Key Points
A feature control frame creates a geometrically shaped tolerance zone within which the feature must lie. The tolerance zones may be a variety of shapes usually defined by the geometric characteristic in the first compartment and other symbology in the second compartment. The tolerance zone may or may not be constrained relative to a datum reference frame. While 3D tolerance zones are the most common, 2D tolerance zones may also be used for cross-sectional control of a feature.
Common tolerance zone shapes:
The Datum Reference Frame is a coordinate system used to constrain tolerance zones and establish the origin of measurement. All basic dimensions are defined from this datum reference frame.
Basic Dimensions (in ASME) or Theoretically Exact Dimensions (in ISO) define the perfect location of a feature. Basic dimensions are indicated by a rectangular box around the value or by a general note on the drawing such as: ALL DIMENSIONS ARE BASIC. The default tolerances (±) stated in the title block of a drawing do not apply to basic dimensions.
