Apparatus and method for electrical discharge machining

A work station for an electrical discharge machining apparatus, including a first mechanism for retaining a workpiece in a predetermined position, at least one electrode for forming a feature on the workpiece, and a second mechanism for positioning the electrode in a manner that is movable into and out of engagement with a designated portion of the workpiece. A portion of the second mechanism is connected to the first mechanism so as to automatically align the electrode with the designated portion of the workpiece. The second mechanism is configured so that the electrode is electrically insulated from the workpiece.

BACKGROUND OF THE INVENTION

The present invention relates generally to an apparatus and method for electrical discharge machining and, in particular, to an apparatus and method for electrical discharge machining which includes an improved work station that provides automatic alignment of the electrode to the workpiece.

Electrical discharge machining (EDM) is a well known process for forming features, such as holes, slots and notches of various shapes and configurations, in an electrically conductive workpiece. Conventional EDM apparatuses typically employ an electrode having the desired shape that is advanced toward the workpiece. A suitable power supply is applied to create an electrical potential between the workpiece and electrode for forming a controlled spark which melts and vaporizes the workpiece material to form the desired feature. The cutting pattern of the electrode is usually computer numerically controlled (CNC) whereby servomotors control the relative positions of the electrode and workpiece. During machining, the electrode and workpiece are immersed in a dielectric fluid, which provides insulation against premature spark discharge, cools the machined area, and flushes away the removed material.

One drawback to electrical discharge machining is that it is a relatively slow process, especially when several distinct features need to be machined into a workpiece. This is particularly so in the aircraft engine industry where electrical discharge machining is widely used for machining various features into aircraft engine parts. To increase the manufacturing output of such parts, it is common to use an EDM apparatus that machines a number of parts at one time. Such an apparatus has a plurality of work stations, each of which has a workpiece fixture located in a single dielectric tank. The work stations are all typically connected to a common power supply. Thus, machining takes place in series one part at a time. That is, a spark will be created in the first work station and then the next work station and so on until each station has a spark supplied. This sequence is repeated until the machining operation is completed for each workpiece.

A problem with this type of apparatus is that whenever one station gets hung up (i.e., fails to discharge for some reason such as electrode misalignment or a EDM particle remaining in the electrode-workpiece gap) all of the stations will become hung up. Since all stations are stopped, it is not evident which station is causing the stoppage. Furthermore, each station must use the same electrode material and polarity because of the series power connection. Thus, each station machines the same feature into the parts. Parts requiring additional features must then be moved to another machine. This means that multiple machines, fixtures and part handling are required before a part is completed Another inefficiency with these conventional EDM apparatuses is that the dielectric tank must be drained and refilled between each cycle to load and unload the parts because the workpiece fixtures are all located and submerged in the tank.

Various approaches to solving such problems are disclosed in U.S. Pat. No. 6,326,576 to Krenz et al., U.S. Pat. No. 6,369,343 to Krenz et al., and U.S. Pat. No. 6,563,071 to Krenz. While each EDM apparatus disclosed in such patents is useful for its intended purpose, it will be appreciated that the slide mechanism which houses the electrode is indirectly connected to a base supporting the workpiece. Accordingly, the slide mechanism moves along a plurality of ceramic rods into position with respect to a workpiece. In this way, the electrode is electrically insulated from the fixtures retaining the workpiece. Not only do such ceramic rods and its related components increase the bulkiness of the slide mechanism, this type of configuration also produces stack-up errors which require manual alignment of the electrode to each workpiece by the machine operator. This introduces a window of error if the electrode is incorrectly aligned, as well as a measure of inefficiency to the process.

Accordingly, it would be desirable for an electrical discharge machining apparatus to be developed which includes a work station which automatically aligns an electrode with a workpiece so as to form a desired feature. It would also be desirable for the work station of such EDM apparatus to be greatly reduced in size and complexity so that multiple features can more easily be formed in a workpiece.

BRIEF SUMMARY OF THE INVENTION

In a first exemplary embodiment of the invention, a work station for an electrical discharge machining apparatus is disclosed as including a first mechanism for retaining a workpiece in a predetermined position, at least one electrode for forming a feature on the workpiece, and a second mechanism for positioning the electrode in a manner that is movable into and out of engagement with a designated portion of the workpiece. A portion of the second mechanism is connected to the first mechanism so as to automatically align the electrode with the designated portion of the workpiece. The second mechanism is configured so that the electrode is electrically insulated from the workpiece. The second mechanism further includes: a stationary first portion including a plurality of shaft members retained in a shaft housing, wherein the shaft members are arranged in a predetermined spaced relation; a second portion for retaining and positioning the electrode, where the second portion includes a plurality of openings therein which are slidably positioned on the shaft members; and, a bearing positioned between a surface defining each opening in the second portion and each shaft member, wherein the bearings have electrically insulating properties.

In a second exemplary embodiment of the invention, an electrical discharge machining apparatus for forming features in a workpiece is disclosed as including: a dielectric tank; at least one work station disposed in the tank, where each work station further includes a first mechanism for retaining a workpiece in a predetermined position, at least one electrode for forming a feature on the workpiece, and a second mechanism for positioning the electrode in a manner that is movable into and out of engagement with a designated portion of the workpiece; a device for supplying power to the electrode; a device for supplying power to the electrode; and, a device for controlling the position of the second mechanism. The second mechanism is configured so that the electrode is electrically insulated from the workpiece.

In accordance with a third embodiment of the invention, a method of electrical discharge machining a workpiece in a work station is disclosed as including the following steps: providing a first mechanism for retaining the workpiece in a predetermined position; providing at least one electrode; providing a second mechanism for positioning the electrode in a desired manner; connecting the first mechanism and a portion of the second mechanism so as to automatically align the electrode with a designated portion of the workpiece; electrically insulating the electrode from the workpiece; loading the workpiece into the first mechanism; and, moving a second portion of the second mechanism with respect to the first mechanism so that the electrode is brought into and out of engagement with the workpiece to form a desired feature therein.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in detail, wherein identical numerals indicate the same elements throughout the figures,FIG. 1depicts an electrical discharge machining (EDM) apparatus10. EDM apparatus10includes at least one work station12disposed in a tank14, which is shown in partial cut-away to reveal work station12therein. Although only one work station12is shown inFIG. 1for purposes of illustration, it should be noted that additional such stations, operating independently, could be disposed in tank14. In this way, EDM apparatus10could machine multiple workpieces at the same time.

As is well known in the field, tank14is filled with a suitable dielectric fluid, such as a dielectric oil, so that a workpiece is immersed in the fluid. The dielectric fluid insulates against premature spark discharge, cools the machined area, and flushes away machining debris. A float switch (not shown) is preferably provided in tank14for detecting when the dielectric fluid reaches a sufficient depth, while a filtering system (not shown) is preferably connected to tank14for filtering the dielectric fluid.

EDM apparatus10also includes a standard EDM control system20which includes a power supply (or other spark generator) and a controller (e.g., a computer numerical control or CNC). The power supply of EDM control system20provides energy to work station12via power cables22. As seen inFIG. 1, the controller is connected to one or more linear motors24which, in turn, control positioning of one or more electrodes during operation of EDM apparatus10is discussed in greater detail herein.

It will be appreciated that other patents, such as U.S. Pat. No. 6,536,071 to Krenz, disclose in detail a work station configuration which involves electrodes being positioned above and/or below a workpiece which are moved to form desired features therein. The present invention is concerned with the positioning of one or more electrodes at either or both sides of a workpiece, as well as at varying angles and orientations thereto, so that additional features may be formed substantially simultaneously in the workpiece. In this way, it is possible that all or most of the features to be formed in the workpiece may be performed in a single operation so as to maximize efficiency and reduce cost.

It will be seen inFIG. 2that work station12preferably includes a first mechanism26for retaining a workpiece28in a predetermined position. Work station12also includes at least one electrode30for forming a feature on workpiece28(seeFIG. 4) and a second mechanism32for positioning electrode30in a manner that is movable into and out of engagement with a designated portion of workpiece28. As discussed in further detail herein, it will be understood that second mechanism32includes a first portion34which is stationary and connected to first mechanism26so as to automatically align electrode30with the designated portion of workpiece28. A second portion40of second mechanism32, also known as a slide herein, functions to move electrode30relative to first portion34and therefore first mechanism26. It will also be noted that second mechanism32is configured so that electrode30is electrically insulated from first mechanism26and workpiece28.

It will be seen that an exemplary workpiece28depicted inFIG. 2is a shroud for use in a gas turbine engine. A typical gas turbine engine employs a plurality of such shrouds arranged in an annular array around the engine's turbine rotor. The shrouds thus define an outer boundary for hot combustion gases flowing through the turbine. Shrouds are ordinarily made by a process in which a casting of the shroud is made and then various features are machined into the casting. It will be understood that certain features may be formed in the shroud by electrodes which are positioned above and below it, respectively, as disclosed in the prior art patents referenced herein. The present invention is concerned with the formation of features in the shroud at its ends, such as a seal slot. It should be noted that a shroud is only an illustrative example of one workpiece that is suitable for use with EDM apparatus10. The present invention is not limited to such workpieces and is applicable to virtually any workpiece in which multiple features are machined.

As best seen inFIGS. 4 and 5, first portion34of second mechanism32further includes a plurality of shaft members36retained in a shaft housing38in a predetermined spaced relation. It will be appreciated that shaft members36are positioned in spaced, parallel relation to each other. While a pair of shaft members36may be utilized, it is preferred that three shaft members36oriented in a substantially triangulated manner be provided. It will also be noted that shaft housing38includes a relatively large opening39formed in a middle portion thereof. This opening39is provided in order that a portion25of an arm member27connected to linear motor24is able to extend therethrough and connect to slide portion40(seeFIG. 1). In this way, linear motor is able to move electrode30into and out of position with respect to workpiece28in accordance with signals from control system20.

As stated above, second mechanism32also includes a second portion40for retaining and positioning electrode30. Second portion40has a locator plate42which includes a plurality of openings44formed therein (defined by a surface46), a holder plate48configured for retaining electrode30in a predetermined location, and at least one clip member50for connecting holder plate48and locator plate42. It will be understood that holder plate48may be removed from locator plate42, and indeed work station12, by means of a knob49so that electrode30may be repaired or replaced. Holder plate48also preferably includes a plurality of openings52formed therein which are positioned adjacent electrode30so as to permit dielectric fluid to flush the area during operation and prevent damage to electrode30.

In order to electrically insulate electrode30from first portion34of second mechanism32, and consequently first mechanism26and workpiece28, a bearing54is preferably positioned between each opening44in locator plate42and each shaft member36. Bearing54is preferably made of an electrically insulating material, such as ceramic, any polymer material, fiberglass or other non-conductive material. One particular example of such a bearing is identified as Part#RJZI-01-06, which is manufactured by Igus, Inc. of East Providence, R.I. It will also be seen fromFIG. 8that bearings54preferably includes a plurality of grooves56formed therein which permit fluid flow between each end of bearings54. This enables the dielectric fluid and other debris to pass therethrough and not affect the ability of second portion40to slide along shaft members36.

It will be seen fromFIGS. 4 and 5that second portion40of second mechanism32also preferably includes a base member58positioned adjacent and connected to holder and locator plates48and42to provide stability to second mechanism32. Base member58likewise has a plurality of openings60formed therein which are sized and spaced to receive a portion of each bearing54. Base member58also has a relatively large opening or pocket62formed therein which is configured to function as a manifold and supply dielectric fluid to openings52in holder plate48. It will be understood that opening62in base member58is in flow communication with a dielectric fluid supply via a tube63connected to an opening (not shown) in base member58. A cover plate59is preferably provided adjacent a side of base member48opposite electrode30in order to retain the fluid within opening62.

As best seen inFIG. 3, first mechanism26includes a base portion64for supporting workpiece28and a housing66positioned adjacent and connected to base portion64. It will be seen that housing66has at least one locator member68associated therewith for aligning workpiece28in a first direction (indicated by arrow70). Base portion64preferably further includes a first base plate72positioned adjacent housing66, a second base plate74positioned in spaced relation to first base plate72, and one or ore spacer members76positioned between first and second base plates72and74, where such items are connected via a plurality of bolts or dowels (not shown). It will be noted that first base plate72preferably includes a locator member78at an end adjacent to second mechanism32so as to align workpiece28in a second direction (indicated by arrow80). Of course, a locator member may be associated with second base plate72in addition to or in place of that located on first base plate72. Such locator member also may be positioned on either side of first and/or second base plates72and74.

It will be seen that the process of loading workpiece28into the predetermined position involves the initial step of placing workpiece28upon base portion64of first mechanism26. Workpiece28is then slid rearwardly (in the direction of arrow70) until it engages locator member68on housing66and laterally (in the direction of arrow80) until it engages locator member78on first base plate72. Thereafter, workpiece28is locked in position by means of one or more spring clips79which preferably are located adjacent and connected to second base plate74(seeFIG. 1).

First mechanism26preferably further includes one or more spacer members81located between housing66and a fixture mount83located aft of work station12. It will be noted that spacer members81extend laterally beyond base portion64of first mechanism26so as to form an arm member85. Each arm member85then includes an opening87formed therein of a predetermined size and shape.

With respect to the direct connection of first mechanism26and first portion34of second mechanism32, it will be seen fromFIG. 7that shaft housing38includes a male portion82and a female portion84which are configured to interface with arm members85in locking engagement. More specifically, male portion82is received in openings87of arm members85while a portion89of arm members85is received within female portion84. A set screw or similar item may be utilized to maintain shaft housing38and arm member85in their connected position. In this way, mating of shaft housing38and arm member85is accomplished in a direction substantially perpendicular to the movement of second portion40of second mechanism32so as to maintain the connection and respective positioning of electrode30and workpiece28during such movement. In this embodiment, it will be noted that electrode30is positioned between shaft housing38and first mechanism26.

In addition to second mechanism32, a third mechanism86(seeFIG. 1) may be located on an opposite side of first mechanism26. Third mechanism86is preferably configured like second mechanism32, but in mirror image, so that it causes relative movement of a second electrode (not shown) with respect to first mechanism26. Similarly, third mechanism86is preferably directly connected to first mechanism26in the manner described hereinabove with respect to second mechanism32so as to automatically align the second electrode with workpiece28. Moreover, third mechanism86is configured so that the second electrode is likewise electrically insulated from first mechanism26and workpiece28.

FIGS. 9-14depict a work station112for an EDM apparatus110having an alternative configuration for positioning electrodes so as to be movable into and out of engagement with a designated portion of a workpiece128. While much of second mechanism132will be similar to that described herein with respect to second mechanism32, it will be understood that a first mechanism126thereof is different from first mechanism26. Accordingly, the manner of connecting a stationary portion134of second mechanism132to first mechanism126, as well as the device utilized to move slide portion140of second mechanism132, will differ. It will also be noted that an exemplary workpiece128depicted is a portion of a hanger, which is utilized in a gas turbine engine to house a shroud.

A rack and pinion device, identified generally by numeral114, preferably receives signals from a controller (not shown) and is utilized to move slidable second portion140of second mechanism132so that electrode130is brought into and out of engagement with workpiece128at the designated portion thereof. In this arrangement, it will be appreciated that shaft housing138is connected to first mechanism126by means of a base member116for rack and pinion system114, as opposed to arm members85of spacers81in the previous embodiment. More specifically, a slot139formed on the bottom of shaft housing138is mated with a portion165of side base portion166so as to be retained in groove118of base member116(seeFIG. 10). As seen inFIG. 11, base member116is connected to a base portion164of first mechanism126via a connecting base portion120. A handle117is also preferably attached to each base member116so as to ease handling and placement of work station112.

It will be seen that connecting base portion120further serves to house a first notched rod member121of rack and pinion system114which interfaces with a gear member122and moves generally in a vertical direction. A second notched rod member123also interfaces with gear member122and moves generally in a horizontal direction. Second notched rod member123includes an end portion124which preferably is connected to slide portion140of second mechanism132(e.g., by a shoulder screw or the like). Accordingly, electrode130is moved into and out of engagement with workpiece128as the rotary movement of gear member122created by first notched rod member is translated by second notched rod member123to slide portion140.

It will be understood that besides interfacing with base portion116and connecting base portion120to establish a predetermined arrangement with shaft housing138, a side portion166of base portion164preferably includes a surface168which functions to support slide portion140as it moves along shaft members136. It will be appreciated that surface168is stepped from a main surface169of base portion164which is utilized to support workpiece128.

Regarding first mechanism126, it will be seen that a locator member172is associated with base portion164so as to position workpiece128properly in a direction indicated by arrow170. A pair of locator plates174and176are preferably positioned at each end of base portion164and held in place by spring clips178and179, respectively, so as to position workpiece128properly in a lateral direction indicated by arrow180. Locator plates174and176will then include openings therein so as to permit the ends of workpiece128to extend past main support surface169and be accessed by electrodes130.

With respect to second mechanism132, it will be seen that at least one connector strip133is utilized to hold holder plate148in position against base member158. Electrode130is retained in position by holder plate148, which is in turn connected to locator plate142. As discussed herein for second mechanism32, locator plate142and base member158include openings therein which each include a bearing154made of a non-conductive material. Thus, slide portion140is able to move along shaft members136while electrode130is electrically insulated from first mechanism126(and therefore workpiece128). At the same time, electrode130is automatically aligned with workpiece128due to the connection of shaft housing138with base portion164of first mechanism126.

Having shown and described the preferred embodiment of the present invention, further adaptations of the electrical discharge machining apparatus and method can be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the invention.