When a technical drawing of a part is done, in order to define its precision tolerances to the nominal dimensions and specifications should be added. To avoid designer having to specify every dimensions tolerances and help manufacturers easily asses the difficulty of manufacturing, the International Standard Organisation (ISO) published a general tolerance norm : the ISO2768.

On a drawing mention of the norm name + two letters : for example ISO2768-mK or ISO2768-fH, will means that the intent of the designer was that all non otherwise specified tolerances on the drawing will have to respect below charts values.

ISO2768

ISO2768-1 general dimensional tolerances

The first part of the norm, refers to the general tolerances, which are the linear and angular dimensions without other tolerances on the specifications.

Table 1 : mm tolerances for linear dimensions excluding machined edge
ReferenceDescription0.5 to 33 to 66 to 3030 to 120120 to 400400 to 10001000 to 20002000 to 4000
fFine requirement±0.05±0.05±0.1±0.15±0.2±0.3±0.5
mMedium requirement±0.1±0.1±0.2±0.3±0.5±0.8±1.2±2
cCoarse requirement±0.2±0.3±0.5±0.8±1.2±2±3±4
vVery coarse requirement±0.5±1±1.5±2.5±4±6±8
Table 2 : mm tolerances for machined edge (chamfer height and external radius)
ReferenceDescription0.5 to 33 to 6More than 6
fFine requirement±0.2±0.5±1
mMedium requirement±0.2±0.5±1
cCoarse requirement±0.4±0.5±2
vVery coarse requirement±0.2±1±2
Table 3 : degrees tolerances for angular dimensions
ReferenceDescriptionUp to 1010 to 5050 to 120120 to 400More than 400
fFine requirement±1°±0°30’±0°20’±0°10’±0°5’
mMedium requirement±1°±0°30’±0°20’±0°10’±0°5’
cCoarse requirement±1°30’±1°±0°30’±0°15’±0°10’
vVery coarse requirement±3°±2°±1°±0°30’±0°20’
Norm ISO2768 mk

ISO2768-2 geometrical tolerances

The second part of the norm is about geometrical tolerances, like perpendicularity, flatness, straightness etc.

Please note that the general dimensions tolerances applies for roundness (through the diameter value) and parallelism.

Table 4 : mm tolerances for straightness and flatness
ReferenceUp to 1010 to 3030 to 100100 to 300300 to 10001000 to 3000
H0.020.060.10.20.30.4
K0.50.10.20.40.60.8
L0.10.20.40.81.21.6
Table 5 : mm tolerances for perpendicularity
ReferenceUp to 100100 to 300300 to 10001000 to 3000
H0.20.30.40.5
K0.40.60.81
L0.611.52
Table 6 : mm tolerances of symmetry with the longest element as reference
ReferenceUp to 100100 to 300300 to 10001000 to 3000
H0.50.50.50.5
K0.60.60.81
L0.611.52

ISO2768 usages

The tolerance ISO norm is very popular for metal sheet parts, precision machining parts and plastic injection parts designing. Casting parts usually use different tolerance norms for the raw casting parts.

Are the tightest tolerances always better ?

Parts designed with tightest tolerances will be closer to nominal values and therefore easier to integrate with other parts in mechanical assemblies. But tighter tolerances means more difficult parts to manufacture and therefore more expensive parts. In order to have effective design to cost approach, tolerance requesting should be matched with manufacturing means default capabilities.

Recommandations

Usage of the ISO2768 norm is very widespread and convenient, but you should not forget that too loose or too tight tolerancing on your drawings will impact the feasibility of your parts.

The version used of the norm used for this article is 1989, please be aware that new version may be published in the future.