Role of Key Terms

Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used in engineering and manufacturing to define the shape, orientation, and allowable variations of a part. It ensures that parts fit and function correctly by providing precise and standardized specifications. 

Understanding key GD&T terms is essential for interpreting technical drawings and ensuring quality control in production. In this guide, we will explore fundamental GD&T terms, each accompanied by drawings to illustrate their meaning and application. These visual examples will help clarify how geometric tolerances control form, orientation, location, and profile, enabling better communication between designers, engineers, and manufacturers. 

 A feature is a general term applied to a physical portion of a part, such as a surface, hole or slot. An easy way to remember this term is to think of a feature as a part surface. The part in figure below contains seven features: the top and bottom, the left and right sides, the front and back, and the hole surface. 

 A complex feature is an irregularly curved surface or a collection of adjoining surfaces or elements that are treated as a single feature. It is a single surface of compound curvature or a collection of other features that constrains up to six degrees of freedom. In the figure below, the entire external surface of the part between points A and B is an example of a complex feature.  

 An interrupted feature is a feature that contains an interruption. The arrow is pointing to interrupted feature in the below example.

Repetitive features or dimensions may be specified by the use of an "X" in conjunction with a numeral to indicate the number of places required. Where used with a basic dimension, the "X" may be placed either inside or outside the basic dimension frame. No space is used between the number of occurrences and the "X." A space is used between the "X" and the dimension.

A feature of size (FOS) is a general term that is used to refer to either a regular feature of size or an irregular feature of a size. 

 A regular feature of size is one cylindrical or spherical surface, a circular element, or a set of two opposed parallel elements, or opposed parallel surfaces, each of which is associated with a directly toleranced size dimension.  

1. Cylindrical 2. Opposed Parallel Lines 3. Opposed Parallel Surfaces 4. Circular Element

 An Irregular feature of size (type A) is a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a sphere, cylinder or pair of parallel planes. Example of type A IFOS is shown below.

An Irregular feature of size (type B) is a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope other than a sphere, cylinder or pair of parallel planes. Example of type B IFOS is shown below.

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