CNC machining nylon guide

• 1
  Why nylon behaves differently from metal
• 2
  Choosing between PA6, PA66 and PA12
• 3
  Feeds, speeds and tooling that work on PA
• 4
  What tolerance is realistic on nylon parts
• 5
  Step by step: from billet to finished PA part
• 6
  Nylon grades and process choices compared

Polyamide is a semi-crystalline thermoplastic. Half of it is ordered crystal, half is loose amorphous chain. That split explains almost everything you see at the spindle: good stiffness in thin walls, poor stiffness everywhere else, and a springy chip that wraps the tool instead of breaking.

The amorphous half absorbs water from the air. PA6 can take up 2.5–3% of its own weight in moisture at 50% relative humidity; PA66 sits slightly lower and PA12 stays under 1%. Water molecules push the chains apart, so the part swells. A 100 mm PA6 bar can grow 0.2–0.3 mm across that range. No machine accuracy fixes that after the fact.

Heat is the second problem. Nylon has low thermal conductivity, roughly 0.25 W/m·K, so cutting heat stays at the edge. Above about 80 °C the material softens, smears and drags. You get melted burrs, a cloudy surface, and a dimension that shrinks back as the part cools.

Add internal stress from the extrusion process itself. Billet that was quenched after extrusion holds locked-in stress. Remove 3 mm from one side and the part bows. This is not tool deflection. It is the material finding a new balance.

PA6 is the workhorse. It has the highest impact strength of the three and the best wear resistance, so it suits bushings, wear pads, rollers and gears that see shock. Its downside is moisture. If the part runs dry and the drawing calls for tight tolerance, PA6 will fight you all year.

PA66 is stiffer and has a higher melting point, around 255 °C against 220 °C for PA6. That matters for parts near a motor or a heat source. It is also more creep-resistant under sustained load. It still absorbs moisture, just a little less, and it is noticeably harder on tooling than PA6.

PA12 is the dimensional one. Water uptake under 1% means a PA12 part holds size across seasons and shipping routes. It also machines to the cleanest finish. The trade-off is price and lower mechanical strength, so it belongs on precision spacers, fluid manifolds and parts that must stay on a CMM print.

Cast nylon, often sold as MC nylon, is a fourth option. It has higher crystallinity than extruded stock, so it is harder and wears longer, but it is only available in thick sections and machines with more internal stress. For thin plates and small turned parts, extruded stock is the safer pick.

• 1
  Pick PA6Impact loads, wear surfaces, cost-sensitive runs.
• 2
  Pick PA66Higher service temperature and creep resistance needed.
• 3
  Pick PA12Tight tolerance across humidity and long shipping cycles.
• 4
  Skip nylonContinuous UV exposure or sustained load above 90 °C.

Use sharp, polished, uncoated carbide. A mirror-flute geometry lifts the chip instead of rubbing it. Two flutes for slotting and deep pockets so the chip has room to clear; three flutes for finishing passes where you want a smoother surface. Avoid heavy TiAlN coatings, which add edge radius and generate heat on plastic.

Run surface speed in the 200–500 m/min range. That sounds fast for plastic, but nylon needs the edge to slice, not to push. A slow spindle with a heavy chip load rubs the material, heats it, and produces the stringy melt you see on a bad part. Feed per tooth of 0.05–0.15 mm keeps the cut cool.

Keep radial engagement modest, 20–40% of tool diameter, and use climb milling. Conventional milling on nylon pulls the workpiece into the cutter and leaves a fuzzy edge. Axial depth of cut of 0.5–1.5 × tool diameter is safe on rigid setups; reduce it in thin walls where the part deflects.

Air blast beats flood coolant on PA. Compressed air clears chips and carries heat away without adding moisture. If you must use coolant, use a synthetic or oil-based type. Water-based flood coolant is the wrong choice because the part absorbs it and changes size after the vise is released.

On a conditioned PA12 part measured in a 20 °C room, ±0.05 mm is routine and ±0.02 mm is achievable on a stable feature. Pushing to ±0.005 mm on nylon is possible on a short, thick-walled feature with the right grade and a stress-relieved billet, but it is not a blanket callout. Put the tight tolerance only where the function needs it.

Thin walls are the trap. A 1.5 mm PA6 wall in a 100 mm part will move more than any cutting strategy can control. If the design allows, thicken the wall or add a rib. Stiffness scales with the cube of wall thickness, so 2.5 mm instead of 1.5 mm changes the picture completely.

Measure after stabilization, not off the machine. A part that reads 50.02 mm hot can read 49.95 mm the next morning. We inspect 100% before shipment, but we schedule the inspection after the part has had time to equalize with the shop atmosphere.

Surface finish follows the same logic. Ra 0.8–1.6 μm is normal on a finished PA face. Ra 0.2–0.8 μm is reachable with a light finishing pass and a sharp tool, though the surface whitens if you push too hard. A cloudy white patch means heat, not a better finish.