Patent Grant
8667824
1. Technical Field
The present invention relates to the structure and operation of electrode assemblies for electro-hydraulic forming processes.
2. Background
Electro-hydraulic forming (EHF) is a process in which a high voltage, stored charge is discharged across spaced electrodes that are disposed in a fluid filled chamber. An EHF system generally includes a chamber that is filled with fluid, for example water with a rust preventative. The electrodes are immersed in the fluid within the chamber. A sheet metal blank is placed on the chamber. A one-sided die is then placed on the blank. Air is then evacuated from both sides of the blank. The capacitor bank stores a charge and may provide between 5 to 50 kV through the electrodes.
The voltage applied to the electrodes creates a high temperature plasma channel. Current from the capacitors expands the plasma channel and fills the region surrounding the plasma channel with gas in the form of super heated steam which then transitions to a steam/water interface. Pressure builds within the chamber and results in a high intensity high velocity shock wave being formed in the liquid that is transmitted to the sheet metal blank. The sheet metal blank is driven into the die by the high velocity shock wave.
Traditional advantages over conventional sheet metal forming processes that are known include reduction in capital cost because only a one-sided die is required to form a panel. Recently, EHF processes have been proposed for forming sheet metal materials that are more difficult to form than low carbon steel. For example, the EHF process could be used to manufacture automotive and truck components from high strength steel, stainless steel or aluminum alloys. EHF processes are generally targeted at low volume stamping processes that may take advantage of the savings resulting from the use of a one-sided die.
The electrode assembly is subject to erosion caused by the shock wave and high temperature plasma. In addition, the fluid in the chamber tends to be corrosive. These conditions lead to erosion of both electrodes and insulation that fills the gap between the chamber and the electrodes. The shock wave resulting from discharge of the capacitors through the electrodes exerts a force on the electrode assembly tending to expel the electrode and insulation from the chamber. The electrode system must withstand the high pressure and high temperature plasma, as well as the corrosive water in the chamber. The electrodes must conduct energy into the chamber, maintain electrical isolation through the insulation, maintain the chamber in a water tight condition and constrain electrode motion during pressure pulses tending to eject the electrode from the chamber.
These and other problems are addressed in applicants' development as summarized below.
Applicants' development, as summarized below, is directed to providing a robust electrode assembly for the EHF forming that resists erosion of the electrode, erosion of the insulation material, expulsion of the electrode or its insulation from the chamber. The electrode assembly must also be adjustable to assure maintenance of the desired inter-electrode gap. In addition, the disclosure also provides for a renewable replaceable electrode tip and ways to reduce erosion and wear of the insulation.
According to one aspect of the present development, an electrode assembly for an electro-hydraulic forming tool that has a fluid filled chamber is provided. The electrode assembly includes an electrode rod upon which an insulation sleeve is assembled. The insulation sleeve has a chamber end and a connector end. The insulation sleeve has a deflection surface at the chamber end that forms a frustum of cone between the electrode rod and the chamber. The deflection surface is narrowest near the electrode to deflect the force of a discharge of the electro-hydraulic forming tool.
According to other aspects of the invention, the deflection surface may be formed as a tapered part of the insulation sleeve on the inner end of the insulation sleeve. Alternatively, the deflection surface may be formed on a metal cap that is assembled over the electrode rod and against the inner end of the insulation sleeve.
According to other aspects of the invention relating to providing an adaptor, an adaptor may be disposed about the insulator sleeve that secures electrode and the insulation sleeve in an opening defined by the chamber. A holder may be provided that receives the electrode and insulation sleeve that is in turn received within the adaptor.
The insulator sleeve may have a reaction surface disposed between the chamber end and the connector end. The reaction surface may be partially conical in shape with its narrowest end at the end of the reaction surface that is furthest from the chamber end of the insulation sleeve. The adaptor may be disposed about the insulation sleeve to secure the electrode and the insulation sleeve in an opening defined by the chamber. A holder may be provided that receives the electrode and insulation sleeve and that is received within the adaptor. The holder may have a conical force receiving surface that is engaged by a reaction surface to hold the insulation sleeve within the holder. A retainer may be attached to the holder to secure the insulation sleeve and the electrode rod within the holder. A lock ring may be provided that locks the retainer to the holder.
According to another aspect of the disclosure, an electrode assembly may be provided for an electro-hydraulic forming tool that has a chamber. The electrode assembly may comprise an electrode rod, an insulation sleeve assembled over the electrode rod with the sleeve having an inner end that is closest to the chamber than the outer end. An adaptor may be disposed about the insulation sleeve to secure the electrode in the insulation sleeve in an opening defined by the chamber.
A conical receptacle may be assembled to the adaptor that defines an opening. A replaceable insulator tip may be assembled within the opening in the conical receptacle. The chamber end of the insulation sleeve may be received within the insulator tip. The conical receptacle may retain the insulator tip when the electrode assembly is discharged and the conical receptacle is removed from the adaptor to replace the insulator tip.
According to other aspects of the invention as they relate to an electrode assembly having a replaceable insulator tip, the adaptor may define the cup-shaped end that is disposed within the chamber. The conical receptacle may be at least partially received within the cup-shaped end of the adaptor.
A holder may be provided that receives the electrode and the insulation sleeve. A retainer may be attached to the holder to secure the insulation sleeve and the electrode rod within the holder. A lock ring may be provided that locks the retainer to the holder.
According to another aspect of the disclosure, an electrode assembly for an electro-hydraulic forming tool may be provided. The tool has a chamber that receives an electrode rod that extends from outside of the chamber to inside the chamber. The electrode rod may be provided with a main portion and a tip. An insulation sleeve is preferably assembled over the electrode rod with the insulation sleeve having a chamber end and a connector end. A holder attaches the electrode rod and insulation sleeve to the chamber.
Other aspects of the disclosure relate to providing a tip portion that is formed with different material than the main portion of the electrode rod with the tip portion being welded to the main portion. The tip portion may be selected from a more erosion resistant and durable material such as low carbon steel, tungsten, molybdenum, or a tungsten-copper alloy. The main portion may be selected from the group consisting essentially of low carbon steel. The tip portion may have a tip profile that is a flat tip with rounded edges that conform to the shape of a cylindrical electrode after a substantial number of pulses.
These and other aspects of the present invention would be better understood with the attached drawings and the following detained description of the illustrated embodiments.
Referring to
An insulation sleeve 24 has an insulation tip 26 on the side of the insulation sleeve 24 adjacent the chamber end 20 of the electrode rod 12. A deflection surface 28 is formed on the insulation tip 26 to deflect the force of an electro-hydraulic discharge from directly impacting the insulation sleeve 24. The deflection surface 28 may be bevelled or formed as a frustum of a cone.
Alternatively, and as shown in
A reaction surface 30 distributes any force applied to the insulation sleeve 24 by the electro-hydraulic discharge to the other parts of the electrode assembly. An engagement surface 31 on the insulation sleeve 24 is in contact with the reaction surface 30 as shown in
An adaptor 40 may be provided between the chamber end 20 of the electrode rod 12 and the insulation sleeve 24 and the chamber wall 16. The adaptor preferably includes a bevelled or a frustoconical surface that extends to a shoulder 42. The shoulder 42 is placed into direct contact with the EHF chamber wall 16. The shoulder 42 is assembled against the EHF chamber wall 16.
A holder 44, as best shown in
A retainer 54 holds the insulation sleeve 24 within the holder 44. The retainer 54 and locking ring 56 combine to hold the insulation sleeve 24 against the holder 44. The holder 44, as shown in
Referring to
The insulation sleeve 72 includes an insulation tip cap 74. The insulation tip cap is a bevelled or frusto-conical surface. A replaceable insulation tip 76 provides the insulation tip cap 74 and is assembled to the chamber end of the insulation sleeve 72. The replaceable insulation tip 76 may be replaced with the electrode tip or independently to provide renewed insulation between the electrode rod 62 and the EHF chamber. A deflection surface 78 is provided on the insulation tip cap. A conical insert 80 is assembled to a cup-shaped receptacle 82. Set screws 84 may be provided to secure the conical insert 80 within the cup-shaped receptacle 82. A holder receptacle 86 is defined within the adaptor 88 that provides the cup-shaped receptacle 82. A holder 92 is received within the holder receptacle 86 and may be adjusted by turning the holder with a wrench that engages the wrench flats 93.
A plurality of vent holes 94 may be provided within the conical insert 80 to allow entrapped gas to be vented from the electrode assembly 60. Any fluid that passes between the conical insert 80 and the insulation insert 76 may be collected within pockets 96 that are provided within the conical insert 80. Any fluid or gas pressure developed within the pockets 96 is vented through the vent holes 94.
As previously described with reference to
Referring to
The electrode must also perform the functions of conducting energy into and out of the chamber, maintaining the electrode isolation from the chamber or a source of ground. The electrode assembly must also maintain the chamber's water tight seal while allowing for measurement and adjustment of the inter electrode gap. The electrode assembly also must constrain motion of the electrode during the pressure pulses that may tend to cause the electrode to be ejected from the EHF chamber.
The profile of the electrode or tip shape can influence the efficiency of the discharge and the voltage differential occurring during a discharge. Finally, the electrode profile can also affect the rate of erosion of the electrode. In some instances, severe tip erosion may be experienced by the electrode tip. Electrodes must be changed periodically during a production operation. The erosion rate substantially impacts the frequency of electrode replacement.
The tip profile shown in
Electrode tips having a pointed or conical cross section tend to focus the discharge energy and erode very quickly to rapidly lose their original design and shape. Flat electrode tips erode more slowly, however, their profile tends to change over time as discharges occur. Electrode tip profiles gradually change during the erosion process with a variable rate of erosion and constantly changing discharge conditions. It is proposed that a stable tip profile be provided initially that has a generally flat tip and rounded edges which asymptotically approach the desired stable electrode tip profile. Such a tip profile tends to exhibit the slowest rate of erosion and is more stable over time.
As shown in
y(x)=yo−A1e(1−x)/t1−A2e(1−x)/t2−A3e(1−x)t3
where e is the base of the natural logarithms (2.718 . . . ), and the constants to be used in the equation describing each profile are found in Table 1 below:
The benefit of an improved tip shape may also be obtained in large measure by providing a tip with a profile that is between lines A and B in
Y=−1.6x2
And that is smaller than a polyline identified by B and defined by the horizontal line (y=0) and the vertical line (x=1) with a radius of ⅛R0.
Where:
y is the electrode tip length, as measured from the end of the tip;
x is the distance from electrode centerline; and
R0 is the electrode radius equal to half of the diameter of the electrode diameter or half of the width of the electrode.
Although embodiments of the invention have been disclosed, it will be apparent to persons skilled in the art that modifications may be made without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be defined by the following claims.
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