Tolerances: tight only where it matters
Every tight tolerance means slower finishing passes, extra measuring and a higher chance of scrap. The cheapest drawing has a sensible general tolerance and tight tolerances only on features that need them.
Start with a general tolerance
Put a general tolerance in the title block, such as ISO 2768-mK. It covers every dimension without its own tolerance. Medium class (m) gives ±0.2 mm on dimensions from 6 to 30 mm, which any decent shop holds without extra effort.
Tighten only functional features
- Bearing seats, shaft fits and dowel holes: use ISO fits such as H7/g6.
- Mating faces and positions of bolt patterns: ±0.05 mm or a position tolerance.
- Everything else: general tolerance.
Rough cost effect
| Tolerance | Typical effect on that feature |
|---|---|
| ±0.2 mm and looser | Standard machining, no extra cost |
| ±0.05 mm | Finishing pass and measuring |
| ±0.02 mm | Careful finishing, temperature control, possibly reaming or boring |
| ±0.005 mm | Grinding, honing or lapping, inspection in a measuring room |
Draw your own part
Drop a STEP file into DrawMyPart. It finds every body, asks you to confirm threads, fits and tolerances, and draws a sheet your manufacturer can build from.
Open the tool