Deep Hole Drilling for the Glass and Optics Industry: Diamond Core Drilling of Fused Silica, Quartz, and Borosilicate for Telescope Mirror Cells and Precision Optics

A manufacturer of large telescope mirror cells (Zerodur glass-ceramic, 0 ± 0.1 × 10⁻⁶ /K CTE, 1.5 m diameter × 100 mm thick, 18 mounting holes Ø25 mm × 100 mm deep, ±0.05 mm position, Ra < 0.4 µm, no microcracks) was using conventional diamond core drilling (metal-bond, 60/80 mesh, 5000 rpm, feed 5 mm/min, water coolant). Exit chipping of 0.1–0.3 mm occurred from thermal stress at breakthrough (Zerodur's zero expansion could not dissipate friction heat). Switching to ultrasonic-assisted diamond core drilling (resin-bond, 200/230 mesh, 20 kHz, 12 µm amplitude, 3000 rpm, feed 2 mm/min, DI water at 5°C, peck 5 mm with 10-second dwell) reduced thrust force at breakthrough by 55%, eliminated exit microcracking (chipping < 0.02 mm), and achieved Ra 0.2–0.3 µm.

Optical Glass Drilling Methods ​

Comparison of Drilling Methods for Optical Glass Materials ​

Recommended Diamond Core Drill Parameters for Optical Glasses ​

FAQ ​

What is the primary challenge in deep hole drilling of Zerodur and fused silica, and why is ultrasonic assistance often required? ​

The primary challenge in deep hole drilling of Zerodur and fused silica is exit chipping — the fracture and removal of material at the bore exit edge, which typically measures 0.1–0.5 mm in width and 0.05–0.3 mm in depth when using conventional diamond core drilling. The exit chipping in these materials is caused by a combination of: their extremely low fracture toughness (0.7–1.2 MPa·m¹/² — comparable to silicate glass), their very low thermal conductivity (1.4–1.6 W/m·K — compared to 25–30 W/m·K for alumina), and in the case of Zerodur, its near-zero coefficient of thermal expansion (0 ± 0.1 × 10⁻⁶ /K). The low fracture toughness means that the material cannot absorb the stress concentration at the exit edge without cracking — the crack propagates from the edge into the workpiece. The low thermal conductivity means that the heat generated at the drill-workpiece interface (from the friction of the diamond grit against the glass) accumulates at the cutting zone rather than being conducted into the workpiece. The temperature at the drill tip can reach 200–400°C during the breakthrough moment (when the drill exits the far surface and the coolant flow path is momentarily interrupted), and the thermal stress from this localised heating adds to the mechanical stress from the cutting force, creating a combined stress that exceeds the fracture strength of the material. In Zerodur, the near-zero CTE means that the material does not expand when heated — instead, the localised thermal stress develops without any thermal expansion to relieve it, concentrating the stress at the crack tip.

Ultrasonic assistance addresses all three mechanisms. The ultrasonic oscillation (20–25 kHz, 10–18 µm amplitude) causes the diamond grit to impact the workpiece at high frequency, which reduces the average cutting force by 40–60% (the impact loading creates fracture more efficiently than the constant force of conventional drilling, so less force is required to achieve the same removal rate). The reduced cutting force at breakthrough directly reduces the mechanical stress on the exit edge. The ultrasonic oscillation also pumps coolant into the drill-workpiece interface during the retraction phase of each oscillation cycle (the gap opens by 10–18 µm at 20 kHz, allowing the DI water to penetrate between the diamond grits), which reduces the interface temperature by 50–100°C and prevents the thermal stress from reaching the fracture threshold. The third mechanism is the modification of the fracture path — the ultrasonic impacts create microcracks (2–5 µm deep) that intersect and remove material in small fragments, rather than the continuous lateral crack propagation that occurs in conventional drilling. At breakthrough, the ultrasonic impacts remove the exit edge material in controlled micro-fragments instead of propagating a single large crack into the workpiece. The combination of these three mechanisms — force reduction, temperature reduction, and fracture path modification — makes ultrasonic assistance essential for deep hole drilling of Zerodur and fused silica when exit chipping must be below 0.05 mm. For borosilicate glass, which has a higher fracture toughness (0.8–1.0 MPa·m¹/²) and a positive CTE (3.3 × 10⁻⁶ /K), ultrasonic assistance is beneficial but not essential — exit chipping can be controlled below 0.08 mm with conventional diamond core drilling at reduced feed rate (50% reduction for the final 2 mm of depth) and chilled coolant (5–10°C).

What coolant is recommended for diamond core drilling of optical glass, and why is deionised water preferred? ​

The recommended coolant for diamond core drilling of optical glass is deionised (DI) water with a resistivity of 18.2 MΩ·cm, at a temperature of 5–20°C, delivered through the centre of the diamond core drill at 3–5 bar pressure. DI water is preferred over tap water, water-miscible emulsion, or oil-based coolants for three reasons. Non-contamination — optical glass components are used in applications where any residue on the glass surface can interfere with the optical performance (a residue film of 0.1 µm thickness can reduce the light transmission by 1–5% depending on the wavelength). DI water leaves no residue when it evaporates — the dissolved minerals that would be left by tap water (calcium carbonate, silica, salts) are removed by the deionisation process. Any residue from a water-miscible emulsion or an oil-based coolant would require an additional cleaning step (solvent cleaning, ultrasonic cleaning in acetone or isopropanol) that risks scratching the polished optical surfaces. The DI water must be filtered to 0.2 µm absolute to remove any particles that could scratch the bore surface during drilling. The second reason is cooling effectiveness — water has a specific heat capacity of 4.2 kJ/kg·K, which is 2× higher than oil and 4× higher than compressed air. The high specific heat capacity allows the water to absorb the frictional heat from the diamond grit-workpiece interface without a large temperature rise. The water temperature must be maintained at 5–10°C for low-CTE materials (Zerodur, fused silica) to provide a thermal sink that prevents the interface temperature from exceeding 100°C. For borosilicate and quartz, the water temperature can be 20°C — a lower temperature is not required because these materials have a positive CTE and can absorb thermal stress without cracking.

The third reason is chip evacuation — the water flow carries the glass debris (silicon dioxide particles, 1–50 µm) out of the bore through the annulus between the drill OD and the bore wall. The glass particles are abrasive (hardness 700–1000 HV for silica particles) and can cause scoring of the bore surface if they accumulate in the annulus. The water flow velocity must be at least 2 m/s in the annulus to keep the particles suspended and prevent them from settling. The water flow rate is calculated from the annular cross-section (the area between the drill OD and the bore ID). For a 25 mm core drill in a 25.1 mm bore (0.05 mm per side clearance), the annulus area is π × (25.1² − 25.0²) / 4 = 3.9 mm². To achieve 2 m/s, the flow rate is Q = V × A = 2 × 3.9 × 60 / 1000 = 0.47 L/min — a very low flow rate. For larger bores, the flow requirement increases proportionally to the annulus area. The water must be continuously recirculated through a 0.2 µm filter to remove the glass particles and prevent them from re-entering the bore. The water pH should be maintained at 7.0 ± 0.5 (neutral) to prevent chemical attack on the glass surface — some optical glasses (NBK7, borosilicate) are susceptible to leaching of alkali ions from the glass surface in acidic or alkaline water, which can damage the optical surface. The DI water system (reservoir, pump, filter, chiller, and deioniser) for a single optical glass drilling machine costs $10 000–30 000, plus $2000–5000 per year for filter cartridges and deioniser resin replacement. The cost is justified by the value of the optical components (a single Zerodur mirror blank for a 1.5 m telescope costs $50 000–200 000, and the drilling is performed after the mirror surface has been polished — any contamination or damage during drilling requires repolishing at a cost of $10 000–50 000).

The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified optical engineers, glass manufacturers, and equipment manufacturers for specific optical glass drilling applications. Data and recommendations are based on published research and industry experience as of 2026.