What are Geometric dimensioning and tolerancing GD & T Symbols

Geometric dimensioning and tolerancing is the representation of engineering tolerances that uses GD&T symbols. Engineering drawing and computer-aided design models use GD & T. GD & T symbols are communicating nominal geometry and acceptable variation in engineering modeling. The purpose of GD&T is to inform engineering staff about the required degree of accuracy and precision of all the features of the part. Stanley Parker initiated GD&T in the form of a true position.

GD & T Rule:

There are several standards for GD&T symbols.

  1. The American Standard of Mechanical Engineers (ASME) Y14.5
  2. The International Organization for Standardization (ISO).

Y14.5 is a single document set of standards for the GD&T symbols. Where 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. Extra dimensions are marked as references.
  4. The dimension should be applied to the feature that represents the function of the features. And it should be used for only one interpretation.
  5. Avoid the explanation of the manufacturing process.
  6. Arrange all the dimensions and tolerances for maximum readability.
  7. Lines at the right angle represent the orthogonal angles.
  8. All the dimensions and tolerances are at 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 symbols feature control frame.

GD & T Symbols:

There is a total of 14 numbers of GD&T characteristics. A separate symbol represents each characteristic. These symbols are the GD&T symbols.  Five tolerances categories classify these characteristics as follows.

  1. Form tolerance
  2. Orientation tolerance
  3. Location tolerance,
  4. Runout tolerance
  5. 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 by attaching the frame. The frame contains only one message if multiple messages are required so multiple frames are attached. The frame contains Geometric characteristics.

  1. The shape of the tolerance zone
  2. Feature tolerance
  3. Feature modifier
  4. Primary datum
  5. Secondary datum
  6. Tertiary datum.

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

Leave a Comment