Abstract:
A method and apparatus for electromechanical grinding is provided. An pulsed alternating waveform is applied between an anodic workpiece and cathodic grinding wheel to physically remove and electrochemically remove material from the anodic workpiece.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrochemical material removal. More specifically, electrochemical grinding methods and apparatus&#39; that includes a mechanical grinding of a workpiece followed by a pulsed electrochemical removal. 
     BACKGROUND OF THE INVENTION 
     Electrochemical machining, electrochemical polishing, electrochemical through-mask etching, and electrochemical deburring are all examples of electrochemical material removal processes whereby metal is removed from a workpiece via an electrochemical reaction. 
     In electrochemical machining, a workpiece and a geometric shape function as electrodes. For example, the workpiece is typically an anode and the geometric shape is typically a cathode or tool electrode. The geometric shape can be a mirror image of a desired shape of the workpiece. During operation, material is typically removed from the workpiece by an anodic electrochemical reaction. An example of an application that employs electrochemical machining processes is in the manufacturing of gun barrels whereby the internal surface is rifled. 
     Other examples of processes that use an anodic electrochemical reaction to modify a workpiece include electrochemical polishing, electrochemical through-mask etching, and electrochemical deburring. 
     Electrochemical material removal has strong utility as a manufacturing technology for fabrication of a wide variety of metallic parts and components. Electrochemical machining has numerous advantages relative to traditional machining, for example, applicability to hard and difficult to cut materials, low tool wear, high material removal rate, smooth bright surface finish, and/or production of parts with complex geometry. 
     Compared to mechanical machining processes where material is typically removed by mechanical cutting and thermal machining processes where material is typically removed by electric discharge machining or laser cutting, electrochemical material removal is a non-contact machining process and typically does not result in a mechanically damaged or thermally damaged surface layer on the machined work piece. 
     While electrochemical material processes are particularly noted for resulting in a typically superior surface finish compared to mechanical machining processes, the material removal rates are typically less than mechanical machining processes. 
     Voltage technologies include electrochemical grinding processes that combine mechanical grinding with an anodic electrochemical reaction. For example, in electrochemical grinding a grinding wheel operates as mechanical grinder and a cathode. Electrochemical grinding processes can be advantageous over mechanical grinding alone and electrochemical processes alone because it increases removal rates typically associated with the mechanical grinding, and improves surface finishes typically associated with anodic electrochemical reactions. 
     During electrochemical grinding, typically, a larger portion of material is removed from the workpiece by an anodic electrochemical reaction (e.g., 90%) and a smaller portion of material (e.g., 10%) is removed from the workpiece by mechanical grinding. Voltage electrochemical grinding can provide surface finishes, typically in the 9 to 20 μinch Ra. For many machining applications improved surface finishes with greater than 1 μinch Ra are required. For example, many military machines (e.g., helicopters or gun systems) are manufactured with new materials that require a high surface finish. 
     Thus, there is a continued need to maintain and even improve the rapid material removal rates associated with mechanical grinding or electrochemical grinding and obtain even better surface finishes such as those associated with electrochemical machining of &lt;1 μinch Ra. 
     SUMMARY OF THE INVENTION 
     One advantage of the invention includes rapid material removal that is near removal rates typically associated with mechanical grinding. Another advantage of the invention includes high surface finishes typically associated with electrochemical material removal. The surface finish can be less than 1 μinch. 
     Other advantages include lack of a mechanically or chemically damaged surface layers or thermally damaged surface layer due to a reduction in the amount of time that a mechanical grinding force is applied to the workpiece. Other advantages include ease of cutting complex geometries due to the electrochemical removal and lower tool wear due to the reduced time that a mechanical grinding force is applied to the workpiece. 
     In one aspect, the invention involves a method for electromechanical grinding. The method involves rotating an electrically conductive grinding wheel. The method also involves applying a first voltage between the electrically conductive grinding wheel and an electrically conductive workpiece. The method also involves applying a first force vector towards the electrically conductive workpiece via the electrically conductive grinding wheel to remove material from the electrically conductive workpiece. The method also involves after removing a desired percentage of material from the electrically conductive workpiece: a) applying a second force vector towards the electrically conductive workpiece, the second force vector being less than the first force vector, and b) applying a second voltage between the electrically conductive grinding wheel and the electrically conductive workpiece to further remove material from the electrically conductive workpiece. 
     In some embodiments, the first voltage is a forward direct voltage or a forward pulse voltage. In some embodiments, the second voltage is a forward pulse followed by a reverse pulse. In some embodiments, the second voltage is a forward pulse followed by a zero pulse followed by a reverse pulse. In some embodiments, a duration of the forward pulse is based on a material of the electrically conductive workpiece. In some embodiments, a duration of the reverse pulse is based an amount of time to depassivate the electrically conductive workpiece. 
     In some embodiments, a duration of the zero pulse is based on an amount of time to dissipate heat of the electrically conductive workpiece, replenish reacting species, remove reaction products, or any combination thereof. In some embodiments, the first force vector is based on a material of the electrically conductive workpiece. In some embodiments, the second force vector is based on a material of the electrically conductive workpiece. 
     In some embodiments, roughness average of the workpiece after the electromechanical grinding is about 8 micro-inches, about 4 micro-inches or less than or equal to 1 micro-inch. In some embodiments, the desired percentage of material removal is about 90%, 95% or 99% material removal. 
     In another aspect, the invention features an apparatus for electromechanical grinding. The apparatus includes an electrically conductive grinding wheel and an electrically conductive workpiece. The apparatus also includes a power source. The power source includes a first control circuit that applies a first voltage to the electrically conductive grinding wheel and the electrically conductive workpiece while a first force is applied toward the electrically conductive workpiece via the electrically conductive grinding wheel until a desired percentage of material is removed from the electrically conductive workpiece, and a second control circuit that, after the desired percentage of material is removed, applies a second voltage between the electrically conductive grinding wheel and the electrically conductive workpiece to further remove material from the electrically conductive workpiece while a second force is applied to the electrically conductive workpiece via the electrically conductive grinding wheel. 
     In some embodiments, the first voltage is a forward direct voltage or a forward pulse voltage. In some embodiments, the second voltage is a forward pulse followed by a reverse pulse. In some embodiments, the second voltage is a forward pulse followed by a zero pulse followed by a reverse pulse. In some embodiments, a duration of the forward pulse is based on a material of the electrically conductive workpiece. 
     In some embodiments, a duration of the reverse pulse is based an amount of time to depassivate the electrically conductive workpiece. In some embodiments, a duration of the zero pulse is based on an amount of time to dissipate heat of the electrically conductive workpiece, replenish reacting species, remove reaction products, or any combination thereof. 
     In some embodiments, the first force is based on a material of the electrically conductive workpiece. In some embodiments, the second force is based on a material of the electrically conductive workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale; emphasis instead is generally placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic representation of an electromechanical grinding system, according to an illustrative embodiment of the invention. 
         FIG. 2  is a schematic illustrating of a grinding wheel in contact with a workpiece according to an illustrative embodiment of the invention. 
         FIG. 3  is a graph of a waveform for an electromechanical grinding system, according to an illustrative embodiment of the invention. 
         FIG. 4A  is a graph of a waveform for an electromechanical grinding system during a first duration, according to an illustrative embodiment of the invention. 
         FIG. 4B  is a schematic illustration of a force for the electromechanical grinding system of  FIG. 4A  during the first duration, according to an illustrative embodiment of the invention. 
         FIG. 5A  is a graph of a waveform for an electromechanical grinding system during a second processing phase, according to an illustrative embodiment of the invention. 
         FIG. 5B  is a schematic illustration of a force for the electromechanical grinding system of  FIG. 5A  during the first duration, according to an illustrative embodiment of the invention. 
         FIG. 6  is a flow diagram exhibiting a method for electromechanical grinding, according to an illustrative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally, the invention involves a method for electromechanical grinding. Electrochemical grinding combines mechanical grinding with electrochemical machining. A grinding wheel is bonded with a metal conductor (e.g., copper, brass and/or nickel). The metal conductor is negatively charged and behaves as a cathode. The workpiece is positively charged and behaves as an anode. An ion conducting electrolyte is positioned in a gap between the grinding wheel and the workpiece. The electromechanical grinding occurs in two phases. During the first phase, a first voltage is applied to the grinding wheel and a first force vector is applied from the grinding wheel towards the workpiece to remove a first desired percentage of material from the workpiece. Once the desired percentage of material is removed from the workpiece, a second voltage is applied to the grinding wheel, and a second force vector is applied from the grinding wheel towards the workpiece to remove a second percentage of material. 
       FIG. 1  is a schematic representation of an electromechanical grinding system  100 , according to an illustrative embodiment of the invention. The electromechanical grinding system  100  includes a grinding wheel  110 , a workpiece  120 , a nozzle  130 , a work pan  140 , a reservoir  150 , an AC power supply  160 , an insulator  170 , a flow meter  180 , a pump  190 , and a clarifier  195 . 
     The AC power supply  160  is in electrical communication with the grinding wheel  110  and the workpiece  120 . The workpiece  120  is positioned inside of the work pan  140 . The workpiece  120  is in fluid communication with an electrolyte inside of the work pan  140  and the electrolyte flowing out of the nozzle  130 . The nozzle  130  is positioned near the grinding wheel  110  and the workpiece  120  at a location that allows the electrolyte flowing out of the nozzle  130  to flow into a gap that is between the grinding wheel  110  and the workpiece  120  and around the grinding wheel  110  and the workpiece  120 . 
     The nozzle  130  is in fluid communication with the flow meter  180 . The flow meter  180  is in fluid communication with the pump  190 . The pump  190  is in fluid communication with the clarifier  195 . The clarifier  195  is in fluid communication with the reservoir  150 . The reservoir  150  is in fluid communication with the work pan  140 . 
     During operation, alternating voltages and one or more grinding forces are applied to cause a physical and anodic electrochemical removal of material from the workpiece. The AC power supply  160  supplies a first voltage to the grinding wheel  110  and the grinding wheel  110  applies a first force towards the workpiece  120 . The nozzle  130  sprays an electrolyte between and around the grinding wheel  110  and the workpiece  120 . 
     The pump  190  pumps the electrolyte from the reservoir  150  through the flow meter  180 , which controls the flow of the electrolyte, to the nozzle  130 . The reservoir  150  receives the electrolyte from the work pan  140 . 
     The first voltage is applied to the grinding wheel  110  and the first force applied towards the workpiece  120 , an anodic electrochemical removal of material and a physical removal of material occur on the workpiece  120  at substantially the same time. 
     For example, turning to  FIG. 2 , an expanded view of contact between the grinding wheel  110  and the workpiece  120  is shown. In  FIG. 2 , the grinding wheel  110  has a point of contact  205  with the workpiece  120 . A gap  210  exists between the grinding wheel  110  and the workpiece  120 . The electrolyte flows through the gap  210 . In this manner, an anodic electrochemical reaction and a physical grinding together cause removal of material from the workpiece. 
     Turning back to  FIG. 1 , once a desired amount of material is removed from the workpiece  110 , a second force and a second voltage is applied to the workpiece  120 . The second force can be substantially zero such that there is no contact between the grinding wheel  110  and the workpiece  120 . With no contact between the grinding wheel  110  and the workpiece  120  and application of the second voltage to the workpiece  120 , only an anodic electrochemical reaction removes material from the workpiece. 
     In some embodiments, during the time duration of the first voltage and the first force, ninety percent of the total material to be removed from the workpiece is removed. In some embodiments, during the time duration of the second voltage and the second force, ten percent of the total material to be removed from the workpiece is removed. 
     In various embodiments, the workpiece  120  is high strength steels, nickel and nickel alloys, titanium and titanium alloys, niobium and niobium alloys, molybdenum and molybdenum alloys, tungsten and tungsten alloys, and aluminum and aluminum alloys, chrome-copper alloys, cobalt-chrome alloys and/or tantalum-tungsten alloys. 
       FIG. 3  is a graph of a waveform  300  for an electromechanical grinding system (e.g., electromechanical grinding system  100 , as described above in  FIG. 1 ), according to an illustrative embodiment of the invention. The waveform  300  has three durations. During a first duration, t a , a positive forward pulse (i.e., a positive anodic pulse) having a voltage amplitude V a  is applied to a workpiece (e.g., workpiece  120  as described above in  FIG. 2 ). During a second duration, time off, a zero pulse is applied. During a third duration, t c , a reverse pulse (i.e., a negative cathodic pulse) having a voltage amplitude V c  is applied. The cathodic pulse can be set to reduce oxide/depassivate a surface. 
     In some embodiments, the second duration is based on an amount of time it takes for heat dissipation, replenishing the reacting species, remove reaction products, or any combination thereof. 
     In various embodiments, the waveform  300  is any of the waveforms as shown in U.S. Pat. No. 6,558,231 to Taylor, the entire disclosure of which is herein incorporated by reference in its entirety. U.S. Pat. No. 6,558,321 shows examples of a two-step electrochemical process comprising a sequence of pulse/pulse reverse electric waveforms for electromachining. The two step process can be tuned to account for the beginning stages of the electromachining process where the surface roughness can be large and consequently the diffusion boundary follows or conforms to the surface profile and the later stages of the electromachining process where the diffusion boundary layer can be larger than the surface profile due to removal of surface asperities. 
     In some embodiments, the waveform  300  is any of the waveforms as shown in U.S. published Patent Application No. 2011/0303553 to Inman, the entire disclosure of which is herein incorporated by reference in its entirety. Inman shows the use of pulse/pulse reverse process for electrochemical machining of metals and alloys that form strongly passive surface layers. The process of Inman can be useful for electrochemical material removal from strongly passive work pieces including electrochemical machining or electrochemical polishing or electrochemical through-mask etching or electrochemical deburring and the like. The process can use aqueous sulfuric acid solutions. 
     In some embodiments, the waveform  300  is any of the waveforms as shown in U.S. Pat. No. 6,402,931 to Zhou, the entire disclosure of which is herein incorporated by reference in its entirety. Zhou shows a pulse/pulse reverse process for electrochemical machining of metals and alloys that form passive surface layers. The process can be useful for electrochemical material removal from passive work pieces including electrochemical machining or electrochemical polishing or electrochemical through-mask etching or electrochemical deburring. The process can use aqueous salt solutions of sodium nitrate and/or sodium chloride. 
       FIG. 4A  is a graph of a waveform  400  for an electromechanical grinding system during a first duration, t a , and a second duration, t o , according to an illustrative embodiment of the invention. During the first duration, t a , a forward pulse having a voltage amplitude, V a , is applied to a workpiece (e.g., the workpiece  120 , as shown above in  FIG. 1 ). At the same time, a first force is applied by an electrochemical grinding wheel  410  toward the workpiece.  FIG. 4B  is a schematic illustration of the first force for the electromechanical grinding system of  FIG. 4A , according to an illustrative embodiment of the invention. Once a predetermined amount of material is removed from the workpiece, the first duration, t a , ends, and the second duration, t o , begins. During the second duration, the waveform  400  transitions to a substantially zero voltage amplitude and the force from the electrochemical grinding wheel  410  towards the workpiece is substantially zero. 
       FIG. 5A  is a graph of a waveform  400  for the electromechanical grinding system of  FIG. 4B  during a third duration, t c , according to an illustrative embodiment of the invention. During the third duration, t c , a reverse pulse having a voltage amplitude, i c , is applied to the workpiece. At the same time a second force that is substantially zero is applied from the electrochemical grinding wheel  410  toward the workpiece.  FIG. 5B  is a schematic illustration of the force for the electromechanical grinding system of  FIG. 5A , according to an illustrative embodiment of the invention. 
       FIG. 6  is a flow diagram exhibiting a method  600  for electromechanical grinding, according to an illustrative embodiment of the invention. 
     The method involves rotating an electrically conductive grinding wheel (step  610 ), for example electrically conductive grinding wheel  110  as shown above in  FIG. 1 . 
     The method also a involves applying first voltage between the electrically conductive grinding wheel and an electrically conductive workpiece (step  620 ), for example workpiece  120 , as shown above in  FIG. 1 . In some embodiments the first voltage is a positive anodic voltage, for example, i a  as shown above in  FIG. 3 . 
     The method also involves applying a first force vector towards the electrically conductive workpiece of the electrically conductive grinding wheel to remove material from the electrically conductive workpiece (step  630 ). 
     The method also involves determining whether a desired amount of material has been removed (step  640 ). The determination can be based on taking a weight of the electrically conductive workpiece. The determination can be based on an amount of time that the workpiece has been under the influence of the forward pulse and the first force vector. If the desired amount of material has not been removed then steps  620  and  630  continue. If the desired amount of material has been removed, then the method involves applying a second force vector towards the electrically conductive workpiece (step  650 ). The second force vector is less than the first force vector. In some embodiments, the second force vector is zero. 
     The method also involves applying a second voltage between the electrically conductive grinding wheel and the electrically conductive workpiece to further remove material from the electrically conductive workpiece (step  660 ). 
     In various embodiments, the method  500  is used for the production of helicopter engines, artillery projectiles, large caliber cannons, turbine cooling technology and/or gun barrels. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.