die
Discharge conditions 80V/470pF
Discharge gap (ȝm) 4
punch
80V/470pF
21
Electrode(ȝm) 100
(b)
100
Fig. 11. SEM of (a) the entrance of die and (b) the end face of micro punch
Table 2. Feature sizes of micro punching mold with complex shape Dimension Code Pieces(ȝm) Die (ȝm) Punch (ȝm)
1 500 500 480
2 300 300 280
3 600 600 580
4 120 120 100
5 100 100 110
6 150 150 140
The dimensions of the machined mold are measured. Deviations are listed in Table 3. The diameter of the tool electrode is 99ȝm. As listed in Table 3, the dimension deviations of mold in X axis (dimension codes 1 and 4) are within 1ȝm. However, some dimension deviations in Y axis are larger than 1ȝm. This might be caused by the thermal deformation of the asymmetrical mechanical structure of equipment in Y axis. Similar process of compensating the deviations in X axis by adjusting the tool path in Y axis might be applied to reduce the dimensional deviations in Y axis (dimensional codes 2 and 3). Further investigation and corresponding compensation have to be done to meet the accuracy requirements.
Due to the serious wear of micro die and micro punch during punching process when the material of Cr12MoV is used, micro die and micro punch shown in Fig. 11 are not used in this study. Fig. 12 shows micro punching results of square micro die and micro mold with workpiece thickness of 80ȝm. Tungsten carbide is used as the material of micro die and micro mold in future study.
(a) Brass punched hole
(b) 3J21 punched hole
Table 3. Deviations of micro punching mold with complex shape Dimension Code
1
2
3
4
5
6
Die (ȝm)
0.2
2.9
1.7
0.6
0.5
-0.1
Punch(ȝm)
-0.3
3.5
4.5
0.4
0.3
-0.4
Punching clearance(ȝm)
0.5
-0.6
-2.8
0.2
-0.2
0.3
(c) Brass blanked part
(d) 3J21 blanked part
Fig. 12. SEM of the punching results of workpiece
Zhijie Zeng et al. / Procedia CIRP 68 (2018) 588 – 593
4. Summary To solve the problem of generate micro mold with complex cross-sectional shape on-line by micro EDM, this paper proposed an novel machining process to prepare the micro punching mold based on the reverse-EDM. The relationship between the deviations of micro die, micro punch and reversed electrode is described. Based on experimental results of deviations, tool paths were generated. The final results indicate that the dimension deviations in X axis have been controlled within 1ȝm. However, it was also found the dimension deviations in Y axis are larger than 1ȝm. It might be caused by the thermal deformation of the asymmetrical mechanical structure of equipment in Y axis. Further investigation is needed. 5. Acknowledgements This work was supported by National Science and Technology Major Projects of China (Grant No. 2013ZX04001091-1) References [1] Fu MW, Chan WL. A review on the state-of-the-art micro forming technologies. Int J Adv Manuf Tech 2013;67:2411–37. [2] Liu H, Yan B, Huang F, Qiu K. A study on the characterization of high nickel alloy micro-holes using micro-EDM and their applications. J Mater Process Technol 2005;169:418–26. [3] Chien W, Hou S. Investigating the recast layer formed during the laser trepan drilling of Inconel 718 using the Taguchi method. Int J Adv Manuf Tech 2007;33:308–16. [4] T. Masuzawa et al., A micro punching system using wire-EDM. Proceedings of ISEM, 1989, 9:86-89. [5] G.L. Chern et al., Study on vibration-EDM and mass punching of microholes. Journal of materials processing technology, 2006, 180: 151-160. [6] G.L. Chern et al., Punching of noncircular micro holes and development of micro-forming. Precision engineering, 2007, 31: 210-217. [7] Xiangwei Gong et al., Fabrication of Micro Punching Mold by MicroEDM. Proceedings of 11th International Conference on Micro Manufacturing, 2016.
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