Single-flute or O-flute bit
An open single flute clears the chip in one piece before it re-melts; a two-flute cutter welds it back into the wall as a frosted, gummy edge.
$34.19
at Amazon
How to cut acrylic on a CNC router
The three things that matter most:
- Use a single-flute or O-flute bit. Two-flute cutters re-melt and weld the chips; a single flute clears them so the edge stays clean.
- Cut, don't melt - chip evacuation is the whole game. Keep the chip moving out of the cut so heat leaves with it.
- Ramp into the cut rather than plunging straight down, which prevents distortion and cracking, and prefer cast acrylic over extruded, which cracks more readily.
Feeds & speeds calculator, pre-filled for acrylic
Set your cutter diameter, flutes and RPM and read the feed rate and chip load. This is the same tool as the acrylic feed & speed page, which carries the full chip-load reference table.
Recommended feed rate
320.0IPM8,128 mm/min
Plunge rate
96.0 IPM
2,438 mm/min
Chip load
0.2540 mm
0.0100″ / tooth
Chip load bandIn range
0.203 mm0.305 mm
Your chip load is 0.2540 mm per tooth, which is within the recommended 0.203–0.305 mm range for acrylic with a 6.35 mm upcut spiral.
Table range for acrylic at 6.35 mm0.203–0.305 mm
Depth factor (cut is 0.47× diameter)×1.000
Geometry factor (upcut spiral)×1.00
Feed = 16,000 × 2 × 0.2540 mm8,128 mm/min
Acrylic. Acrylic fails by melting, not by burning. The chip has to carry the heat away, so an under-fed cut welds the swarf back onto the cutter. Single-flute O-flute geometry is the standard answer.
Upcut spiral. The baseline the published charts assume. Chips are lifted clear of the cut, which keeps heat out of the workpiece at the cost of tear-out on the top surface.
Which bit to use
A single-flute or O-flute (polished, open-geometry) bit is the standard for acrylic. The open flute clears the chip in one piece before it can re-melt; a two-flute cutter traps and re-welds it into the wall as a frosted, gummy edge. An O-flute specifically is ground to leave a near-flame-polished edge in cast acrylic.
Feeds, speeds & depth of cut
Acrylic inverts the wood intuition. Here heat is the enemy and the chip is the coolant, so you want a chip thick enough to carry heat out - but past a point, more speed just makes more heat. The counter-intuitive nuance, which Sienci's own testing documents: if melting persists, sometimes decreasing the feed helps rather than raising it, and the RPM usually wants to be lower than instinct suggests. Start from the calculator's acrylic window below and adjust toward whichever direction stops the melt.
Workholding
Acrylic sheet is smooth and light, so it slips. Tape-and-glue works well for small parts, a vacuum for larger sheet, and clamps at the edges - just don't over-tighten, because acrylic is brittle and will craze or crack under a hard clamp. Support the whole sheet so it cannot chatter, which chips the edge.
Chip evacuation & cooling
Everything about acrylic is chip evacuation. A chip left in the slot re-melts, welds to the wall, and turns a clear edge frosty. Air-assist or a dust boot with strong airflow keeps the cut clear; a single-flute bit does the rest. If the edge comes out frosted or gummy, evacuation - not the bit sharpness - is usually the failure.
Reading your results
A clean cut throws small solid chips and leaves a smooth, near-transparent edge. A frosted, white, or gummy edge with strings of re-melted plastic means too much heat and poor evacuation - fewer flutes, more air, and try decreasing the feed. Cracks radiating from the cut mean you plunged instead of ramping, or clamped a brittle sheet too hard.
Common mistakes
- Using a two-flute bit - it re-melts chips and frosts the edge. Single-flute or O-flute only.
- Plunging straight down instead of ramping in, which cracks and distorts.
- Assuming 'feed faster' always helps - in acrylic, if it is melting, sometimes slow down.
- Choosing extruded acrylic for detailed work - it cracks more than cast.
Tooling & parts for acrylic
The consumables a fix like this usually calls for. Prices are live; we may earn a commission on a purchase, which never affects what we recommend.
In stock
Frequently asked
- Why is my acrylic melting instead of cutting?
- Heat is staying in the cut. Usually the chip is being re-melted and welded back in - use a single-flute or O-flute bit and strong air-assist so the chip leaves cleanly. And unlike wood, if melting persists the fix can be to slow the feed down, not up, and to drop the RPM. Cast acrylic also melts less readily than extruded.
- Cast or extruded acrylic for CNC?
- Cast, for anything detailed. Cast acrylic machines cleaner, resists cracking, and flame-polishes better; extruded is cheaper but softer and cracks more easily under the cutter and around clamps. If you have the choice, cast is the material to learn on.
- Should I ramp or plunge into acrylic?
- Ramp. Plunging straight down concentrates heat and force at one point, which distorts the material and starts cracks. A ramped or helical entry spreads the load and keeps the entry clean.
Related
Tooling and technique are drawn from manufacturer documentation and established references; starting ranges are exactly that - the exact feed and chip load for your cutter come from the calculator above, never invented here.