Multi-wire electrical discharge machining (EDM) machine, multi-wire EDM machining process, process for producing thin plates and process for producing semiconductor wafers

Abstract

Multi-wire electrical discharge machining (EDM) machine (1), comprising: a plurality of guide rollers (3a, 3b, 3c, 3d) for guiding a movement of a wire electrode (6), wherein the wire electrode (6) is wound multiple times at intervals around the plurality of guide rollers (3a, 3b, 3c, 3d); a processing current supply (7) for applying a voltage between each of a plurality of cutting wire sections (6a) and a workpiece (2), wherein the plurality of cutting wire sections (6a) are parallel sections of the wire electrode (6) between a first guide roller (3c) and a second guide roller (3d), wherein the first guide roller (3c) and the second guide roller (3d) are encompassed in the plurality of guide rollers (3a, 3b, 3c, 3d);a dummy workpiece (25) with a contact surface (25b, 25c), wherein the contact surface (25b, 25c) is curved such that it can come into contact with a curved surface of the workpiece (2) except for an endpoint (P1), wherein the workpiece (2) has the curved surface, wherein the endpoint (P1) is reachable by each of the plurality of cutting wire sections (6a) when the machining of the workpiece (2) is complete, wherein the dummy workpiece (25) holds the workpiece (2) from one side in a first direction, wherein the first direction is a direction in which the workpiece (2) moves with respect to the plurality of cutting wire sections (6a);and a pressing mechanism (26) with a pressing section (28) that can come into contact with the curved surface of the workpiece (2), wherein the pressing mechanism (26) presses the workpiece (2) against the dummy workpiece (25) by bringing the pressing section (28) into contact with the workpiece (2) from another side in the first direction.

Description

Technical field The present disclosure relates to a multi-wire electrical discharge machining (EDM) machine, a multi-wire EDM machining process, a method for producing thin plates and a method for producing semiconductor wafers for performing EDM machining simultaneously at a plurality of positions on a workpiece. background In a multi-wire electrical discharge machining (EDM) machine, a single wire electrode is wound multiple times to form multiple parallel cutting wire sections, and the EDM machining process is performed simultaneously on each section. This allows for the simultaneous cutting of multiple thin sheets from a single workpiece. Multi-wire EDM machines are essential for cutting multiple thin sheets from a single workpiece without damaging it. With regard to multi-wire electrical discharge machining (EDM) for cutting semiconductor wafers from a cylindrical semiconductor ingot, patent literature 1 discloses a method for performing EDM machining of a semiconductor ingot by attaching a side face of the semiconductor ingot to a base section of a holding mechanism that holds the semiconductor ingot and moving the holding mechanism relative to a wire. The side face of the semiconductor ingot is attached to a front face of the base section using an adhesive. The base section is a rectangular parallelepiped, and the front face of the base section faces the semiconductor ingot. Patent literature 2 discloses a workpiece holder and a wire electroeroding device for the production of thin disks and semiconductor wafers, wherein the workpiece holder is designed with a fitting section into which a workpiece is fitted without a gap so that a discharge state can be stably maintained during a cutting process by the wire electroeroding device. Citation list Patent literature Patent literature 1: JP 6 397 738 B2; Patent literature 2: DE 11 2011 101 672 T5 Overview of the invention The problem to be solved by the invention In a multi-wire electrical discharge machining (EDM) machine, machining fluid is sprayed from a nozzle onto the side of a workpiece to remove machining chips from the inside of a groove formed in the workpiece by EDM. When thin plates are cut from a cylindrical workpiece, the distance between the nozzle's injection port and the workpiece changes as machining progresses. Depending on this distance, or the amount of machining fluid sprayed, a wire can break due to an unstable machining process. Therefore, a large quantity of machining fluid is sprayed during actual machining, and the workpiece speed relative to the wire is sequentially controlled according to the state of the EDM between the workpiece and the wire. This prevents the machining process from becoming unstable.When the machining fluid is sprayed onto the workpiece in this way, the machining fluid exerts an external force on the workpiece. Immediately before the cutting of the cylindrical workpiece is completed, the cross-sectional area of ​​a connecting section, where the workpiece is to be cut into individual sections (i.e., a section that has not yet been cut), gradually decreases. The cross-sectional area refers to the area of ​​a cross-section perpendicular to the centerline of the cylinder that is the workpiece. In the case of applying the method from patent literature 1, the cross-sectional area of ​​an uncut section decreases, and an external force also acts on the workpiece. Consequently, it is likely that the workpiece will crack when the cutting process is completed. Furthermore, when applying the method described in the aforementioned patent literature 1, the cross-sectional area of ​​the uncut section gradually decreases immediately before the cutting of the cylindrical workpiece is completed, while the machining of the base section begins when the cutting wire segments reach the front surface of the base section. That is, the amount of machining of the base section increases rapidly from zero. Consequently, the workpiece speed relative to the cutting wire segments decreases rapidly with this rapid increase in the amount of machining of the base section. This leads to a local increase in the amount of machining of the workpiece. Because the amount of machining of the workpiece increases locally, there is a risk of the workpiece cracking. Therefore, the technique described in the aforementioned patent literature 1 presents the problem that it is difficult to reduce cracking in a workpiece. The present disclosure was made in view of the above considerations, and one objective of the present disclosure is to provide a multi-wire electrical discharge machining (EDM) machine capable of reducing cracks in a workpiece. Means to solve the problem To solve the problems described above and to achieve the objective, a multi-wire electrical discharge machining (EDM) machine according to the present disclosure comprises: a plurality of guide rollers for guiding the movement of a wire electrode, wherein the wire electrode is wound multiple times at intervals around the plurality of guide rollers; a machining current supply for applying a voltage between each of a plurality of cutting wire sections and a workpiece, wherein the plurality of cutting wire sections are parallel sections of the wire electrode between a first guide roller and a second guide roller, wherein the first guide roller and the second guide roller are encompassed in the plurality of guide rollers;a dummy workpiece with a contact surface, wherein the contact surface is curved such that it is able to come into contact with a curved surface of the workpiece except for one endpoint, the workpiece having the curved surface, the endpoint being reachable by each of the plurality of cutting wire sections when the machining of the workpiece is complete, the dummy workpiece holding the workpiece from one side in a first direction, the first direction being a direction in which the workpiece moves with respect to the plurality of cutting wire sections; and a pressing mechanism with a pressing section capable of coming into contact with the curved surface of the workpiece, the pressing mechanism pressing the workpiece against the dummy workpiece by bringing the pressing section into contact with the workpiece from another side in the first direction. Effects of the invention The multi-wire electrical discharge machining (EDM) machine according to the present disclosure has an effect such that cracks in a workpiece can be reduced. Brief description of the drawings Fig. 1 is a diagram showing a configuration of a multi-wire electrical discharge machining (EDM) machine according to a first embodiment. Fig. 2 is a perspective view of a workpiece clamping device included in the multi-wire EDM machine according to the first embodiment. Fig. 3 is a top view of a dummy workpiece included in the multi-wire EDM machine according to the first embodiment. Fig. 4 is a flowchart showing an operating sequence of the multi-wire EDM machine according to the first embodiment. Fig. 5 is a perspective view of the workpiece clamping device and a plurality of cutting wire sections, showing the state of the workpiece clamping device and the plurality of cutting wire sections in the multi-wire EDM machine according to the first embodiment, which has started the EDM machining process.Figure 6 is a perspective view of the workpiece clamping device and the plurality of cutting wire sections, showing the state of the workpiece clamping device and the plurality of cutting wire sections in the multi-wire electrical discharge machining (EDM) machine according to the first embodiment, which has started pressing the workpiece. Figure 7 is a top view of the dummy workpiece to describe areas of the contact surfaces of the dummy workpiece of the first embodiment. Figure 8 is a first diagram to describe the relationship between the areas of the workpiece that are in contact with a pressing section and the principal stress to be applied to the workpiece in the first embodiment. Figure 9 is a second diagram to describe the relationship between the areas of the workpiece that are in contact with the pressing section and the principal stress to be applied to the workpiece in the first embodiment.Figure 10 is a third diagram describing the relationship between the areas of the workpiece in contact with the press section and the principal stress to be applied to the workpiece in the first embodiment. Figure 11 is a top view of a dummy workpiece according to a modification of the first embodiment. Figure 12 is a perspective view of a workpiece clamping device according to a modification of the first embodiment. Description of the embodiments A multi-wire electrical discharge machining (EDM) machine, a multi-wire EDM machining process, a method for producing thin plates, and a method for producing semiconductor wafers according to one embodiment are described in detail below with reference to the drawings. First embodiment. Fig. 1 is a diagram showing a configuration of a multi-wire electrical discharge machining (EDM) machine 1 according to a first embodiment. In the following description, the X-axis, Y-axis, and Z-axis are three axes that are perpendicular to each other. The Z-axis is a vertical axis. The directions of each axis are defined as follows: a pointing arrow is defined as a positive direction, and a pointing arrow opposite the direction of the arrow is defined as a negative direction. A positive Z-direction is defined as a vertically upward direction, and a negative Z-direction is defined as a vertically downward direction. The multi-wire electrical discharge machining (EDM) machine 1 performs electrical discharge machining (EDM) on a workpiece 2 by moving a multi-wound wire electrode 6 and applying a voltage between each of a plurality of cutting wire sections 6a and the workpiece 2. The plurality of cutting wire sections 6a are sections of the wire electrode 6 that run parallel to each other. The multi-wire EDM machine 1 produces a plurality of thin sheets by simultaneously cutting the plurality of thin sheets from the workpiece 2. The multi-wire EDM machine 1 performs EDM machining while preventing the wire electrode 6 from breaking due to a destabilization of a machining process or mechanical aging by controlling a voltage applied by a machining power supply 7. The multi-wire electrical discharge machining (EDM) machine 1 comprises a wire spool 4, a wire spool drive unit 17, wire discharge rollers 5, a plurality of guide rollers 3a, 3b, 3c, and 3d, and a guide roller drive unit 18. The wire spool 4 feeds the wire electrode 6. The wire spool drive unit 17 rotates the wire spool 4. The wire discharge rollers 5 discharge the wire electrode 6 outwards from the multi-wire EDM machine 1. The plurality of guide rollers 3a, 3b, 3c, and 3d serve to move the wire electrode 6 appropriately. The guide roller drive unit 18 drives the guide roller 3a. The wire electrode 6 is wound multiple times around the guide rollers 3a, 3b, 3c, and 3d at intervals. The guide rollers 3a, 3b, 3c and 3d guide the movement of the wire electrode 6 between the wire coil 4 and the wire ejection rollers 5. The multi-wire electrical discharge machining (EDM) machine 1 comprises a power supply 12 and a drive unit 14. The power supply 12 provides current to the wire electrode 6. The drive unit 14 drives a worktable. The worktable is not shown. A workpiece clamping device is arranged on the worktable. The workpiece 2 is held by the workpiece clamping device. The workpiece clamping device is not shown in Fig. 1. Details of the workpiece clamping device are described below. The drive unit 14 moves the worktable in the direction of the Z-axis, which is a first direction. The first direction is the direction in which the workpiece 2 moves with respect to the majority of cutting wire sections 6a and is the direction in which the EDM machining of the workpiece 2 progresses. The multi-wire electrical discharge machining (EDM) machine 1 comprises the processing power supply 7, a cable 11, a processing control device 9, and a processing status detection device 15. The cable 11 connects the processing power supply 7 and the workpiece clamping device. The processing control device 9 controls the EDM machining process. The processing status detection device 15 detects a machining state. The processing power supply 7 applies a pulse voltage for machining between each cutting wire segment 6a and the workpiece clamping device via the power supply 12. That is, the processing power supply 7 applies the pulse voltage for machining between each of the multiple cutting wire segments 6a and the workpiece 2. Hereinafter, the pulse voltage for machining is referred to as the machining pulse voltage. Each of the guide rollers 3a, 3b, 3c, and 3d has a cylindrical shape. The guide rollers 3a, 3b, 3c, and 3d have parallel central axes that are spaced apart from one another. In Fig. 1, the central axis of each of the guide rollers 3a, 3b, 3c, and 3d is aligned with the Y-axis. In a plane perpendicular to each central axis, the central axes of the guide rollers 3a, 3b, 3c, and 3d are located at positions corresponding to the vertices of a quadrilateral. In Fig. 1, the plane perpendicular to each central axis corresponds to an XZ-plane. Specifically, two of the four guide rollers 3a, 3b, 3c, and 3d, namely guide rollers 3a and 3b, are positioned above the other two guide rollers 3c and 3d. Guide roller 3c is located below guide roller 3b. Guide roller 3d is located below guide roller 3a. In the plane perpendicular to each central axis, a connecting line between the central axis of guide roller 3a and the central axis of guide roller 3b, and a connecting line between the central axis of guide roller 3c and the central axis of guide roller 3d, are aligned with the X-axis. In the plane perpendicular to each central axis, a connecting line between the central axis of guide roller 3b and the central axis of guide roller 3c, and a connecting line between the central axis of guide roller 3d and the central axis of guide roller 3a, are aligned with the Z-axis. A plurality of guide grooves for guiding the movement of the wire electrode 6 are formed in each of the side faces of the guide rollers 3a, 3b, 3c, and 3d. The plurality of guide grooves are arranged at regular intervals along the central axis on each of the guide rollers 3a, 3b, 3c, and 3d. The wire electrode 6, supplied from the wire coil 4, is wound around the guide rollers 3a, 3b, 3c, and 3d along the guide grooves in each of the guide rollers 3a, 3b, 3c, and 3d. The wire electrode 6 is wound multiple times around the guide rollers 3a, 3b, 3c, and 3d and then ejected by the wire discharge rollers 5. Viewed from the side of the plus-Y direction in Fig. 1, each of the guide rollers 3a, 3b, 3c, and 3d rotates clockwise. Each of the plurality of cutting wire sections 6a is a section of the wire electrode 6 that is tensioned between the guide roller 3c, which is a first guide roller, and the guide roller 3d, which is a second guide roller. Between the guide roller 3c and the guide roller 3d, there is a plurality of cutting wire sections 6a running parallel to each other. The plurality of cutting wire sections 6a refers to mutually parallel sections of the wire electrode 6 between the guide roller 3c and the guide roller 3d. In the first embodiment, a second direction, which is the direction of movement of the wire electrode 6 in the plurality of cutting wire sections 6a, corresponds to an X-axis direction. In the example shown in Fig. 1, the guide roller drive unit 18 causes the guide roller 3a to rotate. Furthermore, the wire spool drive unit 17 causes the wire spool 4 to rotate such that the voltage of the wire electrode 6 is kept constant. The multi-wire electrical discharge machining (EDM) machine 1 controls the wire spool drive unit 17 and the guide roller drive unit 18 such that the wire spool drive unit 17 causes the wire spool 4 to rotate and the guide roller drive unit 18 causes the guide roller 3a to rotate such that the wire electrode 6 can move at a desired speed. The machining condition detection device 15 detects a machining condition at each cutting wire section 6a. The machining condition detection device 15 is installed on wires connecting the power supply 12 and the machining power supply 7. The machining condition detection device 15 detects a machining condition by monitoring, via the power supply 12, the gap voltage, the current, the number of times electrical erosion occurs, the number of times a short circuit occurs per unit of time, and the machining pulse voltage, which is the output voltage of the machining power supply 7. The gap voltage is a voltage to be applied to an electrode gap, i.e., a gap between the workpiece 2 and each cutting wire section 6a.The processing state detection device 15 comprises a plurality of processing state detection units 16, such that a one-to-one mapping exists between the processing state detection units 16 and the cutting wire sections 6a. The processing state detection device 15 detects a processing state of each cutting wire section 6a with a corresponding processing state detection unit 16. The machining control device 9 generates a position command, i.e., a feed control command value, based on the machining states detected by the machining state detection device 15. The machining control device 9 outputs the position command to the drive unit 14. The drive unit 14 moves the machining table in the direction of the Z-axis in accordance with the position command. As a result, the position of the workpiece 2 relative to each cutting wire section 6a is changed. The multi-wire electrical discharge machining (EDM) machine 1 adjusts the relative distance between the workpiece 2 and each cutting wire section 6a by means of the drive unit 14. The processing power supply 7 comprises a plurality of processing power supply units 8, such that there is a one-to-one mapping between the processing power supply units 8 and the cutting wire sections 6a. The processing control device 9 issues a voltage application command to each processing power supply unit 8. Each processing power supply unit 8 applies a processing pulse voltage between one of the corresponding cutting wire sections 6a and the workpiece 2. The voltage application command includes each of the command values ​​such as the voltage amplitude, the frequency of the pulse voltage, and the on-time of the pulse voltage. The machining control device 9 controls the relative distance between the workpiece 2 and each cutting wire section 6a by issuing a position command, and effects electrical discharge machining (EDM) between each cutting wire section 6a and the workpiece 2 by issuing a voltage application command. In this way, the multi-wire EDM machine 1 cuts a plurality of thin sheets from the workpiece 2. The workpiece 2 is an ingot to be cut into a plurality of thin wafers. Examples of workpiece 2 include metals such as tungsten and molybdenum, which serve as sputtering targets, and ceramics such as polycrystalline silicon carbide, which are used as components of various structures. The workpiece 2 can consist of single-crystal silicon, which is a material used in semiconductor wafers, or of a semiconductor material such as single-crystal silicon carbide, single-crystal gallium nitride, single-crystal gallium oxide, or single-crystal diamond. Alternatively, the workpiece 2 can be a solar cell material, which is also a material used in solar cell wafers, such as single-crystal silicon or polycrystalline silicon. The multi-wire electrical discharge machine 1 can be a multi-wire electrical discharge machine that produces a plurality of semiconductor wafers by cutting the plurality of semiconductor wafers from an ingot. Among the materials mentioned as examples for workpiece 2, metal exhibits a sufficiently low resistivity. Therefore, metal can be used for electrical discharge machining (EDM) without any problems. Meanwhile, semiconductor and solar cell materials with a sufficiently low resistivity of approximately 100 Ωcm or less, preferably 10 Ωcm or less, can also be used for EDM. Therefore, metal is suitable as workpiece 2. Semiconductor material or solar cell material with a specific resistance in the range between the specific resistance of metal and 100 Ωcm is suitable for workpiece 2. Semiconductor material or solar cell material with a specific resistance in the range between the specific resistance of metal and 10 Ωcm is better suited for workpiece 2. In the multi-wire electrical discharge machining (EDM) machine 1, machining fluid is supplied to the electrode gap, i.e., the gap between the workpiece 2 and each cutting wire section 6a. The machining fluid is supplied to the electrode gap as in a so-called single-wire EDM machine, either by spraying the machining fluid or by immersing the workpiece 2 in the machining fluid. An arrangement for supplying the machining fluid is not shown. The processing power supply 7 generates a processing pulse voltage to be applied to the electrode gap in accordance with the voltage application command of the processing control device 9. The processing power supply 7 generates a processing pulse voltage, for example, by means of a switching power supply. The processing power supply 7 applies the processing pulse voltage separately to the multiple cutting wire sections 6a by means of the plurality of processing power supply units 8. The processing power supply 7 includes a ground electrode 10, which is arranged between the plurality of processing power supply units 8. A ground lead of each processing power supply unit 8 is connected to the ground electrode 10. The ground electrode 10 is connected to the workpiece clamping device via the cable 11. The workpiece 2 is connected to the ground electrode 10 via the workpiece clamping device and the cable 11.It should be noted that the processing power supply 7 can, if necessary, reverse the polarity of the processing pulse voltage to be generated in a suitable manner. The power supply 12 comprises a plurality of isolated power supply units 13. There is a one-to-one correspondence between the plurality of power supply units 13 and the power supply wire sections 6b. In the example shown in Fig. 1, the parallel sections of the wire electrode 6 between the guide roller 3b and the guide roller 3c correspond to a plurality of power supply wire sections 6b. Each power supply unit 13 is in sliding contact with one of the corresponding power supply wire sections 6b. Each power supply unit 13 supplies one of the corresponding power supply wire sections 6b with current, which is supplied by one of the corresponding power supply units 8 for processing. The power supply unit 12 supplies each of the plurality of power supply wire sections 6b separately with current by means of the plurality of power supply units 13.Thus, the processing pulse voltage is applied separately to each cutting wire section 6a by means of a corresponding processing power supply unit 8. Next, details of the workpiece fixture are described. Fig. 2 is a perspective view of a workpiece clamping device 20, which is included in the multi-wire electrical discharge machining (EDM) machine 1 according to the first embodiment. The workpiece clamping device 20 holds the workpiece 2 during the EDM process. In the first embodiment, the workpiece 2 has a cylindrical shape. In Fig. 2, the workpiece 2 is arranged on the workpiece clamping device 20 such that the central axis of the cylinder is aligned with the Y-axis. In Fig. 1, the shape of the workpiece 2 differs from that shown in Fig. 2 to make the individual components of the multi-wire EDM machine 1 easily identifiable. The workpiece clamping device 20 comprises a plate-shaped holder 20a, a reference plate 21, a secondary plate 22, an excitation electrode 23, an electrode holder 24, a dummy workpiece 25, a clamping mechanism 26, and a spacer 27. The reference plate 21 is attached to the holder 20a. The clamping mechanism 26 is mounted on the plus-Z side of the holder 20a. The holder 20a is integrated with the reference plate 21 and the clamping mechanism 26. The holder 20a holds the reference plate 21 and the clamping mechanism 26. The reference plate 21 is integrated with the electrode holder 24, the dummy workpiece 25, and the spacer 27. A first surface, which is one of two surfaces of the workpiece 2 perpendicular to its central axis, is attached to a reference surface of the reference plate 21. The reference surface is a surface of the reference plate 21 opposite the holder 20a, i.e., the XZ plane of the reference plate 21 on the side of the plus Y direction. The electrode holder 24, the dummy workpiece 25, and the spacer 27 are provided on the reference surface of the reference plate 21. The dummy workpiece 25 is located on the side of workpiece 2 in the minus Z direction. Workpiece 2 is attached to the dummy workpiece 25. The dummy workpiece 25 holds workpiece 2 from below. The dummy workpiece 25 is machined by the cutting wire sections 6a, which reached the dummy workpiece 25 during the machining of workpiece 2. A second surface, which is the other of the two surfaces of workpiece 2 perpendicular to its central axis, is attached to the auxiliary plate 22. Workpiece 2 is attached to the reference plate 21, the auxiliary plate 22, and the dummy workpiece 25 using conductive adhesive, conductive wax, or the like. Workpiece 2 is attached to the reference plate 21, the auxiliary plate 22, and the dummy workpiece 25 to be integrated into the workpiece clamping device 20. The reference plate 21 and the auxiliary plate 22 hold workpiece 2 such that it is positioned between them. One end of the excitation electrode 23 is attached to the reference plate 21 by the electrode holder 24, which is located beneath the dummy workpiece 25. Except for the end secured by the electrode holder 24, the excitation electrode 23 is in contact with the secondary plate 22 and the dummy workpiece 25. The excitation electrode 23 and the electrode holder 24 are integrated into the secondary plate 22. The secondary plate 22 is electrically connected to the reference plate 21 via the excitation electrode 23 and the electrode holder 24. One end of the cable 11 shown in Fig. 1 is connected to the excitation electrode 23. The excitation electrode 23 is electrically connected to the machining power supply 7 via the cable 11 and the ground electrode 10. The press mechanism 26 includes a press section 28 that can come into contact with a curved surface of the workpiece 2. The press mechanism 26 moves the press section 28 in the direction of the Z-axis. In the workpiece clamping device 20, the dummy workpiece 25 holds the workpiece 2 from the negative Z-direction side, which is one side in the first direction. After machining of the workpiece 2 has begun, the press mechanism 26 moves the press section 28 in the negative Z-direction to bring the press section 28 into contact with the workpiece 2 from the positive Z-direction side, which is the other side in the first direction. The press mechanism 26 also presses the workpiece 2 against the dummy workpiece 25 by means of the press section 28. The machining control device 9 controls the drive of the press mechanism 26 by issuing a command to move the press section 28.The press mechanism 26 moves the press section 28 in accordance with the command sent by the machining control device 9. The spacer 27 is provided below the electrode holder 24 on the reference surface. The spacer 27 is a component designed to maintain a distance between the reference plates 21 when a majority of the reference plates 21 are arranged in a Y-axis direction. Next, details of the dummy workpiece 25 are described. Fig. 3 is a top view of the dummy workpiece 25, which is included in the multi-wire electrical discharge machining (EDM) machine 1 according to the first embodiment. Fig. 3 shows the dummy workpiece 25 and a portion of the workpiece 2 in contact with the dummy workpiece 25. One end of the curved surface of workpiece 2 in the minus Z direction is a final point to be machined on workpiece 2. Hereinafter, this point is referred to as endpoint P1. Endpoint P1 is a point reached by each of the plurality of cutting wire sections 6a at the end of the machining of workpiece 2. The outer shape of the dummy workpiece 25 is curved along the curved surface of workpiece 2 on the side in the plus Z direction. A recess 25a, extending in the minus Z direction, is formed in the curved section of the dummy workpiece 25. The recess 25a is located in the center of the curved section in the direction of the X-axis. The recess 25a is formed at a position facing a portion of the curved surface of workpiece 2, the portion of which contains endpoint P1. A contact surface 25b comes into contact with the curved surface of the workpiece 2. Contact surface 25b is a portion of the curved section of the dummy workpiece 25 on one side of the recess 25a in the negative X direction. A contact surface 25c also comes into contact with the curved surface of the workpiece 2. Contact surface 25c is a portion of the curved section of the dummy workpiece 25 on one side of the recess 25a in the positive X direction. Each of the two contact surfaces 25b and 25c is curved such that it can come into contact with the curved surface of the workpiece 2, except at endpoint P1. Contact surfaces 25b and 25c are located adjacent to the recess 25a.The workpiece 2 is held by the dummy workpiece 25 in a state in which the endpoint P1 of the workpiece 2 is not in contact with the dummy workpiece 25 and the workpiece 2 is in contact with the contact surfaces 25b and 25c except for the endpoint P1. The dummy workpiece 25 consists of a material that can be machined by electrical discharge machining (EDM). Preferably, the material of the dummy workpiece 25 is a material with machining properties that correspond to those of the workpiece 2. Since the contact surfaces 25b and 25c of the dummy workpiece 25 are curved in such a way that they can come into contact with the curved surface of the workpiece 2, the areas of the regions of the workpiece 2 that are in contact with the dummy workpiece 25 can be increased. As a result, the multi-wire electrical discharge machining (EDM) machine 1 can increase the strength for holding the workpiece 2. Next, details of the press section 28 are described. When pressed against the curved surface of the workpiece 2 by the drive of the press mechanism 26, the press section 28 is deformed to conform to the shape of the workpiece 2. Specifically, a stainless steel plate with a thickness of 0.3 mm or less, or a material such as a urethane sponge, a gel-like material, or a thermoplastic resin can be used as the press section 28. Because the press section 28 can be deformed to conform to the shape of the workpiece 2, the surface areas of the workpiece 2 in contact with the press section 28 can be increased. Consequently, the multi-wire electrical discharge machining (EDM) machine 1 can increase the strength required to hold the workpiece 2.It should be noted that the material or configuration of the press section 28 is not limited to the material or configuration described in the first embodiment, as long as the press section 28 can be deformed in accordance with the shape of the workpiece 2. Next, the operation of the multi-wire electrical discharge machining (EDM) machine 1 will be described. Fig. 4 is a flowchart showing the operating sequence of the multi-wire EDM machine 1 according to the first embodiment. Before the multi-wire EDM machine 1 is put into operation according to the procedure shown in Fig. 4, the workpiece clamping device 20, into which the workpiece 2 has been placed, is positioned on the machining table. In step S1, the multi-wire electrical discharge machining (EDM) machine 1 releases the press mechanism 26 by releasing the press section 28 from the workpiece 2 in the plus-Z direction. At this point, the workpiece 2 is held by the dummy workpiece 25 on the minus-Z direction side and not on the plus-Z direction side. In step S2, the multi-wire electrical discharge machining (EDM) machine 1 begins the EDM machining of the workpiece 2. Fig. 5 is a perspective view of the workpiece clamping device 20 and the plurality of cutting wire sections 6a, showing the state of the workpiece clamping device 20 and the plurality of cutting wire sections 6a in the multi-wire EDM machine 1 according to the first embodiment, which has started the EDM machining. One end of the curved surface of the workpiece 2 on the side of the plus-Z direction is a point where the machining by the plurality of cutting wire sections 6a begins, i.e., a point on the workpiece 2 where the machining is performed for the first time. Hereinafter, this point is referred to as the starting point.The multi-wire electrical discharge machining (EDM) machine 1 performs the machining of the workpiece 2 by applying a machining pulse voltage to the electrode gap while the machining table is moved in the positive Z direction. A machining position on the workpiece 2 moves from the starting point in the negative Z direction. In step S3, the multi-wire electrical discharge machine 1 determines whether the machining position has reached a target position. The target position is a position predefined on the curved surface of the workpiece 2 and serves to determine whether the workpiece 2 can be pressed by the press mechanism 26. If the machining position has not reached the target position (step S3, no), the multi-wire electrical discharge machine 1 repeats step S3 and continues the electrical discharge machining process. Meanwhile, if the machining position has reached the target position (step S3, yes), the multi-wire electrical discharge machine 1 proceeds to step S4. In step S4, the multi-wire electrical discharge machining (EDM) machine 1 causes the press mechanism 26 to begin pressing the workpiece 2. Fig. 6 is a perspective view of the workpiece clamping device 20 and the plurality of cutting wire sections 6a, showing the states of the workpiece clamping device 20 and the plurality of cutting wire sections 6a in the multi-wire EDM machine 1 according to the first embodiment, which has begun pressing the workpiece 2. The press mechanism 26 brings the contact surfaces 25b and 25c into contact with the curved surface of the workpiece 2 by moving the press section 28 in the negative Z direction. Furthermore, the press mechanism 26 presses the workpiece 2 against the dummy workpiece 25 by pressing the workpiece 2 with the press section 28 in the negative Z direction. As a result, the workpiece 2 is held in a state in which it is positioned between the dummy workpiece 25 and the press section 28.The multi-wire electrical discharge machining (EDM) machine 1 performs machining of a section on the side of the minus-Z direction with respect to the destination position. In step S5, the multi-wire electrical discharge machining (EDM) machine 1 determines whether the machining position has reached endpoint P1. If the machining position has not reached endpoint P1 (step S5, No), the multi-wire EDM machine 1 repeats step S5 and continues the EDM machining. If the machining position has reached endpoint P1 (step S5, Yes), the multi-wire EDM machine 1 terminates the EDM machining in step S6. Thus, the multi-wire EDM machine 1 terminates operation according to the procedure shown in Fig. 4. When the electrical discharge machining (EDM) of workpiece 2 progresses to a certain stage, the multi-wire EDM machine 1 positions the workpiece 2 between the press section 28 and the dummy workpiece 25. The multi-wire EDM machine 1 can prevent vibrations at the section where workpiece 2 is to be cut into individual sections by applying an external force to the workpiece 2 with the machining fluid. In this way, the multi-wire EDM machine 1 can prevent cracking of workpiece 2. After the pressing of workpiece 2 by the pressing mechanism 26 is started, the electrical discharge machining (EDM) process continues until the majority of cutting wire segments 6a reach the dummy workpiece 25. When the majority of cutting wire segments 6a reach the dummy workpiece 25, machining of the dummy workpiece 25 is started while machining of workpiece 2 continues. When the majority of cutting wire segments 6a approach the endpoint P1 of workpiece 2 and reach the recess 25a, machining of the dummy workpiece 25 is stopped, and only workpiece 2 is machined. When the majority of cutting wire segments 6a reach the endpoint P1, machining of workpiece 2 ends. The recess 25a is provided in the dummy workpiece 25, and the dummy workpiece 25 comes into contact with the workpiece 2 except at endpoint P1. Consequently, the amount of material being machined on the dummy workpiece 25 does not increase rapidly when the machining position reaches endpoint P1. Therefore, the multi-wire electrical discharge machining (EDM) machine 1 can prevent the amount of material being machined on the workpiece 2 from increasing locally. As a result, the likelihood of workpiece 2 cracking is reduced. Next, the areas of the contact surfaces 25b and 25c of the dummy workpiece 25 are described. Fig. 7 is a top view of the dummy workpiece 25 to describe the areas of the contact surfaces 25b and 25c of the dummy workpiece 25 of the first embodiment. The wider the areas of workpiece 2 that are in contact with the dummy workpiece 25, the greater the frictional force between workpiece 2 and dummy workpiece 25, and the higher the bonding force of workpiece 2. As the areas of workpiece 2 in contact with dummy workpiece 25 become wider, the portion of workpiece 2 that needs to be machined along with the dummy workpiece 25 also increases. Therefore, it is desirable to minimize the amount of machining required on the dummy workpiece 25 while maintaining a certain level of frictional force between workpiece 2 and dummy workpiece 25. A straight line L shown in Fig. 7 is a straight line in the XZ-plane and a straight line passing through a center point O and the endpoint P1 of the workpiece 2 in the XZ-plane. The straight line L is a straight line passing through the endpoint P1 in the direction of the Z-axis. A straight line K is a straight line in the XZ-plane passing through the center point O and any point other than the endpoint P1 on a circle that is an outer edge of the workpiece 2 in the XZ-plane. An angle α is an angle with respect to the straight line L and is formed by the straight line L and the straight line K. An angle between the straight line L and the straight line K that is counterclockwise with respect to the straight line L is defined as a positive angle. An angle between the straight line L and the straight line K that is clockwise with respect to the straight line L is defined as a negative angle.The angle α is an angle in the range of -180 degrees to +180 degrees. In the first embodiment, the contact surfaces 25b and 25c are configured such that the angle α lies in the range of -45 degrees to +45 degrees in order to maintain the frictional force between the workpiece 2 and the dummy workpiece 25 and to reduce the amount of machining required on the dummy workpiece 25. Thus, the contact surfaces 25b and 25c are arranged on the XZ plane, which contains the Z-axis and the X-axis, such that they can come into contact with the curved surface of the workpiece 2 within a range of 45 degrees in any direction with respect to the straight line L. Furthermore, the recess 25a is formed in a specific area, with the endpoint P1, which is a position where the angle α is 0 degrees, located in the middle. The contact surfaces 25b and 25c are arranged such that the contact surfaces 25b and 25c are not located in the area where the endpoint P1 is in the middle. Next, the relationship between the areas of workpiece 2 that are in contact with the pressing section 28 and the principal stress exerted on workpiece 2 is described. Fig. 8 is a first diagram illustrating the relationship between the areas of workpiece 2 that are in contact with the pressing section 28 and the principal stress to be exerted on workpiece 2 in the first embodiment. Fig. 9 is a second diagram illustrating the relationship between the areas of workpiece 2 that are in contact with the pressing section 28 and the principal stress to be exerted on workpiece 2 in the first embodiment. Fig. 10 is a third diagram illustrating the relationship between the areas of workpiece 2 that are in contact with the pressing section 28 and the principal stress to be exerted on workpiece 2 in the first embodiment. The wider the areas of workpiece 2 in contact with the press section 28, the greater the frictional force between the workpiece 2 and the press section 28, and the higher the bonding force of the workpiece 2. The wider the areas of workpiece 2 in contact with the press section 28, the greater the length of workpiece 2 that needs to be cut before the press section 28 begins to press the workpiece 2. Depending on the setting of the areas in contact with the press section 28, the principal stress acting on the workpiece 2 can reach a maximum tensile stress, causing the workpiece 2 to break. Therefore, it is necessary to adjust the areas in contact with the press section 28 so that the principal stress acting on the workpiece 2 remains consistently below the maximum tensile stress during the electrical discharge machining (EDM) process. A region 28a, a region 28b, and a region 28c, shown in Figs. 8 and 9, are distinct areas on the curved surface of the workpiece 2. As shown in Fig. 8, Z1, Z2, Z3, and Z4 denote positions along the Z-axis. The graphs shown in Fig. 10 illustrate the results of the analysis of the relationship between the machining positions and the maximum principal stress values. Fig. 10 shows a graph for a case in which the press section 28 is brought into contact with region 28a, a graph for a case in which the press section 28 is brought into contact with region 28b, and a graph for a case in which the press section 28 is brought into contact with region 28c. When the press section 28 is brought into contact with area 28a, the maximum principal stress value increases significantly as processing progresses. The increase in the maximum principal stress value is less pronounced when the press section 28 is brought into contact with area 28b than when it is brought into contact with area 28a. Similarly, the increase in the maximum principal stress value is less pronounced when the press section 28 is brought into contact with area 28c than when it is brought into contact with area 28b. The maximum principal stress value increases least rapidly when the press section 28 is brought into contact with area 28c, among areas 28a, 28b, and 28c.Therefore, it is possible to reduce cracks in the workpiece 2 by bringing the press section 28 into contact with the area 28c between the area 28a, the area 28b and the area 28c. The straight line L shown in Fig. 9 is the same as the straight line L shown in Fig. 7. The straight line L passes through a starting point P2. A straight line M is a straight line in the XZ plane that passes through the center point O and any point other than the starting point P2 on the circle that forms the outer edge of the workpiece 2 in the XZ plane. An angle β is an angle with respect to the straight line L and is an angle formed by the straight line L and the straight line M. An angle between the straight line L and the straight line M that is counterclockwise with respect to the straight line L is defined as a positive angle. An angle between the straight line L and the straight line M that is clockwise with respect to the straight line L is defined as a negative angle. The angle β is an angle in the range of -180 degrees to +180 degrees. In the first embodiment, the pressing section 28 is formed at an angle β in the range of -75 degrees to +75 degrees such that the frictional force between the workpiece 2 and the pressing section 28 is maintained and the principal stress acting on the workpiece 2 remains constantly below the maximum tensile stress. Thus, the pressing section 28 is arranged in the XZ plane such that it can come into contact with the curved surface of the workpiece 2 within a range of 75 degrees in any direction with respect to the straight line L. Next, a modification of the dummy workpiece is described. Fig. 11 is a top view of a dummy workpiece 25A according to a modification of the first embodiment. Fig. 11 shows the dummy workpiece 25A and a portion of the workpiece 2 in contact with the dummy workpiece 25A. The dummy workpiece 25A holds the workpiece 2 from below. The dummy workpiece 25A comprises two parts 31 and 32. Part 31 and part 32 are arranged such that they can come into contact with the curved surface of workpiece 2, except at endpoint P1. Part 31 makes contact with a section on the side of the negative X direction with respect to endpoint P1. A contact surface 31a of part 31 comes into contact with workpiece 2. The contact surface 31a is curved along the curved surface of workpiece 2. Part 32 comes into contact with a section on the side of the positive X direction with respect to endpoint P1. A contact surface 32a of part 32 comes into contact with workpiece 2. The contact surface 32a is curved along the curved surface of workpiece 2. Both contact surface 31a and contact surface 32a are contact surfaces that are curved in such a way that, with the exception of endpoint P1, they can come into contact with the curved surface of workpiece 2. Workpiece 2 is held by dummy workpiece 25A in a state where the endpoint P1 of workpiece 2 is not in contact with dummy workpiece 25A, and workpiece 2, with the exception of endpoint P1, is in contact with contact surfaces 31a and 32a. Parts 31 and 32 are made of a material that can be machined by electrical discharge machining (EDM). Ideally, the material of parts 31 and 32 should have machining properties that correspond to those of workpiece 2. Since the contact surfaces 31a and 32a of parts 31 and 32 are curved such that they can come into contact with the curved surface of workpiece 2, the areas of workpiece 2 in contact with the dummy workpiece 25A can be increased. As a result, the multi-wire electrical discharge machining (EDM) machine 1 can increase the strength required to hold workpiece 2. Because the dummy workpiece 25A comprises a plurality of parts 31 and 32, it can be designed in a simple shape. A gap is formed below endpoint P1, i.e., between part 31 and part 32. When the machining position reaches endpoint P1, the amount of machining on dummy workpiece 25A does not increase rapidly. Therefore, the multi-wire electrical discharge machining (EDM) machine 1 can prevent the amount of machining on workpiece 2 from increasing locally. As a result, workpiece 2 is less likely to crack. The positions of contact surfaces 31a and 32a can be adjusted in the same way as contact surfaces 25b and 25c described above. It should be noted that the dummy workpiece 25A is not limited to a workpiece with the two parts 31 and 32, but can also comprise three or more parts. The dummy workpiece 25A simply needs to comprise two or more parts that, with the exception of endpoint P1, can come into contact with the curved surface of workpiece 2. Each of the parts comprises a contact surface that is curved such that it can come into contact with the curved surface of workpiece 2, with the exception of endpoint P1. Next, a modification of the workpiece clamping device is described. Fig. 12 is a perspective view of a workpiece clamping device 20B according to a modification of the first embodiment. The workpiece clamping device 20B includes an auxiliary plate 22B instead of the auxiliary plate 22 shown in Fig. 2. The auxiliary plate 22B differs from the auxiliary plate 22. The auxiliary plate 22B has the functions of both the auxiliary plate 22 and the excitation electrode 23. The auxiliary plate 22B is attached to the reference plate 21 by a plurality of electrode holders 24B. By using a plurality of electrode holders 24B, the strength for holding the auxiliary plate 22B can be improved. Even when the multi-wire electrical discharge machine 1 includes the workpiece clamping device 20B, the multi-wire electrical discharge machine 1 can hold the workpiece 2 as if the workpiece clamping device 20 were included. According to the first embodiment, the multi-wire electrical discharge machining (EDM) machine 1 comprises the dummy workpiece 25 or 25A and the clamping mechanism 26, enabling the workpiece 2 to be held with a strong clamping force. The multi-wire EDM machine 1 can prevent vibrations at the section where the workpiece 2 is to be cut into individual sections and can reduce cracking in the workpiece 2. Since the dummy workpieces 25 and 25A are in contact with the workpiece 2 except at endpoint P1, the amount of material being machined on the dummy workpieces 25 and 25A does not increase rapidly when the machining position reaches endpoint P1. The multi-wire EDM machine 1 can prevent the amount of material being machined on the workpiece 2 from increasing locally. As a result, the workpiece 2 is less likely to crack. As described above, the multi-wire EDM machine 1 reduces cracking in the workpiece 2. The configurations shown in the embodiment described above are examples of the subject matter of this disclosure. The configurations of this embodiment can be combined with other known techniques. It is possible to partially omit or modify the configurations of this embodiment without deviating from the scope of this disclosure. Reference symbol list 1 Multi-wire electrical discharge machine; 2 Workpiece; 3a, 3b, 3c, 3d Guide roller; 4 Wire spool; 5 Wire ejection roller; 6 Wire electrode; 6a Cutting wire section; 6b Power supply wire section; 7 Machining power supply; 8 Machining power supply unit; 9 Machining control device; 10 Ground electrode; 11 Cable; 12 Power supply; 13 Power supply unit; 14 Drive unit; 15 Machining condition detection device; 16 Machining condition detection unit; 17 Wire spool drive unit; 18 Guide roller drive unit; 20, 20B Workpiece clamping device; 20a Holder; 21 Reference plate; 22, 22B Side plate; 23 Excitation electrode; 24, 24B Electrode holder; 25, 25A Dummy workpiece; 25a Recess; 25b, 25c, 31a, 32a Contact surface; 26 Pressing mechanism; 27 Spacer; 28 Pressing section; 28a, 28b, 28c Area; 31, 32 Part.

Claims

Multi-wire electrical discharge machining (EDM) machine (1), comprising: a plurality of guide rollers (3a, 3b, 3c, 3d) for guiding a movement of a wire electrode (6), wherein the wire electrode (6) is wound multiple times at intervals around the plurality of guide rollers (3a, 3b, 3c, 3d); a processing current supply (7) for applying a voltage between each of a plurality of cutting wire sections (6a) and a workpiece (2), wherein the plurality of cutting wire sections (6a) are parallel sections of the wire electrode (6) between a first guide roller (3c) and a second guide roller (3d), wherein the first guide roller (3c) and the second guide roller (3d) are encompassed in the plurality of guide rollers (3a, 3b, 3c, 3d);a dummy workpiece (25) with a contact surface (25b, 25c), wherein the contact surface (25b, 25c) is curved such that it can come into contact with a curved surface of the workpiece (2) except for an endpoint (P1), wherein the workpiece (2) has the curved surface, wherein the endpoint (P1) is reachable by each of the plurality of cutting wire sections (6a) when the machining of the workpiece (2) is complete, wherein the dummy workpiece (25) holds the workpiece (2) from one side in a first direction, wherein the first direction is a direction in which the workpiece (2) moves with respect to the plurality of cutting wire sections (6a);and a pressing mechanism (26) with a pressing section (28) that can come into contact with the curved surface of the workpiece (2), wherein the pressing mechanism (26) presses the workpiece (2) against the dummy workpiece (25) by bringing the pressing section (28) into contact with the workpiece (2) from another side in the first direction. Multi-wire electrical discharge machining (EDM) machine (1) according to claim 1, comprising: a reference plate (21) integrated with the dummy workpiece (25) and the press mechanism (26); and a secondary plate (22) integrated with an excitation electrode (23) and electrically connected to the reference plate (21), wherein the excitation electrode (23) is electrically connected to the machining power supply (7), wherein the reference plate (21) and the secondary plate (22) hold the workpiece (2) such that the workpiece (2) is arranged between the reference plate (21) and the secondary plate (22). Multi-wire electrical discharge machining (1) according to claim 1 or 2, wherein the dummy workpiece (25) has a recess (25a) formed at a position facing a part of the curved surface of the workpiece (2), wherein the part of the curved surface has the endpoint (P1), and wherein the contact surface (25b, 25c) is provided adjacent to the recess (25a). Multi-wire electrical discharge machining (1) according to claim 1 or 2, wherein the dummy workpiece (25) comprises two or more parts that can come into contact with the curved surface of the workpiece (2) except for the endpoint (P1), and wherein each of the parts has the contact surface (25b, 25c). Multi-wire electrical discharge machining (1) according to one of claims 1 to 4, wherein in a plane containing the first direction and a second direction, the second direction is a direction of movement of the wire electrode (6) on each of the plurality of cutting wire sections (6a), wherein the contact surface (25b, 25c) is arranged such that it can come into contact with the curved surface of the workpiece (2) at an angle with respect to a straight line through the endpoint (P1), wherein the angle is within a range of 45 degrees in each direction, and wherein the straight line extends in the first direction. Multi-wire electrical discharge machining (EDM) machine (1) according to one of claims 1 to 4, wherein in a cutting plane of the workpiece (2) which includes the first direction and a second direction, wherein the second direction is a direction in which each of the plurality of cutting wire sections (6a) moves, wherein the press section (28) is arranged such that it can come into contact with the curved surface of the workpiece (2) at an angle with respect to a straight line through a starting point at which the machining of the workpiece (2) by the cutting wire sections (6a) is started, wherein the angle is in a range of 75 degrees in each direction, and wherein the straight line extends in the first direction. Multi-wire electrical discharge machining (1) according to one of claims 1 to 6, wherein the press section (28) can be deformed by pressing it against the workpiece (2). Multi-wire electrical discharge machining (EDM) process for performing EDM machining of a workpiece (2) having a curved surface by moving a multi-wound wire electrode (6) and applying a voltage between each of a plurality of cutting wire sections (6a) and the workpiece (2), wherein the plurality of cutting wire sections (6a) are sections of the wire electrode (6) moving parallel to each other, wherein the multi-wire EDM machining process comprises: a step to start the EDM machining in a state in which the curved surface, except for an endpoint (P1), is in contact with a contact surface (25b, 25c) of a dummy workpiece (25), wherein the endpoint (P1) is to be reached by the plurality of cutting wire sections (6a) when the machining of the workpiece (2) is completed, wherein the dummy workpiece (25) holds the workpiece (2) from one side in a first direction.wherein the first direction is a direction in which the majority of cutting wire sections (6a) move with respect to the workpiece (2); and a step to press the workpiece (2) against the dummy workpiece (25) by moving a press section (28) from another side in the first direction towards the workpiece (2) to bring the press section (28) into contact with the curved surface of the workpiece (2) while the electrical discharge machining continues. Method for producing a thin plate, for producing a plurality of thin plates, by cutting the plurality of thin plates from a workpiece (2) having a curved surface by electrical discharge machining (EDM), wherein a wire electrode (6) is wound multiple times and set in motion, and a voltage is applied between each of a plurality of cutting wire sections (6a) and the workpiece (2), wherein the plurality of cutting wire sections (6a) are sections of the wire electrode (6) which move parallel to each other, wherein the method for producing a thin plate comprises: a step to start the EDM machining in a state in which the curved surface, except for an endpoint (P1), is in contact with a contact surface (25b, 25c) of a dummy workpiece (25), wherein the endpoint (P1) is to be reached by the plurality of cutting wire sections (6a) when the machining of the workpiece (2) is completed.wherein the dummy workpiece (25) holds the workpiece (2) from one side in a first direction, the first direction being a direction in which the majority of cutting wire sections (6a) move with respect to the workpiece (2); and a step to press the workpiece (2) against the dummy workpiece (25) by moving a press section (28) from another side in the first direction towards the workpiece (2) to bring the press section (28) into contact with the curved surface of the workpiece (2) while the electrical discharge machining continues. Method for producing semiconductor wafers by cutting a plurality of semiconductor wafers from a billet having a curved surface by electrical discharge machining (EDM), wherein a wire electrode (6) is wound multiple times and set in motion, and a voltage is applied between each of a plurality of cutting wire sections (6a) and the billet, wherein the plurality of cutting wire sections (6a) are sections of the wire electrode (6) which move parallel to each other, the method for producing semiconductor wafers comprising: a step to start the EDM machining in a state in which the curved surface, except for an endpoint (P1), is in contact with a contact surface (25b, 25c) of a dummy workpiece (25), wherein the endpoint (P1) is to be reached by the plurality of cutting wire sections (6a) when the machining of the billet is complete.wherein the dummy workpiece (25) holds the ingot from one side in a first direction, the first direction being a direction in which the majority of cutting wire sections (6a) move with respect to the ingot; and a step to press the ingot against the dummy workpiece (25) by moving a press section (28) from another side in the first direction towards the ingot to bring the press section (28) into contact with the curved surface of the ingot while the electrical discharge machining continues.

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