GD and T | The language of communication for Engineers!

Geometric dimensioning and tolerancing(GD and T) is a structured language of symbols, rules, dimensions, tolerances, and definitions. It allows the designers to define the geometrical features of mechanical parts according to their functional requirements such as location, orientation, size, and form. Hence, making GD and T a precise language that allows the designers to “say what they mean” with respect to their design models in an easy & a simple way.

In this article, I'm going to discuss the history of GD and T and why it is used. Later, I will also discuss how to apply GD and T to the drawings & advantages of using GD and T.

GD and T

GD and T is a language of communication for engineers. It helps the designer to communicate with the manufacturer & the quality inspector using functional features on engineering drawings. It also offers a comprehensive, methodical, logical, cost-saving, feature-based approach to the specification of engineering components. Moreover, with GD and T, one can develop clearly defined designs, taking the function of the part into consideration and allowing for more accurate tolerancing. However, GD and T are more of an addition to the coordinate dimensioning system rather than a complete replacement. 

History of GD and T

The beginning of GD and T has been credited to a man called Stanley Parker. In 1940 while working at the Royal Torpedo Factory, Parker realized that dimensioning the features of components using toleranced nominal dimensions had its own limitations. He also noticed that few components were not manufactured w.r.t drawing and therefore, were termed as scrapped or reworked part. Thus, leading him to develop the concept of "true position" which in turn set the beginning of GD and T.

Since then GD and T have been evolved gradually into a widely used universal language that can be understood by design engineers, manufacturing engineers, quality inspectors, customers and quality personnel. In fact, the concept of GD and T was also adopted by the military to increase production of naval weapons in the 1950s and is now in use in multiple industries around the world. The GD and T language are currently used in Automotive, Heavy Equipment, Aviation and several other industries.

Why GD and T?

GD and T help to fill the communication gap from the designer to all the way to the shop floor. By using the established standards GD and T, the design intent can be fully understood by suppliers all over the world.

There are several other advantages of implementing GD and T which we will discuss later in this article. At first, we have the two main reasons behind the implementation of GD and T.

1. A design model is an idealized representation of a part design. However, the design model by itself does not fully define the design. GD and T permit the designer to fully define the design model in a single sheet of paper. 
2. To reduce the cost many components, parts are currently being sourced from suppliers all over the world such as China and India. The need for accuracy in part design and manufacture is greater now than ever before. This increases the demand for the use of GD and T so that the design intent and functionality of the part can be clearly communicated to the designer and the manufacturer.

How to Apply GD and T to Drawings?

The fundamental rules of GD and T are the foundation on which the system is built. There is a very large amount of information available describing how to apply GD and T to your designs. When these are applied properly, the design engineer can easily define the features, locations, size, shape, and orientation of the part. In this section we will explore some fundamental rules, basic concepts and symbols contained within the GD and T standards. 

1. Datum Reference Frame

The Datum Reference Frame in engineering is a three-dimensional Cartesian coordinate system. It’s one of the most important concepts in GD and T. It describes the creation of three mutually perpendicular planes. These three planes are required for manufacturing and inspecting the part. The parts are mated to the datum reference frame so that measurements, processing, and calculations can be made easily. The three datums are mentioned below: -

1. The primary datum is established with a minimum three contact points.
2. The secondary datum is established with a minimum of two contact points.
3. The tertiary datum is established by a minimum of one point of contact.

In order to design, manufacture, and verify parts, the necessary degrees of freedom must be constrained. This datum reference frame helps the engineers to constrain the six degrees of freedom (3 translational and 3 rotational) with the help of datums.  

2. Language of Symbols

GD and T is basically a language of symbols representing geometric characteristics or tolerance type of a particular feature. These symbols are placed in the first compartment of a feature control frame to define the type of tolerance applicable to the particular feature. The most frequently used tolerance type are as follows: - 

a. Form tolerances 

It controls the shape of features and is often used as a refinement of size.

b. Orientation tolerances 

It controls the tilt of feature and is always associated with basic angle dimensions, often used as a refinement to location.

c. Location tolerances 

It controls the location and is always associated with basic linear dimensions.

d. Runout tolerance

It controls the location of a circular part feature relative to its axis. This is different from circularity, which controls overall roundness.

--> In order to properly apply GD and T, you must become familiar with the language of symbols. There are a total of fourteen GD and T symbols that represent different types of tolerances. It helps the designer to define a precise relationship between the geometrical features of a component by the application of tolerance type to the following characteristics: flatness, straightness, circularity, cylindricity, angularity, perpendicularity, parallelism, position, symmetry, concentricity, profile and runs out.

Use this link to look for the 14 symbols.

3. Feature Control Frame

The feature control frame is attached to a feature stating its requirements or instructions. It consists of several compartments which are used to define a particular feature thoroughly. Only one feature can be defined through a single feature control frame. If there are two feature then there must be two feature control frames. Three main compartments of a feature control frame are mentioned below.

a. First Compartment

The first compartment of a feature control frame contains one of the fourteen geometric characteristic symbols. It is used to define the tolerance type of the feature such as Location, Orientation, Runout, Form. 

b. Second Compartment

The second compartment of a feature control frame contains the total tolerance value for the feature. In some cases, the tolerance may be preceded by a diameter symbol. If the tolerance is preceded by a diameter symbol (?), the tolerance is a diameter as in the position of a hole. However, if there is no symbol preceding the tolerance, the default tolerance zone shape is parallel planes or a total wide zone, as in the position of a slot or profile of a surface.

c. Third Compartment

The third and preceding compartments of a feature control frame contain the datum feature reference(s). Always remember to list the datum planes in the proper sequence i.e from the primary datum.

4. Basic Dimensions

Basic dimensions are theoretically exact numerical values used to define form, size, orientation, or location of a part or feature. These are usually shown on a drawing enclosed in a box, but they can also be invoked by referencing a standard or by a note on the drawing. These dimensions can either be the base dimension or chain dimension depending on the readability of drawing. The diagram below depicts the basic dimensions.

5. Tolerances

An important aspect of a design is to specify the amount that the part features may deviate from their theoretically exact geometry. This problem is overcome by the use of tolerances. These are an allowable amount of variation in dimensions, properties, and conditions without significantly affecting the functioning of systems, machines, and structures. The engineer or designer should attempt to keep tolerances as large as possible because small tolerances can lead to increase in the cost of manufacturing, inspection, and tooling of parts. However, tight tolerances are sometimes necessary for preserving the function of the part.

Advantages of GD and T

From communication to ease of manufacturing, GD and T come with several advantages. Below are the few advantages of using GD and T in your drawings.

1. GD and T is a standardized language of communication between the Customer, Designer, Manufacturer & Quality Inspector.
2. The production department uses this language to understand the design intent and inspection uses this language to determine set-up requirements.
3. GD and T decrease the manufacturing cost as reworked and scrapped part are reduced dramatically.
4. The need for extra drawing notes is also reduced as it easily defines the drawing requirements with the help of symbols.
5. It offers a clear and brief technique for defining a reference coordinate system i.e. datums. These datums are used throughout the manufacturing and inspection process.
6. It tells the manufacturer about the limits of tolerance value within which the part will be functional & easy to assemble. 
7. The application GD and T methodology have also proven to improve quality, reliability, and safety. 
8. It is also used to simplify tolerance analysis.
9. GD and T are also used as a method for calculating the worst-case mating limits.
10. It also leads to better assembling process as the produced parts are qualified parts.

Learning GD and T can help you become an advanced design engineer and serve as a road to your Dream Mechanical Company.

Best of Luck!