Drawing interpretationReading time 12 min
H7/h6 Fits: Read the Hole and Shaft Together
A fit code is not just a pair of letters and numbers. It locates each tolerance zone around the nominal size and tells you the range of possible clearance. Here is how to read it without guessing.
01Reading a fit code
In an ISO fit designation, the letter positions the tolerance zone relative to the basic size; the IT number gives its width. Uppercase letters describe holes and lowercase letters describe shafts. You need both zones to determine the possible clearance or interference.
- 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 |
02Clearance, transition and interference
Place the hole and shaft tolerance zones on the same scale. Their extreme combinations tell you whether the assembly always has clearance, always has interference, or can have either depending on the actual sizes.
| 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
Dimensions without individual tolerances still have limits. Look near the title block for the general-tolerance note and apply the stated class only where no more specific requirement is given.
| 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 |
A note such as ISO 2768-mK combines a class for general linear and angular dimensions with a class for selected geometrical tolerances. Read both parts, and check which edition of the referenced standard the drawing uses.
04Common reading mistakes
- “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.