William Kordonski

Experiments and observations regarding the mechanisms of glass removal in magnetorheological finishing

Recent advances in the study of the magnetorheological finishing (MRF) have allowed for the characterization of the dynamic yield stress of the magnetorheological (MR) fluid, as well as the nanohardness (H(nano)) of the carbonyl iron (CI) used in MRF. Knowledge of these properties has allowed for a more complete study of the mechanisms of material removal in MRF. Material removal experiments show that the nanohardness of CI is important in MRF with nonaqueous MR fluids with no nonmagnetic abrasives, but is relatively unimportant in aqueous MR fluids or when nonmagnetic abrasives are present. The hydrated layer created by the chemical effects of water is shown to change the way material is removed by hard CI as the MR fluid transitions from a nonaqueous MR fluid to an aqueous MR fluid. Drag force measurements and atomic force microscope scans demonstrate that, when added to a MR fluid, nonmagnetic abrasives (cerium oxide, aluminum oxide, and diamond) are driven toward the workpiece surface because of the gradient in the magnetic field and hence become responsible for material removal. Removal rates increase with the addition of these polishing abrasives. The relative increase depends on the amount and type of abrasive used.

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.

Fundamentals of Magnetorheological Fluid Utilization in High Precision Finishing

Magnetorheological finishing (MRF) is an enabling technology that may produce surface accuracy on the order of 30 nm peak to valley (p-v) and surface micro-roughness less than 10 A rms. In MRF, mechanical energy for material removal over the portion of the workpiece surface is generated by the magnetically controlled hydrodynamic flow of a magnetorheological polishing fluid. A fundamental advantage of MRF over existing technologies is that the polishing tool does not wear, since the recirculated fluid is continuously monitored and maintained. Polishing debris and heat are continuously removed. The technique requires no dedicated tooling or special setup. A unique attribute of the MRF process is its determinism that is attained through the use of a well-defined material removal function to eliminate known surface error. The efficiency of material removal and the removal process stability are the crucial factors in MRF. In turn, they are primarily dependent on MR polishing fluid stability. It is shown that the joint use of physicochemical and rheological factors along with specially developed methods of the slurry handling, pumping, and in-line monitoring and maintaining provides a level of MR slurry stability that is quite adequate for high precision finishing. Attention is given to methods of MR slurry property measurements.

Progress Update in Magnetorheological Finishing

In magnetorheological finishing (MRF), magnetically stiffened magnetorheological (MR) abrasive fluid flows through a preset converging gap that is formed by a workpiece surface and a moving rigid wall, to create precise material removal and polishing. 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 surface stress on the workpiece and material removal is obtained. A unique attribute of the MRF process is its determinism. MRF has been successfully implemented to polish optical surfaces to very high precision. MRF reduces the surface micro roughness of optical materials to ≤ 10A. Figure errors are corrected to a fraction of a wavelength of light and subsurface damage is removed. A wide range of optical surface shapes, including aspheres, has been polished on many different materials. Other applications in precision finishing are being considered, including integrated circuits and advanced ceramics.

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.

Magnetorheological (MR) Jet Finishing Technology

One suitable way to polish optics of complex shapes is by using a jet of abrasive fluid. In doing so, the energy required for polishing is supplied by the radial spread of the jet, which impinges upon a surface to be polished. Generally, the jet instability results in a non-deterministic polishing process. A method of jet stabilization has been proposed, developed, and demonstrated whereby the round jet of magnetorheological (MR) fluid is magnetized by an axial magnetic field as it flows out of the nozzle. It has been experimentally shown that in this case a stable and reproducible material removal function can be achieved at a distance of several tens of centimeters from the nozzle. At the same time, the interferometrically derived distribution of material removal for the MR jet coincides well with the distribution of the fluid power density calculated using CFD modeling. Polishing results support the assertion that the MR jet finishing process may produce high precision surfaces on glasses and single crystals.

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.

Material removal in magnetorheological finishing of optics

A concept of material removal based on the principle of conservation of particles momentum in a binary suspension is applied to analyze material removal in magnetorheological finishing and magnetorheological jet processes widely used in precision optics fabrication. According to this concept, a load for surface indentation by abrasive particles is provided at their interaction near the wall with heavier basic (magnetic) particles, which fluctuate (due to collision) in the shear flow of concentrated suspension. The model is in good qualitative and quantitative agreement with experimental results.

Magnetorheological Jet (MR JetTM) Finishing Technology

Conformal (or free form) and steep concave optics are important classes of optics that are difficult to finish using conventional techniques due to mechanical interferences and steep local slopes. One suitable way to polish these classes of optics is by using a jet of abrasive/fluid mixture. The energy required for polishing may be supplied by the radial spread of a liquid jet, which impinges a surface to he polished. Such fluid flow may generate sufficient surface shear stress to provide material removal in the regime of chemical mechanical polishing. Once translated into a polishing technique, this unique tool may resolve a challenging problem of finishing steep concave surfaces and cavities. A fundamental property of a fluid jet is that it begins to lose its coherence as the jet exits a nozzle. This is due to a combination of abruptly imposed longitudinal and lateral pressure gradients, surface tension forces, and aerodynamic disturbance. This results in instability of the flow over the impact zone and consequently polishing spot instability. To be utilized in deterministic high precision finishing of remote objects, a stable, relatively high-speed, low viscosity fluid jet, which remains collimated and coherent before it impinges the surface to be polished, is required. A method of jet stabilization has been proposed, developed, and demonstrated whereby the round jet of magnetorheological fluid is magnetized by an axial magnetic field when it flows out of the nozzle. It has been experimentally shown that a magnetically stabilized round jet of magnetorheological (MR) polishing fluid generates a reproducible material removal function (polishing spot) at a distance of several tens of centimeters from the nozzle. The interferometrically derived distribution of material removal for an axisymmetric MR Jet™, which impinges normal to a plane glass surface, coincides well with the radial distribution of rate of work calculated using computational fluid dynamics (CFD) modeling. Polishing results support the assertion that the MR Jet finishing process may produce high precision surfaces on glass and single crystals. The technology is most attractive for the finishing of complex shapes like free form optics, steep concaves, and cavities.

Magnetorheological Jet Finishing of Conformal, Freeform and Steep Concave Optics

Conformal, freeform and steep concave optics represent important shapes that are difficult or impossible to finish using conventional techniques due to mechanical interferences and steep local slopes. One way to polish these optics is by using a jet of abrasive/fluid mixture. Widely used abrasive water jet machining is not applicable for precision polishing because of natural jet instability, which gives an unstable removal function. Theoretical and experimental results in this paper show how this problem can be addressed with a magnetically stabilized jet of magnetorheological fluid. Polishing results demonstrate the suitability for this technique for precision finishing of complex shapes.