GB2183190A

GB2183190A - Electrode

Info

Publication number: GB2183190A
Application number: GB08529113A
Authority: GB
Inventors: Frederick John Rolinson; Alan Frederick Shaw
Original assignee: ERODEX
Current assignee: ERODEX
Priority date: 1985-11-26
Filing date: 1985-11-26
Publication date: 1987-06-03
Also published as: GB2183190B; GB8529113D0

Abstract

A spark erosion electrode (1, Fig. 1) has at least one bore (6, Fig. 1) extending from a working surface (5, Fig. 1) for the supply of dielectric fluid to the working surface 5 and a flow control plug 7 having an effective length which is of similar cross-section to provide a close fit in the bore and which has at least one groove 9 formed in the peripheral surface 8 to provide a fluid passage for controlling the flow of the dielectric fluid. At the nonworking surface of the electrode, the diameter of the bore may be larger than that of the plug for case of insertion. Alternatively the plug may project from that face of the electrode. <IMAGE>

Description

SPECIFICATION Electrode This invention relates to electrodes and in particular to spark erosion electrodes for use in forming accurately shaped surfaces by the process of spark erosion, also known as electrical discharge machining.

In the process of spark erosion, a working surface of the electrode is accurately machined to be of complementary shape to that to be produced. The electrode and a component on which the surface is to be produced are immersed in a dielectric fluid with the electrode working surface directly opposite and closely adjacent to the surface of the component. A voltage is applied to the electrode causing electrical discharge to occur between the opposed surfaces of the electrode and component which gradually erodes the surface of the component to that required.

The spark erosion process has a number of advantages. In particular components made from very hard materials difficult to machine by conventional methods can be accurately formed to any desired shape using electrodes made from relatively softer materials which are easily machined to the appropriate complementary shape.

in operation of the process, it is necessary to remove the eroded material from the interface between the electrode and component.

It is already known to provide through bores extending between the working surface and a non-working surface through which clean dielectric fluid flows from the non-working surface to the working surface to remove the eroded material from the interface. These bores are usually cylindrical being formed drilling and result in loss of the electrical discharge in the area where the bore opens into the working surface so that the surface of the component directly opposite this area is not eroded. As a result, a number of projections are left in the surface of the component in these areas at the end of the spark erosion process which have to be removed by a further machining operation to obtain the desired smooth surface.This problem occurs even when the smallest diameter bores that can practically be drilled through the electrode are used and the resultant further machining that is required is time consuming and adds to the cost of the finished component.

A solution to this problem proposed in European patent Specification No.0,092,847 is based on the provision of larger diameter cylindrical bores in the electrode in which are located respective plugs to control the flow of dielectric fluid to the working surface.

In one embodiment, the plug is cylindrical and a close fit in the bore. The end of the plug adjacent to the working surface is formed with diametric crossslots extending longitudinally over part of the length of the plug and the other end of the plug is formed with a central opening extending longitudinally over the other part of the length of the plug and opening into the ends of the slots within the plug. The opening and slots together provide a fluid path through which dielectric fluid can flow from the non-working surface to the working surface of the electrode. The slots are of thin cross-section and do not interfere with the spark erosion process. As a result, the surface of the component is uniformly eroded to the desired shape having a smooth surface which does not require further machining.

A disadvantage of this construction however, is that the length of plug that can be used is restricted by practical constraints on the length of cross-slots and central opening that can be provided. Thus the cross-slots split the plug and must be of limited length to maintain structural integrity while, for accurate drilling of the central opening in the other end, the opening must also be of limited length.

By such limitation of the overall length of plug that can be used, problems arise in use of the electrode when the working surface becomes worn and has to be re-machined to the required shape. Such remachining may be effected several times during the life of-the electrode and the cross-slots in the plugs may be removed entirely after only one or two remachining operations necessitating removal and replacement of the plugs which, on account of the close fit, is difficult and can damage the electrode. Also the cost of periodically replacing the plugs during the life of the electrode adds to manufacturing costs.

In another embodiment, the plug is of square cross-section, the diagonals of which are equal to the diameter of the cylindrical bore. In this construction the plug is not a close fit in the bore but leaves gaps between each side of the plug and the bore through which dielectric fluid can flow from the nonworking surface to the working surface of the electrode. The plug is of solid uniform crosssection enabling any length of plug to be used.

A disadvantage of this construction however, is that fitment of the square-section plug in the cylindrical bore relies on point contact between each corner of the plug and the bore to locate the plug in the bore and there are obvious practical problems in accurately machining a plug for this purpose. In particular small variation in the diagonal dimensions of the plug will result in the plug jamming and/or breaking on insertion into the bore.

A further disadvantage is that the size of the gaps increases rapidly with increase in diameter of the bore so that the aforementioned problem of non-erosion of the surface of the component may occur. As a result, the maximum bore diameter and hence maximum transverse diagonal width of square-section plug that can be used is limited, The present invention is intended to remedy the drawbacks and disadvantages afore-mentioned.It solves the problem of how to design a spark erosion electrode incorporating a flow control plug for controlling the flow of fluid to a working surface of the electrode which is not limited as to the length or transverse width of plug that may be used by providing a plug having an effective length which is a close fit in the electrode bore, for example a cylindrical plug in a cylindrical bore, and which has at least one groove formed in the peripheral surface to extend lengthwise of the plug over the entire effective length of the plug to provide a fluid passage for the aforesaid control of fluid flow to the working surface.

The advantages of the present invention are mainly that the structural integrity of the plug is not affected by the provision of the groove in the peripheral surface and the plug can have any desired effective length to permit remachining of the working surface of the electrode without requiring replacement of the plug. Furthermore, this advantage can be obtained for plugs of small transverse width.

Also since flow through the groove is not affected by the transverse width of the plug there is no restriction on the maximum transverse width of plug that can be used.

The plug and bore may have any matching crosssection over the effective length of the plug to provide a close fit of the plug in the bore, for example the plug and bore may be of circular, oval or rectangular cross-section.

The effectjve length of the plug may correspond to the overall length of the plug, for example where the whole plug is a close fit in the bore. or part only of the overall length, for example where the end of the plug remote from the working surface is located in a bore section of increased diameter or projects from the bore.

Advantageously, the plug and bore are cylindrical and the plug is preferably of uniform diameter throughout its length. The plug may have any selected diameter but, in general, a diameter in the range from 1.27mm (0.05") to 12.7mm (0.50") is sufficient for most needs.

The groove conveniently extends the entire length of the plug and is of uniform crosssection throughout its length so that'the dimensions of the groove at the working surface of the electrode remain unchanged on remachining the working surface. The groove may be formed in a plug of the desired length or the plug may be cut to length from a stock of rod in which the groove has previously been formed.

Advantageously, the groove is of narrow width, for example of the order of 0.2mm (0.008") and the depth of the groove selected to provide the required flow rate. Alternatively or in addition a plurality of grooves may be formed in the peripheral surface of the plug, preferably uniformly spaced apart in the circumferential direction, each groove extending lengthwise over the effective length of the plug to provide the required flow rate.

Preferably, the or each grooove extends radially inwards from the peripheral surface of the plug. Where a single groove is provided, the maximum radial depth preferably corresponds to the radius of the plug. Where a plurality of grooves are provided the maximum radial depth is less than the radius of the plug to leave a solid central core to maintain the structural integrity of the plug.

Preferably the electrode is formed with a plurality of spaced apart bores each containing a respective flow control plug according to the present invention.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing, wherein: FIGURE 1 is a perspective view of a spark erosion electrode according to the present invention; FIGURE 2 is a side view of the flow control plug of the electrode shown in Figure 1; FIGURE 3 is a plan view of the flow control plug shown in Figure 2; and FIGURES 4 and 5 are plan views of two alternative constructions of flow control plugs for use in a spark erosion electrode according to the present invention.

Referring first to Figures 1 to 3 of the accompanying drawing, there is shown a graphite spark erosion electrode 1 for forming a recessed curved surface of a turbine blade aerofoil (not shown). The electrode 1 comprises an elongate body having planar side, end and rear surfaces 2,3 and 4 respectively and a raised curved front surface 5 which, in use, is the working surface of the electrode 1 and is complementary to the desired recessed curved surface of the turbine blade aerofoil. The electrode 1 is produced from a rectangular graphite block, the front surface of which is accurately machined to form the raised curved surface 5 shown.

In the spark erosion process, the electrode 1 and the turbine blade aerofoil are immersed in a dielectric fluid such as paraffin with the raised curved surface 5 of the electrode directly opposite and adjacent to the surface of the turbine blade aerofoil in which the recessed curved surface is to be formed. A voltage of the order of 25/100 volts and a current of approximately 30 amps is applied to the electrode 1 causing an electrical discharge to occur between the adjacent opposed surfaces of the electrode 1 and the turbine blade aerofoil which erodes the surface 5 of the turbine blade aerofoil until eventually the desired recessed curved surface complementary to the raised curved surface 5 of the electrode is formed.

Formed in the electrode 1 is a through bore 6 extending from the rear surface 4 to the front surface 5. The bore 6 is cylindrical and of uniform diameter throughout its length. Located in the bore 6 is a flow control plug 7 having one end flush with the front surface 5 of the electrode and the other end terminating within the bore 6. The plug 7 is cylindrical and of uniform diameter throughout its length.

The diameters of the bore 6 and plug 7 are so chosen that the plug 7 is a close fit in the bore 6 over the entire length of the plug 7. In the embodiment illustrated the plug 7 has a diameter of 4.75mm(0.187") and a length of approximately 152.4mm (6.0").

Formed in the peripheral surface 8 of the plug 7 are three similar grooves 9 extending parallel to the longitudinal axis of the plug 7 and over the entire length of the plug 7. The grooves 9 are uniformly spaced apart in the circumferential direction and extend radially inwards from the peripheral surface 8. Each groove 9 is of uniform thin cross-section over the entire length and has a depth less than the radius of the plug to leave a solid centre core 10 to maintain the structural integrity of the plug 7. The grooves 9 are conveniently formed using a grinding wheel or other suitable tool and, in the embodiment illustrated, each groove 9 has a width of 0.2mm (0.008") and a depth of 1.52mm (0.06").

The grooves 9 provide separate fluid passages for controlling the flow of dielectric fluid from the nonworking rear surface 4 of the electrode through the bore 6 to the working front surface 5 for removing, from the interface between the opposed surfaces of the electrode 1 and turbine blade aerofoil, the material eroded from the surface of the turbine blade aerofoil.

The foregoing arrangement in which the end of the plug 7 is flush with the front surface 5 of the electrode and the grooves 9 are of thin cross-section ensures that the supply of dielectric fluid to the working surface 5 does not interfere with the spark erosion process. As a result, the surface of the turbine blade aerofoil directly opposite the grooves 9 is eroded and the finished recessed curved surface of the turbine blade aerofoil is completely smooth and does not require further finishing.

Furthermore, the formation of longitudinal grooves 9 in the peripheral surface 8 of the plug 7 leaving the solid central core 10 ensures that the strength of the plug 7 is not significantly reduced. As a result, there is no restriction on the length of plug 7 that can be used and this enables the plug length to be chosen so that, when the front surface 5 of the electrode becomes worn, the front surface 5 can be re-machined to the required shape without having to remove and replace the plug 7. Indeed the plug length may be selected to permit several re-machining operations to be performed on the electrode 1 without changing the plug 7. This advantage is obtained in the present invention from the use of a bore 6 and plug 7 having similar cross-sections to provide a close fit of the plug 7 in the bore 6.

This considerably simplifies manufacture and assembly of the electrode 1 as the relative dimensions of the bore 6 and plug 7 can be accurately controlled and the plug 7 inserted without risk of jamming and/or breaking.

It will be understood that the invention is not limited to the embodiment above-described. For example, the electrode 1 may have a plurality of spaced apart bores 6 extending between the rear surface 4 and the front surface 5 with a respective flow control plug 7 received in each bore 6.

The diameter of the or each plug 7 and the associated bore 6 may be altered as may the size and/or number of the longitudinal grooves 9 formed in the or each plug 7 to provide any desired rate of flow of dielectric fluid to the front surface 5 of the electrode. Figures 4 and 5 show two alternative constructions of cylindrical plug having different combinations of plug diameter and groove number and size.

In Figure 4, the plug 20 has a uniform diameter throughout its length of 1.27mm (0.05") and is formed with one groove 21 only extending parallel to the longitudinal axis of the plug 20 and over the whole length of the plug 20. The groove 21 extends radially inwards from the peripheral surface 22 of the plug and has a width of 0.2mm (0.008") and a depth of 0.635mm (0.025") equal to the radius of the plug throughout its length.

In Figure 5, the plug 30 has a uniform diameter throughout its length of 12.7mm (0.5") and is formed with four similar grooves 31 uniformly spaced apart in the circumferential direction. The grooves 31 extend parallel to the longitudinal axis of the plug 30 and over the whole length of the plug 30. Each groove 31 extends radially inwards from the peripheral surface 32 of the plug and has a width of 0.2mm (0.008") and a depth of 3.175mm (0. 125") throughout its length to leave a solid central core 33 to maintain the structural integrity of the plug 30.

The or each groove 9,21,31 preferably extends the whole length of the plug 7,20,30 as above-described thus facilitating manufacture by enabling the plugs to be cut to any required length from a stock of rod of the appropriate diameter in which the or each groove has already been formed. However, it will be appreciated that it is only necessary for the operation of the invention for the or each groove to extend over that length of the plug which is a close fit in the bore, i.e. the effective length of the plug within the bore, and the invention includes constructions in which the or each groove extends over the effective length of the plug only.

The or each groove 9,21,31 preferably extends substantially parallel to the longitudinal axis of the plug 7,20,30 but this is not essen tial and the or each groove may extend in both a circumferential and longitudinal direction.

The or each groove 9,21,31 preferably extends radially inwards from the peripheral surface 8,22,32 and is of thin cross-section as above-described but other shapes may be utilised provided that the strength of the plug is not significantly reduced thereby and the required erosion of the surface of the component directly opposite the or each groove is not affected.

The end of the or each bore 6 opening to the rear surface 4 may be of increased diameter, for example stepped or tapered to facilitate insertion of the plug 9,20,30 and flow of the dielectric fluid into the bore 6.

The or each bore 6 and associated plug 9,20,30 may have matching cross-sections over the effective length of the plug other than circular, for example, oval or rectangular.

Finally, the eledtrode 1 and plug(s) 9,20,30 may be made of materials other than graphite, for example, copper, brass, aluminium, copper/tungsten alloys and other materials as are commonly used, to suit the particular application.

Claims

1. A spark erosion electrode having a bore extending from a working surface of the electrode for the supply of fluid to the working surface, and a flow control plug received in the bore for controlling the flow of fluid to the working surface, wherein the plug has an effective length which is a close fit in the bore and which has at least one groove formed in the peripheral surface to extend lengthwise of the plug over the entire effective length of the plug to provide a fluid passage for the aforesaid control of fluid flow to the working surface.

2. A spark erosion electrode according to claim 1 wherein the groove extends parallel to the longitudinal axis of the plug.

3. A spark erosion electrode according to claim 1 or claim 2 wherein the groove is of uniform cross-section.

4. A spark erosion electrode according to any one of the preceding claims wherein the outer end of the plug is flush with the working surface.

5. A spark erosion electrode according to any one of the preceding claims wherein the plug and bore are of similar cross-section over the effective length of the plug to provide the aforesaid close fit of the plug in the bore.

6. A spark erosion electrode according to claim 5 wherein the plug and bore are cylindrical.

7. A spark erosion electrode according to claim 6 wherein the plug has a diameter in the range from 1.27mm to 12.7mm.

8. A spark erosion electrode according to claim 6 or claim 7 wherein the plug has a single groove arranged to extend radially inwards from the peripheral surface.

9. A spark erosion electrode according to claim 8 wherein the single groove has a maximum radial depth corresponding to the radius of the plug.

10. A spark erosion electrode according to claim 6 or claim 7 wherein the plug has a plurality of circumferentially spaced apart grooves arranged to extend radially inwards from the peripheral surface.

11. A spark erosion electrode according to claim 10 wherein each groove has a maximum radial depth less than the radius of the plug to leave a solid centre core.

12. A spark erosion electrode according to any one of the preceding claims wherein the plug is made of a material selected from the group comprising graphite, copper, brass, aluminium and copper/tungsten alloys.

13. A spark erosion electrode according to claim 12 wherein the electrode is made of the same material as the plug.

14. A spark erosion electrode according to any one of the preceding claims having a plurality of spaced apart bores extending from the working surface and a respective flow control plug received in each bore.

15. A spark erosion electrode substantially as hereinbefore described with reference to Figures 1,2 and 3 of the accompanying drawing.

16. A spark erosion electrode substantially as hereinbefore described with reference to Figures 1,2 and 3 of the accompanying drawing as modified by either Figure 4 or Figure 5 of the accompanying drawing.

17. A flow control plug for use in the spark erosion electrode according to any one of the preceding claims.