Tolerance is the most expensive line item on a CNC quote that engineers don’t realize they are buying. Tightening from ±0.005″ to ±0.0005″ can double a part’s cost — yet half the time the looser spec was already adequate.
Standard Bands
Our shop floor defaults follow ISO 2768-mK unless drawings specify otherwise:
- Coarse ±0.010″ — fixturing brackets, weldments, non-mating cosmetic parts
- Medium ±0.005″ — most production milling, default unless called out
- Fine ±0.001″ — bearing seats, alignment surfaces
- Ultra-fine ±0.0002″ — optical mounts, metrology fixtures (grind territory)
If a feature does not mate, slide, or seal — it does not need fine tolerance. Cost is exponential below ±0.001″.
Material matters
Aluminum machines clean to ±0.0005″ without much drama. 304 stainless work-hardens, so the same tolerance on stainless costs roughly 1.6× more in cycle time and tool wear. Hardened tool steels (above 50 HRC) require grinding past ±0.001″.