Key Points
The feature control frame (in ASME) or tolerance indicator (in ISO) is the symbolic means of expressing the tolerance requirements for a feature or set of features. The frame is divided into compartments:
The first compartment contains a geometric characteristic symbol.
The second compartment contains the shape of the tolerance zone followed by the tolerance. The default tolerance zone shape (without a symbol) is parallel planes/total wide zone. A diameter symbol (Ø) or (SØ) may be added to change to a cylindrical or spherical zone, respectively.
Modifiers may be added behind the tolerance value. These modifiers can be MMC, LMC, projected tolerance zone, free state, tangent plane, statistical tolerance, unequal profile, and dynamic profile.
The third, fourth, and fifth compartments contain the datum feature reference(s) to create the datum reference frame. Depending on the specification, these compartments are not always necessary.
The order of precedence reading from left to right is primary, secondary and tertiary. These compartments define the datum reference frame and represent the constraints for the tolerance zone.
Modifiers for the datum features may be added behind each datum feature letter. The modifiers are MMB, LMB, [BSC], and translation.
The tolerance controls the surface when the frame is attached to the surface thru extension lines or leader lines. The tolerance controls the axis or center plane when the frame is placed under the size tolerance.
