Nylon CNC machining guide: how the material actually behaves

• 1
  What nylon is, and why it is not just soft aluminum
• 2
  Moisture absorption: the mechanism behind every nylon tolerance argument
• 3
  Heat, chips and the cutting parameters that keep nylon straight
• 4
  Design rules that keep nylon parts manufacturable
• 5
  Filled and modified grades: when plain PA6 is the wrong answer
• 6
  Nylon grades compared by what they do on the shop floor

Nylon is a polyamide, a semi-crystalline thermoplastic. The crystalline regions give it strength and wear resistance; the amorphous regions let it bend without cracking. That mix is why nylon parts survive repeated impact where acrylic or polystyrene would shatter. Grades differ mainly in how much moisture they absorb and how they were made.

The three you will see most often in a machine shop are extruded PA6, extruded or molded PA66, and cast nylon (often sold as MC nylon or PA6G). PA6 is the general-purpose choice: tough, slightly flexible, easy to source in rod and plate. PA66 is stiffer and has a higher melting point, so it holds shape better in warm assemblies. Cast nylon has a higher molecular weight, which means better wear resistance and less internal stress in thick sections.

Compared with aluminum, nylon is roughly one-seventh the density and about one-thirtieth the stiffness. That sounds bad until you need a part that damps vibration, runs dry against steel, or insulates electrically. Aluminum transmits vibration and conducts heat; nylon swallows both. The trade is dimensional stability, which we will come back to.

One more property matters: nylon is hygroscopic. It will pull water out of the air until it reaches equilibrium with the surrounding humidity. That is not a defect. It is the single biggest reason nylon parts measure differently in Dongguan in July than in a dry warehouse in Arizona.

Water molecules work their way between the polymer chains. They do not dissolve the nylon; they push the chains apart. The result is swelling, and because the effect is volumetric, a thin wall and a thick boss move by different amounts. Extruded PA6 in a 50% relative humidity shop will settle around 2.0–2.5% moisture by weight. Immersed in water, the same grade can climb past 7%.

The dimensional consequence is roughly 2% linear growth from dry to saturated for unfilled PA6. On a 100 mm part, that is about 2 mm. Even the swing from a dry winter warehouse to a humid summer floor can move a 100 mm feature by 0.2–0.3 mm. No machine tool can hold ±0.005 mm against a workpiece that is still drinking water.

The fix is conditioning before the finish cut. Rough machine the part oversize, let it sit in the target environment or in a controlled humidity cabinet until the weight stops changing, then take the last passes. For parts that will live in a humid climate, we condition to that climate, not to a dry lab. For parts headed to a sealed dry enclosure, condition dry.

This is also why drawing a tight tolerance on a large nylon plate is a losing move. If the print calls for ±0.02 mm across 300 mm, the material will not cooperate. Ask instead for a functional fit and let the shop hold what the material allows.

Nylon cuts like a soft, springy metal with a low melting point. It conducts heat poorly, so nearly all the heat from cutting stays at the edge. Keep rubbing and the chip welds to the flute, then tears the surface on the next revolution. The tell-tale is a gummy finish and a faint burnt smell.

The answer is sharp tooling and generous chip load. Use two- or three-flute carbide end mills with high helix and polished flutes. Run 1,500–3,000 rpm on small cutters but keep feed per tooth high, around 0.05–0.15 mm, so the tool is cutting rather than polishing. Depth of cut can be aggressive: 1–3 mm radial is normal because the material offers little resistance.

Cooling matters more than speed. Flood coolant, or better, chilled compressed air, carries heat away and blows the stringy chip clear. Nylon makes long, tough chips that wrap around the tool and re-cut, so air blast plus a pause in the program beats any coating. Never run nylon dry at high rpm on a deep pocket.

Clamping is the other half. Nylon deflects under vise pressure, and a part machined while squeezed springs back once released. Use soft jaws machined to the part profile, light clamping, and support thin walls from behind. For long parts, support the free end. For deep bores, drill in peck cycles and clear chips often.

Wall thickness is the first decision. Below 1 mm, nylon flexes so much that holding a dimension is guesswork; above 6 mm, thick sections cool at different rates and you risk internal voids and sink marks. Keep walls between 1.5 mm and 5 mm where the design allows, and transition thickness gradually rather than stepping abruptly.

Corners should carry a radius. A sharp internal corner in nylon is a stress riser that will crack under repeated load, and it is also hard to machine cleanly because the tool has to stop and reverse. A 0.5–1.0 mm corner radius costs nothing and buys fatigue life. External corners can be chamfered to stop edge chipping during handling.

Threads work well in nylon if you respect the material. Machine-cut threads from M4 up hold up fine; smaller than M3, the crests are fragile and strip easily. For anything that will be assembled and disassembled repeatedly, consider a metal insert or a through-bolt rather than a tapped hole in plastic.

Tolerances should follow function. We can hold ±0.005 mm on a small nylon feature in a conditioned part, and we regularly hold Ra 0.8–1.6 μm on sealing faces. But a ±0.005 mm callout on a 200 mm span is a promise the material cannot keep. Split the print: tight on the mating feature, general tolerance elsewhere.

Unfilled nylon is the default because it is cheap, tough and easy to machine. It is also the wrong choice in a few common situations, and the fix is usually a filled grade rather than a different process.

Glass-filled nylon (PA6-GF30 is typical) raises stiffness and cuts thermal expansion, which helps parts that must hold shape across temperature. The cost is abrasiveness: glass fibers dull tooling fast and the machined surface shows fibers. Expect shorter tool life and budget for more finishing passes.

Molybdenum-disulfide-filled nylon (often called nylon with MoS2) machines to a slippery, low-friction surface. It is the usual pick for bushings, wear pads and dry-running slides where you want the plastic to sacrifice itself rather than the steel shaft.

For high-temperature or high-load parts, PEEK and POM are the alternatives, not nylon. PEEK holds stiffness well past 200 °C and resists chemicals that dissolve nylon. POM is stiffer and more dimensionally stable but has lower impact strength. If your part sees steam, strong acids, or continuous heat above 100 °C, tell us early; switching resin is cheaper than reworking a design.