Donald Golini

Magnetorheological finishing (MRF) in commercial precision optics manufacturing

Finish polishing of highly precise optical surfaces is one of the most promising uses of magnetic fluids. We have taken the concept of magnetorheological finishing (MRF) from the laboratory to the optical fabrication shop floor. A commercial, computer numerically controlled (CNC) MRF machine, the Q22, has recently come on-line in optics companies to produce precision flat, spherical and aspheric optical components. MRF is a sub-aperture lap process that requires no specialized tooling, because the magnetically-stiffened abrasive fluid conforms to the local curvature of any arbitrarily shaped workpiece. MRF eliminates subsurface damage, smoothes rms microroughness to less than 1 nm, and corrects p-v surface figure errors to (lambda) /20 in minutes. Here the basic details of the MRF process are reviewed. MR fluid performance for soft and hard materials, the removal of asymmetric grinding errors and diamond turning marks, and examples of batch finishing of glass aspheres are also described.

Physics of loose abrasive microgrinding

This study examined the physics of loose abrasive microgrinding (grinding with micron and submicron sized abrasives). More specifically, it focused on the transition from brittle to ductile mode grinding which occurs in this region of abrasive sizes. Process dependency on slurry chemistry was the primary area of emphasis and was studied for diamond abrasives varying in size from 3.0 to 0.75 microm on both ULE and Zerodur, with emphasis on ULE. Ductile mode grinding was achieved with smaller abrasives, as expected, however two significant discoveries were made. The first observation was that by simply changing slurry chemistry, it was possible to induce the transition from brittle fracture to ductile mode grinding in ULE. This transition point could be intentionally moved about for diamonds 3.0-0.75 microm in diameter. For any given abrasive size within these limits, either brittle fracture or ductile removal may be achieved, depending on the slurry used to suspend the diamonds. Several slurries were studied, including water, a series of homologous n-alcohols, and other solvents chosen for properties varying from molecular size to dielectric constant and zeta potential. The study revealed that this slurry dependency is primarily a Rebinder effect. The second finding was that a tremendous amount of surface stress is introduced in loose abrasive ductile mode grinding. This stress was observed when the Twyman Effect in ULE plates increased by a factor of 4 in the transition from the brittle to the ductile mode. An assessment of the cause of this stress is discussed.

Surface microroughness of optical glasses under deterministic microgrinding

Deterministic microgrinding of precision optical components with rigid, computer-controlled machining centers and high-speed tool spindles is now possible on a commercial scale. Platforms such as the Opticam systems at the Center for Optics Manufacturing produce convex and concave spherical surfaces with radii from 5 mm to ∞, i.e., planar, and work diameters from 10 to 150 mm. Aspherical surfaces are also being manufactured. The resulting specular surfaces have a typical rms microroughness of 20 nm, 1 μm of subsurface damage, and a figure error of less than 1 wave peak to valley. Surface roughness under deterministic microgrinding conditions (fixed infeed rate) with bound abrasive diamond ring tools with various degrees of bond hardness is correlated to a material length scale, identified as a ductility index, involving the hardness and fracture toughness of glasses. This result is in contrast to loose abrasive grinding (fixed nominal pressure), in which surface microroughness is determined by the elastic stiffness and the hardness of the glass. We summarize measurements of fracture toughness and microhardness by microindentation for crown and flint optical glasses, and fused silica. The microindentation fracture toughness in nondensifying optical glasses is in good agreement with bulk fracture toughness measurement methods.

Magnetorheological finishing: a deterministic process for optics manufacturing

Finish polishing of optics with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the most recently developed process is called magnetorheological finishing (MRF). In MRF, a magnetic field stiffens a fluid suspension in contact with a workpiece. The workpiece is mounted on the rotating spindle of a computer numerically controlled machine. Driven by an algorithm for machine control that contains information about the MRF process, the machine deterministically polishes out the workpiece by removing microns of subsurface damage, smoothing the surface to a microroughness of 10 angstroms rms, and correcting surface figure errors to less than 0.1 micrometers p-v. Spheres and aspheres can be processed with the same machine set-up using the appropriate machine program. This paper describes MRF and gives examples which illustrate the capabilities of a pre-prototype machine located at the Center for Optics Manufacturing.

Precision optics fabrication using magnetorheological finishing

Optical polishing with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the newest process is called magnetorheological finishing (MRF). In MRF, all of the process parameters are controlled by utilizing the state of hydrodynamic flow of a magnetically stiffened magnetorheological abrasive fluid through a converging gap formed by a lens workpiece surface and a moving wall. The shear flow of “plastic” MR fluid results in the development of high stresses in the interface zone and material removal over a portion of the workpiece surface, referred to as the “polishing spot”. The polishing spot is an abrasive-charged, sub-aperture lap that automatically conforms to the local shape of the lens surface. Deterministic finishing is accomplished by mounting a lens on a rotating spindle and sweeping it through the MR fluid with a computer numerical controlled (CNC) machine. A computer program generates both a dwell time schedule for the MRF machine and an accurate prediction of finished surface shape, using a material removal function and initial surface condition information as input. In this paper, we describe the MRF process, a preliminary theory of material removal, properties of the MR fluid, machine configurations, software for finishing, and finishing experiments on a variety of surface shapes (spherical, flat, aspheres) and materials of interest to optics manufacturing. Advantages and current limitations to the process are also described.

Twyman effect mechanics in grinding and microgrinding

In the Twyman effect (1905), when one side of a thin plate with both sides polished is ground, the plate bends: The ground side becomes convex and is in a state of compressive residual stress, described in terms of force per unit length (Newtons per meter) induced by grinding, the stress (Newtons per square meter) induced by grinding, and the depth of the compressive layer (micrometers). We describe and correlate experiments on optical glasses from the literature in conditions of loose abrasive grinding (lapping at fixed nominal pressure, with abrasives 4-400 μm in size) and deterministic microgrinding experiments (at a fixed infeed rate) conducted at the Center for Optics Manufacturing with bound diamond abrasive tools (with a diamond size of 3-40 μm, embedded in metallic bond) and loose abrasive microgrinding (abrasives of less than 3 μm in size). In brittle grinding conditions, the grinding force and the depth of the compressive layer correlate well with glass mechanical properties describing the fracture process, such as indentation crack size. The maximum surface residual compressive stress decreases, and the depth of the compressive layer increases with increasing abrasive size. In lapping conditions the depth of the abrasive grain penetration into the glass surface scales with the surface roughness, and both are determined primarily by glass hardness and secondarily by Youngs modulus for various abrasive sizes and coolants. In the limit of small abrasive size (ductile-mode grinding), the maximum surface compressive stress achieved is near the yield stress of the glass, in agreement with finite-element simulations of indentation in elastic-plastic solids.

Wear and self-sharpening of vitrified bond diamond wheels during sapphire grinding

Abstract Vitrified bond diamond wheels were tested during grinding of single crystal sapphire on a precision CNC grinding platform. Self-sharpening, evidenced by the occurrence of cyclic behavior in the machine deflection (grinding force), was observed for wheels made with one bond/diamond composition. These wheels could remove large volumes of sapphire without re-dressing. The self-sharpening effect was reproducible but showed considerable variability in terms of the detailed cycle shape. Wheel failure eventually occurred due to a loss of the self-sharpening action (overloading) or stall-out of the spindle. Process conditions producing increased wheel wear enhanced the self-sharpening action.

Magnetorheological-suspension-based finishing technology

In magnetorheological finishing (MRF) the mechanical energy for material removal is generated by the hydrodynamic flow of a magnetorheological (MR) polishing suspension through a converging gap that is formed by a workpiece surface and a moving rigid wall. In addition to causing material removal, MRF also reduces the surface micro roughness of optical materials to ≤ 10 Å rms. Shape errors are corrected to a fraction of a wavelength of light and subsurface damage is removed. A theoretical analysis of MRF, based on Bingham lubrication theory, illustrates that the formation of a core attached to the moving wall results in dramatically high stress on the workpiece surface. A correlation between the shear stress on the workpiece surface and materials removal is obtained.

Use of magnetorheological finishing (MRF) to relieve residual stress and subsurface damage on lapped semiconductor silicon wafers

Magnetorheological finishing (MRF) is a novel process demonstrated to be effective for fine figure control and polishing of a variety of optical glasses and crystals. This paper discusses the use of MRF to stress relieve the surfaces of single crystal silicon wafers, of the type used in the semiconductor industry to fabricate integrated circuits. One hundred-mm diameter silicon wafers with a <111> crystallographic orientation were loose abrasive lapped with three different sizes of alumina abrasive to introduce compressive surface stress. The stress generated in the wafer surface was characterized by interferometrically monitoring the bending of the wafer due to the Twyman effect. The thickness of the subsurface damage (SSD) layer was characterized using a dimpling method with a fixture developed at COM. Subsequent polishing by MRF was found to be effective in removing the subsurface damage and associated residual stress generated in the wafer surface during loose abrasive lapping.

Transition between brittle and ductile mode in loose abrasive grinding

This paper examines the transition between brittle and ductile mode in loose abrasive microgrinding (grinding with micron and sub-micron sized abrasives). The work was directed specifically at understanding loose abrasive grinding dependency on slurry fluid chemistry and the swlace stresses that are introduced in the grInding process. Several slurry fluids were investigated including water, a homologous series of n-alcohols, and several other organics selected for various properties including molecular size and dielectric constant. Chemistry was found to play a major role in the process; in fact, by simply changing slurry fluid composition, it was possible to induce the transition from brittle fracture to ductile mode grinding in ULE (Corning Code 7971 Titanium Silicate Low Expansion Glass). Data revealed that the dependency ofloose abrasive grinding on slurry chemistry can best be explained as Rebinder-Westwood chemo-mechanical effects [1,2,3,4]. It was also observed that the grinding surface stresses, known as the Twyman effect, increased dramatically in the transition from brittle to ductile mode grinding.