Machined PA6/PA66 Nylon: How Tight a Tolerance Survives Moisture, Stress and Inspection

Why Metal-Level Tolerances Fail on Nylon

If you currently tolerance nylon like aluminum, moisture is the first thing that makes the drawing unachievable. Nylon is hygroscopic: it absorbs water from the air and grows dimensionally after the cut was made, and one machining source advises a working allowance of 0.2–0.5% swelling in humid environments⁹.

Hygroscopic grades such as nylon therefore grow after machining even when the shop held size perfectly at the machine⁶. The arithmetic makes the point.

For that single-source allowance, a 100 mm dimension can move 0.2–0.5 mm — larger than the entire ±0.01 mm band you would put on a metal part⁹. Metals behave differently: capable shops routinely hold about ±0.005–0.013 mm, within limits set by geometry, machine, thermal environment and measurement system².

Engineering plastics also expand 3–10× more per degree than metals, so temperature adds drift on top of moisture⁶. The decision that follows: tolerance against the part's in-service moisture state, not the state it leaves the machine in.

Storage after machining is part of your specification, not the shop's private problem⁹.

Which Tolerance Band the Drawing Can Honestly Carry

The evidence supports a band comparison, not a nylon-specific tolerance table. It places stable plastics (acetal/POM, PEEK) at ±0.025–0.050 mm and soft plastics (PTFE) at ±0.050–0.130 mm, versus metals down to roughly ±0.013 mm as a routinely achievable tight range^2,6.

Nylon is the hygroscopic case in that plastics group, so treat its achievable band as the plastics level or wider, not the stable-plastics level⁶. Roughly, plastics bands run 2–10× wider than metal practice, driven by moisture uptake and the 3–10× thermal expansion^2,6.

There is a cost reason to stay there: one DFM guide reports that tolerances at ±0.05 mm or larger avoid +25–300%+ lead-time and +50–500%+ cost penalties of extremely tight callouts⁴. Ask each supplier to confirm a band for your specific geometry against their qualified process⁶.

Order Stress Relief and Time the Final Cut

The second expensive failure is the part that passes inspection and then moves. Plastic parts can look dimensionally acceptable immediately after machining and warp or lose flatness hours or days later as internal stresses relax.

Documented by a plastics producer for PEEK, with thin geometries most vulnerable^8,25. Machining releases stresses that were frozen into the stock, so what the cutter leaves behind is not the part's final shape²⁵.

Drift over time can push tight-tolerance assemblies out of spec, producing rework, scrap and supplier risk⁸. For you as the buyer this sets the specification sequence: require stress relief after heavy stock removal and before the final finishing cut, and require final inspection after stress relief, not before it^6,8.

Put the requirement on the drawing or purchase note — one source lists stress relief and fixturing control among the conditions for holding plastics tolerances⁶. The sources give no nylon annealing temperature, time or ramp rates, so treat the specific procedure as something the supplier must state and validate.

Brief the Shop: Machining Practice That Limits Heat and Distortion

Machining practice is the lever the shop controls, and the evidence behind it is generic plastic-machining principle plus a turnability rating — not validated nylon parameters. Nylon 6/6 is rated good for turning⁹, but the cutting details below come from general milling practice⁵.

Put them in the RFQ and ask the supplier for their nylon-specific settings:

• ✓Sharp, positive-rake tooling — reduces cutting resistance and aids chip evacuation
• ✓Climb milling with thick-to-thin chip formation — lower heat and better finish; monitor workpiece displacement from lateral forces
• ✓Optimized feed, speed and depth of cut balanced against heat build-up
• ✓Properly applied coolant or lubrication to manage cutting heat
• ✓Controlled post-machining storage — dimensions change if parts are not stored properly

Inspection That Measures the State the Tolerance Applies To

A tolerance is only as good as the measurement system that proves it, so match the gauge to the band you wrote. Digital calipers resolve 0.01 mm with realistic accuracy of roughly ±0.02–0.03 mm, suiting bands of ±0.10 mm and wider.

Micrometers reach about ±0.010 mm; CMMs achieve ±0.001–0.003 mm and are required for true position, profile, flatness and parallelism²⁶. For the ±0.05 mm-level bands on nylon, a micrometer can verify external dimensions, but any GD&T form or position callout needs a CMM²⁶.

Temperature is part of the measurement spec. Engineering plastics expand 3–10× more per degree than metals — by contrast, aluminum moves only about 2.3 µm per °C on a 100 mm part⁶ (supplier-reported figure).

So the same shop-floor draft that is irrelevant on metal shifts a nylon dimension meaningfully⁶. Define the measurement temperature on the drawing, and time inspection after stress relief as set out above.

The sources do not cover gauge-contact pressure on soft nylon, pre-inspection conditioning, or ISO 1110 conditioning cycles — ask how the supplier verifies soft-plastic parts before you approve the inspection plan²⁶.

Pay Only for the Tolerances the Function Needs

The cheapest tolerance is the one you don't specify. Per the same DFM guide, extremely tight callouts add +25–300%+ lead time and +50–500%+ cost, because they force temperature control, stress relief and specialized equipment⁴.

Profile callouts cost +100–400% in inspection and 100% inspection +300–800% versus AQL sampling, so prefer dimensional callouts and AQL sampling on non-critical features⁴. Since nylon's moisture drift alone can exceed a tight band, spending that premium on features the assembly never checks is waste.

Keep tight callouts on the features a fit or location actually needs, open the rest to ±0.05 mm or larger. And put the difference toward the conditioning and inspection controls that keep the critical features honest (see Tolerance Cost Tradeoff).

Before quoting, confirm which features the supplier would inspect with a CMM and which state — moisture and temperature — their measurement assumes (see Inspection And Acceptance For Cnc Machined Parts).

Before you request a quote

• The evidence leaves this open — put it to the supplier: no source splits PA6 from PA66 on moisture absorption, and none gives saturation-vs-50% RH figures or a dimensional-change-per-%-moisture coefficient — ask the material datasheet holder for grade-specific data
• The evidence leaves this open — put it to the supplier: the 0.2–0.5% swelling figure traces to a single machining publisher as a working estimate for humid environments — verify against a material datasheet or conditioning standard before using it as a tolerance input
• The evidence leaves this open — put it to the supplier: no published nylon-specific tolerance table exists, and no source covers the effect of wall thickness or part size on achievable nylon tolerance — request a supplier-confirmed band for your geometry
• The evidence leaves this open — put it to the supplier: no drying parameters (temperature/time) and no guidance on machining as-received versus moisture-equilibrated stock; no source states which moisture condition a drawing tolerance applies to — ask the supplier to define and control that state
• The evidence leaves this open — put it to the supplier: no nylon annealing or stress-relief procedure is published: no temperature, time, ramp rates, dimensional shift from annealing, or explicit when-required criteria — require the supplier to state and validate their procedure
• Match the supplier documentation to your application and destination market before approval.
• Confirm named service coverage, response expectations and warranty terms for your destination.
• The evidence leaves this open — put it to the supplier: no gauge-pressure or fixturing limits for soft plastics, no measurement timing after machining, and no coverage of ISO 1110 or plastics GD&T practice — ask how the supplier handles each before approving the inspection plan
• Request a written quote with price basis, MOQ, tooling, payment terms and production lead time.