Grinding techniques for fabricating micro-lens array mold made of cemented carbide (Polycrystalline diamond tools and mold surface roughness)
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-01-01 |
| Journal | Transactions of the JSME (in Japanese) |
| Authors | Takuya Semba, Yoshifumi Amamoto, Hitoshi Sumiya |
| Institutions | Fukuoka Institute of Technology, Sumitomo Electric Industries (Japan) |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research developed an ultraprecision grinding technique for fabricating Micro-Lens Array (MLA) molds made of cemented carbide, aiming for a grinding-less finish with a surface roughness (Rz) less than 10 nm.
- Core Achievement: Successful fabrication of an MLA mold (30 ”m diameter, 1027 dimples) on cemented carbide, achieving a minimum surface roughness of 5 nmRz without subsequent polishing.
- Tool Comparison: Nano-Polycrystalline Diamond (NPD) tools significantly outperformed Polycrystalline Diamond (PCD) tools in achieving fine surface finishes.
- NPD Mechanism: NPD tool wear during grinding smoothed the working surface, exposing a high density of fine diamond particles (~50 nm). This mechanism enabled ultraprecision finishing and resulted in a low diameter expansion rate (2%).
- PCD Limitation: PCD tools experienced self-dressing by removed chips, which exposed sharp, uneven cutting edges. This led to increased actual depth of cut, higher surface roughness (best Rz was 15 nmRz), and greater diameter expansion (3%).
- Process Control: Critical thermal management was implemented, controlling the air and grinding fluid temperatures to within ±0.1 °C, limiting thermal displacement of the workpiece to less than ±25 nm.
- Methodology: The tools were prepared using nanosecond laser machining followed by wet lapping with diamond slurry to create a flat, hemispherical tip (0.1 mm radius) for plunge grinding.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| MLA Dimple Diameter | 30 | ”m | Target lens size |
| MLA Pitch | 35 | ”m | Center-to-center distance |
| Number of Dimples | 1027 | - | Total lenses fabricated per mold |
| Work Material | Cemented Carbide (CW500) | - | Ultra-fine grain, high hardness |
| Work Material Hardness | 2620 | Hv | Vickers hardness |
| Tool Material (Best Result) | Nano-Polycrystalline Diamond (NPD) | - | Grain size ~50 nm |
| Tool Tip Radius | 0.1 | mm | Hemispherical profile |
| Achieved Rz (NPD) | 5 | nmRz | Minimum surface roughness |
| Achieved Rz (PCD) | 15 | nmRz | Minimum surface roughness |
| Diameter Expansion Rate (NPD) | 2 | % | Change over 1027 dimples |
| Diameter Expansion Rate (PCD) | 3 | % | Change over 1027 dimples |
| Tool Spindle Speed | 10000 | min-1 | Grinding condition |
| Z-axis Feed Rate | 0.1 | mm/min | Plunge grinding condition |
| Total Machining Time | 104 | min | Time required for 1027 dimples |
| Machine Resolution (X, Y, Z) | 10 | nm | Control resolution of machine tool |
| Grinding Fluid Temperature (Tw) | 26.1 | °C | Set point for MLA fabrication |
| Room Temperature (Ta) | 23.5 | °C | Set point for MLA fabrication |
| Required Thermal Stability | ±0.1 | °C | Control precision for air and fluid |
Key Methodologies
Section titled âKey Methodologiesâ- Tool Pre-Shaping: PCD and NPD raw materials were initially shaped into a hemispherical tip (0.1 mm radius) using nanosecond pulsed laser machining.
- Wet Lapping (Truing): The laser-machined tools were wet lapped using a CVD diamond disk and diamond slurry (0.05-1.2 ”m grain size) to flatten the hemispherical working surface. Lapping conditions were optimized based on tool material (e.g., 0.6 ”m slurry for NPD).
- Workpiece Pre-Grinding: The cemented carbide end face was pre-ground using a 3 mm radius PCD tool to achieve a flatness and roughness of 40 nmRz, ensuring the radial feed (fr) and peripheral feed (fp) were matched at 18 ”m/rev.
- Thermal Acclimation: The workpiece was subjected to a 2-hour operation to acclimate its temperature to the grinding fluid temperature (Tw = 26.1 °C, Ta = 23.5 °C), ensuring thermal displacement was minimized to prevent diameter errors (< ±25 nm).
- Grinding Ratio Determination: A preliminary experiment involving 18 dimples was performed by varying the setting depth of cut (ÎR). The grinding ratio (α = h/ÎR, where h is the actual depth) was measured to predict the necessary setting depth for the target 30 ”m diameter.
- Plunge Grinding: The MLA mold was fabricated by plunge grinding, transferring the 0.1 mm radius hemispherical tool profile into the cemented carbide. The tool rotation speed was 10000 min-1, and the Z-axis feed rate was 0.1 mm/min.
- Roughness and Diameter Measurement: After fabrication, the surface roughness (Rz, Ra) and dimple diameter were measured using a laser scanning confocal microscope and a 3D optical profiler (Zygo, NewviewTM9000).
Commercial Applications
Section titled âCommercial Applicationsâ- High-Volume Optics Manufacturing: Production of durable molds for hot press molding of glass MLA, replacing traditional resin-based MLA in applications requiring high heat and weather resistance.
- Optical Communication Devices: Fabrication of precision micro-lens arrays used in optical devices, such as LD beam profile converters, for improved light collection and distribution.
- Ultraprecision Machining: Application of NPD tools for achieving mirror-like finishes (5 nmRz) on extremely hard materials (cemented carbide, ceramics) without requiring subsequent polishing steps (grinding-less manufacturing).
- Advanced Tooling: Development of high-performance diamond grinding tools (NPD) where controlled wear exposes fine, high-density diamond particles, enabling stable, ultra-fine finishing processes.
- Lithography and Imprinting: Molds created by this method can be used in advanced nanoimprint lithography or thermal imprint processes for replicating micro-optical structures.
View Original Abstract
A grinding technique was developed to fabricate a micro-lens array (MLA) mold made of cemented carbide with a diameter of 30 ÎŒm, a pitch of 35 ÎŒm, a diameter expansion rate of 0%, a surface roughness of less than 10 nmRz and 1027 lenses. Grinding tools made of polycrystalline diamond (PCD) and nano-polycrystalline diamond (NPD) with a tip radius of 0.1 mm and formed by laser machining and wet lapping were employed. Grinding was performed by transferring the hemispherical tool profile to the cemented carbide with a hardness of 2650 Hv. When NPD was employed, the tips of diamond particles, which were not aligned in height on the working surface, were worn down and the particles of the lower layer were exposed. A huge number of diamond particles with a size of 50 nm were engaged in the grinding operation. This phenomenon made it possible to fabricate lenses with a surface roughness of less than 10 nmRz and a diameter expansion rate of 2%. In contrast to NPD, removed chips contributed to dressing the working surface of PCD. The sharp cutting edge newly exposed on the working surface caused a negative effect on creating a fine surface and contributed to increasing the actual depth of cut. Accordingly, the formable surface roughness and diameter expansion rate were 15 nmRz and 3%, respectively.