Guide to Selecting Plastics for CNC Machining

• 1
  Key takeaways
• 2
  Plastic grade comparison for CNC machined parts
• 3
  How to read a plastic datasheet before you cut
• 4
  What changes on the machine for each plastic family
• 5
  What tolerance and finish you can actually hold in plastic
• 6
  Cost, quantity, and when plastic stops being the right answer
• 7
  Step by step: choosing and cutting a plastic part

The first number to look at is not tensile strength. It is the heat deflection temperature under load. A grade that is stiff at 20 °C can sag at 80 °C under the same bolt torque. If the part sits near a motor, a lamp, or an exhaust path, set your temperature ceiling first and only then compare stiffness.

The second number is moisture absorption. PA 6 and PA 66 can take up several percent of their weight in water. The part grows, the bore shrinks, and a shaft that slid in yesterday binds today. POM behaves the same way at a lower level. For any fit that matters, ask for the grade to be dried and cut in one setup.

The third number is the coefficient of thermal expansion. Filled grades move less than unfilled ones. A 200 mm unfilled PA rail can grow more than 0.5 mm over a 40 °C swing, which is ten times the tolerance we can hold on the machine. If the drawing calls for ±0.05 mm over that length, the material and the tolerance are arguing with each other.

Finally, check whether the grade is machinable at all. Extruded rod and plate cut cleanly. Compression molded shapes and some cast nylons contain internal stress that releases when you remove material, so a flat plate can bow after the last pass. We rough, stress-relieve, and finish in separate operations when the geometry is thin.

Soft grades like PP, HDPE, and LDPE cut at high surface speed but need a fast feed to stop rubbing. If you feed too slowly, the tool polishes the surface and melts it. We run sharp two-flute end mills with polished flutes, keep the chip load high, and use air blast instead of flood coolant. Coolant on a soft plastic tends to trap chips and leave a cloudy finish.

POM and PA sit in the middle. They machine well, but both are sensitive to heat buildup in deep pockets. We step down in shallow passes, clear chips with air, and avoid dwelling in a corner. Tapping works with a spiral flute tap and a slightly oversize hole, because these grades relax after the tool leaves.

ABS and PC are common for enclosures. Both take a good finish from a sharp cutter, and both can be vapor polished or bead blasted. PC is the tougher of the two but also the more stress sensitive. A tight thread or a press fit that is 0.05 mm too tight will craze the part weeks later, so we keep interference fits loose on polycarbonate.

PEEK and carbon fibre composites are the abrasive end of the list. PEEK needs sharp carbide, positive rake, and enough rigidity to avoid rubbing; it also costs enough that a scrapped part hurts. Carbon fibre plate wears tools fast, so we budget fresh cutters per job and control dust with extraction. Neither grade is a good candidate for a rushed setup.

The machine can hold ±0.005 mm on metal. Plastic does not always cooperate. Unfilled grades move after cutting, so we hold the tolerance on the machine and then re-check after the part has rested. For long parts, we quote a looser band and say so on the inspection report rather than pretending the number is the same as aluminium.

Surface finish follows the same logic. A sharp cutter on POM or ABS gives Ra 0.8–1.6 μm without extra work. PEEK and PC can reach Ra 0.2–0.8 μm with a finishing pass. Soft PP tends to tear no matter what, so we aim for Ra 1.6–3.2 μm and accept a slightly matte look.

Wall thickness is the other limit. Below about 1.5 mm, soft grades chatter and hard grades chip at the edges. If the design needs a thin web, we prefer PC or a filled PA over PP. When the wall must stay thin and the load is real, it is usually worth switching the part to aluminium and anodizing it.

Threads in plastic are a separate conversation. A machine thread in POM or PA holds well if the engagement length is at least 1.5 times the diameter. In PP or HDPE, a molded-in insert or a metal threaded insert is more reliable than cut threads. We will flag this in the DFM report before cutting.

Plastic stock is priced per kilogram, and the spread between ABS and PEEK is more than an order of magnitude. For a prototype, the material cost is often smaller than the setup time, so picking a cheaper grade to save a few dollars rarely pays off. Pick the grade that meets the function and let the quote reflect it.

There is no minimum order quantity on our side. One prototype and a 10,000-part run go through the same quoting path. Above a few hundred parts, we compare CNC against vacuum casting or injection molding and tell you where the crossover sits for your geometry. Sometimes the honest answer is that CNC is not the cheapest route.

Plastic loses to metal in three common cases. The part runs hot, the part carries a real structural load, or the part needs a hard, thin edge. In those cases aluminium 6061-T6 or stainless 303 will hold tolerance longer and take a thread without an insert. We would rather say that early than ship a plastic part that fails in the field.

Everything we cut is inspected before it ships, with raw material checks, in-process monitoring, and a final report on request. Uploads stay confidential, and an NDA is available if your drawings are not public. If a grade is wrong for the job, that shows up in the DFM analysis we return with the quote, not after the chips are made.