CNC Machining of High-Transparent PC Material

• 1
  What CNC machining of high-transparent PC material does at the edge
• 2
  Which features suit CNC machining of high-transparent PC material
• 3
  Speed, feed, and coolant settings that keep the cut cold
• 4
  Crazing, clouding, and cracks: what actually goes wrong
• 5
  How we run CNC machining of high-transparent PC material
• 6
  PC versus acrylic versus PMMA for clear machined parts

Polycarbonate is amorphous. There is no crystal structure to shear along, so the chip forms by plastic flow, and that flow is what makes or breaks the part. When the edge is sharp and the feed per tooth is high enough, the material shears ahead of the tool and leaves a glossy, transparent surface. When the edge is dull or the feed is too light, the tool rubs, the surface heats, and you get a white smear that no polishing will fully remove.

The second mechanism is residual stress. Extruded PC sheet and molded blanks carry locked-in stress from cooling. Machining removes material unevenly, so the remaining wall redistributes that stress and can bow, crack, or craze days later. A part that measures flat on the bed may not be flat after 48 hours. That is why we rough, stress-relieve, then finish on the same setup whenever the drawing calls for flatness.

The third factor is heat. PC softens near 145 °C and its thermal conductivity is roughly a third of aluminum. Heat does not leave through the chip fast enough, so it stays in the cut zone and builds a gummy edge. Sharp tools, high feed, and air or mist cooling all exist to keep that heat moving.

One more property matters for handling: notch sensitivity. A sharp internal corner concentrates stress, and polycarbonate has no fiber or filler to blunt it. Radii are not cosmetic on this material. They decide whether the part survives a drop test or a bolt torque cycle.

PC machines best when the tool can stay in continuous contact and exit cleanly. Long straight walls, shallow pockets, and open radii all cut well. Thin floor sections are the enemy: a 1 mm floor under a 20 mm pocket will flex under clamping and chatter, and chatter on PC shows up as a visible ring pattern inside the window.

Wall thickness should stay above roughly 1.5 mm for parts that must remain clear. Below that, the wall thins unevenly during polishing and the part reads hazy from certain angles. If the design needs a thinner section, expect to accept a smaller clear aperture or move to injection molding.

Holes are straightforward if you drill with a pilot and ream for fit. Tapping is where most shops get burned. A standard tap in PC generates high radial pressure and often produces a hairline crack at the thread root. Forming taps and thread milling both reduce that pressure considerably.

Curved optical surfaces are a different conversation. A 5-axis machine can produce a spherical or freeform surface, but the as-machined finish will not be optical grade. PC is soft enough that it moves under a polishing pad, so the final figure comes from hand or diamond-turned finishing, not from the milling pass itself.

Tool geometry first. A two-flute carbide end mill with a polished flute and a 10–15° helix clears chips on plastic without dragging them back through the cut. More flutes raise cutting pressure and heat. A dull cutter is the single most common cause of cloudy edges, so change tools on a fixed interval rather than waiting for a bad surface.

Rough with a chipload around 0.05–0.10 mm per tooth at 8,000–12,000 rpm for small cutters. Depth of cut can be generous, up to half the cutter diameter, because PC removes easily when the chipload is maintained. Never let the feed drop to a rub: if the finish starts to look matte, raise the feed before you touch the spindle speed.

Cooling matters for finish, not just for temperature. Flood coolant works but leaves residue in blind pockets. Filtered air or a light mist keeps chips clear and avoids the thermal shock of cold fluid on a hot edge. If the shop uses coolant, dry the part thoroughly before measuring; PC absorbs moisture slowly and can read heavy on tight tolerances.

For finishing passes, take a light radial stepover around 5–10% of the cutter diameter and keep the same feed. A spring pass at the same depth removes the last few microns without adding heat. This is where Ra 0.8–1.6 μm surfaces come from, and where Ra 0.2–0.8 μm is reached on small optical windows.

Cloudy edges almost always trace back to feed, not to speed. The operator sees a white edge, slows the spindle, and makes it worse. The fix is a fresh cutter and a heavier chipload. If the tool has been in the spindle for hours on other plastics, replace it before the PC job starts.

Crazing is a network of fine cracks that appears after machining or after cleaning. It comes from residual stress plus a chemical trigger. Alcohol, acetone, and many degreasers will start it within seconds on a stressed part. Once the network forms, the part is scrap. Wipe with mild soap and water, and anneal blanks before the first cut on any part that must stay clear.

Cracks at tapped holes and sharp internal corners are a design issue more than a machining issue. Adding a 0.5 mm corner radius and switching to a forming tap removes most of them. Where a thread must carry load, an inserted metal boss is a more reliable answer than a deeper thread in PC.

Warping after machining is the slow failure. A part that passes inspection on day one may bow by 0.2 mm by day three. If the drawing has a flatness callout, machine both faces, do not clamp on a finished surface, and let the part rest before final inspection.

Blanks arrive pre-annealed where the drawing demands optical clarity. We rough to within 0.5 mm of final size, then let the part rest so stress can redistribute before the finishing pass. On thin walls, that rest period is not optional.

Fixturing uses soft jaws or vacuum where possible. Hard steel jaws leave marks that survive polishing, and on a clear part every mark is visible. For thin panels we use a vacuum plate and take light depths so the part never lifts.

Inspection happens at three points: incoming material, in-process on critical bores, and full inspection before shipment. Reports are available on request. Tolerances hold at ±0.005 mm ( ±0.0002 in) where the geometry allows it, though we will tell you when a PC feature cannot realistically hold that number.

Finishing is quoted separately from machining. Vapor polishing, flame polishing, and hand polishing all change the surface, and each one has a limit. If your requirement is a clear window with a defined Ra, say so on the RFQ and we will machine to it rather than polish past it.