In mechanical part drawings, parallelism tolerance allows the designer to specify the degree to which a feature‘s orientation may vary with respect to its referenced datumby creating a tolerancezone parallel to that datum. The relevant feature, axis, or center plane must then lie withinthiszone. The parallelism symbol is generallyused toensure features are alignedfor proper function.
In the left figureabove, the boxedparallelismsymbol and toleranceare used to control the center axis of a hole. The boxed symbols can be read “This axis must lie between two planesparallel tothe axis A and spaced 0.3 apart”. Note that if the boxed tolerance includes a diameter symbol, indicating a cylindrical tolerance zone, the tolerance would apply to all views.
The tolerance zone created isindicated by the parallel lines in the rightmost figure, andapplies for the entire length of theaxis.
Parallelism is not often used in a tolerance stack except when it is applied to a surface, in which case it is treated like a flatness control since it refines the orientation of the surface.