Best Router Bits for Acrylic and Plastics on a CNC

Plastics don't fail like wood does. Push a wood bit through acrylic and the problem isn't a dull edge. The chips get too hot, melt, and weld back onto the cutter and the cut wall. The whole job in plastics is getting the heat out with the chip. This guide covers which bit to use for acrylic, polycarbonate, HDPE, ABS, PVC and ACM sign panel, and how to set it up so the edges come off clean instead of gummy. For the general up cut / down cut background, read Up Cut vs Down Cut vs Compression.

Why one flute wins in plastic

A single flute bit has one cutting edge and one large, polished gullet. Each revolution takes a bigger chip and leaves more room to clear it. A big chip carries more heat away than a fine one, and the open flute throws it clear before it can be re-cut. Two or more flutes take thinner chips, pack the gullets faster and rub more, and in plastic rubbing means melting.

That's why our main plastics bit is a single flute up cut. The up spiral lifts chips out of the slot like a drill, so heat leaves the cut instead of building up in it.

Which bit for which plastic

• Acrylic (Perspex), polycarbonate, ABS, HDPE, nylon, acetal, PVC: the 1 flute up cut for plastics. It's the default for profiling and pocketing, in sizes from 0.8mm up to 12mm.
• The best edge finish on acrylic and other soft plastics: the CARBiTOOL single spoon (O) flute. It's ground in Australia from solid carbide, with a large polished gullet, in 3.175mm to 6.35mm on a 6.35mm shank.
• Thin sheet, or when the top face is the show face: the 1 flute down cut. The down spiral presses the top layer flat for a crisp top edge. Use it in thin stock or shallow passes, because it keeps chips in the cut.
• ACM / ACP (Alucobond, Dibond-style panel): the 1 flute down cut for clean top-skin cutting. For folding returns and boxes, use a panel folding V bit. The 1 flute DLC up cut also cuts ACM, and its slippery coating suits the aluminium skins.
• Engraving and lettering in plastic: a 1 flute V bit or a tapered ball nose. Expect a slight burr on softer plastics.

A multi-flute up cut will cut plastic, but only as a 2 flute. On our 2, 3 & 4 flute up cut, the 3 and 4 flute versions are not suitable for plastics.

Signs you're melting, and what to change

• Gummy, welded edges or a plastic "string" wrapped round the bit: the chip is too small or isn't clearing. Raise the feed or lower the RPM to make a bigger chip, and add an air blast.
• Frosted or rough walls: the edge is rubbing. Check the bit is sharp and the collet is clean, then take a light finishing pass of about 0.2–0.5mm at full depth.
• Chips packing the slot: take shallower passes, or use an up cut instead of a down cut.
• Chipped or cracked acrylic edges: usually vibration or a hold-down problem. Clamp or tape the sheet so it can't chatter, and ramp into the cut instead of plunging straight down.

Use our feeds and speeds calculator for a starting point, then make a test cut in scrap. In plastic, the chip itself tells you whether you're right: you want clean, separate curls or flakes, not dust and not melted strands.

Cast vs extruded acrylic

Cast acrylic machines more cleanly than extruded acrylic. Extruded sheet softens at a lower temperature, so it melts sooner and needs a more aggressive chip and better chip clearance. If you have the choice for machined parts, use cast sheet. If you're cutting extruded sheet, run the single flute and keep the air on.

Adam's Tip

Leave the protective film on the acrylic while you cut. It protects the face from chip scratches and holds down any fine burr. Peel it after the job, not before.

Frequently asked questions

What is the best router bit for cutting acrylic on a CNC?

A single flute up cut carbide bit, or a single (O) flute bit for the best edge. One flute takes a bigger chip, which carries the heat away before the acrylic can melt.

Why does my acrylic melt when I cut it?

The chips are too small, or they're being re-cut, so heat builds up in the cut. Increase the feed or reduce the RPM to make a bigger chip, use a single flute bit, and clear the chips with air.

Can I use a down cut bit on plastic?

Yes, for thin sheet or shallow passes where the top edge matters, such as ACM panel and laminated sheet. In thick plastic a down cut packs chips into the slot and melts them, so use an up cut there.

What bit should I use for ACM / aluminium composite panel?

A 1 flute down cut gives a clean top skin when cutting through. For folding the panel, use a panel folding V bit, which grooves the back skin and core so the front skin bends cleanly.