GlossaryKind of term: Standard and conventionGeneral tolerance (ISO 2768)
What are general tolerances?
A general tolerance applies to dimensions and features without a tolerance of their own; ISO 2768 sets the values per class and size range.
A general tolerance is the permitted deviation for every dimension and feature on a drawing that carries no tolerance of its own. It sits in or beside the title block as a reference to a standard and a class, for example ISO 2768-mK. The first letter gives the class for linear and angular dimensions, the second the class for geometrical properties. The value depends on class and nominal size: an untoleranced linear dimension of 80 mm may deviate by ±0.3 mm in class m and by ±0.15 mm in class f.
What does the standard say about the general tolerance?
ISO 2768-1 gives classes f, m, c and v (fine, medium, coarse, very coarse) for linear dimensions, external radii and chamfer heights, and angular dimensions, for parts that are machined or formed from sheet. ISO 2768-2 gives classes H, K and L for straightness and flatness, perpendicularity, symmetry and circular run-out; it contains no values of its own for position. ISO 2768-2 was withdrawn in 2021 and replaced by ISO 22081, which has no H, K and L classes and instead sets general geometrical specifications relative to a datum system. On existing drawings ISO 2768-mK remains common; agree which edition applies. The suffix E, as in ISO 2768-mK-E, applies the envelope requirement to single features of size. Dimensions with their own tolerance, theoretically exact dimensions and auxiliary dimensions are not covered.
How does the general tolerance work in practice?
On a part with many untoleranced dimensions, the class sets how strict most of the geometry is. Whether a finer class means extra work depends on what your process holds anyway: on a stiff machining centre class f may need no extra steps, while on thin-walled, large or sheet work it is less of a given. The class also sets which dimensions you measure and with which gauge. If there is no general tolerance, or a company standard you do not know, agree with the customer which one applies.
What is the general tolerance often confused with?
A general tolerance is not a dimensional tolerance: that sits on one dimension and takes precedence. Nor is it a general roughness: an Ra value by the title block concerns the surface, ISO 2768 concerns size and geometry. And ISO 2768-m is not an IT grade: its values step up per size range in a table of their own.
How do you see the general tolerance in Blake?
In Blake the general tolerance from the title block appears on the part as a proposal, with the class exactly as written on the drawing. Whether you confirm or change it, who did so and when remains visible. More on requests.
Frequently asked questions
What if the drawing gives no general tolerance at all?
Then an agreement you need is missing. Sometimes the customer's company standard or another standard applies through the purchase conditions. Ask which class or standard is meant before you quote; choosing a class yourself is an assumption.
What does the K in ISO 2768-mK mean?
The second letter is the class for general geometrical tolerances from ISO 2768-2: H is the finest, K medium, L the coarsest. It gives values for straightness and flatness, perpendicularity, symmetry and circular run-out. ISO 2768-2 is withdrawn but still found on many drawings.
Is a finer class more expensive?
Not necessarily. If your process holds the finer values without extra steps, the difference is mostly in inspection. If the class calls for a finishing pass, a different setup or more checking, it does. That differs per part and per shop.