What Are CNC Machining Tolerances?
Tolerances define the acceptable variation in a dimension from the specified value. In CNC machining, standard tolerances are typically ±0.05mm (±0.002"), while precision machining can achieve ±0.01mm (±0.0004") or tighter.
Standard Tolerance Classes
| Class | Tolerance Range | Typical Application | Cost Impact |
|---|---|---|---|
| Commercial | ±0.125mm | Non-critical dimensions | Base cost |
| Standard | ±0.05mm | General engineering parts | +10-15% |
| Precision | ±0.025mm | Mating surfaces, bearings | +25-40% |
| Ultra-Precision | ±0.01mm | Aerospace, optical, medical | +60-100% |
Factors Affecting Achievable Tolerances
Material
Aluminum is easier to machine to tight tolerances than stainless steel or titanium. Material thermal expansion also affects dimensional stability.
Feature Type
Holes are generally easier to hold tolerance on than external profiles. Internal corners have natural radius limitations from tool diameter.
Part Size
Larger parts are harder to hold tight tolerances on. A ±0.01mm tolerance on a 10mm feature is very different from the same tolerance on a 500mm feature.
How to Specify Tolerances in Your Drawing
- Only apply tight tolerances where functionally required
- Use GD&T (Geometric Dimensioning & Tolerancing) for complex requirements
- Specify datum references for positional tolerances
- Include surface finish requirements (Ra values) alongside dimensional tolerances
Cost Optimization Tips
Every tighter tolerance adds cost. Apply the principle: as tight as necessary, as loose as possible. Critical mating surfaces need precision; non-functional surfaces can use commercial tolerances.
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