Process and apparatus for electro-erosion machining by means of electrical discharges providing a high rate of material removal

• US 3,875,362 A
• Filed: 12/26/1973
• Issued: 04/01/1975
• Est. Priority Date: 12/29/1972
• Status: Expired due to Term

• Current Assignee: Charmilles Technologies SA (Georg Fischer AG)
• Original Assignee: Francois (Vernier Balleys
• Inventors: Balleys, Francois (Vernier
• Primary Examiner(s): Reynolds, Bruce A.
• Application Number:
  US05/427,489
• Time in Patent Office: 461 Days
• Field of Search: 219/69R,69M,69P,69C
• US Class Current: 219/69.17
• CPC Class Codes: B23H 1/022 for shaping the discharge pulse train B23H1/024 takes precedence
• CPC Class Codes: B23H 7/16 for preventing short circuits or other abnormal discharges by altering machining parameters using adaptive control

Abstract

A process and apparatus for the electro-erosion machining of a workpiece by means of electrical discharges occurring between an electrode tool and the workpiece, permitting a considerable reduction of the wear of the electrode tool and which consist of appropriate logic circuit means for adjusting the duration of voltage cut-off between consecutive voltage pulses applied across the gap between the electrode tool and the workpiece, and in addition, or in the alternative, for adjusting the number of pulses in each pulse train such that at least one of the discharges in each train is effected at a characteristic voltage drop rate which is situated between the voltage drop rate of a normal pulse and the voltage drop rate of an abnormal pulse such as results from arcing or short circuit conditions between the electrode tool and workpiece, and adjusting the cut-off duration between two consecutive pulse trains to a time interval long enough to insure that at least the first discharge pulse in each pulse train is a normal nominal pulse.

Claims

1. A process for machining a workpiece by means of intermittent electrical discharges provided by controlled current pulses whereby consecutive trains of consecutive voltage pulses are applied across a machining gap between said workpiece and an electrode tool by switching on and off across said gap at least one source of direct current such that said source is switched off during pulse cut-off time intervals between two consecutive voltage pulses and during pulse train cut-off time intervals between consecutive pulse trains, each pulse having a no-load voltage amplitude of a predetermined value, said method comprising adjustably increasing the number of said pulses within a single pulse train such that at least one of the discharges across the gap is caused to be a characteristic discharge for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is lower than a predetermined voltage amplitude corresponding to a normal machining discharge and higher than a voltage amplitude corresponding to arcing or a short circuit across the gap, and increasing the duration of the cut-off interval between two consecutive pulse trains to a sufficiently long interval to prevent at least the first discharge within a single train from being one of said characteristic discharges.

2. The process of claim 1 further comprising decreasing the duration of the pulse cut-off time interval between two consecutive voltage pulses within a single pulse train to a relatively short duration.

3. The process of claim 1 wherein said characteristic discharge is detected by measuring the decay slope of said discharge voltage wave a predetermined period of time after the beginning of the current pulse of said characteristic discharge, said period of time being substantially short relative to the period of said discharge.

4. The process of claim 1 wherein said characteristic discharge is detected by measuring at the end of a predetermined time period the voltage amplitude of the machining pulse.

5. The process of claim 4 wherein said characteristic discharge is detected by determining the discharge for which the voltage pulse decay measured at the end of said time period is lower than a first threshold representative of normal machining discharges and higher than a second threshold lower than said first threshold, said second threshold being representative of arcing or short circuits across said gap.

6. The process of claim 1 further comprising providing a constant and predetermined cut-off time interval between two consecutive voltage pulses within a single train.

7. The process of claim 1 wherein the cut-off time interval between two consecutive voltage pulses within a single train is variable such as to maintain a substantially constant ratio of the number of characteristic discharges to the number of voltage pulses across said gap.

8. The process of claim 1 wherein the number of pulses within a single train is adjusted such as to maintain a substantially constant ratio of the number of characteristic discharges to the number of voltage pulses across said gap.

9. The process of claim 1 wherein the duration of the cut-off time intervals between two voltage pulses within a single train is decreased.

10. The process of claim 1 wherein the duration of the cut-off time intervals between two consecutive pulses within a single train is increased.

11. The process of claim 1 wherein each pulse train comprises a predetermined number of voltage pulses.

12. The process of claim 1 wherein each pulse train comprises a predetermined number of current pulses.

13. The process of claim 1 wherein a pulse train is cut off as soon as a predetermined number of characteristic discharges has occurred in a single train.

14. The process of claim 1 wherein a pulse train is cut off as soon as the ratio of the number of characteristic discharges within said train to the number of voltage pulses across the machining gap reaches a predetermined value.

15. The process of claim 1 wherein the cut-off interval between two consecutive pulse trains has a constant and predetermined duration.

16. The process of claim 1 wherein the cut-off interval between two consecutive pulse trains is variable such as to control the number of normal machining discharges which immediately precede the characteristic discharges within a single pulse train.

17. In an apparatus for electro-eroding a workpiece by means of current pulses resulting from applying across a machining gap formed between an electrode tool and a workpiece consecutive voltage pulses by switching on and off across said gap at least one source of direct current such that the source is disconnected from across the gap during cut-off time intervals between two consecutive voltage pulses, each of said trains comprising a plurality of voltage pulses each having a no-load voltage amplitude higher than a predetermined threshold and said trains of consecutive pulses being separated by a cut-off time interval during which none of said discharges occurrs across the gap and means for adjustably controlling the cut-off time interval between two consecutive pulse trains, the improvement comprising means for measuring the machining voltage amplitude of each pulse shortly after the moment at which current flows through the gap, means for effecting a comparison between said voltage amplitude and at least one predetermined value, means for deriving from said comparison a logic signal of alternative values corresponding to a sampling of respectively the presence and absence of characteristic machining discharges, means for storing said logic signal between two consecutive samplings, and means for adjusting as a function of said logic signal the number of voltage pulses within a single pulse train for controlling the rate of occurrence of said characteristic discharge across said machining gap, wherein each of said characteristic machining discharges is a discharge across said gap for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is higher than a voltage amplitude corresponding to arcing or a short circuit across the gap and lower than a predetermined voltage amplitude corresponding to a normal machining discharge.

18. The improvement of claim 17 further comprising means for detecting respectively the presence and absence of a characteristic discharge corresponding to the first discharge within each pulse train, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of said characteristic discharge, and means for respectively allowing and preventing the occurrence of said pulse train after said first discharge according to the significance of said logic signal.

19. The improvement of claim 17 further comprising means for counting the number of machining discharges and means for cutting off a pulse train when the number of said machining discharges within said train reaches a predetermined number.

20. The improvement of claim 17 further comprising means for counting the number of said characteristic discharges and means for cutting off a pulse train when the number of saId characteristic discharges within said train reaches a predetermined number.

21. In an apparatus for electro-eroding a workpiece by means of current pulses resulting from applying across a machining gap formed between an electrode tool and a workpiece consecutive voltage pulses by switching on and off across said gap at least one source of direct current such that the source is disconnected from across the gap during cut-off time intervals between two consecutive voltage pulses, means for supplying across said gap trains of consecutive pulses, each of said trains comprising a plurality of voltage pulses each having a no-load voltage amplitude higher than a predetermined threshold and said trains of consecutive pulses being separated by a cut-off time interval during which none of said discharges occurs across the gap and means for adjustably controlling the cut-off time interval between two consecutive pulse trains, the improvement comprising means for measuring the slope of the voltage amplitude decay of said pulses shortly after the moment at which current flows through the gap, means for effecting a comparison between said slope and at least one predetermined value, means for deriving from said comparison a logic signal of alternative values corresponding to a sampling of respectively the presence and absence of characteristic machining discharges, means for storing said logic signal between two consecutive samplings, and means for adjusting as a function of said logic signal the number of voltage pulses within a single pulse train for controlling the rate of occurrence of said characteristic discharges across said machining gap, wherein each of said characteristic machining discharges is a discharge across said gap for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is higher than a voltage amplitude corresponding to arcing or a short circuit across the gap and lower than a predetermined voltage amplitude corresponding to a normal machining discharge.

22. The improvement of claim 21 further comprising means for detecting respectively the presence and absence of a characteristic discharge corresponding to the first discharge within each pulse train, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of said characteristic discharge, and means for respectively allowing and preventing the occurrence of said pulse train after said first discharge according to the significance of said logic signal.

23. The improvement of claim 21 further comprising means for counting the number of machining discharges and means for cutting off a pulse train when the number of said machining discharges within said train reaches a predetermined number.

24. The improvement of claim 21 further comprising means for counting the number of said characteristic discharges and means for cutting off a pulse train when the number of said characteristic discharges within said train reaches a predetermined number.

25. In an apparatus for electro-eroding a workpiece by means of current pulses resulting from applying across a machining gap formed between an electrode tool and a workpiece consecutive voltage pulses by switching on and off across said gap at least one source of direct current such that the source is disconnected from across the gap during cut-off time intervals between two consecutive voltage pulses, means for supplying across said gap trains of consecutive pulses, each of said trains comprising a plurality of voltage pulses each having a no-load voltage amplitude higher than a predetermined threshold and said consecutive pulses being separated by a cut-off time interval during which none of said discharges occurs across the gap and means for adjustably controlling the cut-off time interval between two consecutive pulse trains, the improvement comprising means for measuring the machining voltage amplitude of each pulse shortlY after the moment at which current flows through the gap, means for effecting a comparison between said voltage amplitude and at least one predetermined value, means for deriving from said comparison a logic signal of alternative values corresponding to a sampling of respectively the presence and absence of characteristic machining discharges, means for storing said logic signal between two consecutive samplings, and means for adjusting as a function of said logic signal the duration of cut-off intervals between two consecutive voltage pulses within a single pulse train for controlling the rate of occurrence of said characteristic discharges across said machining gap, wherein each of said characteristic machining discharges is a discharge across said gap for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is higher than a voltage amplitude corresponding to arcing or a short circuit across the gap and lower than a predetermined voltage amplitude corresponding to a normal machining discharge.

26. The improvement of claim 25 further comprising means for detecting respectively the presence and absence of a characteristic discharge corresponding to the first discharge within each pulse train, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of said characteristic discharge, and means for respectively increasing and decreasing according to said logic signal by progressive steps the cut-off time interval between two consecutive pulse trains.

27. The improvement of claim 26 further comprising means for respectively allowing and preventing the occurrence of said pulse train after said first discharge according to the significance of said logic signal.

28. The improvement of claim 25 further comprising means for respectively allowing and preventing the occurrence of said pulse train after said first discharge according to the significance of said logic signal.

29. The improvement of claim 25 further comprising means for providing an analog magnitude representative of the average value of said logic signal and means for varying the duration of said cut-off interval between two consecutive voltage pulses within a single train as a function of the magnitude of the difference between a reference magnitude of predetermined value and said analog magnitude.

30. The improvement of claim 25 further comprising means for detecting the presence or absence of normal machining discharges, means for providing a corresponding logic signal, means for developing an analog magnitude representative of the average value of said logic signal, and means for varying the duration of said cut-off interval between two consecutive pulse trains as a function of the magnitude of the difference between a reference magnitude of a predetermined value and said analog magnitude.

31. The improvement of claim 25 further comprising means for counting the number of said characteristic discharges and means for cutting off a pulse train when the number of said characteristic discharges within a single pulse train reaches a predetermined number.

32. The improvement of claim 25 further comprising means for counting the number of machining discharges and means for cutting off a pulse train when the number of said machining discharges within a single pulse train reaches a predetermined number.

33. In an apparatus for electro-eroding a workpiece by means of current pulses resulting from applying across a machining gap formed between an electrode tool and a workpiece consecutive voltage pulses by switching on and off across said gap at least one source of direct current such that the source is disconnected from across the gap during cut-off time intervals between two consecutive voltage pulses, means for supplying across said gap trains of consecutive pulses, each of said trains comprising a plurality of voltage pulses each having a no-load voltage amplItude higher than a predetermined threshold and said consecutive pulses being separated by a cut-off time interval during which none of said discharges occurs across the gap and means for adjustably controlling the cut-off time interval between two consecutive pulse trains, the improvement comprising means for measuring the machining voltage amplitude of each pulse shortly after the moment at which current flows through the gap, means for effecting a comparison between said voltage amplitude and at least one predetermined value, means for deriving from said comparison a logic signal of alternative values corresponding to a sampling of respectively the presence and absence of characteristic machining discharges, means for storing said logic signal between two consecutive samplings, and means for adjusting as a function of said logic signal the duration of cut-off intervals between two consecutive voltage pulses within a single pulse train and the duration of the cut-off interval between two consecutive pulse trains for controlling the rate of occurrence of said characteristic discharges across said machining gap, wherein each of said characteristic machining discharges is a discharge across said gap for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is higher than a voltage amplitude corresponding to arcing or a short circuit across the gap and lower than a predetermined voltage amplitude corresponding to a normal machining discharge.

34. The improvement of claim 33 further comprising means for detecting respectively the presence and absence of a characteristic discharge corresponding to the first discharge within each pulse train, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of said characteristic discharge and means for respectively increasing and decreasing according to said logic signal by progressive steps the cut-off time interval between two consecutive pulse trains.

35. The improvement of claim 34 further comprising means for respectively allowing and preventing the occurrence of said pulse trains according to the significance of said logic signal.

36. The improvement of claim 33 further comprising means for respectively allowing and preventing the occurrence of said pulse trains according to the significance of said logic signal.

37. The improvement of claim 33 further comprising means for providing an analog magnitude representative of the average value of said logic signal and means for varying the duration of said cut-off interval between two consecutive voltage pulses within a single train as a function of the magnitude of the difference between a reference magnitude of predetermined value and said analog magnitude.

38. The improvement of claim 33 further comprising means for detecting the presence or absence of normal machining discharges, means for providing a corresponding logic signal, means for developing an analog magnitude representative of the average value of said logic signal, and means for varying the duration of said cut-off interval between two consecutive pulse trains as a function of the magnitude of the difference between a reference magnitude of a predetermined value and said analog magnitude.

39. The improvement of claim 33 further comprising means for counting the number of said characteristic discharges and means for cutting off a pulse train when the number of said characteristic discharges within said train reaches a predetermined number.

40. The improvement of claim 33 further comprising means for counting the number of machining discharges and means for cutting off a pulse train when the number of said machining discharges within said train reaches a predetermined number.

41. In an apparatus for electro-eroding a workpiece by means of current pulses resulting from applying across a machining gap formed between an electrode tool and a workpiece consecutive voLtage pulses by switching on and off across said gap at least one source of direct current such that the source is disconnected from across the gap during cut-off time intervals between two consecutive voltage pulses, means for supplying across said gap trains of consecutive pulses, each of said trains comprising a plurality of voltage pulses each having a no-load voltage amplitude higher than a predetermined threshold and said consecutive pulses being separated by a cut-off time interval during which none of said discharges occurs across the gap and means for adjustably controlling the cut-off interval between two consecutive pulse trains, the improvement comprising means for measuring the slope of the voltage amplitude decay of said pulses shortly after the moment at which current flows through the gap, means for effecting a comparison between said slope and at least one predetermined value, means for deriving from said comparison a logic signal of alternative values corresponding to a sampling of respectively the presence and absence of characteristic machining discharges, means for storing said logic signal between two consecutive samplings, and means for adjusting as a function of said logic signal the number of voltage pulses within a single pulse train, the duration of cut-off intervals between two consecutive voltage pulses within a single pulse train and the duration of the cut-off interval between two consecutive pulse trains, for controlling the rate of occurrence of said characteristic discharges across said machining gap, wherein each of said characteristic machining discharges is a discharge across said gap for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is higher than a voltage amplitude corresponding to arcing or a short circuit across the gap and lower than a predetermined voltage amplitude corresponding to a normal machining discharge.

42. The improvement of claim 41 further comprising means for detecting respectively the presence and absence of a characteristic discharge corresponding to the first discharge within each pulse train, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of characteristic discharge, and means for respectively increasing and decreasing according to said logic signal by progressive steps the cut-off time interval between two consecutive pulse trains.

43. The improvement of claim 42 further comprising means for respectively allowing and preventing the occurrence of said pulse trains according to the significance of said logic signal.

44. The improvement of claim 41 further comprising means for respectively allowing and preventing the occurrence of said pulse trains according to the significance of said logic signal.

45. The improvement of claim 41 further comprising means for providing an analog magnitude representative of the average value of said logic signal and means for varying the duration of said cut-off interval between two consecutive voltage pulses within a single train as a function of the magnitude of the difference between a reference magnitude of predetermined value and said analog magnitude.

46. The improvement of claim 41 further comprising means for detecting respectively the presence and absence of normal machining discharges, means for providing a logic signal of alternative values corresponding respectively to said presence and absence of normal machining discharges, means for developing an analog magnitude representative of the average value of said logic signal, and means for varying the duration of said cut-off interval between two consecutive pulse trains as a function of the magnitude of the difference between a reference magnitude of a predetermined value and said analog magnitude.

47. The improvement of claim 41 further comprising means for counting the number of said characteristic discharges and means for cutting off a pulse traiN when the number of said characteristic discharges within said train reaches a predetermined number.

48. The improvement of claim 41 further comprising means for counting the number of machining discharges and means for cutting off a pulse train when the number of said machining discharges within said train reaches a predetermined number.

49. In a process for machining a workpiece by means of intermittent electrical discharges provided by controlled current pulses whereby consecutive trains of consecutive voltage pulses are applied across a machining gap between said workpiece and an electrode tool by switching on and off across said gap at least one source of direct current such that said source is switched off during pulse cut-off time intervals between two consecutive voltage pulses and during pulse train cut-off intervals between consecutive pulse trains, and wherein characteristic discharges are detected, each of said characteristic discharges being one for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is comprised between predetermined values, the higher of said values corresponding to a normal machining discharge and the lower of said values corresponding to arcing or short circuit across the gap, the improvement comprising varying the length of the cut-off time interval between consecutive pulses within a single pulse train after detection of a predetermined number of successive characteristic discharges as a function of the difference between the total number of pulses within the pulse train and a reference number for maintaining said number of characteristic discharges within predetermined limits.

50. The process of claim 49 further comprising varying the cut-off interval between consecutive pulse trains as a function of said difference between the total number of said pulses within a pulse train and a reference number.

51. In a process for machining a workpiece by means of intermittent electrical discharges provided by controlled current pulses whereby consecutive trains of consecutive voltage pulses are applied across a machining gap between said workpiece and an electrode tool by switching on and off across said gap at least one source of direct current such that said source is switched off during pulse cut-off time intervals between two consecutive voltage pulses and during pulse train cut-off intervals between consecutive pulse trains, and wherein characteristic discharges are detected, each of said characteristic discharges being one for which the discharge voltage amplitude measured between the beginning of a current pulse and the time at which the current is cut off is comprised between predetermined values, the higher of said values corresponding to a normal machining discharge and the lower of said values corresponding to arcing or short circuit across the gap, the improvement comprising varying the length of the cut-off time interval between consecutive pulse trains after detection of a predetermined number of successive characteristic discharges as a function of the difference between the total number of the pulses within a single pulse train and a reference number for maintaining said number of characteristic discharges within predetermined limits.