Symbols, Terms, and Definitions
The vocabulary a senior candidate must own
You must be able to define, without hesitation: feature, feature of size, datum feature, datum, datum reference frame, actual mating envelope, virtual condition, resultant condition, bonus tolerance, basic dimension, and the modifiers MMC, LMC, RFS, and free state. Each symbol in a feature control frame has a precise meaning: the geometric characteristic symbol, the tolerance value, the material condition modifier, and the datum references in order of precedence.
Reading a feature control frame
A feature control frame reads left to right: characteristic, tolerance zone shape and size, material condition modifier, then datums in primary-secondary-tertiary order. The tolerance zone shape is implied by the characteristic (cylindrical for position on a hole, two parallel planes for flatness, two concentric circles for circular runout).
Traps that appear in questions
- Confusing virtual condition (a constant boundary based on MMC or LMC and the geometric tolerance) with resultant condition (the actual boundary after variation).
- Assuming RFS is the default for all controls; RFS is the default for position, but MMC/LMC must be explicitly stated.
- Misreading datum order: the primary datum constrains the most degrees of freedom and appears first.
- Confusing the actual mating envelope (perfect form, smallest circumscribing or largest inscribed) with the actual feature surface.
Rule of thumb
Virtual condition = MMC (or LMC) size ± the geometric tolerance. For an external feature at MMC: VC = MMC + geometric tolerance. For an internal feature at MMC: VC = MMC − geometric tolerance.
Worked example
A pin Ø12.0 ±0.1 with position Ø0.3 at MMC. Virtual condition = 12.1 + 0.3 = Ø12.4. That is the largest boundary the pin can occupy, and the mating hole must clear Ø12.4 to guarantee assembly.