Feature control frame

How to read it

The National Physical Laboratory’s guide describes the frame, which ISO drawings also call the tolerance frame, as a rectangle divided into compartments holding the characteristic symbol, the shape of the tolerance zone, the size of the variation allowed and, where they apply, datum references. The first compartment holds the symbol: one of the 12 geometric characteristic symbols of ASME Y14.5-2018 (Cogorno, whose count reflects the removal of Concentricity (◎) and Symmetry (⌯)). The second is the tolerance, preceded by a diameter sign only when the zone is cylindrical, and followed by any modifier such as a circled M. The remaining compartments hold datum letters in order of precedence from left to right. The number of compartments varies (NPL); Srinivasan’s ISO example for Flatness (⏥) has just two, the symbol and the value 0.12.

Also searched as: fcf, feature control frame gd&t, feature control frame symbol, how to read a feature control frame.

Reading a frame aloud

Cogorno’s guide reads a Position (⌖) frame with a Ø tolerance, a circled M and datums A, B and C as a sentence: the axis of the controlled feature must lie within a cylindrical tolerance zone of the stated diameter, at maximum material condition, oriented and located with basic dimensions to the datum reference frame set up by datum features A, B and C (B and C at their maximum material boundary). NIST’s Vijay Srinivasan describes the ISO frame the same way, as a syntax: symbol and value in compartments of a rectangular frame.

The order of the datum letters

The datum compartments are read in order: primary, secondary, tertiary. Berez’s UNC Charlotte notes explain that a lower datum doesn’t affect the degrees of freedom a higher one has already fixed, so the order is part of the requirement. The letters themselves follow no fixed order: Cogorno notes they need not be alphabetical, though Keyence says designers generally assign letters in order of priority, so the order is often alphabetical in practice.

Two-line frames

A composite frame has one position symbol shared by two rows (Cogorno). The upper row locates a pattern of features to its datums; the lower row holds the features to each other within a tighter tolerance.