Abstract:
An electrical discharge machining head incorporating piezo-electric motors for controlling motion of a slide assembly. The slide assembly is translatable within a machine base and its motion is controlled by the piezo-electric motors. The slide assembly precisely controls the movement of an EDM electrode through the wire guide.

Description:
FIELD OF THE INVENTION 
   This invention is related to a machining device and particularly to an electrical discharge machining (EDM) system. 
   BACKGROUND OF THE INVENTION 
   EDM is a well-known and widely used machining technique. It operates through the erosion of a workpiece, generally metallic, through electrical discharges from an electrode. The process takes place in the presence of a liquid dielectric fluid, such as de-ionized water. An electrode in the shape of an elongated rod, thin wire, or shaped article is put into close contact with the workpiece. Through an electrical potential difference, arcing occurs between the workpiece and the electrode which causes erosion of the workpiece material in a desired manner. 
   EDM processes are used in numerous machining applications. It is especially desirable for its high forming accuracy, the ability to machine extremely hard workpieces, its low applied loading of the workpiece, and its ability to form deep bores in workpieces. 
   During the EDM machining cycle, it is necessary to precisely position the end of the electrode with respect to the workpiece surface being machined as erosion of the workpiece occurs. Due to sacrificial material loss of the electrode, it is also necessary to incrementally advance the electrode tip with respect to the workpiece. Since EDM machining requires maintenance of a critical gap between the electrode and workpiece surface, the positioning of the electrode is provided by precision servo motor heads. A closed loop control system continuously monitors electrical parameters associated with the arc discharge and continuously controls the position of the head to move the electrode with respect to the workpiece. 
   Currently available EDM systems typically utilize a servo motor positioning head using a lead screw type actuator. A precision servo motor rotates the lead screw which causes the head position to be incrementally moved. The present systems have several significant limitations. There is typically a trade-off between the precision of positioning provided by a head and the speed at which it can be moved. The ability to quickly move the head into a machining position and withdrawing from it, while permitting highly accurate positioning is a desired attribute. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an EDM head is provided which utilizes a number of piezo-electric motors used to advance and retract the slide assembly of the head. The piezo-electric motors provide a high degree of positioning accuracy, yet also allows the head to be moved quickly into and out of machining positions. Through the implementation of an air bearing arrangement for the slide assembly, very low friction is provided which enables increased precision in head positioning. 
   Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial view of the EDM head in accordance with this invention; 
       FIG. 2  is another pictorial view of the EDM head in accordance with this invention; 
       FIG. 3  is a top view of the EDM head; and 
       FIG. 4  is a side elevational view of the EDM head. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the figures, EDM head is shown in a completely assembled condition in the figures and is generally designated by reference number  10 . EDM head  10  principally comprises machine base  12  and slide assembly  14 . 
   As shown in the figures, machine base  12  includes mounting bores for mounting the base to an associated EDM machining mounting structure. Fixed to machine base  12  at the forward end of the device, is wire guide clamp  16 . Wire guide clamp  16  provides a mounting location for a wire guide  18  which may be, for example of the type described in U.S. Pat. No. 5,951,883 assigned to the Assignee of this application and which is hereby incorporated by reference. Wire guide clamp  16  is fixedly mounted with respect to machine base  12 . Wire guide clamp  16  further incorporates air cylinder  20  which functions in a so-called “re-feed” cycle which will be detailed later in this description. 
   At the opposite end of machine base  12 , switch bracket  22  is provided which provides a mounting location for a pair of photoelectric home switches  24  and  26 . Switches  24  and  26  are used to indicate the position of slide assembly  14  which will be described in more detail in the following description. Machine base  12  further forms an elongated slide track  28  which allows for linear sliding motion of slide assembly  14 . Two pairs of mounting posts  30  are provided for supporting piezo-electric motors  32 . Displacement transducer  34 , best shown in  FIG. 4 , is affixed to the lower surface of machine base  12  and is an optical device which precisely monitors linear motion of slide assembly  14  which features some type of ruling or index markings. Displacement transducer  34  may be of an optical “bar code” type which measures the motion of slide assembly  14  by changes in its electrical output as the ruling marks move past the transducer. 
   Machine base  12  further forms air flow openings within slide track  28  to provide slide assembly  14  with an air bearing support system. By directing pressurized air to the interface between machine base slide track  28  and slide assembly  14 , motion of slide assembly  14  is accommodated with very low friction. 
   Slide assembly  14  includes slide rail  36  which closely conforms with and fits within mounting base slide track  28 . Tube  38  is mounted to an upper surface of slide rail  36  for guiding an EDM electrode  40  into engagement with wire guide  18 . A pair of friction strips  42  are provided on the opposing side surfaces of slide rail  36  and are provided to engage with piezo-electric motors  32 . An end of slide rail  36  provides mounting head  44  for air cylinder  46  and electrode clamp  48 . When electrode clamp  48  is engaged to clamp EDM electrode  40 , reciprocating motion of slide assembly  14  causes the EDM electrode  40  to move into and out of wire guide  18 . The end of slide rail  36  opposite mounting head  44  features an extending tab or shutter  50  which is provided to interact with photoelectric home switches  24  and  26  to define extended and retracted “home” positions for slide rail  36 . 
   Various types of piezo-electric motors  32  may be employed in EDM head  10 . One type manufactured by EDO Company (Model No. PDA-130) may be used in connection with this invention. These devices provide smooth, ultra-precision motion capabilities through the use of an associated controller which provides driving signals to piezo-electric motors  32 . Their friction shoe  52  interacts with friction strips  42  to precisely position slide assembly  14  as desired. Applied electrical signals to piezo-electric motors  32  cause friction shoes  52  to move in an oscillating manner which causes slide assembly  14  to advance and retract as desired. Piezo-electric motors  32  incorporate internal ceramic crystals which, through the piezo-electric effect, contract and expand in response to an applied electric charge. Motors  32  further maintain the position of slide assembly  14  when they are not energized by an applied signal, enabling the system to remain in a stable condition between movement commands. While it may be possible to use fewer than the four piezo-electric motors  32  shown in connection with this invention, the use of four such devices is presently a preferred implementation. 
   In operation, an EDM electrode  40  is first fed through tube  38 , electrode clamp  48  (with its air cylinder  46  retracted), and through wire guide  18  (with wire guide clamp  16  retracted). Electrode clamp  48  is actuated while wire guide clamp  16  remains retracted so that precision motion of slide assembly  14  causes the EDM electrode  40  to be precisely moved through wire guide  18 . This would be undertaken during an EDM machining operation. Machine base  12  is mounted to a translation stage (not shown) which brings EDM head  10  into position with respect to a workpiece and retracts it following completion of the EDM operation. Accordingly, during machining, an appropriate closed loop control sub-system is used to precisely control the motion of slide assembly  14  and therefore the tip of EDM electrode  40  with respect to the workpiece during a machining operation. 
   Since EDM electrode  40  is partially consumed during EDM operation, it is periodically necessary to advance a length of EDM electrode  40  with respect to slide assembly  14 . When it is desired to “re-feed” EDM electrode  40 , wire guide clamp  16  is activated to clamp against wire guide  18 . Simultaneously, electrode clamp  48  disengages the electrode by retracting air cylinder  46 . Following retraction of slide assembly  14 , wire guide clamp  16  is retracted and air cylinder  46  is activated to frictionally engage EDM electrode  40  through electrode clamp  48 , thus permitting continued EDM operation. 
   While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.