No part will ever be manufactured to the exact nominal dimensions stated on the drawing. This is due to various reasons but for example, the machines used to manufacture the parts can only work within a certain tolerance range, the cutting tools like milling cutters also have a tolerance associated with them. The environmental temperature at the time of manufacture will also pay a role.
The designer is therefore responsible to decide how far dimensions can deviate from the nominal dimensions stated on the drawing so that the part will assemble and function correctly. The designer will state tolerances on the drawing and the manufacturer will make that part within these specified tolerance ranges. Should the part deviate and be outside of these tolerance ranges, the part would most likely need to be rejected and remade.
Adding tolerances to every dimension on the drawing is time consuming and can make for a messy confusing drawing. Most likely only a couple of dimensions really need to be strictly controlled anyway. One solution is to apply ISO 2768 general tolerances. What the ISO 2768 international standard does is to apply a general set of tolerances to your entire part as a default and the designer only then needs to apply tolerances on specific dimensions that require tighter control.
ISO 2768 consists of 2 parts. Part 1 covers linear and angular dimensions and is specified by lower-case letters (f, m, c, v). Part 2 covers geometric tolerances and is specified by uppercase letters (H, K, L).
Using the tables below, decide which of the 2 tolerance classes are most suitable for your parts. Then just add a note onto your drawing sheet to state ISO 2768 general tolerances apply. An example of a typical note is:
“GENERAL TOLERANCES TO ISO 2678 CLASS mK – U.O.S.“
The “U.O.S.” acronym is for “Unless Otherwise Stated”.
Permissible deviations for linear dimensions except for broken edges Tolerance class
(mm) f
(fine)m
(medium)c
(coarse)v
(very coarse)
0.5 up to 3 (see note 1) ±0.05 ±0.1 ±0.2 -
Over 3 up to 6 ±0.05 ±0.1 ±0.3 ±0.5
Over 6 up to 30 ±0.1 ±0.2 ±0.5 ±1
Over 30 up to 120 ±0.15 ±0.3 ±0.8 ±1.5
Over 120 up to 400 ±0.2 ±0.5 ±1.2 ±2.5
Over 400 up to 1000 ±0.3 ±0.8 ±2 ±4
Over 1000 up to 2000 ±0.5 ±1.2 ±3 ±6
Over 2000 up to 4000 - ±2 ±4 ±8
Note 1: For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).
External Radius and Chamfer Heights| Permissible deviations for broken edges | Tolerance class | |||
| (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
| 0.5 up to 3 (see note 1) | ±0.2 | ±0.2 | ±0.4 | ±0.4 |
| Over 3 up to 6 | ±0.5 | ±0.5 | ±1 | ±1 |
| Over 6 | ±1 | ±1 | ±2 | ±2 |
Note 1: For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).
Angular Dimensions| Permissible deviations of angular dimensions | Tolerance class | |||
| (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
| up to 10 | ±1° | ±1° | ±1°30' | ±3° |
| Over 10 up to 50 | ±0°30' | ±0°30' | ±1° | ±2° |
| Over 50 up to 120 | ±0°20' | ±0°20' | ±0°30' | ±1° |
| Over 120 up to 400 | ±0°10' | ±0°10' | ±0°15' | ±0°30' |
| Over 400 | ±0°5' | ±0°5' | ±0°10' | ±0°20' |
| Straightness and flatness tolerances for ranges of nominal lengths | Tolerance class | ||
| (mm) | H | K | L |
| up to 10 | 0.02 | 0.05 | 0.1 |
| Over 10 up to 30 | 0.05 | 0.1 | 0.2 |
| Over 30 up to 100 | 0.1 | 0.2 | 0.4 |
| Over 100 up to 300 | 0.2 | 0.4 | 0.8 |
| Over 300 up to 1000 | 0.3 | 0.6 | 1.2 |
| Over 1000 up to 3000 | 0.4 | 0.8 | 1.6 |
| Perpendicularity tolerances for ranges of nominal lengths of the shorter side | Tolerance class | ||
| (mm) | H | K | L |
| up to 100 | 0.2 | 0.4 | 0.6 |
| Over 100 up to 300 | 0.3 | 0.6 | 1 |
| Over 300 up to 1000 | 0.4 | 0.8 | 1.5 |
| Over 1000 up to 3000 | 0.5 | 1 | 2 |
| Symmetry tolerances for ranges of nominal lengths | Tolerance class | ||
| (mm) | H | K | L |
| up to 100 | 0.5 | 0.6 | 0.6 |
| Over 100 up to 300 | 0.5 | 0.6 | 1 |
| Over 300 up to 1000 | 0.5 | 0.8 | 1.5 |
| Over 1000 up to 3000 | 0.5 | 1 | 2 |
| Tolerance class | ||
| H | K | L |
| 0.1 | 0.2 | 0.5 |
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