CNC Machining Nylon: How PA Grades Behave on the Machine

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  Key takeaways
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  What polyamide does to a cutter
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  Why a nylon part changes size after machining
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  Choosing among PA6, PA66, PA12 and filled grades
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  Tolerances, threads and wall thickness that hold up
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  Where machined nylon earns its place
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  When nylon is the wrong answer
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  Nylon grades against common alternatives

Nylon is a semi-crystalline thermoplastic. Long polymer chains slide past each other under load, which gives the material its toughness and its creep at the same time. On the machine, that same chain structure makes the chip behave unlike metal. Nylon does not form a crisp chip that breaks and falls away. It forms a continuous string or a gummy curl that wraps the tool if the feed is too light.

The practical rule is to take a real cut. A depth of cut below about 0.2 mm per pass lets the edge rub rather than shear, and rubbing generates heat. Heat is the enemy here because nylon softens quickly and its thermal conductivity is low, so the heat stays at the cut instead of flowing into the chip. Push the feed per tooth to 0.1–0.25 mm and keep the surface speed at 300–600 m/min with uncoated or diamond-coated carbide.

Sharpness matters more than coating. A new two-flute end mill with a polished flute cuts clean; the same tool after cutting stainless will push the nylon ahead of it and leave a torn edge. We treat nylon tooling separately from metal tooling for that reason. Single-flute and two-flute cutters clear chips best, and a compressed-air blast keeps the string from wrapping the shank.

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  Climb millingReduces the burr on the exit edge. Conventional milling pulls the part into the cutter and tears the top face.
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  Rough then finishLeave 0.3–0.5 mm for the finish pass. Cutting nylon to final size in one heavy pass distorts thin walls.
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  Cool with airFlood coolant works but the chips absorb it. Air blast is easier to clean and keeps the part dry before measurement.

Polyamide takes water out of the air. PA6 can absorb 2.5–3% of its weight in moisture at 50% relative humidity, and PA66 sits slightly lower. Water molecules push between the polymer chains and act like a lubricant, so the part swells and gets softer at the same time. A 100 mm PA6 bar can grow 0.2–0.3 mm across that range. That is far more than our ±0.005 mm cutting tolerance.

This is the single biggest source of scrap on nylon jobs. A part machined from dry stock measures on size at the bench, then grows overnight in the shop air and fails incoming inspection. The fix is to condition the stock first. We let blanks sit in the shop environment until the weight stops changing, usually 24–48 hours for thin sections and longer for thick bar, then cut to final size in one setup.

For parts that will see wet or humid service, cut them slightly oversize and let them reach equilibrium before the finish pass. If the drawing holds a tight bore, tell us the service humidity. A bore that fits a shaft in a dry plant may bind in a humid one. In a few cases we machine the part wet, then re-check after a controlled dry cycle. That costs time, so it only makes sense on tight interfaces.

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  Dry stock, stable sizeConditioned blanks move less than 0.05 mm on a 100 mm length over a normal week.
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  Wet serviceA part that runs in water or high humidity should be measured wet, not dry.
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  Thin wallsA 2 mm wall reaches equilibrium in hours. A 40 mm block can take days.

Unfilled PA6 is the cheapest and stiffest of the common grades, and it is the one most engineers mean when they say nylon. It machines well, holds threads, and takes impact without cracking. Its weakness is moisture pickup and creep under steady load. PA66 behaves similarly but with a higher melting point and better stiffness at moderate heat, which is why it shows up in engine-bay brackets and under-hood clips.

PA12 costs more and absorbs less water, roughly 0.8–1.0% at equilibrium. That lower pickup makes it the better choice for a part that must hold size in a humid plant or a wet application. It is also easier to print and to machine without stress. When a customer has a fit problem on a PA6 part and the geometry cannot change, moving to PA12 often solves it without a redesign.

Filled grades change the picture again. Glass-filled PA6 raises stiffness and cuts creep, but the glass wears the cutting edge fast and the finish turns matte. It also makes the part more notch-sensitive, so sharp internal corners become crack starters. If you need stiffness close to aluminum with the weight of plastic, filled PA is reasonable, but plan for shorter tool life and radius every internal corner.

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  PA6Baseline toughness and stiffness. Good for fixtures, guards, wear pads and light gears.
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  PA66Higher melting point. Suits brackets and clips that see intermittent heat.
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  PA12Lowest moisture pickup of the three. Pick it when size stability matters more than cost.
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  Glass-filled PAStiffer, but abrasive to tooling and notch-sensitive. Add radii at every internal corner.

We hold ±0.005 mm on metal parts every day, but nylon is a different contract. The material itself moves more than that with humidity, so a drawing that calls ±0.01 mm on a 200 mm nylon part is asking for something the polymer cannot promise. A realistic band for a conditioned part in a controlled shop is ±0.05 mm on small features and ±0.1 mm over long spans. We will still measure and report what we cut, but we tell customers when the tolerance is tighter than the material allows.

Threads work well in nylon. Coarse threads hold better than fine ones because the material deflects rather than shears. A thread that is too fine strips under load. For a fastener that will be removed more than a few times, use a metal insert or a through-bolt with washers. Tapped holes in unfilled PA6 hold a machine screw well if the engagement is at least 1.5× the diameter.

Wall thickness is the other limit. Nylon cools and shrinks unevenly, so a thick section next to a thin one pulls the part out of shape and leaves sink marks. Keep walls between 1.5 mm and 6 mm where you can, and blend thickness changes over a distance rather than stepping them. Ribs should be about 60% of the wall thickness. These are molding rules, but they apply to machined parts too, because the same shrinkage shows up when a thick block relaxes after cutting.

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  Small features±0.05 mm is realistic on a conditioned part. Tighter than that depends on the day.
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  Long spans±0.1 mm over 200 mm. Humidity and room temperature both show up in the reading.
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  ThreadsCoarse pitches hold. Fine pitches strip. Use inserts for repeated assembly.
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  Walls1.5–6 mm, ribs at 60% of wall, blend every thickness change.

Nylon wins where you need toughness, low friction and light weight in one part. Gears and sprockets are the classic case. A machined PA6 gear runs quieter than a metal one and absorbs shock instead of chipping a tooth. Bushings and wear pads do the same job: they sacrifice themselves slowly and protect the shaft. We cut these on 3-axis and 4-axis mills, then tumble the edges so they slide instead of catching.

Guards, covers and cable guides are another fit. Nylon takes a hit without denting, resists oils and greases, and does not corrode. For a machine guard that a technician leans on, that combination beats sheet metal. We have cut these on our 3-axis machines from 20 mm plate, with counterbored holes and a chamfered edge, in runs from one prototype to a few hundred.

Prototype housings and brackets also make sense. A designer can hold a nylon part in hand, check the fit, and change the model before tooling. Because we run no minimum order quantity, a single PA12 bracket is a normal job. For automotive and robotics customers, that speed to a physical part is often worth more than the material cost.

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  Gears and sprocketsQuiet running and shock absorption. Unfilled PA6 is the default.
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  Bushings and padsLow friction against steel. They wear instead of the shaft.
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  Guards and coversImpact-resistant and corrosion-free. Beats sheet metal on abuse.
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  Prototype bracketsOne-off PA12 parts check fit before tooling money is spent.

Nylon creeps. A part held under steady load will keep deforming for months, even at room temperature. A bearing bore that has to stay round under a preload, or a spacer that sets a fixed gap, will lose that dimension over time. POM creeps far less and holds a sliding fit much longer. If the part carries a continuous load and the size matters, use POM, or use a metal and add a plastic liner.

Heat is the second limit. Unfilled PA6 loses most of its stiffness as it approaches 100 °C, and it will not survive a soldering iron or an engine exhaust. Intermittent peaks are fine for PA66 in many under-hood positions, but do not put a nylon part where it sees sustained heat. For that service, PEEK or a metal is the honest answer.

The third case is tight tolerance on a large part. If the drawing calls ±0.02 mm across 300 mm and the part will sit in an uncontrolled warehouse, no machine can promise it in nylon. The polymer moves more than the tolerance. We would rather say that up front than ship parts that fail on receipt.

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  Steady loadCreep deforms the part over months. Choose POM or metal.
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  Sustained heatAbove 100 °C unfilled PA6 softens. Use PA66, PEEK or metal.
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  Tight tolerance, large partHumidity moves the part more than the tolerance allows. Change material or relax the callout.