Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With measuring – testing – or sensing
Reexamination Certificate
1999-04-05
2001-05-15
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With measuring, testing, or sensing
C205S659000, C205S654000, C204S22400M, C204S225000, C204S229500, C204S229600, C204S229800, C204S230600, C204SDIG009
Reexamination Certificate
active
06231748
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method, an arrangement and a power supply for a process of electrochemically machining an electrically conductive workpiece by applying electrical machining pulses between the workpiece and an electrically conductive electrode while electrolyte is supplied between the workpiece and the electrode.
Electrochemical machining is a process in which an electrically conductive workpiece is dissolved at the location of an electrode while electrolyte and electric current is supplied. For this purpose, the electrode is brought into the proximity of the workpiece and, while electrolyte is fed into the gap between the workpiece and the electrode a powerful current is passed through the workpiece and the electrode via the electrolyte, the workpiece being positive with respect to the electrode. The current is applied in the form of machining pulses having a given amplitude and duration. In the intervals between the machining pulses the electrolyte is renewed. During the application of the machining pulses the electrode and the workpiece are moved towards one another with a given feed rate, as a result of which the electrode forms a cavity or eventually a hole in the surface of the workpiece, the shape of this cavity or hole corresponding to the shape of the electrode. This process can be used, for example, for making intricate cavities or holes in or for shaping hard metals or alloys.
The copying precision with which the shape of the cavity or the hole in the workpiece follows the shape of the electrode is important for the quality of the result. Meanwhile, many publications in the form of articles in periodicals and patent documents have appeared in which proposals have been made to improve the copying precision of electrochemical machining.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide an electrochemical machining method and arrangement with a good copying precision. To this end, the method of the type defined in the opening paragraph is characterized in that the machining pulses alternate with electrical passivation pulses of the same polarity as the machining pulses, the voltage of the passivation pulses having an amplitude which is inadequate to dissolve the workpiece and a passivation film on the workpiece.
In the intervals between the machining pulses, during renewal of the electrolyte, passivation pulses are applied deliberately with such an amplitude that a passivation layer of metal oxides is formed around the electrode on the workpiece. During the next machining pulse this layer is removed selectively at the end face of the electrode. In this way the machining pulse is given a higher effectiveness in the feed direction. This improves the copying precision because comparatively more material of the workpiece is dissolved at the end face of the electrode and in the cavity to be machined in the workpiece smaller radii are formed at the edges and slopes of the lateral surface of the cavity. Additional advantages to be mentioned are that the energy consumption of the process is smaller and that the dissolution rate of the workpiece in the feed direction is higher. This is because, owing to the local passivation layers, the energy of the machining pulses is no longer used for the undesired removal of material from the lateral surfaces of the cavity in the workpiece.
Preferably, a distance between the workpiece and the electrode is maintained, which is smaller during the machining pulses than during the passivation pulses. By increasing the distance between the workpiece and the electrode during the passivation pulses it is achieved that owing to the greater distance the electric field of the passivation pulses has less effect at the end face of the electrode than at the lateral walls of the cavity in the workpiece. Thus, it is achieved with greater accuracy that opposite the end face of the electrode, i.e. at the bottom of the cavity in the workpiece, a thinner passivation layer, or even no passivation layer at all, is formed than elsewhere in the cavity to be machined. The amplitude, duration and waveform of the machining pulses are selected in such a manner that activation of the surface to be machined occurs only in the case of the smaller distance during the machining pulses. In that case anode dissolution during the application of machining pulses occurs only at those locations of the workpiece where the gap is smaller than the critical distance. The remainder of the workpiece is protected by a passivation layer and is not dissolved. This results in a very high copying precision. In a variant of the method in accordance with the invention, the workpiece and the electrode perform an oscillating movement relative to one another, the distance between the workpiece and the electrode reaching a minimum during the machining pulses.
The amplitude of the voltage of the passivation pulses plays an important part. Too low a voltage has no or little effect because the passivation layers are too thin. Conversely, too high a voltage causes the previously formed passivation layers to disappear at the lateral surfaces of the cavity and causes the copying precision to deteriorate. An even higher voltage will ultimately yield the same effect as the normal machining pulses and will cause the workpiece to dissolve at various undesired locations in the cavity to be machined. To optimize the amplitude of the passivation pulses a variant of the method in accordance with the invention is characterized in that the amplitude of the voltage of the passivation pulses is adjusted at least once during electrochemical machining, the amplitude of the passivation pulses in a series of successive passivation pulses being changed until the resistance value measured between the workpiece and the electrode has reached a maximum, after which electrochemical machining is continued with an amplitude of the passivation pulse which corresponds to the maximum of the resistance value.
During a series of successive passivation pulses the amplitude of the voltage of the passivation pulses is changed gradually, for example, increased from zero to a given maximum value at which the workpiece begins to dissolve. In each subsequent passivation pulse a slightly higher voltage is applied and the resistance of the gap is measured and stored. From the measured resistance values the highest value is chosen and the corresponding amplitude of the voltage is fixed and sustained for some time during the subsequent machining of the workpiece. The amplitude of the voltage of the passivation pulses is thus optimized for a maximal gap resistance. This implies that the formation of passivation layers on the lateral surfaces is also maximal and the copying precision is optimum.
For this purpose, the arrangement in accordance with the invention is characterized in that the arrangement comprises:
an electrode;
means for positioning the electrode and the workpiece in a spatial relationship so as to maintain a gap between the electrode and the workpiece;
means for supplying electrolyte into the gap;
a first electric power supply source, which is electrically connectable to the electrode and the workpiece to supply machining pulses to the workpiece and the electrode;
a second electric power supply source of the same polarity as the first power supply source and having an output voltage which is controllable by means of a control signal, which second power supply source is electrically connectable to the electrode and the workpiece to supply passivation pulses to the workpiece and the electrode;
means for alternately connecting the first and the second power supply source to the workpiece and the electrode;
means for generating a varying control signal for changing the output voltage of the second power supply source during successive passivation pulses;
means for measuring, during the successive passivation pulses, the electrical resistance of the gap between the workpiece and the electrode at an instant during the passivation pulses and for storing r
Agafonov Igor L.
Alimbekov Rinat A.
Belogorskij Aleksandr L.
Gimaev Nasich Z.
Kucenko Viktor N.
Bartley Ernestine C.
Franzblau Bernard
U.S. Philips Corporation
Valentine Donald R.
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