Method and apparatus for optical polishing

Abstract: A lapping tool for localised optical polishing of a workpiece, the tool having a flexible working surface and being characterised by means for selectively varying the pressure applied, in use, on the workpiece by different regions of the tool working surface whereby to vary the effective area of contact with the workpiece. A method of optical polishing and optical workpiece using a lapping tool whose maximum working surface area is substantially smaller than the workpiece, comprising determining the path to be travelled by the tool across the workpiece, and determining the pressure and effective area of contact of the tool on the workpiece, in order to achieve the next stage of polishing, and then driving the tool over that path while dynamically varying the said applied pressure and effective contact area. Apparatus for guiding a body, such as an optical polishing tool, over a generally flat structure, such as an optical workpiece, comprising a three dimensional drive mechanism for the controlled movement of the body across the surface of the structure, and a pivoted linkage (14,15) linking the drive mechanism to the body such as to constrain the body to pivotal motion about a virtual pivot point (P) which is fixed relative to the drive mechanism (11) and is located at the interface between the body and the workpiece.

Description: This invention relates primarily to the optical polishing, lapping or figuring of optical surfaces, and is particularly useful in the production of large mirrors which may be spherical or aspherical and may be of eccentric shape. However, some of the apparatus disclosed herein has broader applications such as the precision movement of a robot arm relative to a generally flat surface. The well-established process which the invention enhances consists of grinding, polishing and figuring, with loose abrasive or abrasive particles in a softer matrix. A pad or lap is used to apply the abrasive to the workpiece. "Figuring" is continued polishing, applied differentially over the surface to produce very fine changes of surface height. The "tool" is part of the machine. It carries the pad which applies the abrasive to the workpiece. The diameter of this pad is conventionally referred to as the diameter of the tool. The process is usually wet. After some polishing, the workpiece is cleaned and optically tested. The optical test identifies parts of the surface which, although polished, are erroneously proud of the desired profile. Further polishing is applied preferentially in the proud areas. This can be achieved typically by varying the pressure or speed of the polishing pad. Numerous attempts to reduce the errors may be necessary and the process is iterative. The invention aims to enhance the speed and accuracy of this iterative process. The invention provides a new type of tool for localised polishing making possible a fast, automated, computer-controlled machine with applicability to a wide range of products with optical quality surfaces. For surfaces which are not spherical (parts of spheres), iterative figuring is a lengthy process. Mechanisation of the former hand-craft has been attempted with some success (see below), but has not provided a technique with the versatility of hand-craft. Large and small tools have different problems. On the one hand, the best quality is achieved with large tools. They generally conform to the desired aspherical shape of the surface. However, those tools are built and rebuilt for particular workpieces and are often operated in an expensive research and development context with access to the engineers who designed them. On the other hand, small tools may more accurately simulate hand-polishing and are versatile. Unfortunately, if they operated automatically, they tend to create residual defects which are difficult to remove with the same tool. For example, the edges of the tool may create many ridges or grooves which are narrower than the tool itself. They will be visible in the optical test but attempting to remove them with the same tool may create a fresh set of similar defects, slightly moved. Changing the tool introduces further problems. Removing tool-induced features by applying gradually less work in each pass, or by using feathered-out strokes of the tool, is very slow. These problems, whilst not invalidating the process, greatly prolong the series of iterations and the general complexity. With the benefit of the present invention, it is possible, under continuous control, to provide a tapered action with no sharp edge and a variety of profiles for the glass removal function. US patent 4,128,968 (1978) described an automatic polishing machine in which the effect of the edge of a small tool is reduced by local sub-motion of the tool. When the small tool is dithered in position, the centre of the contact area is continuously polished but the edge of the area is less polished. The "removal profile" is specified as being circularly symmetrical. The tool has a pattern of motion which may be helical over the whole surface. The tool itself can rotate on a subsidiary axis which can be the local axis of the tool. Dithering is a method or pattern of operation of the whole tool which attempts to remove the errors it otherwise produces. It is outdated by software: the pattern of motion of the tool can be computed optimally to improve the particular work piece, rather than to produce an arbitrary circular removal profile, which is not necessarily what is required. Also, the dithering effect inevitably makes the locally polished area larger, which again is not necessarily required. US patent 5,157,878 (1992) describes a polishing tool consisting of a running tape pressed against the workpiece. UK patent application 2 259 662 (1993) describes a machine applicable to complex aspherical spectacle lenses which resembles a multi-axis milling machine with an unspecified polishing head substituted for a cutting head. The University of Arizona and University College London have described machines with a complex tool of a diameter approximately half or more the diameter of the workpiece. Essentially all the active area of the tool is available to contact with the workpiece at any instant. The tilt angle of the tool is defined by its contact with the workpiece. An example is published in ESO Conference and Workshop Proceedings No.42, pages 215 - 218, ESO Garching, 27 -30 April 1992, "The Production of Highly Aspheric Secondary Mirrors Using Active Laps" by D.D. Walker et al. The Zeiss company has described a machine with an elongated complex tool whose length is approximately half the diameter of the workpiece. As with the large tool described above, essentially all the active area of the tool is available to contact with the workpiece at any instant. The tilt angle of the tool is defined by its contact with the workpiece. In their Patent Application GB-A-2163076, the complex tool is substantially coextensive with the workpiece. US-A-4 958 463 discloses the features contained in the preambles of claims 1 and 14, respectively. The present invention is defined in the independent claims 1 and 14. The invention provides a tool for localised optical polishing incorporating a principal actuator of position applying a controllable central enhancement of applied pressure of the tool onto the workpiece within the available area of the tool such that the area of contact of the tool with the workpiece (over which area a given useful polishing pressure is applied) is controllable within the overall area of the tool. The total force applied by the tool to the workpiece can be controllable. By adjustment of the total force and the rate of pressure taper, the pressure can taper to zero at a controllable notional boundary (including a circle) which lies within the area of the tool. The tool can be produced and operated such that its internal distribution of polishing pressure has a single peak. In some such embodiments, the edges of the tool outside the effective diameter can be lifted clear of the workpiece. The invention can be operated such that the pressure applied to the workpiece tapers to a lower pressure value from the centre towards the edges of the tool while not necessarily falling to zero within the tool. The rate at which the pressure tapers off from the centre of the tool to the edges is controllable. In some such embodiments, the polishing pressure at the edges of the tool can be reduced to less than ten per cent of the peak pressure within the tool. The overall pressure distribution may correspond approximately to a truncated Gaussian distribution. In an embodiment, the invention is also an apparatus for localised optical polishing as described above incorporating a principal actuator of position and one or more further similar actuators such that the central pressure exerted by the tool or the total force exerted by the tool normally onto the workpiece can be controlled independently of the tapering pressure function. According to the invention, the apparatus for localised optical polishing is such that its radius of curvature on the polishing surface is controllable. Preferably, the apparatus for localised optical polishing as described above incorporates a principal actuator and three further actuators of position such that the angle of attack of the tool against the workpiece is controllable. Further, it is preferred that in the apparatus for localised optical polishing as described above all the parameters described are dynamically controllable as the tool polishes. The diameter of the tool is typically less than 25 per cent of the diameter of the workpiece. The pressure distribution applied by the tool can be axially symmetrical. The tool can be mounted on driven bearings which enable it to sweep over the workpiece in any desired pattern of motion at controllable speeds. The tool can be mounted on a subsidiary motorised spindle which may be on an axis of symmetry of a tool. The tool may controllably rotate on this spindle as part of its polishing motion. The workpiece may be mounted on a turntable which continuously rotates. The tool can be activated to move over a fixed workpiece in a pattern which gives the same effective work as if the workpiece were rotating. In order that the invention may be better understood, a number of embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1A is a schematic axial section through part of a lapping tool embodying the invention; Figure 1B is a plan view of a spring arranged transversely within the tool illustrated in Figure 1A; Figure 1C is a plan view of a set of annular pressure rings within the tool of Figure 1A; Figure 2 is an axial section through a lapping tool in accordance with a further embodiment of the invention; Figure 3 is a schematic cross section, corresponding to the view of Figure 2, of the embodiment of Figure 2, identifying more clearly the support arrangement for the tool head; Figure 4 is a schematic cross section along the line A to A of Figure 3; Figure 5 is a schematic section corresponding to the view of Figure 2, showing the working portion of the tool in different configuration; and Figure 6 is a graph showing the pressure profile of the flexible working surface of the tool of Figure 5 in use, as a function of radius from the centre of the tool. An exemplification of the invention is illustrated in Figure 1. The polishing action is provided by pads of pitch (7). The workpiece (not shown) is in contact with the lower side of the pitch pads. The pads of pitch adhere to a thin membrane of stainless steel (6). The membrane is supported by and cemented to a compressible layer (5) of neoprene, rubber or plastics or a foam of those materials. On the upper side of the compressible layer, a further stainless steel membrane (4) is cemented. Pressure is applied to this upper membrane by a set of annular pressure rings (8). The annular pressure rings (8) are spot welded to a flat eight-armed spring (9). A principal actuator is shown in outline (1) and described in detail here. It is a motorised screw drive in the form of a commercially available stepper motor with a hollow threaded rotor driving a vertical plunger. The plunger itself is extended with a compression spring attached to the acting (lower) end of the plunger. A standard commercially available load cell is fitted within the compression spring. The principal actuator enables the pressure in the centre of the working area of the tool to be increased as the actuator is operated vertically downwards. A stiffening ring (3) is brazed onto the upper side of the eight-armed spring (9). The edge actuator (2) shown is one of three angularly-spaced edge-actuators which are constructed in a similar way to that described above for the principal actuator (1). They are equi-spaced around the periphery and push or pull onto the stiffening ring (3). They also incorporate load cells and serve to control both the tilt of the lower pitch face and the overall pressure and shape, in conjunction with the principal actuator (1). The tool is enclosed in a cast aluminium housing (10) with an access lid (12) which in practice carries electrical connectors and wiring (not shown). The housing (10) has a supporting flange or lugs (11). The stepper motors are computer-driven by means of a standard integrated-circuit stepper-motor controller with a power amplifier stage. The force values indicated by the load cells are available to the computer by means of standard analogue-to-digital converters. The tool is mounted by means of the flange or lugs (11) onto two coordinate cross-slides driven by stepper motors and rack and pinion gearing. This drive system can also incorporate means for spinning the tool and if so also incorporates slip rings for the electrical supplies to the tool. The total downward force of the tool onto the workpiece is in the range 8 to 50 grams per square centimetre of the working area. Active control of the motion of the tool includes control of the pressure distribution exerted by the tool on the workpiece e.g. mirror, and of the distribution of speed of the moving tool in relation to the mirror (stroke, rotation). As disclosed more fully in our paper mentioned above entitled "The production of highly aspheric secondary mirrors using active laps", the control uses feedback to the operator while the machine is running. The machine telemeters pressure distribution, relative velocity distribution between tool and mirror and total lateral frictional force (drag) on the tool (lap). By calibration, it is possible to feedback ablation rate and to do so repeatedly for each point on the mirror surface during polishing. The rates are integrated to estimate how the mirror profile is evolving; this is displayed and the load pattern changed appropriately. At the end of each run, not only is the optical figure compared to that desired, but the predicted change is compared to the observed change. The algorithm for determining ablation is then adjusted accordingly, so the system learns. A second embodiment of the invention will now be described with reference to Figures 2 to 6. The tool (not shown) is attached to a flexible diaphragm 21 which is fixed to the tool-head 22. The tool-head rotates on bearings 23 about the fixed hollow shaft 24 and is driven through the metal bellows flexible coupling 25 which allows the tool to be loaded axially. The channel in the hollow shaft enables the air pressure behind the flexible diaphragm to be varied. The flexible coupling 25 is connected to the rotating pulley wheel 27 by the knurled nut 26 which permits different sizes of tool-holders to be interchanged. The pulley wheel is mounted on the ball-bearing 28 and is driven by the toothed belt 29 from the smaller pulley wheel 30 which is in turn driven through the flexible shaft 31 from an independently mounted electric motor (not shown). The pulley drive assembly is mounted on the plate 32 which in turn is connected to an intermediate mounting plate 33 by two links 34 each hinged at one end to the plate 33 and at the other end to the plate 32. These links constrain the axis of plate 32, and hence the tool, to rock in the plane of the drawing about a virtual pivot point P close to the centre of the tool. This mechanical linkage is shown also in Figure 3, which is diagrammatic only and not to scale. Additional links 44, shown in the orthogonal sectional view of Figure 4, are provided so that there can be a similar movement in the orthogonal plane, with the relative movement of the tool constrained to pivotal rocking motion about the virtual pivot point P. The tilt is controlled by two double-acting hydraulic cylinders 35 (one only is shown) coupled to position transducers (not shown) so that the tool angle can be accurately set using positional information feedback to a control circuit. The fixed hollow shaft 24 is connected to the loading rod 36 which is constrained to move axially, relative to a rigid housing 45 connected to the plate 32, by flexural supports 37. The axial force applied by the loading rod 36 to shaft 24, and hence the tool, is set by the solenoid 38 through the load cell 39 which measures this force. The actual force applied to the tool will differ from this because of the spring constants of the flexural supports 37 and the bellows coupling 25, both acting in the axial direction. The position transducer 40, which in this example is an LVDT (linear variable differential transformer), measures the axial movement of the loading rod and provides a signal from which the axial spring force can be determined, for correcting the load-cell reading. The lateral force exerted on the tool by friction with the work piece is measured by strain gauges 41 mounted on the loading rod 36 which is locally thinned so that it bends in response to this force. The flexible tube 42 is connected to the central channel in the loading rod 36 and feeds the air for pressurising the flexible diaphragm 21. The pressure is controlled as described below, and the air under pressure is supplied from a standard pump (not shown). It will however be appreciated that other fluids including liquids could be used. In this second exemplification, variation in the contact area for polishing is performed by calculated compression of a soft tool under computer control. One effect of the method exemplified is that the polishing pressure is maintained approximately constant for a range of contact areas and total forces. In Figure 2, the soft tool is shown as a rubber diaphragm inflatable by air. It is 50mm in diameter and 2mm thick for a workpiece of 250mm diameter, or larger or smaller in proportion to the workpiece size, or for polishing larger or smaller areas. The diaphragm is inflated with computer-controlled pressure, or temporarily or permanently sealed, so as to bulge. The air pressure is a close approximation to the required polishing pressure. The bulge of the tool is covered on the work side with a polishing material. The polishing material can consist of, for example, cloth, felt, soft polyurethane foam or a mosaic of pitch segments on cloth and can be glued onto the diaphragm. The material can polish in the normal way with the addition of a fine abrasive. The tool is moved towards the workpiece by the machine under computer control, to positions ranging from first contact, then increasingly close so that the bulge is compressed giving increasing contact area. This sequence is illustrated very schematically in Figure 5. When that occurs, the inflated space is compressed, but as its volume is reduced by less than 10 per cent the air pressure increases by less than 10 per cent. Therefore the polishing pressure as determined by the air pressure is constant to within 10 per cent. When the required contact area is obtained in this way the tool and workpiece are rotated or moved by computer-controlled motors in any way required for polishing. The tool may be used in a different mode. In this case, the tool is pressurised or partially evacuated so that the diaphragm approximates to the mean radius of curvature (concave or convex) of the workpiece. The air pressure is then modified slightly to create a distributed PRESSURE exerted by all (or part of) the surface of the tool in contact with the workpiece. An increased air pressure will result in a pressure distribution increasing towards the centre of the tool. The actual pressure distribution is governed by both the air pressure and the tension in the rubber. By choosing an appropriate radial variation in the thickness of the rubber, a pressure distribution may be achieved, which tapers to zero at the edge of the contact area with the workpiece. This is illustrated schematically in Figure 6, showing the pressure profile with radius. Conversely, by decreasing the air pressure, an effect similar to a ring-lap may be achieved. The machine carries the tool on three perpendicular motorised slides. They can change the position of the tool under computer control in relation to the workpiece as described. In addition, a faster change of position and force is provided by means of the solenoid actuator 38. Errors due to friction are reduced by using the flexures 37 rather than slides within the tool. The total force (pressure times area) exerted towards the workpiece increases as the contact area increases. This total force is encoded by a load cell. The applied force as measured by the load cell can be compared with the force predictable from the position in order to validate the operation, as described above. Other load cells such as gauge 41 encode the polishing force in a direction parallel to the surface of the workpiece. The rate of removal of glass (ablation) during polishing depends on several factors including the pressure, speed and lateral drag force of the tool. Force values are available to the computer from load cell readings. The position of the tool is determined by the three perpendicular motorised slides mentioned above, and by the position of the turntable supporting the workpiece, whose rotary position is also encoded. The speed of a spinning tool (if used) is estimated from the motor current or from a rotary encoder. The computer is therefore able to drive the tool at known velocities in relation to the workpiece. Empirical physical laws are defined for a particular workpiece. According to these physical laws, the rate of ablation is proportional to the product of the pressure, speed and polishing time, or the drag force, speed and time. The constants of proportionality are estimated at the start of the work. To do this the rate of glass removal is measured using a conventional optical test before and after a period of work on part of the workpiece area. Having determined the physical law under current conditions, the computer can numerically integrate the instantaneous ablation rate and can therefore estimate a contour map of actual glass removal while working. It can use the estimated contour map to achieve a good approximation to the required result. The constants of proportionality can be redetermined in successive work cycles. The time spent passing over high areas can be increased to ablate them preferentially, which is a well-known process for rotating tools. WITH THE BENEFIT OF THE PRESENT INVENTION, the computer drive can position the tool over a high spot of the workpiece and adjust the contact area of the tool to match the high spot. Thus the high spot can be lowered by polishing without simultaneously reducing surrounding areas. Without the invention, erroneous work is done in areas close to or around high spots and can make the surface too low in those areas. Low areas are more difficult to remove than the original high spots - as glass cannot be added, low areas potentially lead to a need to repolish all the remaining surface. Another method of measuring the force applied to the workpiece is to use load cell devices supporting the workpiece (rather than in the tool). The workpiece is normally on a conventional rotating turntable, and in that configuration the load cells can be between the turntable and the workpiece. They can rotate with the workpiece and can be connected electrically through an axial tubular shaft which drives the workpiece, with the aid of slip rings and/or optical free-space data transmission. A variety of tools can be constructed for one machine and the maximum contact area of a tool will normally be less than one quarter of the area of the particular polished surface. The rubber diaphragm can also be manufactured so that it is set flat or curved under zero pressure. To produce a tapering-off of the applied pressure at the edge of the bulge contact area, increased thickness of diaphragm rubber can be used. Similarly the rubber may have non-uniform thickness. A facility for spinning the tool around its axis is optionally included. It is operated when it is required to increase the polishing speed. This second exemplification also includes a method of pointing the tool directly at the workpiece with the required angle of attack. Normally this angle is such that the axis of the tool is orthogonal to the polished surface at the central contact point. However, the option is included of non-orthogonal axes, in which case the polishing action occurs towards or at the edge of the tool. The method of controlling the tool angle is described as a virtual pivot, as described above in relation to Figures 2 to 4. It consists of hinged plates or rods arranged and actuated to tilt the rest of the tool assembly approximately around the centre of the tool contact area. The advantage of this virtual pivot is that the angle of attack can be changed without swinging the tool across the workpiece, as would occur if the pivot point were distant from the tool. Furthermore, there is little or no reaction of the frictional drag of the polishing tool against the actuators which change the angle of attack, thereby minimising the actuator force requirements. Although the invention has been illustrated by two principal machines for optical polishing, other embodiments of the invention, as defined in the attached claims, are possible. The advantages of the virtual pivot are applicable to a wide range of systems, such as robot wrists. Many other ways of varying the contact area and pressure profile of polishing tools could be employed, not limited to the hydraulic, pneumatic or spring-mechanical examples given.

Claims: A method of optically polishing a surface of a workpiece, the method comprising: positioning a tool (22) having a flexible working surface (7,21) substantially smaller than the surface of said workpiece relative to said surface of said workpiece such that there is an area of contact between said flexible working surface and said surface of said workpiece; and characterised in that said tool comprises a rigid support (22) forming a fluid chamber, and said flexible working surface (21) comprises a diaphragm supported by said rigid support and exposed to pressure in said fluid chamber, the method including: controlling the position of said tool (22) relative to said workpiece so as to control the size of said area of contact; and controlling the pressure in the fluid chamber to control the pressure applied to said surface of said workpiece over said area of contact. A method according to claim 1, wherein pressure applied to said surface of said workpiece by said flexible working surface reduces from a maximum value in a central region of said area of contact to zero at the periphery thereof. A method according to claim 1, wherein pressure applied to said surface of said workpiece by said flexible working surface has a peak value and decreases progressively towards the periphery of said area of contact. A method according to claim 1, wherein pressure applied to said surface of said workpiece by said flexible working surface is distributed in the form of a truncated Gaussian distribution with a maximum at the centre of said area of contact. A method according to any preceding claim, including applying a central force on a central portion of said flexible working surface to provide a central enhancement of pressure applied to said surface of said workpiece. A method according to claim 5, including applying forces around said central portion at a region of said flexible working surface spaced transversely from said central portion to provide said central enhancement of pressure applied to said surface of said workpiece. A method according to claim 1, wherein said diaphragm (21) is selected to have a thickness which varies over its area so as to provide a required pressure distribution. A method according to any preceding claim including rotating said tool about an axis to provide a rotary polishing action. A method according to any preceding claim including controlling the angle of attack of said tool against said surface of said workpiece using a principal actuator (35) and controlling the position of said tool using three further actuators. A method according to claim 9, wherein said tool (22) is held in a body (35) by a pivoted linkage (34,44), and the tool angle of attack is controlled so as to pivot the tool about a virtual pivot (P) located at said surface of said workpiece. A method according to any preceding claim including determining a path to be travelled across said workpiece, determining said area of contact and pressure to be applied over said area of contract at positions in the path to achieve the desired polishing, and moving said tool along said path whilst controlling said pressure and area of contact in accordance with the determination. A method of production of an optical component including optically polishing a surface of said optical component using the method of any preceding claim. A method of production of a mirror including optical polishing a surface of the mirror using the method of any one of claims 1 to 11. Optical polishing apparatus constructed for the optical polishing of a workpiece surface of a predetermined maximum area, the apparatus comprising: a holder for holding the workpiece having said workpiece surface of predetermined maximum size; a tool comprising a flexible working surface (7,21) having an area substantially smaller than said predetermined maximum area; actuator means (38) for moving said tool (22) relative to said surface of said workpiece to form an area of contact between said flexible working surface (7,21) and said surface of said workpiece with said tool head (22) spaced from said surface; and characterised in that the tool comprises a rigid support (22) forming a fluid chamber, and said flexible working surface comprises a diaphragm (21) supported by said rigid support (22); and said polishing apparatus comprises control means for controlling said actuator means (38) to control the position of said tool (22) relative to said workpiece to control the size of said area of contact; and control means adapted to control the pressure in the fluid chamber to control the pressure applied to said surface of said workpiece over said area of contact. Apparatus according to claim 15, wherein said tool is adapted to use said flexible working surface (7,21) to apply a pressure distribution to said surface of said workpiece in which the pressure tapers from a maximum value in a central region of said area of contact to zero at the periphery thereof. Apparatus according to claim 14, wherein said tool is adapted to use said flexible working surface (7,21) to apply a pressure distribution to said surface of said workpiece in which the pressure has a peak value and decreases progressively towards the periphery of said area of contact. Apparatus according to claim 14, wherein said tool is adapted to use said flexible working surface to apply a pressure distribution to said surface of said workpiece which is in the form of a truncated Gaussian distribution with a maximum at the centre of said area of contact. Apparatus according to any one of claims 14 to 17, wherein said tool includes a central actuator (1) for applying a central force on a central portion of said flexible working surface (7) to provide a central enhancement of pressure applied to said surface of said workpiece. Apparatus according to claim 18, wherein said tool includes at least one further actuator (2) for applying forces around said central portion at a region of said flexible working surface (7) spaced transversely from said central portion to provide said central enhancement of pressure applied to said surface of said workpiece. Apparatus according to claim 14, wherein said diaphragm (21) is selected to have a thickness which varies over its area so as to provide a required pressure distribution. Apparatus according to any one of claims 14 to 20, including means (11,30,27,25) for rotating said tool about an axis to provide a rotary polishing action. Apparatus according to any one of claims 14 to 21, including an angle of attack actuator means (35) controllable to control the angle of attack of said tool against said surface of said workpiece and two further actuator means controllable to control the position of said tool in a plane perpendicular to the direction of operation of said actuator means. Apparatus according to claim 22, wherein said tool (45) is held in a body (33) by a pivoted linkage (34,44) to allow said tool to pivot about a virtual pivot (P) located at said surface of said workpiece. Apparatus according to any one of claims 14 to 23, wherein said control means is adapted to determine a path to be travelled across said workpiece, to determine said area of contact and pressure to be applied over said area of contact at positions in the path to achieve the desired polishing, and to control the movement of said tool along said path whilst dynamically controlling said pressure and said area of contact in accordance with the determination.

Inventors: BINGHAM RICHARD GEORGE, KIM SUG-WHAN, PUTTICK KEITH ERNEST, WALKER DAVID DOUGLAS, BINGHAM, RICHARD GEORGE, KIM, SUG-WHAN, PUTTICK, KEITH ERNEST, WALKER, DAVID DOUGLAS
Applicants: OPTICAL INVEST LTD, OPTICAL INVESTMENTS LIMITED
Application year: 1996
Main classification: B24B13
IPC codes: B24B41, B24D13, B24B13