How to Specify General Tolerances in Machined Parts
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 Organisation for standardisation – 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).
At CNC Proto we specialise in the machining of non-ferrous metals, mostly 6000 and 7000 series aluminium alloys, and engineering plastics. On our Quote Request form, we offer by default 3 different choices to our customers for linear and radii dimensions general tolerances (+/- 0.10mm; +/- 0.07mm; +/- 0.05mm). Of course, if you prefer to specify your general part tolerances according to ISO-2768, feel free to let us know in the additional comments field of the form!
For more articles on CNC machining, materials and design of machined parts, be sure to check our Resource Library.
Fernando Duarte Ramos
With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing and building the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost. Learn more About Us.
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