ISO 2768 General tolerance note

How to verify

To check an untoleranced dimension, go to the table in the cited standard and find the row for the corresponding size range: the tolerance depends on the size of the dimension; it is not a single value for the whole drawing. In ISO 2768-1, class m gives around ±0.1 mm from 0.5 to 6 mm, ±0.2 mm from 6 to 30 mm and ±0.3 mm from 30 to 120 mm, and class f roughly half that in the middle ranges. The same applies to general geometrical tolerances: the permissible flatness or perpendicularity value depends on the length of the feature.

Workshop guideline values

General

Class c or v: around ±0.5 mm on dimensions from 6 to 30 mm; common on welded structures and rough parts.

Accuracy

Class m: around ±0.2 mm from 6 to 30 mm; it is the default class on most machining drawings.

High precision

Class f with geometrical class H: around ±0.1 mm from 6 to 30 mm, for precision parts.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Applying the general tolerance to basic (boxed) dimensions or to features that already have their own geometric frame: those are explicitly excluded.
• Treating the class as a single value: the tolerance changes with the size range of the dimension, so ±0.2 mm does not apply to every dimension on the drawing.
• Ignoring the second letter of the note: the geometrical class (H, K or L) imposes general flatness, perpendicularity and runout that many shops never get around to checking.