General tolerances
What are general tolerances?
General tolerances refer to the permissible dimensional and shape deviations according to ISO 2768, applied when individual tolerances are not specifically defined in the technical drawing. In mechanical engineering, general tolerances simplify both design and machining since there's no need to specify a tolerance for every nominal dimension. When a drawing references something like ISO 2768-mK, the machinist immediately knows what dimensional, shape, and positional tolerances apply to the part.
General tolerances and the ISO 2768 standard
General tolerances are based on an international ISO standard, which is divided into two main parts. ISO 2768-1 specifies the allowable deviations for linear dimensions and angular measurements without individual tolerance notations. ISO 2768-2 covers geometric tolerances such as straightness, flatness, and perpendicularity in cases where a more detailed GD&T specification isn’t provided.
In the workshop, this means that the basic dimensioning can be done clearly and efficiently. The designer determines the required accuracy class, and the machine shop manufactures the part according to that class without needing extra interpretation.
General tolerance accuracy classes
ISO 2768 defines different accuracy classes, indicated by the letters f, m, c, and v. These describe the magnitude of permitted deviations, from fine to very coarse. A fine class is used for precise machined components such as sliding surfaces or tight fits, while a coarser class is suitable for structural parts where dimensional deviations have minimal effect on functionality.
For shape and positional tolerances, separate letter classes are used to specify permissible variations for straightness, flatness, etc. This allows the machinist to assess if the part meets requirements without the need for detailed tolerance notation for each surface.
General tolerances in technical drawings
In technical drawings, general tolerances are typically indicated in the title block. Notation such as ISO 2768-mK means that linear dimensions follow the medium class, and geometric tolerances adhere to the corresponding specified level. This streamlines documentation and reduces misunderstandings in the workshop.
However, general tolerances do not replace individual tolerances when they are required for functionality. For example, bearing seats, tight clearance fits, or precise shaft holes are specified separately according to the ISO tolerance system.
General tolerances and the machining process
From the machining perspective, general tolerances directly affect machining settings, measurement, and quality assurance. The tighter the tolerance class, the more attention must be paid to tool condition, machine rigidity, and thermal expansion. Measurement may be performed with, for example, a caliper, micrometer, or CMM depending on required accuracy.
Choosing the right general tolerance class balances costs with functionality. Tolerances that are too tight increase machining and inspection expenses, while overly loose tolerances can impair the compatibility of parts.
Summary
General tolerances are a key aspect of technical dimensioning in mechanical engineering. Based on the ISO 2768 standard, they define permissible dimensional and shape deviations where no individual tolerances are specified. When used appropriately, general tolerances clarify technical drawings, streamline machining, and ensure parts function cost-effectively.
