Structure of an FCF
A Feature Control Frame (FCF) is a rectangular box divided into compartments that defines the geometric tolerance for a feature. It communicates the type of control required, the tolerance value, and the reference datums. The FCF is placed on engineering drawings and connected to the feature it controls.
Components explained
An FCF typically includes:
- Geometric characteristic symbol (e.g., position, flatness, perpendicularity)
- Tolerance value (the allowable variation, sometimes with modifiers like diameter or material condition)
- Datum references (letters indicating the datums used for control, in order of precedence)
Optional symbols, such as modifiers for maximum or least material condition, may also be included to refine the tolerance.
How to read an FCF
An FCF is read from left to right. First, identify the geometric characteristic being controlled. Next, look at the tolerance value to understand the permissible variation. Finally, review the datum references to see how the feature is oriented and located relative to the datum reference frame. This sequence ensures correct interpretation of the design intent.
Common examples
- Position tolerance: Controls the exact location of a hole relative to datums
- Flatness: Ensures a surface lies within two parallel planes
- Perpendicularity: Controls a feature’s orientation at 90° to a datum
- Parallelism: Maintains consistent distance between features
These examples show how FCFs are used to precisely define functional requirements in manufacturing and inspection.
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