Fundamentals and Scope of GD&T per ASME Y14.5
What the standard is and is not
ASME Y14.5 is a drawing-definition standard: it governs how geometric requirements are stated on an engineering drawing, not how parts are manufactured or measured. A candidate must know that the drawing is a legally binding contract, that dimensions and tolerances define the acceptable boundary of a part, and that GD&T exists to control variation in form, orientation, location, profile, and runout that plus/minus tolerancing cannot express cleanly.
Core concepts to command
Know the hierarchy: the drawing's basic dimensions, datum features, feature control frames, and the implicit defaults (Rule #1, also called the envelope principle; Rule #2 for MMC/LMC boundaries). Understand basic dimensions (theoretically exact, no tolerance) versus toleranced dimensions, and why basic dimensions must be tied to a datum reference frame. Understand the difference between a feature of size and a surface, and between regular and irregular features of size.
Traps that appear in questions
- Rule #1 applies to regular features of size only, and is overridden when a straightness or other form control is applied at MMC to the feature.
- Basic dimensions carry no tolerance of their own; their tolerance comes from the geometric control that references them.
- "Perfect form at MMC" is a default, not an absolute rule when overridden.
- Confusing the scope of Y14.5 with Y14.41 (digital data) or Y14.8 (castings).
Rule of thumb
If a question asks how much variation is allowed on a feature, first ask: is it a feature of size? If yes, Rule #1 gives you size plus form. If no, you need an explicit geometric control.
Worked example
A Ø10.0 ±0.2 hole with no geometric control: at MMC (Ø9.8) the hole must be perfectly cylindrical; at LMC (Ø10.2) the hole may be as much as 0.4 out of round. The size tolerance simultaneously limits form.