CNC Machining Tolerances Explained: ISO 2768, GD&T, and What to Specify for Your Parts

Tolerance specification is one of the most consequential — and most misunderstood — aspects of engineering a CNC machined part. Specify too loosely and you get a part that does not assemble correctly. Specify too tightly and you pay significantly more for precision that provides no functional benefit. Getting tolerances right is a skill that comes from understanding what CNC machines can actually achieve and what different tolerance levels cost in practice.

This guide covers the ISO 2768 standard that governs general machining tolerances, the GD&T feature controls used for geometric precision, the tolerance values achievable on different feature types, and the specific tolerance decisions that drive cost most significantly.

ISO 2768 — The Foundation of General Machining Tolerances

ISO 2768 defines general geometric tolerances for machined parts — the default tolerances that apply to any dimension on a drawing that does not have a specific tolerance callout. It has four grades for linear dimensions:

ISO 2768-m is the standard default tolerance class used by the majority of 数控机床 shops for general precision work. When your drawing states ‘General tolerances per ISO 2768-m’ without individual callouts, every unmarked dimension is held to ±0.10mm. This is the right default for most functional machined parts. Only call out tighter tolerances where function genuinely requires them.

Achievable Tolerances by Feature Type

GD&T Feature Controls — When to Use Them

GD&T (Geometric Dimensioning and Tolerancing) is the system for specifying geometric precision — flatness, perpendicularity, parallelism, concentricity, runout, position — in addition to dimensional tolerances. GD&T controls the shape and relationship of features, not just their size.

Key GD&T controls relevant to CNC machined parts:

• Flatness (⏥): Controls how flat a surface is regardless of its position. Use for sealing faces, precision contact surfaces, and optical mounts. Example: 0.02mm flatness on a valve body sealing face.
• Perpendicularity (⊥): Controls how square a feature is relative to a datum. Critical for bores that must be perpendicular to a face for bearing fits. Example: 0.03mm perpendicularity of a bore axis to the base face.
• Position (⊕): Controls the true position of a feature (hole, slot, boss) relative to a datum reference frame. More informative than coordinate tolerances for hole patterns. Example: Ø0.1mm true position for a bolt circle.
• Concentricity / Coaxiality (◎): Controls how well two cylindrical features share a common axis. Critical for rotating assemblies, shafts, and precision couplings.
• Surface profile (⌒): Controls the form of a freeform curved surface. Required for aerospace aerodynamic surfaces and precision contour features.

The Tolerance Decisions That Drive Cost Most

Based on Lewei Precision’s DFM review data across thousands of customer parts, these are the tolerance decisions that most frequently add unnecessary cost:

• Global tight tolerance application: Applying ±0.02mm to an entire drawing when only 3 features need it. Cost impact: 2–5× the cost of selective tight tolerancing.
• Flatness callouts on large surfaces: A 0.005mm flatness callout on a 200mm × 100mm face requires grinding — adding significant cost. 0.02mm is achievable by milling with a proper finishing pass.
• True position on non-critical holes: Applying Ø0.05mm true position to all holes when most are clearance holes that only need ±0.1mm. CMM verification of every hole multiplies inspection time.
• Surface finish callouts tighter than necessary: Ra 0.4µm on structural surfaces where Ra 1.6µm is functionally adequate. The finishing pass cost difference is significant at production quantities.

常见问题

What is the tightest tolerance achievable by standard CNC machining without grinding?

The practical limit for standard CNC milling and turning on a calibrated machine with appropriate fixturing is ±0.005mm on feature size and ±0.01mm on positional accuracy, achieved with a dedicated finishing pass and in-process measurement. Below this level, grinding, lapping, or honing is required.

Should I specify tolerances in metric or imperial on my drawing?

Use whatever system is native to your design environment and consistent throughout your drawing. Mixing metric and imperial dimensions on the same drawing is a quality risk — conversion errors are common. Most global machine shops work in metric; if you design in inches, ensure your machine shop confirms they are working in the correct unit system.

What is the difference between dimensional tolerance and geometric tolerance?

Dimensional tolerance controls the size of a feature — the diameter of a hole, the width of a slot, the length of a step. Geometric tolerance controls the shape or relationship of a feature — how flat a surface is, how perpendicular a bore is to a face, how well a hole pattern is positioned relative to a reference. Both are needed for complete part specification; GD&T adds the geometric control that dimensional tolerances alone cannot provide.