Guide to CNC Machining Tolerances
What Are CNC Machining Tolerances?
A tolerance defines how much a dimension can vary from its nominal value without affecting functionality.
For example, a 2.550″ dimension with a ±0.001″ tolerance allows a range from 2.549″ to 2.551″.
Smaller tolerances mean higher precision, and usually higher cost, while looser tolerances are easier and cheaper to achieve.
Common Standard Tolerances
General Standard
Typical: ±0.005″ (±0.127 mm) for metals
Good for prototypes, general-purpose parts, and non-critical features
High Precision
±0.002″ (±0.051 mm)
Used for tight fits and critical features where performance depends on accuracy
Reamed Holes
±0.0005″ (±0.0127 mm)
For precision fits such as pins, shafts, or press-fit components
ISO 2768 (General Tolerances – Metric)
Fine (f): ±0.05 mm for 0.5–3 mm dimensions
±0.1 mm for 3–6 mm
Common in European manufacturing standards
The Cost Impact of Tolerances
Tighter tolerances often require:
Slower machining speeds
More tool changes
Additional quality checks
This increases lead time and production cost.
Experts recommend applying strict tolerances only to features that truly require them. Over-specifying tolerances can lead to unnecessary expense.
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Practical Design Tips
Be selective: Only specify tolerances on features that affect fit, function, or assembly.
Start standard: Use ±0.005″ (±0.127 mm) unless function demands more precision.
Consider manufacturing limits: Very small features (<0.5 mm) may require explicit tolerances and special tooling.
Use GD&T: Geometric Dimensioning and Tolerancing (ASME Y14.5) controls flatness, concentricity, and position when form and orientation matter more than size.
Example Tolerance Table
| Tolerance Type | Typical Value | Common Use Case |
|---|---|---|
| General standard | ±0.005″ (±0.127 mm) | Most prototype and production parts |
| High precision | ±0.002″ (±0.051 mm) | Tight fits and performance-critical components |
| Reamed hole | ±0.0005″ (±0.0127 mm) | Precision fits for pins/shafts |
| ISO 2768 fine (0.5–3 mm) | ±0.05 mm | European-standard fine machining |
| Tip: default to standard tolerances and tighten only where function requires it. | ||
Key Takeaways
Tolerances directly influence cost, lead time, and performance.
Default to standard tolerances unless precision is critical.
Apply tight tolerances only where they matter.
Leverage GD&T for form, orientation, and positional control.
