CNC Glass Processing: Edging, Drilling, and the Art of Not Breaking Stuff

If tempering is about heat and chemistry is about baths, CNC machining is about brute force with a delicate touch. It's where raw float glass turns into finished product-edges ground, corners shaped, holes drilled. And it's where a surprising amount of glass gets broken before it ever sees a furnace.

 

The Machine Landscape

There are two ways to machine glass, and the choice tells you everything about the shop's business model.

 

Twin-spindle linear edgers are the workhorses. Glass goes in one end, gets ground on both edges simultaneously by vertical and horizontal spindles, and comes out the other end with finished edges. They're fast-like, really fast. A twin-edger can chew through thousands of square feet a day if you're running standard sizes. The trade-off is limited flexibility. You're basically doing straight edges on rectangles.

 

CNC machining centers are the opposite. They use a single spindle with multiple tools-grinding wheels, diamond drills, polishing pads. The glass sits on a vacuum table, and the head moves in X, Y, and Z axes. These machines are slow but incredibly versatile. You can do complex shapes, radius corners, stepped edges, countersunk holes, and custom cutouts. If you're making architectural glass with weird geometries, furniture glass, or anything that isn't a rectangle, you need a CNC.

 

The Tooling: Diamond Is Forever

Everything that touches glass is diamond. Not synthetic grit-actual diamond bonded to metal or resin.

Rough grinding wheels use coarse diamond (60 to 120 grit) and run wet to hog off material fast.

Fine grinding wheels step down to 200 to 400 grit to smooth out the scratches before polishing.

Polishing wheels use 800 to 3000 grit diamond in resin bond, sometimes with cerium oxide or felt wheels for optical clarity on exposed edges.

The wheels wear. A good operator knows when a wheel is "tired" by the sound and the surface finish. Push a worn wheel too long and you'll get burn marks on the edge-localized heat that creates microfractures. Those fractures usually show up in the tempering furnace.

 

Drilling: High Risk, High Reward

Drilling holes in glass is a calculated risk. Every hole is a potential stress concentration. The secret is a two-stage process and the right tool geometry.

Diamond core drills are standard. They look like hole saws. The trick is the pilot-you start with a smaller drill to establish the location, then switch to the full-size core drill. Or you use a drill with a diamond-impregnated "collar" that engages gradually.

Through-spindle coolant is mandatory. Running dry creates heat fractures instantly. The coolant (water or water-soluble oil) has to reach the cutting edge directly through the drill body. If your CNC has poor coolant delivery, you're gambling.

The entry and exit are where breakage happens. When the drill breaks through the bottom surface, the remaining glass thickness can't support the downward pressure. Experienced operators program a "breakthrough" routine-reduced feed rate in the last millimeter, sometimes with a micro-step pause to let the coolant stabilize.

 

Edge Types: What the Customer Sees

Different jobs demand different edges. Each requires a specific wheel set and pass sequence.

Flat edge (seamed) is the minimum. Just knock off the sharp corners with a straight grinding wheel. It's fast, cheap, and goes into frames where nobody sees the edge.

Flat polished takes it further-rough grind, fine grind, then polish until the edge is clear and glossy. It's common for interior glass where the edge is visible but not highlighted.

Beveled is a 45-degree angle ground into the edge. This is a CNC move unless you're running a dedicated beveling machine. Bevels need multiple passes and consistent wheel pressure to keep the line straight.

Pencil edge (radius) is the most common for architectural glass. It's a smooth radius across the edge thickness. You need at least two passes-rough shape then polish-and a machine rigid enough to maintain the radius consistently across a 3-meter sheet.

Arrised is the simplest-just a chamfer at 45 degrees on both sides, usually 1 to 2 mm. It's the default for glass that's going into tempering. Quick, removes the sharp edge, and reduces thermal shock risk in the furnace.

 

The Vacuum Table: Keeping It Still

CNC machining relies on vacuum to hold the glass in place. Lose vacuum, lose the part.

The table is usually segmented, with zones you can turn on and off. For small parts, you use a smaller zone. For large sheets, you light up everything. The rubber seals between zones wear out over time-a tiny leak can drop vacuum pressure enough that the glass shifts during a heavy cut. A shift at 15,000 RPM usually means a broken drill and a scrapped part.

Some shops run mechanical clamps in addition to vacuum for really aggressive work or for parts with holes that break the vacuum seal.

 

The Water Issue

CNC machining is wet work. The coolant keeps the tool cool, flushes glass dust, and prevents heat fractures. That means water management is a bigger deal than most shops admit.

Suspended glass particles are abrasive. If your coolant filtration system is weak, recirculated water becomes a slurry that wears out pumps, seals, and guideways. Good shops use multi-stage filtration-settling tanks, paper filters, sometimes centrifuges-to keep the water clean.

Disposal is another headache. Glass slurry is inert but it's a waste stream. Environmental regulations in most regions require separation and proper disposal. The shops that ignore this eventually get a visit from regulators.

 

Pre-Temper vs. Post-Temper

This is the fundamental decision: machine the glass before tempering or after.

Pre-temper is the standard approach. Cut, edge, drill, then temper. It's faster because you're working with annealed glass, which is more forgiving. The downside is that tempering introduces distortion and slight size changes. If your tolerances are tight, you have to account for that.

Post-temper is rare but necessary for some applications. You temper first, then machine. This is required for holes that need precise positioning relative to coatings, or for glass that warps unpredictably in the furnace. But machining tempered glass is dangerous-you're breaking the compressed surface layer at the edge of every cut. You lose tempering strength locally. And if you're not careful, the whole panel explodes on the machine.

Most shops won't touch post-temper machining. The ones that do charge accordingly.

 

Common Failure Modes

Edge chips. Usually from dull wheels, excessive feed rate, or poor setup. A chip bigger than 2mm is usually a reject. But the real problem is the invisible chips-micro-cracks that don't show until the glass hits the tempering furnace and the panel blows up.

Drill breakout. The bottom surface spalls when the drill exits. This is almost always feed rate or coolant delivery. Sometimes it's worn tooling.

Water marks. Stains on the glass surface from hard water sitting too long. This matters for coated glass especially. A water mark under a Low-E coating looks like a defect after tempering.

Vacuum slips. The glass shifts during machining. You end up with holes in the wrong place or edges that aren't square. Expensive scrap.

 

The Labor Reality

CNC glass machining used to be an operator's job. The machine did what it was told. Today, programming is the bottleneck. A good programmer can set up a complex job with multiple hole patterns, shaped edges, and variable geometries in ten minutes. A bad programmer takes an hour and still crashes the tool.

The best shops I've seen treat their CNC department like a machine shop, not a glass shop. They measure tool wear, track cycle times, and log every break. They know that a $500 diamond wheel needs to run a certain number of meters before it's paid off. And they never, ever let a new operator run a high-value job without a dry run-because glass doesn't give second chances.