GD&T Basics: GD&T Terms, Definitions and Symbols | What is GD&T | CNCLATHING


The base of effective rules is the common standard. GD&T is such a standard for assembly design and production. In this article, we’ll introduce the GD&T terms and definitions, as well as the chart of GD&T symbols.

GD&T Basics - What is GD&T?

GD&T (Geometric dimensioning and tolerancing) is a system used to define and communicate the tolerances for engineering designs and computer-generated 3D solid models. The symbolic language in GD&T describes the geometry and allowable variation or error, it can specify the requirements of accuracy and precision of the CNC machining parts

GD&T Basics - How GD&T Works?

The detailed features with dimensions need to be shown on an engineering drawings, the tolerance limits the minimum and maximum acceptable value. The GD&T language consists of dimensions, tolerances, symbols, definitions, rules, and conventions to set the right variations of location, orientation, size, and form of each feature of the design model, which provides great benefits and flexibility for production. The GD&T symbols are classified into five groups as below. Check out these GD&T terms and definitions.

1. Form controls

Straightness: a tolerance that controls the form of a line somewhere on the surface or the feature.

Flatness: a symbol that references how flat a surface is regardless of any other datums or features.  

Circularity: also called roundness, a 2-Dimensional tolerance that describes how close an object should be to a true circle.

Cylindricity: a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis.


2. Profile controls

Line profile: a two-dimensional tolerance range that that limits the amount of error for line elements relative to their true profile. 

Surface profile: a three-dimensional tolerance limits the amount of error a surface can have relative to its true profile, applies to all line elements collectively.


3. Orientation controls

Angularity: specifies a tolerance zone defined by two parallel planes at the specified angle other than 90 degrees from a datum plane or axis within where the surface or the axis of the feature must lie.

Perpendicularity: a three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from a 90-degree angle

Parallelism: the condition of a surface or center plane equidistant at all points from a datum plane, or an axis. 


4. Location controls

Position: the location of one or more features of size relative to one another or to one or more datums.

Concentricity: a condition in which the axes of all cross-section elements of a feature’s surface of revolution are common to the axis of a datum feature. 

Symmetry:  a three-dimensional geometric tolerance that controls how much the median points between two features may deviate from a specified center plane or axis. 


5. Runout controls

Circular runout: can be defined as two-dimensional geometric tolerance that controls the form, orientation, and location of multiple cross sections of a cylindrical part as it rotates.

Total runout: involves tolerance control along the entire length of, and between, two imaginary cylinders, controls cumulative variations in circularity, coaxiality, straightness, taper, angularity, and profile of a surface. 

GD&T Symbols - GD&T Symbols Chart

Here is a GD&T Symbols Chart including common GD&T symbols can be used in your drawing: