JPH0346245B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electric discharge machining apparatus and a machining method for mirror-finishing a machining surface of a mold or the like including a three-dimensional free-form surface using electric discharge machining.

Recently, research has been carried out on unmanned mold machining, but in order to achieve a mirror finish on the surface of molds, many use mainly rotary grinding wheels.For example, when machining three-dimensional shapes, it is necessary to There are many restrictions. Therefore, attempts are being made to perform mirror finishing using electrical discharge machining.

By the way, mirror finishing using conventional electrical discharge machining technology is only possible on small areas of about 10 cm ² or less, and it usually takes a long time, for example, 20 to 30 hours to process 10 cm ² . When it comes to a large area of practical mold (e.g. ^100-1000cm2 ),
No matter how much time it takes to process the mirror surface (0.4μm)
The current situation is that it is difficult to obtain Rmax (lower than Rmax).

Figure 1 shows an example of a normal electrical discharge machining device. In this figure, 1 is an electrode for electrical discharge machining;
The workpiece 2 and the workpiece 2 are immersed in an insulating processing medium such as white kerosene, facing each other with a gap between them.
3A, 3B, . . . , 3N are a plurality of transistors that generate square wave pulses by intermittent discharge current flowing during machining, and are connected in parallel to each other.
This transistor group may be small in number if the discharge current is small; for example, it may be one transistor. 4A, 4B...4N are resistors to keep the collector current of the transistor within the rating and balance the current, 5A, 5B...5N
6 is a base resistor that limits the base current of each transistor, and 6 is a time counting circuit consisting of a pulse generating circuit, which is composed of an astable multivibrator, a monostable multivibrator, a flip-flop circuit, etc. 7 is an amplifier which amplifies the pulse generated by the time counting circuit 6, and transistors 3A, 3B...
It is given to 3N. Note that E _p indicates a processing power source.

Figures a and b in Figure 2 are diagrams showing the voltage and current waveforms applied to the machining gap by the device shown in Figure 1, where τp is the voltage pulse width, τr is the pause width, and _τN is the null value. The load voltage application time, τ _H is the discharge duration, 8 is the no-load voltage, 9 is the discharge voltage, 10 is the discharge current, Ip is the discharge current peak value, and I _R is the average machining current.

Here, the machined surface is the discharge current peak value Ip,
The smaller the discharge duration τ _H is, the finer the process becomes, but no matter how small the discharge current peak value Ip and the discharge duration τ _H are, there is a limit at which the machined surface will not become a mirror surface.

The inventors of the present invention have found from various experiments and considerations that the reason for the existence of this limit is as follows.

That is, the first reason is that, as equivalently shown in Fig. 3a, there is a stray capacitance C ₀ that is inherent to the processing power source E _p and mechanical equipment (not shown) between the processing lines. In _order to
This is because charge is accumulated in C ₀ and the accumulated charge is discharged, so that the machined surface is roughened by the large current of the capacitor discharge shown in FIG. 3b. To further explain this with specific numerical values, we now have the following: discharge voltage Ed (V), arc voltage between electrodes (voltage between electrodes during discharge) eg (V), circuit inductance L (μH),
If the capacitance acting during machining is C ₀ (μF), then
The capacitor discharge peak current Ip is expressed as Ip≒(Ed-eg)/√...(1), and the discharge duration _τH is expressed as _τH ≒π√...(2).

Here, the normal discharge voltage Ed is determined by the machining power source.
80 to 100 (V), arc voltage eg between electrodes is determined by the combination of electrode and workpiece material, about 20 (V) in the case of copper versus stainless steel, and circuit inductance L is usually about 0.2 (μH). Because there is capacitance
Even if C _p is about 0.01 (μH), Ip≒29 (A) and τ _H ≈0.14 (μS) are calculated from equations (1) and (2). The finished surface roughness of this level is 3 to 4 μmRmax.
The surface roughness is far from mirror-like.

It should be noted that the stray capacitance C ₀ inherent in the machining power source E ₀ and mechanical equipment is approximately 0.01 (μH).

The second reason is the capacitance C _S due to the opposing area between the electrode 1 and the workpiece 2, as shown in FIG.
However, when processing the workpiece 2, it acts superimposed on the stray capacitance C ₀ explained in the above-mentioned first reason.

That is, in FIG. 4, if the opposing area of the electrode 1 and the workpiece 2 is Scm ² , the distance between the opposing surfaces is dcm, and the dielectric constant of the processing medium 11 is ε, then the electrode 1 and the workpiece 2 are
As is well known, the capacitance Cs due to the area facing the workpiece 2 is expressed as Cs=ε·S/d (3), and this affects the machined surface of the workpiece 2.

As mentioned above, since the stray capacitance C ₀ and the capacitance Cs due to the opposing area between the electrode 1 and the workpiece 2 act in a superimposed manner, when the processing area becomes a large area of a practical mold, It was found that no matter how small the discharge current peak value Ip and the discharge duration τ _H were, a mirror surface could not be obtained on the machined surface.

This invention was made based on the above-mentioned second reason from such experimental results and considerations, and it is possible to obtain a mirror surface even if the processing area is a large area of a practical mold. It is an object of the present invention to provide an electric discharge machining device and a machining method having an electrode for mirror finishing.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. That is, FIG. 5 is a schematic configuration diagram of the electrical discharge machining apparatus according to the present invention. In the figure, 50 is a machining tank, 51 is an electrically insulating base installed in the machining tank 50, and 52 is an electrically insulating base installed on the base 51. The workpiece to be placed, 53 is an electrode that faces the workpiece 52 via the processing medium 54, 55 is a processing power source that supplies processing energy between the electrode 53 and the workpiece 52, and this processing power source 55 is High-frequency pulse power supply and high-frequency pulses from this high-frequency pulse power supply (for example, about 2MHz)
It includes a rectifier that rectifies. 56 is an electrode 53
As will be described later, this is an electrode feeding device that provides oscillating motion in addition to main shaft feeding.

Further, the electrode 53 is configured as follows. That is, 57a to 57n are electrode materials such as square rods, hexagonal rods, and round rods made of copper or graphite with a side of about 10 mm, and 58a to 58n-1 are electrode materials 57a to 57.
an insulator that electrically insulates n from each other, 59a~
Reference numeral 59n indicates a current limiting body, for example, a resistor, which has one end connected to each of the electrode materials 57a to 57n and the other end to the current collector plate 60, and 61 indicates a current limiter connected to the electrode materials 57a to 57.
A binding means for binding n, such as a holder,
The current limiters 59a to 59n are housed inside the current collector plate 60 and the holder 61. Further, g indicates the distance between the electrode materials 57a to 57n. Note that when the electrode materials 57a to 57n are metal,
The insulator 58 is formed by filling, welding, or plating.
Electrode materials 57a to 57n via a to 58n-1
If the electrode materials 57a to 57n are graphite, the insulators 58a to 5 may be bonded together.
The electrode materials 57a to 57n may be bonded together using a conductive adhesive or the like via the electrode material 8n-1.

Next, the action of the current limiters 59a to 59n will be explained here.

These current limiters 58a to 59n, as described later, are used to control the current between the other electrode materials 57b to 57n and the workpiece 52, as described above, when an electric discharge occurs between the electrode material 57a and the workpiece 52, for example. The charge accumulated in the capacitance C _S formed in the electrode material 5
7a and the workpiece 52, and limit the current from the machining power source 55 to prevent the current from the machining power source 55 from concentrating on the discharge location of the electrode material 57a. In other words, it functions to limit the maximum current flowing to each electrode material 57a to 57n.

The above is the general configuration of the apparatus, and next, the processing method will be explained in detail. That is, FIG. 6 is a perspective view for explaining the processing method, and in addition to the main shaft feeding in the Z-axis direction shown in this figure, there is a problem that occurs due to the electrode structure consisting of a large number of electrode materials bundled together. In order to remove the residual core on the machining surface and the residual striations on the machining side surface, machining is performed by performing eccentric movement and rocking motion of the electrode 53. First, the main spindle feeding in the Z-axis direction will be explained.

The discharge occurs between the electrode materials 57a to 57n and the workpiece 52.
However, the servo is operated so that the feed of the electrode 53 is maximized within a range where short circuits or arcs do not occur frequently. That is, by detecting the discharge voltage waveform shown in Figure 7, a normal state, a short circuit state, and an arc state are determined, and the number of short circuits or arc discharges that occur within a predetermined period of time (for example, 1 second) is counted, and this total is calculated. The numerical value is compared with the reference value, and if the frequency of short circuits or arc discharges is high, the feed speed is controlled to be low, and if it is low, the feed speed is controlled to be high. Of course, a method may also be adopted in which the average voltage during the machining process that is normally performed is detected and the feed of the electrode 53 is controlled so as to make this equal to the reference voltage.

Next, the eccentric movement and rocking movement of the electrode 53 will be explained.

That is, 62 is an eccentric movement about the reference point 0, and this movement does not give rotation to the electrode 53, and the directions regarding the X-axis and Y-axis do not change. 6
3 and 64 are rocking movements on the X and Y axes,
This movement is controlled to perform reciprocating motion including circular arcs with respect to the X-Z axis and the Y-Z axis.

Generally, the residual core on the bottom surface of the process and the residual streaks on the side surfaces of the electrode materials 57a to 57n can be removed by eccentric movement 62 having an eccentric amount at least equal to the diameter of the electrode materials 57a to 57n. However, as shown in FIG. 6, when forming the workpiece 52 with the electrode 53 having an inclined side surface,
The residual striations due to the n steps cannot be removed by eccentric movement 62 alone, and it is necessary to apply rocking movements 63 and 64 to electrode 53. Note that in the case of electrodes with spherical side surfaces, the rocking movements 63 and 6
4 is preferably performed by subdividing not only the X-Z axis and the Y-Z axis, but also the entire circumference including the intermediate area. In practical terms, a smooth surface can be obtained by dividing the circumference into 8 or 16 equal parts and performing rocking motion.

Next, the relationship between the electrode material used for the electrode 53, the amount of eccentricity, the amount of rocking, and the roughness of the finished surface will be explained.

Regarding eccentric movement, if the amount of eccentricity is equivalent to the diameter of the rod or pipe, all the remaining core can be removed, and the remaining streaks on the sides can be approximated to a nearly straight line or a desired curve. be able to. Therefore, in order to reduce the amount of eccentricity, it is better to use rods, pipes, etc. with small diameters.

Further, the rocking motion is usually determined by the diameter of the rod or pipe and the step determined by the slope of the side surface. FIG. 8 shows the side cross-sectional shape of the finished surface formed by the rocking motion of the electrode, where R is the rocking radius of the rocking motion 63 of the electrode 53, and d is the electrode material 5.
The diameter of the electrodes 7a to 57n, h is the height of the step on the side surface of the electrode, and S is the diameter d of the electrode material 57a to 57n and the height of the step h, given by S=√ ² + ₂ ...(4) This is a level difference pitch. Now, as shown in Fig. 9, the maximum roughness Hmax of the machined surface is S ² /8R<Hmax<d when the step pitch S is larger than twice the swing radius R, that is, when S>2R. (5), and as shown in FIG. 10, when the step pitch S is smaller than twice the swing radius R, that is, when S<2R, Hmax≒d...(6). Therefore, the finished surface roughness Hmax is (4),
As can be seen from equations (5) and (6), the electrode materials 57a to 5
The smaller the diameter d of 7n, the smaller the height h of the step, and the larger the swing radius R, the better the finished surface can be obtained.

According to the inventors' experimental results, using rod-shaped electrode material with a cross-sectional area of 0.5 cm ² , approximately 2000 electrodes are used to process a surface of 1000 cm ² (the electrode material corresponding to the distance g is small). If the electrode 53 is constructed and processed as a collection of
It has been found that a mirror surface can be obtained in ~9 hours.

Further, FIG. 11 shows another embodiment of the present invention, in which one pole of the processing power source 55 and the electrode material 57 are connected.
A series body of transistors 65a to 65n and resistors 59a to 59n is connected between a to 57n,
According to this, for example, the electrode material 57a and the workpiece 5
If a discharge occurs between 2, another electrode material 57b
The electric charge accumulated in the capacitance Cs formed between the electrode material 57n and the workpiece 52 is transferred to the electrode material 57a.
This eliminates any involvement in the electrical discharge of the workpiece 52, making it possible to perform even better mirror finishing.

Note that the transistors 65a to 65n may be replaced with diodes, and the same effect can be obtained even if other unidirectional conduction elements are used.

As described above, according to the present invention, since the current limiter is housed within the current collector and the binding means, it is possible to obtain an electrical discharge machining apparatus in which the electrode for electrical discharge machining is made compact, and a large number of electrode materials are housed in the binding means together with the insulating means. Since the electrodes are bundled together, an electric discharge machining device that is safe and improves workability without exposing the electrodes can be obtained, and mirror surface machining by electric discharge is possible in a practical area with a machining area that was considered impossible with conventional technology. year,
The machining time can also be shortened, and this will greatly contribute to the practical application of mirror finishing of molds using electrical discharge.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing a general electrical discharge machining device, Fig. 2 is a diagram showing the discharge voltage waveform and discharge current waveform by the device in Fig. 1, and Fig. 3 shows the stray capacitance existing in the electrical discharge machining device. An equivalent diagram to be explained and its discharge voltage and discharge current waveform diagram; Figure 4 is a diagram to explain the capacitance existing between the electrode and the workpiece facing each other; Figure 5 is a diagram used in the electric discharge machining apparatus according to the present invention. 6 is a diagram for explaining an embodiment of the machining method in the electrical discharge machining apparatus according to the present invention; FIG. 7 is a discharge voltage waveform diagram for explaining electrode feeding; Fig. 8 is a side sectional view of the finished surface machined by the oscillating motion of the electrode, Fig. 9 is an explanatory diagram when the step pitch in Fig. 8 is larger than twice the oscillating radius, and Fig. 10 is FIG. 8 is an explanatory diagram of the case where the step pitch is smaller than twice the swing radius, and FIG. 11 is a diagram showing another embodiment of the electric discharge machining apparatus according to the present invention. In the figure, 1 and 53 are electrodes, 2 and 52 are workpieces, 51 is an electrically insulating base, 58a to 58n are insulators, 57a to 57n are electrode materials, and 59a to 5
9n is a current limiter, 60 is a current collector plate, 64 is a holder, and 65 to 65n are transistors. In addition,
The same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A large number of electrode materials, means for insulating the large number of electrode materials from each other, binding means for binding the large number of electrode materials together with the insulation means, and a large number of the electrode materials attached to the binding means. a current collector connected to an electrode material; a current limiter configured to be connected in series between the current collector and each of the plurality of electrode materials and housed in the current collector and the binding means; An electrical discharge machining device characterized by having an electrode for electrical discharge machining. 2. A large number of electrode materials, means for insulating the large number of electrode materials from each other, binding means for binding the large number of electrode materials together with the insulation means, and a device attached to the binding means and connected to the large number of electrode materials. A current collector; and a current limiter configured to be connected in series between the current collector and each of the plurality of electrode materials and housed in the current collector and the binding means. The workpiece is machined while the electrode is placed opposite to the workpiece and is moved relative to the workpiece in the opposing direction while giving eccentric motion or spherical motion about a reference point in a plane intersecting the spindle feeding direction. electrical discharge machining method.