Drilling & notching glass
Every hole and notch you cut into glass is a little stress magnet, a spot the glass would rather break at. Get the edge distances, diameters, and (most importantly) the timing wrong and the lite cracks, either on the bench or years later in the wall. Get them right and none of it is a problem. Here are the rules that keep drilled glass alive.
Why drilling must precede heat treatment
If you remember one thing from this whole article, make it this: every hole gets drilled before the glass is heat treated. No exceptions, no "just this once."
Tempering and heat strengthening lock a permanent field of stress into the glass. Touch a diamond core drill to a tempered lite and the micro-fractures it makes reach straight into that stored stress, and the whole thing dices itself into the usual tempered gravel, sometimes the instant you touch it, sometimes hours later when you have already moved on. Even a small notch, chip, or grind mark from a post-treatment drilling attempt will cause this failure.
The same constraint applies to laminated glass, heat treatment must be applied to individual lites before lamination, not after. Once PVB is bonded and autoclaved, the composite unit cannot be drilled without delamination and fracture risk.
Minimum dimensions
These rules exist because glass fails in tension and because the stress concentration around a hole edge scales with proximity to other holes, the lite edge, and the lite corners. These values are typical industry guidelines; always verify with the fabricator for their specific equipment capabilities.
Notching and cut-outs
A notch is a concave cut into the edge of a glass lite, common for hardware clearance, electrical conduit, or point-fixed fitting pockets. Notches concentrate stress at their inside corners more severely than holes because the re-entrant corner has essentially zero radius unless it is finished.
- Inside corner radius: Every notch must terminate in a drilled hole rather than a sharp inside corner. The minimum hole diameter at the corner depends on thickness, but 20 mm (¾") is a common minimum. A sharp inside corner in glass will fracture under almost any load.
- Notch depth: Maximum notch depth is typically limited to 1/3 of the lite width to avoid excessive stress concentration at the root.
- Edge finish: All notch edges must be ground and polished, not left as a raw cut edge, before heat treatment. Raw edges have micro-cracks from scoring that propagate under stress.
Drilling process
You do not drill glass so much as grind through it. The tool is a diamond-tipped core drill, a hollow cylinder with diamond grit on its rim, and the two things that will ruin your day are heat and impatience. Keep it flooded with coolant or you get thermal cracks; go dry or heavy-handed and the edges chip out. The whole technique is basically "let the diamond do the work":
| Parameter | Recommended practice | Why |
|---|---|---|
| Coolant | Continuous water feed at drill face | Prevents thermal crack; flushes glass swarf from cut |
| Feed rate | Light, steady downward pressure | Excessive force chips the entry and exit faces |
| Backing board | Sacrificial wood or MDF under the lite | Prevents exit-face blowout as drill breaks through |
| Through-drill approach | Drill from both faces, meeting in the middle | Eliminates blowout entirely; used for thick glass |
| Edge finish after drilling | Chamfer or polish the hole edge | Removes stress-concentrating micro-cracks from cut |
Annealed vs. heat-treated glass
While heat-treated glass cannot be drilled after treatment, annealed glass has no such restriction. However, annealed glass with holes is structurally weaker than tempered glass with holes because annealed glass lacks the compressive surface layer that makes heat-treated glass resistant to tension-driven crack propagation.