Acrylic sheet looks tough but it is brittle, and a standard twist bit tends to grab and crack it. Drilling acrylic cleanly is entirely doable once you understand why it cracks and how to prevent it.
Why Acrylic Cracks
The sharp lips of a normal metal-cutting bit dig in and try to pull themselves through the sheet, just as they self-feed into wood. In brittle plastic that grabbing action chips the exit side and can split the sheet outright, especially near an edge. Heat is the other enemy: friction melts acrylic, and melted plastic gums the bit and re-hardens into rough, cracked walls.
The Right Bit Geometry
You want a bit that scrapes rather than grabs. Bits ground specifically for plastic have a steep point angle and near-zero rake so they cannot self-feed. If you only have twist bits, you can dull or hone the leading edge slightly, or use a bit with a lower helix. Some people run the bit backwards briefly to knock the sharp edge off. A brad-point or spur bit also gives a cleaner start. Whatever bit you use, make sure it is genuinely sharp but not aggressive, because a chipped or worn edge will chatter and crack the sheet just as badly as one that grabs.
- Plastic-specific bits: ground to scrape, so they will not chip the exit.
- Slow feed: ease off pressure as the tip breaks through the far side.
- Back it up: clamp scrap wood behind the sheet to support the exit hole.
Speed, Feed and Heat
Run a moderate speed with light, steady pressure. Too fast generates heat and melting; too slow and grabby cracks it. Let chips clear and, on thicker sheet, pull the bit out to shed heat. A drop of water or a little soapy water helps keep temperatures down on larger holes. If plastic starts to melt onto the bit, you are going too fast or pressing too hard.
Support and Setup
Leave the protective masking film on the sheet while you drill; it reduces surface scratching and marks your layout. Always clamp a piece of scrap wood tight behind the acrylic so the bit exits into support instead of open air, which is where most blowout cracks happen. Keep holes well back from edges, and if you need a hole near an edge, drill slightly oversize so thermal movement does not stress the plastic.
Finishing the Hole
Ease off dramatically as the tip nears the back face — this last moment is when brittle plastic blows out. For big diameters, step up gradually rather than jumping straight to full size, drilling a smaller pilot first and opening it out in stages. A light countersink or a few passes with fine sandpaper cleans the rim, and you can flame-polish or buff cut edges if the hole will be seen. With the right bit and a patient feed, acrylic drills as neatly as any material.