A major advantage of the geometric tolerancing system is the ability to clearly define exactly what is to be controlled. But determining exactly what geometric entity a feature control frame applies to can involve several different pieces of information.
• The geometric characteristic specified in the feature control frame
• The geometry of the considered feature
• Modifiers in the feature control frame
• Notations under the feature control frame
The geometric characteristic is determined by the symbol in the first compartment of the feature control frame (FCF).
Considered Feature Geometry
The considered feature is determined from the drawing or model. The considered feature will be connected to the FCF by a leader line or extension line.
All geometric tolerance specifications control a particular component of the considered feature. The controlled component is determined primarily by the combination of the geometric characteristic and the considered feature geometry. A given characteristic can be applied to different types of geometry, and a given considered feature can have different geometric characteristics applied to it. The following table shows examples:
Definitions for the controlled components are provided in the ASME Y14.5 standard. Detailed mathematical procedures for extracting controlled components from features are found in the ASME Y14.5.1 standard (Mathematical Definitions of Dimensioning and Tolerancing).
Modifiers in the second compartment of the FCF can make the geometric characteristic apply to a different controlled component. Here are some examples:
Annotations under the FCF can also specify modifications to the controlled component, as shown in the following examples:
Every geometric tolerance controls a certain component of a feature, and this component depends on the specific combination of the considered feature and the information in the feature control frame. A controlled component can be the entire surface of the feature, sets of cross sections, or one of several other types of derived feature components.
To achieve maximum accuracy, inspection for a feature must be planned with the required controlled component(s) in mind. CMM inspection must be done in such a way that the extremities of the correct feature component are captured from the measured points. This involves both point collection strategy and algorithm selection.