Acrylic CNC Machining Service

Acrylic (PMMA) is a thermoplastic with a hardness around 2 to 3 on the Mohs scale and a thermal conductivity roughly a thousand times lower than aluminum. Heat generated at the cutting edge has nowhere to go, so it stays in the chip and in the part. Above about 80 °C the material softens; push it further and the cut edge turns gummy, then re-solidifies with a dull, cloudy band.

That single property drives most of the process decisions. Speeds run higher than for aluminum, feeds are heavier, and the tool must be sharp and polished rather than coated. A two-flute carbide end mill with a high helix and a mirror-polished flute is standard for us. Uncoated tools work better here because coatings add friction and round off the cutting edge.

Chip evacuation matters more than chip load. Long stringy chips wrap the tool, rub the finished wall and leave swirl marks that no downstream polish will fully remove. We use climb milling on finishing passes, air blast instead of flood coolant, and a vacuum shoe on deep pockets. Coolant on acrylic is usually a mistake; mist and air handle the heat without thermal shock.

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  HardnessMohs 2–3, roughly the same as a fingernail
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  Thermal limitSoftens near 80 °C, so heat control sets the feed rate
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  Chip formLong and stringy; evacuation is the main surface-quality lever

Edge chipping is the most common reject. It happens when the tool exits the cut without support, when feed per tooth is too light, or when a dull edge rubs instead of shearing. Light finishing passes are the usual culprit: the tool skates, generates heat and lifts a chip out of the wall. Feeds below about 0.05 mm per tooth on PMMA usually make the finish worse, not better.

Crazing is subtler. Fine hairline cracks appear near holes, slots and machined edges hours or days after the part is made. The cause is locked-in stress from aggressive roughing or from a solvent that attacked the surface. Acrylic is attacked by many common degreasers, so we clean with mild detergent and warm water, never with acetone or IPA on a finished optical face.

Warping shows up on thin plates and on parts with a lot of material removed from one side. Machining releases internal stress that was frozen into the sheet during extrusion. Stress-relief annealing at 70–80 °C before finishing, and symmetric material removal where the geometry allows it, keeps a flat panel flat.

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  Edge chippingCaused by tool exit without support or too-light finishing feeds
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  CrazingResidual stress plus solvent contact; appears after machining
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  WarpingUnbalanced material removal on extruded sheet

We quote ±0.005 mm on metals and on stable plastics, but acrylic moves with humidity and temperature, and it relaxes after machining. A practical tight tolerance on a small PMMA feature is ±0.02 mm to ±0.05 mm, and that is honest engineering rather than a limitation of the machine. On a 300 mm clear panel, thickness and flatness are the numbers that matter, and they drift more than the XY dimensions do.

Optical parts change the conversation. A lens or a light guide needs surface finish and edge quality, not just dimensional accuracy. Diamond fly-cutting on a face mill produces a true optical surface on flat faces; a ball-nose finishing pass on a curved face leaves visible cusps that polishing has to remove. On a curved optical surface, we would rather grind and polish than chase a mirror finish with the cutter alone.

Wall thickness sets the minimum tool diameter. A 1 mm inside radius needs a 2 mm cutter, which deflects and heats quickly. Deep pockets with small radii are where acrylic parts go wrong, so we flag them in the DFM report and suggest a larger corner radius or a split design.

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  Achievable on PMMA±0.02–0.05 mm on small features, tighter on metal inserts
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  Watch insteadFlatness and thickness on large clear panels
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  Design ruleCorner radius at least half the pocket depth where possible

As-machined acrylic is translucent white on the cut face, not clear. That is normal. Restoring clarity takes a sequence: sanding from 400 to 2,000 grit, then a buffing compound on a soft wheel, then flame polishing with a hydrogen-oxygen torch for the final gloss. Flame polishing is fast but it is a skill; a torch held too long leaves a wave in the surface and re-introduces stress.

For parts that need consistent optics, vapor polishing in a controlled chamber gives a more uniform result than hand flame work, at the cost of a longer cycle and some dimensional rounding on sharp edges. We choose between them per part, not per project. If the drawing calls out a sharp edge, vapor polishing will soften it, so we mask or re-machine that edge afterward.

Bonding and assembly are part of the same job. Solvent welding with dichloromethane gives a clear joint but leaves a witness line and can craze nearby material. For visible joints we anneal after welding. Laser marking works on acrylic and gives a frosted white contrast; minimum character height is 1.5 mm, and fine text tends to blur.

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  Hand polishBest edge control, most labor, suitable for low volumes
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  Flame polishFast and glossy, but a skilled operation
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  Vapor polishUniform clarity, softens sharp edges