What is the typical tolerance achievable in metal vs plastic CNC parts?
General Overview of CNC Machining Tolerances
A practical RFQ starting point is ±0.05 mm for rigid metal features and ±0.10 mm for stable plastic features, provided the geometry, stock condition, setup, and inspection plan support those limits. These figures are screening values, not universal capability guarantees. Finished-part tolerance from CNC machining depends on feature size, wall stiffness, tool access, cutting heat, clamping force, residual stress, and measurement conditions. Metal often permits a narrower tolerance because a comparable section usually deflects less during cutting and inspection. Plastic may move under clamp pressure, relax after machining, absorb moisture, or change size as temperature changes. The drawing must therefore separate general dimensions from features that control sealing, alignment, bearing fit, or datum relationships. ASME Y14.5 can define dimensional and geometric requirements, but citing the standard does not prove that a process can meet a particular value. ISO 1 defines 20°C as the standard reference temperature for dimensional specifications and verification; it does not eliminate thermal effects. State the exact grade, condition, critical features, datum system, finish state, acceptance rule, and required inspection record in the RFQ.
Tolerances in Metal CNC Parts
Rigid, well-supported metal features can often be quoted around ±0.05 mm, while tighter values require a feature-specific process and measurement review. General dimensions made by CNC milled or CNC turned routes should not be confused with a guaranteed bore, wall, profile, or true-position result. multi-axis machining may reduce re-clamping between related faces, but machine positioning data still does not establish finished-part tolerance. EDM machining may be considered when tool access or internal geometry makes cutting impractical, subject to material and surface requirements. Stock history matters: aluminum 7075 can move as residual stress is released; stainless steel SUS304 can work-harden and form persistent burrs; and titanium Ti-6Al-4V concentrates cutting heat. Tool wear and deflection also affect Inconel 718 and Hastelloy C-22. For a thin metal wall, compare the clamped in-process dimension with the released, temperature-stabilized result before approving the tolerance.
Tolerances in Plastic CNC Parts
Stable, supported plastic features are commonly screened near ±0.10 mm, but the acceptable value must reflect polymer grade, reinforcement, moisture state, temperature, and load duration. Acetal (POM) is often selected where dimensional stability is important, yet a thin web or interference fit can still deform. Nylon (PA) can change dimensions as moisture content changes, so a measurement taken immediately after machining may not represent the conditioned part. Polycarbonate (PC) requires control of heat, clamping, and edge damage because residual stress can affect both dimensions and service behavior. Higher material cost does not guarantee tighter control: PEEK still needs grade- and geometry-specific review, while PTFE (Teflon) can deform during gripping and measurement. A useful validation keeps the inspection method constant, records temperature and humidity, releases the part from the fixture, and repeats critical measurements after the agreed conditioning interval. For mating plastic features, specify functional clearance or fit rather than copying a metal tolerance without analysis.
Role of DFM and Surface Finishing in Tolerance Control
DFM supports tolerance control only when it converts each functional requirement into a material, setup, finishing, and verification plan. A review by precision machining services should identify the datum features, unsupported walls, deep cavities, long bores, small threads, and surfaces likely to move after unclamping. It should also distinguish size tolerance from flatness, position, and profile; one ± value cannot control all four. Roughing may release stock stress, so a critical metal feature can require an intermediate relaxation step and a separate finishing setup. Plastic features may need low clamp pressure, supported inspection, and a defined conditioning state. Finishing introduces another boundary. electropolishing removes material from exposed metal surfaces, so pre-finish machining targets and final acceptance dimensions must be distinguished. surface treatment for plastics may change appearance, friction, or surface build, but it cannot correct deformation caused by weak geometry or clamping. The purchase specification should say whether inspection occurs before or after finishing, how masked features are treated, and which report proves conformance.
Industry-Specific Expectations
Industry name alone does not determine CNC tolerance; feature function, failure consequence, material behavior, and verification evidence determine the requirement. In aerospace and medical device work, controlled features may require traceable material, a defined datum system, documented inspection, and an explicit acceptance rule. That need does not make every cosmetic surface equally critical. Automotive programs may place more emphasis on repeatable gauging and process evidence at production volume, while a prototype may be accepted from a full dimensional report. The same decision logic applies in other industries. First identify what the feature does; then assign the tolerance and geometric control that protect that function. Next define the material condition, measurement temperature, inspection method, and sample requirement. A metal bearing bore may justify tighter control than its surrounding pocket. A plastic cover may need wider dimensional limits but stricter control of assembly gaps after conditioning. Ask the supplier to identify the tolerance-driving features and any expected movement after unclamping or environmental conditioning. The resulting feature-level plan is more reliable than a single metal-versus-plastic capability number.
