Patent Publication Number: US-9849528-B2

Title: Electrical discharge machining system having independent electrodes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to co-pending patent application Ser. No. 14/854,484, filed concurrently herewith, on Sep. 15, 2015. 
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to electrical discharge machining. Specifically, the subject matter disclosed herein relates to systems for performing electrical discharge machining in turbine components, e.g., gas turbine blades or buckets. 
     Electrical discharge machining (EDM) is a manufacturing process whereby shapes are formed in a subject material using electrical discharge (sparks). An electrode (also called a tool electrode) is placed proximate the subject material (also called a workpiece), and an electrical voltage is applied between the electrode and workpiece. When the intensity of the electric field between the electrode and the subject material exceeds the resistance of the dielectric medium, a current flows from the electrode to the subject material, or vice versa, removing some material from both the electrode and subject material. 
     Currently, EDM is the most reliable technology used to form cooling holes and fuel injection holes in turbine components (e.g., airfoils). As such, EDM is widely used to form these holes in turbine airfoils. However, EDM drilling is relatively slow, even when using a set of multiple electrodes. Further, several EDM machines are typically employed at one time in order to meet production time requirements, which occupies a significant amount of floor space in a manufacturing facility. Even further, in EDM machines employing grouped electrode arrangements, local debris drives the slowdown and withdrawal of not only one or more affected electrodes but also the entire group of electrodes including the unaffected electrodes. The entire group of electrodes feeds or withdraws together according to the worst case electrode (such as the electrode with the most debris buildup or shortest workpiece-electrode gap). As a result, the overall feed rate or machining productivity follows the lowest feed rate of the worst electrode with the most debris buildup or shortest gap at any given time. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Various embodiments of the disclosure include an electrical discharge machining (EDM) system. In some cases, the EDM system includes: a guide structure; a plurality of electrode devices positioned at least partially within the guide structure, the plurality of electrode devices aligned to provide a plurality of electrical discharges to a workpiece, each of the plurality of electrode devices including: an electrode for positioning proximate the workpiece; an electrode holder coupled to the electrode for holding the electrode proximate the workpiece; and a driver coupled to the electrode holder, the driver adapted to modify a position of the electrode holder and the electrode; and a control system operably connected with the driver, the control system configured to provide instructions to the driver of at least one of the plurality of electrode devices to modify a position of the at least one of the plurality of electrodes independently of at least one other one of the plurality of electrodes. 
     A first aspect of the disclosure includes and EDM system having: a guide structure; a plurality of electrode devices positioned at least partially within the guide structure, the plurality of electrode devices aligned to provide a plurality of electrical discharges to a workpiece, each of the plurality of electrode devices including: an electrode for positioning proximate the workpiece; an electrode holder coupled to the electrode for holding the electrode proximate the workpiece; and a driver coupled to the electrode holder, the driver adapted to modify a position of the electrode holder and the electrode; and a control system operably connected with the driver, the control system configured to provide instructions to the driver of at least one of the plurality of electrode devices to modify a position of the at least one of the plurality of electrodes independently of at least one other one of the plurality of electrodes. 
     A second aspect of the disclosure includes: an EDM system having: a guide structure; a plurality of electrode devices positioned at least partially within the guide structure, the plurality of electrode devices aligned to provide a plurality of electrical discharges to a workpiece, each of the plurality of electrode devices including: an electrode for positioning proximate the workpiece; an electrode holder coupled to the electrode for holding the electrode proximate the workpiece; and a driver coupled to the electrode holder, the driver adapted to modify a position of the electrode holder and the electrode; at least one sensor for detecting a position of an electrode relative to a the workpiece; and a control system operably connected with the driver, the control system configured to provide instructions to the driver of at least one of the plurality of electrode devices to modify a position of the at least one of the plurality of electrodes independently of at least one other one of the plurality of electrodes based upon the detected position of the electrode. 
     A third aspect of the disclosure includes an EDM system having: a guide structure; a machine ram coupled to the guide structure; a plurality of electrode devices positioned at least partially within the guide structure, the plurality of electrode devices aligned to provide a plurality of electrical discharges to a workpiece, each of the plurality of electrode devices including: an electrode for positioning proximate the workpiece; an electrode holder coupled to the electrode for holding the electrode proximate the workpiece; and a driver coupled to the electrode holder, the driver adapted to modify a position of the electrode holder and the electrode; at least one sensor for detecting a position of an electrode relative to the workpiece; a control system operably connected with the driver, the control system configured to provide instructions to the driver of at least one of the plurality of electrode devices to modify a position of the at least one of the plurality of electrodes independently of at least one other one of the plurality of electrodes based upon the detected position of the electrode; and a set of independent power supplies connected with each of the plurality of electrode structures such that a loss of power to one of the plurality of electrode structures does not cause a loss of power to a remainder of the plurality of electrode structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
         FIG. 1  shows a schematic depiction of a portion of an electrical discharge machining (EDM) system according to various embodiments. 
         FIG. 2  shows a schematic partial cross-sectional view of an EDM system according to some embodiments. 
         FIG. 3  shows a schematic partial cross-sectional view of an EDM system according to various additional embodiments. 
         FIG. 4  shows a close-up partial cross-sectional view of a portion of an electrode device according to various embodiments. 
         FIG. 5  shows an additional close-up partial cross-sectional view of a portion of an electrode device, including a rotating tube electrode device  40  according to various additional embodiments. 
     
    
    
     It is noted that the drawings of the invention are not necessarily to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As noted herein, the subject matter disclosed relates to electrical discharge machining. Specifically, the subject matter disclosed herein relates to systems for performing electrical discharge machining in turbine components, e.g., gas turbine blades or buckets. 
     In contrast to conventional approaches, various embodiments of the disclosure include an electrical discharge machining (EDM) system having a set of independent electrodes. That is, the EDM system includes a plurality of independently powered, and thus independently controlled, electrodes. These independent electrodes can be insulated from one another, and coupled with at least two distinct controllers (e.g., several distinct controllers). The system can further include independent (electrically isolated) processors to detect different gaps between the electrodes, and opto-couplers to connect the independent processors to a central processor while keeping noise low. The system can include a mounting frame, back plate and guiding plate to stabilize the electrodes. Various features of the disclosure can increase the speed of EDM on turbine components (e.g., airfoils) by several times (e.g., up to ten times). 
     As denoted in these Figures, the “A” axis represents axial orientation (along the axis of the turbine rotor, omitted for clarity). As used herein, the terms “axial” and/or “axially” refer to the relative position/direction of objects along axis A, which is substantially parallel with the axis of rotation of the turbomachine (in particular, the rotor section). As further used herein, the terms “radial” and/or “radially” refer to the relative position/direction of objects along axis (r), which is substantially perpendicular with axis A and intersects axis A at only one location. Additionally, the terms “circumferential” and/or “circumferentially” refer to the relative position/direction of objects along a circumference (c) which surrounds axis A but does not intersect the axis A at any location. It is further understood that common numbering between FIGURES can denote substantially identical components in the FIGURES. 
     Turning to  FIG. 1 , a schematic depiction of a portion of an electrical discharge machining (EDM) system  2  is shown according to various embodiments. As shown, EDM system  2  can include a guide structure  4 , and a plurality of electrode devices  6  positioned at least partially within guide structure  4  (e.g., guide structure  4  at least partially envelops electrode devices  6 ). According to various embodiments, electrode devices  6  are aligned to provide an electrical discharge to a workpiece  8  (e.g., a turbine component such as a gas turbine airfoil). 
     Each of the plurality of electrode devices  6  can include an electrode  10  for positioning proximate (e.g., contacting or nominally separated from) workpiece  8 , and an electrode holder  12  coupled to electrode  10  for holding the electrode  10  proximate workpiece  8 . Each electrode device  6  can further include a driver  14  coupled to electrode holder  12 . Driver  14  can be adapted to modify a position of electrode  10  (relative to workpiece  8 ), held inside holder  12 , with linear bearings. In various embodiments, driver  14  can include at least one of an electric or pneumatic driver, and in some cases, can include a voice coil or a hydraulic driver. 
     EDM system  2  can further include at least one sensor  16  for detecting a position of an electrode  10  relative to the guide structure  4  in the plurality of electrode devices  6 . In various embodiments, the at least one sensor  16  can include a plurality of sensors  16 , and in some cases, at least one of sensor(s)  16  is configured to detect a position of multiple electrodes  10 , as well as a workpiece-electrode gap  11  (distance between each electrode  10  and workpiece  8 ). The at least one sensor  16  can include, e.g., an optical sensor, laser-based sensor, or other conventional sensor capable of detecting the position of one or more electrodes  10 . In various embodiments, sensor(s)  16  can include a gap senor configured to detect the voltage signal between an electrode  10  and workpiece  8 . 
     EDM system  2  can further include a control system (CS)  18  operably connected with driver  14  (e.g., via conventional wireless and/or hard-wired means), where control system  18  is configured to provide instructions to the driver  14  of at least one of the plurality of electrode devices  6  to modify a position (and consequently, the workpiece-electrode gap  11 ) of the at least one of the plurality of electrodes  10  independently of at least one other one of the plurality of electrodes  10  based upon the detected position (and, e.g., workpiece-electrode gap  11 ) of the electrode  10 . In various embodiments, control system  18  is coupled to sensor(s)  16 , and is configured to obtain data about the position of electrodes  10  (as well as gap  11  between each electrode  10  and workpiece  8 ) from sensor(s)  16 , and based upon that position data (and data about gap  11 ), instruct driver  14  of at least one of electrode devices  6  to modify a position of its corresponding electrode  10  (and therefore modify gap  11  between each electrode  10  and workpiece  8 ). 
     Control system  18  may be mechanically or electrically connected to electrode devices  6  (e.g., at driver  14 ) such that control system  18  may actuate at least one of electrode devices  6 . Control system  18  may actuate driver  14  of electrode device(s)  6  in response to a detected position of electrode  10  (and gap  11  between each electrode  10  and workpiece  8 ), e.g., a discrepancy from a predicted electrode position (and gap  11  size), a discrepancy in position (and gap  11  size) with respect to the guide structure  4 , etc. Control system  18  may be a computerized, mechanical, or electro-mechanical device capable of actuating electrode devices  6  (e.g., by initiating or halting driver  14 ). In one embodiment control system  18  may be a computerized device capable of providing operating instructions to electrode devices  6 . In this case, control system  18  may monitor the position of one or more electrodes  10  as well as gap  11  between each electrode  10  and workpiece  8  (e.g., relative to a prescribed pattern and/or predetermined positional information and gap distance, via sensor(s)  16 ), comparing data about the topography of workpiece  8 , along with desired hole locations, depths, etc. with data obtained from sensor(s)  16 ), and provide operating instructions to electrode device(s)  6  to independently modify a position (and consequently, gap  11 ) of at least one of the electrodes  10  in those device(s)  6 . For example, control system  18  may send operating instructions to halt driver  14  of one electrode device  6  under certain operating conditions (e.g., where sensor(s)  16  detect that electrode  10  has reached its desired depth of penetration into workpiece  8 ). In this embodiment, electrode device  6  may include electro-mechanical components, capable of receiving operating instructions (electrical signals) from control system  18  and producing mechanical motion (e.g., pausing, initiating, etc. driver  14 ). In another embodiment, control system  18  may be an electro-mechanical device, capable of electrically monitoring (e.g., with sensors  16 ) parameters indicating positions (and in some cases, gaps  11 ) of one or more electrodes  10 , and mechanically actuating the driver  14  of one or more corresponding electrode devices  6 . While described in several embodiments herein, control system  18  may actuate electrode devices  6  through any other conventional means. 
     In various embodiments, each electrode  10  is controlled independently according to its position and gap  11  between the tip of electrode  10  and workpiece  8  (e.g., opening in workpiece  8  to be formed). As noted herein, electrode  10  can be driven (e.g., via driver  14 ) toward workpiece  8 . Once electrode  10  starts to discharge on workpiece  8 , electrode  10  federate can slow down and CS  18  takes over the feed control. If gap  11  becomes too small with a too low a voltage signal, the feed rate is slowed down (or even reversed) for electrode withdrawal to avoid shorting and overheating to workpiece  8 . Once senor(s)  16  detect the end depth for the process (e.g., hole is formed to desired depth), the CS  18  stops feeding electrode  10  and withdraws electrode  10  independently while other electrodes  10  may still feed, or may be withdrawn, independently. 
     In various embodiments, guide structure  4  is coupled to electrode holder  12 , where guide structure  4  includes a slide bearing  20  ( FIG. 4 ,  FIG. 5 ) allowing electrode holder  12  to move relative to guide structure  4 . In some cases, as shown in  FIGS. 1-3 , EDM system  2  can further include a machine ram  22  coupled to guide structure  4 , and a back plate  24  coupled to guide structure  4 , where back plate  24  at least partially supports the driver  14  in each of the plurality of electrode devices  6 . 
     As shown in  FIG. 1  and  FIG. 2 , according to some embodiments, all of the plurality of electrode devices  6  can be aligned parallel with respect to one another and uniformly angled with respect to the guide structure  4 . That is, all electrode devices  6  can be aligned in a parallel arrangement in order to form equally spaced openings in workpiece  8 , at a same angle of contact. 
       FIG. 3  illustrates an alternative embodiment to the parallel arrangement in  FIG. 2 , where electrode devices  6  are aligned in a multi-directional alignment. In these cases, a first one ( 6   a ) of the plurality of electrode devices  6  is aligned at a first angle (α 1 ) with respect to a second one ( 6   b ) of the plurality of electrode devices  6 , and a third one ( 6   c ) of the plurality of electrode devices  6  is aligned at a second angle (α 2 ) with respect to the second one  6   b  of the plurality of electrode devices  6 , where the second angle (α 2 ) is distinct from the first angle (α 1 ). 
     As shown in  FIG. 1 , according to various embodiments, each of the plurality of electrode devices  6  (several shown) is connected with an independent power supply (IPS)  26 , such that a loss of power to one of the plurality of electrode devices  6  does not cause a loss of power to the remainder of the electrode devices  6 . In some embodiments, power supply  26  can be coupled to more than one electrode device  6 , but it is understood that according to the embodiments disclosed herein, multiple power supplies  26  are employed, such that the entirety of electrode devices  6  is not reliant upon a single power source. It is understood that according to various embodiments, IPS  26  can prevent issues with a consolidated, central power supply, utilized in conventional systems. That is, a shared power supply provides one large spark to one of the multiple electrodes. At any given time in the electric discharging process, that one large spark is limited to only one electrode, so that metal removal rate is low when compared with a multi-spark configuration. The shared power supply (with one large spark) can yield poor surface quality because of the high power level at each electrode. According to various embodiments herein, the independent and dedicated power supplies can delivers precise amounts of power to one electrode at a desired time, based upon local gap control, as described herein. With a small gap and fast feeding, the independent power supply systems according to various embodiments can provide higher power locally when compared with conventional approaches. As each electrode-workpiece gap can differ (e.g., depending on local debris generation and dielectric flushing), independent and dedicated power supplies for distinct electrodes enables multiple sparks, for a high metal erosion rate, and a lower power level for high surface quality finishing after EDM is complete. 
       FIG. 4  shows a close-up partial cross-sectional view of a portion of an electrode device  6  according to various embodiments. As shown In  FIG. 4 , in some embodiments, electrode device  6  is positioned within back plate  24  and slide bearing  20 , where slide bearing  20  is housed within a guide plate  28 . Driver  14  can include a hydraulic cylinder  30  and a piston  32  within that hydraulic cylinder  30  ( FIG. 4 ). Both the cylinder  30  and piston  32  can be held within a bushing  34  at least partially retained by back plate  24 . Driver  14  can be coupled to electrode holder  12  by an insulator connector  36 . In some cases, electrode  10  is held by a clamp (e.g., collet clamp)  38  at the tip of electrode holder  12 . 
       FIG. 5  shows an additional close-up partial cross-sectional view of a portion of an electrode device  6 , where driver  14  includes a rotating tube electrode device  40 , according to various additional embodiments of the disclosure. In these embodiments rotating tube electrode device  14  is coupled to a hydraulic piston  32  (connected with a cylinder  30 ) as shown and described with reference to  FIG. 4 . However, rotating tube electrode device  40  can further include a pneumatic rotary driver  42  including an air motor  44  (having compressed air inlet  46  and outlet  48 ), a water inlet  50  upstream of the air motor  44  (with adjacent seal  52 ), and a rotor  54  coupled with the air motor  44  (where rotor  54  is supported on bearings  56 ). Rotor  54  is connected with an electrode holder  12  (including electrode clamp  38 ). In this configuration, rotating tube electrode  40  is configured to rotate electrode  10  while hydraulic piston  32  drives electrode  10  toward workpiece  8 . This rotating tube electrode device  40  shown in  FIG. 5  can be useful in forming holes in workpiece  8  in spaces requiring minimal clearance and/or a high density of electrode devices  6 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.