Jeffrey L. Ruckman

Analysis of chatter in contour grinding of optical materials

Abstract Chatter that limits ground surface finish has been observed in the deterministic microgrinding of brittle optical materials. In this article, the classical single degree of freedom model for chatter, accounting for both work and tool regenerative effects, is adapted for contour grinding optical surfaces. A linearized expression for the cutting stiffness is developed for the contour grinding geometry based on Preston’s law. Techniques developed to measure the machine frequency response function (FRF) and the Preston coefficient, needed as inputs to the simulations, are described. Numerical simulations based on this model are used to predict the grinding system behavior and to investigate process parameters affecting chatter stability. The stability limits observed during grinding experiments on optical glasses are in good agreement with the simulation results.

Noncontact estimate of grinding-induced subsurface damage

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.

Recent advances in aspheric and conformal grinding at the Center for Optics Manufacturing

Deterministic microgrinding (DMG) process technology, developed by the Center for Optics Manufacturing (COM) at the University of Rochester, has been extended with the development of two new computer-controlled contouring microgrinders that can produce highly accurate aspheric and conformal optical surfaces in minutes. The NanotechTM 150AG Asphere Grinder was designed and built by Moore Tool Company (Bridgeport, CT) with input from the COM Machine Technical Advisory Board. Funded by DARPA, the scope of this COM-led machine development project was to create a cost- effective, high precision machine that would deterministically microgrind aspheric optical components in brittle glass materials. A critical performance requirement dictated that the ground surfaces produced by the NanotechTM 150AG be compatible with the COM and QED-developed magnetorheological finishing (MRF) process. With the new capabilities of this grinder and the magnetorheological finishing process, COM has demonstrated a potential 10X reduction in the cost of asphere fabrication. The NanotechTM 150AG is a CNC controlled, ultra-precision machining system that is capable of deterministically generating axisymmetric aspheric optical surfaces up to 100 mm in diameter in a production environment. The contouring DMG process produces aspheric surfaces within 1 micrometer of the intended surface shape with significantly lower surface roughness and less subsurface damage than conventional grinding processes. Grinding results on a variety of glasses, crystal and polycrystalline brittle materials are reported in this paper. The NanotechTM 500FG ultra- precision Freeform Generator is the second next-generation deterministic microgrinder. Also developed under a DARPA funded program, the NanotechTM 500FG was designed and built by Moore Nanotechnology Systems, LLC with input from a COM-led Machine Technical Advisory Board. The NanotechTM 500FG is a multi-axis, deterministic microgrinding platform capable of generating non-axisymmetric and axisymmetric optical surfaces in brittle materials. This paper reports the first results from this machine.

Deterministic processes for manufacturing conformal (freeform) optical surfaces

This paper describes the computer-controlled machines and deterministic processes developed by the Center for Optics Manufacturing (COM) at the University of Rochester to produce conformal windows and domes that have non-traditional optical surface geometry and unusual shapes. COMs DMG (deterministic microgrinding) technology produces aspheric and conformal (freeform) surfaces in minutes, versus the weeks that are required to produce the surfaces conventionally. The demonstrated techniques and equipment provide a predictable and repeatable optical production process for just about any IR, visible, or UV material. The DMG process, in concert with newly developed CNC machining equipment, typically yields 1.0 wave peak-to-valley form accuracy, 150 Angstroms rms surface finish, and subsurface damage levels low enough that some of the infrared materials do not require additional polishing.

Surface features and residual strains in AlON grinding

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.

Relationship between microgrinding parameters and lens surface features

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.

Modeling of tool shape evolution in conformal (raster) grinding

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.

Chatter in deterministic microgrinding of optical glasses

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.

Minimizing tool marks in deterministic microgrinding

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.

Application of coolants in deterministic microgrinding of glass

Current literature is rich with information on the interaction between coolants and metals in metal working. This includes the study of optimum coolant velocities, nozzle positioning, and coolant formations. Very little information exists on the role of coolants in glass grinding. The CNC deterministic microgrinding machines at the Center for Optics Manufacturing utilize water based coolants to provide lubrication at the part/tool interface, to remove heat from the metal bonded diamond tool, and to help keep the tool surface free of debris. We show that the angle of coolant delivery and the coolant velocity do not affect the rms microroughness of a variety of glasses when ground at commercially relevant in-feed rates. We discuss a preliminary experiment utilizing a high pressure coolant delivery system.