A drawing specifies class m (medium) per ISO 2768-1, with no individual tolerances. What deviation then applies to a 42 mm dimension?
The specified class applies within its scope
ISO 2768-1 gives four classes — f (fine), m (medium), c (coarse), v (very coarse) — each with a table of allowed deviation per size range. Instead of tolerancing every dimension individually, the drawing references the standard and the class once; only applicable dimensions without individual tolerances fall under it, excluding reference and theoretically exact dimensions. Withdrawn ISO 2768-2:1989 contains legacy tables for form and position tolerances (straightness, flatness, perpendicularity, symmetry, run-out) with classes H, K and L.
Worked example: 42 mm, class m
42 mm falls in the >30-120 mm range; class m gives ±0.3 mm there. The table doesn't scale linearly but steps per range: 6 mm in class f falls in the >3-6 mm range and gives ±0.05 mm, while 8 mm in class v falls in the >6-30 mm range and gives ±1.0 mm. Always read the table from the actual size range — two dimensions just either side of a boundary can get a different deviation than their small difference would suggest.
Geometric deviations aren't a ±
ISO 2768-2's tables give the total width of the tolerance zone, not a deviation around a nominal value. Class K run-out, for example, is 0.2 mm — the full zone, not ±0.2 mm. The two read as if they say the same thing, but a ± sign in front would wrongly make the zone twice as permissive and can cause a factor-of-two inspection error. Mechify therefore shows linear/angular deviations with a ± sign and geometric zones without one.
Assumptions and applicability limits
The table values are transcribed from widely used, publicly published summaries of ISO 2768-1/-2. For contractually binding drawings, verify against the specified standard edition. ISO 2768-2:1989 is withdrawn; use these tables only for legacy drawings explicitly referencing it. ISO 22081:2021 provides the current framework for general geometrical specifications and requires deliberate specification choices, not automatic replacement of H/K/L tables. The linear tables only start at 0.5 mm; smaller dimensions fall outside the scope of this class system.