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.
We review extensive data on measured subsurface damage and surface roughness resulting from lapping (loose abrasive grinding under fixed nominal pressure) and deterministic microgrinding (bound abrasive grinding under fixed nominal infeed) of commercial optical glasses with a large range of abrasive sizes. Subsurface damage is measured with the dimple method and related techniques. Surface roughness is measured with white light interferometry. Our results show that subsurface damage and its statistical scatter can both be estimated directly from the non-contact measurement of peak- valley surface roughness.
CNC grinding technology is increasingly used in the manufacture of precision optical components. Grinding performance is strongly influenced by the interaction of the tool and workpiece surfaces on a microscale, which in turn is influenced by the structure (topography) of the two surfaces. Unfortunately, for tool surfaces in particular, relatively little quantitative information has been available on the nature of the surfaces generated during actual grinding operations. However with the availability of advanced metrology techniques, such as optical profilometry and atomic force microscopy, it is now possible to produce detailed three-dimensional images of tool surfaces and from them to extract detailed quantitative information about the surface and its evolution. In this paper we discuss the use of optical profilometry to quantitatively characterize the surface microstructure (topography) of composite diamond tools during grinding of optical glasses. As an alternative to measuring individual diamond profiles, both the size and shape of the active diamonds on the tool surface may be evaluated by examination of the overall bearing ratio of the surface. This is quicker and has the advantage of avoiding potential bias in the selection of diamonds to be measured. Use of a micro- marker technique to precisely measure bond wear rate from a series of surface images using a micro-marker technique is also demonstrated. Grinding performance is dependent on the process conditions, but is also found to be correlated with the tool's surface structure. Moreover since the tool surface evolves during grinding, a complex relationship between the process and performance is produced. Bond wear is found to play an important role in maintaining grinding performance. Process conditions which produce a steady bond wear rate aid in establishing a quasi-equilibrium state ('self-sharpening') under which grinding performance can be maintained indefinitely.
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.
During grinding operations successively 'finer' grinding tools and conditions are employed. Fine grinding serves to improve the surface finish of workpieces but is generally not capable of easily removing the larger volumes of material required initially. Thus, there is a well recognized link between surface finish and the ease of material removal. In this paper we examine this relationship in a quantitative fashion for some optical materials for which a suitable data base is available. Grindability is considered in terms of the volumetric removal obtainable as a function of the applied normal load. For single crystal sapphire ground with a wide range of tools/conditions, this grindability measure is found to correlate approximately with the cube of the workpiece roughness. Similar correlations between grindability and finish are also found in comparing data for different glasses/crystals ground under identical conditions. The origins of this behavior are discussed in terms of possible micromechanical and dimensional explanations. Finally, since ease of material removal and workpiece finish are related, normalization against the roughness is proposed as a means for making comparison between different grinding media/conditions.
Bound abrasive grinding of optical materials with CNC machining equipment can achieve precise control over product configuration. For some materials and grinding geometries, however, there can be cases in which machine deflection is significant and this impacts the shape of the optical parts produced. This machine stiffness issue is associated with the relationship between the material removal rate an the applied normal load, as well as the tool deterioration process. A dynamic model to aid in keeping the process fully deterministic is presented. The validity of this model is discussed according to data collected from plano grinding of BK7 and sapphire with diamond cup tool with wide range of process parameters such as abrasive's grit size, workpiece size, infeed rate, tool dimension, and the level of glazing of the tools. Modifications and application of the model to an sphere grinding geometry is also presented.
Large sapphire components are required to meet the challenging needs of commercial and military optical applications. However, the desirable properties of sapphire also make it difficult to grind and polish. Fabrication costs can represent 50 percent of the price of large sapphire components. Cutting and grinding studies on sapphire were carried out with four types of diamond tools to correlate tool characteristics and process parameters with the grinding mechanism. The Twyman effect was also investigated to relate to crystallographic properties of sapphire with fabrication concerns. If grinding and microgrinding techniques can be optimized, costs associated with fabrication of sapphire and other optical materials will ultimately be reduced.
Material removal during fine grinding operations is accomplished primarily by the action of individual abrasive particles on the glass surface. The mechanical properties of the abrasive are therefore important. Unfortunately it is difficult to directly measure the mechanical response of abrasives once they reach the scale of approximately 10 microns. As a result mechanical properties of fine abrasives are sometimes characterized in terms of an empirical `friability', based on the response of the abrasive to crushing by a metal ball in a vial. In this paper we report on modeling/experiments designed to more precisely quantify the mechanical properties of fine abrasives and ultimately to relate them to the conditions experienced by bound particles during grinding. Experiments have been performed on various types and sizes of diamond abrasives. The response of the particles is a strong function of the loading conditions and can be tracked by changing the testing parameters. Diamond size is also found to play a critical role, with finer diamonds less susceptible to fracture. A micromechanical model from the literature is employed estimate the forces likely to be seen during testing. We are also developing dynamic models to better predict the forces experienced during `friability' testing as a function of the testing parameters.
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.