Acrylic CNC Cutting Secret: Control the Heat, Keep the Clarity

Cast acrylic is the default choice for machined parts. It has higher molecular weight, better chemical resistance, and far less internal stress than extruded sheet. When a cutter loads the edge, cast sheet resists stress crazing instead of cracking weeks later on a shelf. For anything with a polished edge, a load-bearing bracket, or a part that will see solvent or cleaning agents, specify cast.

Extruded sheet costs less and is more consistent in thickness, which suits flat panels that never get machined on the edge. The trade-off shows up in chips and small features. Extruded PMMA softens faster, so it gumdrops onto the cutter and produces a rougher wall. It also machines with more internal stress, so thin ribs can bow after the part is released.

Sheet thickness tolerance matters more than most drawings suggest. Cast sheet often runs ±10% on thickness, so a nominal 10 mm plate may arrive at 9.2 mm. If the part has a critical step or a press-fit bore, measure the actual stock and adjust the setup, or ask us to machine both faces to a fixed thickness before the feature cuts begin.

The single biggest lever on edge quality is the cutter itself. Acrylic wants a sharp, polished tool with a high rake angle and a large flute space. A tool ground for aluminum will cut PMMA, but it usually leaves a duller wall and more melt. Bits sold as plastic-specific are ground with a wider flute and a sharper edge, and they clear chips instead of recutting them.

Two flutes is the usual starting point for acrylic on a router or mill. The open flute form moves soft chips away from the cut fast, and the larger gullet keeps the chip from rubbing. Four-flute tools can work on rigid setups at low depth of cut, but they pack the flutes and raise temperature. If you see stringy melted chips or a frosted wall, drop a flute before you touch the feed.

Diamond-polished carbide lasts far longer than standard grinding on PMMA. The polish slides across the polymer instead of scraping it, and the edge stays sharp for thousands of linear meters. For a production run of clear display parts, the tool cost per part is usually lower with a polished cutter even at three or four times the price.

• 1
  Single-flute O-fluteBest chip clearance in deep pockets and soft sheet.
• 2
  Two-flute upcutGeneral purpose; good wall finish and fast evacuation.
• 3
  Two-flute downcutProtects the top surface; needs a clean spoilboard.
• 4
  Diamond-polished carbideLongest edge life on clear PMMA production.

Acrylic has a narrow window. Too slow and the edge rubs, heats, and melts. Too fast and the tool grabs the chip and chips the wall. The working rule is a high surface speed with a moderate chip load, so each tooth takes a real bite and leaves before the heat soaks in. Spindle speed in the 12,000–18,000 rpm range on a router, or 3,000–6,000 rpm on a mill, is a normal starting point for a 6 mm cutter.

Depth of cut should stay shallow relative to the tool diameter. A common starting point is 0.5 × D radial and 1 × D axial for roughing, then a 0.2–0.3 mm finish pass at full depth. The finish pass is what produces the clear wall. Climb milling on the finish pass leaves a better surface than conventional milling because the tooth enters on the thick part of the chip and exits thin.

Cooling with compressed air beats flood coolant here. Air blows the soft chip out of the pocket and carries heat away without the thermal shock that liquid coolant can cause. A mist of air and a little water-soluble lubricant helps on deep pockets, but avoid flooding. Acrylic absorbs moisture slowly and can cloud or craze if it sits wet with trapped chips.

Acrylic is soft and springy, so clamping force moves the part. A vise tightened like steel will bow the sheet, and the cutter releases the stress as it passes, leaving a tapered wall. Use vacuum tables, double-sided tape on a flat fixture, or soft jaws with light pressure. For thin sheet, a sacrificial backer board supports the underside and stops the edge from chipping out at the exit.

Tabs and onion skinning both help on sheet parts. Leave a 0.3–0.5 mm skin at the bottom of the profile and cut it free by hand or with a second light pass. This keeps small parts from flying and avoids the stress crack that appears when a part is levered off the table. Vacuum fixtures work well above roughly 300 × 300 mm; below that, tape is faster to set up.

Vibration shows up as chatter marks on the wall, and acrylic transmits it more than metal. A short tool, a rigid holder, and minimum overhang solve most of it. If the wall still shows a wave pattern, reduce the radial depth of cut before you change the speed. Running the finishing pass twice at the same setting also cleans up a light chatter mark.

Machined acrylic edges are translucent, not glass clear. Flame polishing passes a hot flame quickly along the edge and melts the surface to a clear finish. It works well on straight edges and simple curves, but it rounds a sharp corner and softens the edge. Keep the flame moving and practice on scrap, because a slow pass boils the surface and leaves bubbles.

Vapor polishing uses solvent vapor in a closed chamber to smooth and clear the whole part. It reaches inside slots and complex shapes that a flame cannot. The trade-off is dimensional change, typically a few micrometers on the surface, and longer cycle time. For optical parts with tight tolerances, order the vapor polish after the critical dimensions are measured and locked.

Sanding and buffing is the slow but controllable route. Wet sand with 400, 800, 1500, then 3000 grit, then buff with a plastic compound. It removes tool marks without changing the part shape, and it works on edges the flame cannot reach. On a clear part, sanding from 400 grit upward in one direction per step is what keeps the finish even.

Laser cutting is fast on flat profiles, but the beam leaves a heat-affected edge. That edge often shows a slight bevel, a brown tint on some sheet, and micro cracks that grow under load. For a sign or a guard panel, laser is fine and cheap. For an optical window, a load-bearing bracket, or a part that will be polished, CNC gives a cleaner, stronger edge.

CNC also machines the third dimension. Counterbores, pockets, threads, chamfers, and stepped edges all come off the same setup, so the part does not need a second process or a bonded joint. That matters on manifolds, brackets, and light pipes where the edge and the feature must line up. A bonded acrylic assembly is weaker than one cut from solid.

The decision usually comes down to tolerance and edge spec. If the drawing calls for Ra 0.2–0.8 μm on a visible edge, or a bore held to ±0.005 mm, run it on a mill. If it is a flat 3 mm panel with a decorative profile and no critical edge, the laser will ship faster and cheaper.