Method of machining by erosive electrical discharges

Abstract

A process and apparatus for achieving relative displacement between the electrode tool and the electrode workpiece of an EDM apparatus about a closed or looped path at a speed varying as a function of the machining gap width, such as to remove material from the machined surfaces at a substantially constant material removal rate.

I claim: 
     
       1. In a process for machining a surface of an electrode workpiece by electrical discharges initiated and supplied with current by a succession of voltage pulses applied to a machining zone across a machining gap between an electrode tool and said electrode workpiece according to which a relative displacement between said electrodes is achieved along a cyclic translation movement following a closed path .[.inscribed in a surface.]. of predetermined .[.shape and.]. .Iadd.contour .Iaddend.in such a manner as to .[.decrease the gap between said electrodes on a portion of the machines surface and to.]. produce a cyclic displacement of the machining zone, the improvement comprising .[.varying the speed of said translation movement.]. .Iadd.adjusting the angular speed of the cyclic translation movement at a predetermined value, adjusting the amplitude of said translation movement at a predetermined value in such a manner as to maintain a machining current at substantially all locations along said path during at least a first cycle of translation, measuring the cyclic variations of the machining gap during the following cycles of translation, and varying said angular speed as a function of said variations of the machining gap .Iaddend.so as to remove for all the machined surface of the workpiece the same quantity of material per unit area during at least one cycle of translation. .[.2. The improvement of claim 1 wherein said path of said translation is maintained in a 
     
     
        predetermined shape..]. 3. The improvement of claim .[.2.]. .Iadd.1 .Iaddend.wherein said .Iadd.angular .Iaddend.speed variation is effected during periods of machining comprising several cycles of translation, said periods being separated from each other by periods of machining during 
     
     
        which said translation has an amplitude which is regulated. 4. The improvement of claim 3 wherein the variations of said speed of translation are memorized and the same speed variations are effected during said periods of machining during which the amplitude of the translation 
     
     
        movement is regulated. 5. The improvement of claim 1 wherein said speed of translation movement is regulated as a function of the .[.variation.]. .Iadd.difference .Iaddend.between a reference value and an electrical 
     
     
        value representing said machining gap and the machining current. 6. The improvement of claim 5 wherein said reference value is a value representing the .[.means.]. .Iadd.mean .Iaddend.value of the .[.value representing the.]. machining gap computed during at least one cycle of 
     
     
        translation movement. 7. The improvement of claim 1 wherein said speed of the cyclic translation movement is controlled according to a program of speeds pre-established as a function of the shape of the workpiece to be 
     
     
        machined. 8. The improvement of claim 1 wherein successive and periodic inversions of the direction of the cyclic translation movement are effected, each period comprising at least one complete cycle of 
     
     
        displacement of the machining zone. 9. An apparatus for machining a surface of an electrode workpiece by electrical discharges initiated and supplied with current by a succession of voltage pulses applied to a machining zone across a machining gap between an electrode tool and said electrode workpiece according to which a relative displacement between said electrodes is achieved along a cyclic translation movement following a closed path .[.inscribed in a surface.]. of predetermined .[.shape and.]. .Iadd.contour .Iaddend.in such a manner as to .[.decrease the gap between said electrodes on a portion of the machined surface and to.]. produce a cyclic displacement of the machining zone, said apparatus comprising means for varying the relative displacement of the electrode tool and electrode workpiece according to a cyclic translation movement along a predetermined path, means for controlling the .Iadd.angular .Iaddend.speed with which said cyclic translation movement is effected, said last mentioned means being responsive to a first electrical value, and means for modifying said electrical value as a function of the .[.variation.]. .Iadd.difference .Iaddend.between a second electrical value representing the machining gap and the machining current and a reference value, wherein said first electrical value comprises two different electrical values one corresponding to the maximum .Iadd.angular .Iaddend.speed of translation movement and the other corresponding to the minimum .Iadd.angular .Iaddend.speed of translation movement, further comprising means for selecting one of said different electrical values, said last mentioned means responding as a function of the difference 
     
     
        between said second electrical value and said reference value. 10. An apparatus according to claim 9, wherein means are provided for counting the number of cycles of translation movement occured during the time interval in the course of which the current of machining is constantly less than a reference current, and means are provided for increasing by a predetermined incremental value said second electrical value as soon as said counting means indicates a predetermined number of cycles of 
     
     
        translation. 11. An apparatus according to claim 9, wherein means are provided for counting the number of cycles of translation movement during the interval of time in the course of which the current of machining is constantly higher than said reference value, and means are provided for giving to said first electrical value said first predetermined value as soon as said counting means indicate a predetermined number of translation 
     
     
        cycles. 12. An apparatus for machining a surface of an electrode workpiece by electrical discharges initiated and supplied with current by a succession of voltage pulses applied to a machining zone across a machining gap between an electrode tool and said electrode workpiece according to which a relative displacement between said electrodes is achieved along a cyclic translation movement following a closed path .[.inscribed in a surface.]. of predetermined .[.shape and.]. .Iadd.contour .Iaddend.in such a manner as to .[.decrease the gap between said electrodes on a portion of the machined surface and to.]. produce a cyclic displacement of the machining zone, said apparatus comprising means for varying the relative displacement of the electrode tool and electrode workpiece according to a cyclic translation movement along a predetermined path, means for controlling the .Iadd.angular .Iaddend.speed with which said cyclic translation movement is effected, said last mentioned means being responsive to a first electrical value, and means for modifying said first electrical value as a function of the .[.variation.]. .Iadd.difference .Iaddend.between a second electrical value representing the machining gap and the machining current and a reference value, wherein means are provided for counting the number of cycles of translation movement during the interval of time in the course of which the current of machining is constantly higher than said reference value, and means are provided for giving to said first electrical value said first predetermined value as soon as said counting means indicate a 
     
     
        predetermined number of translation cycles. 13. An apparatus for machining a surface of an electrode workpiece by electrical discharges initiated and supplied with current by a succession of voltage pulses applied to a machining zone across a machining gap between an electrode tool and said electrode workpiece according to which a relative displacement between said electrodes is achieved along a cyclic translation movement following a closed path .[.incribed in a surface.]. of predetermined .[.shape and.]. .Iadd.contour .Iaddend.in such a manner as to .[.decrease the gap between said electrodes on a portion of the machined surface and to.]. produce a cyclic displacement of the machining zone, said apparatus comprising means for varying the relative displacement of the electrode tool and electrode workpiece according to a cyclic translation movement along a predetermined path, means for controlling the .Iadd.angular .Iaddend.speed with which said cyclic translation movement is effected, said last mentioned means being responsive to a first electrical value, and means for modifying said electrical value as a function of the .[.variation.]. .Iadd.difference .Iaddend.between a second electrical value representing the machining gap and the machining current and a reference value, wherein means are provided for generating a first logic signal indicating the sign of the .[.variation.]. .Iadd.difference .Iaddend.between said second electrical value and said reference value, means are provided for memorizing the chronological order of the changes of state of said first logic signal during a translation cycle, means are provided for generating a second logic signal in which the changes of state correspond with anticipation in the course of a following translation cycle of the memorized chronological order of changes of state of said first logic signal, and means are provided for varying said first electrical value as a function of said 
     
     
        second logic signal. .Iadd.14.  In a process for machining a surface of an electrode workpiece by electrical discharges initiated and supplied with current by a succession of voltage pulses applied to a machining zone across a machining gap between an electrode tool and said electrode workpiece according to which a relative displacement between said electrodes is achieved along a cyclic translation movement following a closed path of predetermined contour in such a manner as to produce a cyclic displacement of the machining zone, the improvement comprising adjusting the angular speed of the cyclic translation movement at a predetermined value, adjusting the amplitude of said translation movement at a predetermined value in such a manner as to maintain a machining current at substantially all locations along said path during at least a first cycle of translation, measuring the cyclic variations of the machining current during the following cycles of translation, and varying said angular speed as a function of said variations of the machining current so as to remove for all the machined surface of the workpiece the same quantity of material per unit area during at least one cycle of translation. .Iaddend. .Iadd.15. The improvement of claim 14 wherein said angular speed variation is effected during periods of machining comprising several cycles of translation, said periods being separated from each other by periods of machining during which said translation has an amplitude which is regulated. .Iaddend. .Iadd.16. The improvement of claim 15 wherein the variations of said speed of translation are memorized and the same speed variations are effected during said periods of machining during which the amplitude of the translation movement is regulated. .Iaddend. .Iadd.17. The improvement of claim 14 wherein said speed of translation movement is regulated as a function of the difference between a reference value and an electrical value representing said machining gap and the machining current. .Iaddend. .Iadd.18. The improvement of claim 17 wherein said reference value is a value representing the mean value of the machining gap computed during at least one cycle of translation movement..Iaddend. .Iadd.19. The improvement of claim 14 wherein successive and period inversions of the direction of the cyclic translation movement are effected, each period comprising at least one complete cycle of displacement of the machining zone. .Iaddend. .Iadd.20. The improvement of claim 14 further comprising measuring the cyclic variations of the machining gap. .Iaddend. .Iadd.21. The improvement of claim 20 wherein said angular speed variation is effected during periods of machining comprising several cycles of translation, said periods being separated from each other by periods of machining during which said 
     
     
        translation has an amplitude which is regulated. .Iaddend. .Iadd.22.  The improvement of claim 21 wherein the variations of said speed of translation are memorized and the same speed variations are effected during said periods of machining during which the amplitude of the translation movement is regulated. .Iaddend. .Iadd.23. The improvement of claim 21 wherein said speed of translation movement is regulated as a function of the difference between a reference value and an electrical value representing said machining gap and the machining current. .Iaddend. .Iadd.24. The improvement of claim 23 wherein said reference value is a value representing the mean value of the machining gap computed during at least one cycle of translation movement. .Iaddend. .Iadd.25. The improvement of claim 21 wherein successive and periodic inversions of the direction of the cyclic translation movement are effected, each period comprising at least one complete cycle of displacement of the machining zone. .Iaddend. .Iadd.26. The improvement of claim 14 wherein said speed of the cyclic translation movement is controlled according to a program of speeds pre-established as a function of the shape of the workpiece to be machined. .Iaddend. .Iadd.27. The improvement of claim 20 wherein said speed of the cyclic translation movement is controlled according to a program of speeds pre-established as a function of the shape of the workpiece to be machined.