Position is used to control the location of features of size. As seen in the previous chapter, position may also be used to locate an irregular closed shape controlled with profile of a surface. Position controls the location and orientation of the feature's center (center point, axis or center plane) not the size and form.
A feature of size must contain or be contained by an actual mating envelope. The 30 mm radius on the drawing at the top may or may not contain an actual mating envelope depending on the actual part variation. Position would be a poor way to control the location of this feature. Making the dimensions basic and using profile of a surface is a better approach. The Ø60 mm feature on the lower drawing clearly can contain an actual mating envelope and therefore may be controlled using position. The profile of a surface tolerance controls the size and form of the feature. The position tolerance controls the location and orientation of the feature with respect to the datum reference frame.
The other features of size on this part are the 4 holes and the 100±0.2 width. The features must be related to the datum reference frame by stated or implied basic dimensions. The width of the part is datum feature B. Its relationship to the first datum plane is perpendicularity. Since it is establishing an origin of measurement, it does not have to be located with position. The 4 holes must be controlled for location relative to the datum reference frame with position.
Notice that most of the dimensions are basic. This is another good time for a general note that dimensions without direct tolerances are basic.
