What are Geometric dimensioning and tolerancing (GD&T) and its purpose

Geometric dimensioning and tolerancing is the representation of engineering tolerances that uses symbols. It is used on engineering drawing and computer-aided design models for communicating nominal geometry and acceptable variation.  The purpose of GD&T is to communicate with engineering staff about the required degree of accuracy and precision of all the features of the part. GD&T was initiated by Stanley Parker in the form of a true position.

GD&T Rules:

There are several standards for symbols in GD&T; American Standard of Mechanical Engineers (ASME) Y14.5 and International Organization for Standardization (ISO). Y14.5 is one document set of standards for the symbols and ISO standards are topic-based categorized.

There are several fundamental rules for dimension and tolerance defined in Y14.5 standards.

  1. All the dimensions must have a tolerance for every feature that will subject to variation.
  2. Scaling is not allowed but only in certain cases.
  3. Engineering drawing defines the finished part. Additional dimensions are marked as reference.
  4. The dimension should be applied to the feature that represents the function of the features and should be used for only one interpretation.
  5. The explanation of the manufacturing process should be avoided.
  6. All the dimensions and tolerances should be arranged for maximum readability.
  7. Orthogonal angles are shown with lines at the right angle.
  8. All the dimensions and tolerances are shown for 20 degrees C and 101.3kPa unless stated otherwise.
  9. All the dimensions are valid for the free state unless stated otherwise.
  10. Dimension and tolerances are applicable for the length, width, and depth of a feature.
GD&T-feature-control-frame.

GD&T symbols:

There is a total of 14 numbers of GD&T characteristics and each of them is represented by a symbol called the GD&T symbol.  These characteristics are categorized into 4 tolerances: Form tolerance, Orientation tolerance, Location tolerance, runout tolerance, and location of derived median point tolerance.

Form tolerances control the shape of a feature. Orientation tolerance controls the tilt of a feature. Runout tolerance controls the co-axiality of the surface. Location tolerance controls the location of the surface, center point, axis, and derived median point of a feature.

Feature control frame:

The feature control frame represents the requirement of a feature to with the frame is attached. The frame contains only one message if multiple messages are required so multiple frames are attached. The frame contains the Geometric characteristics, the shape of tolerance zone, feature tolerance, feature modifier, primary datum, secondary datum, tertiary datum.

GD&T replaces the traditional dimension and tolerances. The use of proper GD&T improves the quality of production and reduce the resources required for production. It is a standardized design method to communicate them clearly and precisely between the production team, customer, and suppliers. The assembly is assured from GD&T produced parts and the production process is repeatable.  GD&T symbols can be inserted in AutoCAD using tolerance command with display the dialog box

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