Patent Application
20170087655
This application claims the benefit and priority of EP 15 186 617.5, filed Sep. 24, 2015. The entire disclosure of the above application is incorporated herein by reference.
Technical Field
The invention relates to an electrode holder for multichannel electrical discharge machining. Further the invention relates to an electrical discharge machine for a multichannel processing electrode holder.
Discussion
Multichannel machining according to the present invention consists in the electrical discharge machining (EDM) with a plurality of work pieces, or a plurality of electrodes, or both: a plurality of work pieces and a plurality of electrodes. Generally, a plurality of individual discharge gaps are created, whereas each individual gap is served by a generator. The principle of multichannel machining has been proposed very early in the history of EDM, for example, in U.S. Pat. No. 2,783,411. Multichannel processing is of great relevance in the production environment, where it is essentially used to parallelize the process to the advantage of productivity. A multichannel EDM machine may have common or individual kinematic, process control and generator.
Typical such cases are direct part production applications, where the processed part is not a casting mold but a final part of the production chain: drilling of injector nozzle holes, drilling of cooling holes for vanes and other turbine components, the texturing of rollers by electrical discharge texturing (EDT), etc. In wire electrical discharge machining (WEDM) multichannel processing has been proposed for multi-wire slicing, for slot forming applications, etc.
However multichannel processing is also applicable with mold manufacturing. In fact, also in the case of injection molding there is a steady trend to enhance production by increasing the parallelization, i.e. the number of casted parts produced by each injection cycle. Thus, multichannel processing is also of great relevance in with common standard electrical discharge machining applications.
Beside the scope of process parallelization, in certain applications multichannel processing is suitable to achieve better technological results. It is known that a factor limiting the surface quality which can be obtained by EDM is the machining surface area. Moreover the machining surface area determines the electrode wear. This is due to the fact that the work piece and the electrode separated by an inter-electrode space (Gap) represent a capacitor which is part of the discharge circuit. The energy storage of said capacitor is proportional to the surface area of the electrode/work piece pairing. The main consequence is that with each voltage variation, especially at the beginning of the discharge, an unwanted additional current discharge adds to the desired discharge, thus changing its physical characteristics, the removal rate, electrode wear, and Gap distance. By dividing the electrode into electrically separated partial electrodes such as disclosed in DE 19540352, the engaged electrode surface area and thus the relevant capacitance is proportionally reduced. The electrically insulated partial electrodes are individually connected to the generator, so that the partial areas of the work piece are machined by the partial electrodes. With a smaller capacitance the wear of the electrode is reduced and improved process stability is achieved.
The capacitance is calculated with the formula for plate capacitors, as follows:
where
C is the capacitance, in Farads;
A is the electrode area, in square meters;
∈R is the dielectric constant of the processing liquid in the Gap;
∈0 is the electric constant; and
Gap is the inter-electrode distance, in meters;
As said it is possible to provide more than one generator, for instance one for each partial electrode. The evolution of electronics leads to a steady higher generator power density and increased efficiency, so that the accommodation of the boards is simplified. Providing a plurality of machine integrated generators for multi-channel die-sinking EDM becomes more and more interesting.
An even more effective use of the generator is possible if a generator is on during a fraction 1/n (with n>2) of the pulse period. In this case one can share the generator on-time between the channels, as described in EP 2397250-A1. For instance, if the on-time is less than 50%, then two channels can be served with one generator: during the pause of the first channel the generator is switched to the second channel so that the generator can be used by turns on one and the other discharge gap.
Usually multichannel processing is implemented with dedicated machines, specially designed to do always the same or similar specific task. A problem with such dedicated machines is that their manufacturing price is heavily determined by the market volume, becoming excessive with low quantities. Even worse, their design and development takes a lot of time, sometimes years, becoming risky for the manufacturer or investor. However, recently some applications have emerged in which it would be desirable to perform a multichannel processing with a plurality of universal electrical discharge machines.
Certain molds are composed of arrays of similar or identical cavities and have very high requisites in terms of surface finish; in most cases it is comparably easy to divide an electrode into partial electrodes having same or similar partial area. As far as possible it would make sense to use universal electrical discharge machines, since these machines are flexible, do implement all required features and options, with the latest manufacturers technologies concerning generator, process control, machine design, etc.
An aspect of the present invention is therefore to provide a device which makes it possible to facilitate the adaptation of standard electrical discharge machines for multichannel processing.
The inventive electrode holder for multichannel electrical discharge machining can be used with an automatic electrode changer (ATC) and suitable electrode holders, to improve the productivity and/or enhance the surface quality.
A first aspect of the present invention is directed to electrode holder for multichannel electrical discharge machining, comprising:
A second aspect of the present invention is directed to a die-sinking ED machine adapted for the best use of such a multichannel electrode holder, the die-sinking electric discharge machine being capable of supplying electric discharge pulses to a plurality of channels, and comprising a current brush assembly, whereas said assembly comprises a plurality of individual current supplying contractors for the supply of the machining current to a respective slip ring.
Further aspects are set forth in the dependent claims, the following description and the drawings. Other features are inherent in the methods and products disclosed or will become apparent to those skilled in the art from the following detailed description of embodiments and its accompanying drawings.
Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings. The figures illustrate the following:
The
The inventive electrode holder (1) comprises on its top, the known and commonly used holding elements (17). The holding elements (17) have the function to hold the electrode holder (1) firmly and detachably fixed to a chuck (not shown) of the machining head in an electrical discharge machine. Part of the holding elements (17) are the so called referencing elements (16) by which the complete electrode holder (1) is precisely positioned on the machine head (not shown) of the electrical discharge machine. The electrode holder (1) further comprises manipulating elements (not shown) by which a fork gripper of an automatic tool changer can grasp the electrode holder (1) for automatic manipulation, e.g. from an electrode magazine to the machine head of an electrical discharge machine and vice versa. The holding elements comprise a drawbar (18) attached axially to the electrode holder (1), by which the electrode holder (1) is clamped in a chuck of the machine head. The referencing elements (16) are made according the state of the art, generally comprising vertical references (Z) in the form of stops, and separate horizontal references (X/Y) in the form of inset features provided in plates being partially elastic in Z direction. By said references (16) the precise and repeatable positioning of the electrode holder (1) with respect to the chuck is achieved.
These holding elements (17) are not described in more detail since they are preferably of standard type, and are available from the known tooling system manufacturers. Additional details regarding the holding elements (17) are illustrated for instance in EP 2361713-A1.
The electrode holder (1) according the invention comprises at the base of the central body (15), an electrode mounting section (19) by which a plurality of electrical discharge machining electrodes (9, 10) are firmly and precisely positioned. The electrodes (9, 10, 11) are mounted in the desired position and are electrically insulated from each other by an air gap or by insulating elements (not shown). Said electrode mounting section (19) is divided in separate clamping sections (22), on which the electrodes (9, 10) are mounted. The clamping sections (22) can be adapted to the number, size and geometry of the used electrodes.
The electrode holder (1) according to the invention comprises a central body (15), with a plurality of separate internal current supplying circuits (20), being electrically insulated from each other, each circuit (20) leading from the periphery of the electrode holder (1) to a separate current feeder sector (21). The separate current feeder sectors (21) (not shown in detail) are embedded in the base of the central body (15) and represent the terminal of the separate internal current supplying circuits (20). Preferably, the individual clamping sections (22) are fixedly mounted on individual current feeder sectors (21), f.i. by means of screws (not shown). However the current feeder sector (21) may comprise movable contacting means to improve the contact with the clamping sections (22). In a possible embodiment the clamping sections (22) and current feeder sections (21) may be integrated to form one element. The current pulses provided by the generator of the electrical discharge machine (not displayed) are conducted to the electrode mounting section (19), to desired one of a plurality of electrodes used in the multichannel process. Here the term “electrode mounting section” is to be interpreted in the widest possible sense; for instance the electrodes may be clamped, screwed, soldered, etc. to the respective current feeder sectors located at the base of the central body.
In a preferred embodiment of the invention, which is also illustrated in
Each slip ring (23) is connected to a separate current feeder sector (21) of the central body (15) and lastly to the individual electrodes (9, 10) by means of separate internal current supplying circuits (20). In this way the current pulses are conducted across the electrode holder (1) to the individual electrodes (9, 10). Preferably the electrode holder (1) has a slip ring (23) for each channel respectively each electrode (9, 10).
Preferably the slip rings (23) constitute a part of the lateral surface of the electrode holder (1) respectively central body (15); here the slip rings (23)—of preferably same diameter—are stapled vertically and are coaxial aligned to the main axis of the electrode holder (1), electrically separated from each other, for instance by means of insulating discs (8). In this way the slip rings (23) can be contacted from the sides, for instance by means of current brushes or other means.
Alternatively, the slip rings (23) may be arranged at the top surface (12) of the central body (15), just below or next to the referencing elements (16), as will be explained further down in respect of
Typically, an electrode holder for multichannel electrical discharge machining according the invention comprises at least the following elements:
Most die-sinking electrical discharge machines comprise a C-axis, cinematically in series to a Z-axis. By means of said C-axis, the electrode holder can be repositioned, for instance to compensate a misalignment of the electrodes with respect to the machine axes x, y or to set a desired angular position. It can also be used as a servo-controlled axis of the electrical discharge machine, for instance to execute a screw-type or helical machining in which the C- and Z-axis are simultaneously interpolated.
The multichannel electrode holder can also be used to execute a machining in which the C- and Z-axis are simultaneously interpolated.
Preferably the electrode holder, respectively its electrode mounting section which includes the slip rings has at least partially cylindrical form. A current brush can be easily brought in contact with the respective slip ring of the electrode holder to conduct the current pulses to the electrode. By the cylindrical constitution of the central body, the rotation of the electrode in C-axis is possible. In this way the machining process can include rotational movement by the electrode holder.
As mentioned, the central body (15) of the electrode holder comprises the internal current supplying circuits (20) and at both ends thereof it has the electrical contacting elements, i.e. slip rings (23) and current feeder sectors (21) by which current pulses are brought to the electrode mounting, respectively to the electrodes. Further, the electrode mounting section comprises insulating elements, preferably having the shape of discs or rings. The electrical components and the insulating elements are mounted together to form a rigid body (compare figures).
The holding elements (17) are located on top of the assembled central body (15) and on the opposite end—respectively at the bottom surface—of the central body (15) the electrode mounting section (19) is allocated with a plurality of mounted electrodes.
Preferably the current brush support (4) comprises an actuator, f.i. a cylinder by which the brush assembly is brought in contact with the slip rings (23) or withdrawn from them. Preferably the current brush assembly (24) comprises one or more springs to press the brushes against the according slip ring (23), to compensate possible concentricity errors of the slip rings (23) and/or to improve the electrical contact. In such a radial configuration, the current brush assembly (24) is preferably mounted laterally to the machine head (or at the rear of the machine head), so that the other side of the machine head remains freely accessible to a gripper of an ATC, and the front side is freely accessible to the operator of the ED machine.
In the embodiment displayed in
The
The design of the electrical connections may be further simplified by replacing the current brush assembly with a plug connector assembly (3) for automatic connection. In such a configuration, the connection of the electrode holder (2) to the electrode cable (5) is established with the axial insertion of the electrode holder (2) into the clutch. More specifically, an automatic tool changer (not shown) moves the electrode holder (2) to the machining head (6), where the electrode holder (2) is clamped in a chuck (7) by means of the drawbar. With said lifting movement a plug connector of the electrode holder is inserted in a socket connector which is fixedly and coaxially mounted on the machining head.
In the configuration showed in
Preferably the components of the central body of the electrode holder for multichannel electrical discharge machining are made modular, i.e. slip rings (27), insulating rings (26) or insulating disks (26), the elements of the internal current supplying circuits (20) and other intermediate and interfacing components are designed in such a way that the electrode holder (1, 2) can be assembled in various configurations and thus extending the number of possible applications.
Preferably the slip rings (27) comprise at least one protrusion to form as linking element a part of the internal current supplying circuit (20); said internal current supplying circuit (20) traverses the central body (15) of the electrode holder vertically, connecting electrically the slip rings (27) with the current feeder sectors (21) on the base of the central body (15). The protrusion respectively linking element is preferably a solid conductor of suitable length or a braid.
The central body (15) of the electrode holder for multichannel electrical discharge machining must be rigid but not particularly accurate, since, as said the current brush assembly (24) comprises springs to press the brushes against the slip rings (23, 27). Thus the manufacturing cost of the multichannel electrode holder (1) according to the invention is comparably low.
The central body (15) must not be accurate due to the fact that the electrodes are machined on the electrode holder (1) itself. Thus only the rigidity of the electrode holder and the accuracy of the referencing elements are important, whereas the accuracy of the central body is not relevant for the manufacturing accuracy.
Preferably, the electrode holder for multichannel electrical discharge machining is provided in a basic configuration for two channels, and comprises further components to form a modular set. In this way the electrode holder (1) for multichannel electrical discharge machining can be assembled to operate with the desired number of channels. Generally said number is between two and four, but the electrode holder according the invention could also be used with a single channel, and, with more than 4 channels.
Preferably, the lowermost layer of the central body (15) is constituted as a terminal insulator disk (25) comprising the desired number of current feeder sectors (21). These sectors (21) are individually contacted by the internal current supplying circuits (20) of the central body (15) for each channel. Preferably the modular set comprises several exchangeable terminal insulator disks (25), each having a certain number of current feeder sectors (21) by which the current pulses are conducted to the clamping sections (22) and thus to the electrodes mounted thereon. For instance the terminal disk (25) illustrated in
According to a further advantageous embodiment of the invention not shown in the figures, the terminal insulator disk (25) of the electrode holder for multichannel electrical discharge machining could be made configurable, i.e. it is designed to fit for a variable number of electrodes and for the largest possible number of application cases.
The central body (15) described in the present embodiment is comparably simple and thus inexpensive, and thanks to the monolithic constitution of the insulating element it is even more rigid. Although the central body (15) comprises a monolithic insulating element (30) the electrode holder (1) still configurable for the desired number of channels, to a certain extent.
Preferably the electrodes (9, 10) are first mounted to the clamping sections (22) of the electrode mounting section (19) of the multichannel electrode holder (1, 2, 14) in the desired configuration, and then they are directly machined altogether on said multichannel electrode holder (1, 2, 14). In this way the relative positional accuracy of the plurality of electrodes mounted is not affected by a clamping operation, but is determined only by the machining accuracy of the machine tool used to prepare the electrodes. Thus also the relative position of the cavities generated by the EDM process is improved.
In another embodiment the multichannel electrode holder bears at least two or more electrodes which are used in sequence with the same work piece. The electrodes are for instance a roughing and a finishing electrode. Here the process is not parallelized; the advantage lies in the fact that the electrodes are mounted on the same electrode holder and fabricated together on a milling machine or WEDM or other machine tool; in this way the accuracy of the process is no longer determined by clamping errors on both milling machine and EDM, but only by the accuracy of the manufacturing processes, and the capacitance is limited because the electrodes are electrically separated.
Obviously it is possible to use a certain configuration of the multichannel electrode holder with a different number of electrodes. For instance, a multichannel electrode holder configured to serve four channels has four separate internal current supplying circuits with four slip rings, four current feeder sectors and four clamping sections. Said multichannel electrode holder configured to serve four channels can also be use with less than four electrodes, i.e. three or two, but also with one single electrode if required. For instance, if said multichannel electrode holder is configured to serve four channels but has only two electrodes, these two electrodes are mounted to separate sectors of electrode mounting section and current pulses are provided to only two slip rings of the current brush assembly.
The present invention discloses a multichannel electrode holder which can be manipulated by means of an ATC with no particular restriction. Although all manufacturers of tooling systems have their own particular design, the multichannel electrode holder according the invention can be used with every specific system, by mounting the specific holding system (17) to the top of the central body (15). Thus it can be manipulated by means of a Robot or an ATC having a suitable gripper.
The multichannel electrode holder disclosed in the present invention may further comprise auxiliary supplying circuits, for instance one or more supplying circuits for the processing liquid and/or compressed air, and/or cabling for sensing means for process monitoring. These auxiliary supplying circuits may be designed to address each individual electrode of the multichannel electrode holder. Here the connection for the processing liquid and compressed air may be common, typically trough the machining head of the EDM machine, or individual.
The present invention is described in relation with an electrical discharge machining process. It is however understood that the same or similar features can be used with other electrical processes, such as electrochemical processes (in particular PECM), with hybrid processes, etc.
The present invention is not limited to the explicitly explained examples and embodiments. The illustrated alternatives are rather to be considered suggestions intended to motivate the person skilled in the art to implement the invention in a most favorable way.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15 186 617.5 | Sep 2015 | EP | regional |
