The form and the size of a drawn object are considered “ideal2 conditions, which cannot be absolutely precise in real life.
Why use the dimensional tolerance?
Manufacturing processes always have geometric or dimensional erros; in the drawing, it is necessary to indicate the maximum permitted variability (known as tolerances) within which the produced parts are acceptable. Tolerances are especially designed to ensure the correct fit between components.
Given two parts designed to fit together (for example, a cylindrical bore and a cylindrical shaft), fitting can occur in two different ways:
- The adjustment – when the pieces to assemble are worked together and adapted during installation. NB: suitable only for small-scale production.
- The interchangeability – when the parts to be assembled are manufactured in large quantities, for example in batches, and must fit together randomly. This method requires the definition of dimensional and geometrical tolerance limits for the components. NB: production can also take place at different times and in different locations. Furthermore, broken pieces can be replaced.
Establishing a dimensional tolerance
Establishing a dimensional tolerance means defining the limits within which a specific dimension is allowed to vary. But how is it assigned?
The limits of dimensional variability (dimensional tolerance) are uniquely determined when the following are specified:
- The tolerance value (the width of the tolerance zone)
- One of the two deviations (the position of the tolerance zone)
By convention, the zero line corresponds to the nominal dimension.
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