Knowledge Base

Geometric Dimensioning and Tolerances Analysis (GD&T)

During World War II, the United States manufactured and shipped spare parts overseas. It was noticed that though they were made to specifications, the parts did not assemble correctly. After the war, a committee was formed to tackle the defective parts problem. The objective of the committee was to find a way to ensure that parts would fit and function properly, irrespective of where they were manufactured. Geometric Dimensioning and Tolerancing (GD&T) is the result of that endevour.

Coordinate Tolerancing System

Before Geometric Dimensioning and Tolerancing standards were introduced, the most popular system for determining tolerances was the coordinate tolerancing system. It is a dimensioning system in which the X, Y, Z coordinates of the centres of features of size and surfaces are located by means of linear dimensions with given tolerances. Though this system was popular when companies were small and there was verbal communication between the design engineer and the machinist, it became redundant when companies grew in size, and designing / production was not restricted to a single location. The shortcoming of this system over geometric dimensioning and tolerances is that it leaves out several important pieces of information about how to inspect the part. It also increases costs and causes safety or functional problems. Geometric Dimensioning and Tolerances (GD&T) standards were introduced to overcome these shortcomings.

Defining GD&T

When design engineers hand over their drawings to production engineers, they need a common technical language for communicating exactly what must be done.  Geometric dimensioning and tolerances is that language. It is a precise language of engineering symbols that clearly communicates the design intent of the part so that everyone is on the same page. These symbols are used to describe a part’s nominal geometry and the allowable tolerance for variation. The result is an improvement in communication between design team and the production tem, and even between design teams. Ultimately, geometric dimensioning and tolerances result in improved part quality. The GD&T methodology is currently used in automotive, heavy equipment, aviation and several other industries. When applied properly, the design engineer can concisely define a features location, size, shape and orientation on the part. Geometric dimensioning and tolerancing is hence intended to be an addition to the coordinate dimensioning system, not as a complete replacement.  The geometric dimensioning and tolerancing methodology takes into consideration the function of a part and how that part functions with related parts. Therefore, for proper application of GD&T design engineers must have a thorough understanding of the function of the part within an assembly. A properly toleranced drawing is a picture that not only communicates the size and shape of the part but also tells a story that explains the tolerance relationships between features.

In short, geometric dimensioning and tolerances (GD&T) is a design tool that uses symbolic language to perfectly capture the design intent.

Geometric dimensioning and tolerances standards should be used when:

  • Drawing delineation and interpretation need to be the same
  • Features are critical to function or interchangeability of mating parts
  • it is important to stop scrapping perfectly good parts and reduce drawing changes

Without common specifications of dimensions and tolerances, engineering drawings would not be understood easily. If there were no GS&T standards, it would be impossible to determine if parts meet their specifications.

There are two major objectives of GD&T analysis:

  •  to define the geometry of perfect, as-designed as-modelled and as-drawn parts and assemblies, which is the basic purpose of dimensioning
  • to define the allowable geometric variation for as-produced parts and assemblies (i.e. the tolerances)

Geometric Dimensioning and Tolerances - Feature Control Frames

When there are mating parts, they should have coordinated datum reference frames, with the interfacing surfaces specified as primary datum features. Two co-ordinate systems are established from these surfaces, one on each part. Relating features on each part to these datum references minimizes variation between related features on each part. While simple parts may have a single datum reference frame, complex parts may have many datum reference frames due to geometry or functional necessity. In such cases, each datum reference frame on a part must be related to the other datum reference frames on the part, either directly or indirectly.

Types of Geometric Dimensioning Tolerances

Form
A form tolerance specifies how far an actual surface or feature is permitted to deviate from the desired form specified in a drawing. It includes flatness, straightness, circularity, profile of a surface and profile of a line.

Orientation 
This type of GD&T specifies how far the actual orientation between two features is permitted to deviate from the perfect orientation given in the drawing. It includes perpendicularity, angularity and parallelism.

Profile
Profile geometric dimensioning and tolerance is a 3D tolerance that defines a uniform boundary around a surface within which the elements of the surface must lie.  It simultaneously controls a feature‘s form, size, orientation, and sometimes location.  It is usually used on parts with complex outer shape and a constant cross-section such as extrusions.

Location and Runout
The location tolerance specifies how far an actual feature is permitted to deviate from the perfect location given in a drawing as related to datum or other features. It includes position, concentricity and symmetry. The runout tolerance specifies how far an actual surface or feature is permitted to deviate from the desired form given in a drawing during full rotation of the part on a datum axis. There are two types of runout - circular and total.

To GD&T or not to GD&T?

In order to apply geometric dimensioning and tolerances properly, the design engineer should know the manufacturing process that will be used to manufacture the part. All dimensioning processes provide a certain degree of geometric control. The decision of whether to apply GD&T or not depends to a large degree on whether the normal control provided by a process meets the design functional requirements. If it can meet requirements do not use GD&T, else use GD&T. In addition, geometric dimensioning and tolerance could be used to ensure consistency of datum references between design, manufacture and verification operations, and also when computer tools are used during design and manufacture.

CAD / CAM in Geometric Dimensioning and Tolerances Analysis

Since perfect dimensioning is virtually impossible to achieve in real world, tolerances were introduced. Tolerance is crucial not only for the functionality of the component, but also for the economy of manufacturing. The increasing use of CAD systems and integrated CAD / CAM systems (like the 3DCS Variation Analyst) involves the development of sophisticated tolerancing techniques. The success of a program like DCS Variation Analyst lies in the intelligent specification of tolerances for design and manufacturing. CAD / CAM systems use geometric model of the numerical world to achieve optimum tolerances.

A good Geometric dimensioning and Tolerance (GD&T) software:

  • Defines feature control panel useful in the monitoring of manufacturing tolerance such as flatness, parallel, perpendicularity, etc.
  • Defines both the feature control and datum symbol for complete manufacturing detailing
  • Applies different dimensioning schemes such as size & location dimension, datum and geometric tolerance
  • Applies the auto-dimensioning scheme for GD&T and checks for complete definition of the manufacturing product