How do I drill plastic and acrylic without cracking it?

Plastics range from soft and forgiving to brittle and unforgiving, and acrylic is at the brittle end. A standard twist bit is ground to bite into metal; in acrylic that same geometry corkscrews the bit in, loads the material sideways, and cracks it — usually at the exact moment the hole was almost finished.

Safety

Step by step

1. Identify the plastic.Acrylic and polystyrene are brittle and crack readily. Polycarbonate, ABS, uPVC and polypropylene are tougher and more forgiving. Acrylic baths and shower trays are usually acrylic sheet backed with resin and board, which behaves like a brittle laminate.
2. Support the material on a flat backing board.Clamp scrap timber tight against the back face. It stops the material flexing, prevents breakout, and gives the bit something to enter rather than air.
3. Use a bit that will not grab.Bits ground for plastic have a shallower point and less aggressive rake. If you do not have one, take an ordinary HSS bit and take the very keenest edge off the leading lips with a slipstone, which stops it corkscrewing.
4. Or step up through sizes.Drill small, then increase in steps. Each step removes less material and loads the plastic far less than jumping straight to the final size.
5. Run at moderate speed with light pressure.Too fast melts the plastic and the swarf welds to the bit. Too slow with heavy pressure cracks it. Aim for continuous curly swarf rather than melted strings or powder.
6. Withdraw to clear swarf.Plastic swarf packs the flutes and then rubs, which generates the heat that melts the hole closed behind the bit. Back out every few seconds.
7. Ease right off at breakthrough.This is where the crack happens. Reduce pressure to almost nothing for the last millimetre and let the bit finish itself.
8. Deburr the hole.A larger bit turned by hand, or a countersink, removes the ridge round the hole. A burr left in place concentrates stress and is where a crack later starts.

Tips

• Acrylic sheet is often supplied with protective film. Leave it on while drilling — it holds the surface together at the rim and prevents scratching.
• A step drill bit is excellent in thin plastic sheet, because its gentle progression avoids the snatch entirely.
• For uPVC window and door frames, use a bit sized for the fixing and keep away from the chambers containing steel reinforcement unless the fixing is meant to go into it.

Common mistakes

• Using a sharp new twist bit at speed in acrylic — The aggressive rake pulls the bit in as it breaks through and levers the material apart. It cracks radially from the hole.
• Drilling without a backing board — Unsupported plastic flexes away from the bit, then snaps back as the bit breaks through. That flex is what cracks it.
• Drilling an acrylic bath or tray without turning the water off — A crack in situ is not repairable to a waterproof standard, and any water in the system will find it.

Questions people ask