Nylon is one of the plastics customers ask for by name most often — and the one most likely to come back out of spec if you machine it the way you'd machine acetal. It's tough, it wears well, and it costs less than most engineering plastics. It also absorbs water, and water changes its dimensions. Most of what's difficult about machining nylon traces back to that one fact.
Here's how we approach nylon parts on the shop floor, and how to decide when it's the right material.
What is nylon?
Nylon is the common name for the aliphatic polyamides, abbreviated PA. They're semi-crystalline, which is what gives nylon its combination of toughness, abrasion resistance, and low friction. You'll see machinable stock sold as Nylatron, Ertalon, and Nylon 101, among others — but they're all polyamide, and most of those brand families span more than one grade, so the brand name alone won't tell you what you have.
The two grades that matter for machined parts are:
- Extruded nylon 6/6 (PA66) — the workhorse for smaller parts from rod and plate. Stronger in tension and much higher-melting than nylon 6 (around 500 °F versus 420 °F). Stiffness depends on which number you're reading: PA66 is stiffer in tension, while cast PA6 is actually stiffer in bending.
- Cast nylon 6 (PA6) — poured and cured rather than extruded. Available in much larger cross-sections, and the usual choice for a big gear, sheave, or wear pad. Worth knowing: cast PA6 is rated for higher continuous service temperature than PA66 despite its lower melting point, and it's at least as hard. Higher melting point does not mean the hotter-running grade.
Choose on section size and service temperature, not on which name sounds more premium.
Nylon 6/6 properties at a glance
Typical values for unfilled extruded nylon 6/6. Moisture moves several of these — see the next section, because with nylon that isn't a footnote.
| Property | Typical value |
|---|---|
| Density | ~1.14 g/cm³ |
| Tensile strength (dry) | ~12,000 psi (83 MPa) |
| Flexural modulus (dry) | ~450,000 psi (3.1 GPa) |
| Melting point | ~500 °F (260 °C) |
| Max continuous service temp | ~175–210 °F, duration-dependent |
| Water absorption, 24 hr (ASTM D570) | ~0.30–0.45 % for machinable stock shapes |
| Water absorption, saturation | ~7–8.5 % |
| Coefficient of friction (dynamic vs. steel, US datasheet method) | ~0.2–0.3 |
| Hardness | ~Rockwell R115 |
Treat these as typical published values, not a specification — grade, fill, stock form, and moisture content all move the numbers, and you should work from the supplier's datasheet for the exact stock on a critical part.
Two rows deserve a caveat rather than a footnote. Continuous service temperature is duration-dependent, and that's the whole reason the published numbers disagree: one manufacturer rates extruded PA66 at 95 °C (203 °F) for 5,000 hours but 80 °C (175 °F) for 20,000 hours, while its US datasheet lists 210 °F as a "long term" figure with no duration stated at all. Treat anything above ~175 °F as a question to ask — for how long? — rather than a number to assume. On a like-for-like basis, cast PA6 is rated higher than PA66. And the friction figure comes from the thrust-washer method US datasheets use — the ISO 7148-2 method returns 0.4–0.6 for the same material, so size a bearing off the wrong one and you can under-predict friction by roughly double.
Where nylon falls short: moisture absorption is the big one — it changes both dimensions and properties. Nylon also has poor resistance to strong acids, oxidizing agents, and halogens, isn't UV-stable in its natural form without additives, and is combustible.
The moisture problem, and what to do about it
This is the whole story with machined nylon. Nylon takes on water from the air and swells as it does — up to around 3% dimensional change at full saturation. A part machined from dry stock and put into a humid plant, or into water, will grow.
It also changes the material's behavior. Absorbed water acts as a plasticizer: nylon gets tougher and more impact-resistant as it takes on moisture, while getting less stiff and lower in tensile strength. The datasheet describes a condition your finished part may not be in.
Three practical consequences:
- Know the stock's condition before you cut. Rod that's been sitting on a shelf in a humid shop is not dry stock, and it won't machine to the same dimension as material from a sealed bag.
- Let parts reach room temperature before you inspect them, and on tight work, condition before final measurement. A part gauged the moment it comes off the machine tells you what it is right then, not what it'll be in service. The good news is that uptake is reversible — a part that has grown can generally be dried back toward its machined size.
- Design around it. If a nylon part has to hold a tight fit in a wet or humid environment, that's a conversation to have before the print is final. Acetal absorbs roughly an eighth as much at saturation and is the usual substitute when dimensional stability matters more than toughness.
What tolerances you can realistically hold
Machined-plastic tolerance is best expressed proportionally — on the order of ±0.001" per inch of dimension — rather than as a single number, because a 1" part and a 12" part are not the same problem. Blanket "standard" figures for machined plastics are published as loosely as ±0.010", which tells you how much the honest answer depends on the part.
With nylon the honest constraint is that moisture and elastic recovery, not the machine, set the floor. Two things work against you:
- Elastic recovery. Nylon is tough and resilient. It deflects away from the tool under cutting pressure and relaxes back afterward, so reamed holes tend to finish undersize and thin, unsupported walls finish oversize. It's why the standard advice is to tap nylon slightly oversize. Support the stock well and take light finish passes.
- Heat and section changes. Nylon is a poor conductor of heat, so warmth from the cut stays local and grows the dimension you're working on. Stock shapes generally ship annealed, so plain extrusion stress is less of a bogeyman than it's made out to be — but removing material asymmetrically or in heavy sections (a U-shape out of plate, a bushing out of solid rod) will still let a part move, and that applies to cast stock too. Rough it, let it relax, then finish.
Tooling and speeds — the short version
- Sharp tools with generous clearance, modest positive rake. Sharpness matters more than any other single factor: a dull edge scrapes instead of cutting, and scraping generates the heat that smears and melts nylon rather than chipping it. Once a surface has smeared you won't polish it out. Rake in the 0–10° positive range works; too much rake causes its own problems.
- Clear the chips — but skip the chip breakers. Nylon produces long, stringy chips that wrap tooling and recut. The counterintuitive part is that chip breakers generally don't work on nylon — it's too tough for them to be effective, which is the opposite of the acetal case where standard chip breakers do fine. Use a pick-off to separate turnings, peck-drill and withdraw the drill frequently, and clear with air.
- Support the part. Nylon isn't as stiff as metal; unsupported thin walls and long features will deflect under the tool and finish out of spec.
- Cooling: air or mist first. Nylon generates more frictional heat than most plastics, and it benefits from coolant more than acetal does. But it's the material whose dimensions move with water — so on tight-tolerance work, reach for air blast or a light mist before water-based flood, which can cost you more in swell than it gains in heat.
When nylon is the right call
Nylon earns its place when a part needs toughness and wear resistance more than dimensional precision:
- Gears, sprockets, and sheaves — absorbs shock loads and runs quietly against metal.
- Wear pads, guides, and slides — genuinely better abrasion resistance than acetal in dry service, and forgiving of marginal lubrication.
- Bushings and bearings — particularly cast and filled grades built for bearing duty.
- A tough stand-in for metal — lighter than bronze or steel, corrosion-free, and quieter running.
Where nylon is not the answer: precision parts that must hold a tight fit in wet or humid service (specify acetal — it's rated better in wet wear anyway), sustained heat beyond its service range (reach for PEEK), and strong acids, oxidizers, or halogens.
The takeaway
Nylon rewards planning more than any other common machined plastic. Pick cast or extruded on section size and service temperature rather than reputation, know the moisture condition of your stock, keep tools sharp and chips clear, and be realistic about tolerance in a material that moves with the humidity in the room. Get those right and nylon does the tough, quiet, wear-resistant work that metal used to.
If you have a nylon part — or you're weighing nylon against acetal and want a straight answer about which will hold your fit in service — our team has been CNC machining engineering plastics since 1968. Send us the print and we'll tell you what's realistic. Request a quote and we'll take it from there.