GD&T symbols

About gD&T symbols

GD&T stands for geometric dimensioning and tolerancing: the symbols on an engineering drawing that say how far a part’s form, orientation and location may vary from the exact geometry the designer drew. A plain plus-or-minus dimension controls size. The GD&T symbols control what size alone can’t, such as whether a face is flat, whether a hole is square to the face it is drilled into, and where the hole sits. Gene Cogorno’s course guide calls it a symbolic language for the size, shape, form, orientation and location of features on a part. ASME, which publishes the US standard, Y14.5, calls it a tool for communicating design intent: that parts made from a drawing have the desired form, fit, function and interchangeability.

Each requirement is written in a feature control frame, a box read from left to right: the characteristic symbol, the tolerance with any modifier, and the datum letters it is measured from. These pages explain the fourteen characteristic symbols, grouped as form, profile, orientation, location and runout, and the modifiers and marks that go with them.

Fourteen symbols, or twelve

Drawings made to ASME Y14.5-2009 and earlier editions use fourteen characteristic symbols. The 2018 edition, which replaced the 2009 one, removed concentricity and symmetry, “term, symbol, and concept” in the words Mitutoyo’s Jim Salsbury quotes, leaving the twelve that Cogorno’s guide counts. NIST and UNC Charlotte’s seminar notes say position and runout now do their work. Both symbols are still found on older drawings, and ISO drawings keep a concentricity and coaxiality symbol and a symmetry symbol, as the UK National Physical Laboratory’s guide to ISO drawings shows.

ASME Y14.5 and ISO GPS

Two standards are in use. ASME Y14.5 is the American one. The international one is ISO 1101, which defines the symbol language for the geometrical specification of workpieces and the rules for reading it. It belongs to the geometrical product specification (GPS) standards of ISO’s technical committee 213, set up in 1996 (Vijay Srinivasan, NIST). Jaime Berez’s UNC Charlotte notes say the symbols and definitions are close enough that learning one teaches about 90% of the other, and list where they part: datums, the envelope principle as a default, third- and first-angle projection, drawing style and some symbols. Salsbury quotes two estimates of the gap. Lowell Foster put agreement at 90 to 95% in 1994; Alex Krulikowski wrote in 2010 that about 65% of possible tolerances are specified or interpreted differently. Each page notes where the two standards differ for its symbol.

Before the symbols

The Y14.5 standard was first published in 1966, NIST’s Shaw Feng and Theodore Hopp wrote in 1991. The engineering drawing notes for Illinois’s ME 170 course describe what the symbols replaced: long and often confusing written notes describing each feature.