Acrylic drilling tips usually start with the bit and stop before the backing, which is where the crack begins. Every fabricator has a box of acrylic parts with a crack running out of a bolt hole. The hole is round, the edge looks clean and the crack appeared a week later, usually after the part was packed and shipped. It is the most common defect in acrylic machining and almost always a drilling decision rather than a material fault.
PMMA is brittle and a poor conductor of heat. A standard twist drill pushes the material rather than cutting it, generates heat at the point, and leaves a ragged exit. The combination of a sharpening crack at the exit and residual stress around the hole is what opens up under load.
Acrylic drilling tips: bit geometry first
A 60° brad-point bit with a relieved cutting edge is the baseline for holes up to 12 mm. It scores the surface before it cuts, which stops the bit from walking, and it leaves a cleaner exit than a 118° twist drill. For holes above 12 mm, use a step drill or bore in two passes, 6 mm then the final size, so the load on the exit face stays low.
For a hole that will carry a bolt, drill 2 mm oversize: a 10 mm bolt gets a 12 mm hole. The clearance allows for thermal movement, takes the load off the hole wall and removes the stress concentration that starts most cracks. Polyethylene or neoprene washers under the head complete the detail.
Speed, feed and heat
Run 500 to 800 rpm for a 6 mm bit and slow down as diameter increases; a 20 mm step drill at 300 rpm is already working hard. Feed steadily and let the bit cut. Dwell in one place polishes the material, raises the temperature past 90 °C and leaves a ring of stress that crazes later.
Air blast or a little water keeps the point cool. Solvent as a coolant is a bad habit: it attacks the hole wall and leaves a stress-cracked surface that fails under the first tightening torque. If coolant is used at all, it should be water with a drop of detergent.
Backing, clamping and exit
Clamp the sheet to a sacrificial backing of MDF or plywood, with the backer in contact across the whole area around the hole. The backing supports the exit face and stops the last 1 mm from tearing out. Freehand drilling with the panel on a bench and air underneath is how a 25 mm hole acquires a star crack at exit.
For stacked parts, clamp the stack and drill through in one pass, then deburr each hole separately with a hand scraper. A deburred hole resists cracking far better than a drilled one, because the micro-crack at the edge is removed rather than left to propagate.
If the part also needs cutting and edge polishing, the sequence in our guide to cutting, drilling and polishing acrylic sheet sets out the order that keeps holes round and edges clear.
One further point for thick sheet: a 20 mm hole in 25 mm acrylic removes a lot of material and generates real heat. Drill 6 mm through, then 12 mm halfway from each face, then finish with a step drill. The two-sided approach keeps the exit stress inside the part rather than at the surface.
Send the drilling layout and sheet thickness to oversales@rpplexiglass.com, and we will supply cast acrylic cut and drilled to your pattern, with clearance holes sized for your fixing hardware.
Frequently Asked Questions
What drill bit should I use for acrylic?
A 60 degree brad-point bit for holes up to 12 mm, and a step drill above that. Avoid standard 118 degree twist drills, which scrape rather than cut and leave a cracked exit. Keep a dedicated set for plastics, because a bit used on metal has a duller edge.
What speed should I drill acrylic at?
500 to 800 rpm for small holes, down to 300 rpm for a 20 mm step drill, with a steady feed and air cooling. If the swarf comes out as a continuous ribbon or the hole edge looks melted, the speed or feed is too high.
Why does acrylic crack after drilling?
Usually three causes together: a tight hole with no clearance, heat from a dull bit, and an unsupported exit face. Drill 2 mm oversize, back the sheet with MDF, deburr the hole and use a washer under the fixing, and the crack will not start.