Quick Answer
CNC machining tolerances — standard CNC tolerance is ±0.05 mm (±0.002 inch) for most features. Tighter tolerances down to ±0.01 mm are achievable but cost more. Looser tolerances (±0.1–0.2 mm) are used for non-critical features. Specifying the right tolerance on each feature — not a global tolerance — is the single most effective way to reduce machining cost without sacrificing part quality.
What Is a Tolerance?
A tolerance is the acceptable range of deviation from a specified dimension. If a dimension is 50.00 mm with a tolerance of ±0.05 mm, any part between 49.95 mm and 50.05 mm is acceptable.
Why tolerances matter:
- Tolerances that are too tight increase machining cost significantly
- Tolerances that are too loose cause assembly problems, poor fit, and field failures
- Every feature should have the tolerance it needs — not the tightest tolerance achievable
Standard CNC Tolerances
| Tolerance Level | Typical Range | When Appropriate | Cost Impact |
|---|---|---|---|
| Standard | ±0.05 mm (±0.002″) | Most machined features | Baseline |
| Precision | ±0.02 mm (±0.001″) | Fit features, bearings, gears | +20–50% |
| High precision | ±0.01 mm (±0.0005″) | Critical fits, optical mounts | +50–100% |
| Ultra-precision | ±0.002–0.005 mm | Instrumentation, metrology | +200–500% |
| Coarse | ±0.1–0.2 mm | Non-critical bulk material, casting cleanup | −10–20% |
Tolerance Chain and Stack-Up
When multiple machined parts must fit together, tolerances stack. The total variation in an assembly is the sum of individual feature variations.
Example: A shaft must fit into a bore with 0.05 mm clearance.
- Bore diameter: 20.00 ±0.05 mm → range: 19.95–20.05 mm
- Shaft diameter: 19.95 ±0.05 mm → range: 19.90–20.00 mm
- Worst-case clearance: 0.00 mm (tight fit) to 0.15 mm (loose fit)
- Statistical stack-up: typically 0.025–0.05 mm clearance in practice
Buyer implication: If you need a specific clearance, each individual tolerance must be tighter than if you calculate the assembly variation. Work with your supplier to specify the assembly requirement, and let them determine the feature tolerances to achieve it.
GD&T: A Better Way to Specify Tolerance
Geometric Dimensioning and Tolerancing (GD&T per ASME Y14.5 or ISO 1101) is more precise than simple plus/minus dimensioning:
| GD&T Symbol | Controls | Example Application |
|---|---|---|
| Position | Location of features relative to datums | Hole location relative to mounting face |
| Perpendicularity | 90° deviation from datum | Bore axis perpendicular to base |
| Parallelism | Parallel deviation from datum | Two surfaces parallel within tolerance |
| Flatness | Surface flatness deviation | Gasket face flatness |
| Circularity (Roundness) | Deviation from perfect circle | Shaft or bore roundness |
| Cylindricity | Combined roundness, straightness, and taper | Bore cylindricity |
| True Position | Actual location vs. theoretically exact location | Hole pattern accuracy |
Why GD&T is better for buyers:
- Clearer communication of functional requirements
- Less ambiguity — one symbol says more than a paragraph of notes
- Enables statistical tolerance analysis
- Used universally in precision manufacturing
Tolerance by Feature Type
| Feature | Typical Achievable Tolerance | Notes |
|---|---|---|
| Linear dimensions (hole depth, step height) | ±0.025–0.05 mm | Standard CNC |
| Diameter (turned or milled bore) | ±0.02–0.05 mm | Standard CNC; turning gives tighter control |
| Hole location (X/Y position) | ±0.05 mm (100% inspection) | Standard CNC; jig boring for tighter |
| True position of hole pattern | ±0.05–0.10 mm (normal) | Tighter requires special fixturing |
| Surface flatness | 0.02–0.05 mm over 100 mm | Surface plate inspection required |
| Perpendicularity (face to face) | 0.02–0.05 mm over 100 mm | Standard for milled parts |
| Surface roughness (Ra) | Ra 0.8–3.2 µm standard | Ra 0.4 µm possible with grinding |
Cost Impact of Tight Tolerances
Tighter tolerances cost more because they require:
- More expensive machine tools (precision CNC vs standard)
- More expensive cutting tools (solid carbide vs coated carbide)
- Temperature control (thermal expansion affects accuracy)
- More measurement and inspection time
- Specialized fixturing and setup
- More skilled operators
- Possibly multiple setups or dedicated machines
Cost curve:
- Standard tolerance (±0.05 mm): baseline cost
- Precision tolerance (±0.02 mm): +20–50% cost
- High precision (±0.01 mm): +50–100% cost
- Ultra-precision (±0.002 mm): 3–5× baseline cost, or more
Specifying Tolerances on Your Drawing
Common mistakes buyers make:
- Specifying a global tolerance (±0.025 mm) on all features — even those that do not need it
- Not specifying GD&T, leaving tolerance interpretation to the machine shop
- Assuming “standard tolerance” means the same thing to every supplier
- Not specifying datum surfaces, making tolerance control ambiguous
What buyers should specify:
- Global tolerance in the title block (standard for most features)
- Specific tolerances on critical features that need tighter control
- GD&T on features where form, orientation, or location matter
- Datum reference frame (which surfaces are the measurement references)
- Maximum material condition (MMC) where applicable for gauge design
- Surface finish requirements (Ra) separate from dimensional tolerance
Tolerance and Process Selection
The achievable tolerance depends on the manufacturing process:
| Process | Best Tolerance | Cost at Best Tolerance |
|---|---|---|
| Laser cutting | ±0.1–0.2 mm | $ |
| CNC milling (standard) | ±0.05 mm | $$ |
| CNC turning | ±0.025–0.05 mm | $$ |
| CNC milling (5-axis, precision) | ±0.01–0.02 mm | $$$ |
| Jig grinding / precision grinding | ±0.002–0.005 mm | $$$$ |
| EDM (wire or sinker) | ±0.01–0.03 mm | $$$$ |
Inspection and Verification
You get the tolerance you measure for. Inspection methods range from simple to complex:
- Calipers and micrometers: For general dimensional checking; ±0.01–0.02 mm accuracy
- Height gauge / surface plate: For flatness, perpendicularity, and 2D measurements
- CMM (Coordinate Measuring Machine): For complex geometries and GD&T verification; ±0.002–0.005 mm accuracy
- Optical comparators: For 2D profile checking of small parts
- Thread gauges: For thread acceptance (go/no-go)
Buyer requirement: Specify inspection method when tolerance is ±0.02 mm or tighter. A CNC machine with good repeatability can hold ±0.01 mm, but verifying it requires CMM or equivalent.
Buyer Checklist: Tolerancing
- Have I specified the tolerance each feature actually needs, not a global tight tolerance?
- Have I used GD&T where form, orientation, or location matters?
- Is the datum reference frame clearly defined?
- Have I separated surface finish (Ra) from dimensional tolerance?
- Is the tolerance achievable by the intended manufacturing process?
- Have I specified the inspection method for tolerances ±0.02 mm or tighter?
- Have I specified GD&T modifiers (MMC/EQR) where needed for gauge design?
Before finalizing the sourcing decision, many OEM buyers also compare CNC Machining for Cast Parts, Casting Tolerances, DFM Review, and First Article Inspection to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
FAQ
What is standard CNC tolerance?
±0.05 mm (±0.002 inch) is the standard achievable tolerance for most CNC machining on standard equipment. Tighter tolerances are possible but cost more. Always specify — do not assume the machine shop will use the tightest possible tolerance.
Can CNC hold ±0.01 mm consistently?
Yes — with the right equipment (precision CNC machines, controlled temperature, quality cutting tools), ±0.01 mm is achievable consistently. However, it requires more care, more inspection, and higher cost than standard ±0.05 mm. Only specify it where it is functionally required.
What is MMC (Maximum Material Condition)?
MMC specifies a feature at its largest condition (maximum material). When used with GD&T position tolerance, it allows a bonus tolerance — the hole can be larger and the position tolerance can increase proportionally. This is useful for producing the easiest-to-manufacture part that still fits the assembly.