What’s the tightest tolerance achievable for CNC milled plastic parts?
What’s the Tightest Tolerance Achievable for CNC Milled Plastic Parts?
There is no universal tightest tolerance for CNC-milled plastic parts. The defensible limit is feature-specific and must be demonstrated for the exact resin grade, stock form, conditioning state, geometry, process route, quantity, and inspection state. A supplier number is only a proposal until trial parts show that the feature remains conforming after unclamping and agreed stabilization. Buyers should define the functional interface, datum system, service temperature and humidity, measurement condition, and acceptance method. Then release the tightest limit that the function needs and the evidence supports, rather than extending one local result across the whole part.
Release a Plastic Tolerance Through Evidence
Release Stage | Required Evidence | Buyer Decision |
|---|---|---|
General drawing limit | Confirm grade, stock form, wall geometry, datum access, delivered state, and a measurement method suitable for noncritical features. | Use only where clearance, envelope, appearance, or handling remains acceptable across expected material movement. |
Critical-feature candidate | Review cutter reach, support, clamping, roughing sequence, conditioning, temperature, moisture, free-state recovery, and measurement uncertainty. | Authorize a trial limit for the named bore, plane, pattern, fit, seal, or motion interface rather than every dimension. |
Released tight limit | Inspect trial parts after the agreed stabilization state and document actual variation, method, environment, datum alignment, and conformity rule. | Release only the limit supported by the trial and define when stock, process, fixture, or environment changes require revalidation. |
A tight local dimension becomes credible only when the selected material and stock state and the inspection condition are controlled. ISO 1 defines 20 °C as the standard reference temperature for geometrical and dimensional property specifications. That reference does not prove that a polymer has reached thermal or moisture equilibrium. State the conditioning procedure, stabilization time or criterion, measurement temperature, humidity when relevant, and whether acceptance is free-state or fixture-restrained. Check both the critical feature and nearby thin geometry after release from the fixture.
Material-Specific Considerations
POM (Acetal): Evaluate the exact grade, extruded or other stock form, section size, stock stress, moisture state, and service range. POM may suit sliding fits and guides, but thin walls or unbalanced removal can move after unclamping.
PEEK: Specify grade, reinforcement, stock manufacturing route, material direction when relevant, annealed state, and required service behavior. High material value increases the cost of unsupported tight limits and late-stage rejection.
ABS: Use grade-specific stock and evaluate heat input, clamping marks, wall flexibility, and relaxed geometry. Broad panels, clips, and cosmetic housings often need functional clearance or profile acceptance rather than tight size on every local feature.
Polycarbonate: Define grade, transparency, stress state, coolant compatibility, polishing, and cosmetic inspection. Dimensional conformity alone does not accept a transparent part if machining or clamping leaves cracks, haze, distortion, or visible stress.
Key Factors Influencing Plastic Machining Tolerance
Thermal expansion: Use supplier data for the exact grade and direction where applicable. Estimate dimensional change across the relevant length and temperature range, then separate machining acceptance from service-performance validation.
Moisture absorption: For moisture-responsive grades, define incoming condition, storage, conditioning medium, equilibrium criterion, and inspection timing. A dry-state dimension may not represent the assembled or service state.
Residual stress: Review stock manufacture, roughing balance, clamp load, internal corners, and material removal sequence. Measure after unclamping and agreed stabilization; an immediate in-fixture reading can conceal later movement.
Surface finish requirement: Coordinate size, form, texture, polishing allowance, and edge condition. A finishing operation can alter the same feature that carries the dimensional requirement, so acceptance must apply to the released final state.
Design Recommendations
Connect each tight limit to fit, sealing, alignment, bearing support, motion, optics, or another measurable function. Use broader envelope or clearance controls for flexible ribs, clips, covers, and unsupported panels when function permits.
Give critical features enough wall support and tool access, and avoid abrupt section changes or strongly one-sided removal when movement would affect the interface. Validate any relief or geometry change against load and assembly needs.
State grade, stock form, conditioning, material direction when relevant, datum scheme, free or restrained state, finish sequence, operating environment, quantity, sample plan, measurement uncertainty, and conformity rule in the RFQ.
Manufacturing Services You May Need
Use the linked plastic CNC machining, POM, PEEK, and polycarbonate pages to route a feature-specific feasibility request. Require the proposed limit, material and stock assumptions, process sequence, trial quantity, stabilization state, measurement method, uncertainty, actual trial results, and revalidation triggers. These records establish the tightest released tolerance for that feature; a general supplier capability statement does not.
Before ordering production, approve the limit only after sample evidence shows the functional plastic feature remains conforming in its defined final measurement state.
