FI ELD OF TH E I NVENTION

The present invention relates to a device and a method for machi ning an electrical cable having a cyli ndrical part and a conical part adjoining the cylindrical part. The invention is particularly suitable for mach ining high and medium voltage cables

PRIOR ART

A high or medi um voltage electrical cable essentially comprises a conductor, a conductor screen su rrounding the conductor and an electrical insu lation layer surroundi ng the conductor screen . Hig h voltage electrical cables have robust di mensions with typical outer diameters between 70 to 1 20 m m . When there is a need to joint two cables, one way is to prepare a flexible vulcanized joi nt between the cable ends. The conductor ends are nor- mal ly joi nted by welding , after wh ich the insu lation layer is restored by melting and curing an i nsu lation material in a pressu rized mould arranged around the cable. For enabling the welding of the conductors, the cable ends have to be stripped. I n order to improve the subsequent restoration of the insu lation layer, it is a common practice to give the stripped cable ends a specific form comprising a conical i nsulation layer end. The tapering of the insulation layer is conventionally done with a tool known as a "pencil sharpener". The tool comprises two cutting edges arranged in the same plane as the axis of the cable, but inclined with respect to the axis of the cable. The cutting edges are tu rned about the cable axis. This tool achieves a sharp transition between the conductor screen and the insulation cone. The transition forms a cone edge that is conventionally shaped usi ng a knife. I n consequence, the cone edge often appears as a circular row of facets. The surfaces are finally given a finishing touch by grinding .

The geometry of the cone edge has a great significance to the cable performance as experience shows that most electrical breakdowns in flexible vu lcanized joi nts at higher voltages occur at the cone edge. I n presence of irregularities like facets following from shaping the cone edge with a knife, the breakdowns are initiated at the edges of the facets. Moreover, irregularities on the cone su rface may lead to locally en hanced electric field strength and can also initiate an electrical breakdown . The described conventional method for stripping an electrical cable is unsatisfactory in that the quality of the resulti ng cable joint is strongly dependant on the skil ls of the person shapi ng the cone edge.

J P06205520 discloses a device for stripping cables enabling formation of a tapered su rface of a cable. The cable is held between a pair of holding bodies. The device comprises a cutter arranged for removing material from the cable, a movable guide frame movi ng the cutter i n the direction of the radi us of the cable at the same time as the cutter is moved along the cable axis, and rotational guiding means for guidi ng a rotation of the cutter about the longitudinal axis of the cable at the same time as the cutter is moved along the longitudinal axis of the cable. This stripping device achieves a helical cutting of the cone, and accordingly the surface of the cone obtains a screw pattern . Fu rther, this device achieves a sharp transition between the cylindrical conductor screen and the cone shaped part of the insula- tion layer. I n order to avoid electrical breakdown in the cable joi nts, the cable has to go through a machini ng step i n which the cyli ndrical surface of the conductor screen , the cone edge, and the su rface of the cone is grinded in order to ach ieve a soft transition between the cyli ndrical part and the conical part of the cable and a smooth surface of the cone as well as the conductor screen . Thus, it is difficu lt to achieve a satisfactory cable joi nt qual ity with the currently available tools. OBJ ECTS AN D SU MMARY OF TH E I NVENTION

The object of the present i nvention is to provide a device for machini ng a cable in order to achieve a soft transition between the cyl indrical part and the conical part of the cable and a smooth surface of the cone to improve quality of cable joints.

This object is achieved by a device as defined in clai m 1 .

Such a device comprises a fixture arranged to receive the cable and to hold the cable i n a fixed position , the fixture defines an elongated space for accom modating the cable, a tool for removing material from the cable incl uding at least one elongated cutter, and rotational guidi ng means for guiding a first rotation of the cutter about a longitudinal axis of said space. The cutter is elongated and the longitudinal axis of the cutter is arranged incli ned with respect to the longitudinal axis of the space so that the angle between the longitudinal axes of the cutter and the space is between 1 0 and 80 degrees. The device further comprises drive means for providing a second rotation of the cutter about a rotational axis th roug h the tool in order to cause the machini ng , and the cutter is adapted to be in contact with the surface of the conical part of the cable as well as the transition between the cylindrical part and the conical part du ring the machi ning .

The device according to the invention uses a cutter that is rotated at a rather h igh cutting speed during the machini ng of the cable. The device makes it possible to machi ne the surface of the cone and the cone edge in one movement, i . e. by rotating the cutter one turn around the cable. The device achieves a smooth surface of the cone and a soft transition between the cone and the cylindrical part, which is necessary for providing as perfect electrical conditions as possible.

Accordi ng to an embodiment of the invention , the device further comprises a radial adj usti ng means for adjusting the distance of the cutter from the axial centre of said space. The device may further comprise an axial adjusting means for adj usting the position of the cutter along the longitudinal axis the space. Before the machining begins, the cutter m ust be positioned so that it is in contact with the surface of the conical part of the cable as wel l as the transition between the cylindrical part and the conical part in order to achieve the desi red resu lt of the machini ng . The radial and axial adjusting means facilitates the adjustment of the position of the cutter relative the cable.

Accordi ng to an embodiment of the invention , the angle between the longitudinal axis of the cutter and longitudinal axis of the space is between 30 and 60 deg rees. The longitudinal axis of the cable coincides with the longitudinal axis of the space when the cable is positioned i n the fixture. The angle between the longitudinal axis of the cutter and the longitudinal axis of the cable depends on the slope of the cone to be machined. With an angle between 30 and 60 degrees the typical cone shapes for cables are covered. Preferably, the angle between the long itudi- nal axis of the cutter and longitudinal axis of the space is adjustable.

Accordi ng to an embodiment of the invention , said drive means for providing the second rotation of the cutter is configu red to provide a cutti ng speed of the cutter that is larger than 0,03m/s. I n order to achieve a smooth machi ning the speed of the cutter must be rather high .

The rotational movement of the cutter about the cable can be made manually or automatically. According to an embodiment of the invention , the device comprises an actuator for providing the first rotation of the cutter. An actuator can perform a more even movement than an operator can perform by a manual movement.

Accordi ng to an embodi ment of the i nvention , the tool for remov- ing material comprises a cyl indrical body having a central axis, the cutter is provided on the body with its longitudinal axis parallel with the central axis of the body, and the cutter is arranged rotational about the central axis of the body. I n th is embodiment, the drive means is arranged to provide the second rotation of the cutter about a rotational axis parallel to a long itudi nal axis of the cutter. Such a tool is easy to manufacture.

Accordi ng to an embodi ment of the invention , the body has a radius larger than 5mm . The diameter of the body determines the shape of the transition between the cone and the cyli ndrical part. The transition will have almost the same radius as the radius of the cylindrical body. With a cyli ndrical body having a radius larger than 5mm a soft transition is ach ieved. Accordi ng to an embodi ment of the invention , the cutter has a length larger than 40mm . I n order to make it possible to machine the surface of the cone and the cone edge i n one movement the length of the cutter should cover the surface of the cone as well as the cone edge. With a cutter havi ng a length larger than 40mm this is achieved for the typical cone sizes of electrical cables.

Another object of the present invention is to provide a method for machi ning a cable by means of a device according to the in- vention , in order to achieve a soft transition between the cylindrical part and the conical part of the cable and a smooth surface of the cone to improve quality of cable joints.

This object is achieved by a method as defined i n claim 1 1 . The method comprises positioning the cable into the fixture, adjusting the position of the cutter so that the cutter is in contact with the surface of the conical part of the cable as well as the transition between the cylindrical part and the conical part, and rotating the cutter about a rotational axis through the tool and at the same time rotating the tool about the longitudinal axial of the cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures. Fig. 1 shows an example of a stripped cable.

Fig. 2 shows a perspective view of a device for machining an electrical cable according to an embodiment of the invention. Fig.3 shows a side view of the device shown in figure 2.

Fig.4 shows a side view of the device holding a cable to be machined. Fig. 5 shows the device holding the cable in a view seen from above.

Fig. 6 shows a perspective view of a device according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION.

Figure 1 shows an example of a stripped cable 1. The cable comprises a conductor 2, a conductor screen 3 surrounding the conductor, and an electrical insulation layer 4 surrounding the conductor screen 3. The conductor 2 and the conductor screen 3 form a cylindrical part 5. The insulation layer includes a conical part 6 adjoining the cylindrical part 5. The transition between cylindrical part 5 and conical part 6 includes a cone edge 7. The cone edge 7 is a part of the transition between the cone 6 and the cylindrical part 5.

Figures 2 - 5 show a device for machining a cable 35 according to an embodiment of the invention. Figures 2-3 show the device without a cable and figures 4-5 show the device holding a cable. The device comprises a fixture 10 arranged to receive the cable and to hold the cable in a fixed position. The fixture includes a first and a second holding element 12, 13 arranged at a distance from each other and adapted to receive the cable and to hold the cable in a fixed position. Each of the holding elements 12, 13 is provided with an opening for receiving the cable and fixating means, for example, in the form of tightening screws 14, for fixating the cable in a define position. Preferably, the holding elements are divisible so that the cable can be entered into the fixture from above. Accordingly it is not necessary to enter an end of the cable through the openings. The fixture defines an elongated space 15 for accommodating the cable. The space 15 has a longitudinal axis 16, which coincide with the longitudinal axis of the cable when the cable is fixated in the fixture.

The device further comprises a tool 17 arranged for removing material from the cable. The tool includes a cylindrical body 20 in the form of a drum having a central axis 21. The drum is provided with at least one elongated cutter 22 arranged with its longitudinal axis parallel to the central axis 21 of the drum. The drum may comprise more than one cutter. In this embodiment two cutters are arranged parallel to the central axis 21 of the drum and on opposite sides of the drum. The drum is arranged rotational about its central axis 21, and accordingly the cutter or cutters are arranged rotational about the central axis 21 of the drum. The central axis 21 of the drum, and accordingly the Ion- gitudinal axis of the cutter, is arranged inclined with respect to the longitudinal axis 1 6 of the space so that the angle v between the central axis 21 of the dru m and the longitudi nal axis 1 6 of the space is between 1 0 and 80 degrees, preferably between 30 and 60 degrees and more preferably between 40 - 50 degrees. The device is provided with drive means 24 including a motor for providing the rotation of the drum 20 about its central axis 21 and accordingly the rotation of the cutter 22. The drive means 24 for providing the rotation of the cutter is configured to pro- vide a cutting speed of the cutter larger than 0,03m/s.

The device further includes a frame 25 i ncluding a first and a second ring shaped element 27, 28 arranged close to the holding elements 1 2, 1 3 of the fixture and includi ng openings for re- ceiving the cable, and two rods 30 arranged between the ring shaped elements 27, 28 and parallel to the long itudi nal axis 1 6 of the space of the fixtu re. The tool 1 7 i ncluding the cutter 22 is con nected to the frame 25. The frame 25 is arranged rotatable relative the fixture 1 0 and accordingly rotatable relative the ca- ble when the cable is held by the fixture. The frame 25 is arranged rotatable relative the fixtu re 1 0 in such a way that the dru m 20 and accordingly the cutters 22 are rotated about the longitudinal axis 1 6 of the space. The cutti ng speed of the cutter is typical ly 20 meter per minute. I n this embodiment, the device comprises an actuator 32 for providing the rotation of the frame relative the fixture. I n this embodi ment the rotational speed of the tool about the longitudinal axis of the cable is about one turn per second. I n an alternative embodiment, the rotation can be made manual ly.

The device comprises axial adjusting means for adj usti ng the position of the cutter along the longitudinal axis 1 6 of the space and according ly along the long itudi nal axis of the cable. I n this embodiment, the axial adjusti ng means incl udes a carriage 34 arranged linearly movable along the rods 30, wh ich are arranged parallel to the longitudinal axis of the space. Accordingly, the carriage is movable in a direction parallel to the direction of the longitudinal axis of the space. The axial adjusti ng means also includes means for locki ng the carriage in a desired position along the rod. The tool 1 7 incl uding the drum 20 and the cutters 22 are arranged on the carriage 34.

The cutter is adapted to be in contact with the su rface of the con ical part as well as the transition during the machin ing , as shown in Fig . 5. The radiuses of the drum as well as the length of the cutter are important features for the resu lt of the machining . The device will provide a soft transition between the cylindrical part and the conical part, the transition having about the same radius as the drum . The radi us of the cutter is preferably in the order of 1 5mm . The cutter 22 and the drum 20 are elon- gated and should preferably have a length which is longer than the lengths of the con ical part of the cable including the transition between the conical part and the cylindrical part. This makes it possible to perform the machining by simply tu rning the tool one turn around the cable.

Fig ure 6 shows a device according to an embodiment of the invention . I n this embodiment the device is provided with a mechanism for moving the tool and accordingly the cutters in an axial direction of the cable. As seen in the figure, the cable in- eludes two conical parts and an intermediate cylindrical part. I n order to provide machi ning of both conical parts and the cyl indrical part, the tool 1 7 is moved along the longitudinal axis of the cable. The movi ng mechanism means can be desig ned in many different ways. I n this embodiment, the moving mechanism includes a screw 43 extendi ng through the frame 1 0 and connected to a threaded piece 36. The feeding mechan ism further includes a th readed gear wheel 37. The gear wheel 37 is opera- tively connected to the carriage 34 and is desig ned to fit to the threads of the piece 36. The carriage 34 is arranged movable along the rods 30. When the carriage 34 incl uding the tool 1 7 has been moved one turn arou nd the cable, the carriage is moved a distance along the axis of the cable in order to be able to machi ne another part of the cable. I n one embodiment of the invention , the moving mechanism is arranged to automatical ly move the tool a predetermined distance along the axis of the cable at each turn the frame 25 is rotated relative to the fixture 1 0.

The device further comprises radial adjusting means for adjusting the distance of tool i ncluding the cutters 22 from the longitu- dinal axis 1 6 of the space and accordingly for adjusting the distance between the cutter and the cable. This distance determines the mach ining depths. The radial adjusting means can be designed in many different ways. I n this embodiment the radial adj usting means incl udes an adjusti ng element 40 provided with an elongated hole 41 and at one end connected to one of the rods 30 and at the other end movably connected to the carriage 34. The adjusting means further incl udes a screw 42 for locking the tool in a desi red radial position . The adj usting element 40 is arranged movably with respect to the carriage 34 in a radial di- rection of the cable thereby achievi ng an adjustment of the distance between the cutter and the cable.

Now, the use of the cable stripping device accordi ng to the figures is described in detail . The insulati ng layers of two cable pieces are first roug hly tapered e.g . by usi ng the conventional "pencil sharpener" or the stripping device disclosed in J P06205520. Some centimeters of the conductor tip is exposed by removing the conductor screen , but on the rest of the conductor tip the conductor screen is left u ntouched under a thin insulati ng layer. The two cable pieces are connected to each other to form a cable 35, and the joint between the cable pieces is covered with an insulating material . The cable 35 is positioned in the fixture as shown in fig ure 4 and 5, and locked i n the fixture by applying the tightening screws 1 4. The position of the cutter is adjusted in the axial direction of the cable by moving the carriage 34 along the rods 30 and locking the carriage when the cutter is in a desired position relative the cable. The position of the cutter is adjusted in the radial di rection of the cable by adjusting the position of the element 40. The position of the tool is adj usted so that the cutter is in contact with the surface of one of the conical parts of the cable as wel l as the transition between the cylindrical part and the conical part, as shown in figure 4 and 5. Preferably, the cutter is i n contact with the surface of the conical part along the enti re length of the conical part i n order to provide mach ini ng of the entire cone in one step as well as i n contact with the cyli ndrical part endi ng at a smal l distance from the cone edge in order to provide a smooth transition between the cyl indrical and the con ical part.

When the position of the tool has been adjusted in a desired manner, the drum 20 is rotated about its rotational axis 21 with a fixed rotational speed at the same time as the frame 25 is rotated relative to the fixture 1 0 about the long itudi nal axis of the cable. Accordingly, the rotati ng cutter is moved along the su rface of the cone and along the transition between the cyli ndrical and conical part i n a direction around the cable axis. If the cutter has been adjusted to be in contact with the su rface of the con ical part along the enti re length of the conical part it is enough to rotate the frame one turn about the cable axis. However, i n order to further improve the resu lt of the machin ing the frame can be rotated more than one turn . If necessary, the distance between the cutter and the cable can be adjusted between each turn in order to adjust the machini ng depth .

When the mach ining of the cone is finished the tool is moved to a new position along the axis of the cable so that the cyli ndrical part of the cable can be machined. When the position of the tool has been adj usted in a desi red man ner, the tool is rotated about the rotational axis 21 at the same time as the frame 25 is rotated relative to the fixture 1 0 about the long itudi nal axis of the cable. When the machini ng of the cyli ndrical part has been finished, the tool is moved to the other conical part and the same machini ng procedure as described above is repeated for the second cone.

Accordi ng to an embodiment of the invention (not shown) the operation of the device is automated. The automation involves providing the device with distance sensors and servo control i n order to replace man ual operation .

The present i nvention is not limited to the embodi ments disclosed but may be varied and modified within the scope of the following claims. For example, the fixture and the frame can be constructed in many different ways. Further, according to an embodiment of the invention , the drum is shaped as a cone and the tool is arranged rotational about the central axis of the cone. I n that case the longitudinal axes of the cutters are not parallel with the rotational axis of the drum . It this embodiment is possible that the rotational axis of the drum is arranged parallel with the longitudinal axis of the space. However, i n that case the rotational axis of the drum is arranged at a distance from the longitudinal axis of the space.