Acrylic is the friendliest plastic to look at and the least forgiving to cut. It machines beautifully right up to the moment it doesn't, and then it cracks—sometimes hours later, sometimes at the very next hole you drill. I've made display cases, a replacement aquarium lid, and a set of machine guards out of cast sheet, and every crack I've caused came from the same two things: heat and a sharp internal corner. Get those under control, and acrylic stops being scary.
What you will learn
- Why cast sheet behaves differently from extruded
- Which bits cut cleanly and which ones melt the edge
- How to keep heat out of the cut
- Why inside corners crack and how to radius them
- How to finish an edge from cut to optically clear
Cast sheet or extruded—does it actually matter?
It matters more than any tool choice. Extruded acrylic is cheaper and more consistent in thickness, but it's softer, it melts at a lower temperature, and it carries internal stress from the extrusion process that shows up as crazing when you machine it. Cast sheet costs more and cuts noticeably cleaner. If you're drilling or routing and you have the choice, buy cast. The practical test: extruded sheet usually has a protective film with printed text that peels off sheets; cast often comes with paper backing. If you're not sure, ask the supplier—and if they can't tell you, assume extruded and cut conservatively. For the actual cutting, a kit with a decent spread of bit shapes matters because you'll want a spiral bit for slots and a separate one for edge work. The HARDELL Orange Advanced 43-piece kit covers both without forcing you to reuse a dulled bit.
Which bit cuts acrylic cleanly?
Single-flute or two-flute cutters with a large flute area, so chips can escape instead of remelting into the cut. The enemy is a bit with fine teeth packed close together—that's a saw designed for wood, and in acrylic it just rubs. Carbide burrs work for shaping and chamfering, but they generate heat fast, so keep them moving. For straight cuts in a sheet up to about 6 mm, scoring and snapping is still the fastest clean method, and the rotary tool isn't involved at all: score deeply along a straightedge, five or six passes, then snap over the edge of the bench. Use the rotary for curves, cut-outs, and everything after the blank is roughly shaped.
How do you keep heat out of the cut?
Speed down, feed up, and never pause in one spot. Acrylic's glass transition is low enough that a bit of dwelling for two seconds will soften the material around it, and when it cools, it shrinks and locks stress into the part. I run around 10,000 to 15,000 RPM on plastic, and I keep the tool moving along the line the entire time. If the chips coming off are stringy or they're welding back onto the edge, you're too slow or feeding too lightly. If the edge is chipping and rough, you're feeding too aggressively. Compressed air is the best cooling you can give it; a quick puff clears chips and drops the temperature. Don't use water—acrylic doesn't care, but your dust turns into a paste, and the paper backing glue gets everywhere.
Why do inside corners crack?
Because a sharp internal corner is a stress concentrator, and acrylic is already carrying residual stress from manufacture. A square cut-out with 90-degree corners will crack from the corner outward, often with no load on it at all. Round every internal corner. A 3mm radius is enough for small parts; go bigger on anything structural. This is also why you should never cut a slot with square ends—drill the ends first with a round hole, then connect them. Drilled round holes don't propagate cracks the way a milled square end does. On the machine guards I made, every slot is a pair of drilled holes joined by a straight pass, and none of them have cracked in four years of vibration.
What about drilling holes near an edge?
Keep at least two hole diameters between the edge of the hole and the edge of the sheet. Less than that, and the thin web between them will bow out under the drill's thrust and crack. Back the workpiece with scrap wood so the bit exits into something solid rather than blowing out the far face. Standard twist drills work fine in acrylic if you sharpen them properly, but a bit ground for plastic has a much more aggressive rake, and it'll pull less on breakthrough. Back off the feed as you feel it start to punch through—that's where the blowout happens. A 6-inch rechargeable caliper is worth having on the bench here, because hole spacing errors are the kind of mistake you only notice at assembly, when the part is already finished.
How do you get from a cut edge to a clear edge?
In stages, and don't skip grit. Scrape or file the edge flat first to remove saw marks, then work through abrasives—something like 320, 600, or 1200—then move to a buffing wheel with plastic compound. Felt bobs on a rotary tool handle the buffing stage well because they stay cool and conform to the edge, and the White Pro 80-piece system has the full grit range plus the felt accessories, so one case covers cutting through to buffing. The temptation is to jump from 320 straight to buffing, which leaves a haze that no amount of polishing removes because you're polishing scratches rather than removing them. On a long straight edge, wet sanding on a flat block beats any power method for the intermediate grits. Finish with a flame polish only if you're experienced; it's fast and gorgeous, and it will ruin the part in under a second if you linger. For most projects, buffing compound gets you 90% of the way with none of the risk.
Can you bend acrylic after cutting?
Yes, and it's the reason people buy sheets in the first place. A heat gun or a strip heater along the bend line, brought up slowly and evenly. Extruded sheet bends at a lower temperature and more predictably; cast sheet needs more heat and holds a tighter radius once formed. Bend against a form, hold it until it cools, and don't rush the cooling with water—quench-bending locks in stress, and it will craze. Keep the protective film on during heating, because it resists scorching from stray contact, and it makes handling a hot sheet easier.
What's a realistic first project timeline?
A simple four-sided display case is an afternoon. Cutting the panels and drilling the fixing holes is maybe two hours. Edge finishing is the other two, and that's the part people underestimate—sanding and buffing always takes longer than the cutting. Assembly with solvent cement is quick, but you get one shot at alignment, so dry-fit everything and mark it before the cement goes near it.
From the workshop: the aquarium lid that cracked on a Tuesday
I cut a replacement lid for a 240-liter tank from extruded sheet because it was what the shop had. I routed a rectangular cut-out for the filter with square internal corners and drilled the four mounting holes about 8 mm from the edge, and it looked fine. Set it on the tank that evening. Tuesday morning—roughly thirty-six hours later—there was a crack running from one corner of the cut-out all the way to the nearest mounting hole. No load, no impact, just residual stress finding the sharp corner and the thin web. I redid it in cast sheet with radiused corners and 20 mm of material around each hole. That lid is still in service. The material cost difference was about eleven dollars, which I'd have happily paid to skip the whole first attempt.
Common mistakes recap
- Square internal corners instead of radiused ones
- Letting the bit dwell and heat the material
- Drilling closer than two diameters from an edge
- Skipping intermediate grits and buffing scratches
- Buying extruded sheet for a job that needs cast
- Quench-cooling a bend
FAQ
Can I use a normal wood saw blade?
You can, but expect melting and a rough edge. Fine-tooth wood blades pack chips and rub. A plastic-cutting blade has fewer, larger teeth, and it's worth the few dollars.
Why does my edge go white when I sand it?
That's heat and fine dust packed into the surface, usually from sanding too fast with a loaded abrasive. Slow down, use fresh paper, and step through the grits instead of jumping.
Is acrylic the same as polycarbonate?
No. Polycarbonate is far tougher and machines more easily, but it scratches readily, and it yellows outdoors faster. Acrylic is more rigid, more scratch resistant, and much better for polishing to clarity.
How thin can I go before it gets unmanageable?
Around 3mm is where hand-cutting gets fiddly and vibration becomes a problem. Below that, scoring and snapping is cleaner than any rotary work.
What glue should I use?
Solvent cement for acrylic-to-acrylic—it welds rather than bonds, and the joint ends up nearly as strong as the sheet. Cyanoacrylate works in a pinch, but it blooms white and the joint stays brittle.
Do I need to remove the protective film before cutting?
Leave it on through cutting and drilling. It prevents surface scratches from the tool base and reduces chip-out at the exit. Peel it before sanding and polishing.
Can I flame polish indoors safely?
Only with good ventilation. It releases unpleasant fumes, and an uneven pass will bubble the surface. For most parts, buffing compound gives a result that's close enough without the risk.
Why did my part crack days after I finished it?
Residual stress is relaxing, almost always from a sharp internal corner or heat damage during cutting. It's the classic acrylic failure, and it's preventable at the design stage, not the finishing stage.
