What standard tolerances can I expect from a professional CNC milling parts factory?

Quick Answer

Professional CNC milling facilities maintain three tiers of tolerance: standard (±0.005", 0.13 mm) for general features, precision (±0.001", 0.025 mm) for mating surfaces and critical dimensions, and high-precision (±0.0005", 0.013 mm) for aerospace and medical applications. Surface finish ranges from 32 Ra (standard) to 16 Ra (precision) and 8 Ra (high-precision).

Tolerance Classes Explained

Standard tolerance (±0.005"): Suitable for non-critical dimensions, clearance holes, general profiles. This is the default for most CNC milling shops and requires no special setup. Precision tolerance (±0.001"): Required for mating surfaces, bearing bores, and locating features. Achievable with quality tooling and proper machine maintenance. High-precision tolerance (±0.0005" or better): Required for aerospace, medical, and precision instrument components. May require temperature-controlled environments, specialized machines, and additional inspection.

Geometric Tolerances

Beyond dimensional tolerances, professional shops also control geometric features: flatness of 0.001" per inch, parallelism of 0.002" per inch, perpendicularity of 0.002" per inch, and concentricity of 0.001" TIR (Total Indicated Runout). These geometric tolerances are equally important for parts that must assemble and function correctly.

Surface Finish Standards

Standard milling produces 63-125 Ra. Precision milling achieves 32-63 Ra. Fine milling with light finishing passes reaches 16-32 Ra. Ultra-fine milling using specialized tools and techniques achieves 8-16 Ra. For reference, a typical automotive bracket might specify 63 Ra, while a hydraulic valve spool might require 16 Ra.

Why Choose SOMI Custom Parts

At SOMI Custom Parts, we clearly communicate our tolerance capabilities before production begins. Our standard practice is to hold ±0.003" on all dimensions unless otherwise specified, with critical features machined to ±0.001". For projects requiring ±0.0005" or tighter, we use dedicated precision machines with temperature control. Every part is inspected using CMM equipment, and dimensional reports are provided with your shipment.

Case Study

An aerospace client required aluminum mounting brackets with ±0.0005" on three critical hole locations, flatness of 0.001" across the mounting face, and 16 Ra surface finish. SOMI programmed the part for a 5-axis machine with a finishing pass strategy and verified every dimension on our CMM. The 300-piece order passed client inspection with zero non-conformances.

Industry Data

The American Society of Mechanical Engineers (ASME) Y14.5 standard defines tolerance classes for machined parts. According to a 2025 SME survey, the median CNC milling shop holds ±0.003" as their standard tolerance. Only 22% of surveyed shops consistently achieve ±0.001" or better on complex parts (SME Machining Survey, 2025).

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