How do I drill a hole in glass without cracking it?

Glass has no plasticity: every bit of stress goes straight into propagating a crack. Drilling it works by abrading a hole rather than cutting one, and everything about the technique is about keeping heat and point loading low. The technique is straightforward. The judgement about which glass can be drilled is the part that matters.

Safety

What you'll need

• Diamond core or spear-point glass bit
• Water, and a putty or plasticine dam
• Soft flat support — timber, felt or a folded towel
• Masking tape (optional)
• Goggles and gloves

Step by step

1. Establish the glass type first.Look for an etched mark in a corner: toughened glass is marked, and it is used in doors, side panels, shower screens, table tops, low-level windows and anything else covered by safety glazing rules. Toughened glass cannot be drilled at all — it shatters completely. Laminated glass cannot be drilled successfully either.
2. Lay the glass flat and fully supported.On a sheet of timber with a towel or felt between. Any unsupported area will flex, and glass does not flex. This also means the bit has something to break into rather than air.
3. Keep well away from edges.Glass is weakest near its edges and holes there crack out almost every time. As a working rule, keep the hole several times its own diameter away from any edge, and further on thin glass.
4. Make a dam and flood the hole.A ring of plumber's putty or plasticine around the spot, filled with water. Water is doing the cooling and carrying the ground glass away, and glass drilling without it fails.
5. Mark and start through tape.A strip of masking tape stops the bit skating on the smooth surface. Some people start with the drill at an angle and bring it upright, as with tile.
6. Run very slowly with almost no pressure.Gear 1, low trigger, and let the weight of the drill do the work. Pressure is the enemy — it is what turns abrasion into a crack.
7. Lift and let the water in regularly.Every few seconds, ease off so fresh water reaches the cutting face. Watch for milky slurry, which means it is cutting; clear water and no slurry means the bit has glazed.
8. Ease right off at breakthrough.The last fraction of a millimetre is where the cone of glass under the bit breaks out. Almost no pressure at all, and if you can, turn the piece over and finish from the other side.

Tips

• Drilling from both sides — halfway from one face, then turning over and meeting it — gives a far cleaner hole with no chipped-out cone on the exit face.
• For bottles, a jig that holds the bottle steady is worth improvising: a cardboard box cut to cradle it, sandbags, or a folded towel in a shallow tray of water.
• If the glass is in something you cannot replace, practise on a piece of scrap of the same thickness first. The feel is unlike drilling anything else.

Common mistakes

• Trying to drill toughened glass — The whole pane disintegrates into fragments the instant the surface tension is broken. There is no technique that avoids this — toughened glass has to be made with its holes already in it.
• Pushing to speed it up — Glass drilling is abrasion at a fixed rate. Pressure adds stress, not speed, and stress is what makes it crack.
• Drilling dry — Local heating creates a thermal stress gradient in a material that cannot accommodate any, and the glass cracks from the heat alone.

Questions people ask