Lens grinding method and apparatus

Abstract: The invention provides a method of generating a lens having desired base and cross curvatures by means of a circular cup shaped diamond tool (42), which is rotated about an axis and is swept repeatedly over a lens blank (68) to remove material so as to generate the required surface, the tool being positioned so as to produce the desired combination of base and cross curvatures on the blank, the method comprising positioning the diamond tool by means of a cross slide (24) and a base slide (22), both of which slides are free to move during the generation of the surface; sweeping the rotating tool over the lens blank and adjusting the cross slide and/or the base slide during the sweep thereby to vary the head angle of the tool during the sweep such that at any position in the sweep the instantaneous head angle is at a value whereby the desired cross curvature produced is constant over the sweep. The invention also relates to a lens generating machine.

Description: The present invention relates to a method and apparatus for generating ophthalmic lenses, in particular spherical and sphero-cylindrical lenses, and more particularly to the generation of ophthalmic lenses with base curves providing optical powers in the range from zero dioptres to at least twenty dioptres. The traditional technique for making ophthalmic lenses involves repeated grinding passes usually known as sweeps across a lens blank with a cutting tool, which is usually a circular cup shaped diamond tool. The sweeps are continued until the blank has been shaped to the desired surface curvature and lens centre thickness. The optical powers of the range of curvatures produced by available conventional lens grinding machines in the spherical meridian is in the range from 3 to 20 dioptres. It is desirable to extend that range down to zero dioptres i.e. lenses with no curvature in the spherical meridian. The extension of the range of lens grinding machines has been the subject of previous proposals. For example, US-A-4 535 566 describes a mechanical system in which the locus of the grinding wheel can be varied to extend the range of the system. The basis of this proposal is to use a cam follower mechanism to radially reposition the diamond tool as it is swept over the lens blank surface. The change in the sweep radius length while sweeping simulates a particular radius of curvature. The operations described in US-A-4 535 566 require a designated cam surface (or template) for every specific base curve it is desired to generate. US-A-4 535 566 maintains the same head angle, i.e. the angle at which the tool head is set to a tangent to the curve being cut throughout the sweep by means of a complex four bar linkage which must be adjusted according to the desired prescription before the lens generation process begins. Thus this prior proposal provides a lens grinding machine which while having an extended range, is only adjustable to produce selected curves within that range, and requires a skilled operator to set the machine up for a particular power. An aim of the present invention is to produce a lens grinding machine and a lens generating method which can be operated with limited operator attention and the use of relatively unskilled operators in that no complex setting up procedures are required by utilizing numerical control procedures. It is a further aim to produce a machine with an extended range, which can be produced by relatively modest machine modifications to existing designs. The present invention provides a method of generating a lens having desired base and cross curvatures by means of a circular cup shaped diamond tool, which is rotated about an axis and is swept repeatedly over a lens blank to remove material so as to generate the required surface, the tool being positioned so as to produce the desired combination of base and cross curvatures on the blank, the method comprising positioning the diamond tool by means of a cross slide and a base slide, both of which slides are free to move during the generation of the surface; sweeping the rotating tool over the lens blank and adjusting the cross slide and the base slide during the sweep thereby varying the head angle of the tool during the sweep such that at any position in the sweep the instantaneous head angle is at a value whereby the desired cross curvature produced is constant over the sweep. The present invention also provides a lens generating machine comprising a base, a tool supporting mechanism and a lens supporting mechanism, both of the mechanisms being mounted on the base, the tool supporting mechanism comprising a diamond tool, a base slide and a cross slide on which the tool is mounted and means for rotating the tool, the tool supporting mechanism being adapted to sweep the tool over the lens blank to generate a lens having desired base and cross curvatures, means for adjusting the cross slide and the base slide during a sweep thereby varying a head angle of the tool during the sweep, sensor means for sensing the instantaneous sweep angle during the sweep, and processing means for processing a signal from the sensor means representative of the instantaneous sweep angle to provide a second signal which is related to any adjustment required to the head angle, the means for adjusting being operable in response to the second signal such that at any position in the sweep the instantaneous head angle is at a value whereby the desired cross curvature produced is constant over the sweep. In the operation of a conventional lens grinding machine, the head angle is fixed during the sweep by clamping the cross slide. The head angle has previously been selected and the adjustment of the cross slide already made prior to clamping. The base slide is then positioned so that the tool edge is the radius of the prescription base curve away from the axis about which the tool is swept. The lens blank is then moved towards the tool so that the finished lens will have a pre-determined lens thickness when the sweeping action is completed. The lens and the tool are stepped toward each other as the sweeping action progresses so that a constant amount of material is removed at each sweep. In the method of the present invention, an additional degree of motion is provided which enables an extended range of curves to be generated. This is achieved by clamping neither the cross slide nor the base slide in a fixed position during the shaping of the lens blank. The cross slide is simply positioned at an initial head angle, and the base slide at an initial distance from the sweep axis. The base slide position when operating the machine in the range feasible in the prior art mode is fixed at a distance away from the sweep pivotal axis equal to the base curve radius. Operating in the range 0-3 dioptres, the base slide is positioned at a known reference radius (which is measured at the centre of the lens) from the sweep pivotal axis, e.g. 170 mm, and this reference radius can be extended or retracted by a particular amount when the base slide is moved away from or towards the lens centre so as to produce the desired base curve. In the case of a 170 mm reference radius, the base slide position can change 6 mm up to 176 mm for an 80 mm diameter lens. As regards the position of the cross slide, if this is positioned so that the diamond tool is at the lens centre, the tool will be at the desired base radius from the sweep pivotal axis, and the real head angle of the diamond tool is at the angle required to generate the desired cross curve, and the sweep angle will be zero. As the tool moves away from the lens centre, and the sweep angle increases, in order to maintain the tool in the correct generating position, the base slide must extend out to enable the tool to move along the path of the desired base curve. The head angle of the diamond tool must be adjusted so that at any particular point on the sweep it is at a constant value equal to the head angle when the sweep angle was at zero. This results in the tool angle being effectively the angle which results in the required cross curve being generated. In order to carry out the above operations, it is therefore necessary to constantly sample the sweep angle during the sweep and adjust the base and cross slide positions in relation to the position of the tool in its sweep so as to maintain the tool at the desired effective head angle to generate the required cross curve, and the desired distance from the sweep pivotal axis to produce the required base curve. The machine must therefore be provided with means to determine the sweep angle, and the base slide must be mounted so as to be able to be positioned at a constantly changing distance from the pivotal sweep axis. An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:- Figure 1 is an elevation of a known form of lens generating machine; Figure 2(a) is a diagrammatic top view of a lens generating machine according to the invention; Figure 2(b) is a diagrammatic front view of a lens generating machine according to the invention; Figure 3(a) is a diagrammatic view of how the desired cross curve is cut by the diamond tool of the machine of Figure 2; Figure 3(b) is a diagrammatic view showing the path followed by the diamond tool of the machine of Figure 2 to generate the desired base curve; Figure 4 is a diagrammatic view of the relationship, in the operation of the machine of Figure 2, between the corrected head angle β which is kept constant throughout the sweep, βo the head angle to produce a reference base radius curve, β' the adjustment to βo to achieve a setting at the head angle β', and β'' the head angle made with the desired curve; and Figure 5 is a flow chart showing the operations performed by the machine of Figure 2 during each sweep in order to maintain the head angle necessary to generate the desired cross curve, and the base slide position to achieve the desired base curve. Referring first to Figure 1, a known lens curve generating machine 10 comprises a base 12 on which are mounted a tool supporting mechanism 14 and a lens supporting mechanism 16. The tool supporting mechanism is similar to that described in US-A-2 806 327 and US-A-3 289 355, the disclosures of which are incorporated by reference herein. Basically, the tool supporting mechanism 14 comprises a plate 18 which is pivotably mounted to the base 12 for rotation about a vertical axis 20. Slidably mounted on a horizontal surface of the plate 18 is a tool support comprising a base curve slide 22, and a cross curve slide 24 pivotably mounted to the base curve slide for rotation about a vertical axis 26 defined by a pin (not shown). The base curve slide 22 can be adjusted horizontally relative to the plate 18 in a fore-to-aft direction toward and away from the lens supporting mechanism. The cross curve slide 24 can be adjusted relative to the base curve slide 22 about the axis 26. Mounted on the cross curve slide 24 is a bearing block 32 which is adapted to slide horizontally relative to the cross curve slide 24 in a direction perpendicular to the fore-to-aft direction. This is achieved by mounting the bearing block 32 by means of a dove-tail track 34 and providing a conventional adjustment means. A spindle housing 38 mounted in the bearing block 32 rotatably carries a shaft 40 on one end of which a diamond grinding tool 42 is supported. The opposite end of the shaft is driven by a belt drive 44 from a motor 46 resting atop the bearing block 32. The tool 42 is cup-shaped and presents a curved cutting edge 45. The curved edge 45 is rounded as viewed in cross-section so as to define a centre of curvature spaced from the plane of the curved edge. The arrangement of the bearing block and spindle housing is such that the vertical axis 26 is intersected by that centre of curvature during each grinding sweep of the tool. The axis 26 thus defines a tool reference axis. The grinding sweep of the tool is effected by oscillating the tool supporting mechanism 14 about the vertical axis 20 after the tool 42 has been properly positioned through appropriate adjustments of the base curve slide 22, the cross curve slide 24 and the bearing block 32. The lens supporting mechanism 16 comprises a support block 50 on which a tailstock assembly 52 is slidably supported. The tailstock 52 includes a housing 53 which can be reciprocated in a horizontal fore-to-aft direction by conventional means. A shaft 56 is mounted in the tailstock for reciprocable movement relative to the housing 53 in the fore-to-aft direction. A front end of the shaft 56 carries a lens holder in the form of a conventional chuck 58. The chuck includes a space ring with a lens blank inserted so that a so-called "front curve" of the lens abuts against a front surface of the space ring. That surface defines a vertical lens reference plane 68 disposed perpendicular to the fore-to-aft direction of movement of the shaft 56 and parallel to the tool reference axis 26. The operation of this machine to generate a particular base and cross curve will now be described. The cross slide 24 is moved to a position at which the head of the diamond tool will be at the head angle necessary to generate the desired cross curve. The cross slide 24 is then clamped in that position. The base slide 22 is then moved to a position such that the tool edge is the radius of the desired prescription curve away from the sweep axis, and the base slide is then clamped in position. The tailstock assembly 52 carrying the lens blank on which the curves are to be generated is then moved to a position such that the lens blank will be reduced to the desired lens centre thickness once the curve generation has been completed. The tailstock slide is then clamped in position and first sweep is commenced. Between each sweep, the relative axial positions of the lens and diamond tool are adjusted so that the diamond tool contacts the blank to remove a further layer of the surface on each sweep until the desired lens thickness is achieved. The lens may then be removed for the further operations necessary to convert it into its final form for filling in frames which comprises at least fining, polishing, and edging, but can also include tinting and coating with such coatings as abrasion-resistant and anti-reflection coatings. Referring now to figures 2 and 3, in the lens generating machine of the present invention, in order to enable the machine to not only generate lenses having curves corresponding to optical power ranging from 3 to 20 dioptres but additionally lenses with curves corresponding to optical power ranging from zero dioptres up to 3 dioptres, two features are required in addition to those conventionally available on a lens grinding machine designed to produce lenses with curves corresponding to optical power in the range 3 to 20 dioptres. The hydraulic cylinder which positions the cross slide must be able to accommodate the increased travel required to enable the additional range of powers to be achieved, and the slide bearing surfaces need to be extended to accommodate the additional travel of the cylinder. Existing machines are available whose design can be simply modified, e.g. the machine sold by Coburn Optical Inc. under the trade name "Coburn Model 2112 generator". The amount of change and additional equipment required depends on the sophistication of the original design, e.g. a hand operated machine would require not only the slide bearing surfaces to be modified but also the addition of powered motion with their associated servo-mechanisms, encoders, and motion control cards with their associated micro-processor equipment. It is essential that an encoder is present on the sweep mechanism to allow the sweep angle to be measured, i.e. the angle that the base slide makes with the machine centre line.The latter is the line joining the point about which the base line is pivoted (the sweep pivot) and the lens centre. This measurement is then used as shown in the flow chart Fig 5 as input to the micro-processor so that the necessary adjustments to be made to the cross-slide, and if necessary to the base slide, can be calculated to maintain the head angle at the value to give an effective head angle at which the required cross curve will be produced. The conversion of the signal received from the encoder to a signal to control the mechanical adjustment of the position of both the base slide and the cross slide, and the use of that signal is carried out in a manner well known to those skilled in the art of servo controlled motion mechanisms. The parts shown in the diagrammatic view in figure 2 are those whose motion is controlled during the operation of the machine. The machine has a base 71 on which there is mounted a sweep platform 72, on which in turn there is mounted a base slide 73, on which the cross slide 74 carrying a diamond tool 75 driven by a motor 76 is mounted. A lens blank supporting mechanism 77 is mounted on the base 71 and the position of the lens blank 80 can be adjusted along an axis A-A in relation to the diamond tool 75. Axis B-B is the sweep axis about which the sweep platform 72 pivots when driven by a hydraulic cylinder (not shown) so that the pivot point 78 is at a distance equal to a desired base curve radius from the sweep axis B-B. The cross slide 74 can be pivoted about the pivot point 78 through which a vertical line C-C passes and which line also passes along the cutting edge 79 of the diamond tool. The diamond tool is then at a head angle to the curve being cut. The head of the diamond tool is of a circular cup shape so that at any head angle other than zero, the result is that the circle is effectively projected as an ellipse when considered in front view. It is a portion of this ellipse which grinds through the lens blank. Figure 3 shows how the ellipse approximates to a circle of the desired radius with a so-called elliptical error occurring at the edges. In Figure 3a, line 100 represents the desired cross curve, line 102 represents an ellipse which approximates to the circle represented by line 104 but leaves an elliptical error 106. The lens blank 80 is moved along with its supporting mechanism 77 to a position such that at the end of the necessary number of sweeps across the lens surface, the lens has a chosen lens thickness as well as the desired surface shape. Figure 3(b) shows the base slide 73 positioned on the sweep platform 72 at the start position of the sweep. The angle of the axis of these two components which passes through the sweep axis and the pivot point 78 with the axis passing through the lens centre being the sweep angle. The sweep about the sweep axis is from this start position to the reciprocal position on the other side of the axis A-A. Figure 4 shows the relationship between the unadjusted head angle at location A, and the adjusted head angle at location B which is achieved with a specific cross slide movement for a particular point in the sweep, i.e. the real time value of the sweep angle α. The sweep angle α is, in the Figure, a value of 48°. Line 108 represents the reference radius of 170.0mm and line 110 represents the desired base curve. Line 112 is the direction leading to the desired centre of curvature. In order to manufacture a lens with a base curve having an optical power of less than 3 dioptrres, the diamond tool is moved to its initial position at say a sweep angle of 48°. The base slide is set so that the radius of the swwp would be 170 mm. The cross slide is then moved to a position such that the head angle of the tool is equal to a value calculated using the reference radius of 170 mm and a sweep angle of zero degrees. This value is a constant for any desired base curve. The diamond tool is then in the position shown as A. In order to put the diamond tool in the correct position for generating the desired base curve as shown, the base slide must be extended to position B, and the diamond tool rotated by a head angle adjustment β' so that the angle β'' is equal to the above constant for the desired base curve. Then: β' + βo = β β is then the corrected head angle. As the sweep angle decreases, the base slide will retract until it reaches the lens centre, after which it will extend. The head angle adjustment required to maintain the effective head angle constant will diminish to zero as the lens centre is reached, as at that point no correction to βo is needed. After the lens centre, the base slide extends, and the head angle adjustment increases. Referring to the flow chart figure 5, the iterative sequence of operations will now be described in more detail. During the sweep across the lens blank from the start position to the end of the sweep, the angle of sweep i.e. the angle to which the sweep platform 71 is pivoted, is sensed by means of a rotary encoder. The signal from the encoder is processed and the adjustment of the cross slide needed to maintain the head angle at the constant value determined. The base slide radius is then determined, and the signal processed so that the base slide may be positioned so as to maintain the sweep along the desired base curve. The process is then repeated until the sweep is complete, and when the sweep is complete, the distance between the lens blank and the tool path is reduced by a pre-determined amount and the next sweep commenced to remove further material and reduce lens thickness. The micro-processor used for these determinations, and the necessary electro-mechanical equipment to carry out the necessary adjustments are both conventional. This method of operating a lens generating machine enables the range of the machine to be extended below 3 dioptres. The machine can of course be operated in the range from 3 to 20 dioptres and when operating in that range, the base slide position does not alter during the generation of the lens curvature.

Claims: A method of generating a lens having desired base and cross curvatures by means of a circular cup shaped diamond tool, which is rotated about an axis and is swept repeatedly over a lens blank to remove material so as to generate the required surface, the tool being positioned so as to produce the desired combination of base and cross curvatures on the blank, the method comprising positioning the diamond tool by means of a cross slide and a base slide, both of which slides are free to move during the generation of the surface; sweeping the rotating tool over the lens blank and adjusting the cross slide and the base slide during the sweep thereby to vary the head angle of the tool during the sweep such that at any position in the sweep the instantaneous head angle is at a value whereby the desired cross curvature produced is constant over the sweep. A method according to claim 1 wherein the said value of the instantaneous head angle represents an effective head angle which is calculated at a prescribed sweep radius and a sweep angle of zero degrees. A method according to claim 1 or claim 2 wherein during the sweep an instantaneous sweep angle is sensed by a sensor and a signal from the sensor representative of the instantaneous sweep angle is processed to provide a second signal which is related to any adjustment required to the head angle, the second signal being employed to adjust the cross slide and the base slide. A method according to claim 3 wherein the second signal is employed initially to adjust the cross slide to a desired head angle and then to adjust the base slide to a desired radius. A method according to any foregoing claim wherein the required surface corresponds to an optical power of from 0 to 20 dioptres. A method according to any foregoing claim wherein the required surface corresponds to an optical power of from 0 to 3 dioptres. A lens generating machine comprising a base, a tool supporting mechanism and a lens supporting mechanism, both of the mechanisms being mounted on the base, the tool supporting mechanism comprising a diamond tool, a base slide and a cross slide on which the tool is mounted and means for rotating the tool, the tool supporting mechanism being adapted to sweep the tool over the lens blank to generate a lens having desired base and cross curvatures, means for adjusting the cross slide and the base slide during a sweep thereby to vary a head angle of the tool during the sweep, sensor means for sensing the instantaneous sweep angle during the sweep, and processing means for processing a signal from the sensor means representative of the instantaneous sweep angle to provide a second signal which is related to any adjustment required to the head angle, the means for adjusting being operable in response to the second signal, such that at any position in the sweep the instantaneous head angle is at a value whereby the desired cross curvature produced is constant over the sweep. A lens generating machine according to claim 7 wherein the means for adjusting comprises a first adjustor means for adjusting the cross slide to a desired head angle and a second adjustor means for adjusting the base slide to a desired radius, the first and second adjustor means being operable in sequence in response to the second signal. A lens generating machine according to claims 7 or 8 wherein the machine is adapted to generate lenses having an optical power ranging from 0 to 20 dioptres.

Inventors: KULAN STEPHEN, KULAN, STEPHEN
Applicants: COBURN OPTICAL IND, COBURN OPTICAL INDUSTRIES, INC.
Application year: 1991
Main classification: B24B13
IPC codes: B24B13