Category: CNC Machining Reading time: 7 min Meta description: A practical guide to CNC machining tolerances — what standards exist, how much precision costs, and when to avoid over-specifying. URL: /blog/cnc-machining-tolerances/ Tags: cnc, tolerances, precision, quality, manufacturing
One of the most common mistakes buyers make is over-specifying tolerances. A drawing calling for ±0.01mm across every dimension drives up cost dramatically — and often doesn't improve the part's function.
Understanding what tolerances are actually achievable, and what they cost, is essential knowledge for anyone buying CNC machined parts.
There is no universal standard, but the industry generally considers:
| Tolerance Class | Range | Typical Application | Relative Cost |
|---|---|---|---|
| Standard | ±0.125mm (±0.005") | General fitting, non-critical surfaces | 1x (baseline) |
| Fine | ±0.05mm (±0.002") | Mating parts, bearing fits | 1.3-1.5x |
| Precision | ±0.025mm (±0.001") | Precision assemblies, jig/fixture critical | 1.5-2x |
| High Precision | ±0.005mm (±0.0002") | Aerospace, medical implants, optical | 2.5-4x |
| Ultra Precision | ±0.001mm (±0.00004") | Specialized (requires grinding/lapping) | 5-10x |
Note: These are linear dimensional tolerances. Geometric tolerances (flatness, concentricity, parallelism) follow different rules and are generally tighter than linear tolerances.
A well-maintained 3-axis VMC (Vertical Machining Center) typically holds ±0.05mm in production. A precision-grade machine can hold ±0.005mm — but only in controlled conditions.
Aluminum (6061) is the most forgiving material for tight tolerances. Materials that create problems:
| Geometry | Tolerance Challenge |
|---|---|
| Thin walls (<3mm) | Deflection during cutting |
| Deep cavities (L:D > 4:1) | Tool deflection, chatter |
| Large parts (>500mm) | Thermal expansion, machine envelope limits |
| Complex freeform surfaces | Machine interpolation accuracy |
Holding ±0.005mm requires temperature-controlled shops. A 10°C temperature change expands a 300mm aluminum part by ~0.07mm — enough to push it out of spec.
Here's a simplified cost breakdown for a typical 100×100×50mm CNC bracket:
| Tolerance | Setup & Inspection | Reject Rate | Cycle Time Impact | Price/part (qty 100) |
|---|---|---|---|---|
| ±0.1mm | Standard | <1% | None | $18-25 |
| ±0.05mm | Extra inspection, offsets checked mid-run | 2-3% | +10% | $25-35 |
| ±0.02mm | Dedicated inspection fixture, CMM sampling | 5-8% | +30% | $38-55 |
| ±0.01mm | 100% CMM inspection, temperature control | 10-15% | +50% | $60-90 |
| Feature | Suggesting Tolerance |
|---|---|
| Non-functional surfaces, clearance holes | ±0.2mm to ±0.5mm |
| General dimensions, mounting holes | ±0.1mm |
| Mating surfaces, shaft fits | ±0.025mm to ±0.05mm |
| Bearing pockets | ±0.0125mm to ±0.025mm |
| Precision bores (H7 fit) | ±0.01mm to ±0.02mm |
Don't call ±0.01mm on:
| Process | Typical Production Tolerance | Best Possible |
|---|---|---|
| 3-axis milling | ±0.05mm | ±0.005mm |
| 5-axis machining | ±0.05mm | ±0.005mm |
| CNC turning | ±0.025mm | ±0.0025mm |
| Swiss machining | ±0.0125mm | ±0.002mm |
| Grinding | ±0.0025mm | ±0.0005mm |
| EDM (wire) | ±0.005mm | ±0.001mm |
| EDM (sinker) | ±0.0125mm | ±0.0025mm |
| Standard | Scope | Commonly Used For |
|---|---|---|
| ISO 2768-f | General tolerances (fine) | European general parts |
| ISO 2768-m | General tolerances (medium) | European most common |
| ASME Y14.5 | GD&T standard | North America |
| ISO 8015 | Tolerancing principles | International |
| GB/T 1804 | Chinese general tolerances | Chinese domestic |
Good practice:
Bad practice:
| If you... | Recommended Approach |
|---|---|
| Don't know what to specify | Use ±0.1mm as default, tighten only on critical fits |
| Need precision mating parts | Call ±0.025mm on mating surfaces, ±0.1mm everywhere else |
| Have a high-volume part | Design for ±0.1mm or looser to maximize yield |
| Need ±0.005mm or better | Choose grinding or precision EDM, plan for 100% inspection |
| Are machining plastic | Loosen tolerances 2x vs metal; material relaxes after cutting |
Submit your drawing with tolerance callouts to app.moldkey.com/quote and our partner shops will verify your requirements before quoting.