JP4956729B2 - Electric discharge machining method and electric discharge machining apparatus - Google Patents

Description

  The present invention relates to an electric discharge machining method and an electric discharge machining apparatus that can easily generate electric discharge at the start of machining and perform stable machining. In particular, the present invention relates to an electric discharge machining method and an electric discharge machining apparatus that increase the stability of electric discharge at the start of machining in micromachining.

  When starting electric discharge machining with a new tool electrode and workpiece facing each other, the servo is unstable for a while after the start of machining, and the machining speed does not increase. A starting phenomenon occurs (see Patent Document 1). This phenomenon is particularly remarkable in microfabrication.

  The cause of instability in micromachining is the lack of processing dust, electrons, and plasma that make it easy to generate electric discharge, and the concentration of surface electric field due to the flat and smooth surface of the tool electrode and workpiece. It is conceivable that the discharge is less likely to occur due to the fact that it is difficult to occur.

  Here, a through hole side surface finishing experiment was conducted. The tool electrode is a cylinder with a copper φ10 mm and is ground, and the workpiece is an SKD11 plate with a thickness of 6 mm and a pilot hole of φ9 mm. The tool electrode is a cathode and the workpiece is an anode. As discharge control, processing is performed by supplying a current having a constant on-time after detecting the occurrence of discharge after an unspecified discharge delay time after applying a voltage.

  As a result, the processing became unstable for about 5 minutes after the start of processing, and then the processing became stable. Comparing the voltage waveform and current waveform when machining is unstable and when machining is stable, when machining is unstable, there is a large variation in the discharge delay time, and the on time and off time are controlled to be constant. Therefore, it was found that large variations in the discharge delay time, in particular, the presence of a pulse having an extremely long discharge delay time, caused the machining to become unstable. In research on general discharge phenomena, it is known that variation in delay time in air discharge under certain conditions can be suppressed by irradiating with ultraviolet rays (Non-Patent Document 1).

Japanese Patent Publication No.53-21557 Kumi Nita, Doctoral Dissertation, The University of Tokyo, “Discharge Phenomenon of Microgap”, December 16, 2004, p19-20

At the start of machining, an unspecified discharge delay time from application of a predetermined voltage to the machining gap to occurrence of discharge may take several seconds. In a general electric discharge machine, the servo response frequency is several tens of Hz, the speed is several tens to several thousand μm per second, and servo control is performed by the average voltage of the machining gap. For this reason, in micromachining with a discharge gap of about several μm , if the discharge delay time is extremely long, such as a few seconds, machining becomes unstable and not only takes machining time, but also the tool electrode and workpiece are The tool electrode may be damaged due to contact, or machining may not proceed.

  An object of the present invention is to provide an electric discharge machining method and an electric discharge machining apparatus capable of easily generating electric discharge at the start of machining and performing stable machining.

In the electric discharge machining method of the present invention, an electric discharge machining liquid is interposed between a tool electrode and a workpiece, and a voltage is applied to form a machining hole in the workpiece. in the method, the cathode by a photoelectric effect to the tool electrode or the workpiece is the cathode in the machining gap formed between the tool electrode and the workpiece when repeatedly the voltage for each single discharge is applied Irradiation with ultraviolet rays having a predetermined wavelength shorter than the longest wavelength from which electrons are emitted is performed.

ただし、ｈはプランク定数、ｅは電子の電荷、ｃは光の速度、Ｖは仕事関数である。 In particular, the wavelength of the ultraviolet rays is represented by the following formula.

Where h is the Planck constant, e is the charge of the electrons, c is the speed of light, and V is the work function.

An electric discharge machining apparatus according to the present invention is a micromachining electric discharge machining apparatus having a discharge gap of several μm for forming a machining hole in a workpiece, and has a predetermined machining gap between the tool electrode and the tool electrode via an electric discharge machining liquid. A workpiece disposed oppositely, a power source connected to the tool electrode and the workpiece, and a cathode in the machining gap when a voltage is applied to the machining gap repeatedly by the power source for each single discharge. and a UV irradiation device for irradiating the ultraviolet rays of the shorter predetermined wavelength than the longest wavelength electrons are emitted from the cathode by a photoelectric effect to the tool electrode or the workpiece.

In particular, when an oil-based electric discharge machining fluid is used as a machining medium, an ultraviolet irradiation device is disposed at a distance where ultraviolet rays reach the cathode metal.

  By supplying initial electrons by irradiation with ultraviolet rays, electric discharge is easily generated at the start of processing, and the discharge delay time can be shortened, so that more stable processing can be performed. As a result, the tool electrode is not damaged or the processing does not progress particularly in the fine processing.

  In general, it is known that there is a so-called photoelectric effect in which electron emission is generated by irradiating ultraviolet rays. The unspecified discharge delay time from the time the voltage is applied to the machining gap until the discharge occurs and the current flows through the machining gap is the “statistical delay” until the initial electrons are generated, and the electron avalanche occurs in all paths. This can be divided into “delay formation” until destruction.

  In the present invention, the photoelectric effect is used to forcibly irradiate the metal of the cathode with ultraviolet rays to generate electron emission, to generate and supply initial electrons for discharge, and to easily generate discharge. Therefore, the statistical delay until the initial electrons are generated is shortened, and the discharge delay time is shortened. By shortening the discharge delay time, machining is stabilized at the start of machining.

  FIG. 1 shows a specific measurement example in which the discharge delay time is shortened by ultraviolet irradiation of the cathode. A voltage is applied by interposing an electric discharge machining liquid 3 between the tool electrode 1 and the workpiece 2 and a photoelectric effect is applied to the tool electrode 1 or the workpiece 2 in a machining gap formed by the tool electrode and the workpiece. An initial electron for discharge is generated and supplied by forcibly irradiating ultraviolet rays having a predetermined wavelength shorter than the level at which electrons are emitted from the cathode.

  In FIG. 1, a tool electrode (anode) 1 and a workpiece (cathode) 2 are arranged in a sunshade frame 4, and an oil-based electric discharge machining fluid 3 is interposed between the tool electrode 1 and the workpiece 2. Let An ultraviolet lamp 5 having a wavelength of about 253 nm is arranged as an ultraviolet light source to irradiate the workpiece 2 with ultraviolet rays. As the ultraviolet irradiation light source, an optical fiber can be used in place of the ultraviolet lamp 5, and the ultraviolet irradiation light source can be disposed in the electric discharge machining liquid close to the machining gap.

  FIG. 2 shows an electric discharge machining circuit at that time. A switching transistor 50 is arranged between the power supply device 10 and the tool electrode 1 that automatically stops output when a current flows for a certain time. When the switching transistor 50 is off, the operation of the power supply device 10 is started, and then the switching transistor 50 is turned on to generate a single discharge between the tool electrode 1 and the workpiece 2. Reference numeral 70 denotes an oscilloscope.

  In FIG. 1, a conical copper tungsten tool electrode (anode) 1 ground at a tip R0.1 mm angle of 50 ° and a flat SKD11 workpiece (cathode) 2 lapped to a surface roughness of 3 nmRz are shown. A voltage was applied with the gap kept constant, and the influence on the discharge delay time when the workpiece 2 was irradiated with ultraviolet rays was examined.

  At this time, in order to increase the reliability of the comparison between ultraviolet irradiation and non-irradiation, single discharge by ultraviolet irradiation and non-irradiation was alternately repeated 50 times and compared by the Laue plot method.

  Here, in the Laue plot method, when a voltage V is applied between electrodes at time t = 0, the discharge delay time is initially determined from the relationship between the number of times N and N that are not destroyed by time t in N trials. This is a method of separating the statistical delay time Ts until electrons are generated and the formation delay time Tf until electron avalanche occurs and the entire path is destroyed, and the relational expression expressed by Equation 1 is used.

ここで、Ｔｓａｖは、統計遅れ時間Ｔｓの平均値である。数１からわかるように、ｌｏｇ（ｎ／Ｎ）とｔとの関係は直線であり、その傾きがＴｓａｖ、ｎ／Ｎ＝１におけるｔの値が形成遅れ時間Ｔｆである。

Here, Tsav is an average value of the statistical delay time Ts. As can be seen from Equation 1, the relationship between log (n / N) and t is a straight line, the slope is Tsav, and the value of t at n / N = 1 is the formation delay time Tf.

  The irradiation start timing was immediately before the voltage application after the contact sensing operation was completed. For this reason, the irradiation time of ultraviolet rays is 10 seconds or less. Although the influence of thermal displacement due to ultraviolet rays on the gap was investigated, the positioning accuracy by the contact sensing operation has hardly changed even after irradiation with ultraviolet rays.

  FIG. 3 shows a Laue plot by ultraviolet irradiation. Data with an extremely long discharge delay time of 100 msec is about 20% when ultraviolet rays are not irradiated, and several percent when ultraviolet rays are irradiated. Therefore, the statistical delay time is shortened by increasing the probability that photoelectrons are generated by irradiation of ultraviolet rays and initial electrons appear between the electrodes.

  In order to generate initial electrons by the photoelectric effect, it is necessary that the wavelength of ultraviolet light is shorter than the level at which electrons are emitted from the cathode by the photoelectric effect. No matter how intensely an ultraviolet ray longer than this wavelength is irradiated, no electrons are emitted. This wavelength is expressed by Equation 2.

ただし、ｈはプランク定数（６．６２６×１０−３４Ｊ・Ｓ）、ｅは電子の電荷（１．６０２×１０−１９Ｃ）、ｃは光の速度（２．９９７９×１０８ｍ／ｓ）、Ｖは仕事関数である。

Where h is Planck's constant (6.626 × 10−34J · S), e is the charge of electrons (1.602 × 10-19C), c is the speed of light (2.9799 × 108 m / s), and V is Work function.

  FIG. 4 shows a calculation result of a work function of a tool electrode and a workpiece generally used for electric discharge machining and a longest wavelength for electron emission. Note that when the work function is small, electrons are likely to be emitted, and discharge formation due to secondary electron emission is quick, and initial electrons for generating discharge are likely to be emitted, thereby shortening the discharge delay time.

  Moreover, the electric discharge machining liquid needs to transmit light. FIG. 5 shows the result of measuring the transmittance of an oil-based electrical discharge machining fluid according to the ultraviolet wavelength. The transmittance is measured with a working fluid layer (depth) of 5 mm and 1 mm.

According to Bouge-Beer's law, the relational expression between the intensity of incident light I ₀ , the intensity of transmitted light I, and the thickness (depth) b of the solution layer is expressed by Equation 3.

The number 3 and results depending on the relationship of the transmittance versus depth shown in Figure 6 in FIG. 5 is obtained. As a result, in the case of the oil-based electric discharge machining fluid, for example, ultraviolet rays reach the cathode metal under the condition of a wavelength of 250 nm and a depth of the electric discharge machining fluid of 3 mm or less, preferably 1 mm or less, and photoelectrons are emitted. As described above, when the oil-based electric discharge machining fluid used in general electric discharge machining is used as the machining medium, the ultraviolet irradiation light source is disposed at a distance where the ultraviolet rays reach the cathode metal.

  The present invention is used for electric discharge machining. In particular, it is possible to improve the stability of electric discharge at the start of processing in fine processing. The present invention extends the possibilities of processing in microfabrication.

It is a block diagram which shows embodiment of this invention. It is a circuit diagram which shows embodiment of this invention. It is a Laue plot figure which shows the ratio of this invention and the conventional discharge delay time. It is a figure which shows the calculation result of the longest wavelength for the work function with respect to material, and electron emission. It is a graph which shows the transmittance | permeability measurement result by the ultraviolet wavelength of a certain oil-based electric discharge machining fluid. It is a graph which shows the relationship of the transmittance | permeability with respect to the depth in the electric discharge machining fluid shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tool electrode 2 Work piece 3 Electric discharge machining fluid 4 Sunshade frame 5 Ultraviolet lamp

Claims (4)

In a micromachining electric discharge machining method in which an electric discharge machining liquid is interposed between a tool electrode and a workpiece and a voltage is applied to form a machining hole in the workpiece, and the electric discharge gap is several μm. longest wavelength electrons from the cathode are emitted by the photoelectric effect in said tool electrode or the workpiece is the cathode in the machining gap formed by a tool electrode and said workpiece when said voltage is applied An electric discharge machining method in which ultraviolet rays having a shorter predetermined wavelength are irradiated. The electric discharge machining method according to claim 1, wherein the wavelength of the ultraviolet light is represented by the following formula.

Where h is the Planck constant, e is the charge of the electrons, c is the speed of light, and V is the work function. In a micromachining electric discharge machining apparatus having a discharge gap of several μm for forming a machining hole in a workpiece, a tool electrode, and a workpiece arranged opposite to the tool electrode with a predetermined machining gap via an electric discharge machining liquid A power source connected to the tool electrode and the workpiece; and the tool electrode or the workpiece that is a cathode in the machining gap when a voltage is applied to the machining gap repeatedly by the power source for each single discharge . discharge machining apparatus and an ultraviolet light irradiation unit for emitting ultraviolet light of the short predetermined wavelength than the longest wavelength electrons are emitted from the cathode by the photoelectric effect. 5. The electric discharge machining apparatus according to claim 4, wherein when the oil-based electric discharge machining liquid is used as a machining medium, the ultraviolet irradiation device is disposed at a distance at which ultraviolet rays reach the metal of the cathode.

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