Abstract:
An apparatus and method for adapting a CNC milling machine for electroerosion machining. The apparatus includes a tubular electrode on the distal end of an adapter shaft. A tool holder on the proximal end of the adapter shaft is mountable in the chuck of a cutter spindle in the milling machine. The adapter shaft is rotatably mounted within a bearing and an electrical brush contact subassembly, both of which are supported by a bracket. The bracket is attached to the milling machine but insulates it from the tool electrode. The bearing supports the adapter shaft in alignment with the CNC spindle. An electrical power supply energizes the electrode and the workpiece for electroerosion in a gap between them. Electrolyte is circulated through the spinning tool electrode during operation. The CNC computer is configured to operate the machine, power supply, and electrolyte flow for electroerosion machining.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 10/842,344, filed May 7, 2004, assigned to the present assignee, and published as U.S. patent application publication 20050247569. The foregoing application is incorporated by reference herein in its entirety. 
     BACKGROUND 
     The present invention relates in general to electroerosion milling (EEM), and more specifically to an adaptive spindle that modifies a Computer Numerical Control (CNC) machine tool, such as a CNC milling machine for EEM. 
     Electroerosion machining is performed by passing an electrical current through a gap between an electrode and a workpiece for removal of material on the workpiece. It uses direct-current (DC) voltages to electrically power removal of the material from the workpiece. An electrolyte is circulated between the tool electrode and the workpiece to facilitate electroerosion of the workpiece material, and to cool and flush the gap region. This process enables a high rate of material removal with low thermal damage to the workpiece. An advanced form of electroerosion machining using a spinning tool electrode is described in related U.S. patent application Ser. No. 10/842,344. 
     EEM provides quicker machining and higher efficiencies than mechanical cutting or other electrical discharge machining (EDM) methods in various applications, such as turbine impeller and bladed disk roughing and machining. It is believed, however, that prior to the present invention there has been no practical way to convert a conventional CNC milling machine for EEM operation. Thus, EEM machines to date have been specialized systems. 
     BRIEF DESCRIPTION 
     An aspect of the invention resides in an adapter spindle assembly for a conventional multi-axis CNC machine tool such as a CNC milling machine that drives and controls movements of a cutter and workpiece to machine complex component geometries under software control. The present adapter converts such a milling machine to operate by EEM. 
     Another aspect of the invention resides in such an EEM adapter spindle assembly including a tool electrode configured to machine a workpiece located across a gap from the tool electrode. Machining is achieved by electroerosion powered by an electrical potential across the gap, and facilitated by an electrolyte flushing liquid circulated into the gap. A power supply is configured to energize the tool electrode and workpiece. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic illustration of an EEM adaptive spindle according to one embodiment of the invention as may be used on a conventional CNC milling machine; 
         FIG. 2  illustrates an EEM adaptive spindle assembly according to a first embodiment of the invention; 
         FIG. 3  illustrates an EEM adaptive spindle assembly according to a second embodiment of the invention; 
         FIG. 4  illustrates an EEM adaptive spindle assembly mounted on a movable spindle carrier of a CNC milling machine. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a CNC milling machine  23  modified for electroerosion machining by installing an EEM adaptive spindle assembly  20  with a spinning tool electrode  21 . The CNC milling machine  23  has a spindle  24  that normally holds a mechanical milling tool as known in the art. A workpiece carrier  27  holds and moves a workpiece  28  relative to the milling tool by rotating and translating the workpiece carrier via servos. A computer  36  executes stored programs to send control signals  38  to the servos and electronics that operate the CNC machine. Signal and control circuits  40  communicate operating conditions to the computer  36 , and may also communicate related control signals from the computer such as automatic system shut-off in case of overheating, low fluid, and the like. 
     To adapt a CNC milling machine  23  for EEM usage, an adaptive EEM spindle assembly  20  is mounted on the CNC tool spindle  24  as later described. Furthermore, an EEM numerical control program is installed in the CNC computer  36 , and a DC power supply  44  is provided to energize the EEM tool electrode  21  for electroerosion in the gap between the tool electrode  21  and the workpiece  28 . A custom plug-in circuit board  42  may be installed in the CNC computer  36  as an interface for a signaling link  46  between the power supply  44  and the CNC computer. A first electrical potential is conducted  48  to the EEM tool electrode from the power supply  44 , and a second electrical potential is conducted  50  to the workpiece  28 , forming an electrical circuit  48 ,  50  including the gap. 
     This generates discharges in the gap  74  between the workpiece  28  and the tool electrode  21 . The CNC computer  36  controls servos in the CNC milling machine  23  to perform relative movements between the tool electrode  21  and the workpiece  28  as known in the art of CNC machine tools, thus controlling the gap  74 . The CNC computer  36  monitors and controls the EEM processes of the EEM adaptive spindle assembly  20 , the associated power supply  44 , and electrolyte pump  60 . 
     A voltage measuring circuit in the power supply  44  senses the voltages across the gap via the power circuit  48 ,  50 , and communicates this data via the signaling link  46  to the custom circuit board  42  in the CNC computer  36 , providing information about the discharging status and condition of the gap  74 . The EEM control program controls the machining feed rate and DC power profile for optimum operation based on this feedback. Circuits in the power supply may include, without limitation, a microprocessor or another computational device, a voltage measurement device, a timing device, a pulse generation device, a voltage comparison device, and a data storage device, among others. All such devices are well known in the art, and any such suitable device may be used without deviating from the scope of the invention. 
     CNC Machine tools are often equipped to spray a liquid on the cutter and workpiece  28  to cool them and to flush away etched particles. A shield  30  may collect the liquid into a collection tank or tub  32  for recycling via a filtration system. EEM uses a liquid electrolyte circulation system that serves these cooling and flushing functions and also enables electroerosion. The EEM electrolyte system may use some existing liquid circulation components of the CNC milling machine  23 , such as the shield  30  and collection tank  32 . Other EEM specialized components may be adapted or added as needed. For example, a separate electrolyte pump and filtration unit  60  may be provided for connection by a fluid return line  62  from the existing collection tank  32 . An internal flushing fluid supply line  64  may provide a first flow of electrolyte from the pump  60  for internal flushing as later described. An external flushing fluid supply line  66  may provide a second flow of electrolyte for external flushing as later described. Filtration may be performed for example as described in US patent application publication 20050218089A1, assigned to the present assignee. The electrolyte pump  60  may be electronically connected  40  to the control computer  36  for data communication to the computer, and for control communication from the computer. This enables flow control and machining shut down for overheating or low fluid conditions. 
       FIG. 2  shows an EEM spindle adapter assembly  20  including a tool electrode  21  positioned across a gap  74  from a workpiece  28 . The power supply  44  generates electrical discharges in the gap  74  that machine the workpiece  28 . The discharges cause particles to separate from the workpiece  28 , thereby machining the workpiece. 
     The tool electrode  21  may be removably mounted on the distal end  81  of a rotatable adapter shaft  80  by means of a collet  82 . A tool holder  84  is fixed to the proximal end of the adapter shaft  80 , and mates with the tool chuck  25  on the CNC spindle  24 . The adapter shaft  80  is electrically insulated from the CNC spindle  24  by insulation  86  between the adapter shaft  80  and the tool holder  84 . The power supply  44  may energize the tool electrode  21  by applying pulses of a voltage difference ΔV between the lead  48  conducting to the tool electrode and the lead  50  conducting to the workpiece. This power is conducted between the lead  48  and the spinning adapter shaft  80  by means of a stationary-to-rotary conduction device, such as a subassembly  88  of electrical brush contacts  89 . Alternately a rotary transformer (not shown) may be used. Rotary transformers induce electrical current into a rotating conductor without physical contact from a stationary conductor. 
     First and second flows  72 ,  73  of liquid electrolyte may be provided for internal and external flushing, respectively. As shown in  FIG. 2  the first flow of liquid electrolyte  72  is supplied to an axially-oriented conduit  90  in the tool electrode  21  via an axially-oriented conduit  92  in the adapter shaft  80 . The electrolyte  72  may enter the conduit  92  in the adapter shaft  80  by means of a fluid input manifold  94  that is sealed around a portion of the adapter shaft  80 . This fluid input manifold  94  passes the fluid  72  to the conduit  92  in the shaft  80  via generally radially-oriented flow paths  95  in the shaft  80 . The electrolyte  72  thus can flow into the conduit  92  while the shaft  80  spins, and into the tool electrode conduit  90 . It exits an opening  91  in the distal end  22  of the tool electrode  21 , where it circulates through the gap  74 , enabling electroerosion, flushing, removing etched particles efficiently, and cooling. 
       FIG. 3  illustrates a second embodiment for supplying the first flow  72  of the electrolyte to the tool electrode. Some CNC milling machines provide “through-the-spindle” flushing via a fluid channel  26  in the spindle  24 . This channel  26  may be utilized by extending the conduit  192  in the adapter shaft  180  through the tool holder  184  as shown. In this case, a fluid input manifold  94  as in  FIG. 2  is not needed. 
     A second flow  73  of liquid electrolyte may be provided for external flushing, in which the liquid  73  is sprayed toward the distal end  22  of the tool electrode  21  from outside the tool electrode.  FIGS. 2 and 3  show a way to provide external flushing by means of a spray manifold  96  mounted around the distal end  81  of the adapter shaft  80 ,  180  and attached to the brush subassembly  88 . The spray manifold  96  has fluid outlets  97  around the tool electrode  21  for spraying the liquid  73  alongside the tool electrode. Alternately or additionally, other external nozzles not shown may be used that are not attached to the adaptive spindle assembly  20 . 
     The adapter shaft  80  is mounted on a low-friction bearing  98 . The bearing  98  and the brush subassembly  88  are supported by a bracket  99  that is attached to the CNC milling machine  23 . The bearing  98  supports the adapter shaft in alignment with the CNC spindle  24 . The adapter assembly  20  is electrically insulated from the CNC milling machine  23 . The bracket  99  may be made of an electrically insulating material as illustrated in  FIGS. 2 and 3 , or it may have an insulating portion  99 i as in  FIG. 4 . The bracket  99  may be attached to a stationary part of the CNC milling machine  23  as shown in  FIGS. 1-3 . In this case, CNC movements of the workpiece  28  relative to the tool electrode  21  are performed by the workpiece carrier  27 , as known in the art of CNC milling machines. Alternately, the bracket  99  may be attached to a movable spindle carrier  29  as illustrated in  FIG. 4 . In this case, CNC movements of the workpiece  28  relative to the tool electrode  21  may be performed by the spindle carrier  29  and/or the workpiece carrier  27 . The movable spindle carrier  29  may be a piston as shown, or it may be a spindle drive mechanism mounted on multiple orthogonal ways or tracks as known in the art of CNC milling machines. 
     An EEM system according to aspects of this invention may use a pulse or continuous direct current power with an open voltage range from about 31V to 70V, and an average current range from about 100 A to 3000 A, with the positive potential connected to the workpiece  28  and negative potential connected to the tool electrode  21 ; an internal/external water-based flushing electrolyte  72 ,  73  with a pressure range from about 100 psi to 1000 psi; a rotary tube electrode  21  with a conductive wall material such as graphite or brass; and a revolution speed range of about 500 rpm to 10,000 rpm. These details are provided as examples only, and are not limiting of the invention. 
     The present adaptive spindle for EEM allows a conventional CNC milling machine  23  to use either electroerosion discharging milling or conventional milling. Example advantages of EEM may include: 1) High material removal rate. Material removal rates of over 20000 cubic mm/min have been demonstrated using a 32 mm diameter tube electrode. 2) Low cutting force. 4) Low tooling cost, since an EEM electrode may be a simple tube of a low cost material compared to conventional milling tools that require high strength, high hardness, and complex cutter shapes. 5) Low tool maintenance, since an EEM tool electrode is not sharpened, but is simply replaced. 
     In operation, EEM provides quicker machining and higher efficiencies than mechanical cutting or other electrical discharge machining (EDM) methods in various applications, such as turbine impeller and bladed disk roughing and machining. In an EEM assembly, a voltage potential is generated across a gap between an electrode and a workpiece to be machined, resulting in an electrical discharge in the gap. When the machining electrode approaches the workpiece surface separated by the gap, an electrical discharge occurs through the gap due to the voltage. The gap, which constitutes a machining zone, is filled with a liquid electrolyte. The EEM system provides a flow of electrolyte, which removes eroded particles from the gap and provides a suitable medium for electrical discharge. 
     Tests have shown that airfoils made of metal alloy, such as Inconel  718  metal alloy, can be produced using the above described process. These tests have indicated a substantial increase in machining speed and a substantial reduction in tooling cost over mechanical machining under test conditions using, for example, a 4-axis numerical control and a pulsed DC power supply. 
     Inconel  718  is one example of a relatively high-strength, high-temperature and corrosion resistant nickel-chromium super alloy. It is suitable for use in air up to 1300 F. It is readily worked and can be age-hardened. This alloy may comprise approximately the following element percentages by weight: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Aluminum 
                 0.2-0.8 
               
               
                   
                 Boron 
                 0.006 max 
               
               
                   
                 Carbon 
                  0.08 max 
               
               
                   
                 Chromium 
                 17-21 
               
               
                   
                 Cobalt 
                    1 max 
               
               
                   
                 Copper 
                  0.3 max 
               
               
                   
                 Iron 
                 Balance 
               
               
                   
                 Manganese 
                  0.35 max 
               
               
                   
                 Molybdenum 
                 2.8-3.3 
               
               
                   
                 Nickel 
                 50-55 
               
               
                   
                 Niobium 
                 4.75-5.5  
               
               
                   
                 Phosphorus 
                 0.015 max 
               
               
                   
                 Silicon 
                  0.35 max 
               
               
                   
                 Sulphur 
                 0.015 max 
               
               
                   
                 Titanium 
                 0.65-1.15 
               
               
                   
                   
               
             
          
         
       
     
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 ELEMENT LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 
                 Adaptive spindle assembly 
               
               
                 21 
                 EEM tool electrode 
               
               
                 22 
                 Distal end of EEM tool electrode 
               
               
                 23 
                 Computer Numerical Control (CNC) milling machine 
               
               
                 24 
                 CNC machine spindle 
               
               
                 25 
                 Chuck on CNC spindle 
               
               
                 26 
                 Through-the-spindle flushing liquid channel 
               
               
                 27 
                 Workpiece carrier 
               
               
                 28 
                 Workpiece 
               
               
                 29 
                 CNC movable spindle carrier 
               
               
                 30 
                 Shield 
               
               
                 32 
                 Liquid collection tank or tub 
               
               
                 36 
                 CNC computer 
               
               
                 38 
                 Data and control signals between the milling machine and 
               
               
                   
                 computer 
               
               
                 40 
                 Data and control signals between the electrolyte system and 
               
               
                   
                 computer 
               
               
                 42 
                 Custom circuit board in CNC computer 
               
               
                 44 
                 DC power supply 
               
               
                 46 
                 Communication link between the power supply and the CNC 
               
               
                   
                 computer 
               
               
                 48 
                 Electrical conductor to tool electrode via brush subassembly 
               
               
                 50 
                 Electrical conductor to workpiece 
               
               
                 60 
                 Electrolyte pump and filtration unit 
               
               
                 62 
                 Fluid return line 
               
               
                 64 
                 Internal flushing supply line 
               
               
                 66 
                 External flushing supply line 
               
               
                 72 
                 Internal flushing electrolyte flow 
               
               
                 73 
                 External flushing electrolyte flow 
               
               
                 74 
                 Gap between tool electrode and workpiece 
               
               
                 80 
                 Adapter shaft 
               
               
                 180  
                 Alternate adapter shaft for through-the-spindle internal flushing 
               
               
                 81 
                 Distal end of adapter shaft 
               
               
                 82 
                 Collet for holding tool electrode in adapter shaft 
               
               
                 84 
                 Tool holder on adapter shaft for mounting in CNC spindle chuck 
               
               
                 184  
                 Alternate tool holder for use with internal through-the-spindle 
               
               
                   
                 flushing 
               
               
                 86 
                 Electrical insulation 
               
               
                 88 
                 Electrical brush contact subassembly 
               
               
                 89 
                 Electrical brush 
               
               
                 90 
                 Conduit in tool electrode for internal flushing 
               
               
                 91 
                 Exit hole in distal end of tool electrode for internal flushing 
               
               
                 92 
                 Conduit in adapter shaft for internal flushing 
               
               
                 192  
                 Conduit in adapter shaft for internal through-the-spindle flushing 
               
               
                 94 
                 Fluid input manifold for internal flushing 
               
               
                 95 
                 Generally radially-oriented fluid flow path in adapter shaft 
               
               
                 96 
                 External flushing spray manifold 
               
               
                 97 
                 Opening in spray manifold for external flushing along tool 
               
               
                   
                 electrode 
               
               
                 98 
                 Bearing 
               
               
                 99 
                 Bracket for mounting adapter assembly to CNC machine 
               
               
                  99i 
                 Electrically insulating portion of bracket