ISO 2768-mK general tolerance for a 250 mm dimension

On the drawing of a welded frame plate, an overall length is written with no specific tolerance: 250. The general title block carries the classic note ISO 2768-mK. The fabrication shop inspector has to approve the laser-cut blank before sending it to the station that machines the seating faces. They need the exact interval that applies, to decide whether the machining allowance is enough or too much.

ISO 2768-mK tolerance values for a 250 mm dimension

Applying the ISO 2768-mK general tolerance to a 250 mm dimension gives a linear tolerance of ±0.5 mm, together with a perpendicularity tolerance of 0.6 mm and a straightness/flatness tolerance of 0.4 mm. The lower-case m stands for the medium dimensional tolerance class, while the capital K specifies the general geometric tolerance class.

Type of specification	ISO 2768 class	Size range (mm)	Tolerance allowed
Linear deviation	m (medium)	Over 120 up to 400	±0.5 mm
Straightness and flatness	K (medium)	Over 120 up to 400	0.4 mm
Perpendicularity	K (medium)	Over 120 up to 400	0.6 mm
Symmetry	K (medium)	Over 120 up to 400	0.8 mm
Circular runout	K (medium)	Independent of length	0.2 mm

Any length not explicitly toleranced on the drawing and between 120 mm and 400 mm automatically takes these limits as soon as ISO 2768-mK appears in the title block.

Linear limits for 250 mm

The first part of the standard (ISO 2768-1) governs linear dimensions. The 250 mm dimension falls in the 120 mm to 400 mm band.

For class m (medium), the table of permissible deviations gives ±0.5 mm over this range.

Minimum size allowed:

Min size = 250.0 - 0.5 = 249.5 mm

Maximum size allowed:

Max size = 250.0 + 0.5 = 250.5 mm

Any direct dimensional measurement taken between the two parallel faces must lie strictly within this 1.0 mm overall interval.

Applying the related geometric tolerances of class K

The second part of the standard (ISO 2768-2) governs geometric features that have no specific tolerance frame. Class K governs form, orientation and runout.

For a 250 mm reference dimension, the flatness requirement on the seating face means the actual surface must lie between two parallel planes 0.4 mm apart.

The perpendicularity tolerance between the reference face and the cut edge allows a maximum deviation of 0.6 mm over the full height of the part.

These geometric requirements come on top of the dimensional limits, and rule out accepting a plate whose average length is 250.0 mm but which is warped by more than 0.4 mm.

The information on automatic dimension detection explains how inspection software picks up these title block rules.

Full example: inspecting a 250 mm base plate

A quality technician receives a machined 6061-T6 aluminium base plate whose design drawing specifies a nominal width of 250 mm with no individual tolerance, covered by the ISO 2768-mK title block.

The check is done on a 3D layout table. The shop is held at 20 °C.

Direct measurement of the distance between the two opposite faces by point probing gives a value of 250.62 mm.

1. Nominal size on the drawing: 250.00 mm.
2. General standard in the title block: ISO 2768-mK.
3. ISO 2768-1 class m range (120 to 400 mm): ±0.5 mm.
4. Allowed interval: [249.50 mm; 250.50 mm].
5. Actual reading: 250.62 mm.

Verdict: the measured value exceeds the upper limit by 0.12 mm (250.62 mm against 250.50 mm max). The part is declared non-conforming. A 0.2 mm corrective facing cut is needed to bring the part back into the allowed range.

Common mistakes in reading ISO 2768

The first mistake is confusing class m (medium) with class f (fine). In class f, the tolerance over the 120–400 mm range drops to ±0.3 mm. Applying the fine-class limits out of habit to a drawing in the medium class leads to wrongly rejecting perfectly usable parts.

The second concerns dimensions in brackets, or reference dimensions. A 250 mm dimension written in brackets, (250), is an auxiliary dimension given for information. It carries no ISO 2768 general tolerance and must not be the subject of a formal inspection record.

The third is applying the linear table of ISO 2768 to radii or chamfers. For a 250 mm chamfer (on a large dome or cone, for instance), the general tolerances on chamfers and radii are completely different from the table for plain linear dimensions.

The fourth happens when inspecting complex welded assemblies, where ISO 2768-mK is applied by mistake instead of ISO 13920, the standard specific to welded structures, which allows larger distortion.

To assess your equipment costs and access our computerised inspection solutions, see the CoteCote pricing page and discover our dimensional inspection management modules.