Patent EA-005146-B1 (EA)
Title: Method of controlling an electrochemical machining process
Inventor: BRUSSE MAARTEN
IPC: B23H7/30
Filed: 2001-04-10
Abstract: 1. A method of electrochemically machining an electrically conductive workpiece by applying an electric current between the workpiece and an electrically conductive electrode while electrolyte is supplied between the workpiece and the electrode, the method comprising: measuring a voltage induced by the electric current, adapting the at least one process control parameter in response to the measured voltage, characterized by determining information relating to the spectral composition of the measured voltage within a predetermined measuring period during the process of electrochemically machining and adapting the at least one process control parameter in accordance with said information. 2. A method according to claim 1, wherein said information comprises at least one amplitude representative of at least one frequency component or at least one range of frequency components of the measured voltage. 3. A method according to claim 2 wherein said information comprises the amplitude representative of at least a harmonic frequency of the waveform constituted by the measured voltage within the predetermined measuring period. 4. A method according claim 3, wherein the method comprises expanding the wave form within the predetermined measuring period in a Fourier series of trigonometric functions and wherein said amplitudes correspond to the Fourier coefficients Ck of said series. 5. A method according to claim 4, wherein the method comprises determining the sign of the Fourier coefficients Ck of a first number of harmonics of said Fourier series and assigning a specific process condition to at least one specific combination of Fourier coefficients indicating absence or presence of a corresponding harmonic and in case of presence, the relative sign of the corresponding harmonic. 6. A method according to claim 5, wherein the method comprises assigning a first process condition of relatively low current density to the absence of a first consecutive number of Fourier coefficients Ck. 7. A method according to claim 5, wherein the method comprises assigning a second process condition of presence of gas-filled cavities in the electrolyte to the presence of second number of consecutive Fourier coefficients Ck with mutually alternating signs. 8. A method according to claim 5, wherein the method comprises assigning a third process condition of relatively high current density to the presence of a third number of consecutive Fourier coefficients Ck with mutually equal signs. 9. A method according to claim 2, wherein said information comprises amplitudes representative of a range of frequency components greater than a predetermined frequency and the adapting of the at least one process control parameter in case of a substantially change of the amplitudes within the predetermined measuring period. 10. A method according to claim 9, wherein said information comprises a running average of said amplitudes across a predetermined time interval. 11. A method according to claim 1, wherein the at least one process control parameter involves changing applying the electric current continuously to applying the electric current intermittently. 12. A method according to claim 11, wherein during applying the electric current continuously, the electrode and the workpiece are moved relatively to each other with a substantially constant speed and during applying the electric current intermittently, the electrode and workpiece are moved relatively to each other in an oscillatory manner or in a repeated manner superposed on a linear movement with applying the current at or near the instant of smallest mutual distance induced by the oscillatory or repeated distance between the workpiece and the electrode. 13. A method according to claim 12, wherein a sequence of intermittently applied electric current pulses is applied when the relative distance between the workpiece and the electrode is small during the relatively oscillatory or repeated movement. 14. A method according to claim 1, wherein the applying of electric current comprises applying electric current pulses of a normal polarity intermittently in pulse like periods, the at least one process control parameter controls applying additionally one or more electric current pulses of an opposite polarity. 15. A method according to claim 1, wherein the applying of electric current comprises applying electric current pulses of a normal polarity intermittently in pulse like periods, the at least one process control parameter controls applying additionally electric current passivation pulses of the same polarity but with an voltage having an amplitude which is inadequate to dissolve the workpiece and a passivation film on the workpiece. 16. A method according to claim 1, wherein the at least one process control parameter controls the application of an electrode cleaning electric current intermittently in one or more pulse like periods with an opposite polarity causing the electrode being cleaned of deposited waste. 17. A method according to claim 16, wherein the electrode and workpiece are moved relatively to each other in a repeated movement, the applying of electric current comprising applying electric pulses intermittently when the distance between the workpiece and the electrode is relatively small, the corresponding position of electrode and workpiece being determined by first bringing the workpiece and the electrode in contact with each other and applying a measurement current in stead of a machining current to determine a contact, the at least one process control parameter controls the applying of one or more electrode cleaning pulses prior to bringing the workpiece and electrode in contact. 18. A method according to claim 1, wherein the electrode and workpiece are moved relatively to each other in a repeated movement, the application of electric current comprising applying electric pulses intermittently in pulse like periods when the distance between the workpiece and the electrode is relatively small, the at least one process control parameter controls changing the duration of the pulse like period. 19. A method according to claim 1, wherein the duration of the pulse like period is reduced to a value smaller then a seeding time required for formation of gas bubbles in the electrolyte. 20. A method according to claim 19, wherein the pulse period is reduced to a value between 10 to 100 microseconds. 21. A method according to claim 20, wherein the corresponding pulse forefront has a value between 100 and 1000 nanoseconds. 22. A method according to claim 19, wherein sequences of intermittently applied electric current pulses are being applied, the pauses between the pulses in a sequence having a value larger than a escape time required for escaping gas bubbles that has been formed in the electrolyte. 23. A method according to claim 22, with a ratio of pause/pulse duration between 2 and 10. 24. A method according to claim 9, wherein the at least one process control parameter controls a fast interrupt of the applied electric current. 25. A method according to claim 1, wherein the electrode and workpiece are moved relatively to each other in an oscillatory movement, the electric current being supplied intermittently in pulse like periods when the distance between the workpiece and the electrode is relatively small, the at least one process control parameter comprises the relative phase shift between the oscillatory movement and the start of applying the electric current each oscillatory movement. 26. A method according to claim 1, wherein the electrode and workpiece are moved relatively to each other in an oscillatory movement, the electric current being supplied intermittently in pulse like periods when the distance between the workpiece and the electrode is relatively small, the at least one process control parameter comprises an electrolyte pressure. 27. A method according to claim 1, wherein the electrode and workpiece are moved relatively to each other in an oscillatory movement, the electric current being supplied intermittently in pulse like periods when the distance between the workpiece and the electrode is relatively small, the at least one process control parameter comprises the relative machining speed the workpiece and electrode are being moved relatively to each other. 28. A method according to claim 1, wherein the process of electrochemically machining comprises applying the electric current in pulse like periods, wherein the predetermined measuring period substantially corresponds to the duration of a pulse. 29. A method according to claim 1, wherein the process of electrochemically machining comprises applying the electric current substantially continuously during a first time duration, wherein the predetermined measuring period is a fraction of said first time duration, such that variations in process conditions may be measured within the measuring period. 30. An arrangement for electrochemically machining an electrically conductive workpiece by applying an electric current between a workpiece and an electrically conductive electrode while electrolyte is supplied between the workpiece and the electrode, the arrangement comprises: an electrically conductive electrode (3); means for positioning (4,5) the electrode and the workpiece (1) in a spatial relationship so as to maintain a gap between the electrode (3) and the workpiece (1); means for supplying (7) the electrolyte into the gap; an electric power supply source (11), which is electrically connectable to the electrode (3) and the workpiece (1) to apply an electric current between the workpiece (1) and the electrode (3), characterized in that, the arrangement further comprises voltage measurement means (17,22) electrically connected with the electrode (3) and the workpiece (1) or with an impedance circuitry in a power supply line between the power supply source (11) and the workpiece (1) or the electrode (3); process adjusting means