Tolerance is critical in the design and manufacturing of CNC parts, to determine the tolerance value for a specific workpiece, we need the help of some international standards. Here we introduce ISO 2768 standard and ISO 2768 tolerance chart, also provide a free PDF file of General Tolerances for Linear and Angular Dimensions for download.
ISO 2768 is an international tolerances standard made by International Organization, it consists of two parts: ISO 2768-1 (Tolerances for linear and angular dimensions without individual tolerance indications) and ISO 2768-2 (Geometrical tolerances for features without individual tolerance indications). The standard simplifies the instructions on the drawing and design of the machining part. In this article, we’ll focus on ISO 2768 Part 1.
An ISO 2768-mK mark means the dimension information for which the tolerances are not specified will be followed according to the m and K class. m class is specified in ISO 2768-1 and K class is specified in ISO 2768-2, which including H, K, and L tolerance levels.
ISO 2768 Part 1 is intended to simplify drawing indications and it specifies general tolerances for linear and angular dimensions without individual tolerance indications in four tolerance classes. It applies to the dimensions of parts that are produced by metal removal or parts that are formed with sheet metal fabrication.
ISO 2768 Part 1 only applies to the following dimensions which do not have an individual tolerance indication:
a) linear dimensions (e.g. external sizes, internal sizes, step sizes, diameters, radii, distances, external radii and chamfer heights for broken edges);
b) angular dimensions, including angular dimensions usually not indicated, e.g. right angles (90°). unless reference to ISO 2768-2 is made, or angles of uniform polygons;
c) linear and angular dimensions produced by machining assembled parts.
It does not apply for the following dimensions:
a) linear and angular dimensions which are covered by reference to other standards on general tolerances;
b) auxiliary dimensions indicated in brackets;
c) theoretically exact dimensions indicated in rectangular frames.
1. These tolerances may be suitable for use with materials other than metal.
2. When selecting the tolerance class, the respective customary workshop accuracy has to be taken into consideration.
3. Smaller or larger tolerances should be indicated adjacent to the relevant nominal dimensions.
4. General tolerances specified in angular units control only the general orientation of lines or line elements of surfaces, but not their form deviations.
5. If general tolerances in accordance with ISO 2768 shall apply, the information shall be indicated in or near the title block, such as ISO 2768-m.
Check out the ISO 2768 tolerance tables corresponding to the 4 classes. Please note that all tolerance in ISO 2768 is given in mm (Metric).
Table 1: Permissible deviations for linear dimensions except for broken edges
Table 2: Permissible deviations for broken edges (external radii and chamfer heights)
Table 3: Permissible deviations of angular dimensions