Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With control responsive to sensed condition
Reexamination Certificate
1999-01-12
2001-11-06
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With control responsive to sensed condition
C205S645000, C205S652000, C204S22400M, C204S228100, C204S229800, C204S230400, C219S069160, C219S069190
Reexamination Certificate
active
06312584
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of electrochemically machining a workpiece by means of an electrode, said method comprising the steps of:
setting a gap between the electrode and the workpiece, which gap is filled with an electrolyte,
feeding an electric current through the electrolyte in the gap via the electrode and the workpiece in order to machine the workpiece,
generating a measurement signal comprising information about possible sparks in the gap, and
protecting the electrode and/or the workpiece against damage by reducing the current when a parameter of the measurement signal exceeds a certain threshold value.
The invention further relates to an apparatus for electrochemically machining a workpiece by means of an electrode, said apparatus comprising:
an actuator for setting a gap between the electrode and the workpiece,
fluid transport means for flushing the gap with an electrolyte,
power supply means for feeding an electric current through the electrolyte in the gap via the electrode and the workpiece in order to machine the workpiece, and damage prevention means comprising
measurement means for generating a measurement signal comprising information about partial discharge precursors of spark discharges in the gap,
detection means for generating a stop signal when a parameter of the measurement signal exceeds a threshold value, and
reduction means for reducing said current upon detection of said stop signal.
The invention also relates to a damage prevention means comprising
measurement means for generating a measurement signal comprising information about premonitory phenomena of partial spark discharges in an inter-electrode gap,
detection means for generating a stop signal when a parameter of the measurement signal exceeds a threshold value, and
reduction means for reducing said current upon detection of said stop signal.
Such a method, apparatus and damage prevention means are known from GB 2 140 824. The known method is applied in an apparatus for electrochemically machining (ECM) a workpiece with an electrode. According to this method a gap is set between the electrode and the workpiece. This gap is filled with an electrolyte and a current from a power supply is fed through the electrolyte via the electrode and the workpiece. In order to prevent the generation of an electric arc across the gap, individual spark discharges are detected by means of an electric circuit connected to the electrode and the workpiece. The electric circuit comprises filter means to distinguish noise caused by spark discharges from noise generated by the power supply. Upon detection of spark discharges the power supply is disabled and damage due to an electric arc is prevented. A disadvantage of the known method, apparatus and damage prevention means is that the spark discharges themselves also damage the electrode and the workpiece. Especially when the inter-electrode gap is smaller than 50 micrometers, spark discharges should be prevented because the resulting damage may require replacement of the electrode.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce the risk of damage to the electrode and/or the workpiece. To this end, the method according to the invention is characterized in that the measurement signal is generated by means of an antenna for detecting HF electromagnetic waves originating from the gap. Due to this measure the measurement signal contains information about the onset of spark discharges. By reducing the current upon detection of this information, the occurrence of spark discharges is counteracted so that the risk of damage is reduced.
The invention is based on the insight that at the onset of a spark discharge, partial discharges occur in an intermediate layer within the gap. Such an intermediate layer may be a passivation layer on the anode, a bubble layer of hydrogen gas on the cathode or a bubble layer of oxygen gas on the anode. Such an intermediate layer has a low conductivity compared to the electrolyte so that almost the entire gap voltage appears across the intermediate layer. Such an intermediate layer has a thickness which is only a part of the gap width so that the electric field strength resulting from the gap voltage can be so high that a partial discharge occurs in this intermediate layer. Such partial discharges cause a local heating of the electrolyte so that a gas bubble is formed. If the volume of gas generated is sufficient it can form an insulating bridge between the electrode and the workpiece and through an avalanche effect a spark discharge between the electrode and the workpiece will result. Thus, it can be understood that the partial discharges are precursors of a spark discharge. It has been found that the partial discharges cause far less damage than the spark discharges. In addition, it has been found that these partial discharges result in HF electromagnetic waves which can be detected with an antenna. Thus, by monitoring electromagnetic waves with such an antenna and reducing the current upon detection of electromagnetic waves corresponding to partial discharges, damage of the electrode and/or the workpiece can be counteracted. As a result, a smaller gap width can be applied resulting in an improved machining speed and accuracy.
It is to be noted that measurement of sparks with an antenna is known from GB 2 095 153. However, this measurement is applied in an electric discharge machine (EDM) which is intended for generating spark discharges recurring at a frequency of 10 to 30 MHz. In such an EDM machine the gap phenomena are completely different and there is no mention of partial discharges. The antenna is used to generate a signal that indicates the frequency and amplitude of the spark discharges and not to detect partial discharges that precede a spark discharge.
A parameter which is suitable and can easily be monitored is, for example, the amplitude of the measurement signal after having been filtered by a band-pass filter which transmits a high frequency band. By filtering the lower frequencies it is avoided that, for example, the mains frequency interferes with the detection of partial discharges.
In an industrial environment there are usually many machines which radiate HF bursts by switching large currents on and off. If these HF bursts are picked up by the antenna this could lead to so called ‘false alarms’. In order to reduce the risks of ‘false alarms’ it is preferred to monitor the amount of energy in a high frequency band of the measurement signal.
The threshold value for the monitored parameter corresponding to a partial discharge can easily be determined by an experiment in which spark discharges are produced deliberately and in which the measurement signal is observed in a time slot preceding the occurrence of a spark. This will be elucidated hereafter in the description with reference to the drawings.
The apparatus and the damage prevention means according to the invention are characterized in that the measurement means comprises an antenna for detecting HF electromagnetic waves originating from the gap.
The embodiment of the invention wherein the antenna is electrically isolated from the electrode and the workpiece has the advantage that HF noise present on the power cords connected to the electrode and/or the workpiece is not directly coupled to the antenna so that the quality of the detection is improved.
The embodiment of the invention wherein the antenna is a dipole antenna has the advantage that the antenna can be positioned outside the working area so that the electrode and the workpiece can easily be accessed and changed without obstruction by the antenna.
The embodiment of the invention wherein the antenna is a loop antenna positioned around the gap has the advantage that the antenna is very sensitive to electromagnetic waves originating from discharges in the gap and is less sensitive to interfering electromagnetic waves from the environment.
The embodiment of the invention wherein the detection means comprise a band-pass filter followed by a level detector to det
Altena Hermanus S. J.
De Silva Anjali K.
Bartlett Ernestine C.
Franzblau Bernard
U.S. Philips Corporation
Valentine Donald R.
LandOfFree
Method and apparatus for electrochemically machining a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for electrochemically machining a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for electrochemically machining a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2616045