CNC machining of optical plastics

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  Why optical parts leave the mold and go on a mill
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  What makes optical plastics different from aluminum
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  Tool geometry, speed and chip control in practice
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  Fixturing, stress relief and the 5-axis case
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  Polishing, coating and inspection after the cut
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  Optical plastic grades compared for CNC work

Injection molding wins on volume. A molded lens can cost cents once the tool exists, and the surface comes off the steel already smooth. The problem is the tool itself. A single-cavity optical mold runs into tens of thousands of dollars and weeks of lead time, and every drawing change means re-cutting steel.

Below roughly a few hundred parts, machining is usually the cheaper route. A mill or lathe needs no cavity, so design revisions cost an afternoon instead of a mold rebuild. Prototype runs, bridge builds, and low-volume sensor windows all fall into this band.

There is a second reason that has nothing to do with cost. Machining lets you cut geometry a mold cannot release: undercut light pipes, deep non-draft bores, and freeform surfaces that would need a collapsible core.

The trade is real. Molded optics repeat to microns once the tool is qualified. Machined optics depend on the machine, the tool, and the operator, which is why process control matters more here than on a metal job.

These polymers are soft, and that single fact drives every decision downstream. PMMA sits around 2 to 3 on the Mohs scale. A carbide tool that would barely mark 6061 will plough through acrylic, so cutting forces have to come down and rake angles have to go up.

Heat is the harder problem. Acrylic softens near 100 °C, polycarbonate near 145 °C, and neither conducts heat away from the cut the way metal does. Friction at the tool tip stays local, so the surface can melt and re-solidify while the bulk of the part is still cool. That is how you get a cloudy band in the middle of a clear window.

Thermal expansion compounds it. PMMA grows about 70 × 10⁻⁶ per °C, roughly ten times aluminum. A 100 mm lens measured at 25 °C will read a different size at 28 °C. If your tolerance is ±0.005 mm, the inspection room matters as much as the machine.

Chemical sensitivity is the third difference. Many of these polymers stress-crack on contact with the wrong coolant or cleaning solvent, and the crack appears days later, not at the machine.

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  PMMA (acrylic)Best clarity and UV resistance; brittle, so avoid sharp internal corners.
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  PC (polycarbonate)High impact strength; cuts gummy and needs sharp, polished edges.
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  COC / COPLow birefringence and low water uptake; good for precision optics.
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  PolystyreneHigh refractive index, cheap, but very prone to stress crazing.

Single-crystal diamond is the tool of choice when the surface has to be optically clear. It holds an edge radius in the nanometer range and leaves a surface that often needs no polishing. Diamond also refuses to cut steel, which is fine here because the workpiece is polymer.

Polycrystalline diamond (PCD) is the workhorse for everything else. It survives the abrasive fillers in some optical grades and costs far less than single crystal. For roughing, a two-flute carbide end mill with a high helix and polished flutes clears chips without dragging them across the finished wall.

Speed and feed follow one rule: keep the chip thick enough to carry heat away, but not so thick that it lifts the material. On acrylic we typically run spindle speeds in the 8,000 to 20,000 rpm range with 0.05 to 0.15 mm per tooth, then finish with 0.02 to 0.05 mm depth of cut.

Chip evacuation is not a detail. A recut chip welds to the surface and leaves a mark that no amount of polishing removes without changing the figure. Air blast beats flood coolant on most optical polymers because it avoids the solvent problem entirely.

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  Rough with air blastClears chips and keeps the cut cool without solvent contact.
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  Finish dry or with a light mistReduces the chance of coolant staining on clear parts.
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  Never dwellStopping the tool in the cut leaves a heat mark on the surface.
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  Climb mill the finish passBetter surface and less edge breakout on thin walls.

Optical polymers scratch if you look at them wrong, so workholding has to spread the load. Soft jaws machined to the part profile, vacuum chucks, and low-pressure clamps all work. A hardened steel clamp jaw leaves a witness mark that becomes a scatter site.

Residual stress is the quiet failure mode. Extruded sheet carries internal stress from the manufacturing process, and machining removes material unevenly, so the part can bow after it is released from the fixture. Annealing the blank before cutting, and again between rough and finish, removes most of it.

For parts with curved or angled optical surfaces, a five-axis machine removes the need for multiple setups. Every re-chuck is a chance to lose alignment, and alignment errors on an optical axis show up directly as beam deviation. Cutting the whole surface in one setup keeps the datum intact.

Thin walls need support. A lens housing with a 1 mm wall will deflect under tool pressure and spring back after the pass. Light finishing cuts, sharp tools, and sometimes a temporary wax or low-melt fixturing compound keep the wall where the drawing says it should be.

Diamond turning can leave a surface at Ra 0.2 to 0.8 μm, which reads as clear to the eye and often passes a basic transmission check. It is not the same as a polished optical surface, and the difference shows up in scatter measurements rather than visual inspection.

When the drawing calls for tighter surface quality, a graded lapping and polishing sequence follows. That is a manual, iterative process, and it can change the radius if the operator is not careful. We measure the figure before and after each stage so the geometry does not drift out of tolerance.

Protective and functional coatings come next. Anti-reflective stacks, hard coats for scratch resistance, and reflective layers all go on after machining. Each one adds a step where the part can be rejected, so it is worth deciding early whether the coating is essential or just nice to have.

Inspection is where optical work differs from standard machining. A CMM confirms dimensions, but it says nothing about transmitted wavefront or surface defects. Parts are checked 100% before shipment, and dimensional and surface reports are available on request.

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  Dimensional reportCMM data against the drawing, on request.
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  Surface finishProfilometer readings at agreed locations.
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  Visual inspectionScratch and dig checks under controlled lighting.
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  Full traceabilityMaterial certificates and process records kept per lot.