Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1998-12-04
2001-06-26
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069120, C219S069160, C219S069170, C219S069180
Reexamination Certificate
active
06252191
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to electrical discharge fine-machining (EDM), and, more particularly, to a method and apparatus for controlling an EDM process.
BACKGROUND OF THE INVENTION
Cavity-sinking or wire-EDM is now already state of the art with excellent surface quality of R
a
<0.1 &mgr;m and at a material defect of the surface with a depth of less than 1 &mgr;m. Such machining requires generators that can generate monopolar or bipolar pulses in the megahertz range with current intensities around 1A This type of generator for ac pulses is disclosed in DE 40 11 752 A1. An oscillating resonance circuit at resonance gap widths to 20 pm and alternating current frequencies to 30 MHz is proposed with the purpose of permitting novel resonance machining during wire-EDM. However, an interfering self-control effect of the wire electrode has to be involved, which can cause errors in contour trueness on the order of the resonance gap width (20 &mgr;m). This effect is known to anyone skilled in EDM, who has already worked with finishing pulses at frequencies above 100 kHz. It is established that, despite trouble-free servocontrol and compensation of the contouring errors of the wire electrode, sharp external contours are rounded and inside corners have material excess. The more often such a fine cut is repeated, the poorer the precision. It is also found that the vertical profile becomes increasingly bulged, convexly bulged in the zones with material excess and concavely bulged in the zones with too much material removal.
It is also known that the electrical resistance of the spark gap in EDM during additionally generated periodic pauses can be determined by means of a measurement current source and the servodrive, rinsing device and generator controlled with it (CH 650 433). This process has the task of determining the degree of contamination, as well as the discharge concentration during cavity-sinkig EDM from the electrical resistance and controlling the process with reference to this value so that higher cutting performance is achieved during unsupervised operation.
In the interest of suppressing wire vibrations that can occur during EDM, it is known that such vibrations can be detected (JP 09248717 A, JP 63-216631, JP 63-22922). Lastly, Japanese Pat. Appi. No. 63-312020 discloses an EDM device wherein the vibration of the working electrode in the direction of machining a workpiece is detected. A machining gain is set in accordance with the amplitude of the vibration of the working electrode. The EDM operation can therefore be constantly performed with optimum machining gain in accordance with the change and fluctuation of the machining conditions.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a process is useful for electrical discharge fine-machining of a workpiece with an electrode, where the electrode and the workpiece are at a fine-machining distance from one another and where smooth finishing pulses are applied to the workpiece in a controlled process. The machining is done intermittently, such that the smooth finishing pulses are applied at the gap during the processing times and the measuring source is applied at the gap at least during the interruption times. During the interruption times, it is determined with the aid of an electrical measurement if the electrode is in contact with the workpiece, and a pulse duty factor is obtained from this measurement, which determines the amount of contact. A control signal is derived from this for controlling the process.
In accordance with another aspect of the present invention, an electrical discharge machining method is useful in connection with a workpiece disposed at a fine-machining distance from an electrode to define a gap. The method includes the step of applying smooth finishing pulses to the gap intermittently such that each smooth finishing pulse is applied during a processing time and not during an interruption time in accordance with a control signal. Next, a determination is made with an electrical measurement at the gap during the interruption time as to whether the electrode is in contact with the workpiece. The method further includes the steps of obtaining a pulse duty factor therefrom and generating the control signal in accordance with the pulse duty factor.
In some embodiments, the determining step includes the step of recording oscillations of the electrode during a measurement time in accordance with a spark gap signal. The pulse duty factor may be defined as that part of the measurement time during which the electrode is in contact with the workpiece. The measurement time may be synchronized with the interruption time such that the measurement time begins after a delay from an onset of the interruption time.
In some embodiments, the generating step includes the steps of generating a first guide quantity in a measurement module from the pulse duty factor and controlling an advance system via providing a first reference signal to a servocontroller. If the pulse duty factor is higher than the first reference value, an advance speed of the advance system is reduced. If the pulse duty factor is lower than the first reference value, the advance speed is increased. A second guide quantity may be generated in the measurement module from the pulse duty factor and the duration of the processing time may be determined from a second reference value. If the pulse duty factor is lower than the second reference value, the duration of the processing time is reduced. If the pulse duty factor is greater than the second reference value, the duration of the processing time is increased.
In accordance with yet another aspect of the present invention, an apparatus is useful for electrical discharge machining of a workpiece disposed at a fine-machining distance from an electrode to define a gap. The apparatus includes a measurement module that develops a pulse duty factor from an electrical measurement regarding whether the electrode is in contact with the workpiece. The apparatus further includes a controller responsive to the pulse duty factor to establish a processing time during which smooth finishing pulses are applied to the gap and an interruption time during which the smooth finishing pulses are not applied to the gap. The electrical measurement occurs during the interruption time.
In some embodiments, the apparatus includes a generator that produces the smooth finishing pulses and a measurement source that produces a measuring voltage during the interruption time. The measurement module may determine the pulse duty factor from electrode oscillations during a measurement time, and may develop from the pulse duty factor first and second guide quantities to control an advance system and to control the duration of the processing time, respectively. The pulse duty factor is preferably representative of that part of the measurement time during which the electrode is in contact with the workpiece. In some other embodiments, the measurement source provides a current source characteristic with adjustable voltage limitation and pulse interruption capabilities. The measurement module may include a comparator that receives a spark gap signal and an adjustable limiting value to develop via a switching device and a low-pass filter an average value during the measurement time. The generator may include a class C amplifier having a modulation input terminal for receiving a machining time signal to establish a modulation gap of the amplifier at 100%. The generator may also include a pair of switching elements disposed in a half-bridge configuration and a pair of antiparallel diodes. The switching elements may be shifted into a nonconducting state in accordance with the machining time signal.
In some embodiments, the generator includes a series output wiring having an adjustable load resistance, an adjustable inductance, and an adjustable capacitance. At least one of these adjustable elements may be adjusted so that a resonance frequency of the series output wiring matches an
AGIE SA
Evans Geoffrey S.
Marshall, O'Toole, Gerstein, Murray & Borun.
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