Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2000-06-08
2002-09-03
Evans, Geoffrey S. (Department: 2125)
Electric heating
Metal heating
Cutting or disintegrating
C279S083000, C403S362000
Reexamination Certificate
active
06444941
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to tool holding systems, and, more particularly, to a system for holding electrodes for accurate machining thereof and electric discharge machining therewith.
BACKGROUND OF THE INVENTION
It is generally understood that to obtain good results in machining operations it is necessary to have both precisely formed cutting tools as well as fixturing for the tools and parts that provides for accurate and predictable relative alignment between the tool and the piece to be machined. This is particularly true in electric discharge machining (EDM). In an EDM process, the cutting tool is brought into close proximity to the metal work surface. The electrode cutting tool serves as the cathode and the metal workpiece is the anode in an EDM operation with voltage developed therebetween creating an electric arc which erodes the metal surface of the workpiece so that it assumes the shape of a mirror image of the proximate surface of the electrode. EDM operations allow for complex shapes such as intricately shaped cavities to be formed in workpieces that otherwise may be difficult to obtain with more conventional metal cutting types of machining operations. As is apparent, the shape and dimensions of the electrode are of paramount importance in terms of the final shape of the workpiece that is being machined with the EDM process.
Accordingly, electrode manufacture must permit for very precise, intricate shapes to be machined onto the electrode blank with a very high accuracy of repetition. Even assuming this occurs, the subsequent set up of the holder and electrode in an EDM machine if not done in a precise and predictable fashion can ruin a machining operation. For instance, it is known to use round shaped electrode posts that have a transverse indexing pin with the post adapted to be received in a round bore of a fixture block. The fixture block has four standing pins that are adjustably spaced ninety from each other about the fixture block bore for abutting the post indexing pin to angularly locate the post in a C direction. The bore includes a hydraulic chuck for clamping onto the post in a direction transverse to its axis to keep it fixed in the X and Y directions.
This system faces several shortcomings. Relying on adjustable standing pins to set the position of the post in the C direction is not very reliable as the pins may be adjusted in a different manner from one worker to the next. Further, it has been found that during machining of the electrode blank such as in milling or grinding operations, the hydraulic clamping provided by the block chuck may not keep the posts sufficiently stationary to provide for precise forming of the electrode.
Another problem with the EDM system described above is in typical post holders the mounting surface at the top of the post or on an enlarged flange thereof is a smooth flat surface with the electrode having a rear flat surface which is then glued to the post mounting surface for attaching the electrode thereto. To provide for extra holding force, a fastener can be threaded into aligned apertures of the post and the electrode through a throughbore countersunk in the post for inserting the fastener through the rear of the post and seating it in the counterbore therein. Where the electrode is adhered to the post, once the electrode is worn so that it is no longer useful for the particular EDM operation for which it is originally designed, both the electrode and the post holder to which it is adhered generally have to be disposed or stored for possible later use such as by modification thereto. This can result in a tremendous waste of electrode post holders as they are not able to be reused with different electrodes. In this regard, it is not unusual to see EDM machine shops stocking hundreds of posts with electrodes adhered thereto. Further, the glue attachment between the mounting surfaces of the post and electrode do not provide for X, Y and C direction locating of the electrode relative to the post.
Because the posts are generally disposed after the electrode is consumed, the fastener is typically advanced into a counterbore of the post for rear mounting of the electrode thereto, as there is no need to be able to access the fastener with the post received and held in its fixture. Accordingly, front mounting of the electrodes to the post such as via a counterbore in the electrode is not typically done.
There are post holders that have been developed which utilize a raised peg for being press fit in a corresponding opening in the electrode. However, this press fit system also relies upon glue to keep the electrode in place during machining operations and thus the post does not provide the advantage of being reusable with different electrodes. Similarly, other mounting systems have been used with cutouts or openings in one of the electrode and post for receiving projecting portions of the other of the electrode and post therein with fasteners for clamping of the electrode to the post also provided. However, each of these systems relies on very precise cutting or drilling operations to form the openings and/or projections in and on the post and electrode that are difficult to achieve to obtain accurate and reproducible mounting of the electrode to the post. Further, these systems do not provide consistent locating of the electrode to the post in the X,Y and C directions, as is desired.
In this regard, machinists have been known to use post holders formed of square stock having raised cross-shaped portions machined on the mounting face thereof for being interfit into a cross-shaped recessed groove machined onto the back of the electrode mounting surface for proper X,Y and C direction locating of the electrode on the post. However, this approach has not seen widespread acceptance due to some significant drawbacks therewith not the least of which is the fact that square tool holder posts are not readily available as standard items for EDM operations similar to round tool holder posts such as those offered by System
3
R International AB of Sweden. One reason for this is the common use of v-fixtures for holding of the posts during machining operations for electrodes and workpieces. Where a square post is clamped onto a v-fixture, the exposed sides of the square post can extend generally at a 45° angle to the vertical which is undesirable for establishing the dimensions of the carbon electrode relative to the electrode holder as there are no vertical or horizontal flats to be used as reference faces.
Accordingly, there is a need for a tool holding system, and in particular a post holder for electrodes that allows for precise and repeatable positioning of the electrode relative to the posts in the X,Y and C directions, and which allows the posts to be reused with different electrodes with the electrodes being able to be repeatably mounted in known predetermined positions relative to the posts. The posts should allow a machinist to obtain accurate machined dimensions on the electrode when it is mounted thereto. In furtherance of the above, an electrode machining system that provides for precise fixturing of the electrode holder in known predetermined positions relative thereto would likewise be desirable.
SUMMARY OF THE INVENTION
In accordance with the present invention, a tool holding system is provided which allows for tools, and in particular electrodes to be removably attached on post holders in precise predetermined positions in terms of the location of the electrode relative to the posts in the X, Y and C directions. As the electrodes are releasably attached to the posts, it is critical for precise machining of the electrodes and for subsequent EDM operations that electrodes be attached to the posts in an accurate and repeatable manner. The present tool holding system utilizes interfitting surface portions which define predetermined positions of the tool on the post relative to the X-Y directions transverse to the post axis and the C-direction angularly about the axis so as to provide the necessary accurate a
Evans Geoffrey S.
Fitch Even Tabin & Flannery
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