Patent Grant
7036408
This invention relates generally to apparatus and methods for cutting lenses and more particularly to turning lathes for cutting non-rotationally symmetrical lenses.
An efficient way to produce rotationally asymmetrical surfaces is with a three-axis single point diamond turning lathe.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The actuator in the above embodiments may be a linear servo-motor.
Alternatively, the actuator may be a rotary servo-motor.
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:
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.
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:
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.
The lens may be turned in three stages with a different of the three lens cutting tools utilized in each stage.
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).
The method may be further improved by including the further steps of:
Preferred embodiments of the invention are described below with reference to the accompanying drawings in which:
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.
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.
Three shuttles are mounted to the base 102 according to the
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 Z1, Z2 and Z3. Although the shuttle axes or paths Z1, Z2 and Z3 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 Z1, Z2 and Z3 to be inclined relative to the lens rotational axis θ. The shuttle axes Z1, Z2 and Z3 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.
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.
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.
Force is determined by the following relationship:
F=m·a
Assuming each of the shuttles 108, 110 and 112 has a mass ms, and the intermediate shuttle 110 is accelerated and decelerated by an amount ai, the accelerative forces Fi associated with the intermediate shuttle 110 may be defined as:
Fi=ms·ai
The outer shuttles 108 and 112 together have a combined mass of 2 ms (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 ai/2. Accordingly, the accelerative force Fo associated with the outer shuttles 112 is:
Fo=2ms·a(−ai/2)
−ms·ai
The total force FL on the lathe 102 at any time will therefore be:
FL=Fi+Fo=ms·ai−ms·ai=0
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.
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” Z1, Z2, Z3 and Z4, 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.
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.
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'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.
As with the
The arrangement illustrated in
Although the shuttle arrangement shown in
In the
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.
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.
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.
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.
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.
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.
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2313830 | Jul 2000 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA01/01012 | 7/11/2001 | WO | 00 | 6/3/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/06005 | 1/24/2002 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5195407 | Takeno et al. | Mar 1993 | A |
| 5320006 | Blocha et al. | Jun 1994 | A |
| 5430504 | Muckenhirn et al. | Jul 1995 | A |
| 6122999 | Durazo et al. | Sep 2000 | A |
| 6523443 | Hof et al. | Feb 2003 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20030183050 A1 | Oct 2003 | US |
