Polycarbonate CNC Machining Service

• 1
  Key takeaways
• 2
  When polycarbonate beats the alternatives
• 3
  Why polycarbonate cnc machining is not like cutting aluminum
• 4
  Tolerance and finish numbers that hold on PC
• 5
  Tools, speeds, and feeds that keep the edge clean
• 6
  How to judge a polycarbonate cnc machining supplier
• 7
  What drives price and schedule on PC parts
• 8
  Step by step: releasing a polycarbonate part

Polycarbonate is an amorphous thermoplastic. It has no melting point you can lean on, it goes soft gradually, and it holds heat far better than metal. That last property is the root of most problems. During polycarbonate cnc machining the tool edge pushes a chip that cannot shed heat into the part, so the material around the cut softens, smears, and then tears rather than shears.

The second issue is stress. PC arrives at the machine carrying internal stress from extrusion or injection. Cutting removes material and redistributes that stress, so a part can measure perfectly today and bow or crack next month. Annealed stock plus light finishing passes are the two controls that keep this out of your build.

The third is chemical sensitivity. Polycarbonate is not attacked by water or alcohol in short contact, but many cutting fluids, degreasers, and thread-locking compounds will craze it. The crazing looks like fine hairline cracks and it starts at machined edges and drilled holes. Once it starts you cannot polish it out.

None of this means PC is hard to machine. It means the process window is narrower than for aluminum or brass. Feed, speed, tool geometry, and clamping all have to agree. When they do, you get a clear, tough part with a good surface. When they do not, you get a part that fails a drop test or a leak check three weeks after assembly.

• 1
  Heat stays in the partPC conducts heat slowly, so the tool has to clear chips fast instead of relying on coolant.
• 2
  Stress moves after cuttingAnnealed stock plus balanced material removal keeps warping predictable.
• 3
  Chemicals craze itConfirm every fluid that touches the part, including the cleaner used before packing.

A ±0.005 mm tolerance is achievable on polycarbonate, but not on every feature. It holds well on bores, slots, and faces where you have a rigid setup and a temperature-stable shop. It is much harder across a long thin wall or an unsupported window, because the material moves under clamping pressure and springs back after the vise opens.

For most PC parts we quote ±0.05 mm as a practical default and reserve ±0.005 mm for the features that actually need it. If you call out a tight tolerance across the whole part, machining time rises and the risk of scrap rises with it. Mark the critical dimensions and let the rest run at general tolerance.

Surface finish follows the same logic. Ra 1.6–3.2 μm is a normal as-machined result. Ra 0.8–1.6 μm needs light finishing passes and a sharp tool. Ra 0.2–0.8 μm is possible on flat faces and outside profiles, but it will show every tool mark, so it only makes sense on parts where the surface is functional, not decorative.

Holes deserve their own note. Drilling PC with a standard 118° point drill pulls the material and leaves a rough exit. Use a sharp drill with a 60–90° point, back the exit face with a sacrificial plate, and peck to clear chips. Reaming gives a clean bore, but only if the drill left enough stock and the part was not already stressed at that wall.

• 1
  Call out critical dimensionsLet non-critical features run at general tolerance to keep cost and risk down.
• 2
  Support the exit faceA backing plate stops breakout on drilled holes and slots.
• 3
  Skip decorative polishingFine Ra on PC shows handling marks and offers little functional gain.

Tool geometry matters more than tool material. We run polished, uncoated carbide with two flutes for most PC work. Two flutes give a bigger chip channel, which matters because PC chips are stringy and pack into a three-flute cutter. Coatings designed for steel add friction on plastic and should stay out of the setup.

Spindle speed sits higher than for aluminum, but not because PC is hard. High rpm with a controlled chip load keeps the contact time short. A starting point for a 6 mm two-flute end mill is 8,000–12,000 rpm at 1,500–2,500 mm/min, with a 0.1–0.2 mm depth of cut for finishing. Climb milling on outside profiles leaves a cleaner edge.

Cooling is where many shops get it wrong. Flood coolant with a general-purpose emulsion will craze the part over time. We use compressed air for chip evacuation and, where heat builds in a deep pocket, a fine water mist that is wiped off immediately after the cut. Any fluid that stays on the part is a risk, so parts are cleaned with a compatible agent and dried before packing.

Clamping pressure is the quiet failure mode. Polycarbonate is soft enough to take a vise imprint, and a tight clamp on a thin wall will leave a stress line that cracks later. Soft jaws machined to the part profile, light clamping, and vacuum fixturing on flat panels all reduce that risk.

• 1
  Two flutes, polished, uncoatedBig chip channel, low friction, no coating built for steel.
• 2
  Air first, mist secondKeep fluids off the part unless heat in a deep pocket forces a light mist.
• 3
  Soft jaws and vacuumLight, supported clamping avoids imprints and stress lines.

Ask what stock they will use before you ask about price. A supplier who quotes on generic extruded PC sheet without confirming annealing is quoting a part that may craze in service. The good answer names the grade, the supplier, and whether the stock is stress-relieved.

Ask how they hold clear parts. Polycarbonate shows every scratch, so a shop that machines it on the same fixture as steel, with the same chip load and the same handling, will deliver a part with cosmetic rejects. Look for dedicated fixturing, protective film, and separate handling for optical surfaces.

Ask what happens after machining. A PC part that is not annealed after heavy material removal can move during inspection or in transit. Post-machining annealing is not always needed, but the supplier should be able to say when it is, and what temperature and ramp they use when it is.

Finally, ask for the paperwork. Dimensional reports, material certificates, and a visual inspection record on clear parts are normal requests. If a shop cannot produce them, the price is not comparable, because you are carrying the inspection cost yourself.

• 1
  Grade and anneal stateThe quote should state the exact PC grade and whether it is stress-relieved.
• 2
  Dedicated handlingProtective film, soft jaws, and separate storage for clear parts.
• 3
  Post-machining annealAsk when it is required and what cycle they run.
• 4
  Inspection recordsDimensional data plus a visual record on optical surfaces.

Polycarbonate stock costs more than ABS or acrylic, and the gap widens for optical grades and thick sheet. That is the smaller part of the price. The larger part is time: slower feeds, lighter passes, more handling, and extra inspection all add machine hours per part.

Quantity changes the picture. A single prototype is mostly setup and programming, so the per-part price is high. Once the program and fixtures exist, a run of 100 or 1,000 parts drops quickly. There is no minimum order quantity here, so a one-off and a 10,000-part run go through the same quoting path.

Lead time on a PC part is usually set by stock availability, not by machining. Standard sheet and rod sizes ship in 3–5 days once production starts, and production can begin within 24 hours of a released order. If you need a non-standard thickness or an optical grade, confirm stock before you commit to a build date.

The cost trap is rework. A part that crazes, warps, or scratches after machining costs far more than the original quote. Paying for annealed stock, correct tooling, and proper handling up front is cheaper than a second run.

• 1
  Stock grade sets the floorOptical and thick PC sheet cost more and may need ordering.
• 2
  Setup dominates small runsOne-off parts are mostly programming and fixture time.
• 3
  Rework is the real costCrazing or warping after delivery costs more than the first quote.