What is the typical tolerance achievable in metal vs plastic CNC parts?

General Overview of CNC Machining Tolerances

CNC machining tolerances depend on material stability, machining process precision, and environmental factors such as heat and humidity. While CNC machining systems are capable of high accuracy, the achievable tolerance differs significantly between metals and plastics due to their mechanical and thermal properties.

Tolerances in Metal CNC Parts

Metals are dimensionally stable, allowing for tighter tolerance control even under aggressive cutting conditions. Typical general-purpose tolerances for CNC milled or CNC turned metal parts are ±0.05 mm. With precision processes such as multi-axis machining or EDM machining, tolerances as tight as ±0.005 mm are achievable for critical aerospace or medical components. Materials such as aluminum 7075, stainless steel SUS304, and titanium Ti-6Al-4V maintain high dimensional consistency and low thermal expansion. For complex, heat-resistant materials like Inconel 718 or Hastelloy C-22, achievable tolerances remain around ±0.01–0.02 mm after compensating for cutting temperature and tool deflection.

Tolerances in Plastic CNC Parts

Plastics are more prone to deformation and temperature-induced dimensional changes, so tolerances are generally wider. For most engineered polymers, such as Acetal (POM), Nylon (PA), or Polycarbonate (PC), the standard tolerance is ±0.1 mm. High-performance materials, such as PEEK and PTFE (Teflon), can achieve a tolerance of ±0.05 mm under controlled temperature and humidity conditions. However, thermal expansion during machining and post-cooling shrinkage may still cause variation, which DFM compensates for by adjusting cutting parameters and fixture design.

Role of DFM and Surface Finishing in Tolerance Control

In both metal and plastic machining, precision machining services rely on DFM principles to set realistic tolerance targets. DFM ensures part geometry, wall thickness, and tool access are optimized to reduce deflection and stress buildup. After machining, finishing operations such as electropolishing for metals or surface treatment for plastics are integrated to refine surface quality without compromising dimensions.

Industry-Specific Expectations

In aerospace and medical device manufacturing, metal parts are routinely produced to meet ±0.01 mm tolerances for both structural and biocompatible components. Automotive parts, balancing cost and scalability, typically have tolerances of ±0.05 mm for metals and ±0.1 mm for plastics. Consumer and industrial products accept wider tolerances when aesthetic fit is prioritized over precision.