The present invention relates to the working of electrically conductive materials by electrical erosion, and more particularly to such working which utilises a segmented electrode arrangement which can be placed adjacent to a workpiece 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 (e.g. 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.

It has previously been proposed to utilise one-piece electrodes in electrical erosion. The elec¬ trode is placed adjacent to the work piece to be ma¬ chined and a voltage is applied across the gap between the electrode and the workpiece to cause electrical discharge across the gap and resultant erosion of the workpiece surface. When applying surface texture to,

for example, elongate roils to be used in the sheet metal industry it is necessary to have a corresponding elongate electrode.

The amount of metal to be removed from the workpiece to produce the required surface roughness in a reasonable machining time is dependent on the elec¬ trical power applied. Increasing the electrical power will cause the surface roughness to be altered. There¬ fore, in order to achieve a desired surface roughness in a minimum machining time it has been proposed that a segmented arrangement of electrodes is utilised in place of a single elongate electrode. A segmented electrode is an electrode divided into a plurality of segments which are insulated electrically from each other and are respectively connected to separate power lines to thereby provide discharges between each independent segment and the workpiece surface. This technique is the "multi channel approach". By this arrangement a specific surface roughness can be achieved in minimum machining time. However, the use of segmented electrodes has led to the problem 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 noticeable on the workpiece itself but when the workpiece is, for example, a roll to be used in milling sheet metal the defects will be reproduced on the sheet metal and will be noticeable.

It has been previously proposed in United Kingdom Patent number 856340 to camber the side of the electrode segments such that the insulating gap between each pair of segments is angled obliquely with respect to the axis of oscillation or rotation of the elec¬ trode, such that during operation of the electrode the break or line between each segment continually changes

its relative position with respect to the workpiece.

This arrangement may reduce somewhat the intensity of any striping, but striping will remain a problem. This technique tends merely to produce stripes which are spread over a broader area corre¬ sponding to the areas of the workpiece which the gap between the electrodes move relatively over during operation.

There is thus still a need to produce elec¬ trical erosion equipment which will minimise both machining time and the production of "stripes".

We have found that by greatly reducing the length of the electrode with respect to the length of the roll whose surface is to be machined and by traversing the electrode along the length of the roll machining times can be reduced and striping eliminated. As before, by dividing the electrode into a number of elements and providing a separate power supply for each electrode element the machining time can also be further reduced. According to one aspect of the invention, the electrode is mounted in a head with the electrode constituted by a plurality of electrode elements arranged, in use, around a position of the circumference of a roll, each electrode element being provided with a respective drive means for moving the element along an axis which is a radius of a circle common to the elements.

Preferably, the electrode elements are copper rods of circular cross-section whose axis lies on the radii of the circle. The rods may be aligned in a direction parallel to the axis of the roll, but are preferably arranged such that in a direction normal to the axis of the roll, the centres of the rods are not in alignment.

The advantage of this arrangement is that

there is no need to machine the end of the electrode elements to match the roll profile. This in turn facilitates the use of a wider range of roll diameters to be machined without the need to replace or re- machine the electrode elements. The diameter of each element is kept as small as possible.

If desired the circular cross-section elements can be rotated about their axes.

The flow of dielectric to the gap between the electrode is preferably controlled in response to monitoring of the machining performance using the emitted RF in the gap and voltage across the gap.

In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawing, in which: -

Fig. 1 shows diagrammatically an apparatus according to the present invention;

Fig. 2 shows a front view of an electrode head assembly as used in Fig. 1 ;

Fig. 3 shows a section side view taken along the line X-X in Fig. 2 ;

Fig. 4 shows a diagrammatic side view similar to that of Fig. 3 but of an atternative disposition of the electrodes head assembly.

Figure 5 shows a modification which may be made to a part of the arrangement shown in Figs. 3 or 4.

One of the basic concepts of the present invention is that an electrode is formed by a plurality of electrode segments bundled together to form a machining surface the length of which is relatively short having regard to the length of the workpiece which is usually a roll for use in a steel mill for texturing the surface of the roll. As a consequence,

it is necessary to traverse the electrode assembly along the length of the workpiece so that the whole workpiece or specifically determined areas thereof can be machined. The means for moving the electrode assembly along the length of the workpiece can be a conventional lead screw or recirculating ball and nut arrangement or in fact any other arrangement which provides a smooth travel of the electrode assembly.

With reference to Figure 1 an electrode 1 to be described in more detail later is shown adjacent an electrically conductive workpiece 2 , which may be a roll intended for use in the milling of sheet metal. A slide unit 3, which may be servo driven, is arranged to move the electrode 1 bodily towards or away from the workpiece 2, as shown by arrow A to allow a roll to be moved in the machine. Sensors 4 and sensor circuitry 11 are arranged to monitor the orientation of the electrode 1 with respect to the workpiece 2, and to monitor various conditions of electrical discharges occurring in the gap between the workpiece 2 and electrode 1 such as gap voltage. Information provided by the sensors 4 is used to control the electric erosion process. A further drive circuit 5 is provided to traverse the electrode to and fro along the length of the workpiece 2, as represented by arrow C.

In more detail, the workpiece 2 is arranged to rotate (arrow B) and it is covered continuously with a thin film of a dielectric liquid from a dielectric supply 8. The electrode consists of a number of conductive elements, for example graphite or copper separated by insulation. An electronic oscillator unit 6 is arranged to generate electric pulses of selected pulse width (on time and off time) chosen via a keypad input 7, and under the control of a control unit 8. A switching unit 9 is arranged to switch electric power

from a power supply 10 across the gap between the electrode 1 and workpiece 2 in accordance with the pulses from the oscillator 6. The power supply may be a power supply such as, for example the TransTec EDM/EDT 30 amp 150 volt. The machining performance is monitored by the various sensors 4 to control the electric erosion process via sensor circuitry 11. The slide unit 3 controls the displacement of the electrode 1 with respect to the workpiece 2, in particular it drives the electrode so that it is held at a datum position, with respect to the workpiece possibly via a servo head (not shown) , to a fixed position for any roll diameter within a specified range. Each electrode element is provided with a separate servo unit 3A and depending on the machinery and other parameters, the electrode elements may be positioned individually closer to or further away from the workpiece. Different distances will give difference finishes to the workpiece, or will cause the workpiece to be machined faster or slower. The sensors 4 will monitor the condition of the electrical discharges occurring in the gap, (e.g. by monitoring gap voltage) and the sensor circuitry 11 and control unit 8 can decide whether the conditions are in accordance with the machining parameters and if not action can be taken (e.g. via the oscillator servo unit or manually) to make corrections until the conditions are in accordance with the chosen machining parameters.

A display may be provided giving a visual display of the conditions so that action may be taken manually.

The switching unit 9 preferably includes MOSFET power devices to control switching. These have the advantage of being fast, voltage controlled devices which can be directly driven by logic circuitry, and

have positive temperature coefficients.

In operation, the slide unit 3 moves the electrode 1 towards the workpiece until it reaches a datum position with respect to the roll axis. The individual electrode segments are then moved radially with respect to the axis of rotation until sparking occurs and monitoring of the condition of the electrical discharges in the gap allows control of the servo units driving the electrode segments and the number of voltage pulses applied.

The sensors 4 may include RF detectors for detecting machining conditions in the gap between the electrode 1 and the workpiece 2. The sensors also include an arrangement for monitoring the surface of the workpiece in order to determine the quality of the machining and hence when numbering may be terminated. At the present, determining when to terminate machining is carried out by the operator using skill and experience.

We propose to use an optical arrangement as a detector. This requires a source of light e.g. laser light which is caused to project a beam of light on to the surface of the workpiece. A light detector is allocated to detect the light scattered from the surface. The sensor circuitry 11 is arranged to monitor the output from the light detector and provide an indication to the control unit 8 which terminates machining when the sensor circuitry detects an appropriate signal pattern from the light detector.

In addition the signal pattern from the light detector can be interpreted to give information concerning areas of the roll which are not machined to the required surface finish. This positional information can be used to position the electrode head in the areas requiring more machining by movement of

the head with the traverse drive arrangement, 13 and 14.

As mentioned above, the electrode 1 is arranged to be traversed along the length of the workpiece 2 which is in the form of a roll to be used in the sheet metal milling industry. Such rolls are typically 98 inches long and it is envisaged that the electrode will only extend along 12 to 20 inches of that length. The construction of one form of electrode is shown in more detail in Figures 2 and 3.

The electrode 1 comprises a mounting plate assembly 21 to which are fixed a number of electrode elements 22. Each element is elongate with one end mounted on the mounting plate assembly 21 and the other end arranged to face the workpiece along the radius whose centre lies on the axis of rotation of the roll. In this embodiment, the electrode elements are cylindrical copper rods of circular cross-section provided with an axial bore 23 extending the full length of the segment. As is shown most clearly in Figure 2, although the electrode elements are arranged on a rectangular grid pattern the overall pattern is angled with respect to the horizontal so that the spaces between electrode segments do not form a line which is either parallel or at right angles to the direction of traverse. This arrangement reduces the striping effect and decreases machining time. It is possible to have the segments horizontally aligned but vertically disposed as above.

The bores 23 are used to conduct dielectric fluid to the gap between the electrode 1 and the workpiece 2. Additional conduits may be provided to supply further dielectric to the gap if desired.

As shown more clearly in Figure 3, the electrode 1 comprises a head member 21 to which a

plurality of electrode elements 22 are mounted. The head member 21 is movable towards and away from the roll 2 by means of a servo motor or a servo hydraulic valve (not shown) driving a threaded member 23. A limit switch 24 is provided so that the electrode head member 21 is brought to a datum position with respect to the axis of rotation of the roll 2.

As mentioned above, each electrode element comprises a copper rod 22 and each arranged along a radius of a circle whose centre lies on the axis of rotation of the roll 2. Each rod 22 is provided with its own electrical and dielectric fluid supply, the dielectric flowing through the an axial bore 23 in the rod to the end face opposite the surface of the roll. Further, each rod 22 is provided with its own servo drive 3A which is under the control of the sensor circuitry 11, EDT oscillator 6 and keypad 7 shown in Figure 1. Once the head member 21 is in the datum position, each rod 22 is individually controlled in order to optimise texturning of the roll.

The working surfaces (ends) of the rods 22 are all of the same configuration and if the diameters of the rods are very small as compared with the diameter of the roll, the ends can be plain and unprofiled. It may be advisable in some circumstances to have slightly curved ends to more accurately conform to the circumference of the roll. In any event, the elements form an arcucate working surface.

As shown in Figure 3, there are four electrode elements mounted one above the other. They are not, in fact, aligned vertically. Further, they are displaced axially along the length of the roll as shown in Figure 2 so that the centres of the rods are not aligned in order to avoid striping. The elements are, however, all disposed on radii of the same circle

- 1 0 -

whose centre, in this case, lies on the axis of rotation of the roll to be machined. The electrode elements provides an arcuate working area which is determined by the number and diameter of elements. The working area as shown, occupies an arc of some 30°. This arc could be longer and it is preferred to locate the arcucate working area at least partially in the upper part of the roll circumference as it is here that the best flushing of the gap between the elements and the roll surface will occur if the direction is supplied from nozzles (not shown) located near the top of the roll.

A tank for dielectric could be provided on the head member 21 with the roll surface forming one wall of the tank. This would enable further electrodes elements to be displaced around the lower half of the roll. A tank could be provided either as a replacement for continuous feed nozzles or in addition to such nozzles.

Alternatively individual dielectric retention rings may be used to overcome the inconvenience of a bath. The action of a dielectric retention ring is to form a reservoir of dielectric at the machining zone between electrode and workpiece and in doing so undertake the function of a bath by creating local reservoirs of dielectric ensuring that the spark gap in the machining process is completely immersed in dielectric fluid and that no sparks escape out from the dielectric fluid.

Figure 5 shows a dielectric retention ring which comprises 61 a spring loaded seal which locates on the diameter of rod 22, 62 a nylon PTFE or other suitable material backing plate to position the sealing mat 63 so as to form an anular pocket 64. In operation the dielectric retention ring is positioned near the

end of rod 22. When machining is taking place the sealing mat 63 is in contact with the roll 2 forming a partial seal between the dielectric retention ring and the roll. Dielectric fluid is supplied through axial bore 23 and fill the anular volume 64, the sealing mat 63 acts as a partial seal and restricts the flow out of the dielectric retention ring thus forming a reservoir of dielectric fluid to completely immerse the spark gap, the seal ring 61 restricts fluid flow along the rod 22. Axial movement of the dielectric retention ring automatically takes place to compensate for wear of the electrode rod 22 through the restriction of forward movement of the dielectric retention ring by roll 2 and the servo movement radially with respect to the axis of rotation of roll 2. The spring loaded seal ring allows the dielectric retention ring to move in these circumstances, but in all other situations keeps the dielectric retention ring by roll 2 and the servo movement radially with respect to the axis of rotation of roll 2. The spring loaded seal ring allows the dielectric retention ring to move in these circumstances, but in all other situations keeps the dielectric retention ring positioned at the end of rod 22.

Preferably, the electrode head is provided with the electronic control circuits for controlling the servo unit and the switching units for the electrode elements themselves. This enables a simpler connection between the main control unit, which fuses the power supply 10, EDT oscillator 6 and control unit 8, and the electrode head.

The electrode 1 extends in an axial direction a distance of the order of about 5 electrode element distances if one is using the arrangement shown in Figure 2 with four "columns" each of four elements.

Because the electrode element is individually servo- driven, care has to be taken at the ends of the roll to ensure even texturing. This may be achieved by monitoring the position of the electrode 1 as it transverses the length of the roll and either slowing it down at the ends of the roll or else over- transversing the electrode so that each of the electrode elements 22 is operable to the very edge of the desired texture surface. In the latter case, it is desirable to inhibit sparking of those electrode elements which have passed beyond the edge of the desired textured surface and to prevent overdriving of the servos of those elements. It will be appreciated that in the latter case also, a bath for dielectric cannot be provided on the head but that dielectric retention rings will continue to function on each individual rod 22 up to the edge of the desired textured surface.

Turning now to Fig. 4, this shows diagrammatically a modification which can be made with advantageous results to the arrangement shown in Fig. 3.

In Fig. 4, the electrode assembly is the same as that described in relation to Fig. 3 and consequently the same reference numerals are used to represent the same parts. The difference lies in the fact that the centre 40 of the circle on whose radii lie the electrode elements 22 is now no longer located on the axis of rotation 41 of the roll 2 to be machined. The centre 40 of the circle is moved a small distance e.g. 2.5cm (1 inch) away from the axis of rotation of a roll of some 65-70cm diameter. Whether the centre of the circle is now located above or below the axis of rotation will depend on the location of the electrode assembly and the direction of rotation of the

rotation of the roll so as to provide easy removal of debris from the gaps between the electrode elements and the roll surface which might otherwise tend to jam the gap. In Fig. 4, with the roll 2 rotating in an anti¬ clockwise direction, the centre of the circle is located above the roll axis. Were the direction of rotation of the roll revered, i.e. to clockwise, it would be necessary to locate the centre of the circle below the axis for the electrode assembly located as shown.

The electrode assembly may be controlled in such a way that machining takes place in only one direction of traverse along the length of the roll or it may be carried out continuously with each traverse.

The exact cross section of the rods may be varied as can the shape of the machining surface when the segments are assembled. Further, more than one electrode assembly may be used simultaneously. In this event, the assemblies may be mounted on the same side of the roll or on opposite side of the roll. If mounted on opposite sides of the roll, the disposition of the electrode segments of one electrode arrangement can be altered with respect to the arrangement of the other electrode assembly so as to further minimise the risk of striping.

It is possible for the rods in both Figs. 2,3 and 4 to be movable and to orbit as the electrode assembly is transversed. This improves the surface finish.

Further, each rod is preferably individually movable towards and away from the surface of the roll either manually or under servo-control or both.

To ensure that each electrode segment matches the roll profile, each individual element is connected to a voltage source via a respective indicator. Each

segment can then be advanced until it contacts the surface of the roll which is earthed to cause the indicator such as a lump or LED to be lit.

Preferably, each element is provided with its own servo-control for moving it towards and away from the roll. This means that not only can the setting up of the electrode assembly be carried out automatically but during machining each segment can be individually controlled in the same way as has been proposed previously.

With both the above described arrangements of electrode it is envisaged that the electrode will be mounted on a pivot permitting the head to remain normal to the surface of the roll. It is to be appreciated that a roll for use ^' in steel manufacture is not in fact a right cylindrical member but in fact has a barrel- shaped surface being of large diameter in the middle of the roll than at the ends. In this case, the sensors 4 can be used to monitor the gap voltage of the electrode segments at the extreme edges of the electrode and monitor any difference in gap voltage and hence correct the attitude of the electrode with respect to the surface of the roll in order to keep the electrode segment voltages equal.

It is also possible if the electrodes are individually movable under servo-control for the individual segments to be moved rather than permitting the electrode to rotate bodily about the pivot.

Further, the supply of dielectric can be automatically controlled in response to monitoring of the efficiency of machining using RF sensors. In this case, any detection of difficult machining conditions will be detected by the control unit 8 and cause a greater supply of dielectric from the dielectric supply 12 to the gap in order to ensure adequate flushing of

the gap .

It is also envisaged that a mechanical wiper could be located against the surface of the workpiece in order to remove debris from the surface and this has been found useful in some circumstances where large quantities of material are being removed quickly.

To accommodate any small variation in the diameter of the workpiece, the gap between the electrode 1 and the workpiece 2 should be kept as wide as possible. To improve the spark gap, the following modifications are proposed:- a) a metallic electrode should be used rather than a graphite electrode; b) the power supply should be capable of working at a much higher voltage than normal, e.g. 300 volts rather than 150 volts or a composite waveform consists of a short pulse of 300 volt at a low current level, superimposed in the machining pulses selected according to the surface requirements at 150 volt; and c) the servo-control unit 3 should be tuned to work at the wider gap and higher voltage at least initially until such time as any eccentricity in the roll surface has been removed.