A method and apparatus for microwave assisted electro-discharge machining of electrically conductive materials

Abstract

The present disclosure relates to the technical field of processing of conductive materials, and particularly relates to a method and equipment for microwave-assisted electric spark processing of conductive materials. The technical solution provided by the present disclosure utilizes the principle of microwave-assisted discharge, and by placing a tool electrode and a workpiece made of a conductive material in a microwave resonant cavity filled with an insulating medium, emitting microwaves of a preset frequency into the microwave resonant cavity, making the microwaves reflect in the microwave resonant cavity to accumulate microwave energy, and connecting a pulse power source between the tool electrode and the workpiece, an electric discharge channel is formed between the tool electrode and the workpiece when the pulse power source is turned on, the workpiece is processed by the influence of the microwave energy on the electric discharge channel between the tool electrode and the workpiece, the cost of the power source is reduced, and the processing efficiency is improved while ensuring the processing precision and surface quality.

Description

Technical Field

[0001] This disclosure relates to the field of conductive material processing technology, specifically to a method and apparatus for microwave-assisted electrical discharge machining of conductive materials. Background Technology

[0002] Conductive materials such as copper, steel, iron, titanium alloys, and high-temperature alloys are widely used in aerospace, electronic devices, medical equipment, and automotive manufacturing industries. They possess excellent comprehensive properties, including superior mechanical, chemical, and physical properties such as ultra-hardness, wear resistance, high strength, high melting point, and good electrical and thermal conductivity. Their applications in microelectromechanical systems (MEMS) are very extensive. However, despite their excellent performance and significant application value, the high hardness and strength of these conductive materials present considerable challenges in processing.

[0003] Existing machining methods mainly include conventional machining and electrical discharge machining (EDM). Conventional machining, including milling and broaching, are the two most widely used and technologically mature methods. However, these two methods are greatly affected by the material, resulting in low machining efficiency, high machining costs, and a tendency for burrs and flash to appear on the machined surface. This is especially true when machining microstructures, where the small tool size makes the tool prone to deformation or even breakage, affecting machining accuracy and stability. EDM is a non-contact machining method, but its pulse power supply is difficult and costly to manufacture, resulting in low machining efficiency. Furthermore, when the pulse width is shortened to a certain extent, discharge cannot be formed, limiting further improvements in machining accuracy. Therefore, there is an urgent need for a conductive material machining method that offers low power supply manufacturing costs, ease of manufacturing, high machining efficiency, and high machining accuracy. Summary of the Invention

[0004] To address the problems in the related technologies, this disclosure provides a method and apparatus for microwave-assisted electrical discharge machining of conductive materials.

[0005] In a first aspect, embodiments of this disclosure provide an apparatus for microwave-assisted electrical discharge machining of conductive materials, the apparatus comprising: a microwave resonant cavity, a microwave feeding device, and a workpiece processing device, wherein...

[0006] The microwave resonant cavity is connected to the microwave feed device and the workpiece processing device respectively, and is used to hold the insulating medium and place the workpiece made of conductive material.

[0007] The microwave feed device is configured to emit microwaves of a preset frequency into the microwave resonant cavity.

[0008] The workpiece processing device includes a workpiece processing mechanism and a tool electrode, and is configured to process the workpiece, wherein the tool electrode is placed in the microwave resonant cavity and connected to the workpiece processing mechanism;

[0009] When the pulse power supply, with its two poles connected to the tool electrode and the workpiece respectively, is turned on, a discharge channel is formed between the tool electrode and the workpiece. The microwaves emitted by the microwave feed device into the microwave resonant cavity are reflected and accumulate microwave energy within the microwave resonant cavity. The workpiece is then processed by utilizing the effect of the microwave energy on the discharge channel between the tool electrode and the workpiece.

[0010] According to an embodiment of this disclosure, the workpiece processing mechanism includes: a fixture, a machine tool guide shaft, a machine tool spindle, a machine tool column, and a machine tool worktable. The tool electrode, the fixture, the machine tool guide shaft, the machine tool spindle, and the machine tool column are sequentially connected and arranged perpendicularly to the machine tool worktable. The microwave resonant cavity includes an end cover and a cavity body, which are capable of relative movement. An openable door is provided on the end cover or the cavity body for placing the workpiece to fix it on the machine tool worktable and removing the workpiece from the machine tool worktable.

[0011] One end of the tool electrode is fixed to the fixture, and the other end of the tool electrode is close to the machine tool table for machining the workpiece;

[0012] The fixture is used to clamp the tool electrode, and the fixture is fixed on the machine tool guide shaft;

[0013] The machine tool guide shaft is fixed to the machine tool spindle through a first through hole provided on the end cover of the microwave resonant cavity, and the machine tool spindle is movably connected to the machine tool column;

[0014] The device also includes a mounting base, and the end cap of the microwave resonant cavity is fixed to the machine tool column through the mounting base, and the cavity is enclosedly connected to the machine tool worktable;

[0015] The device also includes a pulse power supply, one pole of which is connected to the machine tool table and the other pole is connected to the machine tool guide shaft, thereby enabling the pulse power supply to be connected between the tool electrode and the workpiece.

[0016] According to embodiments of this disclosure, wherein,

[0017] The microwave feed device includes a microwave source, a microwave source protection device, a dual-directional coupler, and a power meter. The microwave source, microwave source protection device, and dual-directional coupler are sequentially connected to the tool electrode or the workpiece via coaxial cables. The dual-directional coupler is connected to the power meter via a coaxial cable. The power meter is used to measure the microwave power incident into the microwave resonant cavity. The microwave source protection device includes a circulator or a DC blocker.

[0018] When the dual-directional coupler is connected to the tool electrode, the fixture is also provided with an adapter. The dual-directional coupler and the tool electrode are connected through the adapter on the fixture via a second through hole provided on the end cap of the microwave resonant cavity. When the dual-directional coupler is connected to the workpiece, the dual-directional coupler and the workpiece are connected through a second through hole provided on the end cap of the microwave resonant cavity.

[0019] According to embodiments of this disclosure, wherein,

[0020] The machine tool spindle is configured to reciprocate vertically on the machine tool column by connecting an automatic feed adjustment device, thereby driving the tool electrode to reciprocate vertically with the machine tool spindle;

[0021] The machine tool table is configured to move horizontally by connecting to the automatic feed adjustment device, thereby causing the workpiece to move horizontally with the machine tool table. This is to enable relative movement between the tool electrode and the workpiece through the vertical reciprocating motion and / or the horizontal motion, so as to process the workpiece by combining the relative motion.

[0022] According to embodiments of this disclosure, the device further includes an insulating medium processing device connected to the microwave resonant cavity for filtering and / or cooling the insulating medium.

[0023] Secondly, this disclosure provides a method for microwave-assisted electrical discharge machining of conductive materials, the method comprising:

[0024] The tool electrode and the workpiece made of conductive material are placed in a microwave resonant cavity containing an insulating medium.

[0025] A microwave of a preset frequency is emitted into the microwave resonant cavity, causing the microwave to be reflected within the microwave resonant cavity and accumulating microwave energy.

[0026] A pulse power supply is connected between the tool electrode and the workpiece, thereby forming a discharge channel between the tool electrode and the workpiece when the pulse power supply is turned on;

[0027] The workpiece is processed by utilizing the effect of the microwave energy on the discharge channel between the tool electrode and the workpiece.

[0028] According to an embodiment of this disclosure, the microwave resonant cavity includes an end cap and a cavity body, the end cap and the cavity body being movable relative to each other. An openable door is provided on the end cap or the cavity body for inserting the workpiece to fix it on a machine tool table and removing the workpiece from the machine tool table. The connection of a pulse power supply between the tool electrode and the workpiece specifically includes:

[0029] One end of the tool electrode is fixed to the fixture, and the other end of the tool electrode is close to the machine tool table for machining the workpiece; the fixture is fixed to the machine tool guide shaft;

[0030] The machine tool guide shaft is fixed to the machine tool spindle through the first through hole provided on the end cover of the microwave resonant cavity, and the machine tool spindle is movably connected to the machine tool column.

[0031] The end cap of the microwave resonant cavity is fixed to the machine tool column by a fixing seat, the cavity is enclosed and connected to the machine tool worktable, and the workpiece is placed into the microwave resonant cavity through the openable door and fixed to the machine tool worktable; wherein, the tool electrode is sequentially connected to the fixture, the machine tool guide shaft, the machine tool spindle and the machine tool column and is arranged perpendicular to the machine tool worktable;

[0032] One pole of the pulse power supply is connected to the machine tool worktable, and the other pole is connected to the machine tool guide shaft, thereby realizing the connection of the pulse power supply between the tool electrode and the workpiece.

[0033] According to embodiments of this disclosure, the step of transmitting microwaves of a preset frequency into the microwave resonant cavity specifically includes:

[0034] A microwave source, a microwave source protection device, and a dual-directional coupler are sequentially connected to the tool electrode or the workpiece using a coaxial cable, so that the microwaves generated by the microwave source are emitted into the microwave resonant cavity through the tool electrode or the workpiece. The dual-directional coupler is connected to a power meter via a coaxial cable, wherein the power meter is used to measure the microwave power incident into the microwave resonant cavity. The microwave source protection device includes a circulator or a DC blocker.

[0035] Specifically, when the dual-directional coupler is connected to the tool electrode using a coaxial cable, the fixture is also provided with an adapter. The coaxial cable is passed through a second through hole on the end cap of the microwave resonant cavity and the dual-directional coupler is connected to the tool electrode through the adapter on the fixture. When the dual-directional coupler is connected to the workpiece using a coaxial cable, the coaxial cable is specifically passed through the second through hole on the end cap of the microwave resonant cavity to connect the dual-directional coupler to the workpiece.

[0036] According to embodiments of this disclosure, the method further includes:

[0037] By connecting an automatic feed adjustment device, the machine tool spindle is made to move vertically back and forth on the machine tool column, thereby driving the tool electrode to move vertically back and forth with the machine tool spindle;

[0038] The machine tool table is made to move horizontally by connecting an automatic feed adjustment device, thereby driving the workpiece to move horizontally with the machine tool table.

[0039] The tool electrode and the workpiece move relative to each other through the vertical reciprocating motion and / or the horizontal motion, and the workpiece is processed in combination with the relative motion.

[0040] According to embodiments of this disclosure, the method further includes connecting the microwave resonant cavity to an insulating medium processing device, the insulating medium processing device being used to filter and / or cool the insulating medium.

[0041] The technical solution provided in this disclosure utilizes the principle of microwave-assisted discharge. By placing a tool electrode and a workpiece made of conductive material in a microwave resonant cavity containing an insulating medium, microwaves of a preset frequency are emitted into the microwave resonant cavity. The microwaves are reflected and accumulate microwave energy within the microwave resonant cavity. A pulse power supply is connected between the tool electrode and the workpiece, thereby forming a discharge channel between the tool electrode and the workpiece when the pulse power supply is turned on. The influence of the microwave energy on the discharge channel between the tool electrode and the workpiece is used to process the workpiece, reducing the power supply manufacturing cost and improving processing efficiency while ensuring processing accuracy and surface quality.

[0042] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0043] Other features, objects, and advantages of this disclosure will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings. In the drawings:

[0044] Figure 1 This diagram shows a structural schematic of an electrical discharge machining system in the prior art;

[0045] Figure 2 This diagram shows a structural block diagram of an apparatus for microwave-assisted electrical discharge machining of conductive materials according to an embodiment of the present disclosure.

[0046] Figure 3This diagram illustrates the structural connection of a device for microwave-assisted electrical discharge machining of conductive materials according to a specific embodiment 1.

[0047] Figure 4 This diagram illustrates the structural connection of a device for microwave-assisted electrical discharge machining of conductive materials according to a specific embodiment 2.

[0048] Figure 5 This diagram illustrates the structural connection of a device for microwave-assisted electrical discharge machining of conductive materials according to specific embodiment 3.

[0049] Figure 6 This diagram illustrates the structural connection of a device for microwave-assisted electrical discharge machining of conductive materials according to specific embodiment 4.

[0050] Figure 7 A flowchart illustrating a method for microwave-assisted electrical discharge machining of conductive materials according to an embodiment of the present disclosure is shown.

[0051] Figure 8 A flowchart illustrating the steps of connecting a tool electrode and a workpiece via a pulsed power supply according to an embodiment of the present disclosure is shown. Detailed Implementation

[0052] In the following, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings to enable those skilled in the art to readily implement them. Furthermore, for clarity, portions unrelated to the description of exemplary embodiments have been omitted from the drawings.

[0053] In this disclosure, it should be understood that terms such as “comprising” or “having” are intended to indicate the presence of features, figures, steps, behaviors, components, parts or combinations thereof disclosed in this specification, and are not intended to exclude the possibility of the presence or addition of one or more other features, figures, steps, behaviors, components, parts or combinations thereof.

[0054] It should also be noted that, unless otherwise specified, the embodiments and features described in this disclosure can be combined with each other. This disclosure will now be described in detail with reference to the accompanying drawings and embodiments.

[0055] As mentioned earlier, electrical discharge machining (EDM) is a non-contact machining method with advantages such as no obvious mechanical force during the machining process, a small heat-affected zone on the workpiece surface, and no limitation on the strength and hardness of the workpiece material. It is an important means of machining microstructures. However, in order to ensure machining accuracy and reduce electrode wear, the power supply output pulse width is required to be extremely short. The pulse power supply is difficult to manufacture, has high manufacturing cost, and low machining efficiency. Moreover, when the pulse width is shortened to a certain extent, discharge cannot be formed, which limits the further improvement of machining accuracy.

[0056] Before describing the method of this disclosure in detail, the electrical discharge machining system will be described first.

[0057] Figure 1 A schematic diagram of the electrical discharge machining system is shown.

[0058] like Figure 1 As shown, the basic structure of an electrical discharge machining (EDM) system includes a pulse power supply, a tool electrode, an automatic feed regulator, and a working medium supply system. During EDM, one pole of the pulse power supply is connected to the tool electrode, and the other pole is connected to the workpiece. Both the tool electrode and the workpiece are immersed in a working medium with a certain degree of insulation (usually a liquid medium, but gaseous media can also be used; different media are selected according to different application scenarios). The tool electrode is controlled by the automatic feed regulator to ensure that a very small discharge gap (0.01–0.05 mm) is maintained between the tool electrode and the workpiece during machining. When a pulse voltage is applied between the two poles, it breaks down the working medium, forming a discharge channel. The instantaneous high temperature generated in the discharge area melts or even evaporates the material. The interval between two pulse discharges is very short, and this high-frequency cyclic discharge is repeated continuously while the tool electrode continuously feeds towards the workpiece, thereby completing the machining of the workpiece. Because the cross-sectional area of ​​the discharge channel in electrical discharge machining is very small and the discharge time is extremely short, the energy in the discharge channel is highly concentrated and the energy density is very high. Such high temperature and high energy will cause the workpiece material to melt rapidly, which may lead to machining failure when machining workpieces with high precision requirements.

[0059] To ensure machining accuracy, existing technologies generally shorten the pulse width of the power supply output. However, this leads to difficulties in manufacturing the pulse power supply, high manufacturing costs, and low machining efficiency. Furthermore, when the pulse width is shortened to a certain extent, discharge cannot be formed, which limits further improvement in machining accuracy.

[0060] To further improve the processing accuracy and efficiency of conductive materials, especially microstructures, while reducing the manufacturing cost and difficulty of power supplies, this disclosure provides a method for microwave-assisted electrical discharge machining of conductive materials. According to an embodiment of this disclosure, a tool electrode and a workpiece made of conductive material are placed in a microwave resonant cavity containing an insulating medium. Microwaves of a preset frequency are emitted into the microwave resonant cavity, causing the microwaves to be reflected and accumulating microwave energy within the cavity. A pulse power supply is connected between the tool electrode and the workpiece, thereby forming a discharge channel between the tool electrode and the workpiece when the pulse power supply is turned on. The workpiece is processed by utilizing the influence of the microwave energy on the discharge channel between the tool electrode and the workpiece.

[0061] Unlike existing electrical discharge machining (EDM) processes that place the tool electrode and workpiece in a conventional working medium container, this disclosure utilizes the principle of microwave-assisted discharge. The tool electrode and workpiece are placed within a microwave resonant cavity. This cavity serves two purposes: firstly, it holds the working medium; secondly, when microwaves of a specific frequency are fed into it, microwave resonance occurs within its internal space, accumulating microwave energy. This disclosure leverages the influence of microwave energy on the electric field at the minute gap between the tool electrode and workpiece. By feeding in microwaves of a specific frequency, the cross-sectional area of ​​the discharge channel between the tool electrode and workpiece is increased, and the energy density is reduced. This modified discharge channel is then used to process the workpiece. Thus, even with relatively long voltage pulse widths, precision machining can still be achieved through the reduced energy density discharge channel, reducing the requirements for the pulse width of the pulse power supply output and lowering power supply manufacturing costs. Simultaneously, machining the workpiece through the increased cross-sectional area discharge channel improves processing efficiency while maintaining machining accuracy and surface quality.

[0062] Figure 2 A structural block diagram of an apparatus for microwave-assisted electrical discharge machining of conductive materials according to an embodiment of the present disclosure is shown. Figure 2 As shown, an apparatus for microwave-assisted electrical discharge machining of conductive materials includes: a microwave resonant cavity 210, a microwave feed device 220, and a workpiece processing device 230. The microwave resonant cavity 210 is connected to the microwave feed device 220 and the workpiece processing device 230, respectively, and is used to hold an insulating medium and to hold a workpiece made of conductive material. The microwave feed device 220 is configured to emit microwaves of a preset frequency into the microwave resonant cavity 210. The workpiece processing device 230 includes a workpiece processing mechanism 231 and a tool electrode 8, and is configured to process the workpiece. The tool electrode 8 is disposed within the microwave resonant cavity 210 and connected to the workpiece processing mechanism 231.

[0063] When the pulse power supply connected to the tool electrode 8 and the workpiece is turned on, a discharge channel is formed between the tool electrode 8 and the workpiece. The microwaves emitted by the microwave feed device 220 into the microwave resonant cavity 210 are reflected and accumulate microwave energy in the microwave resonant cavity 210. The workpiece is processed by utilizing the influence of the microwave energy on the discharge channel between the tool electrode 8 and the workpiece.

[0064] The following is combined with Figure 2 The specific components of each structure of the device shown are described through four specific embodiments.

[0065] Figure 3 The diagram shows a structural connection schematic of a device for microwave-assisted electrical discharge machining of conductive materials in specific embodiment 1.

[0066] like Figure 3 As shown, the microwave feed device 220 includes: a solid-state microwave source 1, a microwave source protection device 2, a dual-directional coupler 3, and a power meter 4. The solid-state microwave source 1, the microwave source protection device 2, the dual-directional coupler 3, and the tool electrode 8 are connected in sequence via a coaxial cable. The dual-directional coupler 3 is connected to the power meter 4 via a coaxial cable.

[0067] Microwaves with a preset frequency are generated by a solid-state microwave source 1, transmitted via a microwave source protection device 2 and a dual-directional coupler 3 through a coaxial cable to a tool electrode 8, and finally emitted into the microwave resonant cavity 210 through the tool electrode 8. Additionally, the dual-directional coupler 3 collects the microwave incident and reflected signals and transmits them to a power meter 4, which measures the microwave incident and reflected power. Specifically, the microwave source protection device 2 includes a circulator or a DC blocker.

[0068] The workpiece processing mechanism 231 includes: a fixture 7, a machine tool guide shaft 6, a machine tool spindle 9, a machine tool column 10, and a machine tool worktable 11. The microwave resonant cavity 210 includes a cavity 12 and an end cap 13. The end cap 13 and the cavity 12 of the microwave resonant cavity 210 are movable relative to each other. An openable door 14 is provided on the end cap 13 or the cavity 12 for placing the workpiece 15 to fix it on the machine tool worktable 11 and for removing the workpiece 15 from the machine tool worktable 11. In addition, a first through hole and a second through hole are provided on the end cap 13 of the microwave resonant cavity 210. The tool electrode 8, the fixture 7, the machine tool guide shaft 6, the machine tool spindle 9, and the machine tool column 10 are sequentially connected and arranged perpendicularly to the machine tool worktable 11.

[0069] Specifically, one end of the tool electrode 8 is fixed on the fixture 7, and the other end of the tool electrode 8 is close to the machine tool table 11 for processing the workpiece 15.

[0070] The clamp 7 is used to clamp the tool electrode 8. The clamp 7 is fixed on the machine tool guide shaft 6. The clamp 7 is also provided with an adapter. The dual directional coupler 3 and the tool electrode 8 are connected through the adapter on the clamp 7 via a second through hole provided on the end cap 13 of the microwave resonant cavity. The diameter of the second through hole is the same as the diameter of the coaxial cable used for connection, so as to confine the microwave in the microwave resonant cavity 210 and prevent it from being lost.

[0071] The machine tool guide shaft 6 is fixed to the machine tool spindle 9 through a first through hole provided on the end cover 13 of the microwave resonant cavity 210. The diameter of the first through hole is the same as the diameter of the machine tool guide shaft 6. The machine tool spindle 9 is movably connected to the machine tool column 10. By connecting an automatic feed adjustment device, the machine tool spindle 9 can move vertically reciprocally on the machine tool column 10 (e.g., ...). Figure 3 (As shown in the Z-axis direction), thereby driving the tool electrode 8 to reciprocate vertically with the machine tool spindle 9; the device also includes a fixed base 5, the end cover 13 of the microwave resonant cavity 210 is fixed to the machine tool column 10 through the fixed base 5, the cavity 12 is enclosedly connected to the machine tool worktable 11, and the machine tool worktable 11 can perform horizontal movement (e.g., in the Z-axis direction), thereby driving the tool electrode 8 to reciprocate vertically with the machine tool spindle 9; the device also includes a fixed base 5, the end cover 13 of the microwave resonant cavity 210 is fixed to the machine tool column 10 through the fixed base 5, the cavity 12 is enclosedly connected to the machine tool worktable 11, and the machine tool worktable 11 can perform horizontal movement through the connection of an automatic feed adjustment device (e.g., in the Z-axis direction), thereby driving the tool electrode 8 to Figure 3 (shown in the X and Y axis directions), thereby causing the workpiece 15 to move horizontally along the machine tool table 11.

[0072] The tool electrode 8 and the workpiece 15 are subjected to relative motion through the vertical reciprocating motion and / or the horizontal motion, and the workpiece 15 is machined in combination with the relative motion. The device also includes a pulse power supply 16, one pole of which is connected to the machine tool table 11 and the other pole is connected to the machine tool guide shaft 6, thereby realizing the connection of the pulse power supply 16 between the tool electrode 8 and the workpiece 15.

[0073] When the pulse power supply 16 is turned on, the pulse power supply 16 applies a voltage between the tool electrode 8 and the workpiece 15, causing a spark discharge by breaking down the insulating medium in the microwave resonant cavity 210. At the same time, the solid-state microwave source 1 emits microwaves into the microwave resonant cavity 210. The microwaves are reflected and accumulate microwave energy in the microwave resonant cavity 210. This will affect the discharge channel in the tiny gap between the tool electrode 8 and the workpiece 15, thereby forming a larger field strength. By utilizing the influence of the microwave energy on the discharge channel between the tool electrode 8 and the workpiece 15, combined with the relative motion between the tool electrode 8 and the workpiece 15, the workpiece 15 is processed.

[0074] Specifically, after microwaves are fed in, the energy accumulated by microwave reflection can increase the field strength of the discharge channel between the tool electrode 8 and the workpiece 15, while simultaneously increasing the cross-sectional area of ​​the discharge channel and reducing its energy density. Therefore, precision machining can still be achieved even with relatively long voltage pulse widths, reducing power supply manufacturing costs. At the same time, this equipment improves processing efficiency while ensuring machining accuracy and surface quality.

[0075] Figure 4 The diagram shows a structural connection diagram of a device for microwave-assisted electrical discharge machining of conductive materials in specific embodiment 2.

[0076] compared to Figure 3 The device shown, such as Figure 4 The microwave feed device 220 in the illustrated equipment also includes: a solid-state microwave source 1, a microwave source protection device 2, a dual-directional coupler 3, and a power meter 4. The difference is that the solid-state microwave source 1, microwave source protection device 2, and dual-directional coupler 3 are sequentially connected to the workpiece 15 via coaxial cables. Microwaves of a preset frequency are generated by the solid-state microwave source 1, transmitted to the workpiece 15 via the microwave source protection device 2 and dual-directional coupler 3 through the coaxial cable, and finally emitted into the microwave resonant cavity 210 by the workpiece 15. Additionally, the fixture 7 may not have an adapter interface, as it is not used to connect the dual-directional coupler 3, but only to fix the tool electrode 8. Apart from these two differences, Figure 4 Other structures, their specific components, and their interconnections in the device shown are related to... Figure 3 The devices shown are all the same, so they will not be described again here.

[0077] Specifically, when the dual directional coupler 3 is connected to the workpiece 15, for smaller workpieces, this can be achieved, for example, by clamping the workpiece with a metal clip. Otherwise, any method that allows microwaves emitted by the microwave source to be transmitted through the workpiece 15 into the microwave resonant cavity 210 can be used.

[0078] According to embodiments of this disclosure, the device may further include: an insulating medium processing device connected to the microwave resonant cavity for filtering and / or cooling the insulating medium.

[0079] Figure 5 The diagram shows a structural connection schematic of a device for microwave-assisted electrical discharge machining of conductive materials in specific embodiment 3.

[0080] like Figure 5 As shown, in specific embodiment 3, in Figure 3 An insulating medium processing device 17 is added to the device shown. The insulating medium processing device 17 is connected to the microwave resonant cavity 210 and is used to filter and / or cool the insulating medium.

[0081] Figure 6 The diagram shows a structural connection schematic of a device for microwave-assisted electrical discharge machining of conductive materials in specific embodiment 4.

[0082] like Figure 6 As shown, in specific embodiment 4, in Figure 4 An insulating medium processing device 17 is added to the device shown. The insulating medium processing device 17 is connected to the microwave resonant cavity 210 and is used to filter and / or cool the insulating medium.

[0083] In practical implementation, the insulating medium treatment device can be selected to have only filtration function, only cooling function, or both filtration and cooling functions. The specific selection depends on the specific situation of the equipment and the insulating medium used. For example, when the insulating medium is oil, an insulating medium treatment device with only filtration function can be selected; while when the insulating medium is deionized water, an insulating medium treatment device with only cooling function can be selected.

[0084] Figure 7 A flowchart illustrating a method for microwave-assisted electrical discharge machining of conductive materials according to an embodiment of the present disclosure is shown. Figure 7 As shown, the method for microwave-assisted electrical discharge machining of conductive materials includes the following steps S710 to S740:

[0085] In step S710, the tool electrode and the workpiece made of conductive material are placed in a microwave resonant cavity containing an insulating medium.

[0086] According to embodiments of this disclosure, the structure of the tool electrode can be designed according to the size of the hole or cavity in the workpiece being machined, and its types include forming electrodes and wire electrodes. The forming electrode has the same cross-sectional shape as the surface being machined and is used to machine various irregular holes, micro-holes, and complex cavities; the wire electrode is used to cut workpieces with ruled surfaces. The material of the tool electrode includes, but is not limited to, metallic materials, such as copper, steel, and superhard alloys. The forming size or cross-sectional size of the tool electrode is determined according to the size of the cavity or hole being machined, and depends on the size of the three-dimensional microstructure.

[0087] According to embodiments of this disclosure, the insulating medium is a working medium with a certain insulating strength (10³ to 10⁷ Ω·m), which plays a role in insulating and deionizing the workpiece during processing, cooling the high temperature during processing, and removing discharge substances. The insulating medium is usually a liquid medium, but it can also be a gaseous medium, etc.

[0088] In practice, the choice of insulating medium, as well as the structure, material, and size of the tool electrode, depends on the workpiece to be processed.

[0089] According to embodiments of this disclosure, a microwave resonant cavity is a metallic cavity used as a resonant circuit in the microwave band. It is a closed metallic cavity in which the electromagnetic field is confined. Microwave resonant cavities come in many shapes, the most common being rectangular and cylindrical resonant cavities.

[0090] Unlike conventional containers used in electrical discharge machining (EDM) to hold insulating media, embodiments of this disclosure use a microwave resonant cavity as the container for holding the insulating medium. When machining a workpiece made of conductive material, the tool electrode and the workpiece are placed together in the microwave resonant cavity for machining.

[0091] In step S720, microwaves of a preset frequency are emitted into the microwave resonant cavity, causing the microwaves to be reflected and accumulate microwave energy within the microwave resonant cavity.

[0092] The principle of a microwave resonant cavity is based on the resonance phenomenon. When the frequency of the microwave signal equals the natural frequency of the cavity, energy is maximized within the cavity for transmission and storage. Since resonant cavities are typically made of metal, their smooth internal metal walls reflect the microwave signal, causing it to propagate back and forth within the cavity, forming a standing wave. When the wavelength of the microwave signal is an integer multiple of the cavity's length, the standing wave reaches its maximum value, thus causing resonance. The frequency of the microwave signal is related to the shape, geometry, and waveform of the microwave resonant cavity. In practical implementation, a microwave resonant cavity and a matching microwave frequency can be selected according to actual needs.

[0093] According to embodiments of this disclosure, transmitting microwaves of a preset frequency into a microwave resonant cavity can be achieved in the following two ways:

[0094] Method 1: Use a coaxial cable to connect the microwave source, microwave source protection device, dual directional coupler and tool electrode in sequence, so that the microwave generated by the microwave source is emitted into the microwave resonant cavity through the tool electrode. Connect the dual directional coupler to a power meter through the coaxial cable. The power meter is used to measure the microwave power incident into the microwave resonant cavity.

[0095] According to embodiments of this disclosure, when the microwave source, microwave source protection device, dual directional coupler, and tool electrode are connected sequentially, a coaxial cable is used as the transmission medium. Thus, microwaves of a preset frequency, generated by the microwave source, are transmitted via the microwave source protection device and dual directional coupler, through the coaxial cable to the tool electrode, and finally emitted into the microwave resonant cavity through the tool electrode.

[0096] Method 2: Use a coaxial cable to connect the microwave source, microwave source protection device, dual directional coupler and workpiece in sequence, so that the microwave generated by the microwave source is emitted into the microwave resonant cavity through the workpiece. Connect the dual directional coupler to a power meter through the coaxial cable. The power meter is used to measure the microwave power incident into the microwave resonant cavity.

[0097] According to embodiments of this disclosure, when connecting the microwave source, microwave source protection device, dual directional coupler, power meter, and workpiece, a coaxial cable is also used as the transmission medium. Thus, microwaves with a preset frequency are generated by the microwave source, transmitted to the workpiece via the coaxial cable through the microwave source protection device and dual directional coupler, and finally emitted into the microwave resonant cavity through the workpiece.

[0098] Specifically, a microwave source is a device that generates microwave energy. A microwave source protection device is used to protect the microwave source. A dual directional coupler, connected to a power meter via a coaxial cable, is used to measure the incident and reflected microwave power during the microwave feeding process. The difference between the two yields the actual microwave power fed into the microwave resonant cavity. Additionally, the microwave reflection signal can be used as a feedback signal to adjust the microwave frequency. Therefore, by obtaining the actual fed microwave power and adjusting the microwave frequency emitted by the microwave source using the microwave reflection signal, the microwave frequency matching the microwave resonant cavity can be accurately and efficiently determined, thereby enabling the microwave resonant cavity to resonate more quickly.

[0099] Unlike Method 1, Method 2, in its implementation, replaces the transmission of microwaves from the tool electrode to the microwave resonant cavity with transmission from the workpiece. The advantage of Method 1 is that because the microwaves act directly on the tool electrode, the accumulated microwave energy can precisely influence the discharge channel at the tiny gap between the tool electrode and the workpiece. In practice, different methods can be selected depending on the tool electrode used in the processing. For example, for a forming tool electrode, when the tool electrode is fixed to the guide shaft by a fixture, a microwave feed device can be connected through an adapter on the fixture, thus using Method 1 to transmit microwaves into the microwave resonant cavity via the tool electrode. For a linear tool electrode, Method 2 can be used to transmit microwaves into the microwave resonant cavity via the workpiece.

[0100] In one specific embodiment, the microwave source is a solid-state microwave source. The microwaves generated by this fixed microwave source have a frequency of 2.45 GHz, a power P ranging from 0 ≤ P ≤ 3000 W, a duty cycle DR ranging from 5% ≤ DR ≤ 40%, and a pulse repetition frequency (PRF) ranging from 1 Hz ≤ PRF ≤ 1 MHz. Unlike traditional microwave sources, solid-state microwave sources are a new type of microwave power source that uses solid-state technology to replace traditional mechanical perturbation technology. They mainly consist of a laser, composite glass, and an amplifier, enabling directional control and high-precision positioning. They can generate high-power pulses and wideband modulation signals, and have advantages such as low cost, stable performance, and high safety and reliability.

[0101] In one specific embodiment, the microwave source protection device employs a circulator, which can prevent microwave source damage caused by microwave reflection signals generated during microwave discharge due to impedance mismatch or load changes.

[0102] In another specific embodiment, the microwave source protection device uses a DC blocker. The DC blocker prevents the microwave source from being damaged by high voltage and can effectively filter and block DC current, thereby improving the performance and stability of the fed microwave.

[0103] It should be noted that the methods used in implementing this step are not limited to the two methods described above. As long as the implementation of this method can achieve the following technical effect, namely, after transmitting microwaves of a preset frequency into the microwave resonant cavity, the microwaves are reflected and accumulate microwave energy within the microwave resonant cavity, thereby affecting the electric field at the tiny gap between the tool electrode and the workpiece, resulting in an increase in the cross-sectional area of ​​the discharge channel between the tool electrode and the workpiece and a decrease in energy density, all are within the scope of protection of this disclosure.

[0104] In step S730, a pulse power supply is connected between the tool electrode and the workpiece, thereby forming a discharge channel between the tool electrode and the workpiece when the pulse power supply is turned on.

[0105] According to embodiments of this disclosure, the microwave resonant cavity includes an end cap and a cavity body, which are movable relative to each other. An openable door is provided on the end cap or the cavity body for inserting the workpiece to fix it on a machine tool table and removing the workpiece from the machine tool table; additionally, a first through hole and a second through hole are provided on the end cap of the microwave resonant cavity.

[0106] When connecting the tool electrode and the workpiece via a pulse power supply, such as Figure 8 As shown, this can be specifically performed through the following steps S731 to S734:

[0107] In step S731, one end of the tool electrode is fixed to the fixture, and the other end of the tool electrode is close to the machine tool table for machining the workpiece; the fixture is fixed to the machine tool guide shaft.

[0108] To mount the tool electrode in an electrical discharge machining (EDM) machine, an intermediate tool is needed to hold the tool electrode, and the tool electrode fixture serves this purpose. There are many types of tool electrode fixtures, which can be broadly divided into fixed fixtures and universal (adjustable) fixtures. The specific type of fixture to choose depends on the actual situation.

[0109] Unlike the fixtures used in traditional electrical discharge machining (EDM) equipment, in a specific embodiment, when a coaxial cable is used to connect the microwave source, microwave source protection device, dual-directional coupler, and tool electrode, the fixture used in this disclosure also has an adapter. Besides fixing the tool electrode, this disclosure provides an additional function: when connecting the dual-directional coupler to the tool electrode, the coaxial cable is passed through a second through-hole on the end cap of the microwave resonant cavity and through the adapter on the fixture to connect the dual-directional coupler to the tool electrode, thereby transmitting the fed microwave through the tool electrode into the microwave resonant cavity. However, when a coaxial cable is used to connect the microwave source, microwave source protection device, dual-directional coupler, and workpiece, the fixture does not have an adapter; instead, the coaxial cable is passed through the second through-hole on the end cap of the microwave resonant cavity to connect the microwave feeding device to the workpiece.

[0110] Specifically, the diameter of the second through hole on the end cap of the microwave resonant cavity is the same as the diameter of the coaxial cable, thereby sealing the microwave resonant cavity and confining the microwaves within it to prevent them from dissipating.

[0111] In step S732, the machine tool guide shaft is fixed to the machine tool spindle through the first through hole provided on the end cover of the microwave resonant cavity, and the machine tool spindle is movably connected to the machine tool column.

[0112] Specifically, the diameter of the first through hole on the end cap of the microwave resonant cavity is the same as the diameter of the machine tool guide shaft, thereby sealing the microwave resonant cavity and confining the microwaves within it so as not to dissipate.

[0113] According to embodiments of this disclosure, since the machine tool spindle is movably connected to the machine tool column, the machine tool spindle can perform vertical reciprocating motion on the machine tool column, thereby driving the tool electrode to perform vertical reciprocating motion.

[0114] In step S733, the end cap of the microwave resonant cavity is fixed to the machine tool column by a fixing seat, the cavity is enclosed and connected to the machine tool worktable, and the workpiece is placed into the microwave resonant cavity through the openable door and fixed to the machine tool worktable.

[0115] The tool electrode is sequentially connected to the fixture, the machine tool guide shaft, the machine tool spindle, and the machine tool column, and then vertically positioned to the machine tool worktable.

[0116] In order to keep the microwave resonant cavity fixed during the workpiece processing, according to the embodiments of this disclosure, its end cap can be fixed to the machine tool column by a fixing seat, and the cavity can be fixed to the machine tool worktable. The cavity has no bottom surface, that is, the cavity wall is sealed to the machine tool worktable, and the workpiece is placed in the microwave resonant cavity and directly fixed to the machine tool worktable.

[0117] In step S734, one pole of the pulse power supply is connected to the machine tool worktable, and the other pole is connected to the machine tool guide shaft, thereby realizing the connection of the pulse power supply between the tool electrode and the workpiece.

[0118] According to an embodiment of this disclosure, when a pulse power supply is connected between the tool electrode and the workpiece, one pole of the pulse power supply can be connected to the machine tool table and the other pole can be connected to the machine tool guide shaft. Since the tool electrode is connected to the machine tool guide shaft through a fixture and the workpiece is directly mounted on the machine tool table, the pulse power supply is equivalent to being connected to both the tool electrode and the workpiece.

[0119] In specific connections, the positive terminal of the pulse power supply can be connected to the machine tool table, and the negative terminal to the machine tool guide shaft. This is equivalent to connecting the positive terminal of the pulse power supply to the workpiece and the negative terminal to the tool electrode, thus achieving positive polarity machining. Conversely, if the negative terminal of the pulse power supply is connected to the machine tool table and the positive terminal to the machine tool guide shaft, this is equivalent to connecting the negative terminal of the pulse power supply to the workpiece and the positive terminal to the tool electrode, thus achieving negative polarity machining. Since not only the workpiece but also the tool electrode is etched away during workpiece machining, a DC pulse power supply and a positive polarity machining method can be selected to reduce tool electrode consumption and improve productivity.

[0120] In step S740, the workpiece is processed by utilizing the effect of the microwave energy on the discharge channel between the tool electrode and the workpiece.

[0121] As previously mentioned, when microwaves of a specific frequency are fed into a microwave resonant cavity, the microwaves are reflected within the cavity, resulting in resonance and the accumulation of microwave energy. This microwave energy affects the electric field at the tiny gap between the tool electrode and the workpiece, causing the cross-sectional area of ​​the discharge channel between the tool electrode and the workpiece to increase and the energy density to decrease. This disclosure utilizes the modified discharge channel to process the workpiece, thereby improving processing efficiency while ensuring processing accuracy and surface quality.

[0122] Specifically, when machining a workpiece, the tool electrode can be controlled by connecting an automatic feed adjustment device, thereby maintaining a small discharge gap between the tool electrode and the workpiece.

[0123] According to embodiments of this disclosure, when controlling the tool electrode, the machine tool spindle can be made to reciprocate vertically on the machine tool column, thereby driving the tool electrode to reciprocate vertically along with the machine tool spindle. The machine tool table can also be made to move horizontally, thereby driving the workpiece to move horizontally along with the machine tool table. The vertical reciprocating motion of the tool electrode and / or the horizontal motion of the workpiece create relative motion between the tool electrode and the workpiece, thus combining relative motion to process the workpiece. In addition to vertical and horizontal motion, relative motion also includes relative rotational motion, with vertical and horizontal feed occurring simultaneously during relative rotational motion, and the rotational motion of the tool electrode and the workpiece itself simultaneously performing relative vertical and horizontal feed.

[0124] According to embodiments of this disclosure, the microwave-assisted electrical discharge machining method for conductive materials further includes: connecting the microwave resonant cavity to an insulating medium processing device, wherein the insulating medium processing device is used to filter and / or cool the insulating medium.

[0125] This disclosed technical solution utilizes the principle of microwave-assisted discharge. A tool electrode and a workpiece are placed within a microwave resonant cavity. This cavity serves two purposes: firstly, it holds the working medium; secondly, when microwaves of a specific frequency are fed into it, microwave resonance occurs within the cavity, accumulating microwave energy. This disclosure leverages the influence of microwave energy on the electric field at the tiny gap between the tool electrode and the workpiece. By feeding in microwaves of a specific frequency, the discharge between the tool electrode and the workpiece is stabilized, while simultaneously increasing the cross-sectional area of ​​the discharge channel and reducing the energy density. This modified discharge channel is then used to process the workpiece, enabling precision machining even with relatively long voltage pulse widths. This reduces the requirements for the pulse width of the pulse power supply output, lowering power supply manufacturing costs. Furthermore, it improves processing efficiency while maintaining machining accuracy and surface quality.

[0126] The above description is merely a preferred embodiment of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features disclosed in this disclosure that have similar functions.

Claims

1. A device for microwave-assisted electrical discharge machining of conductive materials, characterized in that, The equipment includes: a microwave resonant cavity, a microwave feed device, and a workpiece processing device, wherein... The microwave resonant cavity is connected to the microwave feed device and the workpiece processing device respectively, and is used to hold the insulating medium and place the workpiece made of conductive material. The microwave feed device is configured to emit microwaves of a preset frequency into the microwave resonant cavity. The workpiece processing device includes a workpiece processing mechanism and a tool electrode, and is configured to process the workpiece, wherein the tool electrode is placed in the microwave resonant cavity and connected to the workpiece processing mechanism; When the pulse power supply connected to the tool electrode and the workpiece is turned on, a discharge channel is formed between the tool electrode and the workpiece. The microwaves emitted by the microwave feed device into the microwave resonant cavity are reflected and accumulate microwave energy in the microwave resonant cavity. The workpiece is processed by utilizing the influence of the microwave energy on the discharge channel between the tool electrode and the workpiece. The workpiece processing mechanism includes a fixture; the microwave resonant cavity includes an end cap; the microwave feed device includes a microwave source, a microwave source protection device, a dual-directional coupler, and a power meter. The microwave source, microwave source protection device, and dual-directional coupler are sequentially connected to the tool electrode or the workpiece via coaxial cables. The dual-directional coupler is connected to the power meter via a coaxial cable. The power meter is used to measure the microwave power incident into the microwave resonant cavity. The microwave source protection device includes a circulator or a DC blocker. When the dual-directional coupler is connected to the tool electrode, the fixture is also provided with an adapter. The dual-directional coupler and the tool electrode are connected through the adapter on the fixture via a second through hole provided on the end cap of the microwave resonant cavity. When the dual-directional coupler is connected to the workpiece, the dual-directional coupler and the workpiece are connected through a second through hole provided on the end cap of the microwave resonant cavity.

2. The device according to claim 1, characterized in that, in, The workpiece processing mechanism further includes: a machine tool guide shaft, a machine tool spindle, a machine tool column, and a machine tool worktable. The tool electrode, the fixture, the machine tool guide shaft, the machine tool spindle, and the machine tool column are sequentially connected and arranged perpendicularly to the machine tool worktable. The microwave resonant cavity further includes a cavity body. The end cover and the cavity body are movable relative to each other. An openable door is provided on the end cover or the cavity body for placing the workpiece in so as to fix it on the machine tool worktable and for removing the workpiece from the machine tool worktable. One end of the tool electrode is fixed to the fixture, and the other end of the tool electrode is close to the machine tool table for machining the workpiece; The fixture is used to clamp the tool electrode, and the fixture is fixed on the machine tool guide shaft; The machine tool guide shaft is fixed to the machine tool spindle through a first through hole provided on the end cover of the microwave resonant cavity, and the machine tool spindle is movably connected to the machine tool column; The device also includes a mounting base, and the end cap of the microwave resonant cavity is fixed to the machine tool column through the mounting base, and the cavity is enclosedly connected to the machine tool worktable; The device also includes a pulse power supply, one pole of which is connected to the machine tool table and the other pole is connected to the machine tool guide shaft, thereby enabling the pulse power supply to be connected between the tool electrode and the workpiece.

3. The device according to claim 2, characterized in that, in, The machine tool spindle is configured to reciprocate vertically on the machine tool column by connecting an automatic feed adjustment device, thereby driving the tool electrode to reciprocate vertically with the machine tool spindle; The machine tool table is configured to move horizontally by connecting to the automatic feed adjustment device, thereby causing the workpiece to move horizontally with the machine tool table. This is to enable relative movement between the tool electrode and the workpiece through the vertical reciprocating motion and / or the horizontal motion, so as to process the workpiece by combining the relative motion.

4. The device according to claim 1, characterized in that, The device further includes an insulating medium processing device connected to the microwave resonant cavity for filtering and / or cooling the insulating medium.

5. A method for microwave-assisted electrical discharge machining of conductive materials, characterized in that, The method is applied to the device according to any one of claims 1 to 4, and the method includes: The tool electrode and the workpiece made of conductive material are placed in a microwave resonant cavity containing an insulating medium. A microwave of a preset frequency is emitted into the microwave resonant cavity, causing the microwave to be reflected within the microwave resonant cavity and accumulating microwave energy. A pulse power supply is connected between the tool electrode and the workpiece, thereby forming a discharge channel between the tool electrode and the workpiece when the pulse power supply is turned on; The workpiece is processed by utilizing the effect of the microwave energy on the discharge channel between the tool electrode and the workpiece.

6. The method according to claim 5, characterized in that, in, The microwave resonant cavity includes an end cap and a cavity body, the end cap and the cavity body being movable relative to each other. An openable door is provided on the end cap or the cavity body for inserting the workpiece to fix it on the machine tool table and removing the workpiece from the machine tool table; the connection of a pulse power supply between the tool electrode and the workpiece specifically includes: One end of the tool electrode is fixed to the fixture, and the other end of the tool electrode is close to the machine tool table for machining the workpiece; the fixture is fixed to the machine tool guide shaft; The machine tool guide shaft is fixed to the machine tool spindle through the first through hole provided on the end cover of the microwave resonant cavity, and the machine tool spindle is movably connected to the machine tool column. The end cap of the microwave resonant cavity is fixed to the machine tool column by a fixing seat, the cavity is enclosed and connected to the machine tool worktable, and the workpiece is placed into the microwave resonant cavity through the openable door and fixed to the machine tool worktable; wherein, the tool electrode is sequentially connected to the fixture, the machine tool guide shaft, the machine tool spindle and the machine tool column and is arranged perpendicular to the machine tool worktable; One pole of the pulse power supply is connected to the machine tool worktable, and the other pole is connected to the machine tool guide shaft, thereby realizing the connection of the pulse power supply between the tool electrode and the workpiece.

7. The method according to claim 6, characterized in that, The emission of microwaves at a preset frequency into the microwave resonant cavity specifically includes: A microwave source, a microwave source protection device, and a dual-directional coupler are sequentially connected to the tool electrode or the workpiece using a coaxial cable, so that the microwaves generated by the microwave source are emitted into the microwave resonant cavity through the tool electrode or the workpiece. The dual-directional coupler is connected to a power meter via a coaxial cable, wherein the power meter is used to measure the microwave power incident into the microwave resonant cavity. The microwave source protection device includes a circulator or a DC blocker. Specifically, when the dual-directional coupler is connected to the tool electrode using a coaxial cable, the fixture is also provided with an adapter. The coaxial cable is passed through a second through hole on the end cap of the microwave resonant cavity and the dual-directional coupler is connected to the tool electrode through the adapter on the fixture. When the dual-directional coupler is connected to the workpiece using a coaxial cable, the coaxial cable is specifically passed through the second through hole on the end cap of the microwave resonant cavity to connect the dual-directional coupler to the workpiece.

8. The method according to claim 7, characterized in that, The method further includes: By connecting an automatic feed adjustment device, the machine tool spindle is made to move vertically back and forth on the machine tool column, thereby driving the tool electrode to move vertically back and forth with the machine tool spindle; The machine tool table is made to move horizontally by connecting an automatic feed adjustment device, thereby driving the workpiece to move horizontally with the machine tool table. The tool electrode and the workpiece move relative to each other through the vertical reciprocating motion and / or the horizontal motion, and the workpiece is processed in combination with the relative motion.

9. The method according to claim 5, characterized in that, The method further includes connecting the microwave resonant cavity to an insulating medium processing device, wherein the insulating medium processing device is used to filter and / or cool the insulating medium.

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