Abstract:
A method and apparatus for substantially nullifying vibration and deflection in a single point lens turning lathe ( 100 ) having a rapidly reciprocating lens cutting tool ( 114 ) and shuttle assembly. The apparatus includes three or more tool shuttles ( 108, 110, 112 ) of similar mass mounted for reciprocating movement along respective generally parallel shuttle paths. The shuttles ( 108, 110, 112 ) are reciprocally moveable by respective actuators ( 118, 120, 122 ) along their respective shuttle paths. The shuttle and tool assemblies are moved by their respective actuators ( 118, 120, 122 ) in opposite directions at a rate which causes forces generated by shuttle and tool assemblies moving in one direction to cancel forces arising from the shuttle and tool assembly movement in the opposite direction. Should an odd number of shuttle and tool assemblies be used, the amount of force generated by the mass moving in one direction may be compensated by having a different rate of movement, and hence a different direction.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to apparatus and methods for cutting lenses and more particularly to turning lathes for cutting non-rotationally symmetrical lenses.  
         BACKGROUND OF THE INVENTION  
         [0002]    An efficient way to produce rotationally asymmetrical surfaces is with a three-axis single point diamond turning lathe. FIG. 1 is an end elevation showing a typical layout for such a lathe and FIG. 2 is a front elevation corresponding to FIG. 1. The lathe which is generally indicated by reference  10  includes a lens supporting assembly  20  and a shuttle  14 . The shuttle  14  is axially movable along a “Z axis” indicated by reference Z by an actuator  16 . A lens cutting tool  18  (typically a diamond tool) is secured to the shuttle  14 .  
           [0003]    The lens supporting assembly  20  supports a lens  22  and rotates the lens  22  about a lens axis indicated by θ. The lens supporting assembly  20  is moveable in a direction Y transverse to the lens axis θ. The lens supporting member typically includes a spindle  21  which rotates the lens  20 . The spindle  21  is mounted to a transversely moveable linear table  23  which in turn is mounted to a base  25  of the lathe  10 .  
           [0004]    Lens cutting is effected by a turning operation. The lens  22  is rotated at a high speed about the lens axis θ. The lens cutting tool  18  is initially placed adjacent an edge  24  of the lens  22 . The lens  22  is moved in the direction Y as the lens cutting tool  18  is moved in the direction Z. Coordinated movement between the lens  22  and the lens cutting tool  18  determines the shape of the lens  22 .  
           [0005]    If the lens  22  is rotationally symmetrical, such as spherical or aspherical, the lathe  10  is operated similarly to a two axis turning lathe. The cut typically starts at the edge  24  and the lens cutting tool is moved both in the Y and Z directions (radially inwardly and toward the lens  22 ). In this instance, the Z position of the lens cutting tool  18  remains constant at any given radial (“Y”) distance from the lens axis θ regardless of rotation about the lens axis θ.  
           [0006]    The relative speed between the lens cutting tool  18  and the respective surface of the lens being cut diminishes to zero as the lens cutting tool  18  approaches the lens axis θ. Accordingly, a very high spindle speed in the lens supporting assembly  20  is desirable in order to maintain an acceptable and productive surfacing operation. Typical spindle speeds are on the order of 3,000 to 10,000 RPM.  
           [0007]    When the desired lens is non-rotationally symmetrical, as for example in the case of toric or progressive lenses, the lens cutting tool  18  must move reciprocally along the Z axis at a frequency proportional to the rotational frequency. Depending on the particular lens  22  being cut, the lens cutting tool  18  may need to be moved by as much as 20 mm at the edge of the lens. In a simple toric lens this would be a substantially sinusoidal motion with a frequency twice that of the rotational frequency.  
           [0008]    A typical actuator  16  would consist of a linear servo motor (such as a voice coil motor) in conjunction with a high speed feedback device which is desirable as being able to produce high speed linear movement at great accuracy. Although such a motor typically has only limited travel, a typical stroke being 30 mm, it may nevertheless be required to achieve velocities as high as 3 to 4 m/s. Such velocities and rapid directional changes can create peak accelerations of 50 to 100 g or even higher. By way of example, if the shuttle  14  and lens cutting tool  18  have a total mass of 2 kg, an actuator acceleration of 100 g will develop reaction forces of 1961 N (approximately 440 lbs).  
           [0009]    It will be appreciated that the above velocity and speed figures are somewhat high for currently available linear servo motors. Such technology is rapidly evolving and to some extent the current invention takes into account desired linear servo motor properties. In any case, the present invention produces a useful result with current linear servo motor technology capable of velocities and forces of about half those set out above.  
           [0010]    The positioning of the lens cutting tool  18  along its tool path needs to be servo controlled to a very high degree of accuracy, typically within 10 nm or less. Assuming that the actuator  16  is capable of such accuracy, the magnitude of the actuating forces could cause structural defections in the lathe  10  which in themselves exceed the accuracy requirements.  
           [0011]    It is an object of the present invention to provide a method and apparatus to cancel vibration caused by actuator forces in a lathe having a reciprocally moveable tool guidance assembly.  
         SUMMARY OF THE INVENTION  
         [0012]    A tool guidance assembly is provided for a lathe. The tool guidance assembly has at least one first shuttle for mounting the tool and a first actuator for causing reciprocal movement of the first shuttle along a first shuttle path. The first shuttle and tool comprise at least part of a first reciprocating mass. The tool guidance assembly has a second reciprocating mass and a second actuator for moving the second reciprocating mass in a direction opposite to the first reciprocating mass. The second reciprocating mass has a mass, a path of movement and a rate of movement selected to substantially cancel accelerative forces caused by the reciprocating movement of the first reciprocating mass.  
           [0013]    The second reciprocating mass may include a pair of second shuttles, each of the pair of second shuttles being disposed on opposite sides of the first shuttle. The second actuator may include respective actuators for each of the second shuttles.  
           [0014]    The first reciprocating mass may also include a plurality of first shuttles and the first actuator may include a respective actuator for each of the first shuttles.  
           [0015]    In one aspect of the invention, a single point diamond turning lathe is provided which has a first shuttle for supporting a cutting tool, the first shuttle being reciprocally moveable along a first shuttle path. A first actuator is connected to the first shuttle for effecting the reciprocal movement of the first shuttle. The lathe has a second shuttle adjacent the first shuttle for supporting a second cutting tool. The second shuttle is reciprocally moveable along a second shuttle path generally parallel to the first shuttle path. The second shuttle has a mass similar to that of the first shuttle. A second actuator is connected to the second shuttle for effecting reciprocal movement of the second shuttle in a direction opposite to that of the first shuttle by an amount of about half that of the reciprocal movement of the first shuttle. The lathe has a third shuttle adjacent the first shuttle opposite the second shuttle for supporting a third cutting tool. The third shuttle is reciprocally moveable along a third shuttle path generally parallel to and coplanar with the first and second shuttle paths, the third shuttle has a mass similar to that of the first shuffle. A third actuator is connected to the third shuttle for effecting reciprocal movement of the third shuttle in a direction opposite to that of the first shuttle by an amount of about half that of the reciprocal movement of the first shuttle. The second and third shuttle balance accelerative forces of the first shuttle to substantially cancel vibration and corresponding structural deflections imparted to the lathe by the reciprocal movement of the first shuttle.  
           [0016]    According to a further aspect of the present invention, a lens cutting lathe is provided which includes a base having a lens support mounted to the base for supporting the lens and spinning the lens about a lens rotational axis. The lens support is transversely moveable relative to the lens rotational axis. A plurality of shuttles for mounting respective cutting tools are mounted to the base for movement along respective shuttle paths toward and away from the lens. The plurality of shuttles are reciprocally moveable by respective actuators mounted to the base. The actuators are arranged to move some of the plurality of shuffles in a direction opposite to a remainder of the plurality of shuttles. The plurality of shuffles are of similar mass and disposed and moved in a manner to maintain a generally fixed center of mass whereby movement of the shuttles in a given direction substantially cancels both linear and rocking forces imposed on said base by movement of the remainder of the shuttles in the opposite direction.  
           [0017]    The plurality of shuttles may consist of two outer shuttles and an intermediate shuttle therebetween. The outer shuttles are arranged to move together in a direction opposite to the intermediate shuttle, and the outer shuttles move at a rate of about one half that of the intermediate shuttle. Accordingly, the total accelerative forces generated by the outer shuttles is generally the same as that generated by the intermediate shuttle.  
           [0018]    According to another aspect of the present invention, the plurality of shuttles may consist of a row of four shuttles arranged in two pairs on either side of a central axis, the shuttles of each of the two pairs being arranged to move in opposite relative directions.  
           [0019]    The actuator in the above embodiments may be a linear servo-motor.  
           [0020]    Alternatively, the actuator may be a rotary servo-motor.  
           [0021]    A method is also provided for nullifying accelerative forces induced in a lathe by movement of a cutting tool secured to a lathe shuttle mounted for reciprocal movement relative to a base of the lathe along a shuttle path. The method comprises the steps of:  
           [0022]    i) providing a balancing mass having a center of mass along the shuttle path; and,  
           [0023]    ii) reciprocally moving the balancing mass in a direction and at a rate which cancels linear forces arising from the movement of the tool without imparting a corresponding rocking force to the structure.  
           [0024]    According to one aspect of the method, the balancing mass may consist of at least two further cutting tools secured to respective shuttles mounted to the base for reciprocal movement by respective actuators along respective generally parallel shuttle paths.  
           [0025]    A method is provided for turning a non-rotationally symmetrical lens on a lens turning lathe having a lens support and at least three cutting tools. The method comprises the steps of:  
           [0026]    i) mounting a lens blank to a lens support assembly;  
           [0027]    ii) rotating the lens blank with the lens support assembly about a lens rotational axis;  
           [0028]    iii) pressing one of the at least three cutting tools against the lens blank;  
           [0029]    iv) moving the lens blank with the lens support assembly in a direction transverse to the lens rotational axis;  
           [0030]    v) reciprocally moving the above one of the at least three lens cutting tools relative to the lens along a first tool path at a reciprocal frequency corresponding to the rotational frequency of the lens blank to produce the non-rotationally symmetrical surface;  
           [0031]    vi) reciprocally moving remaining of the at least three lens cutting tools along respective tool paths generally parallel to and coplanar with the first tool path of the one lens cutting tool in (v) at a reciprocal frequency, in a direction and at a rate which counters and substantially nullifies linear forces imposed on the lathe by the one tool in step v) without imparting a rocking movement on the lathe.  
           [0032]    According to yet another aspect of the method for turning a non-rotationally symmetrical lens, the three lens cutting tools may consist of a first and a last lens cutting tool with an intermediate lens cutting tool disposed equidistantly therebetween and in line therewith. The first and last lens cutting tools are moved in unison contra to the intermediate lens cutting tool at a rate of about half that of the intermediate lens cutting tool.  
           [0033]    The lens may be turned in three stages with a different of the three lens cutting tools utilized in each stage.  
           [0034]    In an alternative embodiment, first, second, third and fourth lens cutting tools may be provided and arranged in line. The first and second cutting tools are moved contra to each other at a similar rate, and the third and fourth cutting tools are also moved contra to each other at a similar rate. The action of the third and fourth tools is rotationally contra to the first and second tools thus simultaneously cancelling any rotational vibration (rocking action).  
           [0035]    The method may be further improved by including the further steps of:  
           [0036]    vii) measuring any resultant imbalance force on the lathe associated with reciprocal movement of the lens cutting tools and generating an output signal;  
           [0037]    viii) sending the output signal to a processor;  
           [0038]    ix) determining how the force may be nullified by varying operation of the actuators;  
           [0039]    x) sending an output to a controller which controls the reciprocal movement of the actuators to cause the controller to vary the movement of the actuators in response to the determination in step (ix) to substantially eliminate the resultant imbalance forces; and,  
           [0040]    xi) repeating steps vi) through x)  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0041]    Preferred embodiments of the invention are described below with reference to the accompanying drawings in which:  
         [0042]    [0042]FIG. 1 is an end elevation of illustrating a typical layout for a prior art single point diamond turning lathe;  
         [0043]    [0043]FIG. 2 is a front elevation corresponding to FIG. 1;  
         [0044]    [0044]FIG. 3 is a front elevation illustrating a three shuttle lens cutting lathe;  
         [0045]    [0045]FIG. 4 is an end elevation corresponding to FIG. 3;  
         [0046]    [0046]FIG. 5 is a front elevation illustrating a form shuttle lens cutting device according to the present invention;  
         [0047]    [0047]FIG. 6 is an end elevation corresponding to FIG. 5;  
         [0048]    [0048]FIG. 7 is a longitudinal section through a typical actuator/shuttle assembly;  
         [0049]    [0049]FIG. 8 is a top plan view of an alternate lathe configuration according to the present invention;  
         [0050]    [0050]FIG. 9 is at front elevation corresponding to FIG. 8;  
         [0051]    [0051]FIG. 10 is a top plan view of another alternate lathe configuration according to the present invention;  
         [0052]    [0052]FIG. 11 is a front elevation corresponding to FIG. 10;  
         [0053]    [0053]FIG. 12 is a top plan view of yet another alternate lathe configuration according to the present invention; and,  
         [0054]    [0054]FIG. 13 is a front elevation corresponding to FIG. 12. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0055]    According to the present invention, accelerative forces arising from reciprocating movement produced by a first mass, which may include one or more shuttles is cancelled by providing a second mass and moving the second mass in a reciprocating movement contra to the reciprocating movement of the first mass. The location and rate of movement of the second mass is selected to create a “balancing” or “cancelling” force opposite to and similar is magnitude to the accelerative forces produced by the first mass. The force created by the second mass should coincide with that produced by the first mass to avoid any undesirable “rocking” motion as a result of the cancelling forces. Although the second mass may simply be present for balancing proposes, as described in more detail below, the second mass is preferably made up of two or more shuttle and lens cutting tool assemblies which may be used as part of the lens cutting operation. Similarly, the first mass preferably consists of one or more shuttle and lens cutting tool assemblies.  
         [0056]    The term “reciprocating” is used herein to refer to a back and forth motion which may, depending on the embodiment of the present invention being described, be either linear or arcuate.  
         [0057]    [0057]FIG. 3 illustrates a lens cutting lathe  100  according to one preferred aspect of the present invention. The lens cutting lathe  100  includes a base  102  mounted to which is a lens support  104  which supports a lens  106  and is capable of spinning the lens  106  about a lens rotational axis θ. The lens support is transversely moveable relative to the lens rotational axis θ as indicated by reference Y.  
         [0058]    Three shuttles are mounted to the base  102  according to the FIGS. 3 and 4 embodiment. These comprise two outer shuttles  108  and  112  an intermediate shuttle  110  therebetween. Respective lens cutting tools  114  are mounted to the three shuttles  108 ,  110  and  112 . The lens cutting tools  114  may be diamond tools of the type currently used for lens cutting.  
         [0059]    The shuttles  108 ,  110  and  112  are reciprocally moveable by respective actuators  118 ,  120 ,  122  along respective shuttle axes or “paths” as indicated by references Z 1 , Z 2  and Z 3 . Although the shuttle axes or paths Z 1 , Z 2  and Z 3  are shown as generally parallel to the lens rotational axis θ, this is not a requirement and it may be preferable for the shuttle axes Z 1 , Z 2  and Z 3  to be inclined relative to the lens rotational axis θ. The shuttle axes Z 1 , Z 2  and Z 3  should be parallel to each other. The actuators  118  and  122  are arranged to move the outer shuttles  108  and  112  in a direction opposite to the intermediate shuffle  110  at a rate half that of the intermediate shuttle  110 . The respective masses of each of the outer shuttles  108  and  112  would typically be generally the same as that of the intermediate shuttle  110 . The lens cutting tools  114  would also be of similar mass.  
         [0060]    [0060]FIG. 7 illustrates a typical shuttle and actuator assembly  200 . The shuttle and actuator assembly  200  includes a linear servo motor  202  which includes a magnet assembly  204  and a coil  206 . The magnet assembly  204  is attached to a housing  208 . The coil  206  is secured to a shuttle  210 . Coil wires  212  provide electrical input to the coil  206  to cause relative movement between the coil  206  and the magnet assembly  204 .  
         [0061]    The shuttle  210  is mounted to the housing  208  for linear movement. Various mounting arrangements may be utilized. A currently preferred mounting arrangement is to use air bearing pads  212  between the housing  208  and the shuttle  210  to allow for smooth, accurate linear motion.  
         [0062]    A position encoder  220  is secured to the shuttle  210 . The position encoder may be a diffraction scale readable by a read head  222  secured to the housing  208  to provide position information to a high speed feedback device  224  which senses the position of the shuttle  210  and provides input to the coil  206  to vary the position of the shuttle  210  in accordance with a pre-determined position stored in a database  226 .  
         [0063]    Force is determined by the following relationship: 
         
       F=m·a 
     
         [0064]    where F=force  
         [0065]    m=mass  
         [0066]    a=acceleration  
         [0067]    Assuming each of the shuttles  108 ,  110  and  112  has a mass m s , and the intermediate shuttle  110  is accelerated and decelerated by an amount a i , the accelerative forces F i  associated with the intermediate shuttle  110  may be defined as: 
         
       F 
       i 
       =m 
       s 
       ·a 
       i 
     
         [0068]    The outer shuttles  108  and  112  together have a combined mass of 2 m s  (the “second mass”). As the outer shuffles  108  and  112  are moved at a rate of half that of the inner shuttle  110 , and in the opposite direction, the acceleration of the outer shuttles  108  and  112  is a i /2. Accordingly, the accelerative force F o  associated with the outer shuttles  112  is: 
           F   o =2 m   s   ·a (− a   i /2) 
         −m s ·a i   
         [0069]    The total force F L  on the lathe  102  at any time will therefore be: 
           F   L   =F   i   +F   o   =m   s   ·a   i   −m   s   ·a   i =0 
         [0070]    If the second mass were other than twice that of the intermediate shuttle  110  (or “first mass” in this case), the rate of acceleration would have to be compensated accordingly. In any case, the acceleration of the second mass should correspond in phase and frequency with that of the first mass and should not induce a resulting moment about the intermediate shuttle. In other words, the forces associated with the outer shuttle  108  should be the same as those associated with the outer shuttle  112 . It is expected that this will usually be accomplished by centrally disposing the intermediate shuttle  110  between the outer shuttles  108  and  112 . It will however be appreciated that other arrangements might work such as compensating for not having the intermediate shuttle  110  centrally disposed by varying the respective masses and accelerations of the outer shuttles  108  and  112 .  
         [0071]    [0071]FIGS. 5 and 6 illustrate another embodiment of the present invention according to which four shuttles  150 ,  152 ,  154  and  156  are provided. The shuttles  150 ,  152 ,  154  and  156  are arranged in a row and may be considered as comprising two pairs of shuttles  158  and  160  respectively on opposite sides of a central axis  162 , with shuttles  150  and  152  comprising a first pair  158  and shuttles  154  and  156  comprising a second pair  160 .  
         [0072]    Respective actuators  164 ,  166 ,  168  and  170  are provided for the shuttles  150 ,  152 ,  154  and  156  to move the shuttles along respective parallel shuttle axes or “paths” Z 1 , Z 2 , Z 3  and Z 4 , all of which while shown as also parallel to the central axis  162  and lens rotational axis θ need not be so. The respective shuttles  150  and  152  of the first pair  158  are arranged to move in opposite relative directions. Similarly, the respective shuttles  154  and  156  of the second pair  160  are arranged to move in opposite relative directions, but in phase with the first pair  158 . In other words, the shuttle  150  would move together with (i.e. in the same direction as) one of the shuttles  154  and  156 . Simultaneously, and in the opposite direction, the shuffle  152  would move together with the other of the shuttles  154  and  156 .  
         [0073]    In the four shuttle embodiment, the total mass of the shuttles moving in either direction is similar and accordingly the rate of acceleration would be similar. An advantage to the four shuttle embodiment is that the stroke length over which each of the shuttles  150 ,  152 ,  154  and  156  moves would be similar.  
         [0074]    In the three shuttle embodiment, using the lens cutting tool  114  associated with the outer shuttles  108  and  112  may, in extreme cases, require a longer compensatory stroke than available from the intermediate shuttle  110 . For example, if the actuator has a 30 mm stroke limit and a 20 mm stroke is required for the outer shuttles, the intermediate shuttle  110  wouldn&#39;t be able to deliver the requisite 40 mm stroke for full cancellation of reciprocally acting forces. It is expected however that this can be tolerated as stroke length diminishes toward the lens axis θ where tolerances are most critical. Accordingly, good force resolution should be possible in the more critical zone nearer the lens rotational axis θ.  
         [0075]    [0075]FIGS. 8 and 9 illustrate yet another embodiment of the present invention somewhat analogous to the embodiment described above with respect to FIGS. 3 and 4. In the FIGS. 8 and 9 embodiment, a lathe  100  has respective outer shuttles  300  and  304  and an inner shuttle  302  mounted to a base  306  for reciprocal movement along respective arcuate paths, as exemplified by arrow  308  in FIG. 9. The shuttles  300 ,  302  and  304  are moved by respective actuators  310 ,  312  and  314 , which may be rotational servo-motors.  
         [0076]    As with the FIGS. 3 and 4 embodiment, the actuators  310  and  314  are arranged to move the outer shuttles,  300  and  304  respectively, in a direction opposite that of the intermediate shuttle  302  and at a rate half that of the rate of movement of intermediate shuttle  302 . The respective masses in each of the outer shuffles  300  and  304  would typically be about the same as that of the intermediate shuttle  302 . Respective lens cutting tools  114  would also be of similar mass. Accordingly, forces imparted by movement of the intermediate shuttle  302  would be cancelled by similar forces imparted by movement of the outer shuttles  300  and  304 .  
         [0077]    The arrangement illustrated in FIGS. 8 and 9 could of course be expanded to more than three actuator/shuttle assemblies, for example, in a manner analogous to the four shuttle embodiment described above with reference to FIGS. 5 and 6.  
         [0078]    Although the shuttle arrangement shown in FIGS. 8 and 9 features the shuttle actuators disposed along a common rotational axis parallel to a base, in certain cases the shuttle actuators may be disposed with respective rotational axes perpendicular (or possibly at some other angle) to the base. FIGS. 10, 11,  12  and  13  illustrate two embodiments of the latter type.  
         [0079]    In the FIGS. 10 and 11 embodiment, four shuttles,  350 ,  352 ,  354  and  356  are provided. The shuttles  350 ,  352 ,  354  and  356  have respective actuators  360 ,  362 ,  364  and  366  which may be servo motors. Analogous to the FIGS. 5 and 6 embodiment, the shuttles  350  and  352  comprise a first pair  370  and the shuttles  354  and  356  comprise a second pair  380 . The shuttles  350  and  352  of the first pair  370  are arranged to move in opposite relative directions parallel to a base  390 . The shuttles  354  and  356  of the second pair  380  are also arranged to move in opposite relative directions parallel to the base  390 , but in-phase with the first pair  370 .  
         [0080]    [0080]FIGS. 12 and 13 illustrate a four shuttle embodiment similar to that illustrated in FIGS. 11 and 12, but having one actuator for each pair of shuttles. According to the FIGS. 12 and 13 embodiment, four shuttles,  400 ,  402 ,  404  and  406  are provided. The shuttles  400  and  402  comprise a first pair  410  and are radially disposed on opposite sides of an actuator  420  which may be a rotary servo motor. The shuttles  404  and  406  comprise a second pair  430  disposed on opposite sides of an actuator  440 . The actuators  430  and  440  are mounted to a base  450  and rotate the shuttles  400 ,  402 ,  404  and  406  in arcuate paths parallel to the base  450 .  
         [0081]    The effect of mounting a pair of shuttles in a radially disposed configuration on opposite sides of a single actuator is much the same from a force cancellation perspective as having a pair of shuttles mounted to separate actuators moving in opposite relative directions.  
         [0082]    Use of a rotary servo-motor generates both a rotational and a linear resultant force when the actuator/shuttle assemblies are not balanced. A linear resultant will be observed if the imbalance masses are 180 degrees out of phase. A rotational resultant will be observed if the imbalance masses are in phase. If the actuators are contra rotating the phase angle will constantly change giving both linear and rotational resultant forces.  
         [0083]    In view of the more complex nature of the resultant forces arising in use of rotational actuators not having a common rotational axis, it would be quite complicated to eliminate resultant imbalance with a third actuator. Having four actuators or four shuttles mounted in two pairs to rotationally balance two actuators does however provide a substantially self-cancelling arrangement.  
         [0084]    In order to compensate for minor variances resulting from such things as differences in combined shuttle and lens cutting tool mass or small amounts of asymmetricality in shuttle positioning, it may be desirable to monitor forces and make compensatory inputs to the actuators. FIG. 5 schematically illustrates one manner in which such a compensation may be effected.  
         [0085]    A measuring device  180  connected to the lathe  100  which measures any resultant imbalance force on the lathe  100  which is associated with the reciprocal movement of the lens cutting tools  114  and generates an output signal indicative of the nature and amount of imbalance force. The measuring device may be any suitable device such as one or more load cells or accelerometers. The measuring device may be connected to any suitable part of the lathe  100  such as the base  102  or the actuators  164 ,  166 ,  168  and  170 .  
         [0086]    The output signal is sent to a processor  182  which determines the nature of the force and whether and how it can be nullified by varying movement of the actuators  164 ,  166 ,  168  and  170 . Factors such as direction and phase of the imbalance force might be considered by the processor  182 . The processor  182  generates and sends one or more output signals to one or more controllers  184  which communicates with and control the movement of the actuators  164 ,  166 ,  168  and  170 .  
         [0087]    The controller(s)  184  receive(s) the output signal(s) and vary the reciprocating movement caused by the actuators  164 ,  166 ,  168  and  170  in response to the output signal(s) to reduce the resultant imbalance force. The monitoring and compensation may be repeated at least periodically.  
         [0088]    Depending on the degree of balance and any harmonic frequencies associated with the spindle rotation, it may prove more effective to do an “air pass” i.e. without cutting and while holding the spindle stationary. This could be repeated for each shuttle/actuator selected for cutting in turn as the dynamics may be slightly different for each shuttle/actuator combination selected for cutting at any given time. The variances might be stored by the processor to provide an initial setting and minimize set-up time.  
         [0089]    The above description is intended in an illustrative rather than a restrictive sense. Variations to the embodiments described may be apparent to persons skilled in such structures without departing from the spirit and scope of the invention as defined by the claims set out below.