UltraForm Finishing (UFF) is a new deterministic subaperture computer numerically controlled (CNC) polisher.
Because UFF uses a compliant tool, the desired depth of removal is achieved by adjusting the tool crossfeed velocity.
Algorithms for determining an optimum crossfeed velocity profile that satisfies tool velocity and acceleration constraints
have been derived for flats, spheres, and mild aspheres. The solutions were validated experimentally. The removal
function that characterizes the interaction between a particular tool and part material is evaluated by making a removal
spot for one set of process parameters. Its variations, as a function of the process parameters, are predicted by using
Hertz contact theory and the Preston equation. Additional algorithms were developed for the evaluation of part and spot
metrology inputs and for tool path generation to prevent tool-part collisions.
UltraForm Finishing (UFF) is a new deterministic subaperture computer numerically controlled (CNC) polisher. Because UFF uses compliant tools with large contact patches, the depth of removal is prescribed by adjusting the tool crossfeed velocity. The equations for the depth of removal as the tool traverses an axisymmetric part are derived. The form correction problem consists in solving these equations by adjusting the tool crossfeed velocity to achieve a desired removal profile. The solution must satisfy constraints on the tool velocity and acceleration. Solutions for flats, spheres and aspheres are achieved by treating the problem as a constrained optimization after writing the depth of removal equations in matrix form. The solutions were validated experimentally. The removal function is evaluated by making a removal spot for one set of process parameters. Its variations, as a function of the process parameters, are predicted by using Hertz contact theory and the Preston equation. To prevent tool-part collisions and to analyze part and spot measurements, algorithms were developed for the tool path and evaluation of metrology inputs.
A new generation of compliant tools and processes called UltraForm Finishing is under development at the Center for Optics Manufacturing (COM) and OptiPro Systems (Ontario, NY). The purpose is to achieve rapid, high quality finishing of hard materials. These compliant tools exhibit a large contact patch that can be up to 1 cm wide. A numerical model was developed to account for finite contact patch geometry on removal for a flat rotating axisymmetric workpiece. This model was used to determine the depth of removal as a function of radial position after the polishing tool has completely traversed the workpiece from its center to a given radial position. The depth of removal was investigated for circular and oval contact patches and a variety of removal functions. A constant removal depth is desired to minimize the induced figure error. Predicted results were compared to experimental measurements of induced figure error.
Contact mechanics was investigated for compliant tools being developed for UltraForm Finishing. Hertz contact theory predictions were compared with experimental measurements. A high speed camera was used to investigate the size and consistency of the contact spots. The contact pressure distributions were measured with a Tekscan tactile grid system. Preston's equation was used to derive a relation between the pressure distributions and the corresponding removal spots. Experimental results were used to estimate Preston's coefficient for this process.
CNC grinding relies on accurate control of the tool shape and position relative to the workpiece. However, tool wear can significantly alter tool shape, potentially producing figure errors. This problem can be particularly important in conformal grinding applications which require the grinding of large areas to optical tolerances and/or the use of relatively small tools (e.g. to grind deep complex shapes). In this study the wear of grinding tools during raster grinding of a conformal component is modeled. The goal is to predict the errors resulting from tool wear and, ultimately, to allow the development of simplified models that can be used to reduce the effects of wear via tool path compensation. In the modeling, wear at each point on the tool is assumed to be proportional to the matching workpiece volumetric removal at that point and, thus, is dependent on the workpiece surface left by the previous raster. An iterative technique is used to predict the tool shape and workpiece surface profile as removal progresses. The effects of process parameters (e.g. raster spacing and tool tilt) are examined. The results are also used in the evaluation and development of a simplified model, which approximates the worn tool shape as a flat bevel.
Aluminum Oxynitride (AlON) is a material of interest to the military for a variety of optical applications, including conformal optics and transparent armor. However, its high hardness and large grain size (on the order of 100-200 micrometers s) produced by current powder metallurgy processes present challenges to deterministic microgrinding. For example, typical contact areas between the tool and work surface for contour grinding are on the order of the AlON grain size. Therefore, individual grains often appear in surface relief (orange peel effect) following contour grinding. In addition, small pits, on the order of 10 micrometers diameter and up to a few microns deep have been observed throughout the grain structure after fine grinding with a 2-4 micrometers diamond tool. In this paper, an overview is given of our experience micro-grinding AlON. First, some of the features observed in fine ground AlON surfaces are described in detail. A theory, based on micro-indentation, is presented to explain the generation of the surface pits. Finally, an estimate of the residual surface stresses after grinding, using x-ray diffraction techniques to measure the strains, is presented.
In this paper, the chatter observed in the deterministic microgrinding of optical glasses is examined. Because chatter adversely affects ground surface quality, understanding and eliminating chatter could significantly reduce total fabrication time. First, a description of the characteristics of chatter marks observed on ground glass surfaces is presented. Next, a model for chatter generation during grinding is described. In this model, a linearized formula for chip area is derived and a parameter (the process cutting stiffness) predicting the possibility of chatter is introduced. Finally, experimental results are presented.
Conformal grinding of optical materials with CNC (Computer Numerical Control) machining equipment can be used to achieve precise control over complex part configurations. However complications can arise due to the need to fabricate complex geometrical shapes at reasonable production rates. For example high machine stiffness is essential, but the need to grind 'inside' small or highly concave surfaces may require use of tooling with less than ideal stiffness characteristics. If grinding generates loads sufficient for significant tool deflection, the programmed removal depth will not be achieved. Moreover since grinding load is a function of the volumetric removal rate the amount of load deflection can vary with location on the part, potentially producing complex figure errors. In addition to machine/tool stiffness and removal rate, load generation is a function of the process parameters. For example by reducing the feed rate of the tool into the part, both the load and resultant deflection/removal error can be decreased. However this must be balanced against the need for part through put. In this paper a simple model which permits combination of machine stiffness and process parameters into a single non-dimensional parameter is adapted for a conformal grinding geometry. Errors in removal can be minimized by maintaining this parameter above a critical value. Moreover, since the value of this parameter depends on the local part geometry, it can be used to optimize process settings during grinding. For example it may be used to guide adjustment of the feed rate as a function of location on the part to eliminate figure errors while minimizing the total grinding time required.
The process for designing optomechanical devices usually involves independent design optimization within each discipline. For instance, an optics engineer would optimize the optics of the device for image quality using Computer Aided Engineering (CAE) tools such as CODE V and OSLO. The structural engineer would then optimize the design to minimize deformation using CAE tools such as SDRC I-DEAS and MSC/NASTRAN. That is, the optics and structure are typically optimized independent of each other. In this paper, two additional methods for optimizing optomechanical devices are investigated. One method involves sequential design optimization. The other method involves the simultaneous design optimization of both the optics and structure of an optomechanical device. Two example problems are used to ex;lore the types of problems that each method is most suitable for. The first example involves an optomechanical device under thermal and gravity load, while the second example involves two thin lenses resting on a cantilevered beam.
In deterministic microgrinding of glass optics with metal bond diamond ring tools, optical surfaces exhibit residual cutting tool marks that can significantly affect the efficiency of the finish polishing process. The tool marks for spherical surface generation appear as curves that follow contact lines between the tool and workpiece from the center to the edge of the workpiece. The tool marks are circumferentially periodic and the number is typically equal to the k-ratio, i.e. the ratio of grinding tool speed to workpiece speed. This paper describes the effect of the k-ratio, the tool cutting face width, and their interaction on tool mark generation. We introduce a new parameter equal to the ratio of tool cutting face width to the k-ratio spacing. Experimental results indicate that this ratio is the critical factor for tool mark generation. For ratios greater than a critical value, the amplitude of tool marks will be reduced to a level not detectable by interferometry. The influences of vibration and tool roughness are also discussed. The model presented provides new insight into the generation of tool marks and optimization of deterministic microgrinding processes.
In deterministic microgrinding (DMG) of glass optics with metal bond diamond abrasive ring tools, cutter marks are generated on the lens surface by the relative motion between the grinding tool and the work piece. The cutter marks for spherical surface generation appear as curves that follow contact lines between the abrasive ring tool and the work piece from the center to the edge of the lens. For DMG surfaces using a three tool process, individual cutter mark heights vary form approximately 5 to 100 nm with a variable spatial separation of from 0.1 to 3 mm along the circumference of the lines. The number of cutter marks generated for one revolution of the work piece is typically equal to the ratio of the tool RPM to the work piece RPM. In this paper we describe experiments designed to investigate the relationship between machine vibration characteristics and cutter mark generation and to identify process parameters that most strongly influence the generation of cutter marks. Machine vibration is monitored during grinding with accelerometers, positioned in the x, y, and z directions and located on the tool spindle. A fast Fourier transform (FFT) is used to identify the dominant frequency components of the machine vibration. The fine ground surfaces obtained with the machine are hen measured with interferometry and also analyzed with a FFT to identify periodic features. An experimental approach is employed to identify the microgrinding process parameters, such as tool speed, work piece speed, infeed rate, cutting edge bevel width, and dwell time that significantly influence the characteristics of the cutter marks. Process parameters can then be chosen to minimize cutter mark generation.
For precise deterministic microgrinding of optical components, the exact location of the region of contact between the machine tool and the glass work piece needs to be specified. Any tool compliance therefore contributes to a lack of precision during grinding and to error in the final surface of the glass work piece. The goals of this work are to analyze the dependence of ring tool compliance changes and to develop simple analytical formulas that express these relationships for machine tools used in spherical lens fabrication. Numerical predictions have also been obtained using NASTRAN, a linear finite element analysis routine. The numerical results have been compared with estimations obtained from approximate analytical expressions to determine the ranges of validity of the expressions. These simplified expressions can be used to aid the design of ring tools for smaller [O(mm) diameter] and larger [O(m) diameter] optical lens fabrication.
We discuss a constitutive model describing the permanent densification of fused silica under large applied pressures and shear stresses. The constitutive law is assumed to be rate- independent, and uses a yield function coupling hydrostatic pressure and shear stress, a flow rule describing the evolution of permanent strains after initial densification, and a hardening rule describing the dependence of the incremental densification on the levels of applied stresses. The constitutive law accounts for multiaxial states of stress, since during polishing and grinding operations complex stress states occur in a thin surface layer due to the action of abrasive particles. Due to frictional and other abrasive forces, large shear stresses are present near the surface during manufacturing. We apply the constitutive law in estimating the extent of the densified layer during the mechanical interaction of an abrasive grain and a flat surface.
To understand and further develop techniques for the deterministic microgrinding of glass, issues in both the materials science and mechanics fields need to be addressed. As part of our efforts at the Center for Optics manufacturing we are working with researchers from the center, other universities, government, and industry in both areas and at the interface between them. In this report we wish to give an overview of some of the efforts we are involved in, including specific examples of the experiments and analyses being performed.