Geometric Dimensioning and Tolerancing (GD&T)

An ideal assembly design comprises of parts which fit each other right….right at the first time! In such a case, the designer is always sure of the assigned clearances and tightness. But in cases where a large number of these parts need replication, conveying the right size and shape (i.e. the nominal dimension of the surface of the part) to the manufacturer is almost impossible.

Given this, manufacturing processes usually boast of an integrated process variation as a result of which variations are reflected in the manufactured parts. For this purpose, designer often needs to analyze the entire assembly and establish variations to be allowed in case of proportions, form, direction and location for each surface of the part to be assembled. These calculated alterations (tolerances) is then communicated to by the designer which in turn impacts both the functional assembly and model part geometry. For this purpose, ‘just words’ are quite inadequate. For instance, a rather simple note instructing the manufacturer to ‘make a perfect cylindrical surface’ would only be effective when both designer and manufacturer-

  • Understand the tongue
  • Understand the intended surface (as per note) and its extent
  • Understand the context in which ‘cylindrical’ is used

While it seems simple in theory, practical examples tell otherwise.

After being severely affected by variations, Geometric Dimensioning and Tolerancing (GD&T) came into existence. A language comprising of specialized characters and graphic representation, GD&T makes communicating variations easy.

What is GD&T?

Geometric Dimensioning and Tolerancing (GD&T) is a standard which defines and communicates mechanical tolerances by way of using symbols on engineering drawings and CAD models. It gives a detailed representation of nominal and allowable variations in geometry. Popular for its lucidity, GD&T guarantees precision and accuracy, each feature of the part reaps the benefit of this standard language. Representation of nominal geometry and assemblies, allowable variations and possible sizes of features (or, parts) of assembly part is made easy by GD&T.

What is GD&T

  • Nominal, as intended to be modelled geometry is defined by dimensional specifications. Basic dimension is an example.
  • Variation to be allowed for size and form of each feature and variation to be allowed in case of location and orientation amidst features is defined by tolerance specifications, For example, datum reference used in linear dimensions and feature control frames.

Therefore, GD&T is often deemed to be a language which facilitates the designers requirement of “saying what they’re intending” pertaining to their model. Manufacturing sector often uses this language to understand the intention of the designer which on inspection helps them to set-up requirements.

Why GD&T is important?

Its characteristic of being unambiguous makes it difficult for users to draw their own interpretation. Due to this very unique characteristic, surfaces of parts are precise and easily controlled.

Following are few benefits of using GD&T:

  • It is easy; for representation & interpretation
  • It is unambiguous; leaves no room for confusion
  • It is simple; a thorough inspection as hard gages and inspection fixtures are allowable

GD&T considers everything -function, manufacturing processes and inspection methodologies! This promises large tolerances with function which in turn reduces cost of inspection and manufacturing. The ‘spare’ or extra available tolerance in certain designs significantly impacts production cost. Further, other integral resources like time (for analyzing the requirement of a missing dimension) is additionally saved.

GD&T Symbols

GD&T Symbols PDF

GD&T Symbols
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