Nylon (PA) CNC Machining

One property decides most nylon jobs

Every material has a property that dominates its specification, and for nylon it is moisture absorption.

Polyamide chemistry attracts water. The polymer chain carries groups that bond with water molecules, so the material absorbs moisture from the surrounding air until it reaches equilibrium with the environment it is in. Two things follow, and both are large:

The part grows. Absorbed water pushes the polymer chains apart, so the dimensions increase. Moving from a dry state to normal indoor equilibrium changes a nylon part by a fraction of a percent — which on a 100 mm part is a fraction of a millimetre, and roughly an order of magnitude larger than the ±0.01 mm we hold on precision features.

The material softens. Water acts as a plasticiser. Dry nylon is stiff and strong; conditioned nylon is neither. The stiffness drop is the larger of the two effects — the modulus can fall by well over half between the dry state and a fully conditioned one, and the tensile strength drops by around a third.

Both effects are reversible. Dry the part out and it returns to its dry dimensions and properties. That is precisely why the effect cannot be treated as a fixed allowance: the part is a different size and a different material depending on the weather.

So the useful question on a nylon enquiry is not “what tolerance can you hold”, but “what state will the part be in when it has to fit, and what state will it be measured in”.

Working with the moisture instead of against it

Three routes work, and which one applies depends on what the part does.

Machine oversize, let it equilibrate, then finish. Machine leaving material on the critical features, let the part reach equilibrium with its service environment, then take the finishing cuts. The most accurate route, and the slowest — it adds a stabilisation period to the schedule.

Start from conditioned stock. Where the material is supplied or stored in a condition close to the service condition, the starting point matches the end point and the change during machining is small. This is the practical route for most production parts, and it needs the service environment stated.

Design the tolerance band around the change. Where the function allows, a tolerance that accommodates the moisture-driven movement is cheaper than fighting it — a clearance fit that still works at both ends of the moisture range is a better design than a nominal dimension that is only correct in one state.

None of these is a machining technique. They are decisions about what the part is for, which is why the service environment belongs on the enquiry rather than in a note after the parts have been made.

Nylon against acetal: which one

Nylon and acetal occupy the same territory — small mechanical parts that move against something else — and the choice between them comes down to a short list.

	Nylon (PA)	Acetal (POM)
Impact toughness	Excellent	Good, but below nylon
Abrasion resistance	Better, especially in dirty conditions	Good
Low-temperature toughness	Retained	Reduced
Dimensional stability	Poor — moisture drives it	Good
Stiffness	Higher when dry, drops sharply when conditioned	Higher and more consistent
Machinability	Gummy, smears and melts	Excellent — short chips, clean finish
Creep	More	Less

The practical reading:

• Nylon wins where the part takes knocks, runs dirty, sees abrasion, or works at low temperature — a mallet head, a conveyor guide, a heavily loaded sprocket in an abrasive environment.
• Acetal wins where the part has to hold a tolerance, run against a clean surface, and stay the same size — a precision gear, a bushing in a controlled assembly, a wear strip with a close fit.

Where a part needs nylon’s toughness and a tight tolerance, that is exactly the case where the conditioning decision above has to be made explicitly. It is doable; it just is not free.

Machining nylon

Nylon is harder to machine than acetal, and the reason is worth understanding because it dictates the fix.

It smears rather than shears. The material is tough and relatively soft in the cut, so instead of forming a clean chip it tends to drag, weld back onto the workpiece and leave a torn surface. On a finishing pass this is the difference between a part that passes inspection and one that does not.

It conducts heat poorly. Friction heat stays at the cutting edge, softens the material further, and makes the smearing worse — a feedback loop that only a sharp edge and a fast cut break.

What works:

• A genuinely sharp, high-rake edge. In plastics, tool sharpness is not a preference — it is the mechanism. A dull edge on nylon rubs, heats and smears.
• High spindle speeds with moderate feeds. Nylon cuts easily, so speed is available; the aim is to remove material before heat accumulates in the workpiece rather than to take heavy cuts.
• Generous chip clearance. Nylon chips are bulky and do not carry heat away. If the chip cannot leave the cut, it recirculates and rubs.
• Air blast rather than flood coolant. Nylon can absorb coolant, and prolonged contact risks staining or property change. Compressed air clears the chip and removes enough heat on most operations.
• Sharp part edges, handled carefully. Nylon burrs are tough and stringy, and aggressive deburring rounds edges that were machined sharp on purpose.

One stock-form note: nylon is commonly supplied as extruded rod or as cast (MC) stock, and the two behave differently. Cast stock is generally more uniform and less stressed; extruded stock can carry residual stress that releases as material is removed, so a thin-walled part may move during machining. For a tight-tolerance or thin-walled part, saying so at the enquiry lets the stock form be chosen.

Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold. In nylon, the tolerance is achievable — the question is at what moisture content it is measured.

Where nylon is the right choice

• Gears, sprockets and worm wheels that take shock or run in dirty conditions
• Bushings, wear pads and slide blocks in abrasive service
• Rollers, guide rails and conveyor components
• Hammer and mallet heads, and other impact tooling
• Insulators, spacers and washers where toughness matters more than precision
• Cable, hose and tube components where flexibility and abrasion resistance are wanted
• Food-handling parts in the grades approved for that service
• Parts operating at low temperature, where nylon retains toughness that other plastics lose

Where nylon is the wrong choice

The part has a tight tolerance and a variable environment. Nylon will move, and if the service humidity is not controlled the part will not hold a nominal dimension. Acetal is dimensionally stable and is usually the better answer; where nylon’s toughness is also needed, the conditioning route has to be planned.

The service temperature is above roughly 80–100 °C. Nylon softens and creeps, and the moisture effect makes it worse.

The part runs continuously under load. Nylon creeps more than acetal, particularly when warm and conditioned. A permanent load needs reviewing against service life.

The part is outdoors without UV stabilisation. Unstabilised nylon chalks and embrittles in sunlight.

Strong acids, phenols or formic acid are involved. All attack nylon. The chemical needs to be named.

A clean, precision sliding surface is required. Nylon’s machining finish and its dimensional movement both work against a precision sliding fit. Acetal or PTFE is the better family.

Finishing nylon

Nylon usually needs nothing beyond deburring, though a few routes are worth knowing.

• Deburring and tumbling — done gently, because nylon burrs are tough and aggressive treatment rounds machined edges
• Annealing — a controlled heat cycle to relieve machining stress, used where a part is thin-walled or will later see heat or solvent exposure
• Conditioning — deliberately bringing the part to a defined moisture content so it measures consistently; this is a finishing step in its own right on a precision nylon part
• Sand blasting — a uniform matte finish where a texture is wanted
• Painting and bonding — possible but not straightforward; nylon’s surface needs treatment before adhesives or coatings will hold reliably

All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.

What to put on the drawing

Instead of	Write
Plastic	Nylon (PA), where toughness or abrasion resistance is the driver
Nylon, tight tolerance	Say which dimensions matter, and the humidity they will be measured and used in
Nylon, wet or humid service	Say so — the part grows and softens as it absorbs moisture
Nylon, continuous load	Flag it — nylon creeps, more so when warm and conditioned
Nylon, thin wall	Say so — residual stress in extruded stock can move the part
Nylon, outdoor service	Specify a UV-stabilised grade
Nylon, strong acid or phenol	Wrong material — nylon is attacked
Nylon above ~80–100 °C	Wrong material — it softens and creeps

The three lines that decide most nylon specifications are what state the part is measured in, what it takes in service, and whether a tighter tolerance is really needed. The first two are the moisture question, and they are the ones most often left off the drawing. The third decides whether nylon is the right material at all, or whether the part belongs in acetal.

Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade and stock form to its mill lot plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the conditioning route and the stock form where either needs settling.