Acrylic CNC machining: a guide to cutting, drilling and bending acrylic

PMMA is a thermoplastic with a low thermal conductivity, around 0.19 W/m·K. Heat from the cut has nowhere to go, so it sits at the tool tip and softens the chip. Once the chip welds to the flute, the edge tears and the surface turns cloudy. Everything below follows from that one fact.

Cast and extruded sheet are not the same material in practice. Cast PMMA has a higher molecular weight, better chemical resistance and machines to a cleaner edge. Extruded sheet is cheaper, thinner in tolerance and more prone to internal stress. When a part carries a polished edge or a bend, we ask which one the drawing calls out.

Acrylic is also brittle. It has no yield point worth speaking of, so a clamp screw tightened by feel will leave a crack that opens three days later. Moisture in the shop air makes it worse. Parts that pass inspection on Friday can craze by the following week if they were stressed during fixturing.

For acrylic CNC machining cutting, a two-flute carbide end mill with a high helix and a polished flute is the default. The polished surface matters more than the coating. Uncoated carbide ground for aluminum works well; TiAlN and other hard coatings raise friction and are best avoided on PMMA.

Run the spindle fast and the feed per tooth moderate. On a 6 mm cutter we typically see 12,000–18,000 rpm and 0.05–0.10 mm per tooth, with a shallow axial depth for finishing. Climb milling gives a better edge than conventional milling on a brittle material. A light finishing pass of 0.2 mm removes the smear left by the roughing pass.

Air blast beats flood coolant. Compressed air clears chips and cools the edge without the thermal shock of liquid. If you must use coolant, use a mist and keep it light. A stream of cold fluid on a warm acrylic edge is a reliable way to start a crack.

Corn-cob and single-flute cutters are for roughing only. They clear chips fast, but the edge they leave needs a finishing pass. On pockets deeper than three times the cutter diameter, use a smaller stepdown and let the air blast do its job.

• 1
  ToolTwo or three flutes, polished, uncoated carbide. High helix for chip evacuation.
• 2
  Speed12,000–18,000 rpm on small cutters; keep surface speed high and chipload light.
• 3
  CoolingAir blast first. Mist only if the cut is deep and the chip load is heavy.
• 4
  Finishing0.2 mm radial pass with climb milling to clear the smeared layer.

A standard 118° twist drill will grab acrylic on breakthrough. The point angle is too sharp and the rake is too aggressive, so the drill pulls itself through the last millimeter and takes a chip out of the exit face. A 60° to 90° point with a zero rake is the fix. We keep a set of drills ground specifically for plastics.

Peck drilling is not optional on holes deeper than one diameter. Retract the drill fully to clear the chip, or the flutes pack and the heat builds. A packed flute on PMMA means a seized drill and a scrapped part. Slow the feed as the drill exits the far face; that is where the damage happens.

Hole size matters for tapped and press-fit features. Acrylic has a high thermal expansion coefficient, around 70 × 10⁻⁶ /°C, so a 20 mm hole drilled at 20 °C will measure smaller in a cold plant. If the hole carries a bearing or a press-fit insert, tell us the service temperature and we will cut to the number that matters.

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  Point angle60°–90°, zero rake, sharp. Standard 118° drills grab on breakthrough.
• 2
  Peck cycleFull retract on holes deeper than 1 × diameter. Clear chips every peck.
• 3
  Exit strategyReduce feed through the last 1–2 mm to avoid exit chipping.
• 4
  Thermal fitQuote the service temperature for press-fit and bearing holes.

Line bending is the common method on sheet up to about 10 mm. The material is heated along a narrow strip to roughly 150–160 °C, then formed over a radius. The strip has to be wide enough to let the material flow, or the outside of the bend whitens and cracks. A bend radius below the sheet thickness is a red flag on any drawing.

We prefer to machine the bend feature after forming, not before. Cutting a pocket or a hole near a bend line concentrates stress, and the heat from the bending operation can distort geometry that was already finished. Sequence the operations so the bend comes first and the critical dimensions come last.

Annealing is the step most drawings leave out. After a bend, the material holds residual stress that shows up later as crazing or a slow spring-back. An oven cycle at 70–80 °C for a few hours and a controlled cool-down relaxes it. The cycle depends on sheet thickness, so we treat it as a process step, not a checkbox.

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  Bend radiusKeep at or above the sheet thickness. Sharper bends whiten and crack.
• 2
  Heat zone150–160 °C along a narrow strip. Overheating blisters the surface.
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  Operation orderBend before final machining so critical dimensions stay true.
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  Annealing70–80 °C oven cycle with controlled cooling to relieve stress.

Acrylic wins on clarity, UV stability and cost. When the part needs impact resistance, it loses to polycarbonate every time. PC takes roughly 200 times the impact energy of PMMA in a notched test, and it bends cold without cracking. A guard, a lens cover or a machine window on a vibrating frame should be PC, not acrylic.

Chemical exposure is the other deciding factor. PMMA is attacked by many solvents, including acetone, and by some cutting fluids. If the part sees solvent wipe-down or fuel contact, we ask about it early. POM or PP handles those environments better, and both machine cleanly.

Optical parts sit in the middle. Acrylic CNC machining cutting can hit ±0.005 mm on a lens holder and Ra 0.2–0.8 μm on a polished face, but it will not produce an optical surface straight off the cutter. A diamond-turned or vapor-polished finish is a separate operation. Specify the optical requirement on the drawing and we will quote the right process.

• 1
  ImpactChoose PC when the part sees shock, vibration or cold bending.
• 2
  SolventsAvoid PMMA with acetone, fuel or aggressive wipe-down chemicals.
• 3
  OpticsMachining holds geometry, not optical figure. Plan a polishing step.
• 4
  CostAcrylic is the lower-cost clear plastic when the duty is light.

Clamping is where most acrylic parts are lost. Vacuum chucks and double-sided tape spread the load over the whole face. Mechanical clamps concentrate it at a point, and PMMA does not forgive that. On thin sheet we back the part with a sacrificial MDF or acrylic plate so the cutter exits into waste, not into the fixture.

Inspection is straightforward on a CMM, but the temperature matters. PMMA grows about 0.07 mm per meter per degree Celsius. A part measured at 18 °C and used at 30 °C will be a different size. We record the shop temperature with the inspection report and quote dimensions at 20 °C unless told otherwise.

Send us the 3D model, the sheet grade and thickness, the bend lines with radii, and any cosmetic requirement on the visible faces. A note about the service temperature and the chemical environment saves a revision. The quote and a free DFM analysis come back within 12 hours, and production can start within 24 hours.

GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants in Dongguan and Singapore, with 16 simultaneous 5-axis centers and a maximum processing size of 4,000 mm. Plastics work is quoted with the same tolerance and inspection standard as metal: ±0.005 mm where the drawing calls for it, 100% inspection before shipment, and reports on request. No minimum order quantity applies, from one prototype to 10,000+ part runs.