Feature Control Frame (FCF)
The feature control frame is the most important symbol in the geometric tolerancing system. It states the requirements or instructions for the feature to which it attached. As its name suggests, the feature control frame controls feature.
A FCF is a rectangular box that is divided into comportments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
Each feature control frame will state only one requirement or one message. There is only one step or only gauge for each feature control frame. If there are two requirements for a feature, it will need two feature control features.
The first compartment of a feature control frame always contains only one of the 14 geometric characteristic symbols. If there are two requirements, we must specify two FCF.
The second compartment of a feature control frame always contains the total tolerance for the feature. The feature tolerance is always specified as a total tolerance. It is never a plus/minus value. And also we can specify modifier symbol if it is applicable along with tolerance.
The third, fourth and fifth compartments always contain datum references. If any of the datums are features of size, a datum modifier such as MMC or LMC may be specified.