DUAL CATHODE TOOLING DEVICE FOR ELECTROEROSION

Electroerosion devices and methods for performing electroerosion machining are disclosed. The electroerosion devices may perform simultaneous electrical discharge machining and pulsed electrochemical machining (S-ED/PEC) through the use of at least two different types of electrodes and a quasi-dielectric working fluid.

BACKGROUND

Electroerosion machining is a machining method that is generally used for machining hard metals or those that would be impossible to machine with other techniques using, e.g., lathes, drills, or the like. Thus, electroerosion machining can be used in trepanning or drilling operations for extremely hard steels and other hard, electrically conductive materials such as titanium, hastelloy, kovar, inconel, carbide, or the like. Different types of electroerosion machining include electrical discharge machining (ED) and electrochemical machining (EC).

Both EC and ED processes use electrical current under direct-current (DC) voltage to electrically power removal of the material from the workpiece. In EC, an electrically conductive liquid or electrolyte is circulated between the electrode(s) and the workpiece for permitting electrochemical dissolution of the workpiece material, as well as cooling and flushing the gap region therebetween. In ED, a nonconductive liquid or dielectric is circulated between the cathode and workpiece to permit electrical discharges in the gap therebetween for removing material from the workpiece.

SUMMARY

Due to differences of the working fluid needed for EC and ED processes, conventional electroerosion devices are directed to performing one process of either EC or ED, but not both.

In one aspect, disclosed are electroerosion devices comprising an electrode assembly defining a central axis, the electrode assembly comprising a first electrode and a second electrode, the second electrode having a first open axial end to receive the first electrode and a second axial end that is at least partially closed by an end wall; a fluid supply containing a working fluid having a resistivity from about 0.01 MΩ·cm to about 1.5 MΩ·cm; a working apparatus configured to translate the electrode assembly relative to the workpiece; a power supply for electrically powering the electrode assembly; and a control system configured to control the power supply and the working apparatus.

In another aspect, disclosed are electroerosion machining methods, the methods comprising driving an electrode assembly towards a workpiece, wherein the electrode assembly comprises a first electrode and a second electrode, the second electrode having a first open axial end to receive the first electrode and a second axial end that is at least partially closed by an end wall; supplying or discharging an electrical current between the electrode assembly and the workpiece while feeding a working fluid from a fluid supply through a gap defined therebetween, wherein the working fluid has a resistivity from about 0.01 MΩ·cm to about 1.5 MΩ·cm and comprises NaBr, NaCl, KCl, Na2SO4, HCl or combinations thereof; and performing electrical discharge machining (ED), pulsed electrochemical machining (PEC), or a combination thereof.

DETAILED DESCRIPTION

Disclosed herein are electroerosion devices for metal removal by simultaneous electrical discharge machining and pulsed electrochemical machining—(S-ED/PEC). The disclosed electroerosion devices can change from electrical discharge machining (ED) and pulsed electrochemical machining (PEC) modes (and vice versa) through the use of a dual-cathode electrode arrangement in combination with a quasi-dielectric fluid. The disclosed electroerosion devices, taking advantage of both ED and PEC processes, are able to provide improved precision, while lowering the overall cost of machining workpieces. In addition, the quasi-dielectric fluids disclosed herein are renewable resources, thereby decreasing the environmental impact of using the electroerosion devices.

The disclosed electroerosion devices combine electro-thermal discharge with electro-ionic dissolution which can provide S-ED/PEC cutting conditions. The pulsed signal can be divided in two on-time conditions for each ED and PEC process. The pulsed signal can be considered to increase the material removal rate (MRR), which is an important factor for ED processes. One of the characteristics of the disclosed ED/PEC hybrid devices is an increase in material removal efficiency, as indicated by a higher MRR, and a reduction in surface roughness. However, it is known that ED processes that can significantly improve MRR also typically result in providing a heat affect zone (HAZ), which can be detrimental to workpieces. In contrast to these known ED processes, the disclosed ED/PEC processes have been found to significantly increase MRR, while also reducing the HAZ by approximately 10 times at about 2 to 20 μm.

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. For example, if a concentration range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, are expressly enumerated in this specification. These are only examples of what is specifically intended, and all possible combinations of numerical values between and including the lowest value and the highest value enumerated are to be considered to be expressly stated in this application.

FIG. 1schematically illustrates an electroerosion device, such as a device10for performing ED, PEC, or both, in accordance with some embodiments of the invention. In an embodiment of the invention, the electroerosion device10is used to remove material from a workpiece70layer by layer to form a desired configuration. The electroerosion device10may comprise a working apparatus20, a numerical control (NC)40, a power supply60, a fluid supply30, an electroerosion controller50, and an electrode assembly100.

The electrode assembly100can be configured to machine any desired configuration in the workpiece70.FIG. 2illustrates an exemplary electrode assembly100of the electroerosion device10. The electrode assembly100comprises a first electrode140and a second electrode150, where the first electrode140may be used for PEC processes, and the second electrode150may be used for ED processes. The first electrode140includes a conductive metal base130and at least one conductive metal pole131having a first axial end arranged on the conductive metal base130and a second axial end extending from the conductive metal base130. In some embodiments including the illustrated embodiment, the at least one conductive metal pole131is a plurality of conductive metal poles. In some embodiments, the at least one conductive metal pole131comprises a set of 4 to 48 conductive metal poles (e.g., 14 poles as illustrated) arranged in a circular pattern on the metal base130. For example, the center of each pole131can be located on a pitch circle diameter centered about a central or “z”-axis of the electrode assembly100, and the poles131can have equal or unequal spacing distances therebetween. In embodiments that include a plurality of conductive metal poles, the poles131can be arranged in variable geometric arrangements. In some embodiments, the conductive metal poles131, at each occurrence, independently have a diameter from about 0.1 mm to about 3 mm. The conductive poles131may be solid or tubular.

The second electrode150may be secured to the electrode assembly100by being positioned within an outer metal ring160. For example, the second electrode150includes a lip or shoulder150A of increased diameter to be retained by the outer metal ring160when inserted through (see, e.g.,FIGS. 2-4, 7A and 7B).

The second electrode150can be formed to include multiple cylindrical walls including an outer cylindrical wall152, an outside of which defines the outer diameter of the second electrode150and an inner cylindrical wall154, an inside of which defines the inner diameter of the second electrode150as shown inFIG. 5D. The second electrode150is open to receive the first electrode140on a first axial end, while an end wall156extends between the outer and inner cylindrical walls152,154to at least partially close an opposite second axial end. An annular pocket or trough157(or plurality thereof), open to the first axial end to receive the first electrode140and having a second axial end proximal to the second axial end of the second electrode150, is defined between a radial inner surface of the outer cylindrical wall152and a radial outer surface of the inner cylindrical wall154.

As shown inFIG. 8, the at least one conductive metal pole131can be positioned within the annular pocket157where there is a gap β between the second axial end of the annular pocket157along the z-axis and the second axial end of the metal pole131along the z-axis. Accordingly, there may be a gap between the first electrode140and the second electrode150. The gap β between the second axial end of the metal pole131and the second axial end of the annular pocket157may be about 1 mm to about 3 mm. In an exemplary embodiment, the gap between the second axial end of the metal pole131and the second axial end of the annular pocket157is about 2 mm.

The second electrode150may have a plurality of fluid outlets151provided in the end wall156and a channel170that allow the working fluid to be fed towards the workpiece70. The channel170can be a central channel arranged or centered on the central or “z”-axis. The plurality of fluid outlets151may be arranged in variable geometric patterns. In addition, the plurality of fluid outlets151may be arranged around the channel170, where the channel170is in a central position. In some embodiments, the plurality of fluid outlets151align with the arrangement (e.g., matching pitch circle diameter) of the at least one conductive metal pole131, which can for example, match the fluid outlets151in number. The poles131can align with respective fluid outlets151or have a predetermined angular offset.

FIGS. 4-7Adepict that the second electrode150may have a patterned surface. As seen inFIGS. 4-7Athe end wall156of the second electrode150can include one or more fluid pathways158on a bottom side thereof (e.g., on the second axial end, opposite the annular trough157). Each of the fluid pathways158can have a spiral shape when viewed along the central “z”-axis. The spiral pathways158can extend radially outward to the radial outer extent of the outer cylindrical wall152to define respective outlets159. At a radial inner end, each spiral pathway158can open into the central channel170. Thus, a fluid dispersion pathway is established between the channel170and the radial outer edge of the second electrode150by each spiral pathway158. One or more of the spiral pathways158can further intersect one or more of the fluid outlets151that extend axially through the end wall156. For example, each one of the spiral pathways158can intersect with one of the fluid outlets151to establish a fluid dispersion pathway from the fluid outlet151to the outlet159of the respective spiral pathway158. The spiral pathways158can number less than the fluid outlets151. For example, the spiral pathways158may be half in number of the fluid outlets151. As such, every other one of the fluid outlets151, taken in a circumferential direction, may connect with one of the spiral pathways158, while the remaining fluid outlets151extend through the end wall156at locations between two adjacent spiral pathways158. Thus, the fluid outlets151that extend between adjacent spiral pathways158extend to outlets at a further extent toward the second axial end.

The first electrode140and the second electrode150are generally manufactured from conductive materials such as graphite, brass, copper, bronze, stainless steel, tungsten or combinations thereof. In an exemplary embodiment, the first electrode140is made of bronze. In an exemplary embodiment, the second electrode150is made of graphite or tungsten.

The electrode assembly100may have a tubular metal piece110. The electrode assembly100may be mounted to the working apparatus20through the tubular metal piece110, and in some embodiments the tubular metal piece110may be a rotative mandrel header. The electrode assembly100may also include a plastic ring120that tightly secures the tubular metal piece110to the base130of the first electrode140.

The electrode assembly100may be in fluid communication with the fluid supply30. As seen inFIGS. 2-4, the electrode assembly may have a channel170running through it, from the tubular metal piece110through the outer metal ring160. This channel allows fluid to flow through the electrode assembly and be fed towards the workpiece70.

The first electrode140may be arranged in the second electrode150with a working fluid (provided by the fluid supply30) inside the electrode assembly100. This can allow an electrical conduction through the working fluid (e.g., quasi-dielectric fluid) by the first electrode140and workpiece70producing an electrochemical reaction of ionic species in low concentrations, which can produce metallic (e.g., Fe) dissolution of the workpiece70and by water electrolysis redox reactions. Accordingly (and as discussed above), the first electrode140can perform PEC processes and the second electrode150can then be used to perform ED processes simultaneously, sequentially, or both on the workpiece70. In some embodiments, the electrode assembly100may be connected with electrical polarity opposite to a pulsed power source60to generate an over potential between the electrode-workpiece system arranged in order to perform the aforementioned ED, PEC, or both.

B. Fluid Supply

The electroerosion device10uses a fluid supply30to provide the working fluid to perform PEC, ED, or both. For example, the fluid supply30contains the working fluid and can be configured to feed the working fluid between the electrode assembly100and the workpiece70. The electroerosion device10uses a working fluid that is a quasi-dielectric fluid that may comprise deionized water. The quasi-dielectric fluid may include salts at varying concentrations. Examples of salts include, but are not limited to, NaBr, NaCl, KCl, Na2SO4and combinations thereof. In addition, the working fluid may include an acid, such as HCl. The quasi-dielectric fluid is a low resistivity working fluid. Low-resistivity, as used herein, refers to a fluid having a resistivity from about 0.01 MΩ·cm to about 1.5 MΩ·cm. In an exemplary embodiment, the working fluid has a resistivity from about 0.1 MΩ·cm to about 0.75 MΩ·cm. The resistivity of the quasi-dielectric fluid can be adjusted by varying small concentrations of total dissolved solids (TDS) within the fluid. For example, TDS can be added to the quasi-dielectric fluid at a few parts per million. The quasi-dielectric fluid is bi-characteristic in that it can act in a weak electrochemical reaction in PEC processes and simultaneously act as a quasi-dielectric for ED processes.

In some embodiments, the working fluid is fed into the inside of the electrode assembly100at high pressure by a pump system, and is ejected through the double electrode configuration to the workpiece70, while the electrode assembly100is rotating.

In some embodiments, the fluid supply30is in communication with and receives pre-programmed instructions from the NC40for feeding the working fluid between the electrode assembly100and the workpiece70. Alternatively, the fluid supply30may be fed separately.

C. Working Apparatus

The electroerosion device10includes a working apparatus20that is configured to move the electrode assembly100relative to the workpiece70, and which in turn can be controlled by a control system. In some embodiments, the NC40device is used to perform conventional automated machining. In some examples, the working apparatus20comprises a machine tool or lathe including servomotors (not shown) and spindle motors (not shown), which are known to one skilled in the art. The electrode assembly100may be mounted on the working apparatus20for performing electroerosion machining. The servomotors may drive the electrode assembly100and the workpiece70to move opposite to each other at a desired speed and path, and the spindle motors may drive the electrode assembly100to rotate at a desired speed. The electroerosion device may perform both milling and drilling processes via electroerosion techniques.

D. Power Supply

The power supply60provides electrical power to the electrode assembly100. In the illustrated embodiment ofFIG. 1, the power supply60comprises a direct current (DC) pulse generator. The electrode assembly100and the workpiece70may be connected to negative and positive poles of the power supply60, respectively. Accordingly, in some embodiments, the electrode assembly100functions as a cathode and the workpiece70may act as an anode. In other embodiments, the polarities on the electrode assembly100and the workpiece70are reversed. The power supply60may function at a range from about 1 kHz to about 100 kHz. In addition, the power supply60may have an open circuit voltage from about 50 VDC to about 80 VDC and about 20 amperes to about 800 amperes.

The electroerosion device10includes a control system, e.g., the NC40, for allowing communication between different elements of the electroerosion device10. The control system40may be configured to control the electrical power supplied by the power supply60, as well as both the rotation and the advance of the electrode assembly100.

In some embodiments, the NC40is a computer numerical controller (CNC). The CNC40comprises pre-programmed instructions based on descriptions of the workpiece70in a computer-aided design (CAD) and a computer-aided manufacturing (CAM), and may be connected to the working apparatus20to control the working apparatus20to drive the electrode assembly100to move, rotate or both according to certain operational parameters, such as certain feed rates, axes positions, or spindle speeds. In some embodiments, the CNC40is a general CNC and comprises central processing units (CPU), read only memories (ROM), random access memories (RAM), or both as known to one skilled in the art.

The CNC40may provide multi-axes movements of the electrode assembly100(e.g., x, y2and z directions, as well as rotation (w)) and workpiece70(e.g., y1direction) for 3D complex machining. For example, the CNC40determines a machining path for the positioning of the electrode assembly100on the workpiece70. The electrode assembly100may comprise electromechanical components that are in communication with the CNC40. The position(s) of the electrode assembly100may be controlled by an algorithm that defines the gap distance80(from nm to μm) on axis “z”, which can be critical for the removal of material in a controlled manner via PEC, ED, or both. In addition, the algorithm may control the movement of the electrode assembly100along the surface contour of the workpiece70with movements on axes “x, y” according to a machining sequence. The algorithm can control the adjustment of penetration position of the “z”-axis to maintain the S-PEC/ED processes. The electroerosion device10may be operated by the algorithm having pulsed electrical signals in voltage and current relationship that are fed into the algorithm, such as fuzzy logical, neural network or heuristic rules. The control algorithm may allow self-adjustment of the gap distance80in “z”-axis between the electrode assembly100and workpiece70, as well as electrical parameters of the discharge condition and electro-dissolution conditions for improved removal of material.

The electroerosion device10may include an electroerosion controller50connected to the power supply60to monitor the status of the power supply60. In some embodiments, the electroerosion controller50comprises one or more sensors (not shown), such as a voltage and/or current measurement circuit for monitoring the status of voltages and/or currents in the gap80between the electrode assembly100and the workpiece70. In other embodiments, the sensor(s) is comprised in the power supply60. In yet other embodiments, the sensor(s) is not comprised in the electrode assembly100or power supply60, but rather are separate individual units relative to the electrode assembly100and power supply60. In some embodiments, the electroerosion controller50comprises a microprocessor or another computational device, a timing device, a voltage comparison device, and/or a data storage device to be served as the sensor(s), as known to one skilled in the art. Additionally, the electroerosion controller50may communicate with the NC40to control the power supply60and the movement of the working apparatus20holding the electrode assembly100.

3. Methods of Using the Electroerosion Devices

Also disclosed herein are methods of performing electroerosion machining methods using the electroerosion device10as described above. The methods may include the first electrode140performing PEC processes and the second electrode150preforming ED processes using the same electrical pole via open circuit voltage under direct-current (DC) supplied by a power supply60capable of providing puled electrical power. This may allow removal of the material from the workpiece70in two simultaneous ways using a fluid medium, e.g., quasi-dielectric working fluid. In PEC processes, the quasi-dielectric working fluid is circulated between the first electrode140and the workpiece70, permitting electrochemical dissolution of the workpiece70material, as well as cooling and flushing the gap region80therebetween. In ED processes, the quasi-dielectric working fluid is circulated between the second electrode150and the workpiece70to permit electrical discharges in the gap80therebetween for removing the workpiece70material. This operation of S-PEC/ED processing may increase the material removal rate with minimum (or without) formation of a recasting molten layer or white-layer when, e.g., drilling holes in a high strength steel (HSS) or other materials having high hardness.

The second electrode150has a depth length F (see, e.g.,FIGS. 5C and 8) that can be greater than the thickness of the workpiece70being machined. For example, the second electrode150may have a depth length F from about 5 mm to about 25 mm. The depth length F may be measured as the axial length of the second electrode150, less the portion forming the lip or shoulder150A, which is not exposed for operation, but rather retained within the outer metal ring160. In addition, the second electrode150may have a separation distance from the workpiece70when performing ED processes from about 10 μm to about 100 μm, such as from about 20 μm to about 90 μm or from about 30 μm to about 80 μm. In an exemplary embodiment, the second electrode150has a separation distance from the workpiece70when performing ED processes of about 60 μm. The combination of depth length F of the second electrode150, the separation distance of the second electrode150from the workpiece70, and the gap β between the first electrode140and the second electrode150are all useful parameters for performing the disclosed electroerosion machining methods.

In some embodiments, the electroerosion device10may provide a diameter of drilling having a range from about ¼ inch to about 3 inches. The diameter of drilling can be in part a function of the dimensions of the first electrode140and the second electrode150. Accordingly, by altering the dimensions and arrangement of the first electrode140and the second electrode150, the diameter of drilling for the electroerosion device10may be altered.