WIRE GUIDE DEVICE FOR ELECTRICAL-DISCHARGE MACHINING, AND DISCHARGE-MACHINING APPARATUS INCLUDING SUCH A DEVICE

Abstract: A guide device (5, 6) for the wire electrode (1) in a discharge machining apparatus is disposed just above or below a workpiece (3). The device may include three or more equally circumferentially spaced guide rings (8, 9, 10) surrounding the electrode (1) and biased outwardly by ten­ sion springs (11, 12, 13) coupled to an enclosing frame member (7) to thereby center the electrode (1) and prevent lateral displacements during use. As an alternative the rings may be replaced by guide balls biased radially in­ wardly against the electrode (1) by compression springs or fluid cylinders. When two guide devices are provided in the apparatus, they may be substantially identical.

Description: This invention relates to a guide device for a wire electrode in discharge machining apparatus, comprising: a centrally apertured frame member for surrounding the electrode; at least three equally circumferentially spaced guide members disposed within the frame member and in individual contact with the electrode at three corresponding equally circumferentially spaced points; and means supported by the frame member and individually coupled to the guide members to cause the guide members for applying equal, radially inwardly directed forces to the electrode at said spaced guide member contact points to thereby resist any lateral displacements of the electrode and maintain the electrode in a centered position to implement accurate machining. Such a device is known from JP-A-56-56346. The invention also relates to apparatus for machining a workpiece by electro-erosion using a wire guided by such a guide device. In such an apparatus the wire electrode is fed at a predetermined longitudinal velocity to compensate for the wear of the electrode, while being transversely displaced relative to the workpiece. The displacement between the wire electrode and the workpiece is generally effected automatically by a servo system under the control of programmed instructions defining a predetermined machining path. It is known to hold an electrode in contact with guide surfaces by mechanical means as described in German Offenlegungsschrift No. 2,207,942 and British Patent No. 1,574,048. WO-A-84/02485 describes an essentially rigid guide device employing ball elements. Fig. 1 of the accompanying drawings is a sectional view of such a conventional guide arrangement for a wire electrode discharge machining apparatus, comprising a wire electrode 1, individual guides 2 disposed above and below a workpiece 3, and a pair of contacts 4 for supplying electrical machining power from a power supply (not shown) to the wire electrode 1. In such an apparatus the wire electrode 1 is fed through the machining zone while being supplied with power through the pair of contacts 4, whereby the workpiece 3 is machined in a controlled manner by electro-erosion. The servo system for effecting the relative displacement between the wire electrode and the workpiece is not shown. The pair of contacts 4 engage the wire electrode 1 at points eccentric to a center line linking the guides 2, as shown in Fig. 1. Since the eccentric contact points are both on the same side of the wire electrode, and the inner diameter of the guides is greater by about 2-5 mm than the diameter of the wire electrode, a gap D is formed between the wire electrode and each guide as shown in Fig. 2. During the machining of the workpiece the wire electrode is acted upon by external forces, such as those produced by a machining fluid or discharge phenomena, which tend to displace the electrode within the range of the gap D between the electrode and the guides. However, since the eccentric force exerted by the contacts 4 is greater than these external forces the electrode movement is constrained in the X direction in Fig. 2. Significant displacements occur in the Y direction, however, which reduces the precision and accuracy of the machining operation. Also, since the inner diameter of the guides 2 is about 2-5 mm greater than the diameter of the wire electrode 1, it becomes necessary to replace the guides whenever the wire electrode diameter is changed. An object of the invention is to provide a guide device for discharge machining apparatus which can guide a wire electrode more positively and precisely. According to one aspect of the invention, the guide device initially defined is characterized in that: the guide members are ball elements individually disposed in guideways for constraining them to radial movement relative to the electrode; and the force applying means comprises fluid cylinders individually coupled between the ball elements and the frame member. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a sectional elevation of a conventional guide arrangement for a wire electrode discharge machining apparatus; Fig. 2 is a schematic cross-section taken on line II-II in Fig. 1; Fig. 3 is a sectional elevation of a guide device arrangement; Fig. 4 is a schematic cross-section of one of the guide devices of Fig. 3; Figs. 5 and 6 are similar schematic cross-sections of alternate guide device arrangements; and Fig. 7 is a cross-section of a guide device embodiment in accordance with the present invention. Referring to the devices as shown in Figs. 3 and 4, a pair of identically configured guide devices 5, 6 engage the wire electrode 1 just above and below the workpiece 3. Each such device comprises a centrally apertured frame member 7 surrounding the electrode and three ring like guide members 8, 9, 10 are arranged in a vertical stack and individually biased towards the frame member by respective tension springs 11, 12, 13. The frame members 7 are fixed to the body of the discharge machining apparatus (not shown) for movement therewith relative to the workpiece in a servo controlled manner. The power supply contacts are not shown, but may advantageously take the form of pinch rollers or the like to avoid the application of any eccentric forces to the wire electrode. The guide member springs are sufficiently strong to override any external forces applied to the electrode during use. The guide members and their respective springs are equally circumferentially spaced at angles of 120°C and the guide members 8, 9 and 10 thus contact the wire electrode 1 at similarly spaced outer radial points A, B and C as shown in Fig. 4. With such an arrangement the wire electrode is positively and continuously retained in a centered position without the formation of any gaps to enable lateral displacements of the electrode, thereby greatly enhancing the accuracy of the machining operation. Moreover, the inherent resiliency of the biasing springs easily accommodates changes in the diameter of the wire electrode without necessitating the replacement of the individual guide members. Although two guide devices 5, 6 are preferably employed in the manner illustrated in Figs. 3 and 4, it would also be possible to use just one such guide device - albeit at the expense of reducing the advantageous effects by a factor of two. Furthermore, although three guide members and associated springs are shown in Figs. 3 and 4, a greater number of such components could also be used. The arrangement of Fig. 5, for example, employs four ring like guide members 14, 15, 16 and 17 outwardly biased by springs 18, 19, 20 and 21 disposed at equal circumferential spacings of 90°C. In the device of Figs. 3 - 5, the guide member rings are outwardly biased by tension springs coupled to the frame members. Fig. 6 shows an alternate device wherein the guide members take the form of resilient ball elements 22, 23, 24 of rubber or the like and instead of being outwardly biased, they are urged inwardly towards the wire electrode by compression springs 25, 26, 27 mounted to the frame member. Reference numeral 28 designates tubular guideways for constraining the ball elements to move in only a radial direction relative to the wire electrode. An embodiment of the present invention is illustrated in Fig. 7 which differs from that of Fig. 6 by the replacement of the compression springs with fluid cylinders 29, 30 and 31 for resiliently urging the ball elements against the wire electrode. Such fluid cylinders could be hydraulic, in which case the necessary resiliency would be provided by the rubber ball elements alone, or pneumatic to provide additional resiliency. As a further alternative the individual biasing springs or fluid cylinders could be replaced by a single biasing member, for example a rubber or elastic band(s) surrounding the ball elements in Figs. 6 or 7. Similarly, the separate tension springs in the Figs. 3-5 devices could be replaced by a single expansion ring surrounding the guide members and coupled to each one for urging them outwardly. Thus, the above-mentioned drawbacks are avoided by providing a guide device for a wire electrode discharge machining apparatus which prevents the movement of the wire electrode in the gap between the electrode and its guide, thereby improving the machining precision. At least one of the upper and lower guide devices has at least three guide members which are in direct contact with the wire electrode at three equally circumferentially spaced radial points thereon. The resilient biasing of the guide members ensures full and constant contact with the wire electrode without any gaps to thereby preclude any lateral displacements of the electrode during machining operations.

Claims: A guide device for guiding a wire electrode in discharge machining apparatus, comprising: a) a centrally apertured frame member (7) for surrounding the electrode (1); b) at least three equally circumferentially spaced guide members (8,9,10; 14,15,16,17; 23,24) disposed within the frame member (7) and in individual contact with the electrode (1) at three corresponding equally circumferentially spaced points; and c) means (11,12,13; 18,19,20,21; 25,26,27; 29,30,31) supported by the frame member (7) and individually coupled to the guide members to cause the guide members for applying equal, radially inwardly directed forces to the electrode (1) at said spaced guide member contact points to thereby resist any lateral displacements of the electrode and maintain the electrode in a centered position to implement accurate machining; characterized in that: d) the guide members are ball elements (22,23,24) individually disposed in guideways (28) for constraining them to radial movement relative to the electrode (1); and e) the force applying means comprises fluid cylinders (29,30,31) individually coupled between the ball elements (22,23,24) and the frame member (7). A guide device according to claim 1, wherein the ball elements are elastomeric. A device according to claim 2, wherein the fluid cylinders are hydraulic cylinders. A device according to claim 2, wherein the fluid cylinders are pneumatic cylinders. A discharge machining apparatus including a circular cross-section wire electrode (1) adapted for relative movement with respect to a workpiece (3), contact means (4) engaging the electrode for supplying electrical power thereto, and a pair of upper and lower guide devices (5,6) individually disposed above and below the workpiece (3) in contact with the electrode (1), at least one of the guide devices being in accordance with any preceding claim. An apparatus according to claim 5, wherein the lower and upper guide devices are identically configured.

Inventors: NAKAYAMA, YOSHIRO C/O MITSUBISI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1985
Main classification: B23H7
IPC codes: B23H7