Patent Publication Number: US-2018036815-A1

Title: Electrical discharge machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-155392 filed on Aug. 8, 2016, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an electrical discharge machine that performs electrical discharge machining by applying voltage across an electrode gap formed between an electrode and a workpiece to make current flow. 
     Description of the Related Art 
     Wire electrical discharge machining has been widely used as a machining method capable of machining metal with high precision. Because wire discharge machining is highly accurate, it is affected by a change in the surrounding environment or a failure of the equipment, which may make the machined workpiece defective. To deal with this, methods for detecting a failure or an abnormality of the wire electrical discharge machine have been devised. 
     International Publication No. WO2013/157071 discloses a wire electrical discharge machine that detects a physical quantity representing the state of a wire electrical discharge machine and displays the detected physical quantity with a normal range of the physical quantity in accordance with the machining condition. This configuration makes possible for the operator to judge whether or not the state of the wire electrical discharge machine is normal. 
     SUMMARY OF THE INVENTION 
     With the configuration disclosed in International Publication No. WO2013/157071, the operator has to grasp the current state of the wire electrical discharge machine while watching the information displayed and manually stop the wire electrical discharge machining if the current state is determined to be abnormal. On the other hand, it is a usual practice to cut out cores in the wire electrical discharge machine. In performing cutting of cores, the operator has to stop electrical discharge machining at suitable timing by monitoring the discharge sound and/or change in processing state. In this way, the operator must monitor the state and perform some operations, resulting in poor production efficiency. 
     It is therefore an object of the present invention to provide an electrical discharge machine which automatically performs predetermined operations without the need of operator&#39;s monitoring and operations. 
     An aspect of the invention resides in an electrical discharge machine that performs electrical discharge machining on a workpiece by applying voltage between an electrode gap formed between an electrode and the workpiece to generate electric discharge in a working fluid, including: a work tank configured to store the working fluid in order to perform electrical discharge machining; a sound collector configured to collect sound generated by electrical discharge machining and transmitted through the working fluid stored in the work tank; and a controller configured to perform a predetermined operation based on sound information collected by the sound collector. 
     With this configuration, since the sound generated by electrical discharge machining is collected via the working fluid, it is possible to correctly grasp the state of the electrical discharge machine and automatically perform a suitable operation in conformity with the state of the electrical discharge machine. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the sound collector is arranged inside the work tank, and a space forming portion configured to define a space accommodating the sound collector is further provided inside the work tank so as to be in contact with the working fluid stored in the work tank. In this way, the space forming portion for defining the space accommodating the sound collector is in contact with the working liquid, so that the sound collector can efficiently collect the sound generated by electrical discharge machining. 
     According to an aspect of the invention, in the above electrical discharge machine, the space forming portion may be formed of conductive material. This makes it possible to reduce the influence on the sound information from the discharge noise arising from electrical discharge machining, hence improve the S/N ratio of the sound information. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the space forming portion is configured to protect against the working fluid stored in the work tank so that the fluid does not enter the space, and the sound collector is arranged adjacent to an outer wall of the space forming portion that is in contact with the working fluid. This configuration does not need to make the sound collector waterproof, which leads to reduction in cost and enables the sound collector to collect the sound generated by electrical discharge machining in a more efficient manner. 
     According to an aspect of the invention, in the above electrical discharge machine, the space forming portion may be formed of mesh so as to introduce the working fluid stored in the work tank into the space. This configuration enables the sound collector to collect the sound generated by electrical discharge machining, directly via the working fluid, in a more exact and efficient manner. 
     According to an aspect of the invention, in the above electrical discharge machine, the sound collector may be arranged outside an outer wall of the work tank and adjacent thereto. This arrangement enables the sound collector to collect the sound generated by electrical discharge machining in a more efficient manner. 
     According to an aspect of the invention, in the above electrical discharge machine, the outer wall around which the sound collector is arranged adjacently may be formed with a sound permeable material that is capable of transmitting sound. This makes it possible to collect the sound generated by electrical discharge machining in a more efficient manner. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the controller includes a storage medium that has stored predetermined reference sound information, and the controller is configured to compare the sound information collected by the sound collector with the reference sound information stored in the storage medium, and based on a comparison result, perform at least one of operations including warning, stoppage of electrical discharge machining and change of machining condition. Thus, use of the reference sound information stored in the storage medium enables more exact grasp of the current state of the electrical discharge machine, whereby it is possible to automatically perform a suitable operation in conformity with the current state. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the controller includes a storage medium that has stored range information indicating a predetermined range of sound information, and the controller is configured to determine whether or not the sound information collected by the sound collector falls within the range indicated by the range information stored in the storage medium, and based on a determination result, perform at least one of operations including warning, stoppage of electrical discharge machining and change of machining condition. Thereby, use of the range information stored in the storage medium enables more exact grasp of the current state of the electrical discharge machine, whereby it is possible to automatically perform a suitable operation in conformity with the current state. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the reference sound information or the range information corresponding to a failure component has been stored in the storage medium, and the controller is configured to detect a component that has failed based on the sound information collected by the sound collector and the reference sound information or the range information, and when a failure of a component is detected, the controller is configured to at least give warning that the detected component has the failure. Thereby, it is possible to accurately detect a failure part in the electrical discharge machine and automatically perform a suitable operation (warning to the operator) accordingly. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the reference sound information or the range information corresponding to completion of a core cutting process has been stored in the storage medium, and the controller is configured to determine whether or not a core cutting process has been completed based on the sound information collected by the sound collector and the reference sound information or the range information, and, when the core-cutting is completed, the controller is configured to at least stop electrical discharge machining. As a result, it is possible to accurately detect whether the core cutting-out process has been completed, and automatically perform an appropriate operation (stoppage of electric discharge machining) accordingly. 
     According to an aspect of the invention, the above electrical discharge machine may be configured such that the reference sound information or the range information corresponding to chattering has been stored in the storage medium, and the controller is configured to determine whether or not chatter occurs based on the sound information collected by the sound collector and the reference sound information or the range information, and, when chatter occurs, the controller is configured to at least perform stoppage of electrical discharge machining or change of machining condition. With this, it is possible to accurately detect occurrence of chattering and automatically perform a suitable operation (stoppage of electrical discharge machining or change of machining condition) accordingly. 
     According to an aspect of the invention, the sound information may include information representing at least one of loudness, sound pitch, tone and frequency of the sound. 
     According to the present invention, since the sound collector collects sound generated by electrical discharge machining via the working fluid, it is possible to grasp the state of the electrical discharge machine exactly, whereby it is possible to automatically perform a suitable operation in conformity with the state thereof. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic mechanical configuration diagram of a wire electrical discharge machine as one type of electrical discharge machine of the embodiment; 
         FIG. 2  is a schematic electrical configuration diagram of the wire electrical discharge machine shown in  FIG. 1 ; 
         FIG. 3  is an illustrative diagram for explaining the arrangement of a sound collector shown in  FIG. 2 ; 
         FIG. 4  is a block diagram showing a controller shown in  FIG. 1 ; 
         FIG. 5  is a flowchart showing the operation control of the controller shown in  FIG. 4 ; 
         FIG. 6  is an illustrative diagram for explaining the arrangement of a sound collector in a variational example 1; and, 
         FIG. 7  is an illustrative diagram for explaining the arrangement of the sound collector in a variational example 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electrical discharge machines according to the present invention will be described in detail hereinbelow with reference to the accompanying drawings, by giving preferred embodiments thereof. The embodiment will be described by giving an example of a wire electrical discharge machine, which is one type of electrical discharge machine, but the present invention may be applied to any electrical discharge machine without use of a wire electrode. 
       FIG. 1  is a schematic mechanical configuration diagram of a wire electrical discharge machine (electrical discharge machine)  10 . The wire electrical discharge machine  10  is a machine tool that effects electrical discharge machining on an object to be machined (workpiece) W, by applying voltage across an electrode gap (clearance) formed between a wire electrode  12  and the workpiece W (see  FIG. 2 ) to generate electric discharge in a working fluid. The wire electrical discharge machine  10  includes a main machine body  14 , a working fluid processor (dielectric fluid unit)  16  and a controller  18 . 
     The wire electrode  12  is formed of, for example, metal material such as tungsten-based, copper-alloy based and brass-based material. On the other hand, the material of the workpiece W is, for example, iron-based material or superhard material such as tungsten carbide. 
     The main machine body  14  includes a supply system  20   a  for supplying the wire electrode  12  toward the workpiece W and a collection system  20   b  for collecting the wire electrode  12  having passed through the workpiece W. 
     The supply system  20   a  includes a wire bobbin  22  with the wire electrode  12  wound thereon, a torque motor  24  for applying a torque to the wire bobbin  22 , a brake shoe  26  for applying a braking force by friction to the wire electrode  12 , a braking motor  28  for applying a braking torque to the brake shoe  26 , a tension detector  30  for detecting the tension of the wire electrode  12  and an upper wire guide  32  for guiding the wire electrode  12 . 
     The collection system  20   b  includes a lower wire guide  34  for guiding the wire electrode  12 , a pinch roller  36  and a feed roller  38  capable of gripping the wire electrode  12  and a wire collection box  40  for collecting the wire electrode  12  transferred by the pinch roller  36  and the feed roller  38 . 
     The main machine body  14  includes a work tank (work-pan)  42  capable of storing a dielectric working fluid such as deionized water or oil used in electrical discharge machining with the upper wire guide  32  and the lower wire guide  34  arranged therein. The work tank  42  is mounted on a base  44 . The workpiece W is placed between the upper wire guide  32  and the lower wire guide  34 . The upper wire guide  32  and the lower wire guide  34  respectively have die guides  32   a  and  34   a  for supporting the wire electrode  12 . The lower wire guide  34  includes a guide roller  34   b  that guides the wire electrode  12  to the pinch roller  36  and the feed roller  38  by deflecting the travel of the wire electrode  12 . 
     The upper wire guide  32  ejects a clean working fluid not containing sludge (machined swarf). This function makes it possible to fill the clearance (electrode gap) between the wire electrode  12  and the workpiece W with a clean working fluid suitable for electrical discharge machining, and also prevent the accuracy of electrical discharge machining from being deteriorated by sludge arising during electrical discharge machining. Also, the lower wire guide  34  may also jet a clean working liquid not containing sludge. 
     The workpiece W is supported by a table  58 , and the table  58  is arranged in the work tank  42  (see  FIG. 3 ). The upper wire guide  32 , the lower wire guide  34 , the table  58  and the workpiece W are immersed in the working fluid LQ stored in the work tank  42 . 
     While the main machine body  14  (wire electrical discharge machine  10 ) moves the position of the table  58  and the position of the wire electrode  12  stretched by the die guides  32   a  and  34   a  relative to each other to machine the workpiece W. 
     The working fluid processor  16  is a device that removes machined swarf (sludge) produced in the work tank  42  and manages the quality of the working fluid by adjusting the electric resistivity and temperature and the like. The working fluid that has been adjusted in quality by this working fluid processor  16  is returned to the work tank  42  again. The controller  18  controls the main machine body  14  and the working fluid processor  16 . 
       FIG. 2  is a schematic electrical configuration diagram of the wire electrical discharge machine  10 . The wire electrical discharge machine  10  further includes a machining power supply  50  and a sound collector  52 . The machining power supply  50  includes a power source E, two pulse application circuits A and B, four relay switches RL 1  to RL 4  and two resistors R 1  and R 2 . The power source E is a DC power supply. The positive electrode (+ electrode) of the power source E is connected to a positive side input terminal PI 1  of the pulse application circuit (first pulse application circuit) A via the relay switch RL 1  as well as to a positive side input terminal PI 2  of the pulse application circuit (second pulse application circuit) B via the relay switch RL 2 . 
     The negative electrode (− electrode) of the power source E is connected to negative side input terminals NI 1  and NI 2  of the pulse application circuit A and the pulse application circuit B. That is, the relay switches RL 1  and RL 2  are relay switches to select either the pulse application circuit A or B as the pulse application circuit for applying voltage (pulse voltage) across the electrode gap. Therefore, only one of the relay switches (first and second relay switches) RL 1  and RL 2  is turned on, and both of them will not be turned on at the same time. 
     The positive side output terminals PO 1  and PO 2  of the pulse application circuits A and B are connected to the wire electrode  12  via a relay switch RL 3  and a first resistor R 1 , and also connected to the wire electrode  12  via a relay switch RL 4  and a second resistor R 2 . That is, the relay switch RL 3  and resistor R 1 , and the relay switch RL 4  and resistor R 2  are connected in parallel. 
     The negative side output terminals NO 1  and NO 2  of the pulse application circuit A and the pulse application circuit B are connected to the workpiece W. That is, the relay switches RL 3  and RL 4  are relay switches for selecting the use of the resistors R 1  and R 2  in order to apply voltage across electrode gap. Accordingly, only one of the relay switch (third relay switch) RL 3  and the relay switch (fourth relay switch) RL 4  is turned on, or both are on. 
     The pulse application circuits A and B generate pulse voltage based on the voltage from the power source E, apply the generated pulse voltage between the electrodes to flow pulse current. That is, a pulse voltage and a pulse current are applied across the electrode gap by the pulse application circuit A or B. The pulse application circuits A and B generate pulse voltages (pulse currents) of different frequencies. Therefore, the energy per pulse of the pulse voltage (pulse current) generated by the pulse application circuit A can be made different from that generated by the pulse application circuit B. The frequencies of the pulse voltages (pulse currents) generated by the pulse application circuits A and B are controlled by the controller  18 . 
     The machining power supply  50  can take multiple different circuit forms by use of relay switches RL 1  to RL 4 . That is, by switching the electrical connections of the relay switches RL 1  to RL 4 , a plurality of circuit states can be formed so that the machining power supply  50  can be set to any one of the multiple circuit modes. The relay switches RL 1  to RL 4  are switched on and off under the control of the controller  18 . 
     The sound collector  52  collects sound given off during electrical discharge machining effected in the working fluid. The sound collector  52  collects the sound transmitted through the working fluid stored in the work tank  42 . That is, the sound generated by the electrical discharge machining performed in the working fluid is transmitted through the working fluid stored in the work tank  42  to the sound collector  52  and collected thereby. The sound collector  52  has at least a microphone (not shown) for converting sound into electric signals (sound information), generates sound information based on the electric signal converted by the microphone and outputs the generated sound information to the controller  18 . 
     This sound information includes information indicating at least one of the loudness, sound pitch, tone and frequency of the sound given off by the electrical discharge machining. In other words, the sound collector  52  includes a sound analyzer (not shown), which analyzes at least one of the loudness, sound pitch, tone and frequency of the sound generated by the electrical discharge machining, based on the electric signal converted by the microphone to generate sound information. 
     Next, the arrangement of the sound collector  52  will be described with reference to  FIG. 3 . The sound collector  52  is arranged in the work tank  42  as shown in  FIG. 3 . In the work tank  42 , a space forming portion  56  defining a space  57  in which the sound collector  52  is disposed is formed. The space forming portion  56  is provided in the work tank  42  so as to be in contact with the working fluid LQ stored in the work tank  42 . That is, part or whole of the space forming portion  56  is immersed in the working fluid LQ stored in the work tank  42 . The space forming section  56  protects against or block out the working fluid LQ so that the working fluid LQ stored in the work tank  42  does not enter the space  57 . That is, the space forming portion  56  partitions the work tank  42  into the space with the working fluid LQ stored therein and the space  57  into which the working fluid LQ does not intrude (the working fluid LQ is not stored). 
     A large discharge noise is generated by electrical discharge machining, so that the discharge noise is superimposed on the sound information collected by the sound collector  52 . To deal with this, it is possible to reduce the influence of the discharge noise by forming the space forming portion  56  with a conductive material. That is, formation of the space forming portion  56  with a conductive material makes it possible to reduce the discharge noise included in the sound information collected by the sound collector  52  and improve the S/N ratio. 
     The sound collector  52  is provided adjacent to the outer wall  56   a  of the space forming portion  56 . Accordingly, the sound generated by electrical discharge machining is transmitted through the working fluid LQ and the outer wall  56   a  of the space forming portion  56  and collected by the sound collector  52 . Therefore, the sound collector  52  can efficiently collect the sound generated by electrical discharge machining. Conversely, when the sound collector  52  is not provided adjacent to the outer wall  56   a  of the space forming portion  56 , the sound given off by electrical discharge machining is transmitted through the working fluid LQ and the space forming portion  56  and then collected by the sound collector  52  via air (the air or other gas), so that the sound cannot be collected efficiently. Here, a reference numeral  58  shown in  FIG. 3  represents a table for supporting the workpiece W. 
     Next, the configuration of the controller  18  will be described briefly with reference to  FIG. 4 . The controller  18  includes an input unit  60 , a control unit  62 , a storage medium  64  and a display unit  66 . The input unit  60  is an operation unit operated by an operator to input information, commands and others. The input unit  60  includes ten keys for inputting numeric data, various function keys (e.g., a power button, etc.), a keyboard, a touch panel and the like. The control unit  62  includes a processor such as a CPU and a memory chip storing a program. This processor, when running this program, provides the function of the control unit  62  of the present embodiment. 
     The storage medium  64  stores data and the like necessary for control by the control unit  62  and also functions as a buffer memory. The storage medium  64  has stored predetermined reference sound information. The display unit  66  is constituted of a liquid crystal display, organic EL display or the like, and displays necessary information and others. The touch panel of the input unit  60  is provided over the display screen of the display unit  66 . 
     The control unit  62  performs predetermined operations based on the sound information collected by the sound collector  52 . More specifically, the control unit  62  compares the reference sound information stored in the storage medium  64  with the sound information collected by the sound collector  52 . When the difference falls within a predetermined range, the control unit  62  performs at least one of warning, stopping electrical discharge machining and modifying machining conditions. 
     When giving a warning, the control unit  62  may warn the operator by displaying the type of warning on the display unit  66  or warn the operator by outputting a sound indicating the type of warning from an unillustrated speaker. Further, when the processing conditions are modified, the control unit  62  performs electrical discharge machining under the modified machining conditions. 
     When a part forming the wire electrical discharge machine  10  has broken, the sound generated by the electrical discharge machining will change. For example, when a fan motor or a pump (not shown) fails, the rotation rate does not change, but abnormal noise is caused. Therefore, storing the reference sound information on each component in accordance with each component failure in the storage medium  64  enables the control unit  62  to determine (detect) the fault location or failure component. When the difference between the sound information collected by the sound collector  52  and the reference sound information is within a predetermined range, the control unit  62  determines that the component stored in association with the reference sound information has failed and issues a warning. At this stage, the control unit  62  may stop electrical discharge machining. 
     Further, in the electrical discharge machining on the workpiece W, it is a common practice to make cores drop away. Conventionally, when performing a cutting-off process, the operator stops electrical discharge machining at appropriate timing by monitoring the change in sound generated by electrical discharge machining and/or the machining state. In order to automatically stop electrical discharge machining, reference sound information corresponding to the finish of the core cutoff process is stored in the storage medium  64  in association with the completion of the core cutoff process. This makes it possible for the control unit  62  to determine (detect) whether or not a core cutoff process has been completed. When the difference between the sound information collected by the sound collector  52  and the reference sound information is within a predetermined range, the control unit  62  determines that the cutting process of the core is completed and stops electrical discharge machining. 
     Further, when the clearance between the upper wire guide  32  that ejects the working fluid LQ and the workpiece W is narrow, the table  58  and other constituents of the wire electrical discharge machine  10  may vibrate due to the flushing pressure of the working fluid LQ, causing so-called chattering. This chartering also produces sound. Since this chatter may occur only at a specific position due to distortion of the workpiece W etc., or may arise at different positions depending on each individual workpiece W, it is difficult to prevent. Therefore, by storing the reference sound information corresponding to chattering in the storage medium  64  in association with chatter, the control unit  62  can determine (detect) whether or not chatter is occurring. When the difference between the sound information collected by the sound collector  52  and the reference sound information is within a predetermined range, the control unit  62  determines that chatter occurs and stops electrical discharge machining, or modifies the machining condition. As a modification of the machining condition, for example, the pressure of the working fluid LQ ejected by the upper wire guide  32  may be changed. In addition, when disconnection of the wire electrode  12  occurs due to the change in the pressure of the working liquid LQ, the rest time of the discharge pulse may be changed. 
     In this manner, storing the reference sound information corresponding to the failure of each component, the reference sound information when a core cutting-out process is completed, the reference sound information when chatter occurs and the like in the storage medium  64  makes it possible to detect which part has failed, whether the core cutting process is completed or whether chatter occurs. 
     Next, the operation control of the controller  18  will be described with reference to the flowchart shown in  FIG. 5 . The controller  18  executes the operation control shown in  FIG. 5  at intervals of a predetermined period. At step S 1 , the controller  18  acquires the sound information collected by the sound collector  52 . Next, at step S 2  the controller  18  compares the sound information acquired at step S 1  with the reference sound information stored in the storage medium  64 . At this time, when multiple pieces of reference sound information are stored in the storage medium  64 , comparison is made with each of the multiple pieces of reference sound information. 
     Next, at step S 3  the controller  18  determines whether the difference between the sound information acquired at step S 1  and the reference sound information falls within a predetermined range. When multiple pieces of reference sound information are stored in the storage medium  64 , it is determined whether or not the difference between the acquired sound information and each of the multiple pieces of reference sound information is within a predetermined range. 
     If it is determined at step S 3  that the difference between the sound information and the reference sound information is within the predetermined range, the control proceeds to step S 4 , where the controller  18  determines whether or not a predetermined time has elapsed in a state in which the difference between the sound information and the reference sound information remains within the predetermined range. The controller  18  has a clock function of clocking the time and determines whether or not a predetermined time has elapsed based on this clock function. 
     If it is determined at step S 4  that the predetermined time has not passed while the difference between the sound information and the reference sound information is within the predetermined range, the control returns to step S 1  and the above-described operation is repeated. On the other hand, if it is determined at step S 4  that the predetermined time has elapsed in a state in which the difference between the sound information and the reference sound information remains within the predetermined range, the control proceeds to step S 5 . 
     At step S 5 , the controller  18  performs a predetermined operation. The controller  18  performs an operation corresponding to the reference sound information based on which the difference of the sound information was determined to be within the predetermined range, and ends this operation control. Also, if it is determined at step S 3  that the difference between the sound information and the reference sound information is not within the predetermined range, this operation control is ended. 
     As an operation corresponding to the reference sound information based on which the difference of the sound information was determined to be within the predetermined range, if, for example, the reference sound information corresponds to a failure of a component, the controller  18  warns that the component has failed. At this time, the control unit  62  may stop electrical discharge machining. 
     When the reference sound information based on which the difference of the sound information was determined to be within the predetermined range corresponds to the completion of a core cutting process, the controller  18  stops electrical discharge machining. At this time, the controller  18  may notify the operator that cutting of the core is completed and electrical discharge machining is stopped. The notice of the finish of core cutting and stoppage of electrical discharge machining may be given to the operator by displaying a necessary message on the display unit  66  or by outputting a sound indication from the aforementioned speaker. 
     When the reference sound information based on which the difference of the sound information was determined to be within the predetermined range corresponds to chattering, the controller  18  stops electrical discharge machining or changes the machining condition. At this time, the controller  18  may inform the operator that discharge machining is stopped due to the occurrence of chatter or that the machining condition has been changed due to the occurrence of chatter. This notice may be given to the operator by displaying a necessary message on the display unit  66  or by outputting a sound indication from the aforementioned speaker. 
     VARIATIONS 
     The above embodiment may be modified as follows. 
     Variational Example 1 
       FIG. 6  is an illustrative diagram for explaining an arrangement of the sound collector  52  in a variational example 1. In the variational example 1, a space forming portion  56 ′ made of mesh is used instead of the space forming portion  56 . That is, the space forming portion  56 ′ with an outer wall  56   a ′ formed of mesh is used. In this case, the working fluid LQ stored in the work tank  42  passes through the space forming portion  56 ′ and enters an interior space  57 . Therefore, when the mesh-formed space forming portion  56 ′ is used, it is not necessary to arrange the sound collector  52  adjacent to the outer wall  56   a ′ of the space forming portion  56 ′, but the sound collector  52  (at least the microphone) needs to be immersed in the working fluid LQ. This enables the sound collector  52  to directly collect the sound generated by electrical discharge machining via the working fluid LQ, so that the sound arising from electrical discharge machining can be collected more exactly. 
     When the space forming portion  56 ′ is formed of a conductive material in order to reduce the influence of the discharge noise, it is necessary to make the mesh finer. Specifically, it is preferable to use a mesh size (openings) that can readily introduce the working fluid LQ into the space  57  and can eliminate the discharge noise to a predetermined level or lower. 
     Variational Example 2 
       FIG. 7  is an illustrative diagram for explaining an arrangement of the sound collector  52  in a variational example 2. In the variational example 2, the sound collector  52  is provided outside the work tank  42 . The sound collector  52  is arranged outside an outer wall  42   a  of the work tank  42  and adjacent to the outer wall  42   a.  Therefore, the sound generated by electrical discharge machining is transmitted through the working fluid LQ and the outer wall  42   a  of the work tank  42  and collected by the sound collector  52 . Therefore, the sound collector  52  can efficiently collect the sound generated by electrical discharge machining. 
     In addition, at least the portion of the outer wall  42   a  of the work tank  42  where the sound collector  52  is disposed is formed of a sound permeable material  70  that can transmit sound. That is, the sound collector  52  is arranged adjacent to the outer wall  42   a  of the work tank  42  formed of the sound permeable material  70 . Thereby, the sound collector  52  can efficiently collect the sound generated by electrical discharge machining. As the sound permeable material  70 , a material that is light in weight and has strength and corrosion resistance is preferable. For example, a metal having a small specific weight such as aluminum, titanium or an iron-based material containing SUS, a resin or the like is suitable. In  FIG. 7 , the portion formed by the sound permeable material  70  is depicted by hatching. 
     Similarly, in the above-described embodiment, at least the portion where the sound collector  52  is arranged adjacently, of the outer wall  56   a  of the space forming portion  56  may be formed of the sound permeable material  70  that can transmit sound. 
     Variational Example 3 
     Though in the above-described embodiment, reference sound information is stored in the storage medium  64 , sound-range information that indicates a sound range is stored in a variational example 3. The storage medium  64  may store, at least, one of the range information corresponding to each component failure, the range information when a core cutting process has been completed and the range information when chatter occurs. In this case, the control unit  62  determines whether or not the sound information acquired from the sound collector  52  falls within the range indicated by the range information stored in the storage medium  64 . Then, when determining that the acquired sound information falls within the range indicated by the range information, the control unit  62  performs an operation (at least one of operations including warning, stoppage of discharge machining and change of machining condition) corresponding to the range information in which the acquired sound information was determined to fall. 
     Variational Example 4 
     Though, in the above embodiment and examples, an item of reference sound information corresponding to each component failure and an item of reference sound information corresponding to chattering are stored in the storage medium  64 , in variation example 4 sound arising from electrical discharge machining in normal operation when neither component failure nor chartering is occurring is stored as reference sound information. In this case, the control unit  62  compares the sound information acquired from the sound collector  52  with the reference sound information stored in the storage medium  64 , determines that an abnormality is occurring when the difference therebetween falls beyond a predetermined range and performs, at least, one of operations including warning, stoppage of discharge machining and change of machining condition. 
     It should be noted that the storage medium  64  may store range information representing a range of sound arising from electrical discharge machining at the time of normal operation in which neither component failure nor chattering is occurring. In this case, the control unit  62  determines whether or not the sound information acquired from the sound collector  52  falls within the range indicated by the range information stored in the storage medium  64 . Then, when determining that the acquired sound information falls beyond the range indicated by the range information, the control unit  62  determines that an abnormality is occurring, and performs, at least, one of operations including warning, stoppage of discharge machining and change of machining condition. 
     Variational Example 5 
     The above variational examples 1 to 4 may be combined as appropriate as long as no contradiction arises. 
     As described above, an electrical discharge machine  10  described in the above embodiment and variational examples 1 to 5 includes: a work tank  42  configured to store a working fluid LQ for performing electrical discharge machining; a sound collector  52  configured to collect sound transmitted through the working fluid LQ stored in the work tank  42 ; and a controller  18  configured to perform a predetermined operation based on the sound information collected by the sound collector  52 . 
     In this way, since the sound generated by electrical discharge machining is collected via the working fluid LQ, it is possible to correctly grasp the state of the electrical discharge machine  10  and automatically perform a suitable operation in conformity with the state of the electrical discharge machine  10 . 
     The sound collector  52  is arranged inside the work tank  42  while a space forming portion  56  (or  56 ′) for defining a space  57  accommodating the sound collector  52  is further provided inside the work tank  42  so as to be in contact with the working fluid LQ stored in the work tank  42 . In this way, the space forming portion  56  (or  56 ′) for defining the space  57  accommodating the sound collector  52  is in contact with the working liquid LQ, so that the sound collector  52  can efficiently collect the sound generated by electrical discharge machining. 
     Since the space forming portion  56  (or  56 ′) is formed of conductive material, it is possible to reduce the influence on the sound information from the discharge noise arising from electrical discharge machining, hence improve the S/N ratio of the sound information. 
     Since the space forming portion  56  protects against the working fluid LQ stored in the work tank  42  so that the fluid LQ will not enter the space  57 , there is no need to make the sound collector  52  waterproof, which leads to reduction in cost. In addition, since the sound collector  52  is arranged adjacent to an outer wall  56   a  of the space forming portion  56  that is in contact with the working fluid LQ, the sound collector  52  can collect the sound generated by electrical discharge machining in a more efficient manner. 
     Since the space forming portion  56 ′ is formed of mesh so as to introduce the working fluid LQ stored in the work tank  42  into the space  57 , the sound collector  52  can collect the sound generated by electrical discharge machining, directly via the working fluid LQ, in a more exact and efficient manner. 
     Since the sound collector  52  is arranged outside an outer wall  42   a  of the work tank  42  and adjacent thereto, the sound collector  52  can collect the sound generated by electrical discharge machining in a more efficient manner. 
     Since the outer wall  56   a  (or  42   a ) around which the sound collector  52  is arranged adjacently is formed with a sound permeable material  70  that can transmit sound, it is possible to collect the sound generated by electrical discharge machining in a more efficient manner. 
     The controller  18  includes a storage medium  64  that has stored predetermined reference sound information. The controller  18  compares the sound information collected by the sound collector  52  with the reference sound information stored in the storage medium  64 , and based on the comparison result, performs at least one of operations including warning, stoppage of electrical discharge machining and change of machining condition. Thus, use of the reference sound information stored in the storage medium  64  enables more exact grasp of the current state of the electrical discharge machine  10 , whereby it is possible to automatically perform a suitable operation in conformity with the current state. 
     The controller  18  includes a storage medium  64  that has stored range information indicating a predetermined range of sound information. The controller  18  determines whether or not the sound information collected by the sound collector  52  falls within the range indicated by the range information stored in the storage medium  64 , and based on the determination result, performs at least one of operations including warning, stoppage of electrical discharge machining and change of machining condition. Thus, use of the range information stored in the storage medium  64  enables more exact grasp of the current state of the electrical discharge machine  10 , whereby it is possible to automatically perform a suitable operation in conformity with the current state. 
     The reference sound information or range information corresponding to a failure component may be stored in the storage medium  64 . In this case, the controller  18  detects a component that has failed based on the sound information collected by the sound collector  52  and the reference sound information or the range information, and when a malfunction of a component is detected, the controller, at least, gives warning that the detected component has a failure. In this way, it is possible to accurately detect a failure part in the electrical discharge machine  10  and automatically perform a suitable operation (warning to the operator) accordingly. 
     The reference sound information or range information corresponding to completion of a core cutting process may be stored in the storage medium  64 . In this case, the controller  18  determines whether or not a core cutting process has been completed based on the sound information collected by the sound collector  52  and the reference sound information or the range information. When the core-cutting is completed, the controller, at least, stops electrical discharge machining. As a result, it is possible to accurately detect whether the core cutting-out process has been completed, and automatically perform an appropriate operation (stoppage of electric discharge machining) accordingly. 
     The reference sound information or range information corresponding to chattering may be stored in the storage medium  64 . In this case, the controller  18  determines whether or not chatter occurs based on the sound information collected by the sound collector  52  and the reference sound information or the range information. When chatter occurs, the controller, at least, performs stoppage of electrical discharge machining or change of machining condition. With this, it is possible to accurately detect occurrence of chattering, and automatically perform a suitable operation (stoppage of electrical discharge machining or change of machining condition) accordingly. 
     Note that the sound information collected by the sound collector  52  includes information representing at least one of the loudness, sound pitch, tone and frequency of the sound. 
     The present invention is not limited to the embodiments described above, and the embodiments can be freely modified within a range that does not deviate from the essence and gist of the present invention as set forth in the appended claims.