Fundamentals of GD&T and Y14.5: Concepts, Symbols, and Rules
Decoding the feature control frame
Nearly every question on this exam reduces to reading one frame: geometric symbol, zone shape and value, material-condition modifier, and datum references in order of precedence. Know the five families cold - form (straightness, flatness, circularity, cylindricity), orientation (parallelism, perpendicularity, angularity), location (position, concentricity, symmetry), profile (line, surface), and runout (circular, total). Only location, profile, and runout can locate a feature; orientation fixes tilt and, implicitly, form.
Two rules generate most of the answers. Rule #1 (envelope principle): unless otherwise specified, the limits of size define a perfect-form boundary at MMC, so size tolerance limits form error on a regular feature of size. Rule #2: regardless of feature size (RFS) is the default for every geometric tolerance; MMC or LMC must be explicitly shown in the frame. Basic dimensions, boxed, are theoretically exact and carry no tolerance of their own - the feature control frame supplies the tolerance, and implied 90-degree angles are basic.
Traps the exam favors: assuming MMC is the default; believing a geometric tolerance replaces the size tolerance (it is an additional requirement, except axis straightness at MMC, which relaxes Rule #1); attaching datums to form controls, which never use them; and putting a material modifier on runout, which is always RFS.
Rule of thumb: read every frame aloud as a sentence - this surface or axis must lie within a zone of this shape and size, at this material condition, relative to these datums in this order.