Basic knowledge of CNC glass processing

Glass has no plastic zone. Load it past its elastic limit and it fractures instead of yielding. That single fact drives every decision in CNC glass processing: tool geometry, feed rate, coolant, and how the part is held. There is no chip formation in the metal sense. Material leaves the cut as fine powder and small conchoidal flakes.

Hardness matters, but fracture toughness matters more. Soda-lime glass sits around 5.5 to 6.5 on Mohs, fused silica a little lower, yet both are brittle enough that subsurface damage extends well below the visible cut. A scratch that looks cosmetic can seed a crack that opens during thermal cycling. This is why finishing passes and edge treatment are not optional steps.

Thermal behavior adds a second constraint. Glass conducts heat poorly, so heat generated at the contact point stays local. Local heating plus coolant produces a steep gradient, and gradients create stress. We keep depth of cut shallow and let the tool pass more often rather than pushing a heavy cut.

• 1
  Brittle-mode removalMaterial is removed by controlled microfracture, not plastic shear.
• 2
  Subsurface damageCracks can run 2–5× deeper than the visible score line.
• 3
  Low thermal conductivityHeat stays at the contact point; light passes beat heavy cuts.
• 4
  Edge quality drives strengthA polished edge can carry several times the load of a rough one.

Almost every glass component starts on a saw or abrasive waterjet. We cut the blank oversize by 0.5 to 1.0 mm per side, because sawing leaves a damaged layer that has to be removed later. For thin sheet under 3 mm, waterjet produces a cleaner edge and less chipping than a diamond blade, so we default to it when the geometry allows.

Milling comes next, usually on a 3-axis or 4-axis machine with diamond tooling. Depth of cut runs 0.05 to 0.2 mm depending on glass type and wall thickness. Spindle speed is high, feed is moderate, and flood coolant keeps the contact zone stable. We do not chase aggressive material removal rates here. Predictable removal beats fast removal.

Drilling follows the same logic. Diamond core drills with a pilot entry, peck cycles that clear the slurry, and a backing plate that supports the exit face. Exit-side chipping is the most common defect in glass drilling, and a backing plate plus reduced feed at breakthrough prevents most of it.

Grinding and lapping set the final dimensions and surface texture. Loose abrasive lapping brings flatness into range, then a polishing step with cerium oxide or colloidal silica brings the surface to Ra 0.2–0.8 μm where the application needs optical clarity. Measurement happens between each step, not only at the end.

Not every glass is worth putting on a CNC. The three families below cover most work we see, and each has a different failure mode. Fused silica (quartz) is the most forgiving for deep features because of its very low thermal expansion. It handles thermal cycling in optical and semiconductor fixtures better than any other common glass.

Borosilicate, including Schott 33 and similar grades, is the workhorse for labware, sight windows and microfluidic chips. It resists thermal shock, machines cleanly with diamond tooling, and is far cheaper than fused silica. If your part does not need deep UV transmission or extreme thermal stability, borosilicate is usually the right call.

Soda-lime is cheap and available in large sheets, which makes it attractive for windows and covers. It is also the most prone to chipping and thermal shock. Thin soda-lime parts with tight edge requirements are the hardest combination we get asked for, and sometimes the honest answer is to switch material.

Chemically strengthened glass is a separate case. Once ion-exchanged, the surface carries high compressive stress. Cutting or drilling it after strengthening releases that stress and typically shatters the part. Machine first, strengthen last.

• 1
  Fused silicaBest thermal stability; suited to optics and semiconductor fixtures.
• 2
  BorosilicateGood balance of cost, thermal shock resistance and machinability.
• 3
  Soda-limeCheap and available; most chip-prone, tight edges are difficult.
• 4
  Strengthened glassMachine before ion exchange, never after.

On well-supported features in borosilicate or fused silica, we hold ±0.005 mm on dimensions and Ra 0.8–1.6 μm as a standard machined finish. Optical faces go finer, to Ra 0.2–0.8 μm, but that requires lapping and polishing steps and changes the cost structure significantly. As-machined surfaces at Ra 1.6–3.2 μm are the economical option when the face is not in the optical path.

Edge quality is the hidden cost driver. A chamfer or polish on every edge adds handling steps, but it also removes the microcracks that cause field failures. If the part sees thermal cycling or mechanical load, specify the edge treatment. If it is purely a cover in a dry environment, a fine grind may be enough.

Feature aspect ratio sets the practical limit. Holes deeper than about 5× their diameter are hard to clear of slurry, and the drill wanders. Slots narrower than 1 mm need special small-diameter tooling and slower feeds. Thin walls under 0.5 mm deflect during milling and often need sacrificial support material.

We inspect 100% of glass parts before shipment, because a crack that survives packing can still fail in the customer's hands. Raw material is checked on receipt, dimensions are monitored in process, and final inspection covers dimensions, edges and visible flaws. Reports are available on request.

• 1
  Standard tolerance±0.005 mm on supported features, glass type dependent.
• 2
  Optical finishRa 0.2–0.8 μm needs lapping and polishing steps.
• 3
  Aspect ratio limitHoles beyond 5× diameter need extra care and slower cycles.
• 4
  Thin wallsBelow 0.5 mm, plan for sacrificial support during milling.

Microfluidic chips are the clearest fit. Channels 100 to 500 μm wide, drilled ports, and a flat bonding face that has to seal. Borosilicate is standard here because the chip often sees temperature cycling during PCR or chemical processing.

Optical and photonics hardware is the second large group. Lens mounts, spacers, windows and light pipes where dimensional stability over temperature matters more than cost. Fused silica shows up often because its expansion coefficient is close to zero.

Vacuum and semiconductor equipment uses glass for viewports, insulator rings and gas distribution plates. These parts usually have tight flatness requirements and see repeated thermal cycles, so edge quality and subsurface damage control are critical.

Instrumentation and medical devices use glass for sample cells, syringe components and sensor windows. These are often small, high-mix parts where no minimum order quantity matters more than unit price. We run from one prototype to 10,000+ part runs without a minimum order quantity on glass work.