CNC acrylic processing techniques: cutting PMMA without chips, melts, or crazing

PMMA comes in two forms that machine differently. Cast sheet is polymerized in a mold, has a higher molecular weight, and machines to a cleaner edge with less gumming. Extruded sheet costs less, holds thickness tolerance tighter, and tends to chip and stress-crack more when the cutter grabs it. For any part that will be polished, bonded, or used in a clean optical path, cast sheet is the safer starting point.

Thickness tolerance on cast sheet is looser, often ±10% or more on nominal. If your drawing calls for ±0.005 mm on a feature, that number has to come from the machined surface, not from the sheet you buy. We measure incoming sheet and adjust the setup so the finished dimension is held after cleanup, not before it. Extruded grades can hold a tighter as-received thickness, which sometimes makes them the better choice for flat panels that need no optical polish.

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  Cast PMMABest edge quality, accepts flame and vapor polish, higher cost.
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  Extruded PMMATighter sheet thickness, cheaper, more prone to stress cracks.
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  Impact-modified PMMATougher, slightly hazy, use where drop resistance matters more than clarity.
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  Cell-cast optical gradesLowest internal stress, for lenses and light guides.

PMMA cuts by shearing, not by chipping like aluminum. A two-flute end mill with a high helix and a sharp, polished flute face clears the soft chip before it rubs. Standard aluminum geometry with a strong edge hone will rub, generate heat, and leave a white, fractured edge. For finishing passes we prefer single-flute or O-flute cutters in the 3 mm to 12 mm range; the single flute gives a larger chip channel and a smaller contact arc.

Cutter sharpness is the variable that degrades fastest and is noticed last. A tool that has already cut aluminum will have a slightly dulled edge and a small built-up edge that shows up as a rough floor in acrylic. We keep a separate set of cutters for plastics and track them by part count rather than by calendar time. When edge roughness climbs or the sound changes, the tool is swapped.

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  Helix angleHigh helix pulls chips up and away from the cut.
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  Rake anglePositive rake lowers cutting force and heat.
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  CoatingUncoated polished carbide; avoid thick coatings that round the edge.
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  RunoutKeep below 0.01 mm; runout doubles the effective feed per tooth.

Chip load does more for acrylic than spindle speed. A thin chip means the cutter rubs instead of cutting, and rubbing is what melts the edge. Keep the feed per tooth in a range where the chip is thick enough to carry heat away. If you hear a high-pitched squeal or see a fine white powder instead of chips, the feed is too low or the tool is dull.

Compressed air is the primary coolant for plastic. It clears chips from the slot and removes the heat that builds in the cut. Misting systems can help on deep pockets, but any liquid that pools in a blind hole will leave a mark after evaporation, and water-based coolant can be absorbed into the surface. We run dry with air blast for most acrylic work and save liquid for deep bores only.

Melting is usually traceable to one of four causes. Excessive friction from a dull or honed tool. A feed rate low enough that the edge rubs. Poor chip evacuation so the same chip is recut. Or a cutter held in a tool holder with runout that makes one flute do all the work. Fix the cause, not the symptom.

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  Air blastAim at the cut, not at the spindle; 4–6 bar is enough.
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  Short flute lengthLess rubbing on the wall, less heat.
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  Climb millingThicker chip at entry, less rubbing on the finished face.
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  Sharp toolTrack by part count; acrylic dulls edges faster than aluminum.

Sheet stock moves when you remove material. A 10 mm cast plate can bow enough to break a tolerance after the first roughing pass. Vacuum tables spread the clamping force over the whole face and let the part relax without lifting. For thin panels we use a spoil board with a gasket channel and dial the vacuum down rather than crushing the sheet.

For small parts, tabs and soft jaws work better than a vise. Vise jaws concentrate force at two points and can leave stress marks that turn into crazing later. If a part has to be held in a vise, we machine soft jaws to the part profile and back the pressure off to the minimum that keeps it still. Torque values matter here, and so does the order of clamp and unclamp on a multi-op part.

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  Vacuum tableBest for flat sheets, low point loading, fast setup.
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  Soft jawsMachined to profile, low clamp pressure, no point marks.
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  TabsKeep small parts connected until the final op.
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  Stress reliefRough, anneal, then finish for tight-tolerance parts.

Machining leaves stress in the surface. A rough pass followed by a finish pass on a thick section can leave enough internal stress for the part to craze weeks later, especially near drilled holes and sharp internal corners. Annealing in a controlled oven at a slow ramp and a slow cool relieves that stress. It is the step most often skipped when a part looks good right off the machine.

Flame polishing with a hydrogen-oxygen torch produces the clearest edge and is fast on straight profiles. It also rounds the edge slightly and needs a steady hand; a pause in one spot turns clear acrylic into a bubble. Vapor polishing with methylene chloride gives a more even gloss on complex geometry but softens fine detail and needs ventilation. For a part with both a critical flat and a visible edge, we polish the edge first and protect the flat.

We hold Ra 0.2–0.8 μm on polished optical faces and Ra 1.6–3.2 μm as machined. The tolerance is ±0.005 mm on critical features, checked with CMM or optical comparator before shipment. Every part is inspected; reports are available on request.

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  As machinedRa 1.6–3.2 μm, visible tool marks, fine for hidden faces.
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  Vapor polishedEven gloss on complex geometry, softens sharp detail.
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  Flame polishedClearest edge, slightly rounded, best on straight profiles.
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  AnnealedRelieves internal stress, reduces later crazing.

PMMA is brittle. A sharp internal corner will concentrate stress and crack under load, and a drop on a hard floor can shatter a part that passed every dimensional check. If the part sees impact, solvent exposure, or a service temperature above roughly 60 °C, another material usually wins. Polycarbonate takes impact better and machines to a duller but tougher edge.

Solvent contact is the other common failure. Alcohol-based cleaners, some thread lockers, and many adhesives will craze a stressed acrylic surface. If a part will be wiped down in service, tell us which cleaner is used and we will test it on a sample before the run. For parts that need both clarity and chemical resistance, we look at coating options rather than changing the base material.

Acrylic still wins in plenty of cases. Light guides, display windows, fluid manifolds, guards, and prototype lenses all benefit from its clarity and its stability under UV. The trick is to design for the material: generous corner radii, no thin unsupported walls, and a surface finish spec that matches how the part is actually used.

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  Choose PMMAClarity, UV stability, low cost, good machinability.
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  Choose PCImpact resistance, higher temperature, still transparent.
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  Choose PEEK or PSUChemical and thermal loads beyond PMMA limits.
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  RedesignAdd radii, thicken walls, move stress away from holes.