Cast iron V-block is a special positioning element. Although it is a separate
positioning element, but the structural elements of its location base surface is
not single. Positioning reference of most positioning elements are positioning
the base surface itself or formed therefrom point, surface, line. According to
this reasoning, positioning reference of cast iron V-block should have two, one
is the intersected line of two positioning plane, another is the plane of
symmetry of the two planes, intersected line as vertical positioning reference;
the plane of symmetry as horizontal positioning reference.
Because cast iron V-block integrated positioning by two planes, it can not use positioning reference method that analyze single surface of the positioning element.