Patent Publication Number: US-8123593-B2

Title: Configuring of lapping and polishing machines

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/126,724, filed May 7, 2008, titled “NOVEL LAPPING AND POLISHING MACHINE CONFIGURATION AND METHOD.” The entire content of the above application is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This document generally relates to configuring of lapping and polishing machines. 
     BACKGROUND 
     A lapping or polishing machine can be used to finish a broad range of materials, e.g., glasses, ceramics, plastics, and metals. The item being finished (lapped or polished) rides on top of the finishing surface (lap), in which the lap is substantially larger than the item being lapped or polished. The lapping or polishing process gradually removes parts of the item to cause the item to have a desired surface profile. The removal process takes place as the result of mechanical interaction or a combination of chemical and mechanical interaction between the item being finished, an abrasive, and the lap surface. This process is generally rate determined by an equation involving both the relative speed between the lap and the finished item and the pressure at the interface. Lapping machine laps can be, e.g., cast iron. Polishing machine laps can include, e.g., any of a family of compounds known as “pitch” or any of a family of synthetic materials such as urethanes. 
     SUMMARY 
     In one aspect, in general, a method of processing a work item includes configuring a first finishing surface of a processing machine using a conditioning tool having a second finishing surface, measuring a contour of the first finishing surface, using the second finishing surface to process the first finishing surface to reduce a difference between the measured contour and a desired contour for the first finishing surface, and using the first finishing surface to process a work item to cause the work item to have a surface contour that is equal to or approximates a specified surface contour. 
     Implementations of the method may include one or more of the following features. Configuring a first finishing surface of a processing machine includes configuring a first finishing surface of at least one of a lapping machine or a polishing machine. 
     Using a conditioning tool having a second finishing surface includes using a conditioning tool having a second finishing surface that has a diameter smaller than a diameter of a surface of the work item. The method includes generating ripples on the first finishing surface as the second finishing surface processes various portions of the first finishing surface, the ripples having a spatial period that is smaller than the diameter of the work item. The method includes canceling effects of the ripples on the first finishing surface when processing the work item using the first finishing surface. 
     Measuring the contour of the first finishing surface includes moving a measurement object across the first finishing surface and measuring positions of the measurement object while the measurement object follows the contour of the first finishing surface. The first finishing surface includes a surface of a material having grooves that cause discontinuities in the first finishing surface, and the measurement object has a diameter that is larger than a maximum width of the grooves. Measuring positions of the measurement object includes at least one of (a) using a linear variable displacement transducer to measure the positions of the measurement object, (b) using a capacitive gauge to measure the positions of the measurement object, (c) using an inductive gauge to measure the positions of the measurement object, or (d) using a displacement measuring interferometer to measure positions of a mirror or retro-reflector attached to the measurement object. 
     The method includes using the first finishing surface to process the work item in parallel to measuring the contour of the first finishing surface and using the second finishing surface to process the first finishing surface. 
     Using the first finishing surface to process a work item includes using the first finishing surface to process a surface includes at least one of glass, ceramic, plastic, or metal. 
     In some examples, configuring a first finishing surface includes configuring a first finishing surface of a solid substrate. In some examples, configuring a first finishing surface includes configuring a first finishing surface of a polishing pitch. 
     In another aspect, in general, a method includes moving a measurement object across a finishing surface of a material of a processing machine for processing surface contours of work items, the material having grooves that cause discontinuities in the finishing surface, the measurement object having a dimension that is larger than a maximum width of the grooves; measuring positions of the measurement object; and determining, using a computer, a contour of the finishing surface based on the measurements of the positions of the measurement object. 
     Implementations of the method may include one or more of the following features. The method includes comparing the determined contour with a desired contour for the first finishing surface, and using a conditioning tool to process the first finishing surface to reduce a difference between the determined contour and the desired contour for the first finishing surface. The method includes using the first finishing surface to process a work item to cause the work item to have a specified surface contour. 
     Measuring positions of the measurement object includes at least one of (a) using a linear variable displacement transducer to measure the positions of the measurement object, (b) using a capacitive gauge to measure the positions of the measurement object, (c) using an inductive gauge to measure the positions of the measurement object, or (d) using a displacement measuring interferometer to measure positions of a mirror or a retro-reflector attached to the measurement object. 
     The material includes polishing pitch. 
     In another aspect, in general, an apparatus includes a material having a first finishing surface to process a surface of a work item; a measuring tool to measure a contour of the first finishing surface; and a conditioning tool having a second finishing surface to process the first finishing surface to reduce a difference between the measured contour and a desired contour of the first finishing surface. 
     Implementations of the apparatus may include one or more of the following features. The material includes grooves that cause discontinuities in the first finishing surface. The material includes polishing pitch. The measuring tool includes a measurement object that moves across the first finishing surface, the measurement object having a size that is larger than a maximum width of the grooves. The measuring tool includes a stage to support a sensor that senses positions of the measurement object, the stage constraining movements of the measurement object. The stage constrains the movements of the measurement object such that the measurement object has a single degree of freedom. The measurement object has a smooth surface that contacts the first finishing surface, the smooth surface having a dimension that is larger than the maximum width of the grooves. The measuring tool includes at least one of a linear variable displacement transducer to measure positions of the measurement object, a capacitive gauge to measure positions of the measurement object, an inductive gauge to measure positions of the measurement object, or a displacement measuring interferometer to measure positions of a mirror or a retro-reflector attached to the measurement object. 
     The second finishing surface has a size that is smaller than a size of a surface of the work item. 
     The second finishing surface has a diameter that is less than one-half of a diameter of a surface of the work item. 
     The substrate includes at least one of cast iron or granite. 
     The substrate has a diameter of at least 3 feet, and the measuring tool has a resolution of 10 microns or smaller. 
     The apparatus includes a controller to control processing of the work item by the first polishing surface in parallel to using the measuring tool to measure the contour of the first finishing surface and using the conditioning tool to process the first finishing surface. 
     The apparatus includes a positioning device to position the measuring tool. The positioning device includes a precision slide mechanism. 
     The apparatus includes a positioning device to position the conditioning tool. The positioning device includes a precision slide mechanism. 
     The apparatus includes a pressure control device to control a pressure applied by the second finishing surface to the first finishing surface. 
     In another aspect, in general, an apparatus includes a material having a finishing surface to process a work item, the material includes grooves that cause discontinuities in the finishing surface; a measuring device to measure a contour of the finishing surface, the measuring device includes a measurement object that moves across the finishing surface and having a dimension larger than a maximum width of the grooves; and a data processor to determine a contour of the finishing surface based on the measurements of positions of the measurement object. 
     Implementations of the apparatus may include one or more of the following features. In some examples, the material includes polishing pitch. In some examples, the material includes a solid substrate. 
     The measuring device includes a measurement object that moves across the first finishing surface, the measurement object having a diameter that is larger than a maximum width of the grooves. The measuring device includes a stage to support a sensor that senses positions of the measurement object, the stage constraining movements of the measurement object. The stage constrains the movements of the measurement object such that the measurement object has a single degree of freedom. 
     The measuring device includes at least one of a linear variable displacement transducer to measure the positions of the measurement object, a capacitive gauge to measure positions of the measurement object, an inductive gauge to measure positions of the measurement object, or a displacement measuring interferometer to measure positions of a mirror or retro-reflector attached to the measurement object. 
     The second finishing surface has a diameter that is smaller than a diameter of a surface of the work item. 
     These and other aspects and features, and combinations of them, may be expressed as methods, apparatus, systems, means for performing functions, program products, and in other ways. 
     Advantages of the aspects, systems, and methods may include one or more of the following. Work items, such as optical elements, can be processed with high accuracy. The lapping machine or polishing machine can be easily operated through a control computer. Other features, objects, and advantages of the invention will be apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are perspective views of an example polishing machine. 
         FIG. 3  is a perspective view of an example measuring stage. 
         FIG. 4  is a cross-sectional diagram of an example conditioning tool. 
         FIG. 5  is a block diagram of an example control computer and the polishing machine. 
         FIG. 6  is a flow diagram of an example process. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
       FIGS. 1 and 2  are perspective views of a polishing machine  100  as viewed from different viewing angles. 
     Referring to  FIG. 1 , the polishing machine  100  includes a layer of polishing pitch  112  having a finishing lap surface  102  that processes work items  104  (e.g., optical elements) to cause the work items  104  to have desired surface contours (e.g., plano, convex, or concave contours). A small conditioning tool  106  operated under computer control configures and corrects the shape of the finishing lap surface  102  based on real-time measurements of the contour of the lap surface  102 . The conditioning tool  106  corrects the shape of the finishing surface  102  as soon as the measurements are made by a measuring device  108 . An error map is generated by comparing the measurements of the finishing surface contour with a desired contour, and the conditioning tool  106  processes the lap surface  102  to reduce the error. 
     In this document, the item being finished is referred to as a “work item,” which can be, e.g., an optical element or any other object that requires fine surface shape and smoothness. 
     The small-tool lap figuring process can be used for flat (plano) surfaces as well as for concave or convex finishing surfaces, or other surface geometries. Based on the measured error in the figure of the lap surface  102  (deviation from a desired shape), a computer algorithm calculates the pressure or dwell profile for control of the motion of the conditioning tool  106  to remove the observed errors. 
     In some implementations, operating the polishing machine  100  to process the work items  104  involve the following steps: (a) measurement of the lap contour and creation of a surface map; (b) comparison of the existing lap surface to a desired surface; (c) calculation of the required motion and pressure profile for the conditioning tool  106  as it moves over the lap surface  102 ; (d) execution of the calculated conditioning tool program; and (e) adjustments by an operator to adjust the desired or target lap shape against which the actual shape is compared to achieve the desired final part requirements. These steps are repeated continuously to converge on a surface figure for the work item that meets design specifications. 
     The polishing machine  100  has several advantages. For example, the measuring tool  108  can measure the lap surface profile with a high resolution to detect small figure errors. The measuring tool  108  can have micron or sub-micron resolution. The conditioning tool  106  can be small, and the motion path and pressure profiles of the conditioning tool  106  can be controlled by computer programming, resulting in a more accurate and predictable lap surface contour. The process for conditioning the lap surface  102  provides for a deterministic means of shaping the lap surface  102  with less operator intervention, as compared to a conventional method of examining a sample work item that is processed by the lap surface  102  to determine how the lap surface  102  should be corrected. 
     The polishing machine  100  may also have the following advantage. Undesirable surface characteristics imparted by small-tool finishing can be avoided by using the small conditioning tool  106  to process the lap surface  102 , and using the lap surface  102  to process the work item  104 , instead of using a small tool to process the work item  104  directly. In conventional small-tool finishing, where a small conditioning tool is used to process the surface of the work item directly, “ripples” may be imposed on the surface of the work item as the small conditioning tool processes one portion after another on the surface of the work item. When using the polishing machine  102 , although the small conditioning tool  106  may cause ripples to occur on the lap surface  102 , the small conditioning tool  106  can be selected so that the ripples have a spatial period smaller than the dimension of the work item  104 . The ripples are not transferred to the work item  104  because of the motion and shape averaging that occurs as the lap surface  102  sweeps under the work item  104 . 
     In the example of  FIG. 1 , the polishing pitch  112  is disposed above a heavy substrate  190 , which is rotated by a belt drive mechanism  192 . Other drive configurations, such as direct drive, are also possible. Each work item  104  is placed in a carrier ring  194 , which has an opening to allow a lower surface of the work item  104  to contact the lap surface  102 . In some examples, the carrier ring  194  rotates as the lap surface  102  rotates, allowing the lower surface of the work item  104  to be uniformly processed by the lap surface  102 . The polishing pitch  112  can be made of a combination of materials, e.g., wax, pine tar pitch, and bituminous elements. The polishing pitch  112  may be viscous and change shape as pressure is applied to it. 
     Process for Conditioning the Lap Surface 
     One of the steps in conditioning the lap surface  102  is to measure the actual lap surface  102 . In some implementations, data that are produced from measurement of the lap surface  102  can be used to generate a motion and pressure profile for the conditioning tool  106 . The measurement can be a densely populated surface map of the entire lap surface  102 , with data points spaced as a function of the overall size of the lap surface  102  and the size of the conditioning tool  106 . For example, the density may vary from several points per square inch to one point for every 2 to 4 square inches. The goal is to have sufficient data to capture the spatial frequencies of concern to the user without over-burdening the computer calculation of overall lap shape. 
     For ease of calculation, the location of each measurement can be planned and executed with computer control of the measuring device  108 . In some examples, algorithms used to analyze regular pixel spaced data from charge coupled device (CCD) arrays can be adapted to measure the finishing surface contour. The measurement accuracy, precision, and resolution of the measuring device  108  can depend on the requirements of the work items  104 . For example, the measuring device  108  can use linear variable displacement transducers (LVDTs), inductive or capacitance gauges, or displacement measuring interferometers (DMIs), each having micron or sub-micron resolution. As described below, lap measurements can be referenced either to a linear slide  110  used to translate the measuring device  108  over the lap surface  102 , or to an external reference such as a precision bar mirror. The data that are collected by the measuring device  108  can be analyzed using a variety of techniques, such as MetroPro software, available from Zygo Corporation, Middlefield, Conn. 
     Once a map of the actual lap shape is generated, a comparison is made to a desired surface profile, which can be mathematically defined. In some examples, the measurement data for the lap surface  102  is reduced (e.g., averaged over small regions) to reduce the computing time. In the case of a plano target shape, any deviation of the measured lap surface profile from a best-fit plane defines the error. For example, in the case of comparison to a desired spherical surface, errors can be defined as deviations of the measured lap surface profile from a mathematically defined spherical surface superimposed on the lap surface  102 . For example, the MetroPro software can be used to generate the error map. 
     After an error map is generated, the motion and pressure profiles for the conditioning tool  106  is calculated. Calculation of the motion and pressure profiles of the conditioning tool  106  follows from an analysis of the error map and can include characterization of a work function, i.e., the surface deformation created by the conditioning tool  106  when applied at a given pressure and with known motion characteristics, and convolution of this work function over the error map. 
     In some implementations, the polishing pitch  112 , the substrate  190 , and associated support mechanism of the polishing machine  100  may have a mass of many tons and do not lend itself well to frequent changes in velocity. Furthermore, changes in the rotation rate of the lap may translate into undesirable errors in the shape of the work item surface. The small conditioning tool  106  achieves variation in local lap surface change through tool rotary speed about its own axis and/or pressure. For example, pressure variation can be applied to the pitch polishing lap where shape change occurs as the result of viscous flow of the pitch. 
     The work function of the conditioning tool  106  is generated by measuring the “before” and “after” of a surface of the polishing pitch  112  that is modified using a known pressure profile of the conditioning tool  106 . For example, a pitch lap is measured and mapped. The conditioning tool  106  is placed at a known radial position and held in this position at a fixed pressure through, e.g., 5 revolutions of the polishing lap. The lap surface  102  is re-measured and re-mapped, and the effect of this particular pressure setting is characterized. Several additional trials may follow with different pressure settings so that a linear (or non-linear) relationship can be established between pressure and lap deformation. This functionally derived relationship is stored in computer memory (or a hard drive or other storage medium) and used in subsequent profile calculations as the required work function. This characterization is repeated and modified as conditions change when, for example, the pitch ages, a new type of pitch is used, the size of the conditioning tool is changed, the temperature of the environment changes. 
     The required motion and pressure profiles for the conditioning tool  106  are translated to a language that can be used by a machine control system that controls the position and pressure of the small conditioning tool  106 . For example, a precision slide mechanism  116  that moves a supporting stage  198  that supports a hydraulic device  114  and the conditioning tool  106  along a radial direction. The hydraulic device  114  determines the pressure the conditioning tool  106  applies to the lap surface  102 . 
     The conditioning tool  106  is controlled by a control computer  160  ( FIG. 5 ) with closed loop position and velocity or pressure control. In some examples, the computer  160  may execute software that controls the conditioning tool  106  according to instructions in a G-Code program. G-Code is a set of commands that define certain types of motions and can be applied across a wide range of commercial products. For example, there are commands that can be used to control peripheral devices, such as turning on or off motors, turning on slurry or cooling systems, varying flow or pressure settings. G-Code can be generated by software or post-processors that take path and speed data from one program (e.g., the error mapping software) and automatically generate the line-by-line instructions needed to cause the conditioning tool  106  to move to selected positions and impart selected pressures to the lap surface. 
     After the G-Code programming is completed, the execution of the program can be performed by the computer  160  without operator intervention. The polishing machine  100  can be set up such that a new metrology sequence is initiated as soon as the program execution has been completed, and a new round of lap surface mapping, error map generation, and correction steps are performed in a continuous loop of automated machine measurement and correction. 
     A human operator may intervene from time to time, such as when parameters (e.g., material type, room environmental conditions, aging of the lap surface) change over time, causing the overall outcome of the polishing process to vary. The operator may perform minor adjustments to compensate for the variations. For example, the operator may make minor adjustments to the shape of the lap surface  102  from time to time. For example, the changes can be small changes in the concave or convex radius of the curvature of the lap. 
     In some implementations, the polishing machine  100  allows an operator to enter the exact radius of curvature through the control computer  160 . This makes the polishing machine  100  easier to operate, allowing lower skilled technicians to process work items with high precision. For example, the change process can be as simple as entering one number—the radius of curvature desired—through a graphical user interface (GUI)  178  provided by control computer  160 . Other types of control change can be used, such as using graphical “sliders” and other GUI tools offered, for example, in LabVIEW, a program available from National Instruments, Austin, Tex. 
     Polishing Machine Features 
     The polishing machine  100  includes a lap metrology system for measuring the contour of the lap surface  102 . The lap metrology system includes the measuring device  108  and a positioning subsystem for controlling the position of the measuring device  108 . The measuring device  108  can be configured to resolve dimensions on the order of 1 micron or less and can interface with a computer data acquisition system. 
     Referring to  FIG. 3 , in some implementations, the measuring device  108  can include a metrology stage  196  having a puck  130  that rides on the surface of the lap as a means to bridge small discontinuities on the lap surface  102  and to provide physical averaging of the lap surface  102 . For example, the measuring device  108  can include an LVDT  132  that measures positions of the puck  130 , or a DMI that measures positions of a mirror or a retro-reflector (not shown in the figure) attached to the puck  130 . 
     On a pitch lap, for example, the surface can be skimmed and roughened with a cutting tool to incorporate a pattern of V-notches as a mechanism for slurry distribution under the work item  104  and as a mechanism for reducing the flow path for the pitch itself as it deforms under pressure from the conditioning tool  106 . In the absence of the puck  130 , the probe of the LVDT  132  may be forced to follow an irregular path and may be bent and damaged by the side loads encountered. 
     In some implementations, the puck  130  is a circular disk of sapphire polished smooth and parallel and sized appropriately for the geometry of the lap deformations and the metrology hardware. For example, the puck  130  has a smooth surface having a diameter that is larger than a maximum width of the grooves on the lap surface  102 . In some examples, the smooth surface has a diameter that is at least three times the maximum width of the grooves so that at least two-thirds of the puck bottom surface is supported at all times. The maximum width of the grooves refers to the maximum width of the grooves in the portion of the lap that needs to be measured. The metrology stage  196  has a lower section  134  having a circular opening  136  to accommodate the puck  130 . The puck  130  is constrained to move with the metrology stage  196 , including maintenance of rotational alignment, but free to follow the vertical contour of the lap surface  102 . In the case of an LVDT measurement, an LVDT probe  138  contacts the upper surface of the puck  130  and provides a constant offset to the lap below. In the case of a DMI measurement, the puck  130  supports a mirror or a retro-reflector and provides a constant offset to the lap. 
     The measurement of the lap surface  102  can be accomplished by moving the measurement point in a generally radial direction while the lap rotates. This can be achieved by using the precision slide mechanism  122  with motorized translation via a ball-screw. This generates a spiral path with an operator-defined pitch and provides an opportunity to efficiently and repeatably interrogate the entire lap surface  102 . When the motions of the lap rotation and the radial motion of the metrology system are servo controlled or monitored using encoder feedback, the exact position on the lap surface for every measurement can be known with precision. 
     The metrology system frame of reference can be, e.g., the linear slide  110  of the precision slide mechanism  122 . The linear slide  110  is positioned on the polishing machine  100  closely perpendicular to the lap rotary axis, but it need not be perfectly perpendicular. The operator can make incremental changes to the shape of the lap until an acceptable surface figure is obtained for finishing the work item  104 . After operating the polishing machine  100  for a period of time, an operator may understand what apparent shape is required without concern for the absolute radius of curvature used to generate the mathematically defined lap surface shape. 
     In addition to slide alignment, the error motion of the metrology stage  196  as it translates across the lap surface  102  is taken into account because any errors in this motion are interpreted as figure error in the lap. To some degree, these errors can be mapped and compensated for in the final calculations since they are generally related to manufacturing tolerances or errors in the slide geometry and are therefore repeatable. 
     Random errors can be accounted for by, e.g., over-sampling and local averaging, or performing multiple scans and averaging the measurements. In some examples where DMI measurements are made, an external optical mirror can be used as a reference frame to provide a high level of accuracy, precision and resolution. The mirror can be mounted above the mechanical slide  110 , supported in a way to minimize gravity sag, and interrogated simultaneously with the lap measurements such that every measurement of the lap is combined with a measurement of the mirror so that error motions in the slide  110  are cancelled out. 
     A feature of the polishing machine  100  is the use of the small conditioning tool  106  to smooth lap anomalies generated by the work items  104  being polished and change the overall degree of curvature of the finishing lap surface  102  to help generate the conditions necessary to achieve the required surface figure on the work item  104  being finished. The operator can use, e.g., radial positioning, pressure, and/or rotation rate of the conditioning tool  106  to manage the correction processes. 
     In some examples, especially using traditional large conditioning tools, the operator may detect excessive edge roll or a 3 rd  order spherical term (hole-and-roll). Some designs, especially traditional designs for the conditioning tool support mechanism, may result in unwanted moments about the support point and cause the leading or trailing edge (relative to the circular motion of the lap) of the conditioning tool  106  to dig into the lap causing an overall toroidal surface figure on the lap. In the case of this small computer controlled conditioning tool  106 , the effect of this moment can be accounted for in the calculated work function that is developed for the conditioning tool  106  and the other machine operating parameters. This is a very significant benefit of the system described herein. Assuming the conditioning tool  106  is small enough (e.g., has a tool diameter that is nominally less than half the diameter of the work item  104 ), the residual ripple left by the work function as it is convolved over the lap surface  102 , is of a sufficiently short spatial period that the normal averaging mechanisms of the work item being processed on the lap of the polishing machine  100  smooth out the ripple effect. 
     When using the small conditioning tool  106  there may also be edge roll on the polishing lap over a distance of approximately the conditioning tool diameter from any free edge on the polishing lap that cannot be bridged by the conditioning tool  106 . The problem of edge roll can be managed by extending the annular boundaries of the lap inward or outward sufficient to insure that the edge roll occurs outside the normal work zone. 
     In some examples, if the work item  104  has a small size, the conditioning tool  106  can be selected to have a size that is larger than the work item  104  so that the spatial period of the ripple is greater than the work item size and goes “unnoticed.” 
     In some implementations, the polishing machine  100  is designed so that a variety of conditioning tools with varying sizes can be easily swapped into the machine as needed. In some examples, the conditioning tool size is about half the size of the work item  104  being finished so the work items  104  can easily bridge the ripples and offset their effects through normal surface averaging principles. In some examples, a conditioning tool is chosen such that the tool is much larger than the work item  104  being finished so the ripple goes “unnoticed.” 
     When using the small conditioning tool  106 , moments about the support point may cause either the leading or trailing edge (relative to the circular motion of the lap) of the tool  106  to “dig into” the lap causing an overall toroidal surface figure on the lap. This effect can be accounted for in the use of a work function that incorporates this effect. 
       FIG. 4  shows an example interface  150  between the conditioning tool  106  and a hydraulic cylinder  152 . Here, the conditioning tool  106  is not actively driven, and relies on the rotary motion of the lap to slowly revolve the conditioning tool  106 . The interface  150  includes spherical roller bearings  154  that allow free rotation while also allowing the conditioning tool  106  to easily conform to the lap surface  102 . This is useful when a non-plano lap shape is desired. In some examples, the interface  150  can include a pair of plain radial bearings that allow rotation but not allow other degrees of freedom. Such interface can be used for a plano lap. In some examples, the interface  150  can include an active rotary drive that drives the conditioning tool  106 . 
     The following describes the motion and pressure control for the conditioning tool  106 . Computer numerical controlled (CNC) programs can be used, with proper feedback from linear and rotary encoders, to allow accurate positioning and speed control for the lap, the conditioning tool, and the metrology instrument stage translation. The conditioning tool pressure is determined by a hydraulic system and can be controlled through use of a digitally controlled pressure regulator that can increase or decrease pressure through a programmable ramp with good accuracy. For an actively driven conditioning tool  106 , the conditioning tool rotary speed can be controlled using CNC programming. 
     Referring to  FIG. 5 , the control computer  160  can control various subsystems of the polishing machine  100 . For example, the polishing machine  100  may include a subsystem  162  for controlling rotation of the lap surface  102 , a subsystem  164  for positioning the measuring device  108 , a subsystem  166  for positioning the conditioning tool  106 , and a subsystem  170  for controlling the rotation speed and/or pressure of the conditioning tool  106 . For example, the subsystem  162  can include the belt drive mechanism  192 , the subsystem  164  can include the precision slide mechanism  122 , the subsystem  166  can include the precision slide mechanism  116 , and the subsystem  170  can include the hydraulic device  114 . 
     The control computer  160  includes a data processor  172  and a storage (e.g., memory or hard drive) to store a measured lap surface map  174  and an error map  176 . A GUI  178  is provided to enable the operator to easily control various parameters of the polishing machine  100 . The computer  160  executes a G-code program  180  to control various components of the polishing machine  100 . 
     In some examples, the computer  160  controls the subsystem  162  to rotate the lap surface  102  at a steady rate. The computer  160  controls the subsystem  164  to position the measuring device  108 , which measures the contour of the lap surface  102  and sends the measurement data back to the computer  160 . The computer  160  generates the measured lap surface map  174  based on the data provided by the measuring device  108 . The computer  160  compares the measured lap surface map  174  with a desired surface contour, and generates an error map  176 . Based on the error map  176 , the computer calculates the motion and pressure profiles of the conditioning tool  106  in order to reduce the error. The computer  160  converts the motion and pressure profiles into a G-code program  180 , then executes the G-code program  180  to control various components of the polishing machine  100  to cause the conditioning tool  106  to have the calculated motion and pressure to reduce the error specified in the error map. The control computer  160  repeats the steps above, including controlling subsystem  164  to position the measuring device  108 , using the measuring device  108  to measures the lap surface  102 , using the measurement data to update the measured lap surface map  174  and error map  176 , generating the G-code program  180 , using the G-code program to control the movement and pressure of the conditioning tool  106  to polish the lap surface  102 , and so forth. At the same time, the work items  104  are placed on the lap surface  102  and processed by the lap surface  102 . 
       FIG. 6  is a flow diagram of an example process  200  for operating a lapping machine or a polishing machine. For example, the polishing machine can be the polishing machine  100  of  FIG. 1 . In the process  200 , the lap contour is measured and a surface map is generated ( 202 ). For example, the measuring device  108  can be used to measure the lap contour, and the control computer  160  can generate the surface map  174 . 
     The measured lap surface is compared to a desired surface to generate an error map ( 204 ). For example, the computer  160  can compare the surface map  174  to a desired surface to generate the error map  176 . 
     The required motion and pressure profile for the conditioning tool as it moves over the lap surface is calculated ( 206 ). For example, the control computer  160  can calculate the required motion and pressure profile for the conditioning tool  106  as it moves over the lap surface  102 . 
     A calculated conditioning tool program is executed to cause the conditioning tool to process the lap surface ( 208 ). For example, the computer  160  can execute the G-code program  180  to cause the conditioning tool  106  to process the lap surface  102 . 
     The lap surface is used to process the work item ( 210 ). For example, the lap surface  102  is used to polish the work item  104 . Processing of the work item  104  can be performed continuously and in parallel to steps  202  to  208  and  212  of the process  200 . 
     The desired or target lap shape against which the actual shape is compared to achieve the desired final part requirements is adjusted ( 212 ). 
     Steps  202  to  212  are repeated. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Any of the methods described above can be implemented, for example, in computer hardware, software, or a combination of both. The methods can be implemented in computer programs using standard programming techniques following the descriptions herein. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output devices such as a display monitor. Each program may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, the program can run on dedicated integrated circuits preprogrammed for that purpose. 
     Each such computer program is preferably stored on a storage medium or device (e.g., RAM, ROM, Flash memory, optical disc, or magnetic disk) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. The computer program can also reside in cache or main memory during program execution. The method can also be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein. 
     Other aspects, features, and advantages are within the scope of the invention. For example, the polishing machine  100  can be replaced by a lapping machine that uses the finishing surface of a hard substrate to process the work item without using a polishing pitch. In this case, the small conditioning tool  106  can be motor driven and achieve variation in local lap surface change through changes in tool rotary speed. For example, the rotating tool surface can wear out parts of the hard substrate, and rotary speed variation can be used to correct lap surfaces. The work function can be generated by measuring the “before” and “after” of the surface of the lap material that is modified using a known rotational speed profile of the conditioning tool  106 . In some examples, the small conditioning tool  106  may process the finishing surface using a combination of rotation and pressure, and the work function can be generated by measuring the “before” and “after” of the surface of the lap material that is modified using a known rotational speed and pressure profile of the conditioning tool  106 . 
     The puck  130  can have any of a variety of shapes, such as an oval shape, and can be made from any of a variety of materials, as long as it has a smooth bottom that can glide over the pitch material and follow the vertical contour of the lap surface  102 . The work item  104  can be of any shape, such as round or rectangular. In the description above, the diameter of the small conditioning tool  106  is smaller than the diameter of the work item  104 . When the work item  104  is not round, the term “diameter” refers to the longest dimension (e.g., diagonal dimension of a square or rectangle) of the surface of the work piece that is in contact with the polishing lap. 
     Various techniques can be used to measure the positions of the puck  130 , such as a proximity gauge that rides along a datum plane developed by a precision slide and measures the gap between the gauge and the puck  130 . For example, the proximity gauge can be a short range position sensor that uses a laser to make a precision distance measurement over a small distance.