Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2001-12-04
2004-03-16
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C219S069110, C219S069160
Reexamination Certificate
active
06707304
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to machine tools, and more particularly to devices and methods for the contactless detection of a processing electrode of a machine tool.
BACKGROUND OF THE INVENTION
Spark erosion machines are used to machine work pieces by means of electrical spark discharge between an electrically conducting workpiece and a processing electrode. In particular, the workpiece can be cut by means of a wire electrode or it can be machined by means of a bar-type cavity sinking electrode in that it is drilled or hollowed. In this process, in addition to removing material particles from the work piece the processing electrode itself also wears off. Consequently, new electrode material has to be supplied continuously to the work area of the spark erosion machine. In a known wire spark erosion machine, for example, the wire electrode moves from a dispenser roll via several pulleys and conveyor lines to the top wire guiding arm from where the wire electrode is moved via additional pulleys and a brake roll that controls the tensile stress of the wire electrode in the work area to the top wire guiding head where a power supply unit is provided for supplying the processing current.
The wire electrode continues from the top wire guiding head through the work area (where a work piece is mounted for machining) to the bottom wire guiding head from where it is guided into the disposal area via another pulley. While the work piece is being machined the wire electrode moves through the wire moving system at a speed of approximately 100-300 mm/sec.
Inserting the wire electrode or reinserting a broken wire electrode into the electrode moving system is a difficult and time-consuming job. The manufacturers of machine tools always try to design user-friendly products and to automate such operations so as to allow the operating personnel to focus on programming and supervising the system. In modern wire spark erosion machines the inserting process is at least partially automated, for example by means of using nozzles that spray a stream of fluid into a conveyor tube, thus pulling in the wire electrode and moving it along.
In order to be able to automate the insertion and movement of the wire electrode in the electrode moving system as fully as possible it is desirable to always know the current position of the electrode tip so as to activate or deactivate suitable moving means at the appropriate time and set suitable control parameters for the wire moving system. Also, insertion errors are easier to diagnose and correct with this method. Therefore, any required manual intervention by the operating personnel can be reduced to a minimum. “Electrode tip” should be understood to mean both the beginning and the end of the wire electrode as it can be very useful to know where the remainder of the wire electrode is located, for example when the wire breaks or when it is cut so as to quickly reinsert it and minimize the system downtime.
In micro-drilling, the position of the electrode end can be detected and, based on this detection, the remaining length of the electrode is determined. Hereinafter, the term electrode tip will be used for such cases as well.
In the prior art, various detection methods are used to detect the electrode tip. For example, U.S. Pat. No. 4,412,118 specifies a device for detecting the position of the wire tip when the wire breaks or when the wire is cut intentionally. In this case, the wire electrode is rewound after the interruption whereby it touches a sliding contact. A comparator circuit detects when the contact between the sliding contact and the wire electrode stops and discontinues rewinding the electrode. A variant is also disclosed where the wire is detected contactless by means of a photo sensor.
Similar wire detection methods are disclosed in U.S. Pat. No. 5,019,684 and U.S. Pat. No. 5,268,551 where a sliding contact or a photo sensor are also used for wire detection. The device specified in U.S. Pat. No. 5,019,684 additionally measures the length of the return path while the electrode is rewound and based on this it determines the location of the wire break.
Another wire detector, which is based on electrically contacting the wire electrode, is disclosed in U.S. Pat. No. 5,523,545.
For completeness' sake German Patent DE 28 26 270 C2 should also be mentioned. This document specifies a different device not related to detecting the wire end. It relates to detecting and compensating the wire deflection caused by the spark erosive process forces (i.e. the deflection of the wire electrode on a plane that is vertical relative to the direction in which the wire advances). The proposed wire position sensor includes four measuring electrodes each of which is spaced apart from the wire electrode in one of the four axial directions +x, −x, +y and −y. A dielectric having a conductivity of 1-100 &mgr;S/cm flows through the overall arrangement. The deflection of the wire electrode in the xy direction is measured in that the change in the resistance is detected between the four measuring electrodes and the wire electrode serving as a common electrode for all four measuring cells.
In the detectors known from the prior art which are based on sliding contacts it was found to be disadvantageous that the detector is in continuous contact with the processing electrode which could adversely affect the electrode moving system. Also, the sliding contacts are susceptible to corrosion and subject to wearing and tend to develop insulating oxide layers. For this reason sliding contacts are unsuitable for low sensor voltages in the wet area of an erosion machine. Even in a largely automated wire moving system of a wire spark erosion machine, for example where a wire electrode is moved via wire guiding tubes and rerouting units, a non-contact detection is advantageous. Although photo sensor arrangements are found in the prior art for contactless wire detection, the contactless detection has to be traded off against various other disadvantages, depending on the structural design of the photo sensors. Because the electrode wire moves continuously, dirt, paraffin, metal and metal oxide deposits have to be expected which contaminate the photo sensor and, thus, cause malfunctions.
It is also known that when the wire is moved via a fluid through a substantially closed tube system it is possible that air bubbles will develop. Optical sensors can be affected by such air bubbles, and they could cause it to operate improperly. Furthermore, a wire detector is frequently subjected to the transport fluid which is under pressure so that the photo sensors have to meet special requirements with regard to tightness. Therefore, optical sensors overall are relatively susceptible to malfunctioning. An ideal wire detection device should be able to detect the full spectrum of wire and bar-type electrodes available on the market. Therefore, it has to be highly sensitive and cover a considerable measuring range because it has to be able to work across the full cross-section of the moving system. Any optical sensors meeting the above requirements are very expensive.
OVERVIEW OF THE DISCLOSED EXAMPLES
A device is disclosed for detecting a processing electrode which is compact and cost-effective. The device is for the contactless detection of a bar-shaped or wire-shaped processing electrode of a machine tool (e.g., a spark erosion machine), having a measuring area through which the processing electrode can be moved. At least one measuring electrode is disposed in the measuring area. The detection is based on an impedance change in the measuring area which is caused by the processing electrode moving through the measuring area.
Furthermore, a wire spark erosion machine is proposed which is equipped with detection devices in multiple places along the moving path of the wire electrode so as to monitor the wire moving system, thereby allowing the wire moving system to be fully monitored.
Additionally, a method is provided for the contactless detection of a bar-shaped or wire-
Bühler Ernst
Cugini Juri
Dal Bò Walter
Tobler Karl
Agie SA
He Amy
Marshall & Gerstein & Borun LLP
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