Effect of System Dynamics on Surface Topography in Fast Tool Servo-Based Diamond Turning of Microlens Arrays
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-05-12 |
| Journal | Nanomanufacturing and Metrology |
| Authors | Takeshi Hashimoto, Jiwang Yan |
| Citations | 3 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: FTS Diamond Turning of Microlens Arrays
Section titled âTechnical Documentation & Analysis: FTS Diamond Turning of Microlens ArraysâReference Paper: Hashimoto & Yan (2025). Effect of System Dynamics on Surface Topography in Fast Tool Servo-Based Diamond Turning of Microlens Arrays. Nanomanufacturing and Metrology.
Executive Summary
Section titled âExecutive SummaryâThis research successfully demonstrates the critical role of Fast Tool Servo (FTS) system dynamics in determining the surface quality of ultraprecision diamond-turned microlens arrays (MLAs). The findings provide essential insights for engineers utilizing FTS systems for complex freeform optics, directly impacting the demand for high-stability diamond tooling.
- Core Problem Identified: Surface waviness (Sdq) in FTS diamond turning is primarily caused by the servo systemâs dynamic response, specifically its resonant frequency (1367 Hz) and settling time (0.001 s).
- Optimization Strategy: A novel machining strategy was developed, incorporating a constant, low surface speed (5 m/min) and a calculated tool offset (5.0 ”m) to ensure the FTS system stabilized before engaging the workpiece.
- Waviness Reduction: The optimized strategy achieved a 53% reduction in average surface waviness (Sdq), improving the result from 93.3 ”rad to 49.9 ”rad.
- Roughness Improvement: Average surface roughness (Sq) was significantly reduced from 2.8 nm to an ultraprecision level of 1.6 nm.
- Repeatability Verified: The optimized process demonstrated high repeatability, reducing the standard deviation of Sdq from 33.5 ”rad to 3.1 ”rad (a 90% reduction).
- Material Implication: The use of Oxygen-free Copper (OFC) was explicitly chosen to minimize tool wear influence, highlighting the necessity of the highest quality diamond tooling when transitioning this process to harder, commercially relevant optical materials (e.g., Nickel, Silicon).
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental results and system analysis:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Workpiece Material | Oxygen-free copper | N/A | Selected for low tool wear influence |
| Lens Array Size | 10 x 10 | N/A | Total 100 lenslets |
| Lenslet Diameter | 1.7 | mm | Concave sphere design |
| Lens SAG (Depth) | 18 | ”m | Maximum depth of the lenslet |
| FTS Maximum Acceleration | 25 | G | Hardware limitation |
| FTS Resonant Frequency (Ïr) | 1367 | Hz | Identified from frequency response plot |
| FTS Settling Time (Theoretical) | 0.001 | s | Within linear system bandwidth (0.2% step input) |
| Initial Average Waviness (Sdq) | 93.3 | ”rad | Constant spindle speed (125 rpm) |
| Optimized Average Waviness (Sdq) | 49.9 | ”rad | Optimized strategy (5 m/min surface speed) |
| Sdq Standard Deviation Reduction | 90 | % | Reduced from 33.5 ”rad to 3.1 ”rad |
| Initial Average Roughness (Sq) | 2.8 | nm | Before optimization |
| Optimized Average Roughness (Sq) | 1.6 | nm | After optimization |
| Optimized Surface Speed | 5 | m/min | Selected to operate within FTS linear range |
| Optimized Tool Offset | 5.0 | ”m | Applied in the uncut zone for stabilization |
Key Methodologies
Section titled âKey MethodologiesâThe research focused on identifying and compensating for dynamics-induced errors in the FTS system during ultraprecision diamond turning.
- Experimental Setup: Machining was performed on a 5-axis ultraprecision diamond turning machine (Nanoform X) utilizing an independent FTS system (FTS5000) as the W-axis actuator.
- Tooling: A natural diamond tool (0.496 mm radius) was used to machine the 10x10 microlens array on an Oxygen-free copper workpiece (50 mm dimension).
- Metrology: Surface topography and waviness were quantified using a Coherence Scanning Interferometric (CSI) optical profilometer. Waviness was quantified using the Sdq parameter (Root Mean Square of the gradient, ISO 25178-2).
- System Identification: White noise was added to the FTS system to derive the frequency response plot, identifying the resonant frequency (1367 Hz) and resonant peak (Mp = +1.0 dB).
- Dynamics Modeling: The FTS system was approximated as a second-order linear system within the small input bandwidth (< 1 ”m step), allowing calculation of the damping ratio (0.523) and natural frequency (2029 Hz).
- Optimization Strategy: The cutting parameters were re-evaluated to maintain a constant, low surface speed (5 m/min) across the entire lens array, ensuring the FTS operated within its stable, linear response range.
- Settling Time Compensation: Based on the derived settling time (0.001 s), a tool offset of 5.0 ”m was calculated and implemented in the machining strategy. This offset ensured the FTS servo system stabilized before the tool engaged the surface, effectively eliminating dynamics-induced waviness.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis study confirms that achieving nanometric surface quality (Sq < 2 nm) and minimizing waviness (Sdq < 50 ”rad) in complex optics requires not only advanced machine control (FTS optimization) but also the highest quality, most stable diamond tooling. 6CCVD specializes in providing the MPCVD diamond materials necessary to replicate and extend this ultraprecision research to industrial scale and harder materials.
| Research Requirement/Challenge | 6CCVD Solution & Capability | Technical Advantage |
|---|---|---|
| Tooling for Nanometric Roughness (Sq 1.6 nm) | Optical Grade Single Crystal Diamond (SCD) | SCD offers superior purity, hardness, and thermal stability compared to natural diamond. This ensures minimal tool wear and maintains the critical edge sharpness required for achieving Ra < 1 nm and Sq < 2 nm results, even under high-frequency FTS operation. |
| Large-Scale Microlens Arrays | Custom Dimensions up to 125 mm (PCD) | We supply SCD and PCD plates/wafers up to 125 mm diameter, enabling the fabrication of larger, high-density optical components for AR/VR and sensing applications, exceeding the 50 mm workpiece used in this study. |
| Ultra-Low Waviness & Form Accuracy | Precision Polishing Services (Ra < 1 nm) | Our internal polishing capability ensures SCD surfaces achieve roughness values of Ra < 1 nm. This foundational material quality is essential for minimizing the baseline surface error before FTS dynamics compensation is applied. |
| Transition to Harder Materials | Heavy Boron-Doped Diamond (BDD) & PCD | The paper used OFC to avoid tool wear. For industrial applications requiring machining of harder substrates (e.g., Silicon, Germanium, Nickel alloys), 6CCVD provides robust Polycrystalline Diamond (PCD) and Boron-Doped Diamond (BDD) materials that withstand higher wear rates while maintaining form accuracy. |
| Custom Tool Trajectory Support | In-House PhD Engineering Support | Our team of material scientists and engineers can assist researchers in selecting the optimal diamond material, crystal orientation, and metalization (e.g., Ti/Pt/Au, Cu, W) to complement complex FTS/STS control strategies and thermal management requirements. |
| Integrated Component Fabrication | Custom Metalization Services | If the microlens array requires integrated sensors or electrodes (e.g., for BDD electrochemical applications), 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu) directly onto the diamond substrate. |
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract A lens array is often used for optical components of sensing devices, requiring high surface quality and form accuracy. Fast tool servo (FTS)-based diamond turning is one of the technologies for manufacturing complicated shapes, such as freeform optics, structured surfaces, and microlens arrays, with high machining efficiency. In this study, lens array machining was performed on copper using an FTS on a diamond turning machine. For evaluating the lens array surface topography, the focus was on surface waviness formation. As a dominating factor of surface waviness, the system dynamics behavior was investigated by capturing and analyzing the position signal. It was found that a specific waviness pattern could be formed on the surface due to the servo response. By considering the dynamics of the FTS system from the captured signals, the FTS system behavior was identified, and optimal machining parameters for the lens array were proposed. A machining test under the optimized cutting conditions reduced the average Sdq used to quantify the waviness amount from 93 to 50 ”rad and the standard deviation from 33 to 3 ”rad, which greatly improved the consistency in accuracy for all lens arrays. This study will contribute to the appropriate utilization of FTS systems in the ultraprecision machining of various advanced optics, such as microlens arrays.