A tolerance is the allowed variation of a dimension. Tighter tolerances need more machine time, more inspection and sometimes different processes — so they cost more. The best drawings apply tight tolerances only where the function requires them.
General Tolerances: ISO 2768
When a dimension has no individual tolerance, the general tolerance class on the drawing applies. ISO 2768-1 defines four classes for linear dimensions:
| Nominal size (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
|---|---|---|---|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 | — |
| over 3 – 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| over 6 – 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| over 30 – 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| over 120 – 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| over 400 – 1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
ISO 2768-m is a common default for machined metal parts.
Our Tolerance Capability
Standard: [TBC: tolerance_standard]. Tightest on critical features: [TBC: tolerance_best].
GD&T
Geometric dimensioning and tolerancing (GD&T) controls form, orientation, location and runout — for example flatness, perpendicularity, position and concentricity — and communicates functional requirements more clearly than +/- tolerances alone.
How to Reduce Tolerance Cost
- Use a general tolerance class and tighten only functional features.
- Dimension from functional datums.
- Avoid tight tolerances on features affected by coating thickness.
- Ask for a DFM review — we will point out tolerances that drive cost.
Frequently Asked Questions
What is a standard CNC machining tolerance?
Many machine shops work to about ±0.1 mm or ISO 2768-m by default when no tolerance is specified. Tighter tolerances are achievable on specific features at higher cost.
What is the difference between ISO 2768-f and ISO 2768-m?
Class f (fine) has roughly half the allowed variation of class m (medium) for the same size range, and costs more to hold.
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