CNC Machining Tolerance Guide
Know what's achievable — and what costs extra
Tolerances are the #1 cost driver in CNC machining. Specifying tighter tolerances than necessary can double or triple part cost. This guide breaks down standard vs. precision tolerances for every feature type, so you can specify exactly what you need — and nothing more.
Overview
CNC machining offers the tightest tolerances of any subtractive process. Standard 3-axis mills hold ±0.05 mm routinely; 5-axis machines and precision lathes can achieve ±0.01 mm or better. The key is knowing which features actually need tight tolerances and which can use standard specs.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Linear Dimensions | ±0.05 mm (±0.002") | ±0.01 mm (±0.0005") | Standard is achievable on most 3-axis mills. Precision requires temperature-controlled shops. |
| Hole Diameter | ±0.025 mm (±0.001") | ±0.005 mm (±0.0002") | Reamed holes achieve precision. Drilled holes are standard. |
| Hole Position (True Position) | ±0.05 mm | ±0.01 mm | Depends on fixturing quality. Multi-setup parts have looser position tolerance. |
| Thread Tolerance | 6H/6g (ISO) | 4H/4g (ISO) | Standard thread class covers 95% of applications. Precision only for high-load fits. |
| Flatness | 0.05 mm per 100 mm | 0.01 mm per 100 mm | Thin parts may warp after machining. Stress relieving helps. |
| Parallelism | 0.05 mm | 0.01 mm | Requires single-setup machining for best results. |
| Perpendicularity | 0.05 mm | 0.01 mm | Critical for assembly interfaces. Specify only on mating surfaces. |
| Surface Finish (Ra) | Ra 1.6 µm (63 µin) | Ra 0.4 µm (16 µin) | Standard is as-machined. Mirror finish (Ra 0.1) requires polishing. |
| Concentricity (Turned Parts) | 0.025 mm TIR | 0.005 mm TIR | Single-setup turning gives best concentricity. |
| Angular Tolerance | ±0.5° | ±0.1° | Standard for most features. Precision for optical or mating angles. |
Key Considerations
Tolerance Only Where It Matters
Apply tight tolerances only to mating surfaces, bearing fits, and critical interfaces. Leave non-functional features at standard.
Cost Jumps at ±0.025 mm
Below ±0.025 mm, inspection moves from calipers to CMM. Every part must be individually measured, adding significant cost.
Use Standard Hole Sizes
Design holes to match standard reamer/drill sizes. Odd sizes require custom tooling or interpolation.
GD&T Over ± Tolerances
Geometric dimensioning and tolerancing (GD&T) communicates intent better. A true position callout is more useful than ± on X and Y.
💰 Cost Impact of Tolerances
Parts with tolerances below ±0.025 mm typically cost 2–3× more due to slower feeds, CMM inspection, and temperature-controlled environments.
Standard tolerances (±0.05 mm) add zero premium — this is normal machining capability for any competent shop.
Start with standard tolerances everywhere, then tighten only the 2–3 features that truly need it. This approach typically saves 30–50% vs. blanket tight tolerances.
⚠️ Common Tolerance Mistakes
- ⚠ Applying ±0.01 mm tolerance to every dimension — only 5–10% of features typically need precision tolerances.
- ⚠ Specifying surface finish Ra 0.8 or better on internal cavities that nobody sees or touches.
- ⚠ Tolerancing features across multiple setups — if two holes must be precisely positioned relative to each other, they should be machined in one setup.
- ⚠ Ignoring material effects: aluminum moves more with temperature than steel. Plastics can absorb moisture and swell.
- ⚠ Calling out flatness on thin sheet-like parts without considering residual stress from machining.
💡 Pro Tips
- ▸ Ask your machinist which tolerances are "free" (achievable without extra effort) — often tighter than you'd expect.
- ▸ For press-fit holes, specify H7/p6 or H7/s6 interference fits rather than arbitrary tight tolerances.
- ▸ If you need Ra 0.4 µm or better, specify it only on the specific faces that require it — not the whole part.
- ▸ Consider post-machining grinding or lapping for ultra-precision surfaces instead of trying to machine to final spec.
- ▸ Our factories' CNC machines are regularly calibrated and certified — ask for the latest calibration report if your application requires documentation.
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