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Example of true position geometric control defined by basic dimensions and datum features. Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof.
The standard (size) tolerances are divided into two categories: hole and shaft. They are labelled with a letter (capitals for holes and lowercase for shafts) and a number. For example: H7 (hole, tapped hole, or nut) and h7 (shaft or bolt). H7/h6 is a very common standard tolerance which gives a tight fit.
ASME Y14.5 is a standard published by the American Society of Mechanical Engineers (ASME) to establish rules, symbols, definitions, requirements, defaults, and recommended practices for stating and interpreting Geometric Dimensions and Tolerances (GD&T). [1]
Other features share the same characteristic. For example, if the drawing shows 8 holes on a bolt circle, and just one is dimensioned, with "TYP" or "(TYP)" following the dimension label, it means that that hole is typical of all 8 holes; in other words, it means that the other 7 holes are that size also.
Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined.
Model-based definition (MBD), sometimes called digital product definition (DPD), is the practice of using 3D models (such as solid models, 3D PMI and associated metadata) within 3D CAD software to define (provide specifications for) individual components and product assemblies.
ISO 129-4:2013 Technical product documentation (TPD) — Indication of dimensions and tolerances — Part 4: Dimensioning of shipbuilding drawings ISO 129-5:2018 Technical product documentation — Indication of dimensions and tolerances — Part 5: Dimensioning of structural metal work
The simplest dimensioning system just specifies distances between points (such as an object's length or width, or hole center locations). Since the advent of well-developed interchangeable manufacture , these distances have been accompanied by tolerances of the plus-or-minus or min-and-max-limit types.