There is variability in any manufacturing process and, as such, tolerances are used to set permissible limits on this variability. Machining is often selected when tolerances are close, as it is more accurate than most other shape-making processes.
Tighter tolerances usually mean higher costs. For example, if the product designer specifies a tolerance of ±0.10 mm on a hole diameter of 6.0 mm, this tolerance could be achieved by a drilling operation. However, if the designer specifies a tolerance of ±0.025 mm, then an additional reaming operation is needed to satisfy this tighter requirement.
This is not to suggest that looser tolerances are always good. It often happens that closer tolerances and lower variability in the machining of the individual components will lead to fewer problems in assembly, final product testing, field service, and customer acceptance. Although these costs are not always as easy to quantify as direct manufacturing costs, they can nevertheless be significant. Tighter tolerances that push a factory to achieve better control over its manufacturing processes may lead to lower total operating costs for the company over the long run.
ISO 2768 is a standard for general tolerances, specifically covering linear dimensions, angular dimensions and geometrical tolerances. It is issued by the International Organization for standardization – ISO in association with Deutsches Institut für Normung – DIN.
The ISO 2768 standard comes in two parts, namely ISO 2768-1 and ISO 2768-2. The first part covers linear and angular dimensions, as well as radius of curvature and chamfer height according to four classes of levels: f (fine), m (medium), c (coarse), v (very coarse). The second parts covers geometrical tolerances in regard of form and position according to three classes or levels: H (fine), K (medium), L (coarse).
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