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Method of machining by erosive electrical discharges |
| RE31493 |
Method of machining by erosive electrical discharges
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| Patent Drawings: | |
| Inventor: |
Pfau |
| Date Issued: |
January 10, 1984 |
| Application: |
06/271,561 |
| Filed: |
June 8, 1981 |
| Inventors: |
Pfau; Jean (Collonge-Bellerive, CH)
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| Assignee: |
Ateliers des Charmilles, S.A. (Geneva, CH) |
| Primary Examiner: |
Shaw; C. C. |
| Assistant Examiner: |
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| Attorney Or Agent: |
Hauke and Patalidis |
| U.S. Class: |
219/69.13; 219/69.16; 219/69.17; 219/69.2 |
| Field Of Search: |
219/69M; 219/69V; 219/69G; 219/69C |
| International Class: |
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| U.S Patent Documents: |
2773968; 3135852; 3322929; 3433919; 3539754; 3564190; 3564190; 3581045; 3591761; 3739136; 3809852; 3816692; 3875374; 3919516 |
| Foreign Patent Documents: |
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| Other References: |
Jacques, "Application in EDM of Synchronized Pulsed Flushing . . . ", SAE 680638, Oct. 1968.. |
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| 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. |
| Claim: |
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 machiningzone 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.]. ofpredetermined .[.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 tomaintain 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 afunction 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 saidpath 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 separatedfrom 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 duringwhich 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 gapcomputed 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 machininggap 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 .[.shapeand.]. .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 therelative 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 themachining 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 whichthe 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 saidreference 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 electrodetool 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 theelectrode 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 beingresponsive 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 areference 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 forgiving 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 amachining 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 varyingthe 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 themachining 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 areprovided 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 thecourse 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 gapbetween 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 acyclic 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 manneras 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 angularspeed 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 ofclaim 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 isregulated. .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 translationmovement 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 themachining 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. Theimprovement 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. Theimprovement 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 oftranslation, 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 machiningduring 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 electricalvalue 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 translationmovement. .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 machiningzone. .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. |
| Description: |
BACKGROUND OF THE INVENTION
The invention relates to a method of machining by erosive electrical discharges initiated and supplied with current by a succession of voltage pulses applied between an electrode tool and an electrode workpiece, according to which a relativedisplacement between these electrodes is achieved along a cyclic translation movement following a closed path inscribed in a surface of predetermined shape and in such a manner as to decrease the gap between these electrodes on a portion of the machinedsurface and to produce a cyclic displacement of the machining zone.
Machining by electro-erosion comprises in general a phase of rough shaping followed by one or several finishing phases during which the machining rate and the gap comprised between the electrode tool and the electrode workpiece are progressivelyand simultaneously decreased. A known machining process (French Pat. No. 1,274,953) permits to effectuate all these operations with the same electrode tool. The process consists in displacing the electrodes one relative to the other according to aloop shaped path such as to reduce the machining gap to the value corresponding to the rate of machining imposed on a portion of the surface to be machined and to provide a cyclic displacement of the machining zone on that surface. This displacement isa translation which may be executed according to a circular path or according to a rectangular path if the surfaces to machine are orthogonal, or again according to a path inscribed in a surface of predetermined form or shape.
This known machining process presents a drawback when machining surfaces having a section of variable curvature or when machining plane surfaces of different sizes. For example, in the case of machining a surface of variable curvature byeffecting a circular translation movement, the machining zone is displaced at a speed varying proportionally to the radius of curvature of this surface. As the removal of material is inversely proportional to the speed of displacement of the machiningzone, the surface portions of small curvature radius are machined more rapidly than the surface portions of large curvature radius, so that the machining gap varies during each translation cycle and causes variations in the machining current, resultingconsequently in a decrease of the efficiency of machining. The machining gap becomes progressively wider and, as a result machining of the surfaces of small radius of curvature stops and an important loss of machining efficiency occurs at each cycle ofthe translation movement.
The same phenomenon arises in the case of plane orthogonal surfaces of different sizes, for example, when machining lateral surfaces of a rectangular cavity. In this case, owing to the fact that each surface is subjected to electrical dischargesat the same time, the density of discharges is larger on the surface of small size than on those of large size, such that the machining progresses more rapidly on the surfaces of small size and the machining current decreases during machining of thesesurfaces until it becomes nil. In addition, the increase of the gap at the angles prevents the electrical discharges from occuring during a relatively long time and results in an important drop in efficiency.
SUMMARY OF THE INVENTION
The new method, according to the present invention, aims to eliminate these losses of time and to permit a maximum machining efficiency. This method is characterized in that, during at least one cycle of translation, the speed of translation isvaried so as to remove for all the machined surfaces of the workpiece the same quantity of material per unit area.
Thus, modifying the translation speed, so as to equalize the material removal on all the portions of the surface subjected to machining, permits to eliminate partly or even completely the machining stoppages and the current variations during thetranslation. The speed of translation may be controlled so as to maintain within predetermined limits during each translation cycle a value representing the machining gap.
The invention has also for object an apparatus for practicing the aforesaid process.
BRIEF DESCRIPTION OF THE DRAWING
Three embodiments of this apparatus are described hereafter by way of examples and with reference to the attached drawing, in which:
FIG. 1 is an explanatory view relating to the machining of a lateral surface of a hole of oval shape;
FIG. 2 is a diagram of machining time of the hole according to FIG. 1;
FIG. 3 illustrates the machining of the lateral surfaces of a cavity of rectangular shape;
FIG. 4 is a diagram of relative time of machining of this cavity; and
FIGS. 5 to 7 show the circuit diagrams of three embodiments of apparatus for practicing .Iadd.the .Iaddend.process according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows an example of machining .[.of.]. the lateral surface of a hole which presents, in section, two different radii of curvature.
The electrode tool 1 is displaced with respect to the workpiece 2 along a circular translation path, so that point A of the electrode follows a circular path C. The machined surface presents two portions S.sub.1 of curvature r1 and two portionsS.sub.2 of curvature r2 larger than r1. When the point A of the electrode 1 follows the circle C at constant speed, the displacement speed of the machining zone, which is defined by the reduced gap dF, is proportional to the radius of curvature of themachined surface so that the speed of displacement of this zone takes the value V.sub.1 when this zone passes the segments ab and cd, and the value V.sub.2 greater than V.sub.1 when ir follows the segments bc and da. The removal of material beinginversely proportional to the speed of displacement of the machining zone, the gap increases more quickly during the machining of portions S.sub.1 than during the machining of portions S.sub.2. This results in a variation of machining current in thecourse of each translation cycle, thus causing a variation of the gap, and consequently a deformation of the workpiece.
The process according to the invention permits to keep the difference between the extreme values of the machining gap dF within predetermined limits in varying the translation speed, so as to maintain, for example, the .[.means.]. .Iadd.mean.Iaddend.machining current within predetermined limits. In these conditions, the variation between the speeds V.sub.1 and V.sub.2 of displacement of the machining zone is reduced in the same proportion during each translation cycle, and the machiningcan progress at the same speed (perpendicular to the surface to be machined) on all parts of the surface of the workpiece, i.e., the same quantity of material per unit of surface is removed on all the areas of the machined surface.
FIG. 2 is a diagram illustrating the advantages of the invention in the case of FIG. 1. The time during which machining proceeds is indicated on the coordinate, and on the ordinate is shown the mean value of the calculated mean machiningcurrent, for example, during each cycle T of a translation, this mean value following the curve C.sub.1 when, according to the known technique of the prior art, the speed of translation and the amplitude of movement remain unchanged.
At the instant t.sub.1, this mean value drops due to the fact that the areas S.sub.1 of the workpiece are no longer subjected to machining. Machining continues on the areas S.sub.2 up to the instant t.sub.2.
In varying the translation speed according to the process of the invention and in imposing the same rate of machining, the mean value of the current during each cycle follows the curve C.sub.2. It is apparent that the mean value of the currentis higher and that machining is terminated at the instant t.sub.3. The gain in machining time is represented by the interval t.sub.2 -t.sub.3. The quantity of removed material being the same in both cases, the surfaces S.sub.3 and S.sub.4 are equal.
FIG. 3 illustrates another example of machining in the case of a rectangular hole in a workpiece.
After the finishing phase, the electrode tool is displaced by translation along a circular path C. In this case, machining of the areas S.sub.5 and S.sub.6 respectively, occurs when the translation trajectory executes the angles a1 and a2,respectively. These angles having equal values, each area is subjected to machining during the same time interval. By contrast, during the translation a0, the machining gap takes too high a value to permit initiation of the discharges. If the areaS.sub.5 is smaller than the area S.sub.6, the density of the discharges on this area S.sub.5 will be the highest, and consequently the progress of machining will be more rapid on the area S.sub.5 than on the area S.sub.6. The mean machining currentoscillates between two values as in the preceding case, but in addition an interruption occurs during the translation a0.
In varying the speed of translation, the "dead time" during which the machining is interrupted is reduced to a minimum and the areas S.sub.5 and S.sub.6 are subjected to machining during different time intervals, so as to maintain the meancurrent within the limits which depend on the sensitivity of the speed adjusting apparatus.
For this particular shape of the workpiece, translation according to a square path a.sub.1, a.sub.2, a.sub.3, a.sub.4 would have the advantage of machining the four corners of the workpiece. In this case also, a variation appropriate to thespeed of translation permits an increase in the efficiency of machining.
FIG. 4 is a diagram illustrating the advantage which can be obtained by the present invention in the case of the example of FIG. 3. According to the known method of machining, the mean value of the current during each translation cycle followsthe curve C.sub.3. From time O to time t.sub.4 .[.only the area S.sub.6 is.]. .Iadd.the areas S.sub.5 and S.sub.6 are .Iaddend.machined. Starting at the instant t.sub.4, the machining occurs only on the area S.sub.6 and the operation is terminated atthe instant t.sub.5. In utilizing the process of the invention, the value of the current follows the curve C.sub.4. There is shown a strong increase of .[.means.]. .Iadd.mean .Iaddend.current owing to the fact that the time intervals without machininghave been reduced, so that the operation is terminated at the instant t.sub.6 and the gain in time represented by the interval t.sub.6 -t.sub.5 takes a value yet larger than in the preceding example.
The process of the invention is also advantageous when finishing operations succeed each other one after the other, the passage of an operation to the following being effected when the mean current or a characteristic value of the machining gapbecomes smaller than a reference value.
FIG. 5 shows an apparatus for carrying out the process of the invention. In this apparatus, a source S delivers the current supply to a machining gap 3 between an electrode 2 and a workpiece 1 through a pulse generator GI .[.controlled by aprogrammer PR.]..
The machining voltage U(t) and the machining current I(t), the latter being a function of the voltage drop across a resistor R in series in the machining circuit, are applied at two inputs of a circuit 4 adapted to deliver a signal g which ischaracteristic of the machining gap width. This signal g is compared with a reference signal Wo in a comparator 25 for supplying a signal representing the difference between the mean voltage between the workpiece 1 and the electrode 2 and the desiredvoltage which is represented by the signal Wo. The output signal of the comparator 25 is fed through an amplifier 5 to a servo-control apparatus GTC controlling the translation of the electrode 2 relatively to the workpiece 1.
The signal W(t) applied to the servo-control apparatus GTC controls the speed of translation such that a drop in the machining voltage with respect to the reference signal Wo, which indicates that the gap is narrower than desired, produces a .[.nincrease.]. .Iadd.decrease .Iaddend.of the translation speed and vice versa.
The apparatus according to FIG. 5 comprises yet different components which permit a correction of the speed of translation with respect to a signal representing the mean value of the machining gap during one complete translation cycle. To thiseffect, an integrator circuit 7 furnishes a signal g corresponding to the integration of the value g during a cycle. Of course, this integrator 7 receives a reset pulse at each cycle of translation, the means for applying this pulse being notrepresented in the drawing.
The signal g is applied to one of the inputs of a comparator 26, the other input of which receives the signal g. The output of the comparator 26 is applied via an amplifier 6 to a switch J1. Thus, by the operation of this latter, it is possibleto compare to the reference Wo either the instant signal g or the difference between this instant signal and the mean value of this signal during the preceding cycle. Thus, when the switch J1 is in its position b, it acts on the speed of translation asa function of the difference between the instant value of the machining gap and its mean value at the time of the preceding cycle. This difference can, of course, be positive or negative, but after passage in the comparator 25, a value which presentsalways the same polarity is obtained.
The signal g is applied to a bistable flip-flop B.sub.1, the second input of which receives a signal go provided by the programmer PR. This signal go defines the mean machining gap desired for the considered machining phase. This flip-flopB.sub.1 responds in delivering a signal on its output Q each time the signal g is greater than go, i.e. each time the effective machining distance is greater than the desired mean value defined by the signal go.
The output signal of the flip-flop B.sub.1 is applied to an input of a transfer register RT, the second input of which receives through an AND gate 16 a signal provided by a counter CT. The counter CT receives at each translation cycle a pulsearriving by a line T and produces a signal each time the number of the received pulses is equal to a number defined by an input NT.
When the mean machining gap, illustrated by the signal g, becomes greater than the gap defined by the signal go, the flip-flop B.sub.1 changes its state. Its output signal is applied, on the one hand to the AND gate 16, and on the other hand toa differentiator 15, the output of which controls the reset of the counter CT. This latter counts a number of translation cycles equal to NT and then gives a signal to a second input of the gate 16. The AND gate 16 is then conductive, and the transferregister RT increases of one step .[.is.]. .Iadd.its .Iaddend.output signal A(t) to increase the amplitude of the translation movement between the electrode and the workpiece. In this way, the correction of the amplitude of the translation is noteffected immediately, but only after the counting by the counter CT of the number of cycles defined by NT.
The circuit is also provided with means permitting alternation of a phase of machining in which the amplitude of the translation is regulated as a function of the instant electrical discharge conditions with a phase of machining during which thespeed is varied as explained above. To this effect, the apparatus comprises a differential amplifier 22 fed with the signals g(t) and go for supplying an instantaneous signal of variation. This latter is applied, on the one hand, to a bistableflip-flop 23 and, on the other hand, to a switch 24. The flip-flop 23 receives on its other input the output signal of the register RT. If the switch J6 is closed and the flip-flop 23 gives a signal indicating that the regulated amplitude is lower thanthe value defined by the register RT, the switch 24 changes its position to apply to the servo control GTC the output signal of the amplifier 22. When the regulated amplitude from the differential amplifier 22 reaches the value defined by the registerRT, the output signal of the flip-flop 23 disappears and the switch 24 returns to its rest position, with the result that the amplitude of the translation is maintained at a fixed value until a new signal is furnished by the flip-flop 23.
The circuit of FIG. 5 shows also that the translation servo control device GTC is driven by the output signal of a counter .[.GI,.]. .Iadd.CI, .Iaddend.the input of which can be connected by a switch J2 to the line T which receives a pulse ateach translation cycle. This arrangement permits to invert the sense of the translation movement after a predetermined number of cycles counted by the counter CI.
The programmer PR has a further input fed by a machining rate control DT for changing the machining .[.rate.]. .Iadd.conditions .Iaddend.according to a pre-established program.
FIG. 6 illustrates another embodiment of the invention in which certain parts are the same as those of the apparatus described with reference to FIG. 5. These common parts carry the same reference numerals and are not described .[.anew.]. .Iadd.hereinafter. .Iaddend.
In the diagram of FIG. 6, the translation servo control GTC differs from that of FIG. 5 in the sense that instead of being controlled by an analogous signal w(t), it is sensitive to the frequency of pulses F(t) formed from a clock H by divider12, and led to the servo control GTC via the AND gates 11 and 13 and the OR gate 14. The pulses of the clock H correspond to the high frequency giving the maximum speed of translation controlled by the servo control GTC, while the frequency coming outof the divider 12 gives the lowest speed of translation.
In the case of FIG. 6, the watched characteristic value is the signal Io furnished by the programmer PR to the pulse generator GI. This signal Io is applied to an input of a flip-flop B.sub.2, the other input of which receives a signal Idefining the .[.means.]. .Iadd.mean .Iaddend.machining current, said signal I being produced by an element 9 such as a current detector. When the signal I is smaller than the reference signal Io, the flip-flop B.sub.2 gives a signal on its output Q.Like in FIG. 5, this signal passes through a differentiator 15 for resetting the counter CT. On the other hand, this same signal can be applied by a switch J3, in its position a, to one of three inputs of an OR gate 10 the output of which drives the ANDgate 11 to lead the pulses of the clock H to the line F(t) via the OR gate 14 and thus controlling the maximum speed of translation. It should be noted that the output signal of the OR gate 10, via an inverter 8, locks the AND gate 13. When the OR gate10 does not provide an output signal, the AND gate 11 is locked, while the AND gate 13 is open, thus giving passage to the pulses from the divider 12 to control the minimum speed of the translation movement.
The control of the translation at the maximum speed is of course operated each time a signal is applied to one of the three inputs of the OR gate 10. This is the case especially when a signal is furnished by a detector DU which is responsive tothe presence of abnormal machining conditions. These conditions are checked by the detector DU in response to the machining voltage U(t) and to the machining current I(t).
The third input of the OR gate 10 is driven by the output of an AND gate 17, the inputs of which are connected respectively to the output Q of the flip-flop B.sub.2 and to the output of the counter CT. In this way, the AND gate 17 initiates themaximum speed of translation after a predetermined number of translation cycles checked by the counter CT has passed during which the mean current was higher than that of the reference Io. This maximum speed of translation is intended to provide abetter cleaning of the machining zone.
When the switch J3 is in its position b, the high speed of translation is switched on when a signal is furnished at the output of an AND gate 18. The AND gate 18 has its inputs connected to a shift register RD which memorizes chronologically thechanges of the output Q. The changes pass across this register RD at a speed determined by the frequency of the signal F1 which proceeds from the servo control GTC. Thus, in the course of a translation cycle which follows the cycle of registration ofchanges of state of the output Q, the changes in state memorized in the register RD are transmitted with an advance of phase in relation to the instantaneous position of the translation .Iadd.by mean of the second input of the AND gate 18. .Iaddend.Thus, an anticipation in the command of the variation of speed is obtained, which permits an increase of rapidity and precision in the regulation of speed.
FIG. 7 shows an example of an apparatus of command of translation movement of variable speed and amplitude, according to a circular or a square path.
The servo control GTC of FIGS. 4 and 5 is replaced by means for controlling the resulting motion of a table of crossed movements 19 by way of stepping motors Mx and My controlled according to the diagram of FIG. 7.
This apparatus utilizes circuit elements known under the name "binary rate multiplier" BRM, into which successive pulses are introduced at a determined frequency and which produce at their output pulses the .[.means.]. .Iadd.mean.Iaddend.frequency of which is proportional to the binary number shown at the input.
Pulses of frequency F1 are applied to the .Iadd.binary rate multiplier .Iaddend.elements BRM1 and BRM2, the output pulses of which .[.the.]. .Iadd.are .Iaddend.respectively introduced into up-and-down preselection counters Cx and Cy counting upto a preselected number Xo and then counting down to zero, the counting up of one counter taking place during the counting down of the other. The numbers calculated by the counters Cx and Cy determine respectively the rate of variation of thefrequencies of output of the elements BRM1 and BRM2. Under those conditions, the relationship of the frequencies at the output of the elements BRM1 and BRM2 is equal to the relationship of the number y and x and, consequently, these numbers correspondto the coordinates of a point being displaced in a circle at a speed proportional to the frequency F1.
The control pulses of the stepping motors Mx and My are supplied respectively by the binary rate multiplier elements BRMx and BRMy fed by the frequency AF.sub.1 which is delivered by a binary rate multiplier element BRM3 controlled by a binarynumber A given by an analog-digital converter 20 fed by the signal A(t) of FIG. 5 or FIG. 6. Besides, the elements BRMx and BRMy are placed under the dependance of the numbers y and x supplied by the counters Cy and Cx.
The orders for inversion of the direction of movement of the motors .[.is.]. .Iadd.are .Iaddend.given by the signals S.sub.1 and S.sub.2 formed by a logic circuit CL as a function of the two signals indicating respectively the counting up andcounting down of the counters Cx and Cy.
Thus, in order to vary the amplitude of the movement without changing the tangential speed, it suffices to vary the signal A(t) and the frequency F.sub.1 in inverse relationship. On the other hand, if one varies F.sub.1 without modifying A(t),the translation speed varies in the same proportions.
The signal of frequency AF.sub.1 is made dependent on the reference frequency F(t) by means of a frequency comparator 21 which modifies the rate of variation of the pilot frequency Fo in acting on the binary rate multiplier element BRM4 so as tomaintain the product A.F1 constant, whatever the value of the number A given by the converter 20. A translation movement according to a square path may be effected by placing the switches J4 and J5 in position b. In this case, the displacement speeds ofthe motors are proportional to XoAF1 or zero, according to the state of the logic signals S3 and S3 formed by the circuit CL, the numer Xo corresponding to the maximum capacity of the counters Cx and Cy.
The invention may be applied to other forms of translation path and to other control systems for these movements, for example electro-mechanical servo-systems with eccentrics or electro-hydraulic .Iadd.translating systems .Iaddend..
Of course, the process which has been described previously by way of example and which may be carried out by the apparatus illustrated in FIGS. 5 to 7 need not necessarily be carried out continuously during the machining. It may be adventageousto alternate the machining periods during which the speed of translation is varied while maintaining constant the path of this translation with other periods during which the path of the translation is not maintained constant, but is modified under theknown action of a servo-mechanism which tends to maintain the machining gap constant.
The alternating of the machining periods as indicated above is advantageous, for it permits a higher machining speed, since the machining gap is continually maintained as small as possible, which gives a maximum machining current during theperiods during which the path of the translation is not strictly determined. The exact outline of the machined surface is then obtained during the following period during which the path of translation is no longer modified, while the speed oftranslation varies in the sense which tends to give a uniform removal of material from all of the machined surface.
Each machining period must obviously comprise at least one complete closed cycle of translation, but in practice, it is advantageous to provide for machining periods, each comprising several tens of cycles of translation.
Finally, in the case where the modifications imposed on the speed are memorized, as explained, for example, with reference to FIG. 6, .[.intersecting.]. .Iadd.interesting .Iaddend.results can also be obtained in effecting a single registrationof speed variations to communicate to the translation during a complete cycle of this latter, this registration of variations being then utilized without further modification up to the end of machining, as well as the action of the servo-mechanism whichmaintains the machining gap at a constant and predetermined value.
Having thus described the present invention by way of examples of embodiments for practicing the invention, modifications whereof will be apparent to those skilled in the art, what is claimed as novel is as follows:
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