ISO 2768 Standard

In the field of mechanical manufacturing, tolerances define the acceptable limits of dimensional variation for a part. They ensure that a part can fulfill its function even if it is not manufactured with absolute precision. This allows for: reducing production costs by avoiding unnecessary precision, ensuring interchangeability of parts, guaranteeing compatibility and assembly quality.

The ISO 2768 standard, applicable to machined, cut, bent sheet metal, welded structures, and standard mechanical subassemblies, helps avoid overloading technical drawings with detailed indications by specifying default general tolerances.

The ISO 2768 standard consists of two parts:

ISO 2768-1: Tolerances for linear and angular dimensions. This part applies to dimensions such as lengths, diameters, radii, angles, chamfer heights, etc.

It defines four tolerance classes:

        –       f: fine (high precision)

        –       m: medium (general use)

        –       c: coarse (wide tolerance)

        –       v: very coarse (for rough parts or where wide tolerances are acceptable)

Examples of linear tolerances:

Nominal Length (mm)	f (fine)	m (medium)	c (coarse)	v (very coarse)
0.5 to 3	±0.05	±0.1	±0.2	—
>3 to 6	±0.05	±0.1	±0.3	±0.5
>6 to 30	±0.1	±0.2	±0.5	±1.0
>30 to 120	±0.15	±0.3	±0.8	±1.5
>120 to 400	±0.2	±0.5	±1.2	±2.5

Angular tolerances:

Nominal Length (mm)	f, m	c	v
Up to 10	±1°	±1°30′	±3°
>10 to 50	±30′	±1°	±2°
>50 to 120	±20′	±30′	±1°
>120 to 400	±10′	±15′	±90′
>400	±5′	±10′	±20′

ISO 2768-2: General geometrical tolerances

This part covers form and position tolerances. It introduces three tolerance classes:

        –       H: strict tolerance

        –       K: medium tolerance

        –       L: large tolerance

Examples:

Flatness and straightness tolerances:

Nominal Length (mm)	H	K	L
Up to 10	0.02	0.05	0.1
>10 to 30	0.05	0.1	0.2
>30 to 100	0.1	0.2	0.4
>100 to 300	0.2	0.4	0.8
>300 to 1000	0.3	0.6	1.2
>1000 to 3000	0.4	0.8	1.6

Perpendicularity tolerances:

Nominal Length (mm)	H	K	L
Up to 100	0.2	0.4	0.5
>100 to 300	0.3	0.6	1.0
>300 to 1000	0.4	0.8	1.5
>1000 to 3000	0.5	1.0	2.0

How to use it

If a part does not require specific precision, general tolerances can be directly applied by adding a note such as:

ISO 2768-mK on the drawing. This means: “m”: medium tolerance for linear and angular dimensions; “K”: medium tolerance for geometrical features

For critical features, specific tolerances can still be applied to the relevant dimensions on the drawing.