If you're sourcing CNC machined parts — whether for a prototype or a 10,000-piece production run — tolerances are the single most important specification on your drawing. They determine how precisely each feature must be manufactured, and they directly impact cost, lead time, and supplier selection.
Yet tolerances remain one of the most misunderstood aspects of CNC procurement. Over-tolerancing drives up costs unnecessarily. Under-tolerancing leads to parts that don't assemble or function correctly. As a factory owner who reviews hundreds of customer drawings every year, I see both mistakes daily. This guide will help you get tolerances right.
A tolerance defines the acceptable range of variation for a dimension. No manufacturing process produces a part at exactly the nominal dimension — there's always some deviation. Tolerances acknowledge this reality and define how much deviation is acceptable for the part to function correctly.
For example, a dimension of 25.00 mm ±0.05 mm means the actual part can measure anywhere between 24.95 mm and 25.05 mm and still be considered acceptable. That 0.10 mm total range is the tolerance band.
Here's a practical reference for what CNC machining can achieve across different tolerance classes:
| Tolerance Class | Typical Range | When to Use | Cost Impact |
|---|---|---|---|
| Commercial / Standard | ±0.10 mm (±0.004") | Non-critical dimensions, clearance holes, general features | Baseline |
| Precision | ±0.05 mm (±0.002") | Fits, alignment features, functional surfaces | 1.2–1.5× baseline |
| High Precision | ±0.025 mm (±0.001") | Bearing bores, sealing surfaces, critical assemblies | 1.5–2.5× baseline |
| Ultra Precision | ±0.010 mm (±0.0004") | Instrument-grade, optical, medical implants | 3–5× baseline |
| Grinding / Lapping | ±0.005 mm or tighter | Gauge blocks, spindle components, semiconductor | 5–10× baseline |
For most commercial CNC machined parts, ±0.05 mm is the sweet spot — achievable on any competent CNC machine without special setups, yet precise enough for most functional requirements.
The most straightforward type — they control the size of a feature. Examples include hole diameters, shaft diameters, lengths, and depths. Expressed as bilateral (±0.05) or unilateral (+0.00/−0.10) tolerances.
Geometric Dimensioning and Tolerancing controls the shape, orientation, and location of features relative to datums. Common GD&T callouts include:
GD&T is more precise than basic dimensional tolerancing because it controls not just size, but the geometric relationship between features. If your parts require critical fits or assemblies, GD&T is worth learning — it communicates your intent much more clearly than ± dimensions alone.
Surface roughness (Ra) is technically a tolerance on surface texture. Standard CNC machining produces Ra 1.6–3.2 µm. Ground surfaces achieve Ra 0.4–0.8 µm. Polished surfaces can reach Ra 0.1 µm or better. Surface finish specifications directly affect machining strategy, cycle time, and cost.
This is the most expensive mistake buyers make, and I see it constantly. A drawing comes in with ±0.01 mm on every dimension — including non-functional features like overall length, chamfer sizes, and clearance hole positions. The result?
The rule of thumb: Only apply tight tolerances to features that actually require them for function or assembly. Everything else should default to standard machining tolerances (±0.10 mm or whatever your general tolerance block specifies).
Every drawing should include a general tolerance block in the title block area. This sets the default tolerance for any dimension not explicitly toleranced. A typical block might read:
This way, you only need to call out specific tolerances on the critical features — everything else falls under the general tolerance.
For features that require tighter control, add the tolerance directly to the dimension. For example:
If your part mates with another component, tell your supplier. Sharing the mating part drawing or at least the mating dimensions helps us understand the intent behind the tolerance. We can often suggest tolerance combinations that are easier to manufacture while still ensuring proper fit.
Not all materials hold tolerances equally well. Material properties significantly affect what's practically achievable:
| Material | Practical Tight Tolerance | Notes |
|---|---|---|
| Aluminum (6061) | ±0.010 mm | Excellent machinability, watch for thermal expansion |
| Stainless Steel (304/316) | ±0.015 mm | Work-hardens; may need stress relief between ops |
| Brass (C360) | ±0.008 mm | Best machinability, excellent for tight tolerance parts |
| Titanium (Ti-6Al-4V) | ±0.020 mm | Springback and heat issues; requires experienced setup |
| Plastics (Delrin, PEEK) | ±0.025 mm | Thermal expansion and moisture absorption are concerns |
For more on material selection and how it affects your CNC project, see our aluminum alloy selection guide and stainless steel machining guide.
After running a CNC shop for years, here's what I wish every buyer knew about tolerances:
Getting tolerances right is one of the most impactful things you can do to reduce costs and improve part quality. For a deeper dive into cost optimization, read our guide on how to reduce CNC machining costs without sacrificing quality, and for RFQ best practices, check out our CNC RFQ guide.
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