ELECTROLYTIC FINISHING METHOD

Abstract: An electrode is positioned to form a predetermined gap between the electrode and a surface of a work, and the electrode and the work are submerged in an electrolyte. Thereafter pulses are applied to the electrode and clean electrolyte is supplied to the gap. A theoretical number of machining necessary for obtaining a desired machining depth is set and an actually cut depth machined on the surface of the work in accordance with the theoretical number is measured. The theoretical number of machining is changed based on the measured cut depth. Machining is further performed in accordance with the changed theoretical number of machining.

Description: The present invention relates to a method for finishing a surface of a work by electrolytic machining, and more particularly to a method for accurately controlling the machining depth. Japanese Patent Application Laid-Open 63-114815 (or equivalent Ep-A-0 266 180) discloses a method for finishing a work machined by electrolytic machining. In the method, the work is secured in an electrolytic tank and is positioned to form a predetermined gap between an electrode and the surface of the work. A pulse is applied to the electrode in a stationary liquid electrolyte, and then clean electrolyte is intermittently supplied to the gap, discharging the electrolyte contaminated with residual products. In the method, a desired machining depth of the work necessary to obtain the required surface roughness of the work without deteriorating the roughness obtained through the electrolytic machining is calculated in dependency on data acquired by experiment and trials. The charge required to machine a unit of area to the desired machining depth is calculated in dependency on an equation relating to an electrochemical equivalent for the particular work material and current efficiency obtained by experiments. If the machine finishing is carried out under the same conditions as the experiments, the dimension of the finished surface is in a small range within +5% of the desired dimensions. However, it is difficult to control the concentration, temperature and pH of the electrolyte in practical machining to be the same as in the experiment. Moreover, the shape of the surface of the work is too complicated to accurately calculate the surface area thereof, causing an error in the calculation. The error affects the calculation of the charge per area unit, so that the machining accuracy is impaired. In addition, it is difficult to flush the liquid electrolyte through the gap between the work and the electrode in the same manner as the experiments. As a result, residual products in the gap cannot be effectively and uniformly removed, deteriorating machining efficiency, particularly in a work with a complicated surface. Thus, the actually machined depth becomes smaller than the desired depth, and hence it is impossible to produce accurate products. It may be of assistance in understanding the invention for the reader to refer to EP-0273380. An object of the present invention is to provide an electrolytic finishing method which can finish a surface of a work with accuracy. According to the present invention, there is provided a method for finishing a work comprising the steps of: securing the work in an electrolytic tank, positioning an electrode to form a predetermined gap between the electrode and the surface of the work, supplying electrolyte to the electrolyte tank so as to submerge the electrode and the work, and, subjecting the work to machining steps, each machining step comprising the application of pulses to the electrode and supplying clean electrolyte to the gap and discharging the electrolyte contaminated with residual products, wherein the method includes the steps of setting a predetermined machining depth; setting a first number of machining steps theoretically necessary to achieve the predetermined machining depth; performing a fraction of the first number of machining steps; measuring the depth of machined surface; calculating a second theoretical number of machining steps which if applied to the workpiece can be expected to machine the work to the desired depth and applying the second theoretical number of machining steps to the work. Other objects and features of this invention will be understood from the following description with reference to the accompanying drawings. Fig. 1 is a schematic illustration showing a system of an electrolytic finishing machine according to the present invention; Figs. 2a and 2b are block diagrams showing a system for supplying current to an electrode and a work; Fig. 3 shows a circuit of a contact detector; Fig. 4 shows a modification of the contact detector; Fig. 5 is a flowchart showing a machining method in a first embodiment; Fig. 6 is a flowchart showing a machining process in the machining method shown in Fig. 5; Fig. 7 is a flowchart showing a process for measuring a machining depth in the machining method; Fig. 8 is a graph showing the relationship between the number of machining and the cut (machined) depth; Figs. 9 and 10 are flowcharts showing the operations of a second and third embodiments of the present invention, respectively; Fig. 11 is a graph showing the relationship between the number of machining and the cut depth in the third embodiment; Fig. 12 is a flowchart showing a measuring process of a cut depth in a fourth embodiment of the present invention; Figs. 13 and 14 are flowcharts showing the operations of a fifth and sixth embodiments, respectively; Fig. 15 is a flowchart showing a measuring process of a cut depth in the sixth embodiment; Fig. 16 is a graph showing the relationship between the number of machining and the cut depth in the sixth embodiment; Fig. 17 is a flowchart showing the operation of a seventh embodiment of the present invention; Fig. 18 is a flowchart showing a centering process in the machining method of the seventh embodiment; Figs. 19 and 20 are flowcharts showing the operations of an eighth and ninth embodiments of the present invention, respectively; Fig. 21 is a graph showing the relationship between the number of machining and the cut depth in the ninth embodiment; Fig. 22 is a flowchart showing the operation of a tenth embodiment of the present invention; and Fig. 23 is a graph showing the relationship between the number of machining and the cut depth in the tenth embodiment. Referring to Fig. 1, the electrolytic finishing machine 1 has an electrode holding device 3 for holding an electrode 2, a work fixing device 5 for fixing a work 4 in an electrolyte tank 15. The work fixing device 5 is a table made of insulation such as granite and ceramic material and the tank 15 contains electrolyte such as sodium nitrate solution. The electrode 2 made of copper or graphite is secured to the lower end of a rod 16 of an electrode holding device 3. The holding device 3 is operatively connected to a servo motor 6 through an electrode driving direction converter 7. The converter 7 is arranged to change rotary output of the motor 6 into axial movement of the rod 16. The work 4 has a three-dimensional recess 4a to be finished, which has been formed by an electrical discharge machine (not shown) with the electrode 2. The output signal of the motor 6 is supplied to a motor control section 9 of a control unit 12, and the motor 6 is operated by a control signal from the motor control section 9. The control unit 12 has a machining condition control section 10 and an electrolyte flow control section 11. The system has a source device 8 which comprises a direct current source 19 and a charge/discharge section 20. The charge/discharge section 20 generates a pulse of a current density (specifically means "peak current density") for a pulse duration dependent on the surface area of the recess 4a, in response to a signal from the machining condition control section 10. The system further has an input device 13 for inputting machining conditions, and an electrolyte filtering device 14 for filtering out residual products in the electrolyte. The input device 13 is arranged to input various machining condition signals such as the material of the work, surface area of the work, machining depth D, grades of dimension accuracy, surface roughness, number of machining times N, and initial dimension δ of the gap 17 between the electrode and the work. The signals are fed to the motor control section 9 and the machining condition control section 10. The filtering device 14 supplies the electrolyte to the tank 15, in response to an output signal of the electrolyte flow control section 11, at the start of the machining and feeds the electrolyte through a nozzle 18 during the machining so as to remove residual products in the gap 17. Referring to Figs. 2a and 2b, direct current source 19 of the power source device 8 comprises a transformer 21 and a rectifier 22. The charge/discharge section 20 has a plurality of capacitors 23-1 to 23-n which are connected in parallel with each other, diodes 24-1 to 24-n for preventing reverse current to the current source 19, switches 25-1 to 25-n for generating pulses, and a charge switch 26 responsive to a signal from a voltage comparator 29 for connecting the direct current source 19 to capacitors 23-1 to 23-n to charge each capacitor to a set value. The machining condition control section 10 comprises CPU 40 applied with signals from the input device 13 to calculate machining and others, a waveform providing section 38 for providing current waveform to be discharged in the gap 17 between the work 4 and the electrode 2, a pulse generator 37 for generating pulses each having a predetermined pulse duration, a charge voltage setting section 36 for setting charge voltage applied to the capacitors 23-1 to 23-n, a current setting section 39, and a D/A converter 33. A voltage detector 27 is provided for detecting the charge voltage applied to the capacitors 23-1 to 23-n. A voltage comparator 29 is provided for comparing a voltage from a D/A converter 28 corresponding to the charge voltage from setting section 36 with the charge voltage detected by the voltage detector 27, and a charge detector 30 is provided for detecting the completion of charging in response to the output of the voltage comparator 29. The control section 10 further comprises a current detector 31 for detecting the current of the electric charge discharged between the work 4 and the electrode 2, a peak current holding circuit 32 for holding a peak current detected by the detector 31, a current comparator 34 for comparing the peak current from the peak current holding circuit 32 with the current supplied from the D/A converter 33, a gate circuit 35 for supplying an operation signal to switches 25-1 to 25-n in response to signals from pulse generator 37 and waveform providing section 38, and a contact detector 41 for detecting the contact of the electrode 2 with the work 4. There is provided a diode 42 for preventing the switches 25-1 to 25-n from breaking down by reverse current. Referring to Fig. 3 the direct current is applied to the charge switch 26 comprising a transistor, the base of which is connected to the CPU 40. The charge switch 26 is controlled in response to the signal from the CPU 40, such that a predetermined charge voltage is applied to the capacitors 23-1 to 23-n through the diodes 24-1 to 24-n. Each of the switches 25-1 to 25-n of the charge/discharge section 20 comprises a transistor, and the base of each transistor is applied with a signal from the CPU 40 for controlling the discharge of the capacitors 23-1 to 23-n. The contact detector 41 has a voltage source 43 for supplying a reference voltage lower than a decomposition voltage to the gap 17 between the electrode 2 and the work 4. The decomposition voltage is a voltage at which the electrolysis current begins to flow passing through an electric double layer comprising the work 4, electrolyte and electrode 2. The electrode 2 is connected to ground through a resistor 50 of the current detector 31. A resistor 44 is connected between inputs of an amplifier 47 through resistors 45 and 46. An output of the amplifier 47 is connected to one of the inputs of a comparator 49. The other output is connected to the ground through a variable resistor 48 to provide a reference current corresponding to the resistance thereof. The output of the comparator 49 is connected to the CPU 40 for supplying a contact signal when the current between the electrode 2 and the work 4 is larger than the reference current. Fig. 4 shows a modification of the contact detector 41. The contact detector 41 has an amplifier 53 having a high impedance. A noninverting terminal of the amplifier 53 is connected to the work through a resistor 51 for preventing overvoltage and a zener diode 52 to detect the current between the electrode 2 and the work 4. An inverting terminal of the amplifier 53 is connected to feedback resistors 57 and 58. An output of the amplifier 53 is connected to a noninverting terminal and an inverting terminal of comparators 54 and 55. An inverting terminal of the comparator 54 and a noninverting terminal of the comparator 55 are connected to variable resistors 59 and 60, respectively. The outputs of the comparators 54 and 55 are connected to an AND gate 56 for supplying a logical product of the outputs to the CPU 40. Thus, the voltage between the electrode 2 and the work 4, amplified by the amplifier 53, is fed to the comparators 54 and 55 to be compared with reference voltages +V and -V. Namely, when the voltage is in the range of +V and -V, the AND gate 56 produces a contact signal. The machining method of the work is described hereinafter with reference to a flowcharts of Figs. 5 to 7 and to a graph of Fig. 8. The electrode 2 which was used at electrical discharge machining for roughly machining the work 4 is attached to the rod 16 and the work 4 is attached to the fixing device 5. The position of the work 4 is adjusted to align the recess 4a with an electrode surface 2a (step 70). At a step 71, machining conditions such as the dimension δ of electrode gap 17, a desired machining depth D, a theoretical number of machining N for obtaining the depth D and others are input by the input device 13. The theoretical number of machining N is calculated based on data obtained from experiments. The electrolyte is supplied to the tank 15 and the operation is started (step 72). An electrolytic machining process (step 73) is described in detail in Fig. 6. The electrode 2 is set to provide the predetermined initial gap δ (step 73a). A counter A which is provided in the CPU 40 is cleared (step 73b). A single pulse current which has a peak current density ip and a pulse duration P for improving the surface roughness of the work is applied to the electrode 2 by the source device 8 in response to the signal from the CPU 40 (step 73c). After the pulse is applied, the electrode 2 is raised by the motor 6 to expand the gap (step 73d). Clean electrolyte is injected through the nozzle 18 to the gap 17, thereby removing the residual products from the gap 17 and the tank 15 (step 73e). After the discharge of the electrolyte, the electrode 2 is lowered to the initial position (step 73f). At the next step 73g, 1 is added to a count Ca of the counter A and the count Ca is compared with one-half of the predetermined number of machining N (N/2). Namely, in the present step, it is determined whether the single pulse current was supplied half the number of machining N which is determined at the step 71. The steps 73c to 73g are repeated until the count Ca reaches N/2. A cut depth Δd1 obtained by machining N/2 times is measured at a step 74 as explained in detail with reference to Fig. 7. A second counter B provided in the CPU 40 is cleared (step 74a), and the counter B is count up (step 74b). The electrode 2 is lowered by a predetermined distance t, for example 1 µm (step 74c), and it is determined whether the electrode 2 has made a contact with the work 4 at the lowered position in dependency on the contact signal from the contact detector 41 (step 74d). When the contact signal is not fed, the program returns to the step 74c so that the electrode 2 is lowered by the distance t at each routine until the contact is made. Thus, the depth Δd1 is calculated as follows at a step 74e. Δd1 = (Cb x t) - δ where Cb is the count of the counter B. At a step 75 of Fig. 5, a new number of machining NO further required to obtain the desired machining depth D is calculated based on the measured cut depth Δd1. Although the number of machining N is calculated so as to increase the cut depth along a dot-dash line a shown in Fig. 8, the actually cut depth in operation is smaller as shown by a line b. Therefore, the new number of machining NO is estimated based on the machining times N/2 and the actual depth Δd1 obtained by machining N/2 times. Since Δd1 x NO = N/2 x (D - Δd1), NO = N(D - Δd1)/2 · Δd1 The calculated number of machining NO is fed to the CPU 40 through the input device 13 at a step 76 and the machining process described in Fig. 6 is carried out accordingly (step 77). That is, at the step 77, the number of machining with which the count Ca is compared is NO/2. A depth Δd2 is obtained by machining NO/2 times as shown by a line c in Fig. 8. The depth Δd2 is measured at a step 78 in the same procedure as the step 74. Thereafter, a further new number of machining NO′ is calculated in accordance with the machined depth Δd2. Namely, Δd2 x NO′ = NO/2 x (D - Δd2 - Δd1) Therefore, NO′ = N(D - Δd1)(D - Δd2 - Δd1) 4 · Δd1 · Δd2 The number of machining NO′ is input at a step 80 and the machining is performed at a step 81 in the same manner as the steps 73 and 77. This time, the machining is performed NO′ times and the machining is finished (step 82). The first embodiment may be modified so that the cut depth Δdi is measured each time when the work is machined a predetermined number of machining, for example at every time of the pulse application. Furthermore, a plurality of pulses may be applied to the electrode 2 at a time. Although the electrode 2 is lowered in order to measure the cut depth, the work 4 may be raised when measuring the cut depth. Fig. 9 shows a flowchart of the second embodiment of the present invention. In the embodiment, the cut depth Δdi is measured a predetermined number n. The number n is input in the CPU 40 along with the desired machining depth D, predetermined number of machining N and the gap dimension δ at the step 71. When the machining is repeated N/2 times, at the step 73 and the depth Δdi is measured, a new number of machining Ni further required to obtain the depth D is calculated at the step 75. At a step 83, it is determined whether the depth Δdi has been measured the predetermined number n. When the answer is no, the calculated number of machining Ni is input in the CPU 40 at a step 84 and the machining process is repeated. When the depth Δdi is calculated n times, the work is machined Ni times at a step 85, which was set at the last routine. Thus, the machining is completed (step 82). Referring to Figs. 10 and 11, the third embodiment of the present invention is characterized in that the number of machining Nd for machining a predetermined desired machining depth ΔD, for example 1 µm, which is smaller than the overall machining depth D is calculated, at the step 71. The machining depth ΔD and the number of machining Nd necessary for obtaining the depth ΔD are input in addition to the machining depth D and the corresponding number of machining N. When the machining is performed Nd times (step 73), the cut depth Δd1 is measured (step 74). The number of machining Nd₁ for obtaining the remainder of the desired depth ΔD is calculated in dependency on the actually cut depth Δd1 at a step 75. Namely, as shown in a graph of fig. 11, Nd x ΔD = Δd1 x Nd₁, Nd₁ = ΔD Nd/ ΔD In the following routines, the number of machining Nd i for i-th machining is calculated as follows. Nd i = ΔD x Nd i-1 / Δdi The number of machining Nd₁ is accordingly input at a step 76. The other operations are the same as in the first and second embodiments. Fig. 12 shows a part of a flowchart of the fifth embodiment of the present invention. In the embodiment, steps 74z to 87 are carried out instead of the steps described in Fig. 7 so that the cut depth Δdi of the work 4 is measured not only in the vertical direction (Z-axis) but also in other directions (X-axis and Y-axis). When the machining process at the step 73 is finished, a depth Δdz on the Z-axis, that is, the vertical axis, is calculated. The electrode 2 is lowered by the distance t until it makes a contact with the work 4. The electrode 2 returns to the initial position at a step 86z after the depth Δdz is calculated by an equation Δdz = (Cb x t) - δ . A depth Δdx in the X-axis direction and a depth Δdy in the Y-axis direction are thereafter measured in the same manner (steps 74x, 74y). Whenever the depth in any of the directions is calculated, the electrode 2 returns to the initial position (steps 86z, 86x, 86y). An average Δdm ( Δdm = ( Δdx + Δdy + Δdz)/3) of the depths Δdx, Δdy, Δdz is calculated at a step 87. The number of machining is calculated based on the average Δdm. Since depths are measured across the whole surface of the three-dimensional recess 4a of the work 4, the work 4 is accurately machined. The depth in another direction such as rotational direction about the Z-axis may be further measured. The fifth embodiment described in Fig. 13 is applied to a machining operation where finishing process is carried out under four different conditions. At the step 71, predetermined number of machining N1 to N4 for each of the machining processes 1 to 4 and the number of machining NR which is the sum of the number of machining carried out in processes 1 and 2 are input. When the machining operation is started (step 72), it is determined whether the number of machining is below the number NR (step 88). In the first routine, since the number of machining is zero, the machining process 1 where the work is machined N1 times is carried out (step 89) and the number of machining N2d is calculated based on the cut depth Δdi measured at the step 74. The machining process 2 is performed in dependency on the calculated number N2d (step 90). The process from steps 89 to 90 is repeated until the work is machined NR times. Thereafter, the finishing process 3 and the finishing process 4 are carried out at steps 91 and 92, respectively. The number of machining calculated at the step 75 may be the total through out the finishing processes 1 to 4, or the total of the processes 1 and 2, or each of the process 1 to 4. Figs. 14 to 22 show embodiments of the present invention applied to machining operation including different finishing processes I and II where the work 4 is machined under different conditions. In the finishing process I, removing pulse current is applied to the electrode for removing an oxide film including electrolytic products formed on the work 4 in the course of the finishing process II so as to provide a lustrous surface. A peak current density ip₁ of the removing pulse current is 30 to 50 A/cm² and pulse duration P₁ is more than 20 msec. In the finishing process II, a finishing pulse current for improving the surface roughness is applied. The pulse current having a peak current density ip₂ of 30 to 50 A/cm² and a pulse duration P₂ less than 10 msec is fed 5 times with a pulse interval of 100 to 500 msec. The current may be a single pulse current as in the finishing process I. In the sixth embodiment shown in Fig. 14, the electrode 2 and the word 4 are set on the electrolytic finishing machine 1 (step 100) and data are input in the CPU 40 (step 101) so that the operation is started (step 102). The input data are total machining depth D, initial gap dimension δ, number of machining NF for the finishing process I, initial number of machining NS₁ for the first time finishing process II, current densities ip₁ and ip₂ and pulse durations P₁ and P₂ for processes I and II respectively. At a step 103, the finishing process shown in Fig. 6 is repeated NF times. In the first routine, after the finishing process I, the program jumps from a step 104 to a step 110 where the removing pulse current is changed to the finish pulse current to perform the finishing process II. When the machining is repeated predetermined times NS₁ for the process II, the program returns to the step 103 where the process I is performed, thereby removing the oxide film. Since it is determined at the step 104 that the present routine is not the first routine, the program goes to a step 105 where the cut depth Δdi is measured. As shown in Fig. 15, the process for measuring the depth Δdi is similar to that shown in Fig. 12. Steps 105z, 105x and 105y are steps for measuring the cut depth Δdz on Z-axis, the depth Δdx on X-axis and the depth Δdy on Y-axis of a coordinate system, respectively. The origin O, having coordinates (0, 0, 0), of the coordinate system is positioned at a certain reference point R at the lower end of the electrode 2 whereby the gap δ is maintained. In the step 105z, the descent of the electrode 2 by the distance t such as 1 µm is repeated until the electrode 2 and the work 4 make a contact with each other (steps 105za to 105zd) as in the previously described embodiment. At a step 105ze, a shifted distance Lz of the electrode 2 is calculated as follows. Lz = Cb x t where Cb is a count of the counter B. The cut depth Δdz on the Z-axis is calculated based on the shifted distance Lz as follows. Δdz = Lz - δ The reference point R of the electrode 2 returns to the initial position at the origin O. Thereafter, the program goes to the step 105x to measure the depth Δdx in the X-axis direction. The counter B is cleared (step 105xa) and the count Cb is started (step 105xb). The electrode 2 is shifted in positive direction (+) of the X-axis, and a shifted distance Lx(+) is calculated at steps 105Xc to 105Xe in the same manner as the steps 105zc to 105ze. At the next step (step 105xf), the depth Δdx(+) is calculated dependent on the quantity Lx(+). When it is determined at a step 105xg that the cut depth in the X-axis direction is obtained, the program returns to the step 105xa to measure the quantity Lx(-) and the depth dx(-) in the negative direction. After both depths Δdx(+) and Δdx(-) are calculated, the cut depth Δdx is calculated by the following equation. Δdx = (Δdx(+) + Δdx(-))/2 The electrode 2 is thereafter shifted to the initial position (step 105xi). The step 105y for calculating the cut depth Δdy on Y-axis is virtually the same as the step 105x so that the explanation thereof is omitted. When all three depths Δdx, Δdy and Δdz are calculated, the depth Δdi is calculated at a step 105a as follows. Δdi = (Δdz + Δdx + Δdy)/3 A total cut depth d is calculated at a step 106 by adding the cut depth Δdi at every routine. The total machining depth d is compared with the designated depth D to determine whether the depth d is within a predetermined range r with respect to the machining depth D (d = D ± r) at a step 107. When the difference between the total cut depth d and the machining depth D is larger than the value r, the program proceeds to a step 108. At the step 108, number of machining NS₂ for obtaining the a predetermined depth α (Fig. 16), for example 10 µm, at the second time of the machining process II is calculated based on the actual depth Δdi as follows. NS₂ = α x NS₁/ Δdi At (i + 1)th time, NS i+1 is NS i+1 = α x NS i / Δdi The calculated number of machining NS i is set at a step 109 so that the machining process II is again carried out, feeding pulse current NS i times at the step 110. The program then returns to the step 103 for the machining process I to apply the removing pulse current. The machining process is finished (step 111) when the total d of the machining depth Δdi is in the range of D±r after repeating the steps 103 to 110. Thus, the work 4 is machined along solid parts of lines b and c in Fig. 16. Since the oxide film is removed after each machining process II, the machining depth can be accurately measured with the contact detector. Figs. 17 and 18 show flowcharts of the seventh embodiment of the present invention, similar to the sixth. In the present embodiment, the centering of the electrode 2 with respect to the work 4 is performed at a step 112 after the total machining depth d is calculated at the step 106. As a result the electrode 2, which was shifted at the step 105 when measuring the depth Δdi, can be positioned more accurately for the machining process II. An example of the centering process is hereinafter described in detail with reference to Fig. 18. At a step 112a, an average shifted distance LxA in the X-axis direction is calculated in accordance with the following equation. LxA=(Lx(+) + Lx(-))/2 where Lx(+) and Lx(-) are shifted distances calculated at the step 105xe (Fig. 15). The electrode is shifted by distance LxA accordingly along the X-axis at a step 112b. So that the x coordinate of the reference point of the electrode 2 becomes zero. The same procedure is performed for adjusting the electrode 2 on the Y-axis. Namely, an average shifted distance LyA is calculated (step 112c) and the electrode 2 is shifted accordingly (step 112d). In order to adjust the vertical position of the electrode 2, that is, in Z-axis direction, the electrode 2 is lowered (step 112e) until it makes a contact with the work 4 (step 112f). Thereafter, the electrode 2 is raised the distance Lz obtained at the step 105ze (Fig. 15). Thus, the dimension of the gap 17 is kept constant at any point of the work surface so that an uniform depth can be formed on the work 4. The steps 112a to 112g may be repeated a predetermined number. The centering process may further be modified to shift the electrode along the X-axis and the Y-axis in each of the (+) and (-) directions until the electrode 2 contacts the work 4, and the electrode 2 is shifted the half of the shifted distance in the x- and Y-axes directions. Alternatively, the centering may be carried out only in X-and Y-axes directions, or the centering direction such as rotational direction about the Z-axis may be added. In the present embodiment, the centering process enables to maintain the dimension between the electrode 2 and the work 4 constant so that the depth across the whole surface of the work 4 can be uniform. Referring to Fig. 19, the eighth embodiment of the present invention is characterized in that the work 4 is prevented from being machined more than a predetermined number of times at each machining process II. The electrode 2 and the work 4 are set (step 120) and the data are input (step 121). The data in the present embodiment include designated machining depth D, number of machining NF for the machining process I and NS₁ for the machining process II-1 (primary), respectively, maximum number of machining NSmax which is the maximum number of machining in the machining process II-2 (secondary) at a step 133, and the initial dimension δ of the gap 17. The maximum machining number NSmax is the number of machining allowed to machine at one machining process II-2. In addition, number of repetition Nr of the machining process II-2 is initialized, that is, set to zero. When the operation is started (step 122), the machining process I and II are consecutively performed, followed by an additional machining process I (steps 123 to 125). A value nr is calculated by substracting 1 from the number of repetition Nr (step 126). In the first routine, since the value of Nr is zero, the value nr is -1. Thus, it is determined at a step 127 that nr is smaller than zero. The cut depth Δdi is measured and the total depth d is calculated in the afore-described manner at steps 128 and 129. When the total depth d is not in the range of D± r (step 130), the program goes to a step 131 where the number of machining NS i+1 for (i+1)th is calculated. Furthermore, the number of repetition Nr and a remainder NS i+1 , are calculated based on the number of machining NS i+1 and the maximum number NSmax at the step 131. Namely, since the work 4 can only be machined NSmax times at the most at each of the machining process II-2, the machining number NS i+1 is divided by the maximum times NSmax to obtain the repeating number Nr and the remainder NS i+1′ , as follows. NS i+1 /NSmax = Nr rem. NS i+1 For example, when the maximum machining times NSmax is 50 and the calculated machining times NS i+1 is 120, 50 is divided into 120, giving quotient 2 and remainder 20. This means that in order to obtain the machining depth ΔD, the machining process II-2 is repeated twice, machining the work 4 50 times each, followed by additional machining process II-2 where the work 4 is machined 20 times. At a step 132, the numbers NS i+1 , NS i+1 ′ and Nr are set so that the machining process II-2 is machined NSmax (50 times), at a step 133. The program then returns to the step 125 for the machining process I, thereby removing oxide film formed by the machining process II-2. Since the repeating number Nr is 2 at the present routine, the value nr calculated at the step 126 is 1. The program consequently proceeds from the step 127 to a step 134 where it is determined that the value nr is zero. Since nr is 1, the program goes to a step 135 where Nr is changed to Nr-1. Further, the machining process II-2 is carried out NSmax times (step 133) for the second time. The program again returns to the step 125 and further to the step 126. Since the repeating number Nr is now one, the value nr is zero so that the program goes to a step 136 through the steps 127 and 134. At the step 136, the number of machining for the machining process II-2 is set to the remainder NS i+1′ , that is, 20 which is calculated at the step 131. Thus, in the third machining process II-2 at the step 133 the work 4 is machined 20 times. As a result, the number of machining becomes NS i+1 (120) as calculated. Thereafter, the program goes to the step 125. The repeating times Nr is zero and hence nr is -1, so that the program further proceeds to the step 128 where the cut depth Δdi as a result of the number of machining NS i+1 is measured. If the total depth d is within the range of D ± r (step 130), the machining is completed (step 137). When the total is smaller than D by the value r, the whole procedure is repeated. Taking into account of the oxide film and eroded metal removed during the course of the machining process I, experiments have shown that the maximum machining NSmax is preferably 25 to 60. In the present embodiment, the process I is necessarily performed after the process II-2. Thus, machining efficiency and accuracy are improved by maintaining the thickness of the oxide film on the work and the quantity of particles of eroded metal in the gap 17 under certain levels. The ninth embodiment shown in Fig. 20 is different from the afore-described embodiments in that the machining depth is adjusted not by changing the number of machining but by changing the peak current density ip. The embodiment is similar to the sixth embodiment shown in Fig. 14 and the same steps in both embodiments are designated by the same reference numerals. After the machining process I and II are finished (steps 103, 110) in the first routine, the cut depth Δdi is measured through the process shown in Fig. 15 (step 105). When the total depth d of the depths Δdi calculated at the step 106 is not within the predetermined range D±r, (step 107), the program goes to a step 140 where a machining efficiency Ki is calculated as follows. Ki = d/NSi At a step 141, it is determined whether the machining efficiency Ki is within a range of K±s where K is a desired machining efficiency calculated based on the designated machining depth D and the predetermined number of machining N(K =D/N) and s is a predetermined value. Referring to Fig. 21, lines a and b represent the desired machining efficiency K and the actual efficiency Ki, respectively. When the actual machining efficiency Ki is in the predetermined range, the machining process II is performed with the pulse current having the same peak current density ip₂. However, when the actual efficiency largely deviates from the efficiency K, the program goes to a step 142 for changing the density ip₂. Namely, when the actual efficiency Ki is larger than K+s, the CPU 40 applies a signal to the charge voltage setting section 36 of the machining condition control section 10 to increase the charge voltage, thereby decreasing the peak current density. To the contrary, when the actual efficiency Ki is smaller than K-s, the set voltage is decreased so as to increase the peak density. The efficiency Ki at the next step (step 110) is changed accordingly. For example, when the actual efficiency is smaller than K as shown by the line b, the peak current density is increased to increase the machining efficiency as shown by a line c in Fig. 21. The other operations are the same as in the sixth embodiment. Thus in accordance with the present embodiment, although the surface area of the recess 4a of the work 4 may not be accurately calculated, the actual machining efficiency is calculated to correct the peak current density appropriate for the actual surface area. In a tenth embodiment of the present invention shown in Fig. 22, the machining depth is adjusted by changing the number of machining of the machining process II as well as the peak current density. Steps 100 to 142 and 110 are identical to the corresponding steps in the ninth embodiment of Fig. 20 and the steps 108 and 109 are identical to the corresponding steps in the sixth embodiment of Fig. 14. The value s in the present embodiment is preferably 50% of the efficiency K. After the peak current density ip₂ is changed (step 142) in the afore-described manner, the number of machining NS i+1 is calculated at the step 108. The machining number NS i+1 is a value required to obtain the predetermined depth ΔD (Fig. 23) if the pulse current having the same peak current density as in the first machining process II the step 110 is applied. As shown in a graph of Fig. 23, the machining number NS i+1 is calculated in the same manner as in the sixeth embodiment based on the predetermined depth ΔD, predetermined machining number NS and the actual cut depth Δd1 obtained by machining NS times as follows. NS i+1 = ΔD x NS/ Δd2 Thus, the work is machined NS i+1 times in the next machining process II. The other operations are the same as in the ninth embodiment. In the present embodiment, the number of machining and the peak current density are both controlled so that the actual machining efficiency Ki approximates the desired machining efficiency K in a short time, thereby shortening the overall machining time.

Claims: A method for finishing a work (4) comprising the steps of: securing the work in an electrolyte tank (15), positioning an electrode (2) to form a predetermined gap (17) between the electrode and the surface of the work, supplying electrolyte to the electrolyte tank so as to submerge the electrode and the work, and subjecting the work to machining steps, each machining step comprising the application of pulses to the electrode, supplying clean electrolyte to the gap (17) and discharging the electrolyte contaminated with residual products, wherein the method includes the steps of setting a predetermined machining depth; setting a first number of machining steps theoretically necessary to achieve the predetermined machining depth; performing a fraction of the first number of machining steps; measuring the depth of the machined surface; calculating a second theoretical number of machining steps which if applied to the workpiece can be expected to machine the work to the desired depth and applying the second theoretical number of machining steps to the work. A method according to claim 1 wherein the fraction of the first number of machining steps is a fixed value. A method according to claim 2 wherein the fraction is a half. A method according to claim 1 wherein the second theoretical number of machining steps is the integer value of the ratio of the desired depth to the machined depth (ΔDi/Δdi). A method according to claim 1 comprising the steps of: calculating a machining efficiency from the machined depth (Δ di) comparing the machining efficiency with a reference value; changing the current density of the pulses based on the result of the comparison to provide pulses having a corrected current density; performing the second machining of the work with the pulses having the corrected current density.

Inventors: KUWABARA, YOUHEI, SUGIYAMA, HARUKI
Applicants: SHIZUOKA SEIKI CO. LTD.
Application year: 1989
Main classification: B23H3
IPC codes: B23H3