GB2033608A - Cutting by a wire electrode - Google Patents

Abstract

A wire electrode 1 cuts a workpiece 3 by electro-erosive or electro- chemical means. The cutting portion of the electrode tends to trail behind the mounting clamps 2 and may introduce errors between the actual cutting path and the desired path. The angle between these paths is evaluated and when it exceeds a predetermined value the direction of movement of the electrode is reversed until the trail is removed. The electrode then continues its forward movement and cutting is resumed from the point where it was left off. The cyclic reciprocatory movement of the wire to remove the trailing error is effected by apparatus including a computer 6 controlling feed motors through a controller 10 causing relative movement between the workpiece 3 and the wire electrode 1. A device 5 senses the voltage and current at the work gap 31 and controls the feedspeed. Deterioration of the erosive pulses in the work gap, through an OR-gate 8, causes a store 9 to effect a backward movement of the wire; the operation of computer being inhibited. <IMAGE>

Description

SPECIFICATION Method and apparatus for paeven8iaag21 praveeu errOG: which arise in the electro-chemical cutin ef a worXsspiece through the deflection of a wire eleGtz The invention relates to a method and also to an apparatus for preventing errors which occur in the course of the electro-erosive or electro-chemical cutting of a workpiece through the relative movement of the cutting portion and the clamping device of a wire electrode.

Electronic computers, numerical controls, movement copying devices (e.g. hydraulic, electric, optic), mechanical devices (e.g. having linear or rotary movements) cause relative movements to be executed as between the electrode and the workpiece.

In this way movement path contours are electroerosively or electro-chemically cut out of the workpiece. Such known apparatus and machines are described in Swiss Patent Specifications Nos.

476,544; 513,693; 513,694 and 521,815, and also in the following articles: the journal "Technische Rundschau" No. 23, of June 1975, "Spark-erosive NC-cutting in the Centre of a Universal Manufacturing System", by Dr. W.

Ullmann; journal "Technische Rundschau", No.61, the 1969 issue, "Wire Erosion now with Numerical Path Control", by Dr. W. Ullman; "Technische Rundschau", lao. 1. February 1971, "Numerically Controlled Spark Erosion", by Dr. W.

Ullmann and Dr. B. Schumacher; "JahrbuchfurOpikund Feinmechanik", 1971, Pegasus-Verlag, Wetzlar, pp. 273ff., article by Ing.

H.A. Schmidt, "Difficult Precision Processing with Single (Simple) Copper Wire"; journal "American Machinist", 21 February 1972, "Sparkling Wire Curves Carbide Dies".

In the course of electro-erosive or electro-chemical cutting of workpieces in the required movement path contours, such as is described in detail in the cited literature, the drawback is experienced that the wire electrode, of diameter of 0.03 to 0.5 mm for example, is deflected during the cutting process proper. This deflection of the wire relative to the wire clamping device is referred to as the trailing of the wire electrode. In a further reference "Proceedings Electro-machining ISEM 5", 21-24 June 1977, Wolf sberg, Switzerland, F. Balleys describes, in the article "Removal Rate vs. Accuracy in Wire Cut" the detailed circumstances which lead, in consequence of the low stiffness of the wire electrode, to the above-mentioned trailing errors.A trailing error of the wire electrode of this kind disturbs the realisation of the desired movement path contour so much that the required manufacturing tolerances of the work pieces to be cut are not met. These workpieces are either rejected or they have to be given further processing. However, this subsequent processing is only possible in some cases. The disadvantageous deviations and distortions from the required move ment path contour occur in the case of transition points of the movement path contour, particularly in the case of deviations from rectilinear movement path sections, such as corners, kinks, turns, ellipses and arcs of circles, and also when the contours are closed. Brief proposals are given, in the lattermentioned article, for removing these movement path contour errors.However, these proposals require elaborate calculations and/or are too inaccurate, as the errors depend both on the size of the angular alteration of the path vector of the contour and also on the workpiece thickness, on the wire diameter, on the materials used for wire and workpiece electrodes, and also on the other technological values, or require a second processing, which does not necessarily give the required accuracy or cannot, in general, be carried out in the formation of "outer cuts", such as are described in the cited literature references.

The invention has the object of removing these movement path contour errors, which are caused by trailing of the wire electrode.

According to one aspect of the invention there is provided a method of preventing errors which arise, in the electro-erosive or electro-chemical cutting of a workpiece, through the trailing (as herein defined) of a wire electrode in a work gap, which electrode is directly held by a wire clamping device, comprising inspecting the angle between the desired path and the trailing of the electrode and, if the angle exceeds a predetermined value, retracting the wire electrode by corresponding output signals of a computer, in the movement path contour already cut until the forces, which are set up during the removal of material, and which are exerted transversely of the electrode axis, and lead to the trailing effect, become so small that the wire electrode is again rectilinearly pulled in consequence of the forces exerted by the wire clamping device, whereupon the rectilinearly pulled wire electrode is again advanced, on the same movement path contour, to the point at which the backward movement began, where-upon the normal electrical removal process is resumed.

According to another aspect of the invention there is provided apparatus comprising a wire electrode held in a clamping device in a work gap for electro-erosive or electro-chemical cutting of a workpiece through the trailing of the wire electrode, feed motors to move the electrode relative to a workpiece, an evaluation device of the work gap, and a computer for controlling the feed motors in response to the evaluation device.

Embodiments of the invention are described in detail below with reference to the accompanying drawings, in which: Figure 1 shows a workpiece which is to be cut out, from a blank, using the method according to the invention; Figure 2 illustrates a first embodiment of the apparatus for cutting out the workpiece in the manner illustrated in Figure 1; and Figure 3 illustrates a further embodiment of the apparatus for performing cutting in the manner illustrated in Figure 1.

As shown in Figure 1, the workpiece is to cut that the apparatus shown in Figure 2 or Figure 3 starts to move the wire electrode, which is clamped in its clamping device, at point PA. The wire electrode is moved to point PO In the course of this movement the wire-shaped electrode is energized to erode the workpiece. At point P,, which is the starting point of the movement path contour, the clamping device of the electrode is moved through angle (p1 = 90" to the left.In the course of its movement between PA and P0 the wire-shaped electrode has already undergone very pronounced deflection and, if this deflection, referred to as trailing, is not taken into account, the electrode would not reach point P0 and would execute a rounded, and therefore incorrect, movement path contour. According to the invention the clamping device of the wire electrode executes an increment of movement in the direction of point PA at the instant at which the computer commands the (forward) feed motors to move the wire-shaped electrode through the angle cp1. As soon as the deflection of the wire electrode has been eliminated in consequence of the backward movement, the wire electrode returns to the point at which it started the backward movement, and executes a further cutting movement.In the course of this latter-mentioned movement the wire electrode is again deflected in consequence of the forces acting on it. As soon as the wire clamping device has again reached the point Pn the wire-shaped electrode again moves back in the movement path contour already cut. In this way the trailing or deflection is reduced to zero.

As soon as this has taken place the wire electrode again advances to the point at which the backward movement started. In this way the path contour is correctly cut, at point P,, in accordance with the required angle cp1. No falsifications or path section errors occur. The wire electrode then moves, along the rectilinear path section ii of Figure 1 to point P1, at which the electrode is cut through the next angle cup2, which amounts to 90 , by a series of cycles of backward movement and advancing. The electrode then moves into the rectilinear path section 12. At point P2 the wire electrode negotiates the angle cup3.

The electrode then moves through the rectilinear path section 13. At the next point P3 the angle ep4 only has a value of a few degrees. The wire electrode is not moved backwards and forwards for this small angle. This is due to the fact that a specific angular value is fed, as reference angle, into the computer and serves as lower limit value. As soon as this fixed angular value has been exceeded by cutting with the wire electrode, the cyclic process of moving the wire electrode backwards and forwards (for preventing the movement path error caused by the trailing of the electrode), takes place; this cyclic movement of the electrodes has been described a number of times above. Let it now be assumed that the reference angle, which has been fed into the computer, is appreciably greater than the angle cp4 at point P3.

The wire electrode then moves, being suitably controlled by the computer, to the new movement path section 14 to the point P4. The angle (p5 at point P4 again only amounts to a few degrees. However, in this case, the computer takes into account the sume of the two angles (p4 + (p5. If this sum exceeds the stored reference angle, the cyclic backwards and forward movement of the wire electrode takes place at point P4 for eliminating the deflection or trailing of the electrode. At this point it should be pointed out that the reference angle, which has ben stored in the computer, has been arrived at on the basis of the required accuracy criteria, and has been programmed. The electrode then cuts the section 15 to point P5. In this case angle (p6 should be as great as the programmed reference angle.In this case the wire electrode is moved cyclically backward and forward, as has been described a number of times before. Let it be assumed that the angle (p7, at the point Pg, is greater than the programmed reference angle, so that a cyclic backward and forward movement of the wire electrode takes place at point P6. The wire electrode cuts section 17 and reaches the next point P7. At this point P7 an arc of a circle begins; this will be briefly described. The wire electrode should now perform an arcuate cutting motion. These individual points of the circle arc are not programmed in the computer. The target point Pn-1 only is stored in the computer. The computer then interpolates the arc point by point.At the same time it monitors the angular rotation of the tangential vector, and as soon as this angular rotation exceeds the value of the reference angle stored in the computer, which should be at point P8 in Figure 1, the cyclic backward and forward movement of the wire electrode takes place. In this way the arcuate path section is cut to the target point Pn-1; deflection or trailing of the wire electrode has been eliminated. The path section has been cut without error. The wire electrode then again travels along the rectilinear path section into the point Pn. At that point the electrode turns through the angle cpn = 90" in a known manner and with the cyclic backward and forward movement of the wire electrode.The wire electrode then cuts through the last path section In + 1 and thus reaches point PO The cutting process normally ends at that point. However, it should be pointed out that, although this point P0 has been reached numerically (through the wire guide arm of the wire clamping device), the cut has not truly been guided right up to point P0 due to the trailing of the wire electrode.

However, the computer is to programmed that, when the cutting process ends, the electrode is moved back and is then again moved forward to the point at which the backward movement started. In this way the last part of the cut is completed, so that the cut has really been completed, in error-free manner, at point PO The electrode then travels in the direction PE. This movement path section PO-PE is not further cut since a working voltage is no longer applied to the wire electrode. Finally, it is to be noted that the workpiece of Figure 1 can be cut both electro-chemically and also electro-erosively.

Figure 2 shows an apparatus which so controls the wire electrode that the tool can be cut in the manner shown in Figure 1.The apparatus shown in Figure 2 is constructed for electro-erosive cutting. The wire electrode 1 is moved, relative to the workpiece 3, whose presence is only symbolically indicated in the figure. The wire clamping device 2 clamps the wire electrode, so that it is rectilinearly pulled. In the present embodiment the wire electrode is moved, as indicated by the direction of the arrow, downwardly in the direction of its longitudinal axis. It is readily possible to arrange matters so that the wire electrode does not move. A generator applies the voltage, required for the cutting process, between the wire electrode 1 and the workpiece 3. This generator is not shown. The literature sources cited at the outset refer to many embodiments of such generator.Figure 2 indicates that the wire electrode 1 is moved in the left-hand direction, as indicated by the arrow. In this case the deflection or trailing of the wire electrode would be such that the wire electrode trails to the right, as indicated in dashed line. An evaluation device 5 for the parameters in the work gap 31 senses the voltage by lines 51, 54 and the current by lines 52, 53. The evaluation device 5 is connected to the computer 6 by way of conductors 100,101,102, 103. A forward feed timing generator7 supplies the computer, by way of conductor 71, with the required timing pulses, which synchronize all the numerical control procedures. The signal for controlling the forward feed timing generator 7 arrives by way of conductor 100.When the cutting conditions alter, e.g. deteriorate, in the work gap 31, the signal in conductor 100 causes the forward feed timing generator 7 to reduce, by way of conductor 71, the timing signals for the forward feed. This means that the relative movement between wire electrode 1 and workpiece 3 is reduced. When the cutting condition in the work gap 31 has improved, the forward feed timing generator 7 is so acted on, by way of conductor 100, that the frequency of the forward feed movements is increased by way of conductor 71. This means a higher speed of relative movement between wire electrode 1 and workpiece 3. The dashed line 103 is intended to indicate that the signals for affecting the forward feed movement in conductor 100 can be readily directly fed into the computer 6 in the absence of a forward feed timing generator 7.The computer produces a similar effect or reaction to that which has just been described in connection with the forward feed timing generator 7.

The evaluation device 5 supplies, by way of conductor 102, a signal indicating deterioration of the erosive work pulses in the work gap 31. Such a deterioration signal is generated when the erosion process no longer takes place in the work gap. This signal passes both into computer 6 and also into the OR-gateS. The OR-gate 8 passes the signal, by way of conductor 81, to the backward store 9. The backward store causes, through the intermediary of conductors 110,111,112, 113,114,115, the feed motors to be controlled by control circuit 10 in such a way that the wire electrode 1 is moved backwards.

The speed of backward movement is determined by the generator 91, which generates the backward movement timing pulses. The conductors designated as 11 lead to the individual stepping motors, which produce the relative movement between the wire electrode 1 and the workpiece 3 both in the X axis and also in the Y axis, or in the Z axis and C axis (when polar coordinates are used for the production of conical cuts).

As has already been mentioned, the computer 6 also receives, by way of conductor 102, the deterioration signal. This signal acts on the computer 6 to cause the movement path information, passing by way of conductors 104,105,106,107,108,109, to the backward store 9 as, by way of conductors 110, 111, 112,113,114,115 and control circuit 10, to the feed motors, to be stopped. Instead of this movement path information, the backward information from the backward store 9 passes, by way of conductors 110 115, to the control circuit 10 for the feed motors. The wire electrode is then moved backwards until the deterioration signal of the work gap decays in conductor 102.This means that the OR-gate 8 can no longer affect the backward store 9, and that the computer 6 prepares the interpolation process for the further guidance of the movement path section beyond the spark deterioration point. However, this interpolation cannot be carried out, as the backward store 9 continues to block it, by way of conductor 92, until the wire electrode 1 has advanced to the point at which the spark deterioration took place and at which the backward movement began. Only when the wire electrode has reached this point does the backward store free, by way of conductor 92, the interpolation process for the further movement contour so that, by way of conductors 104-109 and 110 - 115, the control circuit 10 can appropriately drive the feed motors.

In this way the movement contour of Figure 1 can be cut by the apparatus shown in Figure 2. This can be done in two ways. Firstly, the values of the movement contour, and also the points at which the cyclic backward and forward movements of the wire electrode 1 are to take place, can be specifically programmed on to an information carrier (e.g.

punched tape or magnetic tape). The programme can readily decide at which point the angle of the vectorial rotation is so great that a trailing error is to be expected. This first method has a small drawback in the case of the arc of a circle of Figure 1 (points P7 to Pn-1). The programmer would have to programme appreciably more points than is shown in the figure. However, the programmer can also directly feed these values onto a keyboard 13. The second mode of procedure consists in feeding, into the information carrier 12, the reference angle which indicates the upper limit value for the individual angles Pn-1 of Figure 1. The computer 6 then independently calculates at each point whether the angle rp exceeds or does not exceed this reference value. If the reference value is not exceeded, the cyclic reciprocatory movement of the wire electrode 1 takes place for eliminating the trailing error. The advantage of this second method resides in the fact that the arc of a circle of Figure 1 is no longer so difficult to programme.

The two briefly described methods not only show, as described above, the points at which the wire electrode is to be moved backwards, but these methods also define the path length which is to be travelled through and also how often this path length is to be negotiated. The information concerning the required path length of backward movement of the wire electrode can either be specifically programmed in the information carrier 12 - this will take place in a similar way to the other geometrical data for cutting the movement path contour of Figure 1 - or information concerning the path length of backward movement can be introduced, by means of the manual input 13, into the computer 6 by the programmer. Afurther possible procedure consists in storing so-called technology data in the computer 6, the computer automatically computing the path length, through which the wire electrode must travel backward at each point, on the basis of the stored technology data. If this data is not programmed in the information carrier 12, or is not to be introduced by the manual input 13, or if the technology data are not stored in the computer 6, the path length of backward travel of the wire electrode is determined by the deterioration signal in the conductor 1 02..As was described above, the evaluation device 5 generates, in conductor 102, a deterioration signal of the work gap 31, if the conditions in the work gap have deteriorated. This deterioration for example takes place if there is a short circuit between the wire electrode 1 and the workpiece 3.As soon as the wire electrode 1 has travelled backward, and the deterioration condition is no longer present in the work gap, the deterioration signal is switched off in the conductor 102, so that the computer 6 receives a command to again so act on the backward store 9, the control circuit 10, and the corresponding feed motors through line 11, that the wire electrode 1 is again moved in the forward direction to the point at which the deterioration took place. The next section of the movement path is then electrically removed by the wire electrode 1. This action is continued until the wire electrode again undergoes a certain deflection. The above-described process, whereby the wire electrode 1 is moved back, is then repeated.The information as to how often the wire electrode must travel backward at one of the points Pn of the movement path contour of Figure 1 can also be established in various ways.

This information can be contained, together with all other data and information, in the information carrier 12. The information can also be directly introduced by the operative with the manual input 13. The operative effects manual input by means of so-called technology tables. These tables represent experience values, which have been copiled in the course of actual practice. Another way in which the information concerning the number of backward movements of the wire electrode 1 can be established consists in storing the above-mentioned technology and experience values in the computer 6. By means of this stored information the computer 6 calculates the number of backward movements of the wire electrodes 1.It is also possible to control the number of backward movements by way of conductor 102 and by means of the deterioration signal which is generated by the evaluation device 5. In this case it is no longer necessary to introduce the information, concerning the number of backward movements, into the information carrier 12 or by means of the manual input 13 or by the stored technology and experience values. In this case the technological experience values are no longer necessary at all. It is to be noted that the OR-gate 8 is either controlled by the deterioration signal in conductor 102 or by the output signal of the computer 6 in the conductor 61. The OR-gate actuates, by way of the output conductor 81, the backward store, when the deterioration signal appears in the line 102 or when the output signal appears in the conductor 61 of the computer.The output signal in conductor 61 only appears when the information concerning the length of the backward movement and concerning the number of the backward movements of the wire electrode 1 has either been established in the information carrier 12 or by the manual input 13 or the technology and experience tables stored in the computer 6.

Cases have been described above in which a real deterioration has taken place in the work gap 31 between wire electrode 1 and workpiece 3. It will be made clear below that the tendency itself to such deterioration in the work gap can be detected, and, with the present invention, the wire electrode can be adaptively retracted merely when such a tendency is detected. The evaluation device 5 generates the signal in conductor 101 when the electro-erosive or electro-chemical removal of material between the wire electrode 1 and the workpiece 3 takes place over an angle smaller than 180 subtended by the axis of the wire electrode. In the case of an electro-erosive or electro-chemical removal of material with an angle of 180 relative to the wire electrode 1 removal of material takes place both at the front and also at the two sides.If, due to any circumstances in the work gap 31, this removal angle should be smaller than 180 then, as mentioned above, the signal is generated in the conductor 101 and passes to the computerS. This computer makes a decision, on the basis of the instantaneous process data, whether the wire electrode should or should not be retracted on the occasion of such a tendency to deterioration. If the computer 6 has a decision in favour of retracting the wire electrode 1, this computer controls, by way of conductor 61, the OR-gate which so acts, by way of output conductor 81, on the backward store 9 that signals on lines 11 to the feed motors retract the wire electrode 1 by way of control circuit 10. If the computer 6 decides that the tendency to deterioration (signal in conductor 101) is not serious, no signal passes along conductor 61.

The OR-gate 8 is only acted on at a later point of time, that is to say when deterioration has actually taken place in the work gap 31, for actuating the backward store 9, the control circuit 10, and the feed motors by lines 11.

Figure 3 shows a further embodiment. This example contains components which have already been thoroughly described in connection with Figure 2. A difference in Figure 3 is that the backward store 9 of Figure 2 is no longer contained in the example of Figure 3. The computer 6 is so set out that it replaces the function of a backward store by forward and backward interpolation. The work performed by the OR-gate 8 is also taken over by the computer. All other operations are the same as in Figure 2.

Open parts of the work gap 31 may be sealed off by applying a magnetic field from the outside to act on a liquid containing a mixture of magnetic particles present in the work gap 31. The external magnetic field may be used to improve cleaning conditions in the work gap 31.

Claims (9)

1. A method of preventing errors which arise, in the electro-erosive or electro-chemical cutting of a workpiece, through the trailing (as herein defined) of a wire electrode in a work gap, which electrode is directly held by a wire clamping device, comprising inspecting the angle between the desired path and the trailing of the electrode and, if the angle exceeds a predetermined value, retracting the wire electrode by corresponding output signals of a computer, in the movement path contour already cut until the forces, which are set up during the removal of material, and which are exerted transversely of the electrode axis, and lead to the trailing effect, become so small that the wire electrode is again rectilinearly pulled in consequence of the forces exerted by the wire clamping device, whereupon the rectilinearly pulled wire electrode is again advanced, on the same movement path contour, to the point at which the backward movement began, whereupon the normal electrical removal process is resumed.

2. A method according to claim 1, wherein the points of inspection are controlled by the computer programme.

3. A method according to claim 1, wherein the points of inspection are manually fed into an input device which programmes the computer.

4. A method according to any one of the preceding claims, wherein, when the wire electrode reaches a point of inspection, a signal is generated by a device which evaluates the electrical removal process which simulates a short circuit in the work gap.

5. A method according to any one of the preceding claims, comprising sealing the work gap by a liquid containing an admixture of magnetic particles by application of a magnetic field applied from the outside.

6. A method according to any one of the preceding claims, comprising cleaning the work gap by application of an external magnetic field to a magnetic liquid in the work gap.

7. Apparatus comprising a wire electrode held in a clamping device in a work gap for electro-erosive or electro-chemical cutting of a workpiece through the trailing of the wire electrode, feed motors to move the electrode relative to a workpiece, an evaluation device for the work gap, and a computer for controlling the feed motors in response to the evaluation device.

8. A method according to claim 1, comprising inspecting in the computer said angle and, when the angle has a predetermined value, the wire electrode is retracted, by corresponding output signals of the computer, in the movement path contour already cut until the forces, which are set up during the removal process, lie transversely of the electrode axis, and cause the trailing effect to become so small that the wire electrode is again rectilinearly pulled in con sequence of the forces exerted by the wire clamping device, whereupon the rectilinearly pulled wire electrode is again advanced, on the same movement path section, to the point at which the backward movement began, whereupon the normal electrical removal process is resumed.

9. A method according to claim 1, wherein the computer, which interpolates the movement path contour, generates a signal at the end of the cutting process, and the wire electrode is moved back, in the movement path contour already cut, until the forces which are set up during the removal process, are exerted transversely of the electrode axis, and cause the trailing effect, become so small that the electrode is again rectilinearly pulled in consequence of the forces exerted by the wire clamping device, whereupon the rectilinearly pulled electrode is again advanced, from the same movement path section, to a point at which the backward movement began, whereupon the normal electrical removal process is resumed, so that the end of the cut does not have any contour errors and/or protuberances.