Systems of this type usually consist of a wire supply, e.g., in the form of one or more supply coils, transport means for transporting the wire from the supply coil to the processing section and away from the processing section, tensioning means for subjecting the wire to a suitable tension for processing the workpiece, and disposal means for disposing of the used wire.
One system for the partially automated transport of a wire from the unwinding section (A) to the processing zone is already known from DE 34 19 944 C1 (FIG. 4) by the applicant. This system is very complicated technically. The operational reliability of this system is not satisfactory because the wire can fall out of the transport system in case of a malfunction and lead to interruptions in the spark erosion process. In addition, the wire feed by means of belt guide elements can lead to differences in the speed of the two belt guide elements that surround the wire, in particular, at the deflection points. These differences in speed manifest themselves in the form of fluctuations in the uniform movement of the wire. The belt guide elements shown in this patent are also subject to significant wear. These belt guide elements are not suitable for attaining a comprehensive solution because movements of the machine components that occur cannot be compensated by the belt guide elements and each transfer of the wire influences the operational reliability.
During the removal of the wire, vibrations may occur because the wire is wound onto the supply coil in several layers one on top of the other, as well as side by side. In order to prevent these vibrations, the unwinding direction of the wire should be as vertical as possible with respect to the axis of the supply coil. In U.S. Pat. No. 5,281,787, this is realized with an intermediate roller that is geometrically constructed in such a way that it always unwinds the wire in linear fashion independently of the current separation point from the supply coil. However, such an intermediate roller would require excessive space because, the greater the wire diameter, the larger the curvature radius of the deflection rollers must be to prevent undesired deformations of the wire. This roller would have to have the conventional diameter for deflection rollers even at its narrowest point.
The same objective is attained in DE 89 12 810 C1 by axially displacing the entire supply coil. However, this also requires excessive space as well as a corresponding drive unit.
One known device for controlling the motor of the supply coil is described below. After the stabilizing deflection roller(s), the wire extends around one or two additional deflection rollers in opposite senses, i.e., clockwise around one deflection roller and counterclockwise around the second deflection roller. In this case, at least one of the two deflection rollers is mounted in spring-loaded fashion. Measuring means determine the respective wire tension or tensile force by means of the positions of this deflection roller, the spring element and/or additional auxiliary means, with a control unit controlling the coil motor in accordance with these measured values and/or nominal values. Consequently, a certain basic tension exists between the supply coil and the braking means. This basic tension can be adjusted, which is important when different types of wires are used. For example, the use of thinner wires requires a correspondingly lower basic tension.
Several disclosures pertaining to wire exchange systems or multiple wire supplies have been made. However, all these documents are associated with significant disadvantages, in particular, because the wire must be manually transported to the wire guide head. Improved methods for connecting the old wire and the new wire have been disclosed. U.S. Pat. No. 5,340,958 discloses a wire exchange system in which the end of the old wire is connected to the beginning of the new wire by means of welding in order to insert the new wire into the wire transport system. However, this particular document is also subject to several limitations. In order to insert a new electrode, a wire electrode must already be present in the wire drive because the wire would otherwise have to be inserted manually. In addition, different wire diameters and wire qualities are frequently utilized, i.e., a reliable connection cannot be ensured.
FIG. 6 of DE 34 19 944 C1 shows a multiple wire supply that transfers the wire to a supply section. However, this system requires mechanized holding means that can be controlled individually for each wire coil. A wire transfer without holding means is not reliably ensured.
DE 34 19 944 C1 describes braking means arranged in front of the first guide head which make it possible to process the workpiece under a higher wire tension than that in the wire supply section. Consequently, the components within the latter section are subjected to lesser wear. A significantly lower tension in the wire between ensuing braking and tensioning rollers during the removal from the wire supply coil is particularly practical because an excessively high force within the first section would lead to blocking of the wire on the coil.
In the aforementioned braking means, the threaded wire is wrapped at least nearly once around a braking roller that is driven by a braking motor. In this case, the wire is effectively engaged with the braking roller by means of frictional forces.
EP-541,126 discloses a method in which the tension of the wire electrode is adjusted by different speeds of the tensioning and braking motor.
The previously mentioned braking roller may be arranged in a housing. DE 34 19 344 C1 discloses several devices for automating the threading process within this section of the wire transport system. Among other things, this document describes a device in which the wire enters the housing through an opening and initially adjoins the braking roller in essentially tangential fashion. In this case, at least one additional opening introduces fluid streams at least during the threading process. These fluid streams extend essentially tangential to the braking roller and guide the wire around the braking roller. Consequently, an automatic threading process, in which hardly any wear occurs, is attained.
U.S. Pat. No. 5,073,691 describes wire disposal means. These wire disposal means contain a suction nozzle and at least one pipe arranged behind the second guide head. In this case, the suction nozzle attracts the wire and a fluid by suction during the threading process and transports said wire through the pipe to the tensioning motor. This method makes it possible to automate the threading process within the wire disposal section without having to provide components that are particularly susceptible to wear. In comparison to conventional belt transport system, this is particularly advantageous. Changes in the moving direction of the wire are preferably realized by means of deflection rollers that are driven by a fluid jet at least during the threading process. This allows a precise wire disposal with slight vibration.
In order to reduce the volume of the used wire, a wire cutting device may be arranged behind the tensioning motor. When adjusting a rotational speed that is proportional to the moving speed of the wire, it is ensured that the wire is divided into identically long pieces during its disposal, independently of its moving speed. Consequently, a back-up in the disposal container can be prevented. Such a device is described in EP-198,229 by the present applicant.
The deflection rollers and/or braking roller used are usually constructed as a cylindrical disk, in the outer surface of which a uniform, circumferential groove is arranged. In this case, the groove is tapered symmetrically to the center of the other surface and in the direction toward the disk center. Such rollers allow a precise wire guidance with practically no vibrations and thus improve the processing quality on the workpiece. In addition, rollers of this type can be used for wires with different diameters. When used as a braking roller, such a V-groove increases the friction between the braking roller and the wire.
The individual components of a wire transport system can be realized in various ways. Satisfactory solutions, particularly with respect to the processing section and the disposal section, are disclosed in the state of the art. However, the solutions for the supply section which have been disclosed so far are not satisfactory.