Cutting acrylic on a CNC router
Short answer
Cast acrylic machines far better than extruded. The failure mode is melting: molten chips weld back into the cut. Fix it with a single flute bit, a faster feed, a lower RPM and air on the cut. Slowing down makes it worse.
Cast versus extruded
| Cast | Extruded | |
|---|---|---|
| How it is made | Poured and cured between glass | Pushed through a die |
| Machining | Good. Chips cleanly. | Poor. Melts and gums readily. |
| Thickness consistency | Varies more | Very consistent |
| Internal stress | Low | Higher, so it cracks more easily |
| Cost | Higher | Lower |
| Laser engraves white? | Yes, frosts beautifully | Barely |
Buy cast if you are machining it
The price difference is real and the difference in results is larger. If a sheet is unlabelled and it melts no matter what you do, it is probably extruded.The melting problem
Acrylic does not char like wood, it softens. Softened chips stick to the tool and to the wall of the cut, and then re-harden into a solid rib the tool has to cut again. Every pass after that is worse. Everything about cutting acrylic aims at one goal: get the chip out before it softens.
| Change | Direction | Why |
|---|---|---|
| Flutes | One | Biggest chip, biggest gullet. See O-flute bits. |
| RPM | Lower than wood | Less friction heat |
| Feed rate | Faster | Thicker chips carry heat out |
| Depth per pass | Moderate, not tiny | Very shallow passes make thin chips that melt on contact |
| Air blast | Yes | Cools the cut and clears the slot. The single biggest improvement for many people. |
| Climb cutting | Usually | Chip starts thick, which is what you want here |
Cracking
Acrylic is brittle and it cracks in a few specific situations:
- Inside corners with no radius. A sharp inside corner is a stress concentrator. Design a radius in.
- Screws through holes without clearance. Acrylic expands and contracts more than you expect with temperature. Oversize the hole and never overtighten.
- Clamping pressure on a flexed sheet. Support it flat, do not bend it into position.
- Solvents and some adhesives. Crazing, a network of fine cracks, appears days later. Test any cleaner or adhesive on scrap.
- Extruded sheet with internal stress. Can crack from cutting alone. Another reason to buy cast.
Getting a polished edge
A good machined edge in cast acrylic is nearly clear already. To improve it:
- A light finishing pass of about 0.01 inches at full depth, climb cutting, with a sharp single flute.
- Flame polishing with a small torch passed quickly along the edge. Effective, easy to overdo, and it can introduce stress.
- Sanding through grits then buffing. Slower, safer, and it gives the best result on a visible edge.
Practical handling
- Leave the protective film on while machining. It reduces marking and helps hold chips off the surface.
- Tape and CA glue workholding works well, but a downcut O-flute in shallow passes helps keep thin sheet down without packing softened chips into the cut.
- Thin sheet flexes under the cutter. Support it fully underneath or the cut depth will vary.
- Static makes chips stick to everything, including your dust shoe brush.