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Structure of process energy sources for time-parallel combined processes
Journal of Materials Processing Technology 149 (2004) 519–523
Structure of process energy sources for time-parallel combined processes M. Läuter∗ , H.-J. Trautmann, M. Zybura-Skrabalak, H.-P. Schulze, G. Wollenberg Institute for Fundamental Electrical Engineering and EMC, Otto-von-Guericke-University Magdeburg, Universitetsplatz 2, Magdeburg 39106, Germany
Abstract The structure of the process energy source in the case of combined techniques is determined by the main and assisting processes. At classical combinations of EDM and ECM the changed dynamic load conditions are shown, and the importance of the process-dependent rf noise for process identification is explained. The modular design may be used favorably for shaping the current and/or voltage pulses, especially for changing the rise and fall time of pulses. The structure adaption of the process energy sources to the hybrid time-parallel processes is an important task for process optimization and the effective utilization of the process combination for the provided case of application. © 2004 Published by Elsevier B.V. Keywords: Combined processes; Process energy sources; Electrochemical machining; Spark erosion
1. Motivation The motivation for using hybrid processes is the elimination of the negative or unwanted features of both processes without impeding the ‘good’ technological parameters. Combined machining with time-parallel running processes is featured by alternation and mutual influence between the single processes depending on the conditions at the working area. The used tools and working fluids must allow both processes. In the case of combinations of electrical and mechanical processes, often one of the processes is to improve the conditions for the other process. The energy source needed for the electric part in general does not differ from sources for individual processes. In Fig. 1 the influences of the electrical and non-electrical processes on a hybrid process are shown. Often combinations of electrical processes have larger parameter areas as individual processes. Therefore, specific process energy sources are needed which in their dynamics must satisfy the respective load conditions. Examples of machining processes which depend strongly on the load conditions are the spark erosion (EDM), the arc erosion (ADM) and the electrochemical machining (ECM).
∗ Corresponding author. Tel.: +49-391-67-11077; fax: +49-391-67-11236. E-mail address: [email protected] (M. Läuter).
0924-0136/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2004.02.018
This paper shows the demands on the process energy sources and the dependencies on the generator structure from the case of application. In this paper, only the electric discharge processes (EDM) and the anodic dissolution processes (ECM) are treated because these combinations were frequently used in the last years.
2. Hybrid EDM/ECM processes The combination of EDM and ECM has the aim, to reduce the disadvantages of both processes, • at EDM the small removal rate and the thermal damage of the processed surface, and • at ECM the poor accuracy, damage of faces that are not processed and process interrupting due to passivation and simultaneously to maintain the advantages of the processes • at EDM the high precision and • at ECM the high removal rate. Hybrid machining can be characterized by the definition of a main process and assisting processes. At hybrid processes consisting of EDM and ECM the main process is determined by the selection of the working liquid, the parameter of the energy source and the gap conditions
520
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523 Electrical processes
Spark erosion
Arc erosion
ECM Resistance Heating
Mechanical processes
Working medium
Milling
Gap contamination
Drilling
Gap die
Grinding
Load conditions
Turning
Energy source
Hybrid process Fig. 1. Influences of electrical and mechanical processes in hybrid machining processes.
determined by feed rate and process control. The demands on the energy source are determined primarily by the application and the technological goals. If machines are used for many applications, the energy sources must have a large parameter field [3–6]. Cost-efficient solutions for process energy sources are obtained for a specific or limited field of application. In order to develop basic concepts and structures of energy sources for hybrid machining processes it is necessary to categorize the hybrid process conditions. This division in groups can occur according to different criteria.
general working liquids that are approximately ideal so that no parasitic processes occur. At some applications liquids with a conductivity of a “middle” value are used for technological reasons. An example for this is the wire erosion in de-ionized water with a conductivity of 1–30 S/cm. In this case, additionally to the ED process occurs a very small EC process. Contaminations caused by the removal process of electrical discharges lead partially to a strongly increasing conductivity, so that the EC part increases. Therefore, the wire erosion with de-ionized water represents a process combination of EDM and ECM whereby the working liquid has a low conductivity. Because the parasitic EC process is undesirable for this application, it is necessary that the working liquid regenerates as fast as possible and a pure ED processing can take place again. In the following hybrid, machining processes are considered, where one main process is combined purposefully with one or several assisting processes. The main process is determined by the technological demand for a higher processing accuracy or a higher removal rate. The selection of the working liquid with the corresponding conductivity will then follow. Shown in Fig. 2 is the dependence of the process classification on the conductivity of the working liquid. In the threshold area of dielectric and electrolytic liquids an alternation between main process and assisting process can occur unintentionally. The three main criteria for the working liquid are:
2.1. Working liquid
• the physical and chemical properties; • the contaminations during the machining; • the passivation through subprocesses.
An essential feature of a working liquid for electric processes is the electrical conductivity. Theoretically, the conductivity can have values between an ideal dielectric and an ideal electrolyte. In case of single processes are used
Additionally to the features required from the process, the working liquids must also satisfy the criteria for care of the environment and industrial safety. Not only are the properties of the pure working liquid important in this case, but also the new chemical compounds that come to being
Assisting process
Working liquid
Main process
Dielectric liquid
Pure EDM
With small conductivity
EDM
ECM
With higher conductivity
EDM
ECM
Conductivity without EC process
EDM
Joule heating
Liquid layer
ECM
EDM
Gas layer
ECM
EDM
Solid layer
ECM
EDM
EDM
ECM
Passivation
Electrolyte liquid
Pure ECM
Fig. 2. Influence of the working liquid on the process separation.
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
bubbles/particles of the EC process
dielectric layer (ECM)
two dielectric layers (ECM)
depassivation
sparks
no passivation
arcs
521
bridging
discharge in layer
?
local ECM
partial discharge
arcs
Fig. 3. Types of the passivation through ECM.
during the processing. Hydrocarbons or de-ionized water are used for EDM as dielectrics in general. By additives specific process stages (ignition, flushing conditions, plasma channel expansion) are positively influenced. For ECM aqueous solutions with NaCl or NaNO3 are used mostly as working liquids. In recent time also other electrolytes are used especially for Ti or CrNi alloys. 2.2. Gap conditions In analogy to the single processes the gap conditions have decisive influence on the combined ED/EC processes. By correctly adjusting the flushing system, electrode movement and energy source gap conditions are produced which makes it possible that both processes are effective and optimal machining results are achieved. The alternation between the processes can be caused through different mechanisms. One possibility is the passivation which stops the EC process. The passivation is influenced by the used electrolytes, the current density and the electrode materials. A classification of the passivation is possible by taking into account particles, gas bubbles, chemisorption and dielectric boundary layers [1]. In Fig. 3 the typical forms of passivation are represented. The ECM passivation by gas bubbles and solid particles is called “bridging”. The effect begins locally and expands about the entire machining surface. The parasitic gap capacity and, in the same way, the ohmic resistance of the gap are subjected to a continuous change. This kind of the passivation is very strongly dependent on the flushing system and the geometrical gap conditions. The efficiency of bridging increases with smaller gaps. The time for a passivation phase is in the range from tenth of microseconds until some microseconds. In the working gap dielectric layers and electric as well as dielectric connections of the electrodes are built that produce an electrical breakdown when the voltage drop is sufficiently high. This results in a local destruction of the passivation. For depassivation each kind of discharge can be used if an appropriate process analysis prevents damaging the electrodes. Another form of passivation is the one-sided formation of dielectric layers. These layers become formed
compactly on an electrode in the nanosecond to microsecond time period. This results in a sudden increase of the gap impedance and a great rise of the voltage u(t). The layers are mostly oxide or hydroxyl layers. Their absorption at the cathode causes an increased tunneling of electrons so that more electrons are available for streamer entry. At an anodic oxidation layer a space-charge cloud shape in front of the anode is formed which reduces the effective length of the working gap, but does not create any improved breakdown conditions. The fundamental investigations of the time-parallel combining machining are especially important for the recognition of critical process tasks which must be registered or, if necessary, suppressed by the process control. The space-charge region impedes the streamer entry, starting from the cathode so that the warming of the anodic surface occurs and is delayed. The double-layer formations change the gap conditions in a complex manner. Therefore, precise analysis of the anodic and cathodic layer formation must be carried out in order to determine the load changes. In Fig. 4 a typical u–i characteristic and operating points for a combined EC–ED machining is shown. The working liquid is an electrolyte with a conductivity of 20 S/cm. The test equipment is similar to an EDM equipment. After the connection of the energy source with the working gap current and voltage linear arise. Point 2 as the point of intersection of the EC process characteristic (line 1) and the energy source characteristic (line 3, voltage source with high internal resistance) is the operating point of the EC process. During the following EC phase the dissolution of a little quantity of anodic material takes place, the electrolyte is warmed and gas bubbles are built between the electrodes. After a time of about 1–100 s the igniting of a discharge can occur. The operating point moves within 100 ns from point 2 along the line 3 to point 4 as the operating point of the spark erosion. After an ED time during which a small EC process also occurs, the source of energy is disconnected and voltage and current decrease dramatically. Rise and fall times are recognizable, which give information about the gap impedance and the gap width before and after the machining pulse. Contamination by the used
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
Voltage in V
522
70 60 50 40 30 20 10 0
4
25
Current in A
Current in A
30
20
3 15
-400.0µ
-300.0µ
-400.0µ
-300.0µ
-200.0µ
-100.0µ
0.0
-200.0µ
-100.0µ
0.0
28.0 24.0 20.0 16.0 12.0 8.0 4.0 0.0 -4.0
Time in s
5
10
1
5
2
0 0
10
20
30
40
50
60
70
Voltage in V
1 - Rise and EC-fall time of the pulse 3 - Breakdown for EDM 5 - ED-fall time of the pulse
2 - Operating point ECM (40 V, 5 A) 4 - Burning voltage (16 V up to 26 V)
Fig. 4. u–i characteristic for combined EC–ED machining (voltage source with internal resistance).
materials, their thermal stages and chemical reactions complicate the generalization of gap conditions. In the case of hybrid processes in analogy to single processes, a balance must be found between the removal processes and the feed rate. Technological demands on the hybrid process is a starting point for the calculations for this balance. For example, a smaller working gap is to be set for increasing the processing accuracy. These modified gap conditions change the load conditions because gap impedance and capacity are dependent on the gap width. The base movements and jumping of the cathodic base points lead to further dynamic and/or sudden load changes [2].
which a controlled electrical discharge creates a highly conductive connection between the tool and the workpiece. EDM is a pulsated process, after every discharge a break time for the regeneration of the gap conditions must occur. During the pre-ignition phase a very high load resistance exists. In the channel breakdown phase, this load decreases
i arc
breakdown OP4
2.3. Parameter field of process energy sources The demands on the energy source are determined primarily by the application and the technological goals. In order to develop the basic concepts and structures of energy sources for hybrid machining processes it is necessary to determine the hybrid process parameter. In a process combination including electrical discharge machining and electrochemical machining, two essentially differing load conditions occur. EDM as a separate process takes place in a dielectric in
itotal iedm OP2
0
iecm u bv,ed
u lim
Fig. 5. Characteristic of a current source.
u
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
Generator output
Feeder conditions
u-icharacteristic
Gap conditions
Gap processes
Switching rise-/fall-time
523
Gap liquid
Contamination Passivation
Gap geometry
Electrode material
OPERATING POINTS Fig. 6. Influences on the hybrid process.
by an order of the magnitude of about 107 times within a few hundred nanoseconds. The gap voltage falls from values of 100–300 V onto a fixed value of approximately 25 V in the burning phase. Removal rate, roughness and thermal-influenced zone depend strong on pulse current and pulse duration, thereby the discharge current must be set exactly. This means that using a current source is favorable. On the other hand, ECM is characterized by relatively constant load conditions, which are influenced by the widening of the working gap, the passivation effects and changes of the properties of the working liquid during the machining process. The removal in unwanted areas and the gap width depend on the working voltage. Therefore, an exact adjustable voltage source is desirable for ECM. A pulse energy source is necessary for a combination of the two processes, which can operate both as current source and as voltage source. In Fig. 5 the characteristic (rectangular line) of such energy source is shown. During the movement of the operating point along the vertical part of the line, the energy supply is a constant voltage source. Along the horizontal part the energy supply is a constant current source. The pulses should be as constant as possible so that machining can take place under stable process conditions. The changing between the modes (current source or voltage source) can occur within a few hundred nanoseconds. Thus, the energy source must be highly dynamic. Fig. 6 shows the influences on the combined process.
3. Conclusion The development of the hybrid process energy source is an important contribution to the efforts made to develop a
hybrid machining plant with technological efficiency and cost effectivity. The newly developed source suitable for strongly differing process characteristics represents a great step forward in the development of the process energy sources. As a result, a field for technological studies has now been opened which will make it possible for us to develop optimized machining solutions for a large number of further applications.
References [1] H.-P. Schulze, M. Läuter, G. Wollenberg, A. DeSilva, J.A. McGeough, Applications of ED-process energy sources for combined processes, in: Proceedings of the 18th International Conference on Computer-Aided Production Engineering (CAPE’2003), Edinburgh, UK, 18–19 March 2003. [2] H.-P. Schulze, M. Läuter, W. Rehbein, K. Mecke, Channel spreading during the spark discharge for selected gap conditions and working fluids, in: Proceedings of the IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Albuquerque, NM, 19–22 October 2003. [3] G. Wollenberg, H.-P. Schulze, M. Timm, T. Pape, Simulation of EDM installations, in: Proceedings of the 11th International Symposium on Electromachining (ISEM XI), EPFL, Lausanne, Switzerland, 17–21 April 1995, pp. 153–162. [4] M. Schöpf, I. Beltrami, M. Boccadoro, D. Kramer, ECDM (electro chemical discharge machining), a new method for trueing and dressing of metal-bonded diamond grinding tools, Ann. CIRP 50 (1) (2001) 125–128. [5] J.A. McGeough, A.B.M. Khayry, W. Munro, J.R. Crookall, Theoretical and experimental investigation of the relative effects of spark erosion and electrochemical dissolution in electrochemical ARC machining, Ann. CIRP 32 (1) (1983) 113–118. [6] K.P. Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silvam, New developments in electro-chemical machining, Ann. CIRP 48 (2) (1999) 567–579.
Structure of process energy sources for time-parallel combined processes M. Läuter∗ , H.-J. Trautmann, M. Zybura-Skrabalak, H.-P. Schulze, G. Wollenberg Institute for Fundamental Electrical Engineering and EMC, Otto-von-Guericke-University Magdeburg, Universitetsplatz 2, Magdeburg 39106, Germany
Abstract The structure of the process energy source in the case of combined techniques is determined by the main and assisting processes. At classical combinations of EDM and ECM the changed dynamic load conditions are shown, and the importance of the process-dependent rf noise for process identification is explained. The modular design may be used favorably for shaping the current and/or voltage pulses, especially for changing the rise and fall time of pulses. The structure adaption of the process energy sources to the hybrid time-parallel processes is an important task for process optimization and the effective utilization of the process combination for the provided case of application. © 2004 Published by Elsevier B.V. Keywords: Combined processes; Process energy sources; Electrochemical machining; Spark erosion
1. Motivation The motivation for using hybrid processes is the elimination of the negative or unwanted features of both processes without impeding the ‘good’ technological parameters. Combined machining with time-parallel running processes is featured by alternation and mutual influence between the single processes depending on the conditions at the working area. The used tools and working fluids must allow both processes. In the case of combinations of electrical and mechanical processes, often one of the processes is to improve the conditions for the other process. The energy source needed for the electric part in general does not differ from sources for individual processes. In Fig. 1 the influences of the electrical and non-electrical processes on a hybrid process are shown. Often combinations of electrical processes have larger parameter areas as individual processes. Therefore, specific process energy sources are needed which in their dynamics must satisfy the respective load conditions. Examples of machining processes which depend strongly on the load conditions are the spark erosion (EDM), the arc erosion (ADM) and the electrochemical machining (ECM).
∗ Corresponding author. Tel.: +49-391-67-11077; fax: +49-391-67-11236. E-mail address: [email protected] (M. Läuter).
0924-0136/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2004.02.018
This paper shows the demands on the process energy sources and the dependencies on the generator structure from the case of application. In this paper, only the electric discharge processes (EDM) and the anodic dissolution processes (ECM) are treated because these combinations were frequently used in the last years.
2. Hybrid EDM/ECM processes The combination of EDM and ECM has the aim, to reduce the disadvantages of both processes, • at EDM the small removal rate and the thermal damage of the processed surface, and • at ECM the poor accuracy, damage of faces that are not processed and process interrupting due to passivation and simultaneously to maintain the advantages of the processes • at EDM the high precision and • at ECM the high removal rate. Hybrid machining can be characterized by the definition of a main process and assisting processes. At hybrid processes consisting of EDM and ECM the main process is determined by the selection of the working liquid, the parameter of the energy source and the gap conditions
520
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523 Electrical processes
Spark erosion
Arc erosion
ECM Resistance Heating
Mechanical processes
Working medium
Milling
Gap contamination
Drilling
Gap die
Grinding
Load conditions
Turning
Energy source
Hybrid process Fig. 1. Influences of electrical and mechanical processes in hybrid machining processes.
determined by feed rate and process control. The demands on the energy source are determined primarily by the application and the technological goals. If machines are used for many applications, the energy sources must have a large parameter field [3–6]. Cost-efficient solutions for process energy sources are obtained for a specific or limited field of application. In order to develop basic concepts and structures of energy sources for hybrid machining processes it is necessary to categorize the hybrid process conditions. This division in groups can occur according to different criteria.
general working liquids that are approximately ideal so that no parasitic processes occur. At some applications liquids with a conductivity of a “middle” value are used for technological reasons. An example for this is the wire erosion in de-ionized water with a conductivity of 1–30 S/cm. In this case, additionally to the ED process occurs a very small EC process. Contaminations caused by the removal process of electrical discharges lead partially to a strongly increasing conductivity, so that the EC part increases. Therefore, the wire erosion with de-ionized water represents a process combination of EDM and ECM whereby the working liquid has a low conductivity. Because the parasitic EC process is undesirable for this application, it is necessary that the working liquid regenerates as fast as possible and a pure ED processing can take place again. In the following hybrid, machining processes are considered, where one main process is combined purposefully with one or several assisting processes. The main process is determined by the technological demand for a higher processing accuracy or a higher removal rate. The selection of the working liquid with the corresponding conductivity will then follow. Shown in Fig. 2 is the dependence of the process classification on the conductivity of the working liquid. In the threshold area of dielectric and electrolytic liquids an alternation between main process and assisting process can occur unintentionally. The three main criteria for the working liquid are:
2.1. Working liquid
• the physical and chemical properties; • the contaminations during the machining; • the passivation through subprocesses.
An essential feature of a working liquid for electric processes is the electrical conductivity. Theoretically, the conductivity can have values between an ideal dielectric and an ideal electrolyte. In case of single processes are used
Additionally to the features required from the process, the working liquids must also satisfy the criteria for care of the environment and industrial safety. Not only are the properties of the pure working liquid important in this case, but also the new chemical compounds that come to being
Assisting process
Working liquid
Main process
Dielectric liquid
Pure EDM
With small conductivity
EDM
ECM
With higher conductivity
EDM
ECM
Conductivity without EC process
EDM
Joule heating
Liquid layer
ECM
EDM
Gas layer
ECM
EDM
Solid layer
ECM
EDM
EDM
ECM
Passivation
Electrolyte liquid
Pure ECM
Fig. 2. Influence of the working liquid on the process separation.
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
bubbles/particles of the EC process
dielectric layer (ECM)
two dielectric layers (ECM)
depassivation
sparks
no passivation
arcs
521
bridging
discharge in layer
?
local ECM
partial discharge
arcs
Fig. 3. Types of the passivation through ECM.
during the processing. Hydrocarbons or de-ionized water are used for EDM as dielectrics in general. By additives specific process stages (ignition, flushing conditions, plasma channel expansion) are positively influenced. For ECM aqueous solutions with NaCl or NaNO3 are used mostly as working liquids. In recent time also other electrolytes are used especially for Ti or CrNi alloys. 2.2. Gap conditions In analogy to the single processes the gap conditions have decisive influence on the combined ED/EC processes. By correctly adjusting the flushing system, electrode movement and energy source gap conditions are produced which makes it possible that both processes are effective and optimal machining results are achieved. The alternation between the processes can be caused through different mechanisms. One possibility is the passivation which stops the EC process. The passivation is influenced by the used electrolytes, the current density and the electrode materials. A classification of the passivation is possible by taking into account particles, gas bubbles, chemisorption and dielectric boundary layers [1]. In Fig. 3 the typical forms of passivation are represented. The ECM passivation by gas bubbles and solid particles is called “bridging”. The effect begins locally and expands about the entire machining surface. The parasitic gap capacity and, in the same way, the ohmic resistance of the gap are subjected to a continuous change. This kind of the passivation is very strongly dependent on the flushing system and the geometrical gap conditions. The efficiency of bridging increases with smaller gaps. The time for a passivation phase is in the range from tenth of microseconds until some microseconds. In the working gap dielectric layers and electric as well as dielectric connections of the electrodes are built that produce an electrical breakdown when the voltage drop is sufficiently high. This results in a local destruction of the passivation. For depassivation each kind of discharge can be used if an appropriate process analysis prevents damaging the electrodes. Another form of passivation is the one-sided formation of dielectric layers. These layers become formed
compactly on an electrode in the nanosecond to microsecond time period. This results in a sudden increase of the gap impedance and a great rise of the voltage u(t). The layers are mostly oxide or hydroxyl layers. Their absorption at the cathode causes an increased tunneling of electrons so that more electrons are available for streamer entry. At an anodic oxidation layer a space-charge cloud shape in front of the anode is formed which reduces the effective length of the working gap, but does not create any improved breakdown conditions. The fundamental investigations of the time-parallel combining machining are especially important for the recognition of critical process tasks which must be registered or, if necessary, suppressed by the process control. The space-charge region impedes the streamer entry, starting from the cathode so that the warming of the anodic surface occurs and is delayed. The double-layer formations change the gap conditions in a complex manner. Therefore, precise analysis of the anodic and cathodic layer formation must be carried out in order to determine the load changes. In Fig. 4 a typical u–i characteristic and operating points for a combined EC–ED machining is shown. The working liquid is an electrolyte with a conductivity of 20 S/cm. The test equipment is similar to an EDM equipment. After the connection of the energy source with the working gap current and voltage linear arise. Point 2 as the point of intersection of the EC process characteristic (line 1) and the energy source characteristic (line 3, voltage source with high internal resistance) is the operating point of the EC process. During the following EC phase the dissolution of a little quantity of anodic material takes place, the electrolyte is warmed and gas bubbles are built between the electrodes. After a time of about 1–100 s the igniting of a discharge can occur. The operating point moves within 100 ns from point 2 along the line 3 to point 4 as the operating point of the spark erosion. After an ED time during which a small EC process also occurs, the source of energy is disconnected and voltage and current decrease dramatically. Rise and fall times are recognizable, which give information about the gap impedance and the gap width before and after the machining pulse. Contamination by the used
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
Voltage in V
522
70 60 50 40 30 20 10 0
4
25
Current in A
Current in A
30
20
3 15
-400.0µ
-300.0µ
-400.0µ
-300.0µ
-200.0µ
-100.0µ
0.0
-200.0µ
-100.0µ
0.0
28.0 24.0 20.0 16.0 12.0 8.0 4.0 0.0 -4.0
Time in s
5
10
1
5
2
0 0
10
20
30
40
50
60
70
Voltage in V
1 - Rise and EC-fall time of the pulse 3 - Breakdown for EDM 5 - ED-fall time of the pulse
2 - Operating point ECM (40 V, 5 A) 4 - Burning voltage (16 V up to 26 V)
Fig. 4. u–i characteristic for combined EC–ED machining (voltage source with internal resistance).
materials, their thermal stages and chemical reactions complicate the generalization of gap conditions. In the case of hybrid processes in analogy to single processes, a balance must be found between the removal processes and the feed rate. Technological demands on the hybrid process is a starting point for the calculations for this balance. For example, a smaller working gap is to be set for increasing the processing accuracy. These modified gap conditions change the load conditions because gap impedance and capacity are dependent on the gap width. The base movements and jumping of the cathodic base points lead to further dynamic and/or sudden load changes [2].
which a controlled electrical discharge creates a highly conductive connection between the tool and the workpiece. EDM is a pulsated process, after every discharge a break time for the regeneration of the gap conditions must occur. During the pre-ignition phase a very high load resistance exists. In the channel breakdown phase, this load decreases
i arc
breakdown OP4
2.3. Parameter field of process energy sources The demands on the energy source are determined primarily by the application and the technological goals. In order to develop the basic concepts and structures of energy sources for hybrid machining processes it is necessary to determine the hybrid process parameter. In a process combination including electrical discharge machining and electrochemical machining, two essentially differing load conditions occur. EDM as a separate process takes place in a dielectric in
itotal iedm OP2
0
iecm u bv,ed
u lim
Fig. 5. Characteristic of a current source.
u
M. Läuter et al. / Journal of Materials Processing Technology 149 (2004) 519–523
Generator output
Feeder conditions
u-icharacteristic
Gap conditions
Gap processes
Switching rise-/fall-time
523
Gap liquid
Contamination Passivation
Gap geometry
Electrode material
OPERATING POINTS Fig. 6. Influences on the hybrid process.
by an order of the magnitude of about 107 times within a few hundred nanoseconds. The gap voltage falls from values of 100–300 V onto a fixed value of approximately 25 V in the burning phase. Removal rate, roughness and thermal-influenced zone depend strong on pulse current and pulse duration, thereby the discharge current must be set exactly. This means that using a current source is favorable. On the other hand, ECM is characterized by relatively constant load conditions, which are influenced by the widening of the working gap, the passivation effects and changes of the properties of the working liquid during the machining process. The removal in unwanted areas and the gap width depend on the working voltage. Therefore, an exact adjustable voltage source is desirable for ECM. A pulse energy source is necessary for a combination of the two processes, which can operate both as current source and as voltage source. In Fig. 5 the characteristic (rectangular line) of such energy source is shown. During the movement of the operating point along the vertical part of the line, the energy supply is a constant voltage source. Along the horizontal part the energy supply is a constant current source. The pulses should be as constant as possible so that machining can take place under stable process conditions. The changing between the modes (current source or voltage source) can occur within a few hundred nanoseconds. Thus, the energy source must be highly dynamic. Fig. 6 shows the influences on the combined process.
3. Conclusion The development of the hybrid process energy source is an important contribution to the efforts made to develop a
hybrid machining plant with technological efficiency and cost effectivity. The newly developed source suitable for strongly differing process characteristics represents a great step forward in the development of the process energy sources. As a result, a field for technological studies has now been opened which will make it possible for us to develop optimized machining solutions for a large number of further applications.
References [1] H.-P. Schulze, M. Läuter, G. Wollenberg, A. DeSilva, J.A. McGeough, Applications of ED-process energy sources for combined processes, in: Proceedings of the 18th International Conference on Computer-Aided Production Engineering (CAPE’2003), Edinburgh, UK, 18–19 March 2003. [2] H.-P. Schulze, M. Läuter, W. Rehbein, K. Mecke, Channel spreading during the spark discharge for selected gap conditions and working fluids, in: Proceedings of the IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Albuquerque, NM, 19–22 October 2003. [3] G. Wollenberg, H.-P. Schulze, M. Timm, T. Pape, Simulation of EDM installations, in: Proceedings of the 11th International Symposium on Electromachining (ISEM XI), EPFL, Lausanne, Switzerland, 17–21 April 1995, pp. 153–162. [4] M. Schöpf, I. Beltrami, M. Boccadoro, D. Kramer, ECDM (electro chemical discharge machining), a new method for trueing and dressing of metal-bonded diamond grinding tools, Ann. CIRP 50 (1) (2001) 125–128. [5] J.A. McGeough, A.B.M. Khayry, W. Munro, J.R. Crookall, Theoretical and experimental investigation of the relative effects of spark erosion and electrochemical dissolution in electrochemical ARC machining, Ann. CIRP 32 (1) (1983) 113–118. [6] K.P. Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silvam, New developments in electro-chemical machining, Ann. CIRP 48 (2) (1999) 567–579.