METHOD OF CONTROLLING WIRE CUT DISCHARGE MACHINE

Abstract: A method of controlling a wire cut discharge machine which improves machining accuracy when a corner portion is machined. When a portion to be machined is a corner portion, the pressure and/or quantity of cutting fluid to be fed to a gap between a work and a wire electrode is reduced for only a specific zone. Setting of this zone and feed regulation of cutting fluid are determined on the basis of an allowable radius and the shape and dimension of the corner portion. The discharge energy to be supplied between the work and the wire electrode is decreased simultaneously with the feed regulation of the machining liquid in the zone described above.

Description: The present invention relates to a method for controlling a wire-cut electric discharge machine, more particularly to a control method for machining a corner of the workpiece. A wire-cut electric discharge machine generates an electric discharge between its wire electrode and a workpiece, and cuts the workpiece into a desired shape by shifting the wire electrode with respect to the workpiece according to a machining command data. As shown in Fig. 1, when the wire electrode 1 moves in a predetermined direction (i.e. a right direction in Fig. 1) for forming a slit 3 on a workpiece 2, a pressure is usually generated between the wire electrode 1 and the workpiece 2 due to electric discharge, thereby causing the wire electrode 1 to be bent backward or in a direction of an arrow, i.e. a direction opposite to the direction of movement. For this reason, the wire electrode 1 is bent or curved backward with respect to a straight line connecting a pair of wire guides 4 and 4. In case of a straight-line machining operation, this deflection will not substantially affect the machining accuracy for the worse. However, in order to form a corner, the electrode 1 needs to change its direction of movement in accordance with a machining command, for example, in a normal direction as shown in Fig. 2a. In such a corner formation, the slit 3 having a profile as indicated by a solid line 3b is not formed. An electric discharging portion of the wire electrode 1 is drawn towards an inside of the corner because of the above deflection of the wire electrode 1 to cause a discrepancy from the desired profile, as indicated by a dotted line 3a. An amount of such discrepancy is given as δ. Furthermore, as shown in Fig. 2b, a similar discrepancy will occur in an arc-shaped corner in the case where a straight-line machining operation is followed by an arc machining operation. It is known that the problem of discrepancy such as one occurring when a straight-line machining operation is followed by another straight-line machining operation differing in the direction, or when a straight-line machining operation transfers to an arc machining operation, as is discussed in the foregoing, can be solved to a certain extent by changing the conditions of an electric discharge at the corner. For example, the on-off interval of pulse current supplied can be varied during the corner machining operation so that an electric discharge energy supplied between the wire electrode and the workpiece can be reduced to reduce the deflection of wire and resulting discrepancy of the corner. Moreover, in a wire-cut electric discharge machining, a highly-pressurized machining fluid is usually jetted against the wire electrode in the machined slit from upper and lower sides of the workpiece to remove machining chips and cool down an electrically heated portion. Hence, this machining fluid becomes another cause of the deflection of the wire electrode in the corner machining operation, thereby weakening the above-described corner discrepancy preventive effect due to the control of the discharge energy. EP-A-0151645 discloses a wire-cut electric discharge machining apparatus wherein the pressure of the working fluid at a corner machining portion, or circular machining portion, is reduced to prevent vibration of the wire. EP-A-0378278 discloses the same measure, when linked to a reduction in electric discharge frequency at an angled corner portion or at an arc portion. GB-A-2169536 discloses a method of controlling a wire-cut electrical discharge machine, wherein the pulse width, interval and peak current of the pulse voltage applied to the electrical discharge gap are varied in a suitable manner so as to reduce any drop in machining accuracy at a corner being machined. According to a first aspect of the present invention there is provided a method of controlling a wire-cut electric discharge machine for machining a workpiece into a desired shape by generating an electric discharge between its wire electrode and the workpiece while supplying a machining fluid to a gap between the wire electrode and the workpiece, said method comprising steps of: designating a specified section comprising an arc section and a straight-line section following said arc section when a portion to be machined includes an arc-shaped corner; and controlling a pressure and/or an amount of said machining fluid to levels lower than those of an ordinary straight-line machining operation only within this specified section so that a deflection amount of the wire electrode is suppressed to a predetermined value or lower while the wire electrode is moving within said specified section; characterised in that: said specified section comprises an arc section and a straight-line section ranging from a terminating point of said arc section to a point at a distance L therefrom represented by the following equation: L = ε 0 + (ε 0 - R 0 ) tan {(π-)/2} in the case where an arc radius R₀ of the arc-shaped corner is within a range of 0≦R₀≦ε₀, wherein the arc radius R₀ corresponds to a trace of a center of the wire electrode, ε₀ represents a half width of a machined slit, and  is the angle formed between two straight lines extending from opposite ends of the arc section. According to a second aspect of the present invention there is provided a method of controlling a wire-cut electric discharge machine for machining a workpiece into a desired shape by generating an electric discharge between its wire electrode and the workpiece while supplying a machining fluid to a gap between the wire electrode and the workpiece, said method comprising steps of: designating a specified section on a straight-line section following an intersecting point of two straight lines when a machining section includes a corner formed with two straight lines intersecting with each other; and controlling a pressure and/or an amount of said machining fluid to levels lower than those of an ordinary straight-line machining operation only in this specified section so that a deflection amount of the wire electrode is suppressed to or below a predetermined value while the wire electrode is moving in said specified section; characterised in that: said specified section is defined by a section ranging from said intersecting point of said two straight lines to a point at a distance L' therefrom represented by the following equation: L' = ε 0 + ε 0 tan {(π-)/2}, wherein ε₀ represents a half width of a machined slit and  is the angle formed between said two straight lines. Furthermore, in addition to the pressure and/or amount of the machining fluid, an electric discharge energy supplied between the wire electrode and the workpiece may be controlled to be lower than those in an ordinary straight-line machining operation only within this specified section. The specified section is designated on the bases of the shape and size of the corner. Furthermore, the degree of adjustment of the machining fluid pressure and/or supply amount, as well as the energy of electric discharge energy supplied, may be determined in accordance with the shape and size of the corner. Fig. 1 is a view illustrating a wire electrode deflected during a wire-cut electric discharge machining operation; Figs. 2a and 2b are views respectively illustrating a discrepant portion of a machined shape in a corner machining operation; Fig. 3 is a schematic view showing a machined portion including a small-arced corner; Fig. 4 is a schematic view showing a machined portion including a large-arced corner; Fig. 5 is a schematic view showing a machined portion including a straight-line portion followed by another straight-line portion; Fig. 6 is a schematic block diagram showing basic components of an NC control apparatus for a wire-cut electric discharge machine embodying the present invention; Fig. 7 is a flowchart showing a part of the processing executed by a CPU of the NC apparatus in accordance with the method of the present invention; Fig. 8 is a flowchart showing another part of the flowchart of Fig. 7; and Fig. 9 is a flowchart showing still another part of the flowchart of Fig. 7. A wire-cut electric discharge machine carrying out a method in accordance with the present invention is designed to perform its machining operation by moving the wire electrode, whose feed operation is servo-controlled by an NC apparatus, with respect to the workpiece. Fig. 6 is a block diagram showing basic components of the NC control apparatus. In Fig. 6, a central processing unit (CPU) 10 is connected via a bus 18 to the components such as a tape reader 11 for reading machining command data stored in NC tape 20, a manual data input device (MDI) 12 for inputting machining conditions such as thickness and feed speed of workpiece, a ROM 13 for storing control program, a RAM 14 for storing various set values, and an interface circuit 15. The interface circuit 15 is connected to a servo motor 16 for shifting workpiece mounting base and an electromagnetic valve 17 for adjusting jetting or supply rate of machining fluid. According to the present embodiment, when the portion to be machined is a corner including an arched portion or a corner including two intersecting straight lines, supply pressure and supply rate of the machining fluid is reduced lower than that applicable to the machining of the straight portion for the machining of the portion within a specified region. Such specified region will be explained with reference to Figs. 3, 4 and 5. Furthermore, according to the present embodiment, where the corner includes an arched portion, the region to be specified and the condition for controlling the discharge within the specified region (supply rate and/or supply pressure of the machining fluid and the amount of discharge to be reduced) is varied depending on whether R₀, the arc radius of the locus of the width of the machining, is equal to or smaller than ε₀, 1/2 of the width of the machining slit as shown in Fig. 3 (the case of small arc) or larger than ε₀ as shown in Fig. 4 (the case of large arc). This is necessary because δ, the degree of discrepancy of the machined workpiece, is substantially independent of R₀, the magnitude of arc radius, within the range of 0 ≦ R₀ ≦ ε₀; however, it varies depending on R₀, the magnitude of arc radius, within the range of R₀ > ε₀. In the case of a corner including an arc of small radius as shown in Fig. 3, where the arc as the locus of the center of wire electrode (center of arc is given as 0) is given as ab; the intersecting point of a perpendicular drawn from an inner edge e of the machining slit and a straight line continuously extending from the arc section ab is given as c; and the point at a distance of ε₀ from the point c on the straight line is given as d, the specific section is specified by the arc ab and the straight line bd (= bc + cd) . Where an angle formed between two straight lines continuously extending from both edges of the arc section ab is given as  [rad] (hereinafter, this angle is referred to as a corner angle ), the central angle aob of the arc is equal to an angle obtained by subtracting the corner angle  from π [rad], so that the distance bc is expressed by the following equation: bc = (ε 0 - R 0 ) tan {(π - )/2} Thus, in the corner machining operation including a small arced corner as shown in Fig. 3, the section to be specified is designated as a section including arc ab and point d (point apart by straight distance L from the terminating point b of arc ab). The distance L is expressed by the following equations. L = bd = bc + cd = ε 0 + (ε 0 - R 0 ) tan {(π - )/2} By the way, as the corner angle  is equal to an angle obtained by subtracting the central angle aob of the arc from π [rad], the corner angle  can be determined if the central angle aob of the arc is fixed. If a corner has a small arced shape, an allowable amount of deflection of the wire electrode is calculated on the bases of the corner angle  and a predetermined allowable discrepancy amount δ'. Then, the electric discharge condition is controlled in the specified region in such a manner that a curvature of the wire electrode can be suppressed within the calculated allowable amount of deflection. When a corner has a large arc-shaped portion to be machined as shown in Fig. 4, the specified region is designated to be identical with the arc section ab. That is, no straight-line section is included in the specified region unlike the case of the small arced corner described above. In this case, the allowable deflection amount of the wire electrode is calculated on the bases of the predetermined allowable discrepancy amount δ', the corner angle , and the arc radius R₀. Then, the electric discharge condition is controlled in the specified region in such manner that a deflection of the wire electrode can be suppressed within the allowable deflection amount. When a straight-line portion to be machined has a corner followed by another straight line portion to be machined extending in a different direction as shown in Fig. 5, this corner is regarded as a small arc having an arc radius R₀=0 at an intersecting point a of two straight lines. Thus, the specified region and the electric discharge condition in this specified region are determined in the same manner as the case of small arced corner as is previously described. That is, in Fig. 5, given that the corner angle, i.e. an intersecting angle of above two straight lines is , a power-down distance is designated to be a distance L' between the point a and the point d. This distance L' can be obtained by substituting 0 for R₀ in the equation L = ε 0 + (ε 0 - R 0 ) tan {(π-)/2} . That is, L' = ε 0 + ε 0 tan {(π-)/2}. In this case, the allowable deflection amount of the wire electrode is calculated on the bases of the predetermined allowable discrepancy amount δ' and the corner angle  so that the electric discharge condition can be controlled in the specified region in such manner that a curvature of the wire electrode can be suppressed within the calculated allowable deflection amount. Next, a control method for the wire-cut electric discharge machine as an embodiment of the present nvention, executed by the CPU 10 of the NC control apparatus of Fig. 6, will be explained with reference to the flowcharts of Figs. 7, 8 and 9. By the way, before starting all the procedure, the half width ε₀ of the machined slit can be obtained, by adding a radius /2 of a wire electrode to be used and an estimated machining gap δ₀ (That is, ε 0 = /2 + δ 0 ). These data are stored in the RAM 14 through the MDI 12 in advance. Furthermore, flags Fc, Fs and Fd are all initialized to "0". First, in step S1, the tape reader 11 reads a machining command of the present block from the NC tape 20. Then, in step S2, the CPU 10 makes a judgement as to whether or not the machining command of the present block is a fast feed command. If the machining command is a fast feed command, the CPU 10 proceeds to step S3 to store the command value of the present block in a register Rs. Then, in step S4, the fast feed processing is executed and, subsequently, in step S5, the flag Fs is set to "1". Here, flag Fs = 1 means that the fast feed command has already been executed. Then, the processing returns to the step S1. When the read machining command is not a fast feed command, the processing proceeds to step S6 to make a judgement as to whether or not the machining command of the present block is a straight-line feed command. If the machining command is not the straight-line feed command, the processing proceeds to step S6 to further make a judgement as to whether or not the machining command of the present block is an arc feed command. If the machining command is not the arc feed command, the processing proceeds to step S8 to execute other processing, and then returns to the step S1. By the way, in a case where the above other processing includes a termination command, the processing for the present block will be terminated without returning to the step S1. When the machining command of the present block is an arc feed command, the processing proceeds from the step S7 to step S9 wherein the arc radius R₀ the distance ε₀ are compared to judge as to whether an arc shape of the present block is a large arc (R₀ > ε₀) or a small arc (R₀ ≦ ε₀). If the present arc is a large arc (R₀ > ε₀), the processing proceeds to step S10 to set the flag Fc to "1", and reset the flag Fs to "0". Then, the corner angle  of the large arc is determined in step 11. As the corner angle  is equal to an angle obtained by subtracting the central angle of arc (the angle aob shown in Fig. 4) from π [rad], the corner angle  can be obtained from the central angle of arc included in the present machining command data. After determining the corner angle  of the large arc, the processing proceeds to step S12 to obtain the allowable deflection amount of the wire electrode corresponding to the allowable discrepancy amount δ' on the bases of the corner angle  and the arc radius R₀. In this case, the allowable discrepancy amount δ' is predetermined taking account of the finishing conditions required to be met by the machine. Subsequently, in step S13, not only the degrees to which supply amount and/or supply pressure of the machining fluid are to be controlled on the bases of the allowable deflection amount obtained in the step S12 but also the power reduction amount on the bases of the allowable deflection amount obtained in the step S12. Then, the machining operation is carried out conforming to these conditions. That is, during the machining operation covering the whole of the arc section between the point a and the point b shown in Fig. 4, not only the electric power of the wire electrode is reduced corresponding to the allowable deflection but also the supply amount and/or supply pressure of machining fluid is controlled so that the wire electrode will not deflect exceeding the allowable deflection. Then, the processing proceeds to step S14 to make a judgement as to whether or not the present block has terminated. If the judgement in the step S14 is YES, the processing returns to the step S1. In the step S9, if the present arc is judged to be a small arc (R₀ ≦ ε₀), the processing proceeds to step S15 to set the flag Fd to "1"; after resetting the flag Fs to "0", the processing proceeds to step S16 to calculate the corner angle  of this small arc in the same manner as the large arc. That is, the corner angle  is obtained after obtaining the central angle of the arc included in the present machining command data. After calculating the corner angle  of the small arc, the processing proceeds to step S17 to store the value of the corner angle  in a register R  . Then, the processing proceeds to step S18 to obtain the allowable deflection amount of the wire electrode corresponding to the allowable discrepancy amount δ' on the bases of the corner angle . That is, in the above calculation of the allowable deflection, the arc radius R₀ of the small arc is not used. In step S19, not only the degrees to which supply amount and/or supply pressure of the machining fluid are to be controlled are determined but also the amount power reduction is determined on the bases of the allowable deflection obtained in the step S18. Then, the arc machining operation is carried out conforming to these conditions. That is, not only the electric power of the wire electrode is reduced during the machining covering the whole of the arc section between the point a and the point b in Fig. 3 but also the supply amount and/or supply pressure of the machining fluid are adjusted in accordance with the allowable deflection amount so that the deflection of the wire electrode will not exceed the allowable deflection amount. When the present block is judged to have been finished in step S20, the processing returns to the step S1. In the step S6, if the machining command of the present block is judged to be a straight-line command, the processing proceeds to step S21 to judge the value of the flag Fs. If the value of the flag Fs is "1", it is concluded that the machining command in the preceding processing was a fast feed command, and the present straight-line machining processing follows this fast feed command. Thus, the processing proceeds to step S22 to reset the flag Fs to "0", and proceeds to step S32 to execute an ordinary straight-line machining processing. That is, the processing for power reduction and processing for adjustment of the machining fluid will not take place. When the present block is judged to have been terminated, the processing proceeds to step S34 to store the command of the present block in the register Rs, and returns to the step S1. On the other hand, when the flag Fs is "0" in the step S21 (i.e. when at least one arc or straight-line machining processing has already been performed), the processing proceeds to step S23 to make a judgement as to whether or not the value of the flag Fc is "1". If the value of the flag Fc is "1", it is concluded that the straight-line portion to be machined of the present block follows the large arc-shaped section of the previous block. Then, after resetting the flag Fc to "0" first, the processing proceeds to the step S32 to execute the ordinary straight-line machining processing. Then, in the step S33, when the present block is judged to have been terminated, the processing proceeds to the step S34 to store the command of the present block in the register Rs, and returns to the step S1. When the flags Fs and Fc are "0", while the flag Fd is "1", i.e. when the straight-line portion to be machined of the present block is a machining block following another straight-line portion to be machined, the processing proceeds from step S25 to step S26 to calculate the corner angle  with reference to the command position of the preceding block stored in the register Rs. For instance, the corner angle  is obtained on the bases of the advancing directions of two straight lines of the preceding and present blocks. After obtaining the corner angle , the processing proceeds to step 27 to further obtain an allowable deflection amount of the wire electrode corresponding to the allowable discrepancy amount δ' on the bases of the corner angle . Next, in step S30, not only the degrees to which supply amount and/or supply pressure of the machining fluid are determined but also the amount of power reduction is determined on the bases of the allowable deflection amount obtained in the step S27. Then, the machining operation is carried out conforming to these conditions. The electric power of the wire electrode is reduced during the straight-line section between an initiating point of the present straight line machining and a point at a distance L' (L' is obtained by applying R₀=0 in obtaining L). More particularly, during the machining of the section between the points a and d of Fig. 5, not only the amount of the power reduction is adjusted corresponding to the allowable deflection amount but also the supply amount and/or supply pressure of the machining fluid is adjusted in accordance with the allowable deflection amount so that the deflection of the wire electrode will not exceed the allowable deflection amount. When the progress of the machining is judged to have passed the specified section in step S31, the processing proceeds to step S32 for resuming the ordinary (straight-line) machining operation. Then, when the present block is judged to have been terminated in step S33, the processing proceed to the step S34 to store the command of the present block in the register Rs, and returns to the step S1. When the flags Fs and Fc are "0", and the flag Fd is "1", i.e. when the straight-line portion to be machined of the present block is a straight-line machining block following a small arc-shaped portion to be machined, the CPU 10 proceeds from step S25 to step S28 to obtain the allowable deflection amount in reference to the content (the corner angle ) of the register R  which has been obtained and stored during the processing of small-arced portion in the preceding block. Then, the processing proceeds to step S29 to reset the flag Fd to "0". In step S30, not only the degrees of control for supply amount and/or supply pressure of the machining fluid are determined on the bases of the allowable deflection but also the amount power reduction is determined. Then, the machining operation for the present block is carried out conforming to these conditions. The electric power of the wire electrode and the machining fluid are reduced during the straight-line portion to be machined between an initiating point of the present straight line and a point apart a distance L = ε 0 + (ε 0 - R 0 ) tan {(π-)/2} . To make a judgement when the wire electrode is judged to have passed the predetermined distance corresponding to the designated power-down section. If the judgement in the step S31, the processing proceeds to the step S32 to execute the ordinary straight-line machining operation. Then, in the step S33, when the present block is judged to have been terminated, processing proceeds to the step S34 to store the command of the present block in the register Rs, and returns to the step S1. By controlling the machine in the above-described manner, the deflection of the wire electrode occurring due to the supply of the machining fluid at the corner section and the resulting corner discrepancy can be prevented or at least reduced, to improve the machining accuracy. In the above embodiment, both the power reduction processing and the processing for adjusting the machining fluid supply are executed parallely. However, the greater emphasis may be placed on the adjustment of the machining fluid supply than the power reduction to reduce the deflection the wire electrode during the machining of the corner portion. Further, the power down processing may be omitted so that the deflection of the wire electrode is reduced only by adjusting the supply of the machining fluid. The supply of the machining fluid can be adjusted by adjusting either the supply pressure or supply rate, or by adjusting the both. Furthermore, in the above embodiment, a section between a machining initiation position and a position at a distance of L = ε 0 + (ε 0 - R 0 ) tan {(π-)/2} is designated as a machining fluid adjusting section on the straight-line portion to be machined following the small-arc portion to be machined (Fig. 3) or on the straight-line portion to be machined following another straight-line portion to be machined (Fig. 5). This machining fluid adjusting section is, however, not limited to the above-described region. For example, the terminal point of this machining fluid adjusting section can be further extended.

Claims: A method of controlling a wire-cut electric discharge machine for machining a workpiece (2) into a desired shape by generating an electric discharge between its wire electrode (1) and the workpiece (2) while supplying a machining fluid to a gap between the wire electrode (1) and the workpiece (2), said method comprising steps of: designating a specified section comprising an arc section and a straight-line section following said arc section when a portion to be machined includes an arc-shaped corner; and controlling a pressure and/or an amount of said machining fluid to levels lower than those of an ordinary straight-line machining operation only within this specified section so that a deflection amount of the wire electrode (1) is suppressed to a predetermined value or lower while the wire electrode (1) is moving within said specified section; characterised in that: said specified section comprises an arc section (ab) and a straight-line section (bd) ranging from a terminating point (b) of said arc section (ab) to a point (d) at a distance L therefrom represented by the following equation: L = ε 0 + (ε 0 - R 0 ) tan {(π-)/2} in the case where an arc radius R₀ of the arc-shaped corner is within a range of 0≦R₀≦ε₀, wherein the arc radius R₀ corresponds to a trace of a center of the wire electrode (1), ε₀ represents a half width of a machined slit, and  is the angle formed between two straight lines extending from opposite ends of the arc section (ab). A method of controlling a wire-cut electric discharge machine for machining a workpiece (2) into a desired shape by generating an electric discharge between its wire electrode (1) and the workpiece (2) while supplying a machining fluid to a gap between the wire electrode (1) and the workpiece (2), said method comprising steps of: designating a specified section on a straight-line section following an intersecting point (a) of two straight lines when a machining section includes a corner formed with two straight lines intersecting with each other; and controlling a pressure and/or an amount of said machining fluid to levels lower than those of an ordinary straight-line machining operation only in this specified section so that a deflection amount of the wire electrode (1) is suppressed to or below a predetermined value while the wire electrode (1) is moving in said specified section; characterised in that: said specified section is defined by a section ranging from said intersecting point (a) of said two straight lines to a point (d) at a distance L' therefrom, represented by the following equation: L' = ε 0 + ε 0 tan {(π-)/2}, wherein ε₀ represents a half width of a machined slit and  is the angle formed between said two straight lines. A method of controlling a wire-cut electric discharge machine in accordance with claim 1 or 2, wherein the electric discharge energy supplied between the wire electrode (1) and the workpiece (2) is controlled to levels lower than those in an ordinary straight-line machining operation only in said specified section, thereby to assist in the suppression of the deflection amount of the wire electrode (1) while the wire electrode (1) is moving in said specified section.

Inventors: HIGASHI IZUMI, IZUMIYA SYUNZO, KITA YUKI, SAKURAGAWA TOMONOBU, TSUTSUMI TOSHIRO, HIGASHI, IZUMI, IZUMIYA, SYUNZO, KITA,YUKI, SAKURAGAWA,TOMONOBU, TSUTSUMI, TOSHIRO, HIGASHI, IZUMI, FANUC DAI-3 VIRA-KARAMATSU, KITA,YUKI,ROOM 10-203,FANUC MANSHONHARIMOMI, SAKURAGAWA,TOMONOBU,FANUC DAI-3 VIRA-KARAMATSU, TSUTSUMI, TOSHIRO, FANUC DAI-3 VIRA-KARAMATSU
Applicants: FANUC LTD, FANUC LTD.
Application year: 1993
Main classification: B23H7
IPC codes: B23H7