Engineering Reference
Tolerance & Fit Selection Guide — ISO 2768 & GB/T 1804 Explained
Usage Notes
This guide is for general machining reference only. For precision molds, aerospace, medical devices, and other specialized industries, follow industry-specific standards. Dimensions explicitly annotated on drawings always take precedence over general tolerance standards.
ISO 2768-1 Linear Dimension General Tolerances
For linear dimensions without individual tolerance indications (unit: mm)| Tolerance Grade | 0.5–3 | 3–6 | 6–30 | 30–120 | 120–400 | 400–1000 | 1000–2000 | Application |
|---|---|---|---|---|---|---|---|---|
| f (Fine) | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | ±0.3 | ±0.5 | Precision mating parts, locating surfaces |
| m (Medium) | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | General machining (most common) |
| c (Coarse) | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2.0 | ±3.0 | Weldments, non-mating structures |
| v (Very Coarse) | — | ±0.5 | ±1.0 | ±1.5 | ±2.5 | ±4.0 | ±6.0 | Castings, non-machined surfaces |
Common Fit Selection Reference
Hole-basis fits (H7) — commonly recommended| Fit Type | Fit Code | Characteristics | Typical Applications |
|---|---|---|---|
| Clearance Fit | H7/g6 | Very small clearance, precision sliding | Precision guide pillars, machine spindles, index pins |
| Clearance Fit | H7/f7 | Medium clearance, rotational movement | General shafts & bearing housings, plain bearings |
| Clearance Fit | H7/e8 | Large clearance, high-speed rotation | High-speed shafts, multi-bearing shafts |
| Transition Fit | H7/k6 | Slight interference or clearance | Locating pins, gears on shafts |
| Transition Fit | H7/m6 | Mostly slight interference | Press-fit bronze bushings, fixed pins |
| Interference Fit | H7/p6 | Light press, non-removable | Guide bushings pressed into mold base, bearing outer rings |
| Interference Fit | H7/s6 | Medium press, torque transmission | Gear hubs, couplings |
| Interference Fit | H7/u6 | Heavy interference, shrink fit | Railway wheel rims, large gears |
Recommended Tolerance Grades for CNC Machining
| Process | Typical Achievable Tolerance (mm) | Economical Tolerance (mm) | Notes |
|---|---|---|---|
| 3-Axis CNC Milling | ±0.01 | ±0.05 | Standard tooling, aluminum & steel |
| 5-Axis CNC Milling | ±0.005 | ±0.02 | Multi-face, complex geometries |
| CNC Turning | ±0.005 | ±0.02 | Rotational/cylindrical parts |
| Precision Grinding | ±0.002 | ±0.005 | Guide pillars & bushings, precision shafts |
| Wire EDM (WEDM) | ±0.005 | ±0.01 | Complex contours, sharp corners |
| Sinker EDM | ±0.01 | ±0.03 | Cavities, irregular surfaces |
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