Introduction to GD&T

± tolerances on location create square tolerance zones and ambiguity about where to measure from. GD&T (ASME Y14.5, and the ISO GPS system including ISO 1101) defines exact tolerance zones relative to datums that reflect how the part is actually located and used. Position tolerances, for example, give round zones for holes — about 57% more area than the equivalent square zone.

A feature control frame reads left to right: symbol (e.g. position ⌖), tolerance (⌀0.2), optional modifier (Ⓜ for maximum material condition), then datum references in order of precedence (A, B, C). Example: ⌖ ⌀0.2 Ⓜ | A | B | C — the hole's axis must lie within a 0.2 mm diameter cylinder, located from datums A, B, C by basic (boxed) dimensions.

Datum features (marked with a triangle and a letter) are real surfaces on the part; the datum reference frame they establish is how the part is held for measurement: primary datum contacts at three points, secondary at two, tertiary at one. Choose datums that match how the part mounts.

GD&T is a deep subject with differences between ASME and ISO. This lesson helps you read common callouts; take formal training (and use the standard itself) before specifying GD&T on production drawings.