High Energy Fluid Jet Machining (HEFJet-Mach): From scientific and technological advances to niche industrial applications

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.

Contact mechanics models and algorithms for dome polishing with UltraForm Finishing (UFF)

Moore’s law, predicting a doubling of transistor count per microprocessor every two years, remains valid, demonstrating exponential growth of computing power. This thesis examines the application of numerical methods to aid optical manufacturing for a number of case-studies related to the use of sub-aperture tools. One class of sub-aperture tools consists of rigid tools which are well suited to smooth surfaces. Their rigidity leads to mismatch between the surfaces of tool and aspheric workpieces. A novel, numerical method is introduced to analyse the mismatch qualitatively and quantitatively, with the advantage that it can readily be applied to aspheric or free-form surfaces for which an analytical approach is difficult or impossible. Furthermore, rigid tools exhibit an edge-effect due to the change in pressure between tool and workpiece when the tool hangs over the edge. An FEA model is introduced that simulates the tool and workpiece as separate entities, and models the contact between them; in contrast to the non-contact, single entity model reported in literature. This model is compared to experimental results. Another class of sub-aperture processes does not use physical tools to press abrasives onto the surface. A numerical analysis of one such process, Fluid Jet Polishing, is presented - work in collaboration with Chubu University. Numerical design of surfaces, required for generating tool-paths, is investigated, along with validation techniques for two test-cases, E-ELT mirror segments and IXO mirror segment slumping moulds. Conformal tools are not well suited to correct surface-errors with dimensions smaller than the contact area between tool and workpiece. A method with considerable potential is developed to analyse spatial-frequency error-content, and used to change the size of the contact area during a process run, as opposed to the constant-sized contact area that is state-of-the-art. These numerical methods reduce dependence on empirical data and operator experience, constituting important steps towards the ultimate and ambitious goal of fully-integrated process-automation.

Numerical methods for the manufacture of optics using sub-aperture tools

This thesis provided a basis for deterministic optical polishing of aspheres and ogives with compliant tools. A new deterministic subaperture computer numerically controlled (CNC) polisher called UltraForm Finishing (UFF) is developed by OptiPro Systems (Ontario, NY). The UltraForm Finishing process was designed to remove midspatial frequency surface errors generated by grinding tools, preserve or correct the form of parts and polish the concave surface of tangent ogives made of tough optical materials. The three main goals of this thesis are 1) to develop polishing algorithms that specify collision free tool paths to correct part form, 2) to investigate a local removal model and 3) to investigate sources of any induced surface errors. The form of the part after polishing can be predicted by expressing the removal profile as a convolution integral of the tool removal function and crossfeed velocity. For form correction, the inverse problem is solved to obtain a tool crossfeed velocity profile that can achieve a desired removal profile. Explicit expressions for form correction problem were defined for plano and spherical parts. An approximate formulation was introduced for aspheres and ogives. The form correction problem was solved as a constrained optimization problem using regularization to achieve feasible solutions and overcome ill-conditioning. Algorithms establishing a collision free tool path have been developed. For that purpose, a two-dimensional model was introduced to detect tool-part collisions. Search strategies were defined to adjust the tool position and prevent such collisions.

/pdf/investigation-of-polishing-algorithms-and-removal-processes-43agmhphic.pdf

Investigation of Polishing Algorithms and Removal Processes for a Deterministic Subaperture Polisher

A polishing tool useful for grinding and high precision, fine polishing of flat or curved optical surfaces, as well as for the optical flattening of semiconductor and metallic surfaces. The tool does not make contact with the surface to be polished and lacks moving parts; but produces a high velocity flux “cushion” that expands radially and parallel to the working surface, generating a stable, uniform and repeatable annular abrasion footprint. Due to the hydrodynamic characteristics of the tool, it can create polished surfaces of high-precision optical quality starting from the grinding process up through the final fine polishing process without having to change the tool, thereby avoiding friction against the work surface and tool wear. It can polish thin membranes and does not require a rigid or active support for the working surface. This invention considerably simplifies optical polishing processes and reduces costs with respect to other known methods.

Hydrodynamic radial flux polishing and grinding tool for optical and semiconductor surfaces

A new tool for hydrodynamic radial polishing, HyDra, allows for the local polishing of optical surfaces with a controllable wear rate. The results of the removal rate for different polisher types and sizes, applied air pressures for slurry expulsion, and tool height with respect to the working surface, are reported. We present a numerical analysis of the volumetric removal rate for the dynamic experiments as well as a comparison with a similar technique.

Static and dynamic removal rates of a new hydrodynamic polishing tool

A novel hydrodynamic radial polishing tool (HyDRa) is presented. It performs corrective lapping and fine polishing of diverse materials by means of a low-cost abrasive flux and a hydrostatic suspension system that avoids contact of the tool with the working surface. With this tool it is currently possible to polish aspheres and free-form optics of up to 2.5 meters in diameter. It has the advantage of avoiding fallen edges during the polishing process as well as reducing tool wear out and deformation. The functioning principle is based on the generation of a high-velocity, high-pressure abrasive emulsion flux with radial geometry. The polishing process is repeatable and achieves high degrees of precision and accuracy on optical and semiconductor surfaces. An additional advantage of this new tool is the possibility to perform in-process interferometric measurements. Recent results of polished aspheres are discussed.

HyDra: A novel hydrodynamic polishing tool for high quality optical surfaces; Techical Digest

This work presents the deterministic hydrodynamic polishing system (HyDRa (HyDRa­ dynamic Radial Polishing Tool) based on a polishing tool developed at the Instituto de Astronomía, UNAM. HyDRa is one of several modern deterministic polishing techniques that allow for computer controlled finishing of complex optical surfaces. The HyDRa system is based on a hydrodynamic polishing tool that generates a variable­density abra­ sive flow that makes possible the production of high-quality optical surfaces of diverse materials. The tool comprises several stacked operational stages that generate a graz­ wing abrasive flow with a predictable, zero-force erosion footprint on the workpiece that removes material. A recent innovation to the hydrodynamic polishing tool adds a switch­ ing capacity to the tool that allows the polishing action to be interrupted at will, without losing the stability of the operating parameters. This further increases the versatility and performance of the tool, since it is now possible to polish only the areas where the surface needs correction. Pulsed polishing also adds several techniques to this polishing method that further increase the system’s versatility.

/pdf/hydra-vortex-polishing-with-a-deterministic-hydrodynamic-1g4r9da6gu.pdf

Erika Sohn

Papers

Static and dynamic removal rates of a new hydrodynamic polishing tool

HyDra: A novel hydrodynamic polishing tool for high quality optical surfaces; Techical Digest

HyDRa: Vortex Polishing with a Deterministic Hydrodynamic Radial Polishing Tool