GB2116103A - A method and apparatus for electroerosively machining a workpiece - Google Patents

Abstract

A multi-strand wire electroerosion machining method and apparatus are disclosed wherein a plurality of wires (1, 2, 3) dispensed from respective wire sources (5, 6, 7) are fed separately into a wire travel path (11) and are assembled via wire guides (8, 9, 10, 16, 17) into a bundle (4) thereof. The bundle (4) of the wires traversing a fluid-flushed cutting zone (12) defined between machining electrode guides (13, 14/113, 114) constitutes a single travelling electrode for electroerosively machining a workpiece (15). The wires (1, 2, 3) from the cutting zone (12) are taken up into a collection site (22) while they are held sufficiently taut between the electrode guides. The workpieces (15) is displaced relative to the bundle (4) of travelling wires transversely to the axis thereof to machine a desired contour in the workpiece. Preferably, the wires (1, 2, 3) in the bundle (4) are twisted. Solenoid units or rotary dies may be provided for shaping the wires in their travel path to provide the bundle (4) with a predetermined cross-sectional contour; one or more of the wires may travel in the opposite direction to the other(s). <IMAGE>

Description

SPECIFICATION A method and apparatus for electroerosively machining a workpiece The present invention relates to an improved travelling-wire eiectroerosive machining method and apparatus.

The term "electroerosive machining" or "electroerosion" is used herein to refer to a process of electrical machining in general, including electrical discharge machining (EDM) in which a workpiece material is removed by the action of successive electrical discharges, electrochemical machining (ECM) in which a workpiece is machined by electroly ticsolubilization or removal ofthe material and electrochemical-discharge machining (ECDM) in which material removal is effected by a combination of the actions of electrical discharges and electrolytic metal solubilization, or rnmoval.

In atravelling-wire electroerosion process, a con tinuous electrode wire is axially transported by a wire axial drive means from a supply means to a takeup means. In the path of wire travel, a pair of machining guide members are disposed at opposite sides of an electrically conductive workpiece to define a straight line path therebetween through which the electrode wire axially is passed while traversing the workpiece, thus positioning the electrode wire in a precise machining relationship with the workpiece. Tension means is provided to hold taut the traveling electrode wire across the supply and take up sides and between the positioning guide means.An electrical machining current, typically or preferably in the form of a succession of time-spaced electrical pulses, is applied between the traveling electrode wire and the workpiece across a machining gap flooded with a machining liquid, e.g. a water liquid of a dielectric nature of low conductivity, or an aqueous solution of electrolyte, to electroerosively remove material from the workpiece. As the material removal proceeds, the workpiece is displacedtransverselytothe longitudinal axis orthe straight line path of the traveling wire electrode along a prescribed two-dimensional machining feed path underthe command, advantageously, of a numerical controller, so that a desired contour of machining is generated in the workpiece.

It is importantthatthe wire electrode be of good electrical conductivity and composed so as to afford a satisfactory rate of material removal and yet be subject two less electroerosive wear itself. It is desir able thatthe wire electrode be heat-resistant and retain sufficient tensile strength at a temperature created bythe passage of a machining current of high amperage or current density, to be free from breakage in operation. Customarily, the wire electrode is constituted as a single strand wire having a diameter of 0.05to 0.5 mm composed of a copper metal or alloy such as brass. Such a wire has been provided by drawing it through a die and usually has had a circular cross section.

The machining liquid is supplied, typically from one or more nozzles, into the machining gap to serve on the one hand as a gap machining medium to carry the discharge and/or electrolytic current and on the other hand as a coolant two dissipate heat developed by the passage of the machining current of high amperage orcurrent density. Higher amperage or current density is desirable to achieve greater removal rate and efficiency, and necessitates removal of the machining liquid in the gap at a higher rate.

It has, however, been experienced that the continued supply ofthe machining liquid n an ample amount towards the machining gap often causes wire breakage and does not allow the use of a greater machining current. When the electrode wire is excessively heated or insufficiently cooled, it tends to break. There is thus a severe limitation in the heat-dissipation ability of a conventional electrode wire traversing the machining gap.With a conventional electrode wire having a regular machining surface, it has also been observed that gases produced by discharges and/or electrolytic decomposition of the delivered machining liquid tend to be adherent to the electrode surfaceto separate the latterfrom the coolant liquid and thus to act as a thermal insulatortherebetween, and furtherto allow gaseous discharges essentially of a thermal nature to develop thereacross. Furthermore, a number of electrode wires of different thicknesses have had to be replaced by one after another depending upon particularconfigurationsto be machined in a workpiece.

It is, accordingly, an important object of the present invention to provide a travelling-wire electroerosion machining method and apparatus which facilitate delivery of the machining liquid onto the electrode surface and cooling of the latter, make it unnecessary to replace electrode wires to machine diverse machining configurations and allow the electrode to resist breakage.

Another important object ofthe present invention isto provide an electroerosion machining method and apparatus oftravelling-wire type which allow a machining current of greater amperageto be delivered through the machining gap, thereby affording greater machining efficiency than the prior art.

Other and specific objects of the invention will become apparent as the description thereof which follows proceeds.

In accordance with the present invention there is provided, in a first aspect thereof, an improved method of electroerosively machining an electrically conductive workpiece with a traveling elongate electrode in a cutting zone defined between a pair of machining electrode guides and flushed with a machining fluid, which method comprises the steps of: a) continuously dispensing a plurality of electrically conductive wires from respective wire sources in a wire supply site to feed the wires separately into a wire travel path towards the cutting zone; b) guiding the separate traveling wires in the path to bring them together into an essentially mutual parallel contact at least between the machining electrode guides and therebyforming a bundle thereoftraversing the cutting zone and consitutlng the traveling elongate electrode: cj passing an elecWroerosion machining current between the bund'e ofthe traveling wires and the workpiece to electroerosively remove material from the workpiece in the fiuid-fiushed cutting zone; d)continuouslytaking up the wires from the cutting zone into a collection site while holding the traveling wires taut between the machining electrode guides; e) displacing the workpiece relative to said wire bundle trans'jersely to a longitudinal axisthereofto machine a contour in the workpiece.

The wire bundle provides instep c) an electroero sive machining surface collectively formed by the individual wires, which acts against the workpiece.

Specifically, te individual wires may be of a regular circular cross-section and may be of a diameter or thickness rt;saging between 0.05 and 0.5 mm and preferably between 0.1 and 0.5 mm. All such wires may be of an equal thickness or diameter. Otherwise one or more of the wires may be of a thickness or diameter greaterthan that orthose of the reminder.

Thenumberandsizeorsizesofthewires used may be selected depending upon a particular crosssectional shape ofthe surface contour ofthe bundle which is collectively defined by the individual wires as weil as upon a particular cross-sectional area of the wire bundle desired. The number of wires for assembling should typically be 2 to 5. Advantageous ly,the wires in step b) be shaped to provide the wire bundle with a preselected surface contour.

The invention also provides, in a second aspect thereof, an apparatus for electroerosively machining an electrically conductive workpiece with a traveling elongate electrode in a cutting zoneflushedwith a machining fluid, which apparatus comprises: a pair of machining electrode guides for defining the cutting zonetherebetween; a plurality of wire sources in a wire supply site for dispensing wires respectively therefrom to feed them separately into a wire travel path towards the cutting zone; wire guide means for assembling the separate traveling wires in the path to bring them together into an essentially mutual parallel contacting relationship at least between the machining electrode guides wherebyto form a bundlethereoftraversing the cutting zone and constituting the traveling elongate electrode; a pow ersupplyforpassing an electroerosion machining current between the bundle oftraveling wires and the workpiece to electroerosively remove material from the workpiece in thefluid-flushed cutting zone wire drive means for continuously taking upthewires from the cutting zone into a collection site; tension meansforholdingthetraveling wires taut between the machining electrode guides; and machining feed means for displacing the workpiece relative to the wire bundletransverselyto a longitudinal axis thereof to machine a contou r in the workpiece.

These and otherfeatures ofthe present invention and advantagesthereofwill become more readily apparentfromthefollowing description made with reference to the accompanying drawing in which: FIG. lisa diagrammatic cross-sectional view, largely magnified, of a conventional, single-strand wire electrode electroerosively cutting a workpiece; FIG. 2 is a similar view illustrating a bundle offour similarwires electroerosively cutting a workpiece in an arrangement embodying the principles ofthe present invention; FIG. 3 is a side-eievational view, essentially in cross section diagrammaticily illustrating an apparatus according to the invention utilizing fixed electrode guide members;; FIG. 4 is a similar view diagrammatically illustrating another apparatus ofthe invention utilizing rotary electrode guide members.

FIGS. 5 and 6 are cross-sectional views ofthe upper, and lower machining electrodeguideswhich may be used in the arrangements of FIGS. 3 and 4, taken along V-V and VI-VI, respectively, in FIG. 3; and FIGS. 7(A) - (F) are cross sectional views of wires in bundle arranged therein in variousformatsfor electroerosively cutting a workpiece.

FIG. 8 is a cross-sectional view of another embodiment of machining electrode guide for use with the invention.

In FIG. 1,a conventional, single-strand wire electrode Eo is shown in a process of electroerosively cutting a workpiece Wand machining a contour M therein.Theworkpiece Wmay be assumed to be stationary, the wire electrode Eo, while axially traveling,then moving from the right-hand sideto the left-hand side as viewed in the FIGURE to continue material removal from the workpiece Wacross a narrow gap G uniformly maintained in front of the wire electrode Eo.The machining fluid must be consecutively delivered into the gap G and gases as well as other machining products must be consecu tivelydischargedtherefrom in orderforthegapto be held in an ideal condition.The gap G is of limited space, however, and theseflushing activities, as noted previously, tend to be unsatisfactory so that the wire Eo becomes excessively heated, limiting the currentwhich can be passed therethrough to reduce the risk of breakage.

As shown in FIG. 2, a plurality ofthinnerwires, shown by four, el, e2, e3 and e4, may, in accordance with the invention, be assembled together to form a bundle thereof Ewhich has a cross-sectional area defined bythe sum of those ofthe individual wires equal to that ofthe single-strand wire Eo shown in FIG. 1. It will be seen thatthe bundle Eprovides greater resistance to breakage because the tensile strength ofthe sum of individual wires el, e2, e3 and e4 is greaterthan the single-strand wire Eo which is equal in cross-sectionai area to the sum ofthe individual wires ofthe bundle E. Moreover, the cooling rate is markedly increased because for a given length the su rface a rea of the bundle E is greaterthan that of the single-strand wire Eo given the same cross-sectional area. In addition,the rugged peripheryofthe bundle E,which comprises grooves and ridges, offers greater spaceforfluid flow and hence assures thorough delivery ofthe machining fluid and facilitates removal ofthe gap products.

These improved flushing actions area especially enhanced when the wire bundle Eistwisted or driven, as will be described hereinafter, to reciprocate angularly or rotationally about its axis. In that case, the machined contour M being developed in the workpiece Wis, further advantageously, made to conform to the semi-cylindrical machining surface effectively created by the wires e1 - e4which axially move while turning about the axis of the bundle E. In other words, an effective increase in the gap spacing G is provided without effectively changing the machining electrode surface. The increased cooling action permits greater erosion current to be passed and hence results in enhancement in the rate of material removal.

FIG. 3 shows a traveling-wire electroerosion machining arrangement embodying the present invention. In the arrangement shown, three wires, 1, 2 and 3 are dispensed from respective spools 5,6 and 7 and fed separately via respective guide rollers 8,9 and 10 into wire travel path 11 towards a cutting zone 12 defined between a pair of machining electrode guides 13 and 14 and having aworkpiece 15therein In the path ofwire travel 11, wire assembling guide rollers 16 and 17 are provided to bring the separate wires 1, 2and 3togetherinto an essentially parallel- contacting relationship to form a bundle4thereof which traversesthe cutting zone 12 between the machining electrode guides 13 and 14.

In the wire travel path 11 there are provided, immediately beneath the lower machining electrode guide 14, a pair of wire guide rollers 18 and 19 and, downstream thereof, a further pair of wire guide rollers 20 and 21 to take up the wire bundle 4 into a collection site. The rollers 18 and 21 are constituted by pinch rollers pressing the wire bundle 4 againstthe rollers 19 and 20 which are constituted by capstans that apply a forward traction drive to the wires 1,2 and 3 stretched between the supply reels 5,6,7; and rollers 20,21. The wire bundle 4 going between the rollers 20 and 21 is shown as collected into a receptacle 22 accommodated in a column 23 of the machine.A braking traction drive is applied to the wires 1,2 and 3 by each of capstan and pinch roller units 24,25 and 26 arranged between the supply reel and the guide roller, and 8,6 and 9,7 and 10 respectively to hold the wire bundle 4 sufficiently taut between the machining electrode guides 13 and 14.

Accordingly, the individual separate wires 1,2 and 3 and hence the bundle 4thereof are advanced to travel axially at a given rate by the traction drives 18,19; 20, 21 against the breaking force produced by the units 24,25 and 26.

The reels 5,6 and 7 are shown as mounted at an upper portion of the column 23 of the machine and the braking units 24,25 and 26 of the guide rollers 8,9 and 10 are shown as mounted on a machine head 24 extending horizontally from the upper portion of the column 23. A housing 25 is attached to the head 24 as depending therefrom and includes thewire-assembling guide rollers 16and 17.The upper machining electrode guide 13 is provided at a lower end ofthe housing 25 whereas the lower machining electrode guide 14 is provided on an arm 26 extending horizontally from the lower portion of the column 23.

A nozzle 27 is disposed above the workpiece 15 and fed with the machining fluid e.g. a distilled water medium, from a reservoir 28 to direct it onto the bundle4ofwires 1,2 and3 entering intothecutting zone 12 in the workpiece 15. A machining power supply 29 has one output terminal electrically connected to the workpiece 15 and the other output terminal electrically connected to the electrically conductive guide roller 16 which constitutes one of the wire-assembling guide rollers 16,17. An electrical machining current is thus passed from the power supply 29 between the bundle4 of wires 1,2 and 3 and the workpiece i 5 through the machining fluid to electroerosively remove material from the workpiece 15 acrossthe machining gap G.

While the wires 1,2 and 3 are shown to be taken up altogether or in the bundle into the collection site, they may aftertraversing the lower machining guide 14 be separated from one another and separately taken up into respective collectors, e.g. reels. It should also be noted that all wires need not be advanced in the one direction. Thus one or two of them may be advanced from up to down as shown while the other may be advanced from down toup through the cutting zone 12. Then the bundle 4 is formed by wires traveling in opposite directions in the cutting zone 12 between the machining electrode guides 13 and 14.

The workpiece 15 is supported on a workstand which is, in turn, mounted on a worktable 31 comprising anX-axiscomponenttable32anda Y-axis component table 33 arranged in a cross-feed configuration. The tables 32 and 33 are driven by respective motors controlled with command signals from a NC (numerical control) unit to displace the workpiece 15 in a plane transverse to the axis of the bundle 4 of wires 1,2 and 3 along a prescribed cutting path which defines the contour of cut to be electroerosively machined in the workpiece 15.

Each of the machining electrode guides 13 and 14 are shown as formed with a guide opening 13a, 14a, which may be trinagular in cross section, for slidably guiding wires 1,2 and 3 in the bundle 4therethrough.

Preferably, the guides 13 and 14are arranged, as shown in FIGS. Sand 6, to twistthe wires 1,2 and 3 therebetween. This arrangement causes thewires 1, 2 and 3to be axially displaced while turning about the axis ofthe bundle 4 in the cutting zone 1 2to develop, juxtaposed with the workpiece wall 1 5a being machined, a moving electrode surface effec tively semi-cylindrical effective in cross section and to allowthewall 1 Sato conform to the semi-cylindrical electrode surface.

In the embodimentof FIG.4 in which the same reference numerals are used to designate the same parts as in FIG. 3, the machining guides 13 and 14are rotational. In this embodiment, thus, the guide blocks 13 and 14 are secureiy received in pulleys 41 and 42 which are rotationally supported through the lower plate 24a ofthe machine head 24 and through the upper plate 26a ofthe arm 26, respectively. Each of the pulleys 41 and 42 is connected via an endless belt 43,44 with a pulley 45,46 rotationally driven by a servo motor 47,48. It is apparent that the pulleys and endless belts may be replaced by gears and gear transmissions. A reciprocation signal may energize the motors 47 and 48 to rotationally reciprocate the electrode guides 13 and 14 synchronously but always out of phase or in opposite direction to continuously or periodicallytwistthe wires 1,2 and 3 in the bundle 4 in the cutting zone 12. The electrode surface is in effect a semi-cylinder and is thus here again provided bythe axially moving bundle 4 of wires 1, 2and 3 in the cutting zone 12.

A pair of movable electromagnetic solenoid units 50 and 51 diagrammatically shown disposed in the path of wire travel between the machining guides 13 and 14 may be employed and may operate to regulate the cross-sectional arrangement of wires 1,2 and 3 in the bundle 4 in the cutting zone 12 and hence the arrangement of wires 1,2 and 3 relative to the direction of cut in theworkpiece 15 in any desired format such as shown in FIG. 7(A)-- (F) in the course of a given intricate contour-machining operation. In this manner, the cross section ofthewire bundle is shaped readily in any desired format according to the particular configuration and geometry which are to variable appear in a given machining contour.

Obviously, the solenoid units are operable with a ferromagneticwire and may be replaced by mechanical die sets designed to be displaced buy a numerical controllerto alterthe shape of the die openings. To this end, it will be apparentthatthe basic structure of the rotary die sets 13 and 14 may be employed with three wires 1,2 and 3 as shown to be assembled. The die opening 13,14should betriangularin cross section and sized to slidably accept the wires as shown in FIG. 5 or 6. The servo motors 47 and 48 operate to variably establish an angular position of the guide openings 13a and 14a so that the wires 1,2 and 3 are arranged in any one or another of the formats as shown in FIGS. 7(A), 7(B) and 7(C).To enablethewires 1,2 and 3 to align in a row as shown in FIG. 7(C), 7(D) or7(F), a pair of further rotary rotary die sets 113,114 similarto die sets 13, 14may be provided each having a rectangular die opening 1 13a, 1 l4afor accepting the wires 1-3 in row as shown in FIG. 8 and the triangular die sets 13 and 14may be disassembled and retracted. A desired angle of orientation of the row of wires can then be established bythe corresponding angular positioning of thediesll3,li4through motors.

There is thus provided, in accordance with the invention, an improved electroerosive machining method as well as apparatus thereforwhich facilitates delivery ofthe machining fluid onto the electrodesurfaceand cooling thereof, hence permitting a machining currentofgreateramperageto be delivered through the electrode whereby to afford enhanced cutting efficiency. The unique traveling elongate electrode constituted by a bundle of wires is, further advantageously, greater in breaking strength for a given electrode cross section. In practice, the number in relation to the thickness of strand or componentwires may simply be selected to establish a desired electrode thickness and cross section. Still further advantageously, the invention offersthe shaping ofthe electrode in the cutting zoneto effectively alter its machining surface area so that cutting at each portion ofthe maching contour can be achieved with both a maximum precision and efficiency.

Claims (20)

1. A methodfo electroerosively machining an electrically conductive workpiece with a traveling elongate electrode in a cutting zone defined between a pair of machining electrode guides and flushed with a machining fluid, the method comprising the steps of: a) dispensing a plurality of electrically conductive wires from respective wire sources in a supply site to feed the wires separately into a wire travel path toward rd said cutting zone; b) assembling said separate traveling wires in said path to bring them together into a bundle thereof traversing said cutting zone, said bundle of wires consituting said traveling elongate electrode;; c) passing an electroerosion machining current between said bundleofthetraveling wires and said workpiece across a machining gap to electroerosively remove material from the workpiece in said fluidflushed cutting zone; d) taking up said wires from said cutting zone into a collection site while holding the traveling wires taut between said machining electrode guides; e) displacing said workpiece relative to said wire bundletransverselyto a longitudinal axisthereofto machine a contour in the workpiece.

2. A method according to claim 1 wherein said wires have a thickness ofthicknesses ranging between 0.05 and 0.5 mm.

3. A method according to claim 1 wherein said wires have a thickness orthicknesses ranging between 0.1 and 0.5 mm.

4. A method according to claim 1 2 or3,wherein said wires are essentially circular in cross section.

5. A method claim 1,2,3 or4whereinthe number said wires is 21to 6.

6. A method according to claim 1,2,3,4 or 5, further comprising the step of shaping the wires in said step (b) of assembling said wires of provide said wire bundle with a predetermined cross-sectional contour spaced from the wall of said workpiece across said machining gap.

7. A method according to claim 6wherein said wires are shaped by changing the arrangement thereof in said bundle with respecttothedirectionof said displacement of said workpiece relative to the longitudinal axis of said bundle.

8. A method according to any one of the preceding claims,furthercomprising the step oftwisting said wires in said bundle at least between said machining electrode guides.

9. A method according to claim 8 wherein said wires are twisted cyclically.

10. Apparatus for electroerosively machining an electrically conductive workpiece with a traveling elongate electrode in a cutting zone flushed with a machining fluid, said apparatus comprising: a pair of machining electrode guidesfordefining said cutting zonetherebetween; a plurality of wire sources in a wire supply site for dispensing wires respectively therefrom to feed them separately into a wire travel path towards said cutting zone; wire guide means for assembling said separate traveling wires in said path to bring them together into a bundle thereoffortraversing said cutting zone, to thereby constitute said traveling elongate electrode;; a powersupplyfor passing electroerosion machining current between said bundle oftraveling wires and said workpiece across a machining gap to electroerosively remove material from theworkpiece in said fluid-flushed cutting zone; wire drive meansfortaking up said wires from said cutting zone into a collection site; tension means for holding said traveling wires taut between said machining electrode guides; and machining feed means for displacing said workpiece relative to said wire bundle transversely to a longitudinal axisthereofto machine a contour in said workpiece.

11. Apparatus according to claim 10 wherein said wire guide means comprises a pair of rollers arranged to be rotatable while pressing said wires therebetween.

12. Apparatus according to claim 10 or 1 1,further comprising means fortwisting said wires in said bundle at least between said machining electrode guides.

13. Apparatus according to claim 12, wherein said twisting means includes respective guide openings in said machining electrode guides, equally shaped to allow said wires in said bundle to be slidably passed therethrough but arranged with different orientations relative to one other.

14. Apparatus according to claim 12 or 13, where- in said twisting means includes at least one rotary guide member constituting at least one of said machining electrode guides and adapted to be reciprocated rotationally about an axis of said bundle.

15. Apparatus according to claim 14, including two such rotary guide members constituting said machining electrode guides and adapted to be rotatable in mutually opposite directions in each cycle of said rotational reciprocation.

16. Apparatus according to any one of claims 10 to 15, further comprising means in said wire travel path for shaping said wires to provide said bundle with a predetermined cross-sectional contour spaced from the wall of said workpiece across said machining gap.

17. Apparatus according to claim 16 wherein said shaping means includes means for changing the arrangement of said wires in said bundle with respect to the direction ofthedisplacementofsaidworkpiece relative to said axis ofthe bundle effected by said machining feed means.

18. A method of electroerosively machining an electrically conductive workpiece substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings.

19. Apparatus for electroerosively machining an electrically conductive workpiece substantially as hereinbefore described with reference to and as shown in Figures 2 to 8 ofthe accompanying drawings.

20. Aworkpiece machined buy a method as claimed in any one of claims 1 to 9 and 18 or by apparatus as claimed in any one of claims 10 to 17 and 19.