Drawing interpretationReading time 12 min
Does H7 on h6 come out loose or tight?
Those five characters, Ø20 H7, carry three pieces of information: the basic size, where the tolerance zone sits, and how wide it is. Read them properly and you stop reaching for the table every time.
01The two halves of a tolerance designation
An ISO 286 designation is a letter plus a number. The letter sets where the tolerance zone sits relative to the basic size; the number (the IT grade) sets how wide it is.
- Upper case is used for holes (internal features), lower case for shafts (external features).
- Letters run A to Z. A–G give a hole on the large side (clearance), H puts the lower deviation exactly at zero, and J–ZC give a hole on the small side (interference). Shafts are the mirror image: a–g run small, h puts the upper deviation at zero, and j–zc run large.
- IT grades run from IT01 to IT18, and the smaller the number the tighter the tolerance. The same IT grade means a different actual value at different basic sizes, the larger the size, the wider the tolerance.
| IT grade | Value at Ø20 | Typical use |
|---|---|---|
| IT5 | 9 μm | Gauges, precision bearing fits |
| IT6 | 13 μm | Precision fits, gear shafts |
| IT7 | 21 μm | Ordinary fitting surfaces (by far the most common) |
| IT8 | 33 μm | Looser fits |
| IT9 | 52 μm | Non-mating surfaces, keyways |
| IT11 | 130 μm | Stampings, machined faces on castings |
02Three kinds of fit
Overlay the hole and shaft tolerance zones and you get the character of the fit.
| Nature | Definition | Common combinations (hole basis) | Uses |
|---|---|---|---|
| Clearance fit | The hole is always larger than the shaft | H7/g6、H7/f7、H8/e8 | Plain bearings, guide pillars |
| Transition fit | May end up with clearance or with interference | H7/k6、H7/m6、H7/js6 | Needs location but must come apart |
| Interference fit | The shaft is always larger than the hole | H7/p6、H7/r6、H7/s6 | Bearing outer races, pressed-in bushes |
What to think about with an interference fit
- Press force is proportional to the interference; too much and a thin-walled part splits.
- Shrink and freeze fitting. Heating the outer part or chilling the shaft cuts assembly force dramatically, but check that the temperature swing will not affect material properties.
- Surface roughness gets flattened, so the real interference is less than the drawing dimension. That is why interference surfaces usually carry a tighter Ra requirement as well.
03General tolerances: the dimensions with nothing written on them
No drawing tolerances every dimension. The ones left bare fall under the general tolerance, set by a line near the title block such as “untoleranced dimensions per ISO 2768-mK”.
| Class | Code | Linear tolerance, 6–30 mm | What it asks |
|---|---|---|---|
| Fine | f (fine) | ±0.1 mm | Precision machinery |
| Medium | m (medium) | ±0.2 mm | The most common |
| Coarse | c (coarse) | ±0.5 mm | General structural work |
| Very coarse | v (very coarse) | ±1.0 mm | Castings, flame cutting |
The second letter in the code (H, K or L) refers to general geometric tolerances: straightness, flatness, perpendicularity and symmetry also have defaults when nothing is called out. So “ISO 2768-mK” actually sets two default systems at once.
04Three common misreadings
- “H7 means ±0.01.” It does not. H7 always has a lower deviation of zero, so the whole tolerance sits on the plus side, and the value changes with basic size.
- “Tighter is safer.” Putting IT6 on a dimension that does not need accuracy raises cost without changing function. Tolerances should be derived backwards from what assembly requires.
- “General tolerances do not matter.” Eighty per cent of the dimensions on a drawing depend on them. Swap 2768-m for 2768-f and the difficulty of the whole drawing changes.