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Geometric Dimensioning & Tolerancing

Geometric dimensioning and tolerancing (GD&T) is a method of specifying geometry of physical objects on conventional or CAD drawings, along with allowable variations (tolerances) in form, size and orientation.

Note: Use of GD&T is optional and NOT needed to design and make parts in eMachineShop – see Tolerancing.

One of the most common standards for GD&T is ANSI Y14.5.

GD&T uses rectangular blocks:

block

… which include:

  1. A GD&T symbol
  2. A tolerance
  3. When needed, up to 3 letters that correspond to datums (from which measurement is to be made).

The above block would be read, "The position of the feature shall be according to the dimensions given, +/- 0.003, as measured from datums A and B." Datums are the lines or planes from which measurements are to be taken – often the lowest and leftmost part of a basically rectangular workpiece or the center of a basically round part.

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