Tolerance callouts on drawings directly drive machining cost. Tight to ±0.005 mm vs ±0.1 mm can mean 5-10× the price.
Three tolerance systems
- ISO 2768 — general tolerances (default when not otherwise specified). Four grades: f (fine), m (medium), c (coarse), v (very coarse).
- IT grades — precision fits for shafts and holes. IT5-IT6 for bearings, IT7 standard for gears and slides, IT8 for general fits.
- GD&T — geometric dimensioning & tolerancing. Symbols for straightness, roundness, perpendicularity, parallelism, concentricity, position, profile. Always paired with a Datum.
Surface roughness Ra
Ra 6.3 for rough machining → Ra 0.4 for sealing fits → Ra 0.1 for mirror polish. Each step tighter adds ~20-50% cost.
Common over-specification mistakes
- Marking every dimension to ±0.005 mm (most don't need it — use ISO 2768-m default + tighten only the critical ones)
- Specifying Ra 0.4 across the board (Ra 1.6 covers most fits)
- Using GD&T without a Datum reference (meaningless — the shop can't measure "relative to what")
Sensible starting spec
- General dimensions: ISO 2768-m
- Shaft/hole fits: IT7 (H7/h6 or H7/g6)
- Surface: Ra 3.2 general, Ra 1.6 for fits
- GD&T: 2-3 critical features at ~0.05 mm
If you're unsure about tolerances, share the part's use case + mating partner + load conditions — we'll help define a sensible spec.
