An electrical machining device

Abstract

An electrical machining device 10 (EDM) includes a nozzle 14 connected by a flow path 20 to an electrolyte reservoir 18. The flow path has a gas inlet 22 and the device injects a gas to form gas bubbl

Description

FIELD OF THE INVENTION The present invention relates to an electrical machining device for machining a 5 surface of a workpiece. BACKGROUND OF THE INVENTION Electrochemical machining and electrical machining, known as electrical arc discharge machining, are known processes for machining a surface of a workpiece. Electrochemical machining machines the surface of a workpiece using 10 electrolysis, whereby two conductive electrodes of opposite polarities form a cell in an ionic solution in order to dissolve a surface of an electrode (i.e. the surface of a workpiece). Electrical arc discharge machining machines the surface of a workpiece using ablation caused by electrical arc discharge between two opposing conductive electrodes as current is attempted to be put through a dielectric 15 therebetween. Through both of these processes, machining of surfaces can be used to roughen the surface to improve bonding for mounting components and applying coatings to the surface. Surface machining can also be used for modifying the optical and / or tribological properties of the surface, or polishing the surface to produce a homogenous surface finish. However, both of these 20 processes require the surface of a workpiece to be conductive so as to form the counter electrode. Electrical machining typically requires complex pulsing and very high electrical power to create arc discharge, and may generate considerable amounts of heat at the surface of the workpiece as well as causing electrode wear. Electrical machining also traditionally requires the machining process to be 25 carried out under submerged or confined conditions. The present invention seeks to overcome or at least mitigate one or more problems associated with the prior art. SUMMARY OF THE INVENTION According to a first aspect, there is provided an electrical machining device for 30 machining a surface of a workpiece, the electrical machining device comprising: a nozzle connectable to an electrolyte reservoir, the nozzle configured to dispense an electrolyte jet towards a surface of a workpiece, in use; and an electrolyte flow path for conveying electrolyte from an electrolyte reservoir to the nozzle; wherein the electrolyte flow path comprises a gas inlet therealong, and wherein, in use, the electrical machining device is configured to inject a gas into electrolyte flowing 5 along the electrolyte flow path via the gas inlet so as to form gas bubbles in said electrolyte. The electrolyte machining device is configured to adjust the size of the gas bubbles formed in the electrolyte. This arrangement is advantageously provides a new approach to large area surface texturing by creating a high-speed discharge type machining phenomenon 10 that can be carried out on nonconductive surfaces without the need of adaption of the materials to be machined only requiring simple power supply producing comparatively low electrical power thus creating a much more efficient and effective method of surface texturing large areas of nonconductive materials by inclusion of gas bubbles in the electrolyte stream to increase resistance at a 15 discrete point causing arcing. The electrical machining may be configured to apply a voltage to the nozzle such that the nozzle defines a first electrode. The electrical machining may be configured to apply a voltage to surface electrolyte on a surface of a workpiece, in use, such that said surface electrolyte 20 defines a second electrode. The electrical machining may comprise a conductive member configured an arranged to apply a voltage to surface electrolyte on a surface of a workpiece, in use. The conductive member may be configured and arranged to apply a voltage to 25 surface electrolyte on a surface of a workpiece, in use, at a position laterally spaced from the nozzle. The electrical machining may be configured to apply a voltage to the nozzle and surface electrolyte on a surface of a workpiece, in use, to generate an electrical arc discharge. 30 The electrical arc discharge may be generated at the nozzle and impacts upon a surface of a workpiece, in use. The nozzle may be arranged so as to be spaced apart from a surface of a workpiece, in use. The electrical machining may comprise an electrolyte reservoir, and the electrolyte flow path may be connected between said electrolyte reservoir and the 5 nozzle. The electrical machining may comprise a gas source connected to the gas inlet for supplying gas thereto. The electrical machining may be configured to inject compressed gas into the electrolyte flow path. 10 The compressed gas may comprise a pressure in the range 0.5-5 bar, for example approximately 2 bar. The gas may be selected from one or more of compressed air, argon, nitrogen, helium, neon, krypton, xenon, radon, and carbon dioxide. The gas inlet may define an area, and the size of said area may be adjustable. 15 The electrical machining may comprise a valve at or near the gas inlet to adjust the area of the gas inlet. The electrical machining may be configured to adjust a pressure of the gas injected into the electrolyte flow path. The electrolyte may comprise an ionic solvent. 20 The ionic solvent may have a conductivity of at least 8000 pS / cm. The ionic solvent may comprise an inorganic salt solution. The inorganic salt solution is of a molar concentration of at least 0.1M. The inorganic salt solution may comprise compounds of the formula MX, where M is selected from Na+, K+, Ca2+, Mg2+, Cu2+ and Zn2+, or combinations 25 thereof, and X is selected from F-, CI-, Br-, I-, NO3- and SO42-, or combinations thereof. The electrolyte may comprise a water based solution According to a second aspect there is provided an electrical machining process for machining a surface of a workpiece using an electrical machining device comprising a nozzle connectable to an electrolyte reservoir, an electrolyte flow path for conveying electrolyte from said electrolyte reservoir to the nozzle, said 5 the electrolyte flow path comprising a gas inlet therealong, the electrical machining process comprising the steps of: conveying electrolyte along the electrolyte flow path; injecting a gas into the electrolyte within the electrolyte flow path via the gas inlet so as to form gas bubbles in said electrolyte; dispensing a jet of the electrolyte from the nozzle towards a surface of a workpiece; applying a 10 charge to the nozzle and applying a charge to dispensed electrolyte solution on a surface of a workpiece at a position laterally spaced from the nozzle, such that the nozzle and the electrolyte on said surface of a workpiece form first and second electrodes; and generating an electrical arc discharge at the nozzle. The process includes the step of adjusting the size of the gas bubbles formed in the 15 electrolyte. I U / The process may comprise applying applies a voltage in the range 10-600V. CM LO The process may comprise applying a current in the range 1-10 Amps. BRIEF DESCRIPTION OF THE DRAWINGS CM Embodiments of the invention will now be described with reference to the 20 accompanying drawings, in which: Figure 1 shows a schematic view of an electrical machining device according to an embodiment; and Figure 2 shows a schematic view of a nozzle of the electrical machining device of Figure 1. 25 DETAILED DESCRIPTION OF EMBODIMENT(S) Referring firstly to Figure 1, an electrical machining device 10 for machining a surface 12 of a workpiece is illustrated. The electrical machining device 10 may be considered to be an electrical arc discharge machining device. The electrical machining device 10 includes a nozzle 14. The nozzle 14 is 30 configured to dispense electrolyte towards the surface 12 of a workpiece. Put another way, the nozzle 14 is configured to dispense an electrolyte jet 16 towards the surface 12 of a workpiece. It will be appreciated that the nozzle 14 may be removably mounted to the electrical machining device 10 to allow for different nozzles to be used for different machining operations. The electrical machining device 10 includes an electrolyte reservoir 18 for 5 supplying electrolyte to the nozzle 14. The nozzle 14 is connected to the electrolyte reservoir 18. In alternative arrangements, it will be appreciated that the electrical machining device 10 may not include the electrolyte reservoir 18, and may instead simply be connectable to a reservoir separate from the electrical machining device 10. In such alternative arrangements, it will be appreciated that 10 the nozzle 14 is connectable to the electrolyte reservoir. An electrolyte flow path 20 is provided so as to convey electrolyte from the electrolyte reservoir 18 to the nozzle 14. The electrolyte flow path 20 is connected between said electrolyte reservoir 18 and the nozzle 14. The electrolyte flow path 20 is provided with a gas inlet 22 therealong. The electrical machining device 10 15 includes a gas source 24. The gas source 24 is connected to the gas inlet 22 via a gas flow path 26 for supplying gas to said gas inlet 22 and injecting gas into electrolyte flowing along the electrolyte flow path 20. The electrical machining device 10 is configured to apply a charge to the nozzle 14 such that the nozzle 14 defines a first electrode. In alternative arrangements, 20 the electrical machining device 10 may include a first electrode that is separate from the nozzle 14, and the electrical machining device 10 may be configured to apply a charge to said first electrode. The separate electrode may be positioned to be adjacent to the nozzle 14, for example. As electrolyte is dispensed from the nozzle 14 towards the surface 12 of the 25 workpiece, the dispensed electrolyte builds up on said surface 12. Put another way, dispensed electrolyte builds up as surface electrolyte 17 on the surface 12 of a work piece during use of the electrical machining device 10. The electrical machining device 10 is configured to apply a charge to surface electrolyte 17 on the surface 12 of a workpiece. In this way, the surface 30 electrolyte 17 forms a second electrode. The electrical machining device 10 includes a conductive member 28 arranged to terminate at or proximate to the surface 12 of a workpiece. The electrical machining device 10 is configured to apply a charge to said conductive member 28 to apply a charge to the surface electrolyte 17. The conductive member 28 is arranged to apply a charge to the surface electrolyte 17 at a position laterally spaced from the nozzle 14. The conductive member 28 may be referred to as a contact electrode. The electrical machining device 10 is arranged such that the nozzle 14 is spaced apart from the surface 12 of a workpiece such that the first and second electrodes 5 are spaced apart. The spacing between the electrode 14 and the surface 12 of the workpiece may affect the machining of said surface 12. The nozzle 14 may be moveable / adjustable such that the spacing between the nozzle 24 and the surface 12 can be adjusted to suit a particular machining operation. The electrical machining device 10 is configured to apply an electrical charge to 10 the first and second electrodes, in use, to generate an electrical arc discharge. Put another way, the electrical machining device 10 is configured to apply an electrical charge to the nozzle 14 and the surface electrolyte 17, in use, to generate an electrical arc discharge. The electrolyte in the electrolyte jet 16 is conductive such that said electrolyte jet 15 16 provides an electrical connection between the first and second electrodes. Put another way, the electrolyte jet 16 provides an electrical connection between the nozzle 14 and the surface electrolyte 17. The ionic solvent has a conductivity of at least 8000 pS / cm, for example at least 8500 pS / cm. In some arrangements, the ionic solvent has a conductivity of at 20 least 9000 pS / cm. Providing the electrolyte with a conductivity above this minimum facilitates the generation of an electrical arc discharge. The electrolyte solution may be provided in the form of an ionic solvent. In the embodiment, the electrolyte is a water-based electrolyte. The electrolyte is provided as a water-salt solution, for example a water based solution comprising 25 an ionisable compound. It will be appreciated that any suitable electrolyte may be used, such as a substantially water free ionic solvent. It will be appreciated that any suitable solvent may be used that is capable of having a salt dissolved therein, for example the solvent may be a substantially water free solvent such as one or more of ethylene glycol, glycerol, methanol, ethanol, 1-propanol, 2-30 propanol and / or propylene glycol. The ionic solvent comprises an inorganic salt solution. The inorganic salt solution is of a molar concentration of at least 0.1M, for example at least 0.5M. The inorganic salt solution comprises compounds of the formula MX. M is selected from Na+, K+, Ca2+, Mg2+, Cu2+ and Zn2+, or combinations thereof. X is selected from F', Cl’, Br, T, NO3- and SCU2‘, or combinations thereof. M will often by a group I metal, such as Na+ or K+ and X will often be a halogen, such as F‘, Cl’, Br , r. 5 Whilst not essential, in order to maintain the sustainability of the machining process in an industrial setting, the electrolyte usefully may not be a highly toxic and / or a highly acidic / alkali solution. This also allows the electrolyte solution to maintain a low environmental impact solution. The electrolyte may be substantially neutral. Put another way, the pH of the electrolyte solution may be 10 in the range of 5 to 9, or 6-8. In order to be able to apply a charge to the nozzle 14 and the surface electrolyte 17, the electrical machining device 10 includes a power source 30. It will be appreciated that in order to supply power to the electrical machining device 10, the power source 30 may include one or more batteries or may be connectable to 15 an external power source. Referring now to Figure 2, the nozzle 14 and gas inlet 18 are illustrated in more detail. As discussed above, the electrical machining device 10 is configured to inject a gas into electrolyte flowing along the electrolyte flow path 20 via the gas inlet 22. 20 The injection of gas into the electrolyte forms gas bubbles 32 in said electrolyte. Imparting or injecting gas bubbles 32 into the electrolyte jet 16 displaces the conductive electrolyte with a dielectric gas so as to reduce the conductivity of the electrolyte jet 16. Put another way, imparting or injecting inert gas bubbles 32 into the electrolyte jet 16 works to block the electrical path between the 25 electrodes (i.e. between the nozzle 14 and the surface electrolyte 17) through the electrolyte jet 16. The gas bubbles created in the electrolyte jet 16 result in an increase in the resistivity of the electrolyte solution (i.e. of the electrolyte jet 16). When the electrolyte jet 16 impacts the surface 12 of a workpiece, the gas bubbles 32 30 contained therein are expelled from the electrolyte onto the surface. This results in an increased conductivity of the surface electrolyte 17 compared to the electrolyte jet 16. Due to the increased electrical resistance of the electrolyte jet 16 by the presence of the gas bubbles 32, when the electrical machining device 10 applies a sufficient voltage to the nozzle 14 and the surface electrolyte 17, an electrical arc discharge occurs. The electrical arc discharge occurs at the nozzle 14. The electrical arc 5 discharge extends to the surface electrolyte below the nozzle 14 and impacts the surface 12 of the workpiece. The generated electrical arc discharge machines the surface 12 of the workpiece. In this way, electrical arc discharge machining is performed due to the presence of a conductive surface electrolyte, even if the surface 12 of the workpiece is not conductive. 10 It will be appreciated that the electrical machining device 10 may be configured to inject compressed gas into the electrolyte flow path 20. Put another way, the gas source 24 may be a source of compressed gas. The compressed gas may have a pressure in the range 0.5-5 bar, for example approximately 2 bar. It will be appreciated that the electrical machining device 10 may be configured to adjust 15 the pressure of the gas so as to adjust the size of the gas bubbles formed in the U / electrolyte. CM LO The gas injected into the electrolyte flow path may be one or more of helium, neon, argon, krypton, xenon, radon, carbon dioxide, compressed air or any other suitable gas. The gas injected into the electrolyte flow path is used to increase the 20 electrical resistance of the electrolyte jet 16, and it will be appreciated that a different gas may be used to machine different surfaces for different purposes. The gas may be an inert gas. The electrical machining device 10 is configured to adjust the size of the gas bubbles 32 formed in the electrolyte. The inlet aperture of the gas inlet 22 may be 25 adjustable to adjust the size of the gas bubbles 32 formed in the electrolyte. Put another way, gas inlet 22 defines an area, and the size of said area is adjustable. The electrical machining device 10 may include a valve that is operable to adjust the area of the gas inlet 22. The electrical machining device 10 may be configured to adjust the pressure of the gas injected into the electrolyte flow path 20 to 30 adjust the size of the gas bubbles 32 formed in the electrolyte. Although the electrical machining of a material surface has been described with reference to the electrical machining device illustrated in Figures 1 and 2, it will be appreciated that any suitable electrical machining device configured to dispense electrolyte solution towards a surface may be used. In some 35 arrangements, the nozzle 14 form part of a machining unit that may be controlled by a robotic arm 52, e.g. as a part of an automated production line. In such arrangements, the machining unit may be configured such that it is able to be moved over a surface of a workpiece by the robotic arm without having to remove and the re-attach said machining unit. This arrangement enables machining to be 5 carried out continuously over the surface. In further arrangements, the nozzle 14 may form part of a machining unit that is configured to be driven over the surface. In order to drive the machining unit over the surface, a drive arrangement may be provided to move the machining unit over said surface. 10 An electrical machining process for machining a surface 12 of a workpiece using an electrical machining device 10 will now be described. Electrolyte is conveyed along the electrolyte flow path 20. A gas is injected into the electrolyte within the electrolyte flow path 20 via the gas inlet 22. The _ injection of the gas into the electrolyte forms gas bubbles 32 in said electrolyte. 10 CM 15 The electrolyte containing the gas bubbles 32 continues to flow along the electrolyte flow path 20 into the nozzle 14. LO Electrolyte is dispensed from the nozzle 14 towards the surface 12 of a workpiece. Put another way, the nozzle 14 dispenses an electrolyte jet 16 towards the surface 12 of a workpiece. 20 The electrical machining device 10 applies a charge to the nozzle 14. The electrical machining device 10 applies a charge to dispensed electrolyte solution on the surface 12 of a workpiece. The electrical machining device 10 applies a charge to dispensed electrolyte at a position laterally spaced from the nozzle 14. In this way, the nozzle 14 and the electrolyte on said surface 12 form first and 25 second electrodes of a cell defining a gap therebetween. The electrical machining device 10 applies an electrical charge to the first and second electrodes, in use, to generate an electrical arc discharge. The electrical machining device 10 may be configured to apply a potential of less than 600 V. Often, the electrical machining device 10 may apply a potential in the 30 range 10V to 600 V. The electrical machining device 10 may be configured to apply a charge of less than 10A. Often, the electrical machining device 10 may apply a charge in the range 1-10A. LD Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. An electrical machining device for machining a surface of a workpiece, the electrical machining device comprising:a nozzle connectable to an electrolyte reservoir, the nozzle 5 configured to dispense an electrolyte jet towards a surface of a workpiece,in use; andan electrolyte flow path for conveying electrolyte from an electrolyte reservoir to the nozzle, said electrolyte flow path comprising a gas inlet therealong,10 wherein, in use, the electrical machining device is configured toinject a gas into electrolyte flowing along the electrolyte flow path via the gas inlet so as to form gas bubbles in said electrolyte, andwherein the electrolyte machining device is configured to adjust the size of the gas bubbles formed in the electrolyte.inCU 15 2. The electrical machining device according to claim 1, configured to apply avoltage to the nozzle such that the nozzle defines a first electrode.LOC*^ 3. The electrical machining device according to claim 1 or claim 2, configuredto apply a voltage to surface electrolyte on a surface of a workpiece, in ■-j— use, such that said surface electrolyte defines a second electrode.20 4. The electrical machining device according to claim 3, comprising aconductive member configured an arranged to apply a voltage to surface electrolyte on a surface of a workpiece, in use.

5. The electrical machining device according to claim 4, wherein the conductive member is configured and arranged to apply a voltage to 25 surface electrolyte on a surface of a workpiece, in use, at a positionlaterally spaced from the nozzle.

6. The electrical machining device according to any preceding claim, configured to apply a voltage to the nozzle and surface electrolyte on a surface of a workpiece, in use, to generate an electrical arc discharge.30 7. The electrical machining device according to claim 6, wherein the electricalarc discharge is generated at the nozzle and impacts upon a surface of a workpiece, in use.

8. The electrical machining device according to any preceding claim, wherein the nozzle is arranged so as to be spaced apart from a surface of a workpiece, in use.

9. The electrical machining device according to any preceding claim,5 comprising an electrolyte reservoir, wherein the electrolyte flow path isconnected between said electrolyte reservoir and the nozzle.10.The electrical machining device according to any preceding claim,comprising a gas source connected to the gas inlet for supplying gasthereto.10 11.The electrical machining device according to any preceding claim,configured to inject compressed gas into the electrolyte flow path.12.The electrical machining device according to claim 11, wherein the compressed gas comprises a pressure in the range 0.5-5 bar.13.The electrical machining device according to any preceding claim, wherein 15 the gas is selected from one or more of compressed air, argon, nitrogen,helium, neon, krypton, xenon, radon, and carbon dioxide.14.The electrical machining device according to any preceding claim, wherein the gas inlet defines an area, and the size of said area is adjustable.15.The electrical machining device according to claim 14, comprising a valve 20 at or near the gas inlet to adjust the area of the gas inlet.16.The electrical machining device according to any preceding claim, configured to adjust a pressure of the gas injected into the electrolyte flow path.17.The electrical machining device according to any preceding claim, 25 comprising an electrolyte in the electrolyte flow path, wherein theelectrolyte comprises an ionic solvent.18.The electrical machining device according to claim 17, wherein the ionic solvent has a conductivity of at least 8000 pS / cm.19.The electrical machining device according to claim 17 or claim 18, wherein 30 the ionic solvent comprises an inorganic salt solution.20.The electrical machining device according to claim 19, wherein the inorganic salt solution is of a molar concentration of at least 0.IM.21.The electrical machining device according to claim 19 or claim 20, wherein the inorganic salt solution comprises compounds of the formula MX, where 5 M is selected from Na+, K+, Ca2+, Mg2+, Cu2+ and Zn2", or combinationsthereof, and X is selected from F', Cl’, Br, T, NCb’ and SO42’, or combinations thereof.22.The electrical machining device according to any preceding claim, wherein the electrolyte comprises a water based solution.10 23. An electrical machining process for machining a surface of a workpieceusing an electrical machining device comprising a nozzle connectable to an electrolyte reservoir, an electrolyte flow path for conveying electrolyte from said electrolyte reservoir to the nozzle, said the electrolyte flow path comprising a gas inlet therealong, the electrical machining process 15 comprising the steps of:conveying an electrolyte along the electrolyte flow path;injecting a gas into the electrolyte within the electrolyte flow path via the gas inlet so as to form gas bubbles in said electrolyte;dispensing a jet of the electrolyte from the nozzle towards a surface 20 of a workpiece;applying a charge to the nozzle and applying a charge to dispensed electrolyte solution on a surface of a workpiece at a position laterally spaced from the nozzle, such that the nozzle and the electrolyte on said surface of a workpiece form first and second electrodes; and25 generating an electrical arc discharge at the nozzle,wherein the process further comprises the step of adjusting the size of the gas bubbles formed in the electrolyte.24.The electrical machining process according to claim 23, comprising applying applies a voltage in the range 10-600V.30 25.The electrical machining process according to claim 23 or claim 24,comprises applying a current in the range 1-10 Amps.

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