Engineering guide to CNC machining tolerances, ISO 2768 Fine/Medium standards, surface roughness Ra, concentricity, and cost vs precision trade-offs.
ISO 2768 is the international standard governing general dimensional tolerances for linear and angular dimensions produced by machining without individual tolerance indications on technical drawings. It is split into ISO 2768-1 (linear/angular) and ISO 2768-2 (geometrical tolerances).
At Divine Brass Industries, our Swiss CNC turning center routinely operates within ISO 2768 Fine (f) and custom micro tolerances down to ±0.005 mm (±0.0002") for critical B2B assemblies.
| Tolerance Class | 0.5 to 3mm | 3 to 6mm | 6 to 30mm | 30 to 120mm | 120 to 400mm | Recommended OEM Application |
|---|---|---|---|---|---|---|
| f (Fine) | ±0.05 mm | ±0.05 mm | ±0.10 mm | ±0.15 mm | ±0.20 mm | Swiss CNC Micro Turned Parts & Sensor Bodies |
| m (Medium) | ±0.10 mm | ±0.10 mm | ±0.20 mm | ±0.30 mm | ±0.50 mm | Commercial Fittings, Fasteners & Threaded Inserts |
| c (Coarse) | ±0.20 mm | ±0.30 mm | ±0.50 mm | ±0.80 mm | ±1.20 mm | Unmachined Closed-Die Forging Blank Features |
| Surface Finish (Ra μm) | Microinch (N-Scale) | Manufacturing Process | Typical Part Feature Application |
|---|---|---|---|
| Ra 0.4 μm (N5) | 16 μin | Precision Diamond Burnishing / Lapping | Valve stem sealing surfaces & O-ring grooves |
| Ra 0.8 μm (N6) | 32 μin | Swiss CNC Fine Turning / Grinding | Precision sensor shells, hydraulic spools |
| Ra 1.6 μm (N7) | 63 μin | Standard CNC Lathe Turning / Milling | Pipe fitting threads, knurled insert bodies |
| Ra 3.2 μm (N8) | 125 μin | Rapid Rough CNC Turning | Internal non-critical bores, structural clearance |
