Tolerances and fits
No part is made exactly to size. Tolerances state how much variation is acceptable — and fits define how mating parts go together. Learn to read and specify them sensibly.
In this lesson you will
- Explain why tolerances are needed
- Read and write limit and ± tolerances
- Use general tolerance notes
- Understand clearance, transition and interference fits
Why tolerances
Every process has variation. A tolerance defines the acceptable range: tight enough for the part to work, loose enough to make economically. Over-tight tolerances increase cost and inspection effort without benefit.
Writing tolerances
- Bilateral: 50 ±0.1.
- Unilateral: ⌀10 +0.1/0.
- Limits: 49.9 / 50.1.
- General tolerances: a title-block note such as ISO 2768-m applies default tolerances to all untoleranced dimensions, so only critical ones need individual tolerances.
Fits
When a shaft goes into a hole, the combination of their tolerances gives a fit:
- Clearance fit: always a gap (sliding parts).
- Transition fit: may be a small gap or small interference (accurate location).
- Interference fit: always an overlap (pressed parts).
ISO 286 defines standard tolerance grades and fit designations such as H7/g6 (a close running clearance fit, hole basis). Use tables or calculators to find the actual limits.
| Fit (hole basis) | Type | Typical use |
|---|---|---|
| H11/c11 | Clearance (loose) | Loose running |
| H7/g6 | Clearance (close) | Sliding, precise running |
| H7/k6 | Transition | Accurate location |
| H7/p6 | Interference | Press fits |
Surface finish
Surface texture affects function (sealing, sliding, fatigue). It's specified with a surface texture symbol and a roughness value such as Ra 1.6 µm (ISO 21920 replaced ISO 1302 for indication). Specify it only where it matters.
Exercise
Specify a fit
- A 20 mm shaft must slide smoothly in a bearing housing bore.
- Choose a suitable ISO fit and look up the limits for hole and shaft from an ISO 286 table.
- Write both toleranced dimensions and the fit designation on a sketch.
- Add a general tolerance note to the title block.
You should end up with: A correctly specified clearance fit.
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Lesson quiz
A fit that always leaves a gap is…
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