⚡ electrical engineering

Development of a Wire-Renewal EDM Milling Electrode for Fabricating Microlens Array Molds with High Shape Accuracy on Tungsten Carbide

This study proposes and validates a novel wire-renewal EDM milling electrode that overcomes traditional electrode wear limitations to fabricate high-accuracy microlens array molds on tungsten carbide through optimized rotationally averaged annular discharge.

Original authors: Md Nasir Uddin, Fubin Ma, Tianfeng Zhou, Liheng Gao, Gang Wang, Weijia Guo

Published 2026-07-06

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CC BY 4.0

Original authors: Md Nasir Uddin, Fubin Ma, Tianfeng Zhou, Liheng Gao, Gang Wang, Weijia Guo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you need to carve a perfect array of tiny, smooth glass lenses (like a honeycomb of microscopic magnifying glasses) onto a piece of metal so hard that it's almost impossible to cut with traditional tools. This metal is Tungsten Carbide, which is as tough as a diamond and used for things like drill bits.

The problem is that the metal is too hard for regular drills, so engineers use Electrical Discharge Machining (EDM). Think of EDM not as a drill, but as a lightning storm. You use a tool (an electrode) to zap the metal with tiny electrical sparks. These sparks are so hot they melt and vaporize a tiny bit of the metal, carving out a shape.

The Problem: The "Worn-Out Pencil"

In a standard EDM process, the tool (the electrode) acts like a pencil. As you try to carve a perfect circle, the tip of the pencil gets worn down and rough.

The Issue: If you are carving 25 tiny lenses in a row, by the time you get to the 5th one, your "pencil" is so worn out that the shape changes. The lenses become misshapen, uneven, or rough. To fix this, you'd usually have to stop, throw away the old tool, and grind a brand new one. This is slow, expensive, and makes it hard to keep all the lenses perfectly aligned.

The Solution: The "Self-Sharpening Wire"

The researchers at the Beijing Institute of Technology invented a clever new tool to solve this. Instead of a solid, static metal tip, they built an electrode that holds a thin copper wire inside a curved groove on a spinning ball.

Here is how it works, using a simple analogy:

The Spinning Ball: Imagine a bowling ball spinning on a finger.
The Wire: Now, imagine a thin copper wire wrapped around the equator of that ball, held tight like a belt.
The Magic: As the ball spins, the wire rubs against the hard metal surface, creating the electrical sparks to carve the lens.
The Renewal: The moment the wire gets a tiny bit worn or rough from the sparks, the machine automatically shifts the wire forward by a tiny fraction of a millimeter. This exposes a brand new, fresh, smooth section of the wire to the work.

It's like having a pencil that automatically slides forward to reveal a fresh, sharp point every time you write a single letter. You never have to stop to sharpen it or replace it.

How They Made the Perfect Lenses

The researchers didn't just build the tool; they figured out exactly how to spin it and zap it to get the best results. They treated the process like tuning a radio or cooking a meal:

Spinning Speed (The Dance): If the ball spins too slowly, the sparks hit the same spot too often, creating a deep hole in the middle (like a dimple). If it spins too fast, the sparks don't have time to do their work. They found a "Goldilocks" speed (80 rotations per minute) where the sparks sweep evenly across the surface, creating a perfect bowl shape.
The Spark Timing (The Rhythm): They adjusted how long the sparks lasted and how much time they waited between them. Too short, and the metal doesn't melt enough. Too long, and the heat gets uneven. They found the perfect rhythm to ensure the whole lens is carved evenly.
The Power (The Intensity): They tweaked the voltage and current. Too much power makes the sparks too wild and rough; too little makes the carving slow.

The Result: A Perfect "Lens Honeycomb"

Using this self-renewing wire tool and the perfect settings, they successfully carved a 5x5 grid (25 lenses) onto the Tungsten Carbide.

Uniformity: Every single lens was the exact same depth (about 164 micrometers deep).
Shape: They were perfectly round and smooth, with no "dimples" in the center or rough edges.
Efficiency: They didn't have to stop to change tools or sharpen anything. The wire kept renewing itself, making the whole process fast and reliable.

Why This Matters

This invention is like giving a sculptor a chisel that never dulls. It allows manufacturers to create high-precision molds for microlenses (used in cameras, sensors, and medical devices) out of the hardest metals available, without the headache of constant tool changes or imperfect shapes. It turns a difficult, stop-and-start job into a smooth, continuous, and highly accurate process.

Technical Summary: Development of a Wire-Renewal EDM Milling Electrode for Fabricating Microlens Array Molds

Problem Statement
Microlens arrays (MLAs) are critical components in modern optical systems, including imaging, LiDAR, and biomedical applications. Precision glass molding is the preferred mass-production method, requiring high-fidelity molds typically fabricated from tungsten carbide (WC) due to its hardness and thermal stability. However, machining WC into precise micro-optical structures is challenging. Conventional ultra-precision techniques like diamond turning suffer from rapid tool wear and poor form accuracy. While Electrical Discharge Machining (EDM) offers a non-contact alternative for hard materials, it faces a significant limitation: electrode wear. In conventional EDM, the electrode head degrades after machining even a single microlens, leading to progressive deterioration in form accuracy, surface quality, and uniformity. This necessitates frequent electrode replacement or trimming, which interrupts the process, increases costs, and complicates the alignment required for dense MLA fabrication.

Methodology
To address these limitations, the authors developed a novel wire-renewal EDM milling electrode. The core innovation is a rigid cylindrical shank transitioning into a spherical head featuring a precision-contoured groove. A copper wire is tensioned within this groove between upstream and downstream spools.

Mechanism: As the electrode rotates, the wire conforms to the spherical curvature, creating a stable, controlled inter-electrode gap. Crucially, the system allows for in-situ wire renewal: after machining a cavity, the wire is indexed forward by a controlled increment ( $\Delta w$ ), exposing a fresh, unworn segment for the next operation. This prevents the accumulation of wear-induced distortions on the active discharge interface.
Experimental Setup: Experiments were conducted on a CHARMILL ROBOFORM die-sinking EDM platform using WC workpieces (94% WC, 6% Co). The study utilized a parametric optimization approach, systematically varying spindle rotation speed (20–100 rpm), pulse width, pulse interval, servo voltage, and peak current.
Theoretical Modeling: The authors developed a comprehensive electro-thermal model to describe the process. They analyzed the transition from an instantaneous elliptical discharge region (due to the wire-electrode geometry) to a rotationally averaged annular discharge region. A heat flux model was established to explain the material removal evolution toward a spherical-cap morphology.

Key Contributions

Novel Electrode Design: The introduction of a wire-renewal mechanism that maintains a constant effective discharge radius without reshaping or replacing the entire electrode, significantly reducing electrode management complexity.
Mechanism Analysis: Theoretical and experimental validation of the "rotationally averaged annular discharge." The study elucidates how spindle rotation transforms an elliptical wear zone into a circular annular discharge field, which is critical for achieving uniform material removal.
Parametric Optimization: A systematic investigation identified that spindle rotation speed and pulse timing (width and interval) are the dominant factors in mitigating "center-biased" material removal (where the center of the cavity is deeper than the edges). In contrast, servo voltage and peak current were found to primarily influence the overall material removal rate and aperture size.

Results
Under optimized conditions (80 rpm rotation, 25 µs pulse width/interval, 80 V servo voltage, and 1 A peak current), the study achieved the following:

High Shape Accuracy: Fabricated microlens cavities exhibited a sag depth of approximately 160 µm with a low form error (RMS $\approx$ 3.4 µm; NRMS $\approx$ 2.07%).
Uniformity: The process successfully eliminated the central dimple often seen in rotational EDM, resulting in symmetric, spherical-cap profiles.
Surface Quality: The surface roughness ( $S_a$ ) was measured at 3.2 µm. SEM analysis revealed uniform micro-crater structures and resolidified features, with no significant burrs or edge ridges.
Array Fabrication: A (5×5) MLA mold was successfully fabricated on WC. The array demonstrated high uniformity with a depth consistency of $164 \pm 0.3$ µm across all 25 lenslets and excellent coplanarity.
Material Integrity: EDS analysis confirmed that while the WC composition changed slightly post-EDM (increased carbon and oxygen due to thermal effects and oxidation), the material integrity remained consistent with the EDM process.

Significance and Claims
The paper claims that the proposed wire-renewal EDM milling strategy provides a robust solution for scalable and precise micro-optics manufacturing. By enabling in-situ wire renewal, the method overcomes the primary bottleneck of electrode wear in conventional EDM, allowing for the reproducible fabrication of high-accuracy microlens molds on hard materials like tungsten carbide without frequent interruptions. The study concludes that this approach significantly enhances the efficiency and accuracy of MLA mold fabrication, offering a pathway for cost-effective, large-scale production of structured optical molds. The authors note that future work will focus on further refining surface quality and investigating post-processing techniques.