Definition
A feature control frame is the rectangular box, split into compartments, that specifies a geometric tolerance. The first compartment holds the symbol, flatness or position for example. The second holds the tolerance value, sometimes with a diameter symbol in front of it. The compartments after that hold the datum letters in order.
What it means for the calculation
The frame is the shortest route to what a feature costs. The symbol says which kind of control is needed, the value says how tight, and the datums say how the part must be clamped and measured. Three frames on a single hole can together force an extra setup and an inspection report that the plain dimensions would never reveal.
Where it appears on the drawing
As a rectangular box split into compartments, with a leader line to the feature or attached to its dimension line.
Common mistakes
Reading only the number and skipping the symbol or the datums, which prices a position tolerance as if it were a plain size deviation. Also missing the diameter symbol in the second compartment, which turns a square tolerance zone into a round one, with a different allowable deviation.
The frame is the shortest route to what a feature costs. The symbol says which kind of control is needed, the value says how tight, and the datums say how the part must be clamped and measured. Three frames on a single hole can together force an extra setup and an inspection report that the plain dimensions would never reveal.
See how Blake reviews a partFrequently asked questions
What is in the first compartment?
The geometric symbol, which says the kind of control: form, orientation, location or runout.
Do all feature control frames carry datums?
No, form tolerances such as flatness and circularity appear without one.
What does a ⌀ before the tolerance value mean?
That the tolerance zone is round rather than a strip or a block.