GB2220162A

GB2220162A - Electrical discharge machining apparatus

Info

Publication number: GB2220162A
Application number: GB8914979A
Authority: GB
Inventors: Vries Hendrik Lute De
Original assignee: CAMEL SPARK Ltd
Current assignee: CAMEL SPARK Ltd
Priority date: 1988-06-29
Filing date: 1989-06-29
Publication date: 1990-01-04
Anticipated expiration: 2009-06-29
Also published as: GB8914979D0; GB2220162B; GB8815485D0

Abstract

The apparatus, for machining e.g. a press tool (10, Fig. 1) for forming, with a complementary tool, an ashtray, has a number of electrodes 16 - 38 to which power is supplied for machining. The current supplied to each electrode is controlled independently of the other electrodes and the electrodes are movable independently through preselected paths to effect machining. This enables in situ machining of complicated shapes to be effected accurately and relatively easily and cost effectively. Electrodes 16- 20 erode the shape of the bowl (42) of the ashtray, electrodes 24 - 30 the rim (44) and electrodes 32 - 38 the depressions (46, Fig. 1). <IMAGE>

Description

Title: Electro-Discharge Machininq This invention relates to the electro-discharging machining (spark erosion) of an electrically conductive material.

Dies inking by a spark erosion machine involves customarily an electrode which is profiled to the shape of the cavity to be achieved. The electrode may be a machined piece of metal, or have a profile that has been deposited on a substrate by plating or, as is generally the case, consist of machined graphite. Graphite is often employed because of its machinability and low tool wear during the erosion process.

Apart from the investment in time, machines and materials for the profiling of the electrode for the majority of spark erosion tasks, the utilisation of such an electrode during the erosion process is limited to the time it takes to create a crater in a workpiece together with a complementary period of shutdown of power (pause) to remove erosion products and to allow dielectric fluid used in the process to reconstitute itself. Thus only a minute area is eroded at any one time and at a load factor well below unity whilst the remainder of the surface remains unchanged.

Furthermore, in order to remove the products of erosion the dielectric fluid is admitted usually through one or more channels in the electrode for distribution throughout the gap between the electrode and the workpiece. Because of the narrow dimensions of the gap a high resistance is offered to the flow of fluid in the gap and therefore the dielectric fluid has to be pumped into the gap at relatively high pressure. Dynamic forces due to the formation and collapse of gas bubbles in an incompressible medium add to the fluid pressure in the gap required for flushing.In order to minimise variations of the size of the gap due to the fluctuation of fluid pressure in the gap (which variations may be significant relative to the nominal size of the gap) single electrode diesinking spark erosion machines customarily require heavy section C frames for securing the position of the electrode relative to the workpiece.

The present invention seeks to provide an improved method and apparatus for electrode discharge machining.

Accordingly, the present invention provides an electrical discharge machining apparatus comprising a plurality of electrodes, means for supporting said electrodes and control means for controlling the supply of electric currents to said electrodes independently of one another.

The present invention also provides a method of machining a workpiece by spark erosion in an electrical discharge machining apparatus having a plurality of electrodes, comprising supplying power to said electrodes independently of one another to effect machining.

Each of the electrodes is conveniently controlled independently as regards the initiation, sustaining and termination of its eroding current. The resulting effect is that as many separate eroding currents are generated as there are electrodes with the operating periods and non-operating periods of the electrodes occurring simultaneously or overlapping in time.

Another effect of the present invention is that the projected area presented by an electrode to the material to be eroded may be smaller than the projected area of the cavity to be formed less any lateral gaps or may be as small as the cutting edge of a knife or as the point of a pin. This eliminates substantive pressures between an electrode and the material to be eroded and enables an assembly of electrodes and their actuators to be made of a lightweight, portable construction as well as obviating a requirement to pressurise the dielectric fluid to ensure flushing in the gap between the electrode and the workpiece.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings in which: Figure 1 is an example of a cavity which has been eroded into a workpiece in the form of a female press tool; Figure 2 is a diagrammatic representation in plan view of one possible arrangement of and the relative movements in the horizontal plane of twelve electrodes for use in the erosion of the tool of Figure 1; and Figure 3 is a perspective view of one example of an electrode in which the portion of the electrode which conducts current to the workpiece is knife edged.

Referring to the drawings, Figure 1 shows a block 10 of an electrically conductive material such as metal in the form of a female pressing tool in which a cavity 12 has been eroded. The bottom of the cavity is the shape 14 to be imparted by pressing to a flat disc of metal by a co-operating and correspondingly profiled male pressing tool (not shown in the drawings). In Figure 1, the tool is for pressing an ash tray.

In Figure 2, electrodes 16-22 serve to erode the shape of the bowl 42 of the ash tray, electrodes 24-30 serve to erode the shape of the rim 44 of the ash tray and electrodes 32-38 serve to erode the shape of the semi-circular depressions 46 in the rim of the ash tray.

In order to effect the erosion the the electrodes are sunk individually or in one or more groups into the workpiece as well as moved individually or in one or more groups in horizontal planes (linearly or rotationally) in the directions of the arrows in Figure 2. The physical travels in any one plane or direction may be influenced by a requirement to maintain dimensions of the spark gaps within desired upper and/or lower limits and by a requirement to prevent electrodes from making electrical contact with each other if the possibility of this happening could arise. For example, in the horizontal plane the electrodes 16-22 are moved in an arcuate path, as are electrodes 24-30. In the horizontal plane electrodes 32-38 are moved along a linear path. Each electrode can also be rotated about its axis during machining.

Each electrode is operated by a control system electrically independently of each other electrode for the initiation, maintenance and termination of an eroding current regardless of any other electrode, although all of the electrodes may share a common power supply.

Figure 3 shows an example of an electrode 48 with a mounting lug 50 and a machining edge 52. The machining edge is preferably a knife edge which ensures that the forces to be absorbed by the mounting lug 50, by way of which the electrode is secured to a supporting head to control its rotary and/or horizontal and vertical movements, are negligible. This allows a lightweight construction of the supporting head and its drive mechanism. In addition, the use of a knife edge enables excellent heat transfer to be achieved to a moving supply of dielectric fluid moving between the electrode and the workpiece which reduces the rate of wear of the electrode and minimises metallurgical changes in the outer layers of the material of the workpiece.

The flow of dielectric fluid may be put through one or more tubes which may also be the carrier(s) of an electrode into which an electrode is clipped or affixed in some manner.

As will be appreciated, other shapes of electrode may be used and the edge 52 may, for example, be in the form of a stylus for creating writing or an artistic impression or specific details in the workpiece.

In the illustrated example, each electrode is designed to erode a specific section of the workpiece and the required horizontal and vertical movements of each electrode may be programmed into a microprocessor or computer in the form of mathmetical equations or of algorithms or, for example, to be stored as a three dimensional array of cells in which certain cells would control the formation of the cavity to be eroded, thus eliminating the requirement to profile an electrode in more than one dimension.

In other examples each electrode may be independently moved in one or more oblique plane(s) and/or curved path(s) in any direction from the vertical including upwardly.

Electrodes may also be physically arranged to erode, simultaneously, more than one surface or volume, such as for example the combustion chambers in the head of a multi-cylinder internal combustion engine. This may be effected by means of a single assembly of electrodes and electrode actuators or drives, the overall movement of the electrodes being controlled by a single control unit with ganged or individual control of each electrode and powered by a single power supply with ganged or individual control of each electrode.

Since the electrodes can be of lightweight construction, as is mentioned above, an assembly of electrodes may be taken to a workpiece and be positioned onto it or relative to it or affixed to it for subsequent erosion of surfaces and/or volumes to make it possible for example to upgrade and/or machine large castings in situ or to carry out in situ machining operations on large or heavy structures such as legs of an oil production platform which cannot easily be moved. In the latter case the provision of for example a suitable sealing box would allow machining to be carried out underwater.

The present invention enables, for example, simultaneous employment of a plurality of electrodes to profile by straight line generation a blade for a steam or a gas turbine where twisting of the blade through an uneven application of heat may be prevented by a suitable organisation of sparking sequences. Having finished the erosion process to fine tolerances, to a smooth surface and minimal metallurgical changes in the outer layers of the blade, the dielectric fluid may in the same operation be replaced by a fluid as used for electrochemical machining.

The polarity of the currents can then be reversed and the surface be electrochemically machined to polish off any hardened or metallurgically unsound layers with considerable savings in setting up time, tooling costs and production time and greatly improved quality control.

The electrodes and their actuators may be built as standardised modules which may be arranged in various combinations and be controlled by a microprocessor or a computer so as to provide a flexible assembly of electrodes and a flexible means of controlling the movements and the eroding functions of the electrodes. For example, the application of knife edged and/or pointed electrodes in computer controlled modules would enable the tooling up for a complex machining profile or profiles to be accomplished within a short period of time or almost instantly. It would also enable any desired profile or profiles to be machined to close tolerances with minimum or negligible metallurgical changes in the surface(s) of the profile(s) and also without imposing for example macroscopic thermal stresses.If the example is applied to difficult or impossible to move structures it will be seen that the present invention may realise huge cost reductions.

Since an assembly of electrodes and their actuators can be of lightweight construction, the whole of the assembly may easily be moved along a workpiece by suitable guide means, for example, to profile and finish surfaces of heavy rollers as are used in steel and paper mills.

The present invention also provides a method of machining a plurality of workpieces by spark erosion in an electrical discharge machining apparatus having a plurality of electrodes, comprising supplying power to said electrodes independently of one another to effect machining, the plurality of workpieces not necessarily being numerically equal to the plurality of electrodes. For example, a requirement may exist to produce valves with conical sealing surfaces to control the flow of fluids.The sealing surfaces of a batched number of valve assemblies may be produced by spark erosion whereby, for example, the mating male and female parts alternately function as electrode and as workpiece after each spark cycle or number of spark cycles by alternately reversing the polarity of the power supplied by a single means and controlled by a single means whilst in each assembly rotating one part relative to the other and maintaining a presence or a supply of dielectric fluid between the mating surfaces. The material of the valves in the example may be metal or electrically conductive ceramics depending on for example requirements such as resistance to corrosion, resistance to erosion or abrasion, suitability to operate at designated temperatures, freedom from maintenance.

Finally, the volume of metal removed by the eroding current is roughly proportional to the strength of the current.

By distributing the supply of power over a number of independently controlled electrodes during finishing operations, the current per electrode may be kept low so as to reduce the thickness of the layer of metal in which properties may change due to local high temperatures, as well as slowly to ramp up the current to minimise wear of the electrode and slowly to ramp down the current so as to promote a degree of annealing in the outer skin of the eroded surface, whilst reducing the length of time for the operation relative to the length of time taken by a single electrode machine in proportion to the number of the plurality of the electrodes employed.

In order to effect fine finishing of a workpiece, the electro-discharge machining may be switched to electro-chemical machining. During electro-discharge machining, the workpiece serves as a negative electrode whilst the electrode has a positive polarity. Dielectric fluid is pumped through the gap between the two during machining. In order to change to electro-chemical machining for fine finishing of the workpiece, the polarity of the workpiece and the electrode is reversed. The workpiece becomes the positive electrode and the electrode itself has a negative polarity. In addition, the dielectric fluid is replaced by an electro-chemical fluid which is typically water.

During electro-chemical machining the voltage between the workpiece and electrode is typically 5 volts with a current of typically 200 amps, compared with a discharge initiating voltage of 150 volts and an operating voltage of 15 volts with a pulsating current of 25 amps average in electro-discharge machining.

Claims

1. An electrical discharge machining apparatus comprising; a plurality of electrodes, support means for supporting said electrodes, and control means for controlling the supply of electric current to at least one of said electrodes independently of the other electrodes.

2. An apparatus as claimed in claim 1 further comprising control means for controlling movement of at least one of said electrodes independently of the other electrodes.

3. An apparatus as claimed in claim 1 or 2 wherein at least one said support means of said electrodes is supported for movement relative to a workpiece for effecting in situ machining of the workpiece and wherein said control means is operable for controlling movement of said at least one support means along a preselected path.

4. An apparatus as claimed in claim 3 wherein a plurality of said support means of said electrodes are supported for movement relative to a workpiece and said control means is operable for controlling movement of said plurality of support means of said electrodes along preselected paths, at least two of said support means being movable independently of one another.

5. An apparatus as claimed in any preceding claim wherein at least one of said electrodes is supported by said support means for movement simultaneously in at least two orthogonal axes and further comprising control means for controlling movement of said at least one electrode along a preselected path.

6. An apparatus as claimed in claim 5 wherein a plurality of said electrodes are supported by said support means for movement in at least two orthogonal axes and said control means is operable for controlling movement of said plurality of electrodes along respective preselected paths, at least two of said electrodes being movable independently of one another.

7. An apparatus as claimed in claim 5 or 6 wherein said axes lie in a substantially horizontal plane.

8. An apparatus as claimed in claim 5 or 6 wherein said axes lie in a substantially vertical plane.

9. An apparatus as claimed in any of claims 5 to 8 wherein said at least one electrode is movable simultaneously in three orthogonal axes.

10. An apparatus as claimed in any of the preceding claims wherein at least one of said electrodes is supported by said support means for rotation about the electrode axis and wherein said control means is operable for controlling rotational movement of said at least one electrode.

11. An apparatus as claimed in any of the preceding claims wherein a plurality of said electrodes are supported by said support means for rotation about the electrode axes and said control means is operable for controlling rotational movement of said plurality of electrodes, at least two of said electrodes being rotationable independently of one another.

12. An apparatus as claimed in any of claims 1 to 11 whereinssaid control means is a microprocessor.

13. An apparatus as claimed in any of claims 1 to 11 wherein said control means is a computer.

14. An apparatus as claimed in any of claims 1 to 11 wherein said control means is a controller with an embedded processing element.

15. An electrical discharge machining apparatus substantially as hereinbefore described with reference to the accompanying drawings.

16. A method of machining a workpiece by spark erosion in an electrical discharge machining apparatus having a plurality of electrodes comprising supplying power to said electrodes independently of one another to effect machining.

17. A method as claimed in claim 16 comprising controlling movements of said electrodes independently of one another to effect machining.

18. A method as claimed in claim 16 or 17 further comprising moving at least one of said electrodes along a preselected path during machining.

19. A method as claimed in claim 18 wherein said preselected path is in at least one of a substantially horizontal and substantially vertical direction.

20. A method as claimed in claim 18 or 19 wherein a plurality of electrodes are moved along respective preselected paths and at least two of said electrodes are moved independently of one another.

21. A method as claimed in any of claims 16 to 20 for machining complementary surfaces on cooperating workpieces wherein said cooperating workpieces alternately serve as an electrode comprising applying an electric potential between said cooperating workpieces and periodically reversing the polarity of said potential.

22. A method as claimed in any of claims 16 to 21 further comprising reversing the polarity of electric power applied to said electrodes and workpiece and replacing dielectric fluid of the machining apparatus by an electro-chemical fluid to effect electro-chemical machining of the workpiece.

23. A method of machining a workpiece by spark erosion in an electrical discharge machining apparatus, substantially as hereinbefore described with reference to the accompanying drawings.