Acrylic CNC Mechanical Cutting Guide

PMMA cuts cleanly with a sharp cutter when the heat leaves with the chip. That last part is the whole job. A laser vaporizes the kerf and leaves a polished edge, but it also leaves a heat-affected zone and a slight taper. A saw is fast on straight lines and useless on a pocket. Mechanical cutting with a milling cutter removes material as chips, so the bulk of the sheet stays at room temperature and stays at full strength.

The trade-off is fixturing and cycle time. A laser needs seconds per part; a mill needs a program, a fixture and a cutter that is actually sharp. For one flat panel with an outline, the laser wins. For a 20 mm deep pocket with a 0.4 mm floor, a threaded bore and a counterbore in the same setup, mechanical cutting is the only way to hold the dimensions.

The material also matters. Cast acrylic machines to a duller, more forgiving chip and holds a flame-polished edge well. Extruded sheet is cheaper and cuts faster, but it is more prone to stress crazing around tight radii and press fits. If the part will see solvent, we ask which sheet grade it is before we choose the cutter.

We keep PMMA on the same 3-axis, 4-axis and 5-axis platforms as aluminum, with plastic-specific cutters, vacuum fixtures and air blast. Travel goes up to 4,000 × 400 × 150 mm on the large frame, which covers most signage and lighting panels in one setup.

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  Best forPockets, bores, threads, undercuts, 3D contours and any feature with a floor
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  Weak spotThin webs and long slender ribs, where cutter pressure can deflect the part
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  Edge resultMatte-to-satin as machined; vapor polishing or flame polishing for clarity
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  Sheet gradeCast PMMA for optical parts, extruded for flat panels and covers

A two-flute end mill with a high helix is the standard start for PMMA. The wide flute space lifts the chip fast, and the polished flute surface reduces friction. We run uncoated carbide almost all the time. Coatings such as TiAlN add a hard layer that holds heat at the cutting edge, which is the opposite of what you want in plastic.

Rake angle does the cutting work. A positive rake of about 10–15° shears the material and keeps the chip flowing. A neutral or negative rake scrapes instead of shears, and scraped PMMA goes cloudy. For finishing passes we switch to a single-flute cutter or a diamond-coated tool when the surface callout is tight.

Tool diameter sets the smallest internal radius you can cut. A Ø6 mm cutter leaves a 3 mm corner radius; a Ø2 mm cutter leaves 1 mm but deflects roughly nine times as much under the same load. Design corners at 1.5× the cutter radius or larger so the tool can clear the corner without rubbing.

Drills need care too. A 118° point drill grabs acrylic and can crack the exit face. We use 60–90° point drills with a slow feed and a backing plate, or helical interpolation with an end mill for bores above Ø8 mm. Reaming is rare; a bored hole with a sharp cutter holds ±0.025 mm in most cases.

Heat is what ruins acrylic. It softens the chip, the softened chip welds to the flute, and the welded chip rubs the wall. The wall goes cloudy and the cutter starts to squeal. The fix is not more coolant. It is a bigger chip load and a shorter path out of the cut.

We run air blast, not flood coolant. Compressed air clears the chip and cools the cutter without thermal shock. A mist of water-soluble coolant can help on deep pockets, but it must be light and constant, because a wet-then-dry cycle promotes crazing. For deep pockets we program a peck cycle so the cutter leaves the cut every few millimeters and lets the air clear the cavity.

Clamping pressure is the other silent failure. Acrylic is about 30 times more flexible than aluminum, so a vise tightened the way you would tighten it on steel will bow the part and cut an inaccurate pocket. We use vacuum fixtures on flat sheet, low-pressure toggle clamps on edges, and we never clamp across an unsupported span.

Thin walls deserve a different plan. Below about 2 mm wall thickness, cutter pressure pushes the wall away and the finished dimension drifts. We leave a 0.3–0.5 mm finishing allowance, take a light spring pass, and support the wall from the back with a soft fixture or a wax fill on very thin sections.

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  Chip checkChips should look like small commas, not powder and not welded strings
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  Air blastPoint the nozzle at the cut, not at the part, to push chips out of the pocket
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  Coolant ruleAir only for most jobs; light mist only on deep pockets with a continuous supply
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  Clamp ruleSupport under every clamp point; no overhang longer than 3× the part thickness

As-machined PMMA is translucent, not clear. A sharp cutter with a good finish pass reads around Ra 0.8–1.6 μm, which is fine for light pipes and diffusers, and not fine for a view window. For optical clarity we either run a diamond finishing pass or send the part to vapor polishing, which dissolves a thin surface layer and leaves a glass-clear face.

Flame polishing works on edges and simple outlines. It is fast and it closes fine tool marks, but it rounds the edge and it can craze a stressed part. For a flat window, mechanical polishing with a sequence of grits gives the flattest result. We keep laser engraving for part numbers, with a minimum character height of 1.5 mm so the mark stays legible after polishing.

Not every acrylic part belongs on a mill. A 1 mm-thick flat gasket with a complex outline is cheaper die-cut or laser-cut. A part with a 0.2 mm internal corner radius cannot be milled, because no cutter that small survives the load. A long thin tube is better on a lathe or extruded. We tell you when the geometry suits another process, because a bad process choice shows up as scrap.

For parts that do belong on a mill, we quote from your STEP file and return a DFM note within 12 hours. Prototypes run from one piece; production runs go to 10,000+ parts. Every part gets 100% inspection before shipment, and inspection reports are available on request.