Tolerances determine both the function and the price of a part. Too loose, and it does not fit; too tight, and it becomes unnecessarily expensive. The good news: for most dimensions a single note in the title block is enough.
General tolerances to ISO 2768-1
Dimensions without their own tolerance are governed by the general tolerance in the title block, usually “ISO 2768-m”. The standard has four classes: f (fine), m (medium), c (coarse) and v (very coarse). For machined parts, m is the usual standard; if nothing is specified, we also apply ISO 2768-m.
| Nominal size (mm) | f (fine) | m (medium) | c (coarse) |
|---|---|---|---|
| 0.5 to 3 | ± 0.05 | ± 0.1 | ± 0.2 |
| over 3 to 6 | ± 0.05 | ± 0.1 | ± 0.3 |
| over 6 to 30 | ± 0.1 | ± 0.2 | ± 0.5 |
| over 30 to 120 | ± 0.15 | ± 0.3 | ± 0.8 |
| over 120 to 400 | ± 0.2 | ± 0.5 | ± 1.2 |
| over 400 to 1000 | ± 0.3 | ± 0.8 | ± 2 |
| over 1000 to 2000 | ± 0.5 | ± 1.2 | ± 3 |
The former part 2 of the standard (geometrical tolerances, “ISO 2768-mK”) has been replaced by ISO 22081. Part 1 for linear and angular dimensions remains valid and appears on most drawings.
Fits: H7, h6 and others
Where two parts must fit together — a shaft in a bearing, a pin in a hole — a fit according to ISO 286 is specified. The letter defines the position of the tolerance zone (upper case for holes, lower case for shafts), the number the tolerance grade (IT grade): the smaller, the tighter.
| Fit | for Ø over 10 to 18 mm | typical use |
|---|---|---|
| H7 (hole) | 0 / + 0.018 mm | bearing seats, locating holes |
| h6 (shaft) | 0 / − 0.011 mm | shafts in bearings, guides |
| g6 (shaft) | − 0.006 / − 0.017 mm | close running fit |
| k6 (shaft) | + 0.001 / + 0.012 mm | transition fit, e.g. rolling bearings |
H7/h6 gives a clearance fit that can be assembled by hand; H7/k6 a transition fit that is usually lightly pressed in. We normally produce fits down to IT7 directly by turning and milling; tighter tolerances and hardened surfaces are ground.
What tight tolerances cost
Every step tighter means additional finishing passes, slower cutting data, more inspection time and a higher risk of scrap. A fit at the bearing seat makes sense — a fit on a free outer contour does not. Ask yourself for every toleranced dimension: what happens if it is 0.1 mm out? If the answer is “nothing”, the general tolerance is enough.
When drawing and 3D model differ
With us, the 3D model is decisive for the geometry; tolerances, fits and surface finishes from the drawing apply in addition. If a drawing dimension is to take precedence, please mark it explicitly as binding. The details are in our terms and conditions.