Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1999-03-15
2001-05-01
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069160, C219S069170, C219S069200
Reexamination Certificate
active
06225589
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improvements in electric discharge machining (EDM)apparatus, including improved electrodes, and, more particularly, to an improved numerically controlled EDM apparatus for three-dimensionally machining a desired pattern, such as a cavity, in an electrically conductive workpiece with minimal down time. The improvements include a new form of generic electrode for roughing operation that essentially scoops out metal stock from the workpiece; and also a wire electrode that serves as a point source of the sparks to erode the workpiece. The invention further relates to automatic replenishment of spent wire electrode portions to compensate for portions eroded away during machining operation. And the invention relates still further to a novel EDM electrode position sensor and control system that maintains the virtual position of the electric sparks.
BACKGROUND
When a high voltage is applied between two metal pieces separated by a small gap, the voltage difference between the metal pieces stresses the insulating medium situated in the gap. That insulating medium could be a gas, such as the air, or a liquid that is dielectric in character. Should the gap be small, the voltage could be large enough to ionize and electrically break-down the insulating medium, producing an electric arc, or, as variously termed, a spark that jumps across the gap between those opposed surface locations on the metal pieces which, as viewed microscopically, are closest to one another. The spark conducts electrical current from the source of high voltage through the metal.
The electric spark releases energy in various forms, including visible light, ion and electron acceleration, acoustic energy and heat. That released energy, principally believed to be the heat, has an erosive effect on the metals, referred to as electroerosion. Minute amounts of metal sputters from the surface of both metal pieces. Although noting the physical effect of sparks on the metal, the theoretical physics underlying that erosion is not fully understood by the present applicant and, as becomes apparent, is not necessary to the understanding of the present invention.
Because of its erosive effect, electrical sparks have heretofore been used to cut metals, form complex three dimensional cavities within metals and otherwise shape metal surfaces in three dimensions, a process referred to as electric discharge machining (EDM). Apparatus to perform EDM usually employs a shaped metal piece, called the electrode, to electroerode the other metal piece, usually larger in size, the workpiece, which is to be cut and/or shaped. To date EDM has become quite sophisticated. Three types of EDM apparatus are presently used for discharge machining: sinking EDM, numerically controlled three dimensional generic electrode EDM, and wire EDM.
Sinking is the main type of EDM used for making mold parts and other work, where the cut is not composed of straight lines going through the workpiece. In the sinking process, one or more electrodes are prepared to mimic a “positive” image to the “negative” image of the cavity desired. With both the electrode and workpiece immersed in a dielectric “flushing” fluid, the electrode is gradually pushed into the mold material as sparks erode the mold. Debris is flushed out from the spark gap by circulating the flushing fluid.
With sinking, electrode wear is usually an important factor: sharp edges and fine details need sequential roughing and finishing operations with re-surfacing of the electrode in between. However, flushing is the single most important limitation of sinking process speed and accuracy. Since the gap between electrode and workpiece is typically less than 0.005 of an inch and gap areas often exceed one or more square inches, special provisions are often needed to help dielectric (machining) fluid flow through the gap. Regular, frequent stops in cutting action and withdrawal of the electrode from the workpiece to increase the gap and permit a higher rate of fluid flow, are normal functions of existing EDM sinking operations. Material removal rate is very slow, usually less than one cubic inch (1 in
3)
per hour); electrode preparation is expensive and operator involvement is often required.
Where the size of the workpiece permits, conventional machining, being much faster than sinking EDM, is often used initially to rough out the final shape as a shortcut to speed up the metal removal process. After such pre-machining, the workpiece is often heat treated to its final hardness, and then finished with sinking type EDM.
As an advantage, an aspect of the present invention permits removal of large volumes of metal from a workpiece, expeditiously, much more quickly than by the sinking process, effectively scooping out a chunk of metal at a time from the workpiece, and is particularly useful in working hard or brittle materials, such as carbides, or hard-to-machine work pieces, such as fragile or thin walled shapes and the like.
The numerically controlled three-dimensional EDM process employs a generic electrode, such as a small tube. The term “generic electrode”, as accepted in the art, refers to an axial non-formed tool electrode of a simple machining surface contour, which may be cylindrical, triangular or square in cross section and which is generally dissimilar or independent of the three-dimensional shape of a final cavity or contour to be machined in a workpiece. Such a “generic electrode” is distinguished from the formed tool electrode used in the sinking process that is a mirror image or a scaled-down or scaled-up image of the three-dimensional cavity or contour desired in the workpiece.
In the three-dimensional EDM process with at least one generic tool electrode having a machining surface contour at an end portion thereof, the tool electrode is axially juxtaposed with a workpiece to position the machining surface contour in spaced juxtaposition therewith across an EDM gap and the gap is supplied with a machining liquid, a dielectric fluid. A succession of electrical discharges are produced across the EDM gap to electroerosively remove stock from a localized portion of the workpiece. To advance the process, the tool electrode is displaced relative to workpiece the along a three-dimensional path, typically under numerical control, a digital computer, while maintaining the EDM gap, whereby the desired cavity or contour, dissimilar to the generic electrode and basically determined by the path of the three dimensional feed displacement effected between the tool electrode and the workpiece, is carved out in the workpiece. Examples of such numerically controlled EDM apparatus is presented in patents to Inoue, U.S. Pat. No. 4,543,460, granted Sep. 24, 1985, and U.S. Pat. No. 4,606,007, granted Aug. 12, 1986 and in Shimizu, U.S. Pat. No. 4,608,476 granted Aug. 26, 1986.
The advantages of the generic electrode EDM process over the conventional “sinking” EDM process, which makes it essential to use one or more similar formed electrodes of mirror images of a desired cavity or contour, are increasingly recognized in the art. It is very difficult to prepare a formed tool electrode of a precise mirror image of a desired cavity or contour required in the sinking EDM process. In addition, several such electrodes of slightly varying sizes are often required to allow repetition of the process in different modes ranging from roughing to finishing. Because of such factors, the sinking EDM process for machining a three-dimensional cavity or contour has been very costly and laborious.
By contrast, in the three dimensional EDM process a simple tool electrode in the form of a cylinder of small cross-section or the like, or more than one such simple electrode varying in size can simply be employed to machine a large and/or intricate cavity or contour. The cavity or contour is machined in the workpiece by displacing the generic electrode and the workpiece relative to each other, under numerical control or sequence copying control, along a prescribed three-dimensional path pro
Evans Geoffrey S.
Goldman Ronald M.
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