Abstract:
An EDM machine with a plurality of separate electrodes selectively driven by the same actuator on the same head of the EDM machine is provided. Preferably, all of the electrodes are not used to cut the workpiece at the same time. Rather, the actuator has a drive mechanism, which is preferably servo-controlled, and each electrode is selectively engaged with the drive mechanism to advance and retract only the desired electrode relative to the workpiece. Either the workpiece or the EDM head is preferably manipulated using a CNC positioning system to accurately locate the desired electrode relative to the workpiece.

Description:
CO-PENDING APPLICATION 
     This application claims the benefit of U.S. application Ser. No. 60/119,859 filed on Feb. 12, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electrical discharge machining and more particularly, to an electrical discharge machine having more than one electrode. 
     BACKGROUND OF THE INVENTION 
     Conventional machines for electrical discharge machining (EDM) have a single electrode carried by a holder, movable relative to a metallic workpiece by a servo-controlled actuator and energized by a pulsed electrical current to form holes. slots or cavities in the metallic workpiece. An operative end of the electrode is spaced from the workpiece providing a spark gap therebetween which permits a spark discharge to pass between the electrode and the workpiece. Each spark produces heat sufficient to melt or vaporize the adjacent portion of the workpiece leaving a pit or cavity in the workpiece. EDM is ideal for applications involving workpieces that are difficult to machine, have a complex shape, require small slots, holes or passages and close tolerances. 
     The single electrode of a conventional EDM machine enables only one hole size to be formed into a single workpiece. To form different sized holes in a workpiece, the machine must be taken out of production for a tooling change which greatly increases the down time of the system and significantly increases the time and cost to manufacture a workpiece. Still further, the operator of the machine may not be able to change the tooling requiring another individual to perform this operation which extends the down time and further increases the cost to manufacture the workpiece. Alternatively, the workpiece may be carried on a fixture to multiple workstations, each having a separate EDM machine, to form different sized holes in the workpiece. This significantly increases the complexity and cost to manufacture a workpiece because multiple EDM machines are needed, and the fixture and transport mechanisms must be extremely accurate and reliable to form consistent workpieces. Introducing multiple EDM machines, fixtures and a transport system significantly reduces the reliability of the system as a whole thereby increasing the system down time and increasing the cost of manufacture. 
     SUMMARY OF THE INVENTION 
     An EDM machine with a plurality of separate electrodes on the same head of the EDM machine and selectively driven by the same actuator, is provided. Preferably, all of the electrodes are not used to cut the workpiece at the same time. Rather, the actuator has a drive mechanism, which is preferably servo-controlled, and each electrode is selectively engaged with the drive mechanism to advance and retract only the desired electrode relative to the workpiece. Either the workpiece or the EDM head may be manipulated using a CNC positioning system to accurately locate the desired electrode relative to the workpiece. 
     The EDM machine preferably has a carriage driven between retracted and advanced positions relative to a base by the drive mechanism. Each electrode is selectively coupled to the carriage for co-movement therewith between corresponding retracted and advanced positions, by a separate drive clamp. To advance or retract one of the electrodes, the drive clamp is closed and the carriage is moved relative to the base to move the electrode with the carriage. When the drive clamp of an electrode is open, the electrode is not coupled to the carriage so that the carriage may be moved relative to the electrode which remains in its retracted position when it is not needed. 
     To account for wear of the electrodes and to reset their position relative to the carriage, a re-feed clamp carried by the base may be provided for each electrode. As the electrode wears away, it can be replenished or reset by advancing the remaining electrode relative to the carriage. To do this, on the retract stroke of the carriage the re-feed clamp of the electrode is closed to clamp the electrode and the drive clamp of that electrode is opened, releasing the electrode from the carriage. Continued movement of the carriage to its retracted position and relative to the electrode will, in effect, advance the electrode relative to the carriage to reset the position of the operative end of the electrode relative to or pull the electrode further out of the carriage. Each re-feed clamp may also be used to maintain its corresponding electrode in its retracted position when the carriage is advanced and the electrode is not needed. 
     According to one aspect of the invention, each electrode may be of a different size or shape to permit differently sized holes, slots or cavities and the like to be formed in a workpiece at the same workstation in a single fixture. Another object and advantage of the invention is that a wide range of holes, slots or cavities may be formed with a single EDM machine using a single head driven by a single drive actuator, typically servo-controlled, and without having to change tooling or provide multiple EDM heads, multiple drives and controls therefor, or multiple workstations and a system to transport the workpieces between workstations. Yet another advantage of the invention is that multiple workpieces requiring different holes, slots and the like may be machined by a single EDM without any tooling change. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiment and best mode, appended claims and accompanying drawings in which: 
     FIG. 1 is a plan view of an EDM machine embodying the present invention with a center electrode and tube removed to simplify the view; 
     FIG. 2 is an end view of the EDM machine of FIG. 1 illustrating electrode guides and their mounting to a base; and 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring in more detail to the drawings, FIGS. 1-3 illustrate an EDM machine  10  embodying the present invention and having a plurality of electrodes  12 , 14 , 16  carried by a single EDM head  18  and selectively coupled to a carriage  20  slidable relative to a base  22  of the head  18  to dispose an electrode adjacent to a workpiece to be machined. Desirably, each electrode  12 , 14 , 16  may have a different size or shape and may be individually advanced toward and used to machine the workpiece to enable the single EDM machine  10  and head  18  to cut a variety of shapes and sizes of holes, slots, cavities and the like into the workpiece. Preferably, to facilitate aligning a desired one of the electrodes  12 , 14 , 16  with the workpiece, either the head  18  or the workpiece is carried and manipulated by a CNC positioning system  24  operated by a controller  26 . 
     The carriage  20  is slidably carried by the base  22 , such as by spaced apart, parallel rails (not shown) so that it can move relative to the base between retracted and extended positions. The carriage  20  has a base plate  28 , and a mounting plate  30  fixed to the base plate  28 . 
     To move the carriage  20  between its retracted and extended positions, a drive mechanism  32  carried by the base  22  is operatively connected to the carriage  20 . More specifically, in the embodiment shown in FIGS. 1-3, the drive mechanism  32  is a servo-controlled reversible motor  34  which is connected to the carriage by a ballscrew  36  driven by the motor  34  and a nut  38  received on the ballscrew  36  and connected to the carriage  20 . As the motor  34  rotates the ballscrew  36 , the nut travels axially along the ballscrew  38  in a direction corresponding to the direction of rotation to cause linear reciprocation of the carriage  20  relative to the base  22  in a conventional manner. Alternatively, the drive mechanism  32  may be of substantially any other design suitable to cause linear reciprocation of the carriage  20 , such as, for example, a pneumatic or hydraulic cylinder, piezoelectric motor, linear motor and so forth. 
     According to the present invention, a plurality of electrodes  12 , 14 , 16  are disposed on the carriage  20  and may be individually used to machine a workpiece as desired. Each electrode  12 , 14 , 16  may be of substantially any size or shape and is preferably elongate to account for wear in use and extend their useful life. Each electrode  12 , 14 , 16  is carried by a separate tube  42 , 44 , 46  carried by the mounting plate  30  which physically and electrically isolates the electrodes from the surrounding components of the machine  10 . Each electrode  12 , 14 , 16  extends out of its respective tube  42 , 44 , 46 , through a corresponding clamp assembly  48 , 50 , 52 , respectively, fixed to the mounting plate  30  of the carriage  20 , and then through a corresponding electrode guide  54 , 56 , 58  which properly orients the operative end of its electrode  12 , 14 , 16 . Each electrode guide  54 , 56 , 58  is fixed to a corresponding guide support  60 , 62 , 64  which are in turn fixed to a sub-base  66  fixed to the base  22  of the head  18  all by cap screws. A thermally insulating pad  68  may be disposed between the guide supports  60 , 62 , 64  and the sub-base  66  to limit the heat transfer between them. Thus, the electrode guides  54 , 56 , 58  are not carried by or movable with the carriage  20 . 
     Each clamp assembly  48 , 50 , 52  has a pneumatic drive clamp  76 , 78 , 80  and optionally and preferably has an electrode position sensor clamp  70 , 72 , 74 . The electrode position sensor clamps  70 , 72 , 74  each have a plunger  82  (FIG. 3) driven by a pneumatic signal between an open position spaced from its electrode  12 , 14 , 16 , and a closed position bearing on its electrode  12 , 14 , 16 . A sensor (not shown) carried by the plunger  82  of each sensor clamp, determines the existence of its electrode under its plunger  82 . If no electrode is sensed under the plunger  82 , a signal is sent to the controller indicating that the electrode is nearly completely used up and should be replaced by a new electrode. 
     Each drive clamp  76 , 78 , 80  has a plunger  84  (FIG. 3) driven by a pneumatic signal between an open position spaced from its corresponding electrode  12 , 14 , 16  and a closed position bearing on its electrode and releasably coupling its electrode with the carriage for co-movement therewith. The plunger  84  of each drive clamp  76 , 78 , 80  may be moved independently of the plungers  84  of the other drive clamps to selectively couple the electrodes, as desired, to the carriage. 
     Optionally, a separate re-feed clamp  86 , 88 , 90  may be provided on each electrode guide  54 , 56 , 58 . Each re-feed clamp  86 , 88 , 90  has a plunger  92  driven by a pneumatic signal between an open position spaced from its electrode  12 , 14  or  16  in its corresponding electrode guide  54 , 56 , 58  and a closed position bearing on the electrode. In use, as the electrodes become worn or are “used up” they must be replenished or reset by advancing the remaining electrode relative to the carriage  20  so that a sufficient portion of the electrode extends beyond its guide  54 , 56 , 58  and may be disposed adjacent to a workpiece. To accomplish this, the plunger  92  of the associated re-feed clamp  86 , 88 , 90  is driven to its closed position during at least a portion of the return stroke of the carriage  20  (when it moves toward its retracted position) and the associated drive clamp  76 , 78 , 80  for that electrode is released to permit relative movement between the carriage  20  and the electrode. Continued movement of the carriage  20  to its retracted position, with the electrode trapped against movement by its re-feed clamp, effectively extends the electrode relative to or pulls the electrode out of the carriage  20  a short distance. With the electrode in its desired position, its re-feed clamp is released or opened. 
     In use, to machine a workpiece, the workpiece is aligned with the electrode  12 , 14  or  16  having the desired characteristics. This is done under CNC control by moving the head  18  relative to the workpiece, or the workpiece relative to the head  18  if the workpiece is carried by the CNC positioning device  24 . To permit the selected electrode  12 , 14  or  16  to be advanced, a pneumatic signal is provided to its drive clamp  76 , 78  or  80  causing its plunger  82  to move to its closed position coupling only the selected electrode  12 , 14  or  16  to the carriage  20 . The carriage  20  is then moved from its retracted position toward its advanced position adjacent to the workpiece via the motor  34  through the ballscrew  36  and nut  38 . The selected electrode is advanced with the carriage toward the workpiece and the final distance between the selected electrode and workpiece is controlled by the servo-controller of the motor  34 . The appropriate electrical current is applied to the selected electrode to machine the workpiece as desired. 
     When the machining operation is complete, the carriage  20  is moved toward its retracted position by the motor  34 . To account for wear of the electrode, prior to the carriage  20  reaching its fully retracted position, the drive clamp plunger  82  associated with the selected electrode is moved to its open position and the corresponding re-feed plunger  92  is moved to its closed position locking the selected electrode to the base  22  through its electrode guide, its guide support, and the sub-base. Further movement of the carriage to its retracted position causes relative movement between the selected electrode and the carriage to increase the length of the selected electrode which extends from the carriage  20 . 
     When the selected electrode and carriage are initially advanced, the drive clamps  76 , 78 , 80  of the other or non-selected electrodes are maintained in their open positions permitting relative movement between these electrodes and the carriage. If desired, the re-feed clamps of the non-selected electrodes can be closed to ensure that the electrodes are not advanced with the advancing carriage. Alternatively, the electrode guides may have a spring loaded holder or other mechanism which provides sufficient drag to prevent movement of a non-selected electrode without having to close its re-feed clamp. However constructed, preferably, only one electrode is advanced to the workpiece at a time. 
     While the invention has been described with reference to a preferred embodiment, it will be readily appreciated by those skilled in the art that modifications can be made without departing from the spirit and scope of the invention as defined by the appended claims. For example, while three electrodes have been shown and described, any number of electrodes (more than one) may be provided.