Drill slowly with a sharp bit, a backing board and light pressure.
What the message means
Acrylic is brittle and heats up. A fast, dull or grabbing bit stresses the sheet and cracks it near the hole.
Fix it in this order
- Clamp the sheet to a backing board of scrap wood.
- Leave the protective film on while you drill.
- Start with a small pilot hole, then step up to the size you need.
- Use low speed and let the bit cut, do not push hard.
- Keep holes away from edges by at least twice their diameter.
- Wear eye protection and keep hands clear.
Why it happens
Acrylic is hard and brittle and a poor conductor of heat. A bit that grabs the sheet breaks out a chip around the hole, and a bit that rubs without cutting melts the plastic, which then re-hardens and stresses the edge. Cast acrylic tends to chip and crack, while extruded acrylic tends to melt and gum up the bit, so the right technique depends a little on which you have.
A bit ground for plastics, with a flatter cutting angle, is less likely to grab than a standard metal twist bit. A backing board stops the bit tearing out the far side as it exits.
If you see this, do this
| You see | Do this |
|---|---|
| Crack at the hole | Too fast or too much pressure. |
| Melted plastic on the bit | Slow down and clear the chips. |
Related
robot chassis design, screws in plastic.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.
Why this happens
Acrylic is stiff and brittle, and it conducts heat poorly. When a bit grabs, it pulls a chip out and starts a crack that follows the stress. When a bit rubs without cutting, it melts the plastic, which then sets again around the hole and leaves stress inside the edge. The two kinds of sheet react differently: cast acrylic tends to chip and crack, while extruded acrylic tends to melt and gum the bit.
Stress can remain after drilling. A screw tightened hard into a hole presses on its edge, and some cleaners, such as alcohol-based sprays, can cause fine surface cracks called crazing in stressed acrylic. A crack that appears hours later is usually stress from a hole that was cut roughly, tightened too much or placed too close to the edge.
How to confirm the cause
- Look at the cracked hole in good light. Fine cracks in a star pattern point to a bit that grabbed, and a melted rim points to too much speed.
- Look at the chips from drilling. Long, curled gummy strings suggest the plastic is melting, and small clean chips suggest it is cutting properly.
- Measure how far the hole is from the edge with calipers. A hole closer than about twice its diameter is a likely crack site.
- Test the same settings on an offcut of the same sheet before drilling the part.
- Check the bit. A dull or damaged tip rubs, and a standard metal twist bit with a steep tip tends to grab.
Worked example
A worked plan for an M3 mounting hole, with the arithmetic for the edge distance:
Clearance hole for M3 screw: about 3.2 mm (use 3.4 mm if you need slack)
Minimum distance from edge: 2 x 3.2 = 6.4 mm (round up to 7 mm)
1. Clamp the sheet to scrap wood, protective film still on.
2. Pilot hole 1.5 mm, slow speed, light pressure, clear chips often.
3. Step up to 3.2 mm with the same slow speed.
4. Deburr gently, then tighten the screw only until snug.Common mistakes
- Drilling at high speed with a steel twist bit that has a steep point.
- Removing the protective film before drilling, which leaves the surface open to scratches and chips.
- Forcing the bit through instead of letting it cut.
- Tightening screws until the plastic whitens around the head.
- Cleaning the sheet with solvents and then mounting it under stress.
If this did not fix it
- Screw stress after assembly: use a washer, leave a little clearance and avoid over-tightening.
- Sharp inside corners in a cut-out act as stress points; round them with a small radius.
- If acrylic keeps cracking, a tougher plastic such as polycarbonate is less brittle, though it must be drilled with its own care.