How to Cut Acrylic with a CNC Router: Bits, Speeds and Feeds
Acrylic (PMMA) is one of the most satisfying materials to machine — and one of the easiest to ruin. Cut it right and you get crystal-clear edges that need no finishing. Cut it wrong and you get cloudy, frosted edges, melted chips welded back onto the workpiece, and cracks running along the cut line.
The good news: acrylic is completely predictable. Every failure has a mechanical cause, and every cause has a fix. This guide covers the bit geometry, the parameters, and the cutting strategy that turn acrylic from a frustrating material into one of your most profitable ones.
Quick answer: cut acrylic with a single-flute (O-flute) up-cut carbide bit, run 16,000-18,000 rpm at a chip load of about 0.08-0.12 mm per tooth, and cut thin sheet in one full-depth pass. Melting and cloudy edges come from rubbing — a chip load that is too low — or from a dull edge, not from cutting too fast.
Why Acrylic Is Hard on Bits
Acrylic is a thermoplastic. Unlike wood, it does not chip away cleanly — it melts. The polymer softens around 160°C, and every cutting edge generates heat as it shears the material. If that heat stays in the cut, three things happen:
- The chip melts and re-welds. The molten chip sticks to the cutting edge, then re-bonds to the freshly cut edge. That rewelded material is what creates the frosted, cloudy edge — and it also sticks to the bit, making the next cut worse.
- The edge cracks. Acrylic is notch-sensitive. A dull or wrong-geometry bit doesn’t cut — it pushes. That compression stress starts micro-cracks at the edge, which propagate into the sheet.
- The bit overheats. Heat that isn’t carried away by chips goes into the carbide. A single overheated run can visibly dull an edge that should last months.
The core principle for acrylic: get the hot chip out of the cut as fast as possible, and let the cutting edge do the work instead of rubbing. Every choice below — bit, speed, feed, depth, cooling — serves those two goals.
The Right Bit: Single-Flute and O-Flute
For acrylic, the bit geometry matters more than almost anything else. The rule is simple: use a single-flute (or O-flute) bit, and avoid multi-flute bits.
A single-flute bit has one cutting edge and one large chip channel. That channel is exactly what acrylic needs — it carries the melted chip out of the cut in one continuous spiral, before it can re-weld. Multi-flute bits (two-flute and up) divide the chip into smaller pieces that pack into tighter channels, and in acrylic that means heat builds, chips stick, and edges cloud over.
What to look for in an acrylic bit:
- Single or O-flute design — the large polished gullet is the whole point
- Sharp, polished flute surface — a smooth chip channel releases the chip instead of holding it
- Upcut spiral — pulls chips up and out of the cut (for sheet machining; a downcut is better only when the top surface must stay mark-free, at the cost of chip evacuation)
- Solid carbide — holds an edge long enough to justify the setup
YINGBA’s single-flute router bit range is built around this geometry — large polished gullets and sharp mirror edges, including O-flute variants and coated options for longer runs. A 3.175 mm shank single-flute bit covers most hobby and light-production acrylic work.
Feeds and Speeds for Acrylic
Start from the two formulas:
RPM = (Cutting speed x 1000) / (PI x Tool diameter)
Feed rate (mm/min) = RPM x Flutes x Chip loadFor acrylic, use a cutting speed of 200–350 m/min and a chip load of 0.04–0.08 mm per flute — the same values built into the YINGBA speed & feed calculator, so you can get exact numbers for your bit and machine in seconds.
Starting values for a single-flute bit (mid-range settings):
| Tool diameter | Spindle speed (RPM) | Feed rate (mm/min) |
|---|---|---|
| 1 mm | ~24,000 (machine limit) | ~1,400 |
| 2 mm | ~24,000 (machine limit) | ~1,400 |
| 3.175 mm | ~24,000 | ~1,400 |
| 4 mm | ~20,000 | ~1,200 |
| 6 mm | ~13,000 | ~800 |
| 8 mm | ~10,000 | ~600 |
| 10 mm | ~8,000 | ~500 |
Reading the table: small diameters spin at the machine limit — the chip load still matters, so keep the feed proportional. Larger diameters need less RPM and proportionally less feed. These are starting values: if the edge clouds, increase feed or reduce RPM; if the bit chatters, reduce feed.
The one parameter that matters most is chip load. Acrylic fails from too little chip load (rubbing → melting) more often than from too much. A common beginner mistake is slowing everything down “to be safe” — on acrylic, that is exactly how you melt the edge. When in doubt, err toward the higher chip load.
Step-Down Strategy and Cooling
Depth of cut. For solid acrylic sheet, a good starting rule is a step-down of no more than half the bit diameter per pass for roughing, and 0.5–1 mm for the final finishing pass. Cutting the full sheet thickness in one pass is possible with small bits and high RPM, but it concentrates all the heat at the deepest point — two or three lighter passes produce better edges and longer bit life.
Air cooling, not liquid. Acrylic should be cooled with an air blast (or at least a strong vacuum extraction), not flood coolant. Liquid coolant causes thermal shock on PMMA — the surface cools faster than the interior, and the sheet cracks. A focused air nozzle aimed at the cutting zone carries chips away and keeps the tool temperature stable.
Climb cutting. Use climb milling — climb cuts push the sheet down onto the table and leave a cleaner edge in acrylic. For thin sheets, secure the full surface — double-sided tape or a vacuum bed — because any vibration at the edge will turn into micro-cracks.
Edge Quality Checklist
| Symptom | Cause | Fix |
|---|---|---|
| Cloudy / frosted edge | Chip rewelding — chip load too low, or multi-flute bit | Increase feed, use single-flute bit |
| Melted, rough bottom edge | Chip packing at full depth | Reduce step-down, stronger air blast |
| Hairline cracks from the edge | Notch sensitivity — dull bit, too deep a pass, or vibration | Sharpen or replace bit, lighter passes, secure sheet |
| Chattering / screaming | Feed too high for the bit and machine | Reduce feed 10–20% |
| Burning smell, yellowing | Rubbing — chip load way too low | Increase feed significantly, check RPM |
One practical tip: keep a test scrap of every sheet thickness you cut. When a parameter set produces a glass-clear edge, note it — diameter, RPM, feed, step-down, air pressure. After a few sessions you will have a table tuned to your own machine that beats any generic chart.
Cut Acrylic Like a Pro
Acrylic rewards clean, confident cutting: single-flute geometry, moderate speeds, a healthy chip load, air cooling, and shallow finishing passes. Get those five things right and the material does the rest — transparent edges, no polishing, no cracked sheets.
FAQ
What router bit cuts acrylic without melting? A single-flute (O-flute) up-cut carbide bit. The single flute gives a large chip channel so chips leave the cut quickly, and the up-cut direction lifts heat and debris away from the edge.
What speed and feed should I use to cut acrylic? Start around 16,000-18,000 rpm with a chip load of roughly 0.08-0.12 mm per tooth, which typically lands near 1,300-2,200 mm/min for a 1/4-inch single-flute bit. Cast acrylic tolerates a slightly heavier chip load than extruded.
Why does acrylic melt or cloud when I cut it? Melting and clouding are heat symptoms: a chip load that is too small makes the edge rub instead of shear, and a dull bit does the same at any setting. Increase the feed, keep the bit sharp, and make sure chips are clearing.
How do I get a polished, clear edge on acrylic? Use a sharp single-flute bit, a moderate feed, and leave the protective film on during cutting. A properly sheared edge on cast acrylic comes off the machine clear; cloudy edges point to rubbing or overheating.
What depth of cut should I take in acrylic? For thin sheet, one full-depth pass is best. On thicker material, step down in passes no deeper than one tool diameter to keep the load and the heat manageable.
Need the right bit for your acrylic work? Contact YINGBA with the sheet thickness, tool diameter and machine type — we will confirm the right geometry and starting parameters before you order.
YINGBA manufactures solid carbide router bits and CNC carving tools for distributors, manufacturers and OEM/ODM projects worldwide. Standard models are kept in stock; custom geometry, coatings, marking and packaging are produced to your drawings.
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