Analysis of Polishing Mechanism and Characteristics of Aspherical Lens with MR Polishing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-06-30 |
| Journal | Journal of the Korean Society of Manufacturing Process Engineers |
| Authors | Jungwon Lee, MyeongâWoo Cho, Seok-Jae Ha, KwangâPyo Hong, Yong-Kyu Cho |
| Citations | 3 |
| Analysis | Full AI Review Included |
Technical Analysis and Documentation: Ultra-Precision Finishing of Aspherical Lenses via MR Polishing
Section titled âTechnical Analysis and Documentation: Ultra-Precision Finishing of Aspherical Lenses via MR PolishingâDocument Generated for 6CCVD (6ccvd.com)
Executive Summary
Section titled âExecutive SummaryâThis analysis focuses on research demonstrating the effectiveness of Magneto-Rheological (MR) polishing for achieving ultra-precision surface finishing on complex optical components, specifically PMMA aspherical lenses. The findings directly support the demand for high-quality Chemical Vapor Deposition (CVD) diamond materials essential for the diamond turning (DTM) tools and molds utilized in such demanding optical applications.
- Application: Ultra-precision finishing of free-form, high-curvature aspherical lenses, overcoming the limitations of conventional lapping or grinding.
- Mechanism: Utilized a custom 4-axis MR polishing system with sophisticated geometric control (tangent/normal vectors) and Linear Velocity Control.
- Key Achievement (Roughness): Surface roughness ($R_a$) was improved dramatically, achieving a near 90% reduction from 40.99 nm (after CNC turning) to an exceptionally smooth 4.54 nm (after MR polishing).
- Process Validation: The MR polishing process proved capable of replacing or enhancing the finishing quality traditionally requiring expensive, dedicated Diamond Turning Machines (DTM).
- Material Relevance: The requirement for high-accuracy DTM and high-precision molds demands extreme stiffness and smoothness in the tooling material, positioning 6CCVDâs Single Crystal (SCD) and Polycrystalline (PCD) diamond as the optimal solution for manufacturing these tools and molds.
- Alternative Tooling: Demonstrated the potential to achieve optical-grade surface quality using a combined CNC lathe + MR polishing process, reducing reliance on specialized DTM equipment if precision CVD diamond tools are utilized.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points define the parameters and outcomes of the optimized MR polishing process applied to PMMA aspherical lenses.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Initial Roughness (Ra) | 40.99 | nm | Measured after CNC Diamond Turning |
| Final Roughness (Ra) | 4.54 | nm | Measured after MR Polishing |
| Initial Roughness (Rmax) | 357.16 | nm | Measured after CNC Diamond Turning |
| Final Roughness (Rmax) | 35.72 | nm | Measured after MR Polishing |
| Magnetic Field Strength | 15.92 | kA/m | Constant applied magnetic field |
| Polishing Gap Distance | 1.0 | mm | Gap between MR Wheel and Workpiece |
| Wheel Linear Speed | 1236 | mm/s | Fixed wheel velocity during operation |
| Optimized Workpiece Linear Speed | 418 | mm/s | Maintained for uniform material removal |
| Material Removal Rate (Max) | 3.65 | ”m | Achieved at 1.0 mm polishing depth |
| Aspherical Lens Diameter (Tested) | 18 | mm | Diameter of the fabricated PMMA lens |
| Aspherical Lens Height (Center) | 5.28 | mm | Dimension of the fabricated PMMA lens |
Key Methodologies
Section titled âKey MethodologiesâThe MR polishing study relied on precise control of geometric parameters and operational conditions to ensure uniform material removal across the complex, free-form aspherical surface.
- Workpiece Fabrication: PMMA aspherical lenses were initially fabricated via CNC turning (using high-speed steel or diamond tools) to a preliminary roughness of $R_a$ 40.99 nm.
- System Setup: A custom 4-axis position control system (X, Y, Z, and C-axis rotation) was established using a PMAC controller for ultra-precise movement.
- MR Fluid Composition: The fluid was a suspension containing 30-40% volume concentration of strong ferromagnetic Iron (Fe) particles, enabling viscosity control via external magnetic fields.
- Geometric Analysis & Tool Path Generation:
- The aspherical lens shape was analyzed using reverse engineering (curve fitting) to derive the Aspheric Equation.
- The equation was differentiated to calculate the Tangent Vector and Normal Vector at every point on the surface.
- An Offset Curve was generated along the normal vectors, corresponding to the required polishing depth ($g$), to define the precise tool path coordinates.
- Kinematic Control (Linear Velocity Control):
- The system implemented Constant Linear Velocity Control ($v_i = r_i \omega_i = constant$) by dynamically adjusting the angular velocity ($\omega$) of the workpiece as the polishing radius ($r$) changed.
- This critical control step ensured consistent relative velocity between the MR fluid ribbon and the workpiece, maintaining uniform material removal rate and minimizing shape errors.
- Optimization: Initial experiments determined the optimal workpiece linear speed (418 mm/s) required before conducting the final aspherical lens polishing trial.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe requirement for ultra-precision optical finishing, whether achieved via DTM or post-polishing, relies fundamentally on the mechanical stability, thermal properties, and ultimate smoothness of the diamond tools, molds, and substrates utilized in the manufacturing process. 6CCVD provides the enabling materials to replicate or exceed the precision demonstrated in this research.
Applicable Materials
Section titled âApplicable MaterialsâThe rigorous demands of DTM and MR polishing tool manufacturing require diamond substrates optimized for optical quality, wear resistance, and polishing capability.
| Research Requirement | 6CCVD Recommended Solution | Key Advantage for this Application |
|---|---|---|
| Ultra-Hard Tooling/Molds | Optical Grade Single Crystal Diamond (SCD) | Excellent purity, high thermal conductivity, and unmatched hardness for DTM inserts/molds. Easily polished to $R_a$ < 1 nm. |
| Large-Area Optical Substrates | High Purity Polycrystalline Diamond (PCD) | Available in large formats (up to 125 mm wafers). Ideal for large-area molds or wear-resistant components in the polishing system. |
| Wear-Resistant Components | Custom-Thick CVD Diamond Substrates | Thicknesses up to 10 mm available, ensuring maximal rigidity and lifespan for critical positioning or reform components. |
| Abrasive Wheel Core Material | Boron-Doped Diamond (BDD) | Potential use in conductive or structurally rigid components within the MR polishing apparatus where standard diamond properties are required alongside conductivity. |
Customization Potential
Section titled âCustomization PotentialâThe successful implementation of MR polishing requires tools and systems built to micron (”m) and sub-micron tolerances. 6CCVD capabilities meet these exacting standards:
- Ultra-Low Roughness Certification: 6CCVD guarantees surface roughness of $R_a$ < 1 nm for SCD and $R_a$ < 5 nm for inch-size PCD wafers. This is essential for diamond tools used to create the smooth initial surfaces (DTM inserts) or for ultra-precise metrology standards.
- Custom Dimensions and Shapes: We provide custom plates and wafers up to 125 mm diameter (PCD), along with precise laser cutting and shaping services to produce specialized DTM inserts or MR polishing system components tailored to specific geometries.
- Advanced Metalization Services: Should the MR polishing system or the DTM chucking system require specialized mounting (e.g., for thermal management or vacuum chucking), 6CCVD offers in-house metalization with thin films of Au, Pt, Pd, Ti, W, and Cu.
Engineering Support
Section titled âEngineering SupportâThe complexity of controlling linear velocity, generating offset curves, and optimizing the 4-axis system for aspherical finishing demands deep material knowledge.
6CCVDâs in-house PhD engineering team possesses extensive expertise in CVD diamond applications for ultra-precision machining and advanced optics. We can assist partners in:
- Selecting the optimal diamond grade (SCD vs. PCD) based on the required tool geometry and maximum processing temperature.
- Designing diamond inserts and tools for DTM applications aimed at minimizing subsurface damage and maximizing initial surface quality before post-polishing.
- Consulting on material integration and stability for highly sensitive polishing and metrology equipment, such as the MR polishing system discussed in this research.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
The aspherical lens was designed to be able to array a focal point. For this reason, it has very curved surface. The aspherical lens is fabricated by injection molding or diamond turning machine. With the aspherical lens, tool marks and surface roughness affect the optical characteristics, such as transmissivity. However, it is difficult to polish free form surface shapes uniformly with conventional methods. Therefore, in this paper, the ultra-precision polishing method with MR fluid was used to polish an aspherical lens with 4-axis position control systems. A Tool path and polishing mechanism were developed to polish the aspherical lens shape. An MR polishing experiment was performed using a generated tool path with a PMMA aspherical lens after the turning process. As a result, surface roughness was improved from <TEX>$R_a=40.99nm$</TEX>, <TEX>$R_{max}=357.1nm$</TEX> to <TEX>$R_a=4.54nm$</TEX>, <TEX>$R_{max}=35.72nm$</TEX>. Finally, the MR polishing system can be applied to the finishing process of fabrication of the aspherical lens.