Specifying tolerances is one of the fastest ways to move part cost — in either direction. This guide covers what's standard, what's premium, and how to keep drawings manufacturable without giving up what matters.
What 'standard tolerance' actually means
Unless a drawing says otherwise, machine shops quote to ISO 2768-m (medium) for machined features — roughly ±0.1 mm below 100 mm and ±0.2 mm above. At SkillsProto, our default general tolerance for CNC machining is ±0.02–0.05 mm on critical features, and we always ask which dimensions truly matter before pricing.
When you actually need ±0.005 mm
Bearing bores, sealing faces, dowel-fit holes and gauge-critical interfaces justify tight tolerances. Cosmetic features, pocket floors and hole positions in non-locating patterns usually don't. A useful discipline: mark only functional dimensions with tolerances and leave the rest general — every tight callout adds inspection time and scrap risk.
- General features: ISO 2768-m (±0.1 mm class)
- Functional fits: ±0.02–0.05 mm
- Precision interfaces (bearings, gauges): ±0.005–0.01 mm with CMM verification
Cost drivers hiding in tolerances
Halving a tolerance rarely doubles cost at the ±0.05 mm level — but pushing below ±0.01 mm triggers a different league: slower feeds, extra setup, climate-controlled measurement and higher scrap allowance. If only one bore needs it, isolate that bore with a local callout instead of tightening the whole drawing.
How we handle it
Every SkillsProto quote includes a DFM pass. If a tolerance looks unnecessarily tight — or dangerously loose for a sealing feature — the engineer flags it with a recommendation. Get in touch with your STEP file and we'll review it within 24 hours.
Frequently Asked Questions
What is a standard CNC machining tolerance?
Unless the drawing specifies otherwise, machined features are quoted to ISO 2768-m — roughly ±0.1 mm for features under 100 mm. On critical features SkillsProto holds ±0.02–0.05 mm as standard practice, with ±0.005–0.01 mm available on precision interfaces and verified by CMM.
When is ±0.005 mm actually worth specifying?
Only on true functional interfaces: bearing bores, dowel or press fits, sealing faces and gauge-critical datums. Cosmetic surfaces and non-locating hole patterns don't need it — every tight callout adds inspection time and scrap risk, so mark only the dimensions that carry function.
Does a tighter tolerance always cost more?
Below roughly ±0.05 mm, yes — slower feeds, extra setups, climate-controlled measurement and a higher scrap allowance all add up. If only one bore needs precision, isolate it with a local GD&T callout instead of tightening the whole drawing.
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