CNC Machining Tolerance Guide — ISO 2768 and Beyond

Understand CNC machining tolerances, ISO 2768 standards, and how to specify precision for your custom parts.

What Are Machining Tolerances?

Machining tolerances define the acceptable variation in a part dimension. In CNC machining, tolerances are typically expressed as ± values. The tighter the tolerance, the more precise—and expensive—the part.

ISO 2768 Standard

ISO 2768 is the most widely referenced general tolerance standard. It has two parts:

• ISO 2768-1 — Linear and angular dimensions. Classes: f (fine), m (medium), c (coarse), v (very coarse)
• ISO 2768-2 — Geometrical tolerances. Classes: H, K, L

For most general machining, ISO 2768-m is standard. This gives ±0.1 mm for dimensions up to 6 mm, ±0.2 mm for 6-30 mm, and ±0.3 mm for 30-120 mm.

What CNC Machines Can Achieve

• Standard CNC Milling: ±0.1 mm (±0.004")
• Precision CNC Milling: ±0.05 mm (±0.002")
• High-Precision / Jig Grinding: ±0.005 mm (±0.0002")
• CNC Turning: ±0.05 mm (±0.002") standard

How Tight Should You Specify?

Only specify tight tolerances where the function demands it. Every 0.01 mm reduction in tolerance can increase cost by 15-30 percent due to additional inspection, slower feeds, and higher reject rates.

• Non-critical surfaces: ±0.5 mm
• General fits: ±0.1 mm
• Precision fits: ±0.02 mm
• Bearing seats / critical interfaces: ±0.005 mm

Surface Finish vs Tolerance

These are related but distinct. A part can have a tight tolerance but rough surface finish. Specify both:

• Ra 3.2 μm (125 μin) — Standard machining finish
• Ra 1.6 μm (63 μin) — Fine machining
• Ra 0.8 μm (32 μin) — Precision/grinding finish

Geometric Dimensioning and Tolerancing (GD and T)

For complex parts, GD&T (ASME Y14.5) provides a richer language to define form, orientation, location, and runout tolerances. Key symbols include flatness, parallelism, concentricity, and true position.