FEATURE CONTORL FRAME

Feature Control Frame

The feature control frame states the requirements or instructions for the feature to which it is attached. Simply put, the feature control frame controls features. Each feature control frame contains only one message (requirement); if two messages for a feature are necessary, two feature control frames are required.

The first compartment of a feature control frame contains one of the fourteen geometric characteristic symbols. Only one of the symbols can be placed in a feature control frame; if there are two requirements for a feature, there must be two feature control frames or a composite tolerance. The symbol will specify the requirement for the feature, such as, “this feature must be flat,” or “this feature must be positioned.”

The second compartment of a feature control frame contains the total tolerance for the feature. The feature tolerance is always a total tolerance, never a plus/minus value.

If the tolerance is preceded by a diameter symbol (?), the tolerance is a diameter or cylindrical shaped zone, as in the position of a hole. If there is no symbol preceding the tolerance, the default tolerance zone shape is parallel planes or a total wide zone, as in the position of a slot or profile of a surface.

Following the feature tolerance in the feature control frame, a material condition modifier, such as MMC or LMC (see Material Condition Modifiers) may be specified if the feature has size, such as a hole. If the feature has size, and no modifier is specified, the default modifier is RFS. If the feature has no size, such as a plane surface, then the modifier is not applicable.

The third and following compartments of a feature control frame contain the datum feature reference(s) if they are required. For example, if a form tolerance, such as flatness or straightness, is specified, then no datum feature reference is allowed. However, if a location tolerance like position is specified, the datum feature references are usually specified.

The alphabetical order of the datum references has no significance—the significance is their order of precedence, reading from left to right as primary, secondary, and tertiary. The primary is the first feature contacted (minimum contact at 3 points), the secondary feature is the second feature contacted (minimum contact at 2 points), and the tertiary is the third feature contacted (minimum contact at 1 point). Contacting the three (3) datum features simultaneously establishes the three (3) mutually perpendicular datum planes or the DRF. The DRF is created by so-called Datum Simulators which are the manufacturing, processing, and inspection equipment, such as surface plate, a collet, a three jaw chuck, a gage pin, etc.

In certain situations, the datum feature modifiers Maximum Material Boundary (MMB) or Least Material Boundary (LMB) may be applied to the datum feature. The default modifier is Regardless of Material Boundary (RMB). Since the datum feature has size (it can get larger and smaller), information is necessary on the size condition of the datum feature to which the datum feature reference applies. The modified condition of the datum feature (MMB, LMB, RMB) defines the size or condition of the datum feature simulator