CNC Machining Tolerances Explained
What ±0.05 mm and tighter callouts actually mean: which features they apply to, why setup count dominates stack-up, and where thermal expansion takes over.
Xinyuanmu Engineering · Published September 7, 2026 · Updated October 12, 2026 · 3 min read
A CNC machining tolerance is the total permissible variation on a dimension or geometric characteristic. General machined dimensions are typically held to ±0.05 mm, with tighter bands on small critical features; achievable tolerance depends on feature size, material, setup count and measurement temperature rather than on the machine specification alone.
Key points
- A ±0.05 mm callout permits 0.1 mm total variation, not 0.05 mm.
- Setup count is usually a larger error source than the machine itself.
- Thermal expansion dominates on parts above roughly 200 mm.
- Tolerance tighter than the process can hold repeatably should trigger a process change, not more inspection.
What the number on the drawing means
A tolerance defines the total permissible variation around nominal. A bilateral callout of ±0.05 mm allows 0.1 mm of total movement; a unilateral callout of +0.02/−0.00 mm allows a total of 0.02 mm. The distinction matters because quoting or inspecting against the wrong interpretation doubles or halves the real requirement.
Feature versus process
The same process produces very different results depending on the feature:
| Feature | Realistic band | Dominant error source |
|---|---|---|
| Turned diameter, small part | ±0.05 mm | Tool wear and machine positioning |
| Milled pocket depth | ±0.05 mm | Tool deflection |
| Hole position, one setup | ±0.02 mm | Machine positioning |
| Hole position, second setup | ±0.05–0.1 mm | Repositioning and fixture repeatability |
| Flatness, thin plate | Controlled band, by inspection | Residual stress release |
| Surface roughness | Ra 0.4 µm achievable | Finishing pass strategy |
Why setup count dominates stack-up
Every time a part is re-clamped, the fixture contributes a new positional error and the datums may shift. On multi-setup work, this is usually the largest single contributor to out-of-tolerance features — larger than the machine’s positioning accuracy. This is why 5-axis machining often improves accuracy, not just speed: it cuts the number of setups.
The practical rule: minimise setups on tolerance-critical features. If two features must be related tightly, machine them in the same setup wherever the geometry allows.
Where thermal expansion takes over
Above roughly 200 mm, thermal movement becomes the dominant error. Aluminium moves about 23 µm per metre per degree Celsius; steel about 11 µm. A long aluminium part measured in a warm inspection area can read outside a tight tolerance without anything being wrong with the machining. Temperature-controlled inspection and consistent measurement conditions matter as much as the machine for large parts.
Inspection cannot create capability
If a process cannot hold a band repeatably, inspecting more often does not fix it. The correct response is a drawing change, a process change, or a different operation such as grinding. A supplier that accepts an impossible tolerance and inspects around a known capability gap is transferring the failure to your line.
What this means for your drawing
- Specify tight tolerances only on the features that actually control function.
- Allow general tolerances elsewhere, and state the standard (for example ISO 2768-m).
- Think about datum strategy before release: a well-chosen datum can remove a setup.
- Expect tighter bands to be confirmed by first-article inspection, not assumed.
Related reading
- What is CNC machining?
- DFM checklist for CNC parts
- Tight-tolerance machining
- CNC machined precision parts
This article describes general manufacturing practice for machined and fabricated components. It is not a substitute for a drawing-specific engineering review — tolerances, material selection and process capability depend on the individual part.