Acrylic CNC Machining Services for Clear, Crack-Free Parts

Acrylic is PMMA, and the two sheet types behave differently under a cutter. Cast acrylic has a higher molecular weight, machines to a cleaner edge and polishes back to optical clarity. Extruded sheet is cheaper and more uniform in thickness, but it chips more readily and shows stress whitening around holes and threads. For a clear lens cover or a fluidic manifold, cast is the safer default.

Thickness also drives the choice. Cast sheet is commonly available from 1.5 mm to 50 mm and beyond; extruded is usually thinner stock. If your part is a 3 mm window, extruded may be fine. If it is a 20 mm block with an internal channel, cast gives you better internal stress distribution and less risk of crazing after solvent bonding.

Acrylic absorbs moisture, roughly 0.3 to 0.4 percent at equilibrium in humid air. That matters for two reasons. First, a wet blank machines with a rougher finish and can leave a milky edge. Second, a part measured wet will shrink slightly as it dries. For tight work we dry blanks before the finishing pass and let parts stabilize before final inspection.

PMMA is soft and has poor thermal conductivity, so the heat from cutting stays in the chip and the tool edge. A dull cutter rubs instead of shearing, and the result is a gummy, welded edge. We use sharp two-flute carbide end mills with polished flutes and high rake angles, which clear chips fast and cut at lower temperature. Diamond-coated or PCD tooling is reserved for long production runs where edge life pays for itself.

Spindle speed runs high and feed per tooth stays moderate. A typical starting point for a 6 mm two-flute cutter is 12,000 to 18,000 rpm at 0.05 to 0.10 mm per tooth, then adjusted by chip evacuation and surface finish. Climb milling on the finishing pass gives a cleaner wall. Ramping into a pocket beats plunging straight down, because a plunge concentrates heat and often leaves a mark you cannot polish out.

Cooling is mostly about chip removal, not flood coolant. Compressed air or a mist keeps the cut zone clear. Flood coolant can work on thick sections, but many acrylic grades are sensitive to some coolants and will craze. If a part needs flood, we test on scrap first. For thin sheet under 5 mm, we often cut on a vacuum table with a backing board to stop vibration and edge chipping.

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  Two-flute carbidePolished flutes, high rake, sharp edge. Best all-round choice.
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  Climb millingUse on finishing passes for a cleaner wall and fewer pull-outs.
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  Ramp entryAvoid straight plunges; they trap heat and leave witness marks.
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  Air or mistClear chips first. Test any flood coolant on scrap before a real part.

Acrylic likes generous radii and thick walls. An internal corner should carry a radius at least one third of the cutter diameter, ideally more, so the tool can sweep through without dwelling. Sharp inside corners force a small cutter, and a small cutter deflects. If the corner is cosmetic, tell us and we can leave a radius and note it on the drawing.

Threads are the weak point. A tapped hole in acrylic holds far less load than the same hole in aluminium, and a coarse thread in thin sheet will strip. For anything that will be assembled more than a few times, use a metal insert or a through-bolt with a nut. We can machine a pocket for a heat-set insert, but keep the boss wall at least 1.5 times the insert diameter.

Deep pockets and tall thin walls are where acrylic punishes you. Roughing a 40 mm deep pocket with a 6 mm cutter means a long reach, and the tool will chatter. Chatter in acrylic does not just sound bad; it leaves a frosted band that no amount of flame polishing fully removes. If your part needs a deep cavity, expect to open the tolerance on the floor finish or split the part and bond it.

Machined acrylic is translucent, not transparent, because the cutter leaves fine tool marks that scatter light. To reach clarity you remove those marks in stages: hand sand from 400 grit up through 1500, then polish with a compound. Flame polishing with a hydrogen-oxygen torch is faster and gives a glass-like surface on edges, but it introduces heat. On a part with internal stress, that heat can trigger crazing days later.

Vapor polishing uses solvent vapor to melt the surface layer smooth. It reaches internal features that a buffing wheel cannot, which makes it useful for fluidic channels and small bores. The trade-off is dimensional change. Vapor polishing removes a thin skin of material, typically a few microns to tens of microns depending on dwell time, so we mask or compensate on features that must hold tolerance.

Annealing is the step most people skip. Machining leaves residual stress in the part, and stress plus a solvent or a warm environment equals cracks. A proper anneal cycle holds the part below its glass transition temperature for a controlled period and cools slowly. For parts that will be solvent bonded, drilled after delivery, or used near windows, we recommend annealing before the final finish.

We machine acrylic to ±0.005 mm on features that matter, and that number holds on a stable part measured at a controlled temperature. It does not mean every dimension on a 300 mm clear panel comes in at ±0.005 mm. Acrylic has a thermal expansion coefficient around 70 × 10⁻⁶ per °C, roughly seven times that of steel. A 100 mm dimension moves about 0.007 mm for every 1 °C change. Ship that part from a 22 °C shop to a 30 °C lab and the number moves.

Moisture adds a second drift. A part machined dry and then exposed to humid air will swell slightly. If your assembly has a hard interference fit, the fit can tighten or loosen with the season. Design acrylic assemblies with a small clearance rather than a press fit, or use a compliant element to take up the change.

The honest answer for optical surfaces is that flatness and parallelism are harder than hole position. A 200 mm wide window will not stay flat to ±0.005 mm across the face after machining and polishing; internal stress release causes a slight bow. If flatness is functional, tell us the allowable bow and we will rough, stress-relieve, then finish machine.

Acrylic is brittle. It does not yield before it breaks, so a part that sees impact, drop loading or repeated flexing will eventually crack. If the application is a guard that gets bumped or a bracket under vibration, polycarbonate is the better material. PC takes roughly 200 times the impact energy of acrylic, at the cost of lower clarity and worse UV resistance.

Chemical exposure is another limit. Acrylic crazes on contact with many solvents, alcohols and some cleaning agents. If the part will be wiped down with IPA every shift, plain PMMA will develop a web of fine cracks. A hard-coated or chemically resistant grade can help, but the honest move may be to switch to a different polymer.

Heat is the third limit. PMMA softens near 100 °C and starts to deform well before that under load. Near an engine, a lamp housing or a heated chamber, acrylic will creep out of tolerance. For those jobs we would look at PEEK, PEI or a thermoset. As acrylic CNC machining services go, part of the value is telling you when not to specify acrylic, and our engineers will say so before the chips fly.