Tolerance is the permitted deviation on a characteristic of a part. No machine produces an exact dimension: the drawing therefore gives a nominal value and a range, between a lower limit and an upper limit, within which the part remains compliant. A dimension “20 ± 0.1” accepts any measured value between 19.9 and 20.1 mm; the tolerance, the difference between these two limits, is here 0.2 mm.
There are several families. Dimensional tolerances apply to lengths, diameters or angles. They are written as a value (± 0.05) or by standardised class according to ISO 286, such as H7 for a bore: on a 25 mm diameter, H7 allows 0 to +21 µm. Geometric tolerances control form, orientation or position (flatness, perpendicularity…) and belong to GPS dimensioning. Finally general tolerances, under ISO 2768, apply to all dimensions without an individual indication: classes f, m, c and v for linear and angular dimensions, H, K and L for geometry. They avoid having to dimension every detail.
The tighter the tolerance, the more demanding the manufacture: fine adjustments, finishing passes, sometimes grinding, longer inspection, a higher scrap rate. Cost rises quickly, hence the rule of imposing a tight tolerance only where the function requires it. Temperature, material (plastics expand far more than metals) and surface treatments also influence what can be held. To find the values of an ISO 2768 class, use the ISO 2768 tolerances tool.
In a quote request
Give the general tolerance of the drawing in the title block (for example ISO 2768-mK), mark the functional dimensions with tight tolerances and justify them, then leave everything else under general tolerances. State whether inspection of these dimensions is required, by what means, and whether they apply before or after treatment. See the guide on machining tolerances and the definition of fit, which is the application of tolerances to assembled parts.