JPH0521688B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a forming electric discharge machining method such as so-called die-sinking, and the present invention relates to a forming electric discharge machining method such as so-called die-sinking. After machining is performed under machining conditions such as rough machining and semi-machining using the divided full-form machining electrode, a smoother mirror finish is applied to the machining area formed by the machining in the previous process. The present invention relates to an electric discharge machining method that achieves the purpose of machining by performing electric discharge machining at a high speed.

[Prior art]

Processing is performed by placing a workpiece and a processing electrode facing each other with a minute interval in a predetermined processing liquid such as kerosene, and applying voltage pulses intermittently between the workpiece and the processing electrode. In electrical discharge machining, if the amount of machining per pulse is large, the machining speed will be faster, but the disadvantage is that the machined surface will be rougher, and if the amount of machining per pulse is small, the machining speed will be slower. However, the machined surface is smoothed. In finishing machining, in order to make the finished surface as smooth as possible and increase the machining speed, it is theoretically possible to reduce the amount of machining per pulse and shorten the pulse interval. There is a limit to reducing the frequency and shortening the pulse interval (that is, increasing the frequency). In other words, in the discharge circuit, there is a capacitance corresponding to the machining area between the conductive cable and the workpiece and the machining electrode, that is, the machining area, and an inductance of the conductive cable. Even if it is not provided, it takes time for the voltage between the workpiece and the machining electrode to rise, and there is also a limit to the rise of the discharge current. In electrical discharge machining that forms and supplies a voltage pulse train, it is generally not possible to reduce the generation period of the discharge pulses to 1 to 2 μS or less. For this reason, in finishing machining, a capacitor charging/discharging method has often been used, but satisfactory results have not been obtained in terms of machining speed, electrode wear, etc. Furthermore, when the period of generation of discharge pulses is shortened, there is a problem in that so-called arc discharge is more likely to occur.

For this reason, finishing machining methods have been proposed in the past in which machining is performed by charging and discharging stray capacitance in a discharge circuit including the machining interval (for example,
According to this method, extremely fine discharge pulses of about 0.1 μs or less can be generated at a frequency on the order of MHz.

[Problem to be solved by the invention]

However, in this method of charging and discharging the stray capacitance of the discharge circuit, the discharge pause time is extremely short, so concentrated discharge progresses and a continuous arc discharge state occurs, and the machined surface is affected by heat, making it impractical. It was difficult to perform proper processing. In addition, in order to perform mirror finishing using charging and discharging of stray capacitance in a discharge circuit, the stray capacitance should be 50 to 100 pF or less, preferably
It is desirable to set it to 30 pF or less, and for this reason, it is necessary to keep the area between the processing holes to approximately several cm ² or less, usually around 1 cm ² or less, making it impossible to mirror-finish a wide processing surface.

In view of these conventional problems, the present invention prevents the occurrence of continuous arc discharge in finishing processing by charging and discharging only the stray capacitance of the discharge circuit, and enables mirror finishing processing of a wide area. It was invented for the purpose of

[Means to solve the problem]

Therefore, the electric discharge machining method according to the present invention uses a full-form machining electrode that is divided into one or a small number of parts formed according to the dimensions and shape of the workpiece to be machined, and also controls the on/off switching of a switch element. Using the intermittent voltage pulses generated by this as a machining power source, one machining process can be performed under desired machining conditions such as rough machining, rough machining and semi-machining, and semi-machining and semi-finishing, while replacing electrodes as necessary. In an electric discharge machining method in which finishing machining is performed by changing the machining conditions after performing the above machining, after completing one or more machining steps such as the rough machining, the full-form machining electrode is moved to the machining surface of the electrode. Processing time for a processing area of several ^cm2 or less, which is sufficiently small compared to the area〓
The finishing electrode is replaced with a finishing electrode that faces a part of the machined surface in the pre-processing process, and a DC voltage or voltage pulse is applied between the finishing electrode and the workpiece with a short circuit current limited to less than 1A. The machining is performed by charging and discharging only the stray capacitance of the discharge circuit including the small-area machining gap formed between the finishing machining electrode and the workpiece. This electric discharge machining method sequentially processes the machined surfaces in the machining process while sequentially moving the opposing parts by applying a relative translational motion.

[Effect]

According to the present invention, when performing finishing machining, the electrical machining conditions for electrical discharge machining up to the immediately preceding machining process are changed, and the machining electrode is replaced with a previously prepared finishing machining electrode, thereby reducing the area 〓 between machining. is switched to a small value of several cm ² or less, the stray capacitance of the discharge circuit is controlled to be small, and machining is performed at a high frequency on the MHz order using miniaturized discharge pulses with low discharge energy per pulse. ,
In addition, by sequentially moving the finishing electrode for a small machining area to the opposite part of the workpiece over a large machining area, continuous arc discharge is prevented and a smooth mirror finish is achieved. Machining can be performed at high speed, and even a wide area can be easily polished to a mirror finish.

[Embodiments of the invention]

The details of the present invention will be explained below with reference to embodiments shown in the drawings. As shown in FIG. 1, in this embodiment, an AC power source 1 is provided, in which a commercial AC power source or commercial AC is once rectified into DC, and then converted to AC at a predetermined high frequency by switching with a switch element. , the peak voltage is 80V ~ due to transformer 2
After transforming the voltage to about 1500V and rectifying it with a rectifier 3 to make it direct current, it is passed through a current limiting resistor 4 between the facing electrodes 7 of the discharge circuits 5 and 6 and the workpiece 8.
to be applied. The discharge circuits 5 and 6 are configured to reduce the short-circuit current by the current-limiting resistor 4, as described in the experimental example described below, and as disclosed in the publication regarding the prior art of stray capacitance charging and discharging. is limited to a small value of 1A or less.

Here, the finishing machining electrode 7 refers to electrical machining conditions such as voltage pulses, machining feed control conditions, or machining fluid control conditions when performing finishing machining in the electric discharge machining method of the present invention, as will be described later. In addition to switching various processing conditions such as supply conditions, a finishing electrode is used to form a machining gap with a sufficiently small machining area below a predetermined value by opposing a part of the machining surface of the workpiece up to the pre-machining process by changing the electrode. It was replaced by
In other words, up to the processing step before this finishing process, the normal 1
Rough machining, rough machining and semi-machining, and semi-machining and semi-finishing are performed by using a total machining electrode divided into a single or a small number of multiple parts and using voltage pulses formed by turning on and off switch elements as a machining power source. One or more machining steps are performed under desired machining conditions, replacing electrodes as necessary, and a cavity to be finished is formed in the workpiece.

Therefore, the machining area of the finishing electrode 7 being less than a predetermined value means that the facing area of the machining gap formed from time to time during machining is always less than a predetermined value, for example, approximately 5 cm ² or less, preferably 2 to 3 cm ² The stray capacitance C between the discharge circuits 5 and 6 is made small by controlling the formation of the opposing area so as to maintain the following small value of about 1 cm ² . Specifically, the facing area of the finishing electrodes is generally about 5 cm ² or less, preferably about 1 cm 2 or less, which is about 2 to 3 cm ² or less, and the capacitance is about 50 to 100 pF, preferably about 1 cm ² or less. 30pF
The following is desirable.

In other words, mirror finishing is possible when the processing area of conventional electrodes is around a few ^cm2 or less, but mirror finishing is almost impossible when the processing area is several hundred ^cm2 , for example, because of the wide processing area. One of the causes is thought to be that the charge stored in the stray capacitance formed in the gap generated a discharge pulse with a peak current value larger than the predetermined value, which roughened the machined surface. be. In this case, it is of course preferable that the inductance of the discharge circuits 5 and 6 be made as small as possible.

Further, if necessary, a switch element 10 is provided so that the circuit 5 connected to the finishing electrode 7 and the circuit 6 connected to the workpiece 8 can be short-circuited at a predetermined period.
A control circuit that divides the output of an oscillator with a preset frequency at a constant ratio, or changes the division ratio according to the output of the machining state discriminator 9 between machining and rectifies and adjusts the level of this frequency-divided signal. 11 as a control signal to the switch element 10 to turn the switch element 10 on and off. The switch element 10
The period of the pulse applied to is the sum of the times T1 shown by τon and τoff in FIG. During the period τoff when the switch element 10 is controlled to be on, the discharge circuit is short-circuited and high-frequency discharge is stopped between machining operations. This period T1 is selected to be several times or more the electric discharge repetition period between the finishing electrode 7 and the workpiece 8.
For example, depending on the period of high-frequency discharge due to charging and discharging of stray capacitance by the lead wires of the discharge circuits 5 and 6, machining is performed for 0.1 to 500 μs, and processing is performed for 0.1 μs to 10 ms.
Make it interrupt.

As mentioned above, the stray capacitance C is preferably
By setting the value to 30 pF or less, as shown in V in Figure 3, the charging/discharging voltage characteristics of the stray capacitance of the discharge circuit due to the DC voltage applied to the discharge circuit are such that the voltage rise change is V1 in a general discharge circuit. Compared to the delay as shown in the figure, the rise is steeper as shown in V2, which makes it possible to perform machining by high-frequency charging and discharging, and a small predetermined energy discharge is generated between the finishing electrode 7 and the workpiece 8. Machining can be progressed by applying current.

On the other hand, machining is performed using a so-called full-type electrode, which is usually divided into one or a small number of parts and is used for processing up to the processing steps such as the above-mentioned semi-processing, without considering the area between the machining stages. In this case, the area between machining is large, and this area may increase as machining progresses, so the value of stray capacitance is also large.
The discharge energy per discharge is large, making mirror finish machining difficult, and the speed of finishing machining is slow because high-frequency discharge is not used.Also, the discharge current peak of each discharge is small in proportion to the time width of the discharge pulse. The current is not very high, and there is little or no pause time between each discharge, so what used to be a high-frequency arc discharge, the area between machining is set or maintained below a predetermined value, and the floating static in the discharge circuit is reduced. By controlling the capacitance to a small level, machining is performed at a high frequency on the MHz order using miniaturized discharge pulses with low discharge energy per pulse, making it possible to perform smooth mirror finish machining at high speed. In addition, since machining is performed while moving the facing portion of the finishing electrode and the workpiece, it is possible to prevent the occurrence of continuous arc discharge and to easily mirror-finish a wide area of the machined surface. .

Next, in the present invention, the machining interval, that is, the opposing area between the machining electrode and the workpiece 8 is limited to a predetermined small value by replacing the machining electrode with the finishing machining electrode 7 according to the present invention. A specific example of processing in this state will be explained.

FIG. 4 shows a circular hole-shaped workpiece 8 in the workpiece 8.
This is an example of finishing processing according to the present invention when a is formed, and this processed part 8a is processed by a round bar-shaped electrode having a size that is approximately inserted into the processed part 8a in the previous process, for example, under semi-finishing processing conditions. It is processed. Therefore, in the device in this example, either the table 13 or the arm 14 is configured to be movable by an X-axis or Y-axis motor, or either the table 13 or the arm 14 is configured to be movable on the X-axis. It has a mechanism for positioning the workpiece 8 on the X and Y planes, the other being driven by a motor and a Y-axis motor. At the tip of the arm 14, a rod 16 having a rack 15 connects a Z-axis motor 17 mounted on the arm 14.
The output shaft pinion 18 of the rod 16 is engaged with the rack 15 so that it can be moved up and down, and an arm 20 is rotatably attached to the lower end of the rod 16 by a motor 19.
The block 21 is fitted so as to be able to swing freely along the
A threaded rod 23 connected to the output shaft of a motor 22 attached to one end of the arm 20 is screwed into the screw hole of the block 21, and a finishing electrode 7 is attached to the block 21.
It has a holder 24 for mounting. As shown in the figure, the finishing electrode 7 has a small cylindrical shape with a surface area much smaller than the area of the processed portion 8a, but various shapes other than the cylindrical shape can be adopted.

When performing finishing using this equipment,
NC the position setting mechanism of the workpiece 8 on the X and Y planes.
By operating a device (none of which is shown) or the like, the center of the processed portion 8a is aligned with the center of rotation of the arm 20. Further, the motor 22 is operated so that a minute gap is formed between the finishing electrode 7 and the inner wall of the processing section 8a, and the motor 17 is operated.
is activated to lower the finishing electrode 7 so that its bottom surface faces the bottom surface of the processing section 8a with a small distance therebetween, and then, with the processing section 8a immersed in the processing liquid, finish processing electrode 7 or its vicinity is lowered. While spouting machining fluid from the machining fluid (the jetting mechanism for this machining fluid is not shown), the motor 19 is operated to rotate the finishing electrode 7 around the rotation center of the arm 20, and the finishing electrode 7 and the workpiece are rotated. The discharge circuits 5 and 6 outputs are connected between the workpiece 8 and a voltage is applied.
In this way, the opposing area between the machining parts 8a depends on the diameter of the finish machining electrode 7, but it is around several ^cm2 or less, and the machining part 8a can be sequentially scanned and machined, and the finishing machining electrode After rotating the finishing electrode 7 once or several times, the rotation radius of the finishing electrode 7 is made smaller and the rotational speed is increased, and the same processing operation is repeated. On the contrary, the machining may be performed while gradually increasing the radius of rotation, or the material may be moved in a spiral manner.

FIG. 5 shows the spherical surface 8 of the workpiece 8A in the previous process, for example, under medium machining conditions, as in the case of FIG. 4 above.
This is an example of mirror-finishing the spherical surface 8b machined using a full-form electrode that fits almost the entire surface of b. Less than 1/2nd, i.e. several cm ²
The workpiece 8A is placed in a machining fluid 28 in a machining tank 27 placed on a rotary table 26 rotated by a motor 25, using a spherical surface with opposing areas of the front and back or less. The rod 16 is positioned and immersed so as to coincide with the center of rotation by the motor 25. On the other hand, a guide 29 having an arc-shaped guide hole 29a is attached to the rod 16 whose position in the Z-axis direction can be adjusted.
6, the guide hole 29a is positioned concentrically with the spherical surface 8b, and the arc-shaped guide body 30a, which is integrated with the arm 30 to which the finishing electrode 7A is attached, is positioned so that the guide hole 29a is concentric with the spherical surface 8b. The rod 16 is pivotably fitted into the guide hole 29a.
The arm 30 is connected to the screw rod 33 that meshes with the output screw rod 32 of the motor 31 attached to the motor 2.
5 while rotating the workpiece 8A,
By operating the motor 31, the finishing electrode 7A is moved continuously or stepwise in the direction of the arrow 37 or in the opposite direction along the spherical surface 8b of the workpiece 8A, and the workpiece is moved as described above. The outputs of the discharge circuits 5 and 6 are connected between the finishing electrode 7A and the finishing electrode 7A, and the electrodes are applied, and machining is performed while machining liquid is spouted from the finishing electrode 7A etc. by a mechanism not shown. It is.

Note that instead of rotating the table 26, the rod 16 may be rotated.

FIG. 6 shows another example of finishing the spherical part 8b, in which a bowl-shaped finishing electrode 7B prepared in advance, which is slightly larger than the radius of the spherical part 8b, is used as an exchanger during finishing. When finishing the side surface a of the spherical part 8b, the finishing electrode 7 is set so as to cover the spherical part 8b.
B is displaced to the left in the drawing as shown by the solid line, and when machining the top b or its vicinity, the finishing electrode 7B is lowered as shown by the two-dot chain line 7B'. By changing the height, position, or inclination, or by rotating the finishing electrode 7B and the workpiece 8A relative to each other, the actual opposing area can be reduced to a small value of around several cm ² or less. This allows processing to be carried out while being held.

FIG. 7 shows an example in which the spherical recess 8c of the workpiece 8B is finished in the same manner as described above, and the motor 25
By operating the motor 40 attached to the rod 16 while rotating the workpiece 8B, the motor 40 is actuated to the rod 16 via the threaded rod 42 that meshes with the output threaded rod 41 and the link 43. The arm 39, which is swingably attached by the pin 38, is moved in the direction of the arrow 44 (or in the opposite direction).
The arm 39 is rotated continuously or stepwise.
The finishing electrode 7C (this finishing electrode has an arcuate surface along the spherical concave surface 8c and is positioned so that the arcuate surface faces the spherical concave surface 8c) attached to the spherical concave surface 8c is moved along the spherical concave surface 8c. The processing is performed by still,
In this case as well, instead of rotating the table 26, the rod 16 may be rotated.
Also, contrary to the case shown in FIG. 6, a spherical finishing electrode having an outer diameter slightly smaller than the spherical concave surface 8c is used, and the spherical finishing electrode is inserted into the spherical concave surface 8c as explained in FIG. By displacing the spherical finishing electrode in the spherical concave surface 8c, electric discharge may be generated at a position where the spherical finishing electrode is close to the spherical concave surface 8c.

FIG. 8 is an example of finishing a groove 8d machined in a workpiece 8C, and
The workpiece 8C is positioned on a table 46 that is moved by a shaft motor 45 so that the moving direction of the table 46 and the direction of the groove 8d match, and the tip angle is, for example, relative to the bottom angle of the groove 8b, for example about 60 degrees. The finishing electrode 7D, which is as small as about 50 degrees and shorter than the length of the groove 8d, is attached to the rod 16 by, for example, a vertical movement cylinder 47 as shown in the figure, and by extending the cylinder 47, the finishing electrode 7D is inserted into the groove 8d.
The bottom angle of the groove 8d and the tip angle of the finishing electrode 7D are set in the cylinder 47 in a direction perpendicular to the axial direction of the groove 8d so that the tilting fulcrum is near the tip of the finishing electrode 7D while facing each other with a small interval between them. The X-axis or Y-axis motor 45 slowly tilts back and forth at an angle corresponding to the difference in angles several times per second or less.
The table 46 is continuously or as shown in a, b,
The machining is performed by moving intermittently in each range of c and d.

Next, to explain an example of machining, in the example shown in FIG. When the workpiece 8 is made of S55C material, the area of the finishing groove 8d facing 8 cm ² is 0.4 cm ² ,
The capacitance between machining is 20 pF, and the final machining electrode 7 is applied at a speed of 3 cm/sec using direct current with a voltage of 300 V and a short circuit current of about 0.5 A, and the on-time τon shown in FIG. 2 is controlled by the switch 10. When 1 μsec and off time τoff is 2 μsec, the machining speed is 2.7
mg/min, the machined surface roughness was 0.28μmRmax, the maximum machining current was 0.2A, and the peak current was 20A. on the other hand,
When machining is performed using the so-called full-form electrode used in the previous semi-finishing process, that is, a machining electrode shaped to fit over the entire machining groove 8d, the machining speed is 3.
mg/min, the machined surface roughness was 1.1μmRmax, the machining current was 0.2A, and the peak current was 35A. This result shows that according to the present invention, mirror finish processing with extremely good machined surface roughness can be performed at a relatively high speed. Furthermore, if the position coordinates of the finishing electrode are corrected during processing depending on the degree of wear of the electrode, finishing processing can be performed with higher precision. Furthermore, the present invention can be applied not only to finishing parts having a simple shape as described above, but also to finishing parts having a complicated shape. The process may be performed using multiple types of electrodes with different shapes.

As described above, according to the present invention, during finishing machining, electrical machining conditions are changed for electric discharge machining up to the previous process, and machining is performed by replacing the finishing machining electrode prepared in advance. By doing this, the area between machining is kept at a small value below a predetermined value, the stray capacitance of the discharge circuit is controlled to be small, and the discharge pulse is miniaturized with small discharge energy per pulse. Machining is performed using high frequencies on the order of MHz, and processing is performed while sequentially moving opposing parts so that the finishing electrode with a small machining area and the workpiece face the part with a large machining area. This makes it possible to perform smooth mirror finishing at high speed while preventing the occurrence of sustained arc discharge, and also enables easy mirror finishing of a wide area of the machined surface.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of an electrical discharge machining apparatus that implements the method of the present invention, Fig. 2 is a waveform diagram explaining the operation of Fig. 1, and Fig. 3 is the relationship between the discharge voltage and discharge current of the present invention. 4 to 8 are diagrams showing examples of electric discharge machining equipment that implements the machining method of the present invention. 1... AC power supply, 7,7A to 7D... Finishing electrode, 8,8A to 8C... Workpiece, 8a to 8d
...Processed part, 10...Switch element, 11...
control circuit.

Claims (1)

[Scope of Claims] 1. A machining electrode is opposed to a workpiece with a minute gap in between, and an intermittent voltage pulse is applied between the two with machining fluid interposed in the gap to generate electrical discharge. The machine generates 300 ms and applies a relative machining feed in the opposing directions, and uses a full-form machining electrode shaped according to the dimensions and shape of the workpiece to be machined, as well as turning on and off a switch element. Using the generated intermittent voltage pulses as a machining power source, one or more machining steps can be performed under desired machining conditions such as rough machining, rough machining and semi-machining, and semi-machining and semi-finishing, while replacing electrodes as necessary. In an electrical discharge machining method in which finishing machining is performed by changing the machining conditions after machining, after completing one or more machining steps such as the rough machining,
The overall machining electrode is replaced with a finishing electrode that forms a machining gap with a machining area of several cm ² or less, which is sufficiently small compared to the area of the electrode machining surface. A DC voltage or voltage pulse with a short-circuit current limited to less than 1A is applied between the finishing electrode and the workpiece, and the small-area machining gap 〓 formed between the two is Machining is performed by charging and discharging only the stray capacitance of a discharge circuit including a discharge circuit, and relative translational motion is applied between the finishing electrode and the workpiece to sequentially move opposing portions of the two. An electrical discharge machining method characterized in that the machining surfaces in the pre-machining step are sequentially machined.