Converting an ISO 286 fit into numerical limits
On a drawing, a dimension toleranced with a fit can’t be inspected as it stands.
⌀24 H7 gives no limit: you have to look up, in ISO 286, the fundamental
deviation of the letter and the tolerance interval of the number, then apply
them to the nominal diameter. It’s a calculation every inspector redoes dozens
of times per drawing.
Here is the method, the values used most often, and an example carried through to the conformity verdict.
What a designation contains
⌀24 H7 reads in three parts.
- 24 is the nominal size, in millimetres.
- H is the position letter, or fundamental deviation. It places the tolerance interval relative to the zero line. Capital letters denote holes (internal features), lower-case letters denote shafts (external features).
- 7 is the grade, or tolerance degree. It gives the width of the interval, written IT7.
The letter says where the interval sits, the number says how wide it is. Both combine with the size range the nominal falls into.
Interval widths for grade 7
The IT width depends on the range the nominal falls into, not on its exact value. A ⌀81 and a ⌀119 share the same width, because both fall in the 80 to 120 mm range.
| Nominal range (mm) | IT7 |
|---|---|
| over 3 up to 6 | 12 µm |
| over 6 up to 10 | 15 µm |
| over 10 up to 18 | 18 µm |
| over 18 up to 30 | 21 µm |
| over 30 up to 50 | 25 µm |
| over 50 up to 80 | 30 µm |
| over 80 up to 120 | 35 µm |
| over 120 up to 180 | 40 µm |
Watch the range boundaries: they are excluded at the bottom and included at the top. A nominal of 30 mm belongs to the 18 to 30 range, not to the 30 to 50 range. It’s a classic mistake when reading the table quickly.
The most common positions
Three letters cover the vast majority of mechanical drawings.
H, hole with zero lower deviation. The lower deviation is 0, the upper deviation is IT. The size can only be larger than nominal.
h, shaft with zero upper deviation. The upper deviation is 0, the lower deviation is minus IT. The size can only be smaller than nominal.
js, symmetrical interval. The interval is split evenly either side of the nominal, i.e. plus or minus IT divided by two.
For a g shaft, the upper deviation is negative and also depends on the range: minus 6 µm between 10 and 18 mm, minus 7 µm between 18 and 30, minus 9 µm between 30 and 50, minus 10 µm between 50 and 80.
An example carried through to the end
Take a hole dimensioned ⌀24 H7, measured at 24.026 mm.
- Range. 24 falls over 18 and up to 30. The table gives IT7 equal to 21 µm, i.e. 0.021 mm.
- Position. The letter H sets the lower deviation at 0. The upper deviation is therefore plus 0.021 mm.
- Limits. The size is good between 24.000 and 24.021 mm.
- Verdict. The reading, 24.026 mm, exceeds the upper limit by 5 µm. The characteristic is out of tolerance.
Five microns are hard to spot on an inspection sheet filled in by hand. The measured value looks like the nominal, the line looks right, and you have to redo the limit calculation to notice the overshoot.
The same reasoning for a ⌀112 h7 shaft gives, in the 80 to 120 range, an IT7
of 35 µm and a zero upper deviation: the limits are 111.965 and 112.000 mm.
Where the calculation goes wrong in practice
Three situations come up again and again.
A nominal exactly on a range boundary. A ⌀30 h7 and a ⌀31 h7 don’t have the same tolerance, even though the dimensions look alike on the drawing.
Mixing up capital and lower case. An H7 read as h7 reverses the direction of the interval: a part inspected as good becomes bad, and the other way round.
A dimension without a fit. When a dimension has no tolerance of its own, it falls under the general tolerance given in the title block, such as ISO 2768-mK. So it has limits too, also depending on the size range, which have to be looked up in a second table.
This table work is exactly what CoteCote does when it reads the drawing: the detected fit is converted into numerical limits for the nominal it read, and the comparison with the reading happens as you enter it. The features page details the exact coverage.