5 Essential CNC Plexiglas Techniques to Avoid Costly Mistakes

Standard metal-cutting end mills ruin acrylic. The positive rake and sharp edge that shear aluminum cleanly will fracture PMMA ahead of the cutting edge, leaving subsurface cracks and edge chipping that only show up after polishing. Acrylic is brittle. It does not form a continuous chip that carries heat away, so the tool has to cut it, not push it.

For milling, use two-flute carbide end mills with a low helix, typically 0–30°. The lower helix reduces upward force on the part and limits the tendency to lift thin sections. A zero-rake or slightly negative-rake geometry scrapes rather than digs. For drilling, use 60–90° point angles with slow spiral flutes; standard 118° metal drills grab and crack the exit face.

Sharpness matters more than coating. A dull edge rubs, and rubbing generates heat. Change tools before they feel worn. For deep pockets, a single-flute or two-flute cutter evacuates chips better than a four-flute tool, which packs material into the flutes and re-cuts it.

Polished flutes help. So does a slight edge hone. The goal is a clean shear at low cutting pressure, not a mirror finish on the tool.

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  End mill2-flute carbide, low helix 0–30°, polished flutes
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  Drill60–90° point angle, slow spiral, sharp lip
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  AvoidHigh-helix 3–4 flute tools meant for aluminum

Acrylic has a narrow window between a clean cut and a melted edge. Surface speed for PMMA usually sits between 150 and 300 m/min for finishing, but the feed per tooth does most of the work. Chip thickness must be high enough to carry heat out of the cut. Too light a chip load lets the tool rub, and rubbing is what creates the melt zone.

A practical starting point: 0.10–0.20 mm feed per tooth on a 6 mm two-flute cutter, spindle around 12,000–18,000 rpm for small tools, and a radial depth of cut no more than 25–50% of the cutter diameter. Axial depth can be deeper, but keep it consistent so the chip load stays even.

If you mill acrylic with a 3-flute or 4-flute cutter, reduce feed by 30–40% compared to aluminum. More flutes mean more friction with no gain in removal. Watch the chips. Fine powder or a gummy string means you are rubbing; clean, comma-shaped chips mean the parameters are close.

Climb milling gives a better edge on acrylic. Conventional milling tends to pull the material and leave a ragged wall. On a finish pass, take a light but constant cut, never a spring pass with no load.

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  Feed per tooth0.10–0.20 mm on a 6 mm two-flute cutter
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  Radial engagement25–50% of cutter diameter
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  Chip signalComma-shaped chips = good; powder = rubbing
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  Cut directionClimb milling for a cleaner wall

Acrylic cracks where stress concentrates. A hard clamp point on a thin wall will hold the part flat until the tool releases it, then the stored stress opens a crack. The fix is to spread clamping force over a large area and keep the part supported underneath.

For flat sheet work, a vacuum table with a sacrificial MDF or acrylic spoilboard is the safest option. It holds without point loads and leaves no clamp marks. When vacuum is not possible, use soft jaws machined to the part profile, or nylon-tipped clamps with a torque limit.

Thin-walled parts need support inside the cavity. Pot the part in a low-melt wax or use a machined plug that fills the bore. For long, narrow parts, machine from a larger blank and leave a web that you cut last. Do not clamp across an unsupported span.

Tabs and onion skin work well for small parts. Leave 0.3–0.5 mm of material at the bottom, then cut the tabs by hand or with a finish pass. This avoids the part shifting when it is freed.

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  Vacuum tableBest for flat sheet; no point loads
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  Soft jawsMachined to profile, nylon tips
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  Thin wallsWax pot or internal plug for support
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  Small partsTabs and 0.3–0.5 mm onion skin

Acrylic does not conduct heat well, so the heat stays at the edge. Flood coolant removes heat but can stain or cloud some grades. Air blast and mist are common compromises. For optical parts, use clean, dry compressed air with a cold-air gun; for general work, a light mist of water-soluble coolant is fine if you dry the parts promptly.

Chip evacuation matters as much as cooling. Re-cut chips scratch the finished wall. Use an air blast aimed at the cut, and add a vacuum shoe for pockets. On deep cavities, peck with a retract so chips clear. Never let chips pile up around the cutter.

Temperature monitoring is practical, not theoretical. A non-contact IR thermometer pointed at the cut zone will show the rise. If the edge starts to look glossy or drags, stop and check the tool. A dull cutter raises temperature fast.

Do not use coolant that attacks PMMA. Some solvents and cutting oils cause crazing. Check the data sheet before you introduce a new fluid.

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  Air blastClean and dry; good for optical parts
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  MistWater-soluble, dry parts promptly
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  Chip controlVacuum shoe, peck on deep cavities
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  Check fluidAvoid solvents that cause crazing

Machining leaves stress in the part, and stress shows up as crazing or warping later. Annealing is the standard relief method for acrylic. Typical cycles hold the part at 70–80 °C for a period based on thickness, then cool slowly, often less than 10 °C per hour. This is done before final finishing if the part will be exposed to solvents or heat.

Not every part needs a full anneal. If the part is thick, has deep pockets, or will be polished with a solvent, anneal it. Thin flat parts with generous radii often survive without it. Judge by the risk of crazing in service, not by habit.

For optical clarity, the finish pass is what counts. A sharp cutter with a light finishing pass leaves Ra 0.8–1.6 μm on PMMA. If you need better, polish with a soft pad and a fine compound. Flame polishing works but can round edges and cause stress; use it only where geometry allows.

Verify with a visual check under side light and a dimensional check. Report the finish and any visible crazing. Do not ship a part with subsurface cracks, even if the dimensions are in tolerance.

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  Anneal70–80 °C, slow cool below 10 °C/hour
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  When to annealThick parts, deep pockets, solvent polishing
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  FinishLight pass, Ra 0.8–1.6 μm on PMMA
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  CheckSide light for crazing, dimensional report