WO1988004217A1

WO1988004217A1 - Optic fiber lapping or polishing apparatus

Info

Publication number: WO1988004217A1
Application number: PCT/GB1987/000896
Authority: WO
Inventors: Gareth Lloyd; Desmond Ernest Richardson
Original assignee: Engis Limited
Priority date: 1986-12-09
Filing date: 1987-12-09
Publication date: 1988-06-16
Also published as: GB8629400D0; AU1040388A

Abstract

Apparatus for finishing articles, particularly the ends of fibre optic cables, with a convex surface, comprises a lapping wheel (61) rotatable about a vertical axis and having a concave cavity (90) with a radius of curvature corresponding to the desired radius of curvature for the surface of finished articles. A carriage (30) holds the articles against the surface of the cavity and is mounted with a ball bearing (75) for universal swivelling movement. The centre axis of the cavity (90) is offset from the axis of rotation of the lapping wheel so that as the wheel rotates and the cavity moves around the axis of the rotation of the lapping wheel the carriage is caused to rock and the articles move over the surface of the cavity.

Description

Optic fiber lapping or polishing apparatus.

The present invention relates to improvements in lapping or polishing apparatus and is concerned especially with lapping or polishing apparatus for use in preparing the ends of optical fibres. Optical fibres are used in many applications such as telephone systems and computer data transmission cables. When joining lengths of optical fibre cable it is important that a good light- transmitting connection be formed in order to avoid light- transmission losses. Usually the ends of the optical fibres are finished flat and polished to reduce reflection. Connectors for joining the lengths of cable are usually constructed so as to position the cables to be joined with the end faces of the optical fibres parallel to one another but spaced apart axially by a small distance, typically 6 microns. A problem with this arrangement is that if the axes of the cables are tilted relative to one another so that the end faces are not parallel, increased light losses occur. To overcome this problem some optical fibres are now finished with a part-spherical or other none-flat shaped end surface. The connectors for such optical fibre cable are designed so that when connected together the ends of the adjacent optical fibres touch. Such an arrangement can accommodate small inaccuracies in the axial alignment of the cables without substantially increasing the light losses. However in order to avoid significant light losses it is necessary to achieve a high degree of polish on the ends of the optical fibre cables and ensure that the centre of curvature of the end face lies substantially on the axis of the fibre cahle. This requirement makes finishing the ends of the fibre cable more difficult and expensive than with the conventional flat-ended cable. In one known apparatus for polishing the ends of optical fibre cables to a spherical finish three cables are mounted in three grooves in the edge of a generally triangular carrier plate. The axes of the grooves all converge towards a point above the plate which lies on an axis passing perpendicular through the middle of the carrier plate. The cables are clamped in the grooves by bridging members which straddle the cable and are screwed into the edge of the plate. To lap and polish the ends of the optical fibre cables on this carriage to a spherical shape, the carriage is lowered on to a lapping or polishing wheel which has a concave annular lapping surface of the desired radius of curvature. Abrasive paste is applied to the surface of the wheel with a brush or with a spray and the lapping or polishing wheel is then rotated about an axis passing through the centre of curvature of the lapping or polishing wheel surface. The carriage plate is positioned so that its vertical axis coincides with the axis of rotation of the lapping/polishing wheel.

A disadvantage of such an arrangement is that all wear on the lapping/polishing wheel surface is confined to a narrow zone. In practice only a small number of fibre optic cables can be polished with each lapping wheel before the surface becomes out of true and the lapping/polishing wheel has to be discarded or reconditioned by grinding. Typically, only nine fibre optic cables can be polished before the wheel has to be replaced.

According to, the present invention in a first aspect, lapping or polishing apparatus for finishing articles with a convex surface comprises a lapping or polishing plate rotatable about an axis, a concave cavity in the surface of the wheel with its axis offset from the axis of rotation of the wheel, the radius of curvature of the cavity corresponding to the radius of curvature desired on the finished article, and means for holding one or more workpieces against the surface of the cavity, said means permitting universal swivelling movement of the workpiece, whereby the workpiece is caused to rock about its pivot point as the cavity moves around the rotation axis of the lapping or polishing plate and the workpiece thus moves over the surface of the cavity. When the workpieces are fibre optic cables preferably there are at least three cables carried by the holding means and the assembly of cables and holding means is supported on the surface of the cavity by the ends of the cables.

With this arrangement, the area of the cavity which is used for lapping or polishing the workpiece is greater than that of the known arrangement. This is particularly useful where the workpiece is a plurality of fibre optic cables. By spreading the wear over a larger area of the lapping or polishing plate the life of the lapping plate is substantially increased.

When the lapping/polishing plate is so worn that it can no longer be used to produce a satisfactory finish it may be discarded or it may be redressed on a lathe. Where the lapping/polishing plate is a circular disc with the axis of concave cavity offset from the centre axis of the disc this can be an inconvenient and difficult process because the lapping/polishing plate must be mounted on the lathe with the axis of the cavity, and not the centre axis of the plate, mounted along the turning axis of the lathe. According to the present invention in a second aspect, a method for reconditioning or trueing a lapping/polishing plate in apparatus as described above comprises replacing the said holding means with a tool of the desired radius of curvature and operating the apparatus with the tool in the cavity. This method enables the plate to be redressed without the problems of mounting the plate on a lathe as described above.

Reconditioning a lapping/polishing plate using the method described above may be inappropriate for some applications. It may for example produce too smooth a finish on the surface of the cavity for rapid grinding of the fibre optic cable ends.

An alternative embodiment of the invention uses a lapping/polishing plate in which the axis of the cavity coincides with the axis of the plate but the plate is mounted in the apparatus for rotation about an axis offset from the axis of the plate. Conveniently the apparatus has a turntable and a circular mounting plate is mounted coaxially on the turntable with its centre axis coinciding with the axis of the turntable. The lapping/polishing plate is removably mounted on the circular mounting plate with its centre axis offset from the axis of the mounting plate. Such a lapping plate can more easily be reconditioned on a lathe. It can also be made of smaller size using less material and is therefore less expensive to manufacture.

The means for holding the workpieces may comprise a known arrangement of a plate with grooves for receiving the ferrules of fibre optic cables running across its peripheral edge, and individual clamping means for holding a ferrule in each of the grooves. The clamping means may comprise a bridging member secured in position by two screws. This arrangement permits the axial positioning of the ferrules to be set individually. However, a disadvantage of the known arrangement is that it is difficult and time consuming to fasten the fibre optic cables in the grooves In the carriage. A jig may be required to ensure that the cables are correctly positioned and each cable has to be clamped in position with two screws. These disadvantages mean that finishing cable ends with the known apparatus is time consuming and expensive.

According to the present invention In a third aspect, a carriage for holding a plurality of fibre optic cables whose ends are to be lapped or polished, comprises a first plate with a plurality of bores extending therethrough, each bore having a counterbore at one end for receiving an enlarged portion of the fibre optic cable ferrule, and a second plate mounted against the side of the first plate which has the counterbores, the second plate having a series of holes corresponding with the holes of the first plate, each hole in the second plate being of generally key-hole shape, with an enlarged portion corresponding substantially in diameter with the diameter of the counterbore and a smaller portionof substantially smaller diameter than the diameter of the counterbore, the second plate being movable between a first position in which the enlarged portions are aligned with the counterbores to permit the ferrules of the fibre optical cables to be inserted through the second plate into the bores in the first plate, and a second position in which the smaller portions of the holes are aligned with the counterbores in the second plate whereby the fibre optic ferrules are retained in the carriage.

Where the carriage is to be used for finishing the ends of the fibre optic cables to a flat surface the bores in the first plate should be parallel to one another. Where the carriage is to be used for finishing fibre optic cables to a spherical surface the axes of the bores should lie on radii from a centre point positioned above the plate. The fibre optic cables should project so that their ends lie at the desired radius of curvature from the said centre point. Conveniently, the first and second plate can be mounted together so that they can rotate about an axis perpendicular to their surfaces whereby the second plate can be moved between the first and second positions by twisting the first plate relative to the second plate about the axis.

With this arrangement, mounting the fibre optic cables in the carriage is much simpler than with the known device. The second plate is rotated into the position where the enlarged portion of the hol e is aligned with the counterbore in the first plate. The cable ends are inserted into the bores so that the end of the fibre optic cable projects through the bottom of the first plate. When all the connectors have been inserted, the second plate is twisted relative to the first plate so that the small portion of the holes is brought into register with the counterbores and the ferrules are locked in position.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings of which:

FIGURE 1 shows a typical optical fibre cable with mounting ferrule;

FIGURE 2 shows a fibre optic cable connector; FIGURE 3 shows a vertical section through a carriage for supporting fibre optic cables in a lapping machine;

FIGURE 4 shows a plan view of the carriage of FIGURE 3; FIGURE 5 shows a plan view of the lower plate for the carriage of FIGURE 3;

FIGURE 6 shows a perspective view of a lapping/polishing machine;

FIGURE 7 shows a section through the lapping/polishing machine of FIGURE 6;

FIGURE 8 shows a detail of the section of FIGURE 7 on an enlarged scale;

FIGURE 9 shows a tool for reconditioning the lapping/polishing plate of the machine of FIGURES 6, 7 and 8;

FIGURE 10 shows a perspective view of an alternative arrangement for the lapping/polishing wheel of the machine of FIGURES 6 to 8;

FIGURE 11 shows a plan view of an alternative form of carriage; and

FIGURE 12 shows a section on the line 12-12 of FIGURE 11. Referring to Figure 1, this shows the end of a typical fibre optic cable 10. The cable consists of a central optical fibre 11, typically having a diameter of 8 to 10 microns and an outer sheath 12 of plastics material having an outside diameter, typically, of 125 microns. The optical fibre and the cladding have differing refractive indices such that light signals passing through the optical fibre are totally contained within the sheath by internal reflection. At the end of the fibre optic cable the plastics material sheath 12 is replaced by a ceramic sleeve 96. The end of the optical fibre is secured in a steel mounting ferrule 13 which has a cylindrical outer surface 14 coaxial with the optical fibre at its forward end and a peripheral flange 15 at its rear end for locating the ferrule axially in its connector. The front end face 16 of the fibre optic cable and ferrule assembly is finished to a convex spherical shape, the centre of curvature of the face lying on the axis of the optical fibre. Typically the radius of curvature of the end face is 60 mm, although other radii and even non-spherical non-flat shapes may be used in some instances.

Figure 2 shows how optical fibre cables as shown in Figure 1 are joined in a typical connector. Two cables 17 and 18, each terminating with a ferrule 13, are mounted respectively in male and female connector parts 19 and 20. An internally screw-threaded ring 21 which is captive on the female connector engages external screw threads 22 on the male connector to hold the two connector parts together. When the connector parts are properly connected together the end faces of the two fibre optic cables touch in the regions of their optical fibres so that light can pass from one cable to the other without crossing an air gap.

Figures 3 to 5 show a carriage 30 for holding optical fibres during the lapping/polishing process for producing the spherical end faces. The carriage 30 consists of two plates 31 and 32 pivotally connected together. The lower plate 31 has a central opening 33. The upper plate 32 has a hub 34 projecting downwardly from its centre which fits In the hole 33 so that the two plates can pivot relative to one another about a central vertical axis. A wave spring washer 36 is located around the hub 35 between the lower surface of the lower plate 31 and a circlip 37 which fits in an annular groove 38 at the lower end of the hub 34. The wave spring serves to hold the two plates together resiliently.

The upper surface of the lower plate and the lower surface of the upper plate are horizontal in their central regions 39 but are inclined upwardly and outwardly at an angle θ to the horizontal in their peripheral regions. Typically the angle θ is 20°.

Nine equi-spaced cylindrical bores 40 pass through the lower plate 31. The bores are all located in a circle at the same distance from the central vertical axis 35. The axes of the longitudinal bores 40 all pass through a common centre point 41 on the vertical axis 35 so as to make an angle θ to the vertical. Each bore 40 has a diameter at its lower end corresponding to the diameter of the cylindrical portion 14 of the ferrule and a counterbore 42 at its upper end of a diameter corresponding to the diameter of the flange 15. The axial length of the counterbore 42 corresponds to the axial dimension of the flange 15. The axial dimension of the lower portion of the bore 40 is less than the axial dimension of the cylindrical portion 14.

The upper plate 32 has a circle of nine holes 44 corresponding to the bores 40. Each hole 44 has a generally keyhole shape with a large diameter portion 45 corresponding in diameter to the diameter of the counterbore 42 and a smaller diameter portion 46 extending circumferentially from the larger diameter portion.

A vertical bore 47 in the lower plate receives a helical spring 48 and a ball 49. Two axial bores 50 and 51 of smaller diameter than the ball 49 pass through the upper plate 32. The bore 50 is positioned so as to coincide with the bore 48 when the smaller diameter portions 46 of the holes 44 are aligned with the holes 40. The bore 51 is positioned to align with the bore 47 when the larger diameter portions 45 are aligned with the counterbores 42. Thus, the spring loaded ball bearing 49, by engagement with the ends of the holes 50 and 51 respectively, serves to locate the two plates when the portions 46 and 45 respectively are aligned with the counterbores.

To mount the fibre optic cables to be polished in the carriage 30, the two parts of the carriage 31 and 32 are twisted relative to one another so as to the bring the large diameter portions 44 of the holes 42 into register with the counterbores 42 in the lower plate. In this position nine fibre optic cable ends are inserted through the holes in the upper plate into the bores 40 in the lower plate. The forward part of each connector is received by the lower diameter portion of the bore 40 and the flange 15 is received by the counterbore 42. Once all nine fibre optic cable ends are inserted in the holes 40 the upper plate 32 is twisted relative to the lower plate so as to bring the smaller diameter portion 46 into register with the holes 40. The parts of the upper plate on either side of the small diameter portion pass over the rear surface of the flanges 15 to hold the ferrules 40 in position in the holes 40. The diameters of the smaller portion are sufficient to pass freely around the outer sheath 12 of the fibre optic cable.

Once the fibre optic cables have been mounted in the carriage with their end faces projecting below the lower face of the lower plate, the carriage is ready for mounting in a lapping machine as shown in Figures 6 and 7.

The lapping/polishing machine consists of a turntable 60 which carries an interchangable lapping/polishing plate 61.

The turntable 60 is rotatable about a vertical axis 62 on a spindle 63. The spindle is rotated by a motor 64 and belt 65 drive located inside a housing 66. A circular interchangeable lapping/polishing plate 61 for carrying the abrasive material is placed on the turntable 60. Three drive pegs 130 on the upper surface of the turntable engage in three holes 131 in the underside of the lapping plate 61 to transmit drive from the turntable to the plate. Only one peg 130 and one hole 131 are visible in Figure 7. The pegs and holes are equi-spaced around the centre axis 62. The lapping plate 61 sits in a circular recess 67 in the upper surface of the housing 66.

A post 68 projects vertically from the housing at the side of the recess 67 and carries a horizontal arm 69. A lever 70 is pivoted at the outer end of the arm and carries a vertical rod 71 which passes through a bore 72 in the arm 69. An adjustable weight 73 can be moved along the lever 70 to vary the pressure on the vertical rod 71.

A universal swivel ball bearing 75 projects downwardly from the lower end of the rod on a shank 76 of reduced diameter. The parts of the machine so far described are of conventional construction and as in our machine sold as the Kent 4B Automatic Fibre Optic Polishing Machine.

The swivel bearing 75 engages in a spherical socket 80 in the upper surface of the upper plate 32 of the carriage 30. The rod, lever and weight assembly serve to hold the carriage with the tips of the fibre optic cables held against the surface of the lapping plate 61 whilst the swivel 75 and socket 80 permit universal swivelling movement of the carriage against the surface of the plate in a manner that will be described below. A bracket 82 mounted on the rod 71 engages a slot 83 in the carriage 30 and prevents the carriage rotating in a horizontal plane as the lapping wheel rotates.

A stand 85 at the back of the machine supports the fibre optic cables 10 and prevents them interfering with the operation of the machine.

In accordance with the invention, the upper surface of the lapping/polishing plate has a spherical depression 90 with a radius of curvature corresponding to the "desired radius of curvature of the end faces of the fibre optic cables. A recess 95 is formed at the bottom of the depression to provide clearance with the hub 34 projecting from the bottom of the carriage. The centre of curvature of the depression 90 is offset from the axis of rotation 62 of the lapping plate, typically by a distance of 6 mm. The ball swivel 75 is positioned to lie on the axis of rotation of the lapping plate but lies below the centre of curvature of the depression 90. The distances of the tips of the fibre optic cables from the centre point 41 corresponds to the radius of curvature of the depression 90 and the desired radius of curvature of the end faces. As the lapping plate 61 Is rotated on its vertical axis the depression precesses around it because the depression is eccentric and it causes the carriage to rock on the swivel 75 as the plate is rotated.

An atomiser arrangement 100 has a spray nozzle 101 located above the plate alongside the depression 90 and can be operated to spray a suspension of diamond or other abrasive in a liquid medium such as for example is sold under the trade mark HYPREZ, onto the surface of the lapping plate. Such an atomiser arrangement is sold as our Hyprez Liquid Diamond Electronic Dispenser. A reservoir 102 containing the suspension of abrasive is placed on a electronic stirrer 103. A rotating magnet on the stirrer causes a rod 104 of magnetic material in the reservoir to rotate with it, thus stirring the abrasive mixture in the reservoir and maintaining the abrasive particles in suspension. Other kinds of stirrer might be used such as an air jet stirrer.

As the lapping/polishing plate is rotated and the carriage swivels on the swivel bearing 75 the ends of the cables are ground and polished by the abrasive medium carried on the surface of the depression 90 to produce the desired finish. Because the ends of the cables are constantly moving across the surface of the lapping plate the wear on the lapping plate is not confined to a small zone. Thus the lapping plate can be used to finish a larger number of fibre optic cable ends before it requires reconditioning than is the case with the known machine.

The lapping/polishing plate may conveniently be formed of a composite material consisting of metal powder bonded in a resin such as is sold under the trade mark KEMET. A sequence of different lapping/polishing plates and diamond suspensions of different coarsenesses may be used in order to produce the desired finish. For example, the ends of the optical fibres may initially be shaped using a coarse lapping plate such as a KEMET iron or KEMET copper lapping plate with a coarse slurry and then finally polished using a fine polishing plate such as a KEMET tin plate with a fine diamond slurry.

When it is desired to recondition the plate this can be achieved simply by replacing the carriage 30 with a dressing tool or puck of cast iron as shown in Figure 9. The dressing tool 120 has an outer surface with a curvature corresponding to the desired curvature of the depression 90. The outer surface has grooves 121 for collecting abraded material from the lapping wheel. A central spherical socket 122 receives the swivel bearing 75 to hold the dressing tool in the depression. The bracket 82 engages a slot 83 to prevent the tool rotating in a horizontal plane. The lapping plate Is rotated and abrasive paste is applied to the surface of the lapping tool to restore the depression to the desired curvature.

The puck of Figure 9 uses a loose abrasive for reconditioning the plate. Alternatively, the abrasive may be fixed on the puck for example, by plating the part-spherical surface of the puck with a material, for example, copper in which an abrasive material, for example diamond, is embedded.

Figure 10 shows an alternative arrangement of lapping/polishing plate which may be used in the machine of Figures 6 to 8. The lapping/polishing plate 140 is a circular disc with a circular depression 141 in its upper surface. The radius of curvature of the depression corresponds to the desired curvature of the ends of the fibre optics cables. The centre axis of the depression 141 coincides with the centre axis of the disc. The lapping/polishing plate 140 is, however, mounted eccentrically on the turntable 60 of the lapping/polishing machine by means of a circular mounting plate 142. Three drive pegs 143 on the upper surface of the plate 142 engage in three holes 145 in the underside of the lapping/polishing plate 140 to hold the lapping plate in position. Rubber O-rings 144 fitted in .grooves on the pegs 143 ensure a tight fit with the holes 145. The pegs 143 are offset from the centre axis of the mounting plate so that the centre axis of depression in the lapping/polishing plate is offset from the axis of rotation of the turntable. The drive pegs 130 on the turntable 60 engages three holes 146 in the underside of the mounting plate 142 equi-spaced around its centre axis. In use the lapping/polishing plate 140 functions in the same manner as the plate 61. However when it is desired to recondition the lapping/polishing plate, it can be removed from the mounting plate and mounted on a lathe with its axis on the turning axis and reground. Furthermore, since the lapping/polishing plate 140 is smaller than the lapping/polishing plate 61, less material is used in its construction. The plate 140 is therefore less expensive to manufacture.

Figures 11 and 12 show an alternative carriage 159 for holding the fibre optic cables. An octagonal plate 160 has seven grooves 161 extending across its edge. The axes of the grooves 161 converge at a point 170 above the plate on the centre axis of the plate. The grooves 161 have a V-shaped cross-section. Seven bridging members 162 are screwed to the edge of the plate 160 with V-shaped grooves

163 on the bridging members in register with the seven V-shaped grooves 161 on the plate. Each bridging member is held in position by a pair of clamping screws 165. The bridging members serve to hold the fibre optic cables by their ferrules in the grooves 161 of the plates. The flanges 15 of the ferrules abut the upper surface of the plate.

The plate 160 Is dimensioned so that the ferrules of the fibre optic cables lie on radii of a sphere with a radius of curvature corresponding to the radius of curvature of the depression 90. It has a central socket 166 for receiving the swivel ball bearing 75 of the lapping machine and a recess 167 for engaging the bracket 82.

The carriage 159 may be used in place of the carriage 30 for holding the fibre optic cables In position. It may be preferred to use this carriage where the manufacturing tolerances for the fibre optic cable is specified in terms of the distance between the front of the flange 15 and the end of the cable, since the position of the ferrule in the carriage 159 is set by abutment of the front of the flange 15 against the top surface of the plate 160. In the carriage 30 the position of the ferrules is determined by abutment of the rear surfaces of the flange 15 with the underside of the upper plate.

It will be appreciated that various modifications of the apparatus described above are possible. For example, although carriages with seven and nine bores arranged in a circle for receiving fibre optic cables are described, carriages with different numbers of holes are possible and they may be arranged in a different way, for example in two concentric circles. Preferably the carriage carries at least three fibre optic cables so that the cable ends can support the cable and carriage assembly on the surface of the lapping/polishing plate.

Claims

CLAIMS :

1. Apparatus for finishing articles with a convex surface comprises a lapping or polishing plate rotatable about an axis, a concave cavity in the surface of the wheel with its axis offset from the axis of rotation of the wheel, the radius of curvature of the cavity corresponding to the radius of curvature desired on the finished article, and means for holding one or more workpieces against the surface of the cavity, said means permitting universal swivelling movement of the workpiece, whereby the workpiece Is caused to rock about its pivot point as the cavity moves around the axis of the lapping or polishing plate and the workpiece thus moves over the surface of the cavity.

2. Apparatus according to claim 1 in which the holding means is adapted to hold fibre optic cables.

3. Apparatus according to claim 2 in which the holding means is adapted to hold at least three cables and the assembly of cables and holding means is supported on the surface of the cavity by the ends of the cables.

4. Apparatus according to claim 1, 2 or 3 and including a tool for reconditioning the surface of the cavity in the lapping plate, the tool being interchangeable with the said holding means and having a part-spherical surface with a radius of curvature corresponding to the desired radius of curvature of the cavity.

5. Apparatus according to claim 4 in which the part-spherical surface has grooves In it for collecting abraded material.

6. Apparatus according to claim 1, 2 or 3, in which the holding means comprise a carriage comprising a first plate with a plurality of bores extending therethrough, each bore having a counterbore at one end for receiving an enlarged portion of the fibre optic cable ferrule, and a second plate mounted against the side of the first plate which has the counterbores, the second plate having a series of holes corresponding with the holes of the first plate, each hole in the second plate being of generally key-hole shape, with an enlarged portion corresponding substantially in diameter with the diameter of the counterbore and a smaller portion of substantially smaller diameter than the diameter of the counterbore, the second plate being movable between a first position in which the enlarged portions are aligned with the counterbores to permit the ferrules of the fibre optic cables to be inserted through the second plate into the bores in the first plate, and a second position in which the smaller portions of the holes are aligned with the counterbores in the second plate whereby the fibre optic ferrules are retained in the carriage.

7. Apparatus according to claim 6 in which the bores lie on radii from a centre point positioned above the plate.

8. Apparatus according to claims 6 or 7 in which the first and second plate are mounted together for relative movement about an axis perpendicular to their surfaces whereby the second plate can be moved between the first and second positions by twisting the first plate relative to the second plate about the axis.

9. Apparatus according to any of the preceding claims in which the centre axis of the cavity coincides with the centre axis of the lapping or polishing plate and the axis of rotation of the lapping or polishing plate is offset from the centre axis of the lapping or polishing plate.