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Every tolerance on a drawing is a promise the workshop has to keep, and an inspector has to check. The tighter the number, the more it costs to keep that promise. A good drawing puts tight tolerances only where the part's function needs them.
Why tighter costs more
Moving from a normal tolerance to a tight one rarely changes the machine. It changes how the part is made: slower finishing passes, an extra operation such as grinding, more careful setups, temperature control, and more measuring. Each step down the scale adds time, and sometimes a scrapped part.
Start from what the feature does
- Fits: bearing seats, bores for shafts and locating pins need a defined fit, such as H7/g6 or H7/k6. Use a fits table rather than guessing a number.
- Locating features: faces and holes that position the part in an assembly may need a position or flatness tolerance relative to a datum.
- Everything else: outside profiles, clearance holes and pockets usually work at general tolerances.
Use general tolerances for the rest
A note such as ISO 2768-mK sets sensible default tolerances for every dimension without its own tolerance. It keeps the drawing readable and stops the workshop from guessing. Only the features that matter then carry their own callouts.
Match the tolerance to the process
Scroll the table sideways to see all columns on a small screen.
| Process | Normal practice | Tight, at extra cost |
|---|---|---|
| CNC milling and turning | ±0.1 mm | ±0.01 mm |
| Grinding | ±0.02 mm | ±0.002 mm |
| Laser cutting | ±0.25 mm | ±0.1 mm |
| Investment casting | ±0.5 mm | ±0.1 mm |
| Sand casting | ±3 mm | ±0.5 mm |
These are general figures for features up to about 100 mm. Size, material and setup shift them. If a cast or cut part needs a tighter number on one feature, it is usually cheaper to machine that feature afterwards than to tighten the whole process.
A quick checklist
- Does every tight tolerance protect a fit, a seal or a location?
- Is there a general tolerance note for everything else?
- Are datums chosen from the surfaces that locate the part in use?
- Can the chosen process hold the number without an extra operation?
