GB2202481A - Electrode and its use in electrical discharge machining - Google Patents

Abstract

In the present invention a flexible elongate electrode (1) is arranged such that profile may be adjusted as desired. The electrode is mounted on a flexible arbor (4) which is deformed by the application of a load thereto, to bring about a desired deformation of the electrode. A suitable servo mechanism is used to apply the desired load to the arbor. The electrode is preferably comprised of a plurality of electrode segments (2,26), which are shaped and placed relative to each other to minimise any striping effect (Fig. 4). An arrangement is described for accurately machining a roll using two such electrodes (Fig. 5, not shown). The ED machine may have a reverse polarity capability whereby initially the electrode itself may be machined to an accurate profile. <IMAGE>

Description

forking of Electrically Conductive Materials by Electrical Erosion The present invention relates to the working of electrically conductive materials by electrical erosion, and more pPrticuarly to electrical erosion working in which an electrode is placed adjacent a orkpiece and is arranged to have a voltage applied thereto such that sparking occurs across the gap between the electrode and workpiece causing erosion of the workpiece surface.

Electrical erosion techniques are used in the manufacture of, for example, metal rolls which are to be used in the sheet metal industry. The surface texture of such rolls has to be engineered to within very precise parameters, as the rolls are used to press sheet metal (eg for use in the manufacture of car bodies) and any defect in the roll surface texture will be reproduced on the sheet metal. Electrical erosion techniques are suitable for precisely machining the surfaces of such rolls.

Known electrical erosion techniques utilise an electrode which is place adjacent a roll to be textured.

A pulse voltage is applied across the gap (uslIlly throrh an electrolytic fluid) between the roll and electrode.

A spark discharge will occur at the point where the electrode and roll are closest, the spark beating the surface of the roll and creating a minute crater therein.

Repeated discharges at high frequency cause steady highenergy electrical discharges which produce rany minute craters per second in the roll at various points on the roll surface, depending which point ta closest to the electrode at the time a resulting in a uniform surface finish. Electrical erosion techniques are very controllable and are easily able to surface texture hard rolls.

The electrodes utilised are generally elongate in form, to match the length of the workpieces to be machined. Workpieces destined to be used in rolling sheet steel can be very long.

In order to speed up the erosion process it has previously been proposed to utilise a segmented electrode arrangement. This comprises an elongate electrode divided into segments by separating insulating gaps. A power supply is provided for each separated segment so that sparking may occur between each segment and the workpiece every time a voltage pulse is applied. This results in an increased machining speed over a corresponding unsegmented electrode with only 8 single power supply (ie only one spark per voltage pulse is obtained).

The conditions in the gap between the electrode and the workpiece effect the machining parameters and are very important. It is important to monitor the voltage or equivalent parameter across the gap to detect whether correct desired machining of the workpiece is taking place.

One important factor which effects the gap conditions is the relative orientation of workpiece and electrode.

If the electrode comes too close to the workpiece undesirable arcing across the gap may occur, or if the electrode is tilted relative to the workpiece one part of the workpiece may be machined more than the other part, resulting in an unevenly machined workpiece.

Further, the profile presented to the workpiece by the electrode is an important factor in deciding the final machined shape of the workpiece. For example, a 'crowned" profile As often desired for rolls used in the steel milling industry, as these tend to bend in the middle due to the tress applied to them in pressing plate, so a roll with a slightly large diameter towards its middle than at its ends is desirable. An electrode having a profile substantially "mirroring" the desired crown profile can be used to produce the desired profile in the workpiece.

It has previously been proposed in electrical erosion apparatus to provide servo control to adjust the relative orientation of electrode and workpiece. For example, it has been proposed to control the position of the elongate electrode by a single servo arranged to move the electrode towards and away from the workpiece and tilt it relative to the workpiece. A problem with such an arrangement is that if the orientation of the electrode becomes undesirable, in order to change the orientation it may be necessary to move the whole electrode away from the workpiece before adjusting orientation. This may cause an interruption in sparking and thus a reduction in speed of machining.

Further, if a particular profile of workpiece is desired it is necessary to provide an electrode which has been specially saed to produce the desired profile.

It has further been proposed, in conjunction with a segmented electrode, to provide a separate servo control for each segment in order to reduce some of the above problems eg if incorrect sparking is occurring between a segment and the workpiece it is possible to adjust the position of only that particular segment, rather than having to move the whole electrode. However, the provision of separate servo control for each segment is complex and expensive.

The present invention provides an electrode for use in an electrical discharge machining apparatus, the electrode being elongate and being relatively deformable, whereby the profile of the electrode presented to a workpiece in machining may be adjusted.

Preferably, the electrode comprises a plurality of electrode segments separated from each other by insulative gaps. Any material in the gaps acting as insulation, is preferably flexible to allow for deformation of the electrode.

The present invention further provides electrical discharge machining apparatus, comprising an elongate relatively deformable electrode which is arranged to be placed adjacent a workpiece, and means arranged to control deformation of the electrode, whereby the profile of the electrode presented to the workpiece in machining may be adjusted.

The electrode is preferably mounted on a flexible arbor. To cause the electrode to deform a load is applied to the arbor at the desired position. This will cause the arbor to flex, which, in turn, will cause the electrode to deform to the desired profile.

The electrode preferably comprises a plurality of electrode segments separated from each other by insulative gaps. Any material in the gaps, acting as insulation is preferably flexible to allow for deformation of the electrode.

An advantage of this arrangement is that it is not necessary to provide separate servo control for each segment, whilst still retaining the ability to adjust the position of eath segment relative to the workpiece.

Preferably a servo arrangement is also provided in addition to the flexing ability, so that any large changes in the orientation of the electrode may be made.

Further, in any electrical erosion process employing an elongate electrode it has been found that the sparking frequency between the workpiece and electrode at the ends of the electrode is less than the sparking frequency in the middle. Unless this is compensated for it can result in uneven machining. The ability to flex the electrode provided by the present invention means that compensation can be made merely by adjusting the electrode such that the ends are positioned closer to the workpiece than the centre.

A problem with using segmented electrodes for electrical discharge machining processes is the phenomenom of "striping" of the workpiece surface. Defects in the workpiece surface are produced which are in substantial alignment with the corners of the segmented electrodes.

Such defects may not be particularly noticable on the workpiece itself but when the workpiece is, for example, a roll to be used in pressing sheet metal the defects will be reproduced on the sheet metal and will be noticable.

It has previously been proposed to reduce the striping effect by forming electrode segments such that the gaps between them run at an angle to the axis of the electrode This has been found to reduce the intensity of the striping, although the striping still remains a problem.

We believe that the striping phenomenom may occur with the angled electrode due to the gap between adjacent electrode segments which causes the volume of the electrode as "seen" from the workpiece to change along the length of the workpiece.

Where a segmented electrode is utilised in the present invention, we prefer a shape of electrode which minimises the change in volume of active electrode along the length of the electrode which is "seen" by the workpiece as the electrode is operating. The shape of the electrode segments in the electrode we prefer is such that any line passing through the longitudinal axis of the electrode and being perpendicular thereto will alway pass through or be contiguous to an insulating gap Advantageously this means that during operation of the electrode the volume of active electrode along the length thereof when viewed from parallel points on the workpiece will remain the same as the gap between each segment changes position relative to the workpiece, eg.

by rotation of the electrode relative to the workpiece.

Preferably, the angle which the plane of the insulating gap makes with the longitudinal axis through the electrode is 450 We have also found that we can make a further reduction in striping, and at the same time increase the speed of machining, by using at least one extra electrode.

An extra electrode may be placed at the side of the workpiece opposite from the first electrode, with the insulating gaps between the extra electrode segments offset with respect to the insulating gaps in the first electrode; Preferably, in operation, the respective electrodes are arranged such that movement of the insulating gaps of the first and extra electrode with respect to the workpiece are "out of phase" with each other. This may be achieved, for example, by contra rotation of the electrodes.

As an electrode is used in a process of electrical erosion it tends to wear. After a time wear could occur to such an extent that the shape of any electrode segment is changed from, for example, the shape of the present invention, thus leading to striping. We further may prefer to arrange the shape of the insulating gap between a pair of segments such that even if the electrode wears the volume of electrode either side of the gap seen by the workpiece remains generally constant as the electrode is operated, e.g the profile at the gap could be designed such that it is wider at some points than at others, or could be designed to follow a "sinuous" path (as the electrode is viewed in profile).

As well as flexing of the electrode to alter its profile some initial shaping of the electrode profile may be desirable. Preferably, we do this by applying reverse polarity between the electrode and workpiece in order to machine the electrode to cause a change in the overall shape of the electrode with respect to the workpiece. Applying reverse polarity involves reversing the voltage between the electrode and the workpiece.

In electrical erosion processes it is known to monitor the voltage of sparking occuring in the gap between the electrode and workpiece in order to determine the prevailing machining conditions.

A problem involved with monitoring the voltage is that due to losses (eg cable losses) between the voltage monitors at the gap and the control machinery the voltage readings at the control machinery may be inaccurate.

In order to avoid this difficulty we propose that, on setting up the electrical erosion apparatus, we take a reading of the voltage at the gap and compare it with the reading given in the control machinery. If the control reading is inaccurate we can adjust the control to give a correct reading so as to compensate for any losses.

Features and advantages of the present invention will become apparent from the following description of an embodiment theredf, by way of example only, with reference to the accompanying drawings, in which, Figure 1 is a schematic block diagram of an electrical erosion apparatus in accordance with an embodiment of the present invention, also illustrating an electrode in accordance with an embodiment of the present invention; Figure 2 shows a plan view of an electrode in accordance with an embodiment of the invention mounted for control of deformation of the electrode; Figure 3 shows a plan view of part of a segmented electrode;; Figure 4 shows a plan view of a part of a segmented electrode the segments of which are shaped so as to minimise any striping effect, and Figure 5 shows a schematic block diagram of an electrical erosion apparatus in accordance with a further embodiment of the present invention, utilising two elongate electrodes.

With reference firstly to Figure 1, an electrical erosion apparatus is illustrated, comprising a cylindrical elongate electrode 1 which is divided into a number of conducting segments 2 (which may, for example, be of a graphite) separated by insulative gaps 3.

The electrode 1 is mounted for rotation on an arbor 4. The arrangement comprising the electrode 1 and arbor 4 are mounted with respect to a servo head 5 which can move the arrangement under control of the master control and power supply unit 6. A sub servo 12 is provided for causing flexing of the electrode 1.

A workpiece 8 (for example a metal roll destined for use in the milling industry) is mounted for rotation on a bed 9. A workpiece drive unit 10, having a workpiece drive motor 11, is mounted adjacent the workpiece 8 for driving the workpiece 8. Workpiece steady's 13 are provided to support the-workpiece 8.

Dielectric fluid 14 (eg oil) is applied to the gap 15 between the electrode 1 and workpiece 8. A dielectric storage tank and filter system 15 is provided for circulation of the fluid 14 and a drip tray 16 is provided to catch the fluid.

Slip rings and brushes 7 are provided to enable electrical power to be supplied to each segment 2 of the electrode 1. Power supply feeders 17 provide power to the slip rings and brushes 7 and to the workpiece 8 from the master control and power supply 6.

In operation the electrode 1 is brought close to the workpiece 8 by operation of the main servo head 5.

A voltage is then applied between the electrode segments 2 and workpiece 8 to cause sparking across the gap 15, resulting in machining of the workpiece 8 The master control 6 can be programmed with the desired machining parameters to control machining of the workpiece 8. Feedback from monitors monitoring the condition in the gap 15 provides information on the machining conditions to the master control 6. If the conditions are not those desired the master control can take action to alter the conditions, eg by varying the voltage applied to the gap 15 and/or altering the orientation of the electrode relative to the workpiece.

The electrode 1 and workpiece 8 are rotated in opposite directions, as indicated by arrows E and F.

Referring now to Figure 2 of the drawings an embodiment of electrode mounting (as used in the Figure 1 apparatus) which allows flexing of an electrical erosion electrode is illustrated. The relative alignment of workpiece and electrode is an important factor in determining machining speed. By providing this flexible arrangement it is possible to finely adjust the alignment and position of the electrode segments relative to the workpiece.

The arrangement illustrated in Figure 2 16 essentially a relatively enlarged view of the electrode/ arbor arrangement illustrated in Figure 1.

In Figure 2, reference numeral 1 indicates an elongate electrode which may be used in an electrical discharge machining process. Electrode segments 2, which may be supplied separately with electricity, are separated by gaps 3- which contain insulating material. -The insulator is a material which is capable of deformation. For example, rubber.

The electrode 1 has a longitudinal axis, illustrated by line AA when the electrode is undeformed.

The electrode 1 is mounted for rotation on an end bearing support 20 and spindle head 21 respectively.

These are fixed to a hollow arbor 4, the length of which runs generally parallel to the length of the electrode 1.

The arbor is mounted on fixed pins 22,23 and is capabl-e of flexing when a load is applied thereto, due to the moment created between the load and fixed pins 22,23 Deformation of the arbor 4 to a load applied will be transmitted to the electrode via bearings 26,21,, causing the electrode to deform.

For example, if a load is applied towards the centre of the arbor 4 in the direction of arrow A (away from the electrode), the arbor will flex and the ends will tend to pivot such that they will lie generally in line with line C,C. This pivoting moment will be transmitted to the electrode 1 tia the bearings 20;21 causing the electrode to deform as illustrated by broken line DD (note that the, deformation may be somewhat exaggerated in the drawings so as to assist clarity.

By choosing the load and where the load is applied on the arbor any desired deformation may be given to the electrode for the electrical discharge process, whereby the electrode may be correctly aligned with the workpiece.

As an example of the amount of deformation normally required, for an 80 inch electrode the amount of deformation available in the present invention may be in the range of 2 inch.

Referring again to Figure 1, the sub servo 12 is arranged, under control of the master control 6 to apply a load to the arbor to cause flexing of the electrode to obtain a desired machining profile.

With reference now to Figure 3 and 4 an electrode in accordance with the present invention and arranged such that any striping effect is minimised, will be described.

With reference, firstly, to Figure 3, part of a cylindrical segmented electrode 30 is illustrated as viewed from the side. Discontinuous edges 31 of the electrode 30 in the drawing indicate that the electrode could be longer than is shown.

The electrode 30 is representative of the type which could for example, be used to machine the surface of metal rolls for use in the milling industry. A workpiece surface is schematically shown as designated by reference numeral 32.

The electrode 30 is divided up into segments 33 which are separated from each other by insulating gaps 34. For the sake of clarity, only one segment and gap in the Figure 1 are designated by reference numerals. The same reference numerals, it will be appreciated, apply to each of the other scements -nd gaps shown. The insulating gaps 34 are cambered at an angle with respect to the longitudinal axis YY through the cylindrical electrode 30.

In operation the electrode 30 is positioned adjacent to a workpiece surface 32, leaving a gap P. For the purposes of clarity the relative size of the gap P in the drawing is exaggerated. In actual operation the electrode will be quite close to the workpiece surface so as to enable electric discharges to occur. A dielectric fluid (liquid) is usually utilised in the gap P in order to promote the occurrence of discharges.

- Each segment 32 is provided separately with electrical power, such that electrical discharge will take place between each electrode segment 33 and the workpiece surface 32. thus resulting in machining and texturing of the workpiece surface.

t The electrode 30 o'( is arranged to rotate about its longitudinal axis YY as machining is occurring.

Rotation causes the relative position of the insulating gaps 34 as seen from the workpiece surface 32 to change relative to the workpiece surface 32. This is illustrated by considering the view from point M at the workpiece surface 32.

The line 32 represents the part of the workpiece surface which is closest to the electrode 30 -at any time.

Viewing from point W along a line bi which is perpendicular to the workpiece surface 32 - position 0 of the insulating gap 34 trill be seen. However, as the electrode rotates in the direction of arrow I the position of the insulating gap 34 will move as seen from M along M. The position of the insulating gap 34 with respect to N will actually be seen to oscillate during rotation in the path illustrated by double beaded arrow R..

As discussed above in the preamble, although cambering of the insulating gaps such as is shown in the arrangeient described above may reduce the intensity of any striping but it still remains a problem. In fact it merely may produced less intense strips which are spread over a broader area.

Referring now to Figure 4, a side view of a cylindrical electrode 25 in accordance with the present invention is shown. The electrode 25 is comprised of segments 26 separated from each other by insulating gaps 27, in a similar manner to the electrode described above.

However, in the case of this embodiment in accordance with the present invention the cambering of the insulating gaps 27 and the shape of the segments 26 are arranged such that any line passing through the longitudinal axis Y'Y' of the segments 26 and being perpendicular thereto will always pass through or be contiguous to an insulating gap 27.

This is illustrated in the Figure by linesp,R,S which are drawn perpendicular to the axis Y'Y' at different points along its length. It will be seen that wherever along the electrode 25 such a line as Q or R or S is taken, the line will always pass through an insulating gap.

The same is not the case with the electrode 30 of Figure 2. Lines such as T can be constructed which are perpendicular to the line YY but which do not pass through, or close to, any insulating gap 34.

In the embodiment of the invention illustrated in Figure 2, considering a point N' on a workpiece surface 28, viewing from N' along a line M' the portion 0' of an insulating gap 27 closest to the workpiece surface 28 at any one time will oscillate to the extent of R' relative to the workpiece surface as the electrode 25 rotates in the direction of arrow I'.

The shape of the segments 26 and arrangement of the insulating gaps 27 is such that the volume of actual active electrode adjacent the workpiece surface 28 changes only minimally for any point along the workpiece surface 28.

Portion E is taken approximately through the corners of a segment 26 and portion F approximately through the centre of the top line of the segment. Both portions defined by E and F include substantially the same volume of active electrode. The volumes will be substantially unchanged as the electrode rotates.

This is not the case with the electrode of Figure 3.

The volume of active electrode in the area adjacent to the insulating gap 31 will be different from the volume of electrode through the centre of an electrode segment 33. A section through the centre of a segment 33 would not take in any part of an insulating gap 34.

With reference to Figure 5 a further embodiment of electrical discharge machining apparatus is illustrated schematically. This apparatus comprises two segmented electrodes in accordance with the present invention 50A and 50B. The electrode 50A comprises 16 segments (a to p, the numerals b to o are left off the drawings for the sake of clarity) which are divided from each other by respective insulating layers 51. Electrode 50B comprises similar components which are reference numeralled in a similar manner.

Each of the electrode segments a to p are shaped such that the volume of electrode remains substantially unchanged along the length thereof as seen from the direction of the workpiece 52 (as the electrodes 50A,50B rotate the line of each insulative layer which is nearest to the workpiece 52 will change position relative to the length of the workpiece). Both electrodes 50A, 50B are arranged to rotate in the contra directions illustrated by arrows A,B.

The workpiece 52 is also arranged to rotate in the direction of arrow C.

Further components of the electrical discharge texturing apparatus are illustrated in block diagram form and are similar to the components of Figure 1.

The master EDT control section 54 deals with data entry (eg one can input desired surface parameters to be produced by the EDT process) and control of the apparatus.

A servo control unit 55 control the relative positions of the workpiece 53 and electrodes 50A,B by movement of all or some, and receives instructions from the master control 54. A power supply 56 supplies power to the electrode segments a to p via conductive means 57 which may be, for example, commutator brushes. The power supply 56 is also connected to the master control section 54.

Finally, a gap analyser 58 is arranged to monitor conditions within the gap z between the workpiece 52 and electrodes 50A,B so as to provide feedback to master control 54 regarding the texturing conditions. Dielectric material is preferably utilised in the gap Z in order to promote the occurrence of electrical discharges.

The electrodes 50A,B are arranged such that in operation they will effect opposite sides of the workpiece 3 to be machined.

The electrodes 50A,B are arranged with resepct to each other such that the insulating gaps 51A in the electrode arrangement 50A are offset with respect to the insulating gaps 51B in electrode arrangement 50B.

This can best be seen by viewing along the broken line Z-Z which runs perpendicularly to the length of the electrode arrangements in the drawing. The insulating gaps are offset such that when viewed along this line the position of the insulating gaps SB lie approximately midway the insulating gaps 51A, as illustrated at points P and 0.

The insulating gaps of each respective electrode are arranged such that they will rotate out of phase with each other.

In operation, because of the use of two electrodes and the offset of the insulating gaps the workpiece 52 will be machined very evenly. Using two electrodes also will give an increase in the speed of machining.

Note that it would be possible to use more than two electrodes, and even machining could be maintained by appropriately offsetting the insulating gaps of the respective electrodes with respect to each other.

The present invention can be used satisfactorily in the surface texturing of any type of conductive workpiece - not only metal rolls for use in the milling industry.

It will be appreciated that the type of surface texture applied to a workpiece may be varied depending upon the chosen texturing parameters.

In most applications dielectric will be used in the gap between the electrode and workpiece.

It will be appreciated that any number of segments may be utilised in the electrodes employed.

In all the embodiments of the invention illustrated, the insulative gaps between electrode segments have been shown as being straight and symmetrical. As machining is carried out, electrode wear occurs due to erosion of the electrode due to sparking. As the electrode wears the segments will change shape and there will come a time when the geometry of adjacent segments is such that a constant electrode volume is not being presented to the workpiece as rotation of the electrode and/or workpiece is occuring. This will result in an increase in the striping effect.

It is possible to offset or delay the occurrence of this phenomenom by specially shaping the electrode segments so that electrode wear does not result in an increase in the intensity of striping, at least for a substantial time. This can be done, for example, by shaping the segments such that the insulative gaps thereby produced are sinuous in shape, as seen from the side, so that even when wear occurs any imaginary line perpendicular to the longitudinal axis of the electrode will always pass through or be contiguous to an insulative gap. Other shapes than sinuous will also suffice for this function.

The shapes can be determined by calculation.

The dimensions of the electrode segments can also be calculated given the desired length and width of the electrode, and the number of electrodes. A typical electrode could be 2m long and have 16 segments.

As discussed in the preamble, it is also desirable sometimes that the profile of the electrode be shaped prior to machining, and this can be done by applying a reverse polarity between the workpiece and electrode causing the electrode to be machined itself until the profile is as desired. For example, a negative polarity could be applied to the electrode to cause machining of the electrode profile. The apparatus of the present invention is preferably provided with a reverse polarity capability.

Also, the apparatus of the invention preferably provides a means for determining threshold voltage, in order to compensate for losses when monitoring gap conditions (eg cable loses), as also discussed in the preamble.

Claims (13)

1. An electrode for use in an electrical discharge machining apparatus, the electrode being elongate and deformable, whereby the profile of the electrode presented to a workpiece in machining may be adjusted.

2. An electrode in accordance with claim 1, comprising a plurality of electrode segments separated from each other by insulative gaps, the shape of the segments and gaps between them being such that the change in volume of active electrode relative to the workpiece during machining, where the electrode and workpiece are oscillated or rotated relative to each other, is minimised whereby to reduce any striping effect.

3. An electrode in accordance with claim 2, wherein the shape of the electrode segments and gaps between them are designed such that the minimum change in volume of active electrode relative to the workpiece is maintained even as the electrode wears during machining.

4. Electrical discharge machining apparatus, comprising an elongate, relatively deformable electrode which is arranged to be held adjacent a workpiece, and means arranged to control deformation of the electrode, whereby the profile of the electrode presented to the workpiece in the machining may be adjusted,

5. Electrical discharge machining apparatus in accordance with claim 4, wherein the means arranged to control deformation of the electrode comprises a flexible arbor to which the electrode is mounted, the arbor being arranged to flex under a load to cause deformation of the electrode.

6. Electrical discharge machining apparatus in accordance with claim 5, further comprising servo means for applying a variable load to the arbor.

7. Electrical discharge machining apparatus in accordance with any of claims 4,5 or 6, wherein the electrode comprises a plurality of electrode segments separated from each other by insulative gaps, the shape of the segments and gaps between them being such that the change in volume of active electrode relative to the workpiece during machining, where the electrode and workpiece are oscillated or rotated relative to each other, is minimised, whereby to reduce any striping effect.

8. Electrical discharge apparatus in accordance with claim 7, wherein the shape of the electrode segments and gaps between them are designed such that the minimum change in volume of active electrode relative to the workpiece is maintained even as the electrode wears during machining.

9. Electrical discharge machining apparatus in accordance with any of claims 4 to 8, wherein a further electrode is provided for machining.

10. Electrical discharge machining apparatus in accordance with any of claims 4 to 9, wherein means are provided for applying reverse electrical polarity between the workpiece and electrode, whereby to machine the profile of the electrode.

11. Electrical discharge machining apparatus in accordance with any of claims 4 to 10, wherein means are provided for monitoring the electrical voltage between the workpiece and electrode and displaying the monitored voltage, and means are provided for adjusting the displayed voltage to compensate for any losses occurring between monitoring of the voltage and display thereof.

12. An electrode for use in electrical discharge machining apparatus, the electrode being substantially as described herein with reference to Figures 1,2,4 and 5 of the drawings.

13. Electrical discharge machining apparatus substantially as described herein with reference to Figures 1,2,4 and 5 of the drawings.