ELECTRICAL DISCHARGE MACHINING APPARATUS FOR FORMING MINUTE HOLES IN A WORKPIECE

Abstract: An electrical discharge machining apparatus for forming minute holes in a workpiece has a machining electrode com­ posed of an electrode core wire, a support tube having a bore in which the electrode core wire is slidably supported and an outer peripheral surface held in highly concentric relation to the bore, and a rotatable support body for transmitting rota­ tive power to the support tube fitted therein, the machining electrode being rotatable about its own axis which is substan­ tially aligned with the central axis of the electrode core wire. The machining electrode is rotatably supported in V-shaped grooves in a support and movable toward and away from the workpiece. The workpiece is movable between a position below the machining electrode and a position below a posi­ tioning means held in juxtaposed relation to the machining electrode. A position to be machined on the workpiece can determined by the positioning means.

Description: DESCRIPTION ELECTRICAL DISCHARGE MACHINING APPARATUS FOR FORMING MINUTE HOLES IN A WORKPIECE The present invention relates to an apparatus for forming minute holes in a workpiece through an electrical discharge machining (EDM) or electroerosive machining process Electrical discharge machining (EDM) has conventionally been deemed inappropriate for forming minute holes in a workpiece for the reasons that machining accuracies such as surface roughness and out-of-roundness become more and more problematic as the diameter of a hole formed is smaller, and it is difficult to feed the electrode in minute steps. There are two EDM machining processes for forming a minute hole having a diameter of 0.1 mm or smaller in a workpiece. In one process, the machining electrode is held at rest, and in the other process, the machining electrode is rotated about its own axis. Rotating the machining electrode could increase the machining speed, and improve out-of-roundness and accuracy of the shape of the hole formed. However, the machining process in which the electrode is held at rest during operation has heretofore been prevalent in the art since the electrode is not subjected to displacement and the time required for the electrode to form the hole is shorter. Various electrode constructions are known for use in forming holes while the electrode is held at rest. Where a thin wire which requires no cutting is employed as an electrode, it is directly soldered to an electrode holder, or is plated, reinforced, and chucked, or protected and guided by a body of glass, ruby or the like and chucked. Where a relatively thick wire is used for an electrode, the tip end thereof is thinned by electrolytic grinding so as to serve as a machining electrode. The above electrode constructions however fail to provide a degree of concentricity of a few microns or less between the machining tip and outer periphery of the electrode. It has been impossible to use such electrode in machining a hole while rotating the electrode since it is difficult to attach the electrode perpendicularly to the workpiece. When a workpiece is subjected to an EDM process with the wire electrode held at rest, the shape of the wire electrode is transferred to the workpiece. No hole with small out-of-roundness would be formed in the workpiece if a commercially available thin wire finished to insufficient dimensional accuracies were used for machining. One electrode construction used for machining a workpiece while the electrode is rotating is composed of a round rod having a diameter of about 1 mm which can be chucked. The electrode material as it is rotated is shaped into an electrode of a desired diameter by a reverse electrical discharge. Although the electrode thus formed has sufficient accuracy, it will take about one hour to form an electrode for forming a minute hole 30 micrometers across from an electrode material having a diameter of 1 mm. The thined portion of the electrode which has a diameter of 30 micrometers is short in length and hence in service life. Because the electrode will be displaced once detached from the chuck, the electrode has to be formed again each time it machines a minute hole in a workpiece. Conventional EDM machines employ a ball-and-roller bearing by which the electrode is rotatably supported. The ball-and-roller bearing however cannot avoid displacements of the electrode by a few micrometers. The electrode assembly comprises a spindle construction as a whole which has a large stray capacity and hence cannot reduce the capacitance of an RC circuit. Where holes having a diameter of about 50 micrometers are formed by such an electrode assembly, accuracies such as out-of-roundness and surface roughness become extremely poor. The electrode assembly is heavy and its speed of response for feeding the electrode is necessarily reduced. Therefore, the electrode assembly has no smooth machining capability, and can machine holes in a practical range of diameters down to about 100 micrometers at minimum. Another problem with the prior EDM machines is that it is quite a complex task to position the electrode accurately for forming a minute hole. Two processes have been available for positioning the machining elecrode and workpiece with respect to each other in forming a hole having a diameter of 0.3 mm or smaller. One positioning process is known as a contact sensing process in which an electric short circuit is sensed between the machining electrode and the workpiece, a contact position is read on a position scale or the like, and the electrode and the workpiece are relatively moved to a desired position according to the reading on the position scale. According to this process, a weak voltage is applied between a machining electrode and a workpiece placed in an insulative liquid contained in a machining bath on an X-Y table, while at the same time the machining electrode is moved horizontally toward the workpiece. When the machining electrode contacts the workpiece, a shortcircuiting current flowing between the machining electrode and the workpiece is detected and the reading on the position scale at this time is set as 0. Asssuming that a desired position where a hole is to be pierced is spaced a distance a in the direction of X from the position in which the workpiece and the machining electrode are in contact with each other, the X-Y table is moved a distance a + d/2 in the direction of X taking the diameter d of the machining electrode into account, so that the central axis of the machining electrode will be in registration with the desired position. When the X-Y table is moved another distance b + d/2 in the direction of Y, the center of the machining electrode is brought accurately into registration with another position in which to pierce another hole. The above process can provide a sufficient degree of accuracy where the machining electrode has a relatively large diameter. However, in case the machining electrode has a diameter of a few tens micrometers for forming minute holes, the electrode lacks desired rigidity and tends to flex upon contact with the workpiece, resulting in the failure of accurate positioning. Another positioning process uses a stereomicroscope for observing the tip of the machining electrode and the workpiece while positioning them relatively to each other. This positioning process however also fails to effect accurate positioning because of the parallax since the workpiece is obliquely observed. It is an object of the present invention to provide an electrical discharge machining apparatus for forming minute holes in a workpiece, which will eliminate the drawbacks of the conventional EDM apparatus. Another object of the present invention is to provide an electrical discharge machining apparatus for forming minute holes in a workpiece, which requires no spindle construction, has a light and small-size electrode assembly, has an electrode support electrically insulated from the other parts for greatly reducing a stray capacity, and hence is capable of forming minute holes of small surface roughness and out-of-roundness and diameters ranging from 10 to 200 micrometers at high machining speeds. Still another object of the present invention is to provide an electrical discharge machining apparatus for forming minute holes in a workpiece, which presents no parallax and can position a thin machining electrode relatively to a workpiece with high accuracy. Still another object of the present invention is to provide an electrical discharge machining apparatus for forming minute holes in a workpiece, which has a support tube supporting an electrode core wire and a rotatable support body, the support tube and the rotatable support body having a central axis and a center of rotation which are aligned with each other highly accurately for preventing the electrode from wobbling movement or displacement during rotation thereof, so that minute holes of small out-of-roundness and good accuracy of shape can be formed in a workpiece. A still further object of the present invention is to provide an electrical discharge machining electrode for forming minute holes, which can be fed out of a rotatable support body as the electrode is progressively consumed by electrical discharges, so that the electrode can be used for a long period of time. A still further object of the present invention is to provide an electrical discharge machining electrode for forming minute holes, which can easily be manufacured from a commercially available thin core wire. A still further object of the present invention is to provide an electrical discharge machining electrode for forming minute holes, which allows easy removal of machined chips produced upon electrical discharge machining for an increased machining speed. According to the present invention, there is provided an electrical discharge machining apparatus for forming a minute hole in a workpiece, comprising a machining electrode for forming the minute hole in the workpiece by an electrical discharge between the machining electrode and the workpiece, a machining electrode moving means for moving the machining electrode toward and away from the workpiece, a positioning means for establishing a machining position with respect to the workpiece, and a workpiece moving means for moving the workpiece between a first position below the machining electrode and a second position below the positioning means. The machining electrode is composed of an electrode core wire slidably supported in a support tube having an outer periphery and a bore in which the electrode core wire is inserted, the outer periphery and the hole being in highly concentric relation to each other. The support tube is coupled to a rotatable support body by which rotative power is transmitted to the support tube, with their axes of rotation in alignment with each other. The machining electrode is supported by a support means having V-shaped grooves and will be rotated at a constant speed while an electrical discharge is produced between the machining electrode and the workpiece. The machining position can be established by piercing a minute hole in the workpiece at any desired position, moving the workpiece to the position below the positioning means to bring the pierced minute hole into alignment with a prescribed position in the field of view of a microscope so that the positions directly below the machining electrode and the microscope will be in conjugate relationship, then setting a position in which to machine the workpiece at the prescribed position in the field of view of the microscope, and thereafter returning the workpiece to the position directly below the machining electrode. The present invention will be described in detail by way of illustrative example with reference to the accompanying drawings, in which; FIG. 1 is a perspective view of an electrical discharge machining apparatus for forming minute holes in a workpiece according to the present invention; FIG. 2 is a perspective view of a machining head in the electrical discharge machining apparatus of FIG. 1; FIG. 3 is a perspective view of a machining electrode used in the electrical discharge machining apparatus FIG. 4 is a longitudinal cross-sectional view of the machining electrode shown in FIG. 3; FIG. 5 is a front elevational view of the machining electrode shown in FIG. 3; FIGS. 6A and 6B are side elevational and crosssectional views of a die for fabricating a wire guide for the machining electrode; FIG 7 is a perspective view of an electrode core wire in the machining electrode; ; FIGS. 8A, BB, and 8C are plan, front elevational, and perspective views of a support for the machining electrode; FIG. 9 is a schematic side elevational view, partly in cross section, of a machining electrode moving mechanism in the electrical discharge machining apparatus; FIG. 10 is a fragmentary side elevational view of the machining head; FIG. 11 is an exploded perspective view of a fine adjustment mechanism for positioning a workpiece in the electrical discharge machining apparatus; FIG. 12 is a perspective view of a workpiece moving mechanism in the electrical discharge machining apparatus; FIG. 13 is a circuit diagram, partly in block form, of a circuit arrangement for an electrical discharge unit and the machining electrode moving mechanism in the electrical discharge machining apparatus; ; FIG. 14 is a perspective view of a machining electrode according to another embodiment of the present invention; FIG. 15 is a fragmentary longitudinal cross-sectional view of the machining electrode shown in FIG. 14; FIGS. 16A, 16B, and 17 are views illustrative of a process of assembling a wire guide and a mandrel of the machining electrodes of the present invention; FIG. 18 is a transverse cross-sectional view of a machining electrode according to still another embodiment of the present invention; FIGS. 19 and 20 are side-elevational views, partly in cross section, showing the manner in which machining electrode of still further embodiments are used; FIGS. 21A and 21B are plan and front elevational views of a machining electrode support according to a still further embodiment of the present invention; ; FIG. 21C is a cross-sectional view of a protruding member in the machining electrode support illustrated in FIGS. 21A and 21B; FIGS. 22A and 22B are diagrams illustrative of an angular error caused at the time a number of holes are formed by the electrical discharge machining apparatus of the present invention; and FIGS. 23A and 23B are diagrams explanatory of a process of positioning a workpiece when a number of holes are formed by the electrical discharge machining apparatus of the present invention. FIG. 1 shows an overall arrangement of an electrical discharge machining (EDM) apparatus for forming minute holey in a workpiece according to the present invention. The electrical discharge machining apparatus includes a mac ining electrode 4 and a machining head 10 supporting the machining electrode 4. A workpiece 1 is placed on a support base 11 disposed in a machining bath 2. As the machining electrode 4 icj moved toward the workpiece 1 while rotating the machining electrode 4, an electrical discharge is initiated between the machining electrode 4 and the workpiece 1. A minute hole can be formed in the workpiece 1 by successively feeding the machining electrode 4 toward the workpiece 1. The EDM apparatus also includes a positioning microscope 12 movable in three directions, that is, X and Y directions extending parallel to a base 3 and perpendicularly to each other, and a Z direction extending perpendicularly to the base 3 and the X and Y directions. The machining bath 2 can be fed in the X direction by sliding a slide plate 29 of the bath 2 on a slide table 14 with a fine adjustment screw 13. The machining bath 2 can be angularly adjusted in an X-Y plane with a fine adjustment screw 27. The slide table 14 can be moved in the X direction until it abuts against stoppers 31, 32. When the slide plate 14 is fixed in abutment against the stopper 31, the workpiece 1 is positioned directly below the machining electrode 4. When the slide plate 14 is fixed in abutment against the stopper 32, the workpiece 1 is positioned directly below the microscope 12. An eccentric cam 33 is angularly movable by a lever 34 as described later on. A table 17 movable in the Y direction is disposed beneath the slide table 14 and can be moved by a feed screw 18. The machining bath 2, the slide table 14, and the table 17 jointly constitute a support base assembly. The machining head 10, the microscope 12, and the support base assembly are mounted individually on the base 3. The microscope 12 is mounted on a holder having a slide mechanism having three slides. More specifically, a slide 21a extends in a driection perpendicular to the workpiece 1 or the Z direction for adjusting the focus of the microscope 12. The slide 21a can be replaced with a mechanism for moving an objective lens of the microscope upwardly and downwardly. A slide 21b serves to move the microscope 12 in the same direction as that of movement of the slide table 14, or the X direction. A slide 21c serves to move the microscope 12 in a direction, or the Y direction, normal to the direction in which the slide table 14 moves. The microscope 12 has an eyepiece bearing cross hairs 15 which will appear in the field of view of the viewer, the intersection of the cross hairs 15 serving as a sight. FIG. 2 shows the machining head 10 in greater detail. The machining electrode 4 is rotatably supported by a support or bearing 6 having V-shaped grooves, and will be rotated by a belt 5 trained around a pulley 7 on the machining electrode 4 and a pulley 9 on a motor 8, while at the same time the machining electrode 4 is held against the bearing 6. Since the pully 9 is located upwardly of the pulley 7, the machining electrode 4 is biased to move upwardly under the tension of the belt 5 while being rotated by the motor 8. The machining electrode 4, the bearing 6, and the pulley 7 jointly constitute an electrode assembly electrically insulated by an insulator 16 as of ceramics from a body 35 of the machining head 10. The electrode assembly is electrically connected to a DC power supply 37 by means of a brush 36. The machining electrode 4 can be fed selectively in two modes, that is, rough feeding and fine feeding. In the rough feeding mode, the body 35 is vertically slid along a post 38 on the base 3 to bring the machining electrode 4 together with the body 35 to a position near the workpiece 1 disposed in the machining bath 12. Thereafter, in the fine feeding mode, a pinion gear 42 is rotated by a motor 43 to rotate a gear 19a integral with a feed screw 19 for thereby vertically moving the feed screw 19, causing a pivot lever 44 to move the machining electrode 4 for a small vertical interval. A machining circuit is composed of the DC power supply 37, a capacitor 45, and a resistor 36 through which the brush 36 is connected to the DC power supply 37. A machining current flowing through the machining circuit is detected by a current monitor 47 and utilized for enabling a servo control circuit 47 to control the speed and direction of rotation of the motor 43 for controlling the speed and direction of fine feeding movement of the machining electrode 4. When an ordinary electrical discharge is generated between the machining electrode 4 and the workpiece 1, the motor 43 is rotated counterclockwise at a prescribed constant speed to feed the machining electrode 4 at a predetermined speed toward the workpiece 1. When the machining electrode 4 and the workpiece 1 in the machining bath 2 are short-circuited, the current monitor 47 detects a short-circuiting current and causes the servo control circuit 48 to rotate the motor 43 clockwise at a high speed for moving the machining electrode 4 away from the workpiece 1 at a high speed. After the short circuit has been eliminated, the motor 43 is rotated counterclockwise again to feed the machining electrode 4 at the prescribed speed toward the workpiece 1. The workpiece 1 is progressively machined while the above cycle of operation is repeated. FIGS. 3 through 5 illustrate the machining electrode 4. The machining electrode 4 is constructed of a thin electrode core wire 51 slidably guided in a wire guide or support tube 52 made of ceramics which is attached by a guide sleeve or tube 54 to a metal mandrel or rotatable support body 53. The wire 51 has an end connected to a pipe 56 made of an electrically conductive material and fixedly mounted in the mandrel 53 by means of a metal screw 55. As shown in FIG. 4, the wire guide 52 is fitted in the guide sleeve 54 with a clearance of 2 micrometers or smaller therebetween. The mandrel 53 is fitted in the guide sleeve 54 with a clearance as defined in JIS (Japanese Industrial Standards) h6 - P6. According to the above embodiment, the wire guide 52 comprises a ceramic pipe for use as an optical fiber connector core, and the electrode core wire 51 is fitted therein with a clearance of 1 micrometer or less. The wire guide 52 has an outer peripheral surface and a bore which are concentric with each other with an error of 1 micrometer or less. The bore of the guide sleeve 54 is finished by lapping so that the inside diameter thereof differs from the outside diameter of the wire guide 52 by the range of 0 to 2 micrometers. The mandrel 53 is disposed in the guide sleeve 54 with a close fit having a clearance as defined in JIS h6 - P6. Since the wire guide 52 is supported by the guide sleeve 54 with the outer peripheral surface of the mandrel 53 serving as a positioning reference, the machining electrode 4 is of high accuracy with the wire 51 displaceable to 4 micrometers at maximum. A method of manufacturing the wire 52 with an error of concentricity between the outer peripheral surface and bore thereof being 1 micrometer or smaller will now be described with reference to FIGS. 6A and 6B. Where the wire guide 52 is made of a sintered material such as ceramics, the wire guide 52 can be fabricated by vacuum extrusion molding. FIGS. 6A and 6B show a die used in vacuum extrusion molding. The die is composed of an outlet piece 57, a pin 58, and a pin holder 59 in which an end of the pin 58 is embedded. A kneaded mass 60 of ceramics is fed in the direction of the arrows to the right (as shown in FIG. 6B) through gaps 59' in the pin holder 59 into the outlet piece 57. The neaded ceramic mass 60 is then extruded out of the die in the form of a pipe by means of the pin 58 and the outlet piece 57. Through increased accuracies of the combined pin 58 and the outlet piece 57, the error of concentricity between the bore and outer peripheral surface of the extruded ceramic pipe becomes 10 micrometers or smaller. After the extruded ceramic pipe has been subjected to firing, a wire is threadd through the bore of the ceramic pipe and the outer peripheral surface is cylindrically ground by a centerless grinder with the wire serving as a guide, thus producing a high-precision wire guide 52 having a concentricity error of 3 micrometers or smaller. In producing pipes through extrusion molding, no manufacturing problem arises out of the fact that the outside diamter of the pipe becomes large with respect to the diemeter of the bore therein, or the bore is enlarged. The guide sleeve 54 may be machined by various processes such as lapping or honing since only the bore in the guide sleeve 54 in which the wire guide 52 is fitted should meet the prescribed dimensional tolerance. It is easy to keep the dimensions of the guide sleeve 54 within the tolerance ranging from 0 to 2 micrometers. A fit as prescribed by JIS h6 - P6 can easily be achieved between the mandrel 53 and the guide sleeve 54 simply by grinding the outer peripheral surface fo the guide sleeve 54 with a cylindrical grinding machine. In order to hold the thin wire 51 serving as the machining electrode highly linearly within the mandrel 53 and also to provide electric conductivity between the wire 51 and the mandrel 53, a pipe 56 is staked or soldered or otherwise connected to a rear end of the wire 51, as shown in FIG. 7, and then is secured to the mandrel 53 by a metal setscrew 55, as illustrated in FIG. 4. The wire guide 52 is fitted in the mandrel 53 with a clearance as prescribed by JIS h6 - H7. The wire guide 52 may be made of ruby, sintered metal, cemented carbide, for example, other than ceramics. The pipe 56 may be made of any electrically condictuve mateirals. The guide sleeve 54 may also be made of ruby, sintered metal, ordinary metal, for example, other than ceramics. According to the present embodiment, the clearance between the wire 51 and the wire guide 52 is on the order of 1 micron and hence is negligible. FIGS. 8A, 8B, and 8C illustrate the bearing 6 in detail. The bearing 6 is composed of two upper and lower V-grooved members 61 attached to a support body 62 and having V-shaped grooves or notches in which the machining electrode 4 is disposed and held against the V-grooved members 61. The V-grooved members 61 are made of ruby in the illustrated embodiment, but may also be made of hard metal, sapphire, diamond, quenched steel, or other wear-resistant materials. The V-grooved members 61 and the support body 62 may be of an integral construction of the same material. Each of the V-shaped grooves should preferably flare at an angle rangeing from 300 to 1500 FIG. 9 illustrates the fine feed mechanism in greater detail. The lever 44 is pivotally supported by a pivot C. The machining electrode 4 and the feed screw 19 have semispherial or conical ends held pivotally against contact members 321 on the lever 44 which are made of a highly wear-resistant material. The contact members 321 are in the form of sapphire blocks in the illustrated embodiment, but may be made of ruby, diamond, ultra high strength steel, or quenched steel, for example. The feed screw 19 has an externally threaded lower portion threaded in an internally threaded hole in the body 35. Upon rotation of the motor 43, the pinion 42 and the gear 19a meshing therewith are rotated to cause the feed screw 19 to move vertically as the gear 19a meshingly engages the pinion 42 at different positions. The vertical movement of the feed screw 19 is transmitted through the lever 44 to the machining electrode 4 which is then fed in its axial direction. In the present embodiment, the feed gear l9a and the pinion 42 have a gear ratio of 5 : 1, the external threads on the feed screw 19 comprise righthand screw threads having a pitch of 0.5 mm, and the pinion 42 has a teeth length of 10 mm. When the motor 43 rotates counterclockwise, the feed screw 19 is rotated clockwise at a speed which is 1/5 of the speed of rotation of the motor 43, and is moved upwardly by a distance of 0.1 mm per revolution of the motor 43. The maximum distance the feed screw 19 can move vertically is determined by the teeth length of the pinion 42. As a result, an end A of the lever 44 which contacts the feed screw 19 is lifted upwardly by a distance of 0.1 mm. With the lever 44 having a leverage L1 : L2 = 1 : 1 in the illustrated embodiment, an end B of the lever 44 opposite to the end A is depressed by a distance of 0.1 mm. The machining electrode 4 is operatively connected to the motor 8 by the pulleys 7, 9 and the belt 5. Since the machining electrode 4 is normally biased upwardly by the belt 5, the machining electrode 4 is held in contact with the lever 44 at all times. Accordingly, the vertical displacement of the lever end A is equal to the distance the machining electrode 4 is fed vertically. The foregoing mode of operation is effected when a stable electrical discharge is produced. When the workpiece 1 and the machining electrode 4 are short-circuited, the motor 43 is rotated clockwise to lift the machining electrode 4 off the workpiece 1. The tension of the belt 5 can be reduced by a weight 63 attached to the end A of the lever 44. FIG. 10 shows the manner in which the bearing 6 is electrically insulated for reducing any stray capacity and the brush 36 is in contact with the machining electrode 4. The body 35, and the machining electrode 4 and the bearing 6 are electrically insulated by the insulator 16 which is made of ceramics, paper, vinyl, or glass, for example. The machining electrode 4 and the machining power supply are held in electric contact directly by means of the brush 36. This arrangement allows the discharge circuit small in size to reduce any stray capacity which would influence electrical discharge machining. The machining bath 2 and the slide table 14 are illustrated in greater detail in FIG. 11. A rotatable slide 26 is mounted on the underside of the machining bath 2 and has a cylindrical portion slidably fitted in a hole 25 defined in an upper surface of the slide table 14. By pushing a projection 28 on the machining bath 2 with the adjustment screw 27, the machining bath 2 is angularly moved through a small angle about its center through the slidable fitting engagement between the slide 26 and the slide table 14. A ball bearing may be disposed between the slide 26 and the slide table 14 instead of fitting engagement therebetween. The machining bath 2 is movable in the direction of the arrow A (corresponding to the X direction in FIG. 1) by the slide plate 29 attached to the underside of the machining bath 2 and guided by a guide 29a on the slide 26. The distance that the machining bath 2 is moved in the A direction can be adjusted by a feed screw 30. FIG. 12 shows the stoppers 31, 32 for the slide table 14. The stopper 31 disposed on the lefthand side (as shown) of the slide table 14 comprises a micrometer head, while the stopper 32 on the lefthand side of the slide table 14 comprises a permanent magnet having a ground end surface. The slide table 14 can be positioned at a lefthand limit position by pressing a lefthand end 14a of the slide table 14 against the micrometer-head stopper 31 with a spring (not shown) disposed within the slide table 14, and at a righthand limit position by magnetically attracting a righthand end 14b of the slide table 14 to the end surface of the magnetic stopper 32. The slide table 14 which has been attracted to the magnetic stopper 32 can be disenaged therefrom in one operation by angularly moving the eccentric cam 33 with the lever 34. The released slide table 14 is then pressed at its lefthand end 14a against the micrometer-head stopper 31 under the tension of the spring in the slide table 14, whereupon the machining electrode 4 of the machining head 10 is held in confronting relation to the workpiece 1 in the machining bath 2. Operation of the EDM apparatus thus constructed for forming a minute hole in the workpiece 1 will be described with reference to FIG. 1. The workpice 1 is placed in the machining bath 2 filled with an insulative machining solution such as illuminating kerosene. The workpiece 1 may roughly be positioned at this time. Then, the slide table 14 is moved into abutment against the lefthand stopper 31 and stopped. The workpiece 1 is non; positioned substantially below the machining electrode 4. The machining electrode 4 is rotated and lowered against the workpiece 1 to form a marking hole in the wo kpiece 1. The marking hole may be of a size which can be visually recognized. Then, the machining electrode 4 is raised, and the slide table 14 is moved into abutting engagement with the righthand stopper 32 and stopped there under magnetic forces of the stopper 32. At this time, the workpiece 1 is disposed substantially below the microscope 12. The microscope 12 is then moved by the slides 21a, 21b, 21e attached to the holder 20 for focusing adjustment until the crossing point, serving as the sight, of the cross hairs 15 in the field of view of the microscope 12 is aligned with the center of the marking hole in the workpiece 1. With such adjustment, the position H (FIG. 1) of the machining electrode 4 and the crossing point I of the cross hairs 15 of the microscope 12 are brought into a conjugate relationship through the movement of the slide table 14. The term "conjugate relationship" used herein means the condition in which when the slide table 14 is moved into engagement with the stoppers 31, 32, a desired position on the workpiece 1 in which to form a hole and which is established by the crossing point I of the cross hairs 15 under the microscope 12 coincides with the position of the hole which will actually be formed by the machining electrode 4 at all times. Accordingly, once such a congugate relationship is established, any desired position on the workpiece 1 which is to be perforated will be positioned directly below the machining electrode 4 by first bringing such a desired position on the workpiece I into alignment with the crossing point I of the cross hairs 15 under the microscope 12 and then moving the slide table 14 into abutment against the stopper 31 and stopping the slide table 14 there. The manner in which a desired position on the workpiece 1 to be perforated is aligned with the crossing point I of the cross hairs 15 in the field of view of the microscope 15 will be described hereinbelow. After the conjugate relationship has been established between the machining electrode position H and the crossing point position I, the slide table 14 is fixed in abutment against the stopper 32. Then, the workpiece 1 is moved by sliding the slide 14 in the Y direction and the slide 29 in the X direction with the feed screws 18, 13 so that the desired position to be machined will be aligned with the center I of the cross hairs 15. Since the feed screws 18r 13 comprise micrometer heads having a lead of 0.5 mm in the illustrated embodiment, such positioning adjustment can be carried out easily and highly accurately. In this adjustment, the slide table 14 is also moved in the Y direction as the slide 17 below the slide table 14 is moved in the Y direction. However, with the slide table 14 and the slide 17 being movable in mutually perpendicular directions, the conjugate relationship established between the crossing point position I on the microscope 12 and the machining electrode position H is not impaired at all. After the above positioning adjustment has been effected, the slide table 14 is moved again into abutment against the stopper 31. The desired position on the workpiece 1 to be perforated is now positioned directly below the machining electrode 4 with accuracy since the conjugate relationship has already been established by previously positioning the slide table 14 in engagement with the stoppers 31, 32. The workpiece 1 can then be machined to form a minute hole in the desired position by lowering the energized machining electrode 4 while rotating the same. As described above, once the conjugate relationship has been established, no further positioning adjustment is necessary in successive cycles of machining operation. When the wire 1 is consumed as the machining proceeds, the setscrew 55 is loosened, the wire 1 is allowed to drop by gravity or pulled out by forceps, and then fixed in position by the screw 55 for further machining operation. FIG. 13 schematically illustrates a motor control circuit in which a stepping motor is used for feeding the machining electrode 4 toward the workpiece 1, and also shows an electrical discharge machining circuit. A portion enclosed by a dotted line is the electrical discharge machining circuit, while the remainder serves as the motor control circuit corresponding to the servo control circuit 48 shown in FIG. 2. Denoted at 4 is the machining electrode, 1 the workpiece, 45 the capacitor, 23 the resistor, and 37 the power supply. The motor control circuit includes a reference voltage detecting resistor 64, a comparator 65 for comparing voltages generated by the resistors 47, 64, an AND gate 66 for rotating the motor 43 at a low speed, an AND gate 67 for reversing the motor at a high speed on short-circuiting, an oscillator 68 for generating a reference rotation signal, a 1/N counter 69, and a driver amplifier 70 for driving the motor 43. During a normal electrical discharge, the reference rotation signal is frequency-divided by the 1/N counter 69. A signal from the 1/N counter 69 is applied through the AND gate 66 to the driver amplifier 70 for rotating the motor 43 at a contant low speed. When the machining electrode 4 and the workpiece 1 are short-circuited to cause the comparator 65 to produce a short-circuit signal, the signal from the oscillator 68 goes through the AND gate 67 to the driver amplifier 70 which rotates the motor 43 at a high speed in the opposite direction. FIGS. 14 and 15 are illustrative of a machining electrode according to another embodiment of the present invention. According to this embodiment, the mandrel 53 and the guide sleeve 54 in the embodiment of FIGS. 3 and 4 are integrally formed of the same material. A thin electrode core wire 51 is guided by a wire guide or support tube 52 of ceramics which is disposed in and attached to a mandrel or rotatable support body 53 made of metal such as quenched steel. The wire guide 52 is fitted in the mandrel 53, as shown in FIG. 15, with a clearance as prescribed by JIS h6 - P6. The wire guide 52 is composed of a cermics pipe for use as an optical fiber connector, with the error of concentricity bewteen the outer peripheral surface and the bore being 1 micrometer or less. The mandrel 53 is machined to render its outer peripheral surface and bore concentric with each other, with a concentricity error being 1 micrometer or smaller. The wire guide 52 is fitted in the mandrel 53 in a manner to reduce any possible diplacement of the electrode core wire 51 to a minimum. The distance that the wire guide 52 is inserted longitudianlly in the mandrel 53 is sufficiently larger as compared with the diameter of the wire guide 52, so that the wire guide 52 will not be tilted appreciably with respect to the longitudinal axis thereof. As a result, the electrode core wire 51 is subjected to only a greatly reduced displacement. Where the wire guide 52 has an outside diameter of 2.5 mm or larger, the mandrel 53 may be bored in an ordinary manner, such for example as by being drilled and then finished with a reamer. Since the tool used is of a sufficient rigidity, the bore formed in the mandrel and the outer peripheral surface thereof can be concentric with each other with a concentricity error of a few micrometers. A process of fitting the wire guide 52 in the mandrel 53 in a manner to minimize any possible displacement of the electrode core wire 51 will be described with reference to FIGS. 16A, 16B, and 17. FIG. 16A illustrates the positional relationship between the bore and outer peripheral surface of the wire guide 52, and FIG. 16B shows the positional relationship between the bore and outer peripheral surface of the mandrel 53. As shown in FIG. 16A, the center 0' of the bore in the wire guide 52 is radially displaced rl from the center 0 of the circular outer peripheral surface of the wire guide 52 at an angular position el, As illustrated in FIG. 16B, the center 0"' of the bore in the mandrel 53 is radially displaced r2 from the center 0" of the circular outer peripheral surface of the mandrel 53 at an angular position e2. When combining the wire guide 52 and the mandrel 53 together in fitting engagement with each other, the mandrel 53 is angularly moved clockwise about its own axis through an angle of e2 in FIG. 16B, and the wire guide 52 is angularly moved counterclockwise about its own axis through an angle of 1800 - el The resulting relative angular positional relationship between the wire guide 52 and the mandrel 53 is illustrated in FIG. 17. More specifically, the error of concentricity R between the bore in the wire guide 52 and the outer peripheral surface of the mandrel 53 can be expressed by the following equation: 2 1 As a consequence, the concentricity of the bore in the wire guide 52 fitted in the mandrel 53 with respect to the outer peripheral surface of the mandrel 53 is better than the concentricity of the bore in the wire guide 52 with respect to the outer peripheral surface thereof. The machining electrode with no substantial core wire displacement can therefore be achieved by combining the wire guide 52 and the mandrel 53 which have bore concentricity errors of a few micrometers. In an actual assembling procedure, a tool microscope having a magnification of 800x is used to determine concentricity errors of the bores in the wire guide 52 and the mandrel 53, and the angular positions elt e2 to determine a relative angular position in which to assemble the wire guide 52 and the mandrel 53, followed by forcing the wire guide 52 into the mandrel 53 in the determined relative angular position. With the above machining electrode construction, the bore in the wire guide 52 and the guide sleeve 54 or the mandrel 53 have a concentricy error of 2 microns or less, which substantially prevents the electrode core wire 51 from being displaced during electrical discharge machining so that a minute hole can be formed which has quite small out-of-roundness and reduced surface roughness. When it is necessary to make the concentricity error smaller, the position of the wire guide 52 with respect to the guide sleeve 54 or the mandrel 53 may be adjusted in the radial direction as shown in FIG. 18. More specifically, the guide sleeve 54 or the mandrel 53 surrounding the wire guide 52 has three radial adjustment screws 71 threadedly disposed therein at about 1200-spaced intervals and having tip ends positionable in contact with the outer peripheral surface of the wire guide 52. By adjusting the adjustment screws 71 while observing the position of the wire guide 52 as with a microscope, the wire guide 52 can be positioned into highly concentric relation to the guide sleeve 54 or the mandrel 53. FIGS. 19 and 20 illustrate machining electrodes having chip discharge passages according to further embodiments of the present invention. In FIG. 19, an electrode core wire 51 has a recess 72 extending upwardly from a tip thereof. The workpiece 1 is immersed in an insulative oil 73 such as illuminating kerosene in the machining bath 2. During an initial stage of machining of the workpiece 1 by an electrical discharge between the wire 51 and the workpiece 1, chips 74 produced from the machined workpiece 1 can easily find their way into the oil 73. When the wire 54 is moved downwardly as the machining process proceeds, chips 74 are discharged through the recess 72 serving as a chip discharge passage into the oil 73. As a result, there is no danger of short-circuiting between the wire 51 and the workpiece 1 or of a secondary electrical discharge produced between the wire 51 and the chips 74 or the workpiece 1 and the chips 74, and the speed at which the workpiece 1 is machined can be increased. According to the embodiment of FIG. 20, a machining electrode core wire 51 has a chip discharge passage in the form of a helical groove 75 for allowing chips 74 from the workpiece 1 to be discharged along the helical groove 75 into the oil 73. FIGS. 21A, 213, and 21C show a support or bearing 77 according to a still further embodiment of the invention for supporting the machining electrode 4 through a pivot bearing construction. The bearing 77 has upper and lower V-grooved or notched members 77a each accommodating two protruding members 76 having semispherical distal ends projecting on surfaces defining the V-shaped groove. The machining electrode 4 is therefore supported by the four protruding members 76. Each V-shaped groove should preferably have an opening angle a in the range of from 300 to 1500. As shown in FIG. 21C, the protruding member 76 has a semispherical member 78 and a cover body 79 in which the semispherical member 78 is partly embedded. The cover body 79 is threaded or fitted in a hole defined in the V-grooved member 77a. The semispherical member 78 is made of ruby in the illustrated embodiment, but may also be made of a highly wear-resistant material. The member 78 may be flat at the distal end. A mode of operation of the electrical discharge machining apparatus in which a number of holes are formed in a workpiece will be described with reference to FISS. 22A, 22B, 23A, and 23B. FIG. 22A illustrates a workpiece 1 with a number of holes formed therein along a straight line. For forming a number of such holes K along a straight line in a workpiece, it should generally be questioned how parallel are a reference surface A and a line B connecting the centers of the holes K. Stated otherwise, the angle e (FIG. 22B) formed between the reference surface A and the line B should be as small as possible. Such desired parallelism between the surface A and the line B can be achieved in the following manner: The machining electrode position H and the microscope position I (FIG. 1) are first brought into conjugate relationship, and then the center of a desired position B1 on the workpiece which is to be perforated is aligned with the crossing point of cross hairs 81 in a field of view 80 of the microscope, as shown in FIG. 23A. In such a condition, the slide 17 (FIG. 1) is moved in the Y direction by means of the feed screw 18 to observe desired positions B2 through BN succssively with the microscope 12. If the straight line along which holes are to be formed in the workpiece 1 is not parallel to the Y direction in which the slide 17 is fed, or is inclined to the Y direction through an angle of , then the desired machining positions B2 through BN will be seen progressively deviated from the crossing point of the cross hairs 81 of the microscope as the slide 17 is moved in the Y direction. To cope with this, the rotatable slide 26 (FIG. 11) is angularly moved through e by means of the adjustment screw 27 until the straight line on the workpiece 1 coincides with the Y direction so that the desired machining positions B2 through BN will not be progressively deviated from the crossing point of the cross hairs 81 when the positions B2 through BN are successively observed with the microscope 12. After the above adjustment has been performed, the slide table 14 is moved toward the machining electrode 4, and the slide 17 is moved progressively in increments in the Y direction by means of the feed screw 18 while holes are formed in the workpiece 1 successively at the desired positions B1 through BN. The formed holes are accurately aligned along a straight line without suffering from any unwanted angular error with respect to the Y direction or the reference surface. The positioning mechanism in the EDM apparatus according to the present invention can be incorporated not only in electrical discharge machining for forming minute holes, but also in microdrilling, laser machining, and other types of machining employing the machining head for forming small holes in workpieces With the arrangement of the present invention, minute holes can be formed at a high speed in a workpiece at positions accurately determined in a non-contact fashion without any parallax. When the machining electrode is rotated, the electrode core wire therein is subjected to no appreciable displacement relative to the mandrel or rotatable support body, whereby minute holes of small out-of-roundness, high shape accuracy, and surface roughness of 1 micron or smaller can be formed in the workpiece. As the electrode core wire is consumed, it can be fed out from within the rotatable support body. Since a commercially available thin core wire can be used as the electrode core wire, a machining electrode of a long service life can easily be manufactured. Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims: CLAIMS: 1. An electrical discharge machining apparatus for forming a minute hole in a workpiece, comprising a machining head having a machining electrode for forming the minute hole in the workpiece by an electrical discharge between the machining electrode and the workpiece, machining electrode moving means for moving the machining electrode toward and away from the workpiece, positioning means for establishing a machining position with respect to the workpiece, and workpiece moving means for moving the workpiece between a first position below the machining electrode and a second position below the positioning means. 2. An electrical discharge machining apparatus for forming a minute hole in a workpiece, comprising a machining electrode for forming the minute hole in the workpiece by an electrical discharge between the machining electrode and the workpiece, support means having V-shaped grooves for supporting said machining electrode therein, rotating means for rotating said machining electrode about its own axis, machining electrode moving means for moving the machining electrode in a direction toward and away from the workpiece, workpiece moving means for holding and moving the workpiece bewteen a first position directly below said machining electrode and a second position spaced from said first position in a plane to which said direction of movement of said machining electrode extend perpendicularly, first fixing means for fixing said workpiece moving means in said first position, second fixing means for fixing said workpiece moving means in said second position, a microscope disposed upwardly of said second position and movable in said plane, said microscope having a sight for observing the workpiece from above, and positioning means for aligning a position to be machined on the worpiece with said sight of said microscope. 3. An electrical discharge machining apparatus according to claim 2, wherein said support means comprises a pair of upper and lower members having said V-shaped grooves, respectively, said machining electrode being supported in said V-shaped grooves against said upper and lower members. 4. An electrical discharge machining apparatus according to claim 2, wherein said support means includes protruding members pro3ecting on surfaces defining said V-shaped grooves. 5. An electrical discharge machining apparatus according to claim 1 or 2, wherein said machining electrode moving means includes a lever having an end against which an upper end of said machining electrode is held, a motor, a feed screw axially movable in response to rotation of said motor and held against an opposite end of said lever, said lever having a pivotally supported intermediate portion. 6. An electrical discharge machining apparatus according to claim 2, wherein said rotating means comprises a first pulley mounted on said machining electrode, a motor, a second pulley rotatable by said motor, and a belt trained around said first and second pulleys, said second pulley being disposed upwardly of said first pulley. 7. An electrical discharge machining apparatus according to claim 1, wherein said machining electrode moving means is incorporated in said machining head, said machining head being movable toward and away from the workpiece. 8. An electrical discharge machining apparatus according to claim 2, wherein said machining electrode and said support means are electrically insulated from said rotating means and said machining electrode moving means. 9. An electrical discharge machining apparatus according to claim 1 or 2, wherein said workpiece moving means comprises a machining bath for placing the workpiece therein and a table on which said machining bath is mounted, said machining bath being slidable and angularly movable on said table. 10. An electrical discharge machining apparatus according to claim 2, wherein said first and second fixing means comprises stoppers, respectively, said workpiece moving means being spring-biased into abutment against one of said stoppers and magnetically attractable against the other stopper. 11. An electrical discharge machining apparatus according to claim 1 or 2, wherein said machining electrode comprises an electrode core wire, a support tube having a bore in which said electrode core wire is slidably supported, said support tube having an outer peripheral surface substantially coaxial with said bore, a rotatable support body connected to said support tube, and means providing electric conduction between said electrode core wire and said rotatable support body. 12. An electrical discharge machining apparatus according to claim 11, wherein said machining electrode also includes a hollow guide sleeve in which said support tube and said rotatable support by are closely fitted and retained. 13. An electrical discharge machining apparatus according to claim 11, wherein said rotatable support body has a bore in which said support tube is supported and an outer peripheral surface substantially coaxial with said bore in siad rotatable support body. 14. An electrical discharge machining apparatus according to claim 11, wherein said means providing electric conduction comprises an electrically conductive pipe mounted on an end of said electrode core wire in said rotatable support body and a setscrew by which said electrically conductive pipe is held against and secured to said rotatable support body. 15. An electrical discharge machining apparatus according to claim 13, wherein said support tube is fitted in said rotatable support body such that a direction in which there is any deviation between centers of the bore and outer peripheral surface of said support tube coincides with a direction in which there is any deviation between centers of the bore and outer peripheral surface of said rotatable support body. 16. An electrical discharge machining apparatus according to claim 11, 12, 13, or 15, wherein said support tube comprises a ceramic tube. 17. An electrical discharge machining apparatus according to claim 11, 12, 13, 15, or 16, wherein said support tube is formed from a kneaded mass by vacuum extrusion molding. 18. An electrical discharge machining apparatus according to claim 12, wherein said support tube and said rotatable support body are integrally formed of the same material. 19. An electrical discharge machining apparatus according to claim 12, wherein said support tube has three radial screws at substantially 120 0-spaced intervals, said three radial screws having tip ends positionable in contact with the outer peripheral surface of said support tube. 20. An electrical discharge machining apparatus according to claim 11, 14, or 15, wherein said electrode core wire has a passage for discharging chips produced from the workpiece by the electrode core wire. 21. An electrical discharge machining apparatus according to claim 20, wherein said passage comprises a recess extending from a tip end of said electrode core wire toward said rotatable support body. 22. An electrical discharge machining apparatus according to claim 20, wherein said passage comprises a groove extending helically from a tip end of said electrode core wire toward said rotatable support body. 23. An electrical discharge machining apparatus according to claim 13, wherein said support tube has three radial screws at substantially 120 0-spaced intervals, said three radial screws having tip ends positionable in contact with the outer peripheral surface of said support tube. 24. An electrical discharge machining apparatus according to claim 5, including a motor control circuit for rotating said motor at a certain speed to feed said feed screw at a constant speed to move said machining electrode toward the workpiece while an electrial discharge is being produced between said machining electrode and said workpiece, and for reversing said motor at a higher speed when said machining electrode and said workpiece are short-circuited. 25. An electrical discharge machining apparatus for forming a minute hole in a workpiece, comprising a base, a machining head mounted on said base and having machining means for forming the minute hole in the workpiece, a microscope mounted on said base for observing the workpiece in a direction perpendicular thereto, and a table assembly movably mounted on said base for moving the workpiece in at least first and second directions parallel to said base and perpendicular to each other, said machining head and said microscope being juxtaposed in said first direction in confronting relation to said table assembly, said microscope being movable in said first and second directions with respect to said base, said table assembly comprising a first table for supporting the workpiece, a second table supporting said first table, and a third table mounted on said base and supporting said second table, said first table having at least one of means for moving the first table in said first direction and means for angularly moving the first table through a small angular interval, one of said second and third tables being slidable in said first direction and the other in said second direction, said table assembly having first holder means for holding said one of the second and third tables in a first position in which said machining head confronts the workpiece, and second holder means for holding said one of the second and third tables in a second position in which said machining head confronts said micrscope. 26. An electrical discharge machining electrode for forming a minute hole in a workpiece, comprising an electrode core wire, a support tube having a bore in which siad electrode core wire is slidably supported, said support tube having an outer peripheral surface substantially coaxial with said bore, a hollow guide sleeve in which said support tube is supported, and a rotatable support body fitted and held in said guide sleeve, said electrode core wire and said rotatable support body being in electrically conductive relation to each other. 27. An electrical discharge machining electrode for forming a minute hole in a workpiece, comprising an electrode core wire, a support tube having a bore in which siad electrode core wire is slidably supported, said support tube having an outer peripheral surface substantially coaxial with said bore, and a rotatable support body having a bore in which said support tube is fitted and supported, said rotatable body having an outer peripheral surface substantially coaxial with said lastmentioned bore.

Inventors: ARAKI, NOBUHIRO, KAWATA, KOICHI, MATSUSHITA, HISATO, SATO, TAKEO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1984
Main classification: B23H7
IPC codes: B23Q17, B23H7, B23H9