Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1999-12-14
2002-06-11
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069150
Reexamination Certificate
active
06403910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to apparatuses and methods for performing high-speed electric discharge machining operations. More particularly, the present invention relates to an apparatus and a method for performing a high-speed electric discharge machining operation, wherein the apparatus is adaptable to perform a plurality of electric discharge machining operations simultaneously and in close proximity to one another, and wherein the method provides simultaneous electric discharge machining operations in close proximity to one another.
2. Description of the Related Art
Electric discharge machining (EDM), sometimes referred to as “electro-discharge machining” or “electrode discharge machining”, is a known process for drilling deep, small diameter holes in a metal workpiece, such as a turbine blade of a gas turbine engine. EDM operates on the principle that, if a electrically-charged EDM tool (typically, a negatively-charged copper-alloy electrode) is brought within close proximity to a electrically-charged (for example, positively-charge if the electrode is negatively-charged) workpiece which is sometimes submerged in a bath of dielectric fluid (typically, water), an electric potential difference exists between the electrode and the workpiece and a spark discharge will arc the gap therebetween, thereby eroding a small amount of material from the workpiece adjacent the electrode. If the negative charge to the electrode is in the form of a series of pulse charges, the electric potential difference between the electrode and the workpiece is systematically repeated such that spark discharges occur at a rapid rate, and a hole can be “drilled” into the workpiece if the electrode is incrementally advanced into the workpiece as workpiece material is slowly eroded therefrom. The dielectric fluid assists in the formation of the spark discharges, cools the workpiece during repeated spark discharges and carries away material eroded from the workpiece.
The EDM apparatus typically includes a translatable arm connected to a precision drive device, such as a servomotor, which is adapted to advance the arm (and the electrode held thereby) linearly towards, and away from, the workpiece with high tolerances, for example, under computer control. One complete EDM drilling operation typically is a slow process, whereby the arm smoothly advances the electrode into the workpiece at a rate of only about 1 linear inch per minute, which is due to the minute amount of material that is removed from the workpiece during a single spark discharge. Accordingly, where it is desired to drill numerous holes in the workpiece, such as, for example, in the turbine component described in U.S. Pat. No. 5,605,639 to Banks, et al., it is necessary for the EDM drilling operation to be performed once for each hole, with the operator's repositioning the workpiece relative to the electrode in between each operation. It will be apparent to one of ordinary skill in the art that such an operation is inefficient, time-consuming and increases the likelihood of adjacent hole misalignment. It is therefore desirable to provide an EDM apparatus, wherein a plurality holes can be drilled simultaneously thereby.
Attempts have been made to provide an EDM apparatus adapted to drill simultaneously a plurality of holes. For example, the aforementioned Banks '639 patent discloses a multiple piece electrode having a plurality of rods extending therefrom in fixed spaced relation to one another for simultaneously drilling a plurality of holes thereby. However, where a new turbine component is to be manufactured having a different distribution of diffusion holes, an entirely new electrode must be constructed having the rods arranged accordingly. Therefore, it is desirable to provide an EDM apparatus including a plurality of electrodes to drill simultaneously a plurality of holes thereby, wherein the spacing between adjacent electrodes can be adjusted to permit adjustable spacing of the holes.
It is also known that, during each EDM drilling operation, the leading tip of the electrode erodes therefrom slightly, albeit at a rate which is slower than the rate at which material is eroded from the workpiece. As such, the arm of a typical EDM apparatus is equipped with a conventional chuck-type gripping device to permit the electrode to be removed therefrom and to be replaced from time-to-time, particularly, when insufficient material remains on the electrode to permit one complete EDM drilling operation. However, it is oftentimes desirable for a single electrode to perform numerous EDM drilling operations, such that replacement thereof is not required after every EDM drilling operation, but instead, replacement of a worn electrode is required only after a predetermined number of EDM drilling operations have been performed. Accordingly, it is desirable for the electrode to be of a suitable length such that, as the tip thereof is eroded therefrom, sufficient material remains thereon after a predetermined number of EDM drilling operations to permit at least one additional complete EDM drilling operation to be performed.
It has been observed that the leading tip of the electrode typically does not erode uniformly, and as a result, the hole being formed thereby tends to curve, or wander, from the intended central axis thereof. Where the hole is but one hole in an array of closely-spaced holes, such as in the arrangement of diffusion holes of the turbine component shown in the aforementioned Banks '639 patent, curvature thereof as a result of uneven electrode wear will result in adjacent holes intersecting one another. This result becomes significantly aggravated where the hole is of a very small diameter. Accordingly, it is desirable to provide an EDM apparatus and method whereby electrode wear is uniform such that a hole formed thereby is substantially straight.
It is known to rotate, or spin, the electrode around its central axis while performing an EDM operation, such that the electrode will wear uniformly, thereby forming a substantially straight hole. For example, U.S. Pat. No. 3,764,771 to Roy teaches an EDM apparatus including a rotatable chuck which holds an electrode for performing an EDM operation. The EDM apparatus of Roy '771 is typical of prior-art EDM apparatuses utilizing a conventional chuck-type gripping device for holding an electrode used in an EDM operation in that a locking device, such as a set screw, is used to clamp the chuck onto the electrode. Repositioning, removing or replacing the electrode from the chuck of an apparatus such as is shown in Roy '771, requires the operator to manually manipulate the locking device, such as, by unthreading the set screw with an Allen wrench. The area surrounding the chuck, then, must be substantially free of any obstructions which would prevent the operator from gaining access to the set screw with an Allen wrench to thereby release the electrode from within the chuck. Accordingly, two or more chucks typically cannot be spaced in close proximity to one another by a distance less than a minimum distance required to permit access to each chuck's locking device. It is therefore desirable to provide an EDM apparatus which is adaptable to perform a plurality of electric discharge machining operations simultaneously and in close proximity to one another. It is also desirable to provide a method of forming a plurality of holes in a workpiece using electric discharge machining, wherein a plurality of electric discharge machining operations are performed simultaneously and in close proximity to one another.
U.S. Pat. No. 3,919,515 to Bangs teaches a manually-manipulated mechanical locking device for clamping an electrode in a non-rotatable collet, wherein a tool, such as an Allen wrench, is not required to release the electrode from within the collet. However, it is furthermore desirable to provide an EDM apparatus, wherein an electrode may be clamped and released from a rotatable gripping device thereof wit
Stang Scott A.
Turner, Jr. Levi
Dunn Tom
Hi-Tek Manufacturing, Inc.
Killworth, Gottman Hagan & Schaeff LLP
Pittman Zidia
LandOfFree
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