cnc tolerance chart
CNC Tolerance Chart — What Each Band Requires
A tolerance is a cost instruction before it is a quality requirement. Put ±0.01 mm on a dimension that only locates a cover and the part gets slower to make and more expensive to inspect, with nothing gained. Put no tolerance on a dimension that does need one and the first article fails for a reason nobody agreed on.
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- Standard
- ISO 2768-m general tolerance
- Turning
- ±0.01 mm routine, ±0.005 mm with grinding
- Verification
- Micrometer / gauge, 20 °C reference
- Cost step
- Tighter than ±0.01 mm adds an operation
The chart below is the one we work to internally. It is not a marketing claim about capability — it is the band each process holds before the price steps up, which is the part a buyer needs in order to specify a part that can be made at a sensible cost.
With a drawing vs without: what we default to
Most tolerance disputes do not start with a wrong number on the drawing. They start with a dimension that was never toleranced at all, where the supplier has to assume something. This is the assumption we make, and it is worth knowing before the quote rather than after the parts arrive.
| Dimension | Metal, drawing given | Metal, no drawing | Plastic, drawing given | Plastic, no drawing |
|---|---|---|---|---|
| Linear dimension | ±0.01 mm | ISO 2768-fine | ±0.05 mm | ISO 2768-medium |
| Hole diameter | ±0.02 mm | ISO 2768-fine | ±0.06 mm | ISO 2768-medium |
| Shaft diameter | ±0.02 mm | ISO 2768-fine | ±0.07 mm | ISO 2768-medium |
| Flatness / parallelism | ±0.01 mm | ISO 2768-fine | ±0.03 mm | ISO 2768-medium |
| Perpendicularity | ±0.01 mm | ISO 2768-fine | ±0.03 mm | ISO 2768-medium |
Read the table as two questions in one grid. The first is material: plastics move more than metals under the same cut, because they deflect under clamping force and expand with heat, so the same process holds a wider band. The second is information: when a dimension is toleranced on the drawing, we cut to that number; when it is not, we fall back to the ISO 2768 class that applies, and the number on the right is what the part will actually measure.
Two consequences that affect the price. A dense drawing is not necessarily expensive — it is the opposite: a dimension that is toleranced is a dimension we can plan around. An empty drawing is not cheap either, because we have to quote the default band and the inspection that goes with it, and a part that later fails against an unstated expectation costs a rework that a drawing would have prevented.
Final achievable tolerance depends on the material, the geometry, the part size and the operation. The table is the starting point for a quotation; the drawing review is what confirms it.
Specifications
| General tolerance | ISO 2768-m unless stated on the drawing |
|---|---|
| Turning | ±0.01 mm routine · ±0.005 mm with grinding |
| Milling | ±0.02 mm profile · ±0.01 mm bored feature |
| Drilled / reamed hole | ±0.1 mm drilled · ±0.02 mm reamed |
| Surface finish | Ra 1.6 standard · Ra 0.8 by grinding |
| Verification | Micrometer, bore gauge, go/no-go gauges, 20 °C reference |
Frequently asked questions
What tolerance can CNC machining hold?
±0.01 mm on turned diameters and bored features as standard. ±0.005 mm is achievable with grinding plus an agreed datum and measurement temperature. Below that, the limit is measurement rather than the machine.
What is ISO 2768-m?
A general tolerance standard for linear and angular dimensions that are not individually toleranced. 'm' is the medium class, which is the usual default for machined parts that are not precision features.
Is a tighter tolerance always better?
No. A tolerance tighter than the function needs adds cost and inspection time without improving the part. The test is whether the part fails at the loose end of the band — if not, the callout is buying nothing.
Why does grinding cost extra?
Because it is a separate operation with its own setup, wheel and fixture, and it is the route for bands below ±0.005 mm or finishes below Ra 0.8. It is quoted as a line rather than absorbed into the turning price.
How is a thread tolerance different?
Threads are specified by class — 6g external, 6H internal — not by a millimetre band. A millimetre tolerance on a thread callout does not describe the pitch diameter, and the two are verified with gauges rather than micrometers.
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