CNC Milling Tolerances Explained: Standards, Costs & When to Use Each
Standard CNC milling tolerance is ±0.1mm (ISO 2768-m). Precision tolerances of ±0.01mm are achievable but cost 2–4× more and require fewer setups. Specify tight tolerances only on critical dimensions — typically mating surfaces, bearing fits, and assembly interfaces.
Tolerance choices are the single biggest cost driver in CNC milling. Over-specifying tolerances on non-critical dimensions can double your part price. This guide explains the standards, real-world costs, and how to specify tolerances correctly.
ISO 2768 General Tolerance Classes
ISO 2768 defines four general tolerance classes for linear dimensions without explicit tolerance callouts: - **Fine (f)**: ±0.05mm — high precision, expensive - **Medium (m)**: ±0.1mm — standard CNC milling, default at FormHub - **Coarse (c)**: ±0.3mm — rough machining, low cost - **Very coarse (v)**: ±0.5mm — preliminary or sand-cast parts
When to Specify Tight Tolerances
Tight tolerances (±0.01–±0.025mm) are justified for: - Bearing seats and shaft fits (ISO H7/h6) - Mating surfaces in assemblies - Sealing surfaces (O-rings, gaskets) - Optical or measurement reference surfaces For non-critical features (clearance holes, external profiles, decorative surfaces), use ISO 2768-m and save 30–60% on cost.
Cost Impact of Tolerance Tightening
Tightening tolerance from ±0.1mm to ±0.01mm typically requires: 1. Slower feed rates (1.5–2× cycle time) 2. Additional finishing passes 3. Climate-controlled measurement (CMM verification) 4. Tool wear monitoring per part Result: part cost increases 2–4× for tight-tolerance features.
Achievable CNC Milling Tolerances by Process
| Process | Standard | Precision | Cost Impact |
|---|---|---|---|
| 3-axis milling | ±0.1mm | ±0.025mm | 1.0× / 2.5× |
| 5-axis milling | ±0.05mm | ±0.01mm | 1.3× / 3.5× |
| Swiss turning | ±0.025mm | ±0.005mm | 1.5× / 4.0× |
| Wire EDM | ±0.005mm | ±0.002mm | 3.0× / 6.0× |
| Surface grinding | ±0.005mm | ±0.001mm | 2.5× / 5.0× |
Frequently asked questions
- What is the tightest tolerance achievable with CNC milling?
- On a calibrated 5-axis machine, ±0.005mm (5 microns) is achievable on small features under climate-controlled conditions. For tighter tolerances (1–2 microns), use surface grinding or wire EDM.
- How much does tighter tolerance cost?
- Going from ±0.1mm to ±0.025mm typically adds 30–80% to part cost. Going to ±0.01mm or tighter adds 150–300% due to slower cycle times, additional finishing, and CMM verification.
- What tolerance should I specify if I am unsure?
- Default to ISO 2768-m (±0.1mm) for general dimensions and only specify tighter tolerances on features that physically need them — typically bearing fits, sealing surfaces, and assembly interfaces.
- Do you measure every part?
- Standard inspection covers 3 critical dimensions per part with first-article inspection on every batch. Full CMM inspection of all dimensions is available on request and recommended for tolerances tighter than ±0.025mm.