Introduction to GD&T
What is GD&T?
A design model is an idealized representation of a part design. However, the design model by itself does not fully define the design. Due to imperfections in manufacturing processes, physical parts never match the design model exactly. An important aspect of a design is to specify the amount the part features may deviate from their theoretically exact geometry (as defined in the design model).
Geometric dimensioning and tolerancing, often referred to as GD&T, is a symbolic language used on engineering drawings and models to define the allowable deviation of feature geometry. The language of GD&T consists of dimensions, tolerances, symbols, definitions, rules, and conventions that can be used to precisely communicate the functional requirements for the location, orientation, size, and form of each feature of the design model. Thus, GD&T is an exact language that enables designers to “say what they mean” with regard to their design models. Production can then use the language to understand the design intent and inspection looks to the language to determine set up requirements.
In order for any language to be effective, it must be based on a common standard. ASME and ISO both have standards related to the application and use of GD&T. The ASME standards are generally used by U.S. companies while the ISO standards are commonly used by companies based in other countries. Within each of those organizations there are several standards that are applicable to the topic of GD&T. Furthermore, each standard is periodically revised, with each version of the standard identified by the year of its acceptance.
GD&T Advisor supports the following tolerancing standards. (Click on the standard to link to a page that provides details about the specific standards that are referenced by that rule set).
General Approach to Applying GD&T to a Design Model
A design model consists of a collection of geometric shapes, referred to as features (see Feature Descriptions for a complete list of supported feature types). In a completed design, the ideal geometry of each feature must be defined, along with the permissible deviation of its geometric attributes (e.g., location, orientation, size, and form). There are two common approaches to defining the permissible deviation of the geometric attributes of the features in the design model. GD&T may be applied to each feature based on how the part functions or it may be based on how the part is manufactured. It is generally considered best practice to apply GD&T based on part function. The first step in applying GD&T to a design model is to establish a datum reference frame. The first datum reference frame that is established is called the predominant datum reference frame. In a functional approach to GD&T, the surfaces that orient and locate the part in its assembly are typically used as the datum features that are used to establish the predominant DRF. There are also additional considerations for datum feature selection:
Manufacturing considerations – You may choose to select the surfaces that orient and locate the part in the machining fixture as the datum features.
Inspection considerations – You may choose to select the surfaces that orient and locate the part during the inspection process as the datum features.
Once the predominant DRF has been established, you can use GD&T to constrain the geometry attributes of each feature in the model with respect to that DRF based on function. In complex parts it may be necessary to establish multiple DRFs, but ultimately all features in the part must be constrained, either directly or indirectly, to the predominant DRF. GD&T Advisor provides convenient tools for defining the features on your part, applying GD&T to those features, and then evaluating each feature and the applied GD&T on the feature to determine whether all of the geometry attributes of the feature are fully constrained.
In addition to annotations applied directly to the features of the model, the tolerancing standards also require various general notes. GD&T Advisor provides tools to help you to manage those notes. See Properties & Notes, ASME Standard Reference Notes and ISO Standard Reference Notes for more information.