Geometric Dimensioning & Tolerancing. Explained practically.

GD&T expresses functional requirements (how a part must behave in assembly), while plus/minus tolerances only limit dimensional extremes without context. A hole tolerance of ±0.05 mm tells you the hole size, but says nothing about whether it's concentric to the shaft axis or perpendicular to the mounting face. GD&T position, concentricity, and perpendicularity express the actual functional requirement. Result: easier-to-manufacture parts that still work correctly.

A feature control frame is the rectangular box containing GD&T callouts on a drawing. Reading left to right: geometric symbol (e.g., position ⌖), tolerance value (e.g., 0.05), material condition modifier (e.g., ⓂMMC), then datum references (A|B|C). Example: | ⌖ | ⌀0.05 Ⓜ | A | B | C | means "position tolerance of diameter 0.05 at maximum material condition, referenced to datums A, B, and C."

Maximum Material Condition is the drawing size that leaves the most material on the part. For external features (shafts, pins): MMC is the largest size. For internal features (holes, slots): MMC is the smallest size. The Ⓜ modifier means "tolerance applies at MMC" — which allows bonus tolerance as the feature moves away from MMC. Ⓛ (LMC, least material condition) is the opposite. For rigid assembly fits, MMC is often appropriate.

When a feature is toleranced at MMC (Ⓜ), additional tolerance is allowed as the feature departs from MMC. For example: ⌖ ⌀0.05 Ⓜ on a Ø10 +0.1/-0 hole. At MMC (Ø10), position tolerance is 0.05. As hole grows larger (⌀10.05), bonus tolerance of 0.05 is added → effective position tolerance becomes 0.10. Bonus tolerance makes parts easier to manufacture while guaranteeing assembly fit.

For 80% of drawings: position (⌖) for hole and slot locations, perpendicularity (⊥) for critical mating faces, parallelism (∥) for flat-to-flat relationships, flatness (⏥) for surfaces that must be flat, and concentricity (⊙) or runout (↗) for rotating features. Advanced callouts (profile of surface ⌔, profile of line ⌓, composite tolerancing) are valuable for complex curves but not needed for typical machined parts.

Choose datums that match functional mating sequence: primary datum = largest/most stable feature (floor of part, largest face). Secondary datum = second most important mating feature (typically a pin or edge that locates the part rotationally). Tertiary datum = the final locating feature. For a flanged part, this is usually: primary = flange face, secondary = bolt hole, tertiary = outside edge. Matches how the part mates during assembly.