The GD&T Model

What you need to know about model-based GD&T for the manufacturing enterprise.

Model-based Geometric Dimensioning and Tolerancing (GD&T) is powerful digital intelligence that can automate and improve quality across the entire manufacturing enterprise. A snapshot of the manufacturing companies implementing it and involved in the standards process points to the growing importance of this 3-D digital manufacturing technology: Spirit AeroSystems Inc.; Raytheon Missile System; Northrop Grumman Corp.; Caterpillar Inc.; The Boeing Co.; Lockheed Martin Aeronautics Co.; Ford Motor Co.; General Motors Corp.; Rolls-Royce Corp.; GE Aviation, Goodrich Aerostructures; Bombardier Inc.; BAE Systems; Rockwell Collins; and Eaton Corp.

The Model Based Enterprise

The Model Based Enterprise (MBE) organization is a great source of information and defines MBE as “an integrated and collaborative environment, founded on 3-D product definition (model-based definition) shared across the enterprise, enabling rapid, seamless and affordable deployment of products from concept to disposal.” They state that “The practice of using 2-D drawings as the master deliverable in the design process is fading fast, as many companies are adopting a new approach called MBE, where the 3-D model is the central source of all design information. A key driver to this trend is the development of new standards for 3-D model annotation which specify that 3-D drawings now include all information to satisfy the needs of downstream users.”

Technical Tips

  • The more CAD file formats your inspection software can import, the more customers you can better support.
  • The inspection software’s ability to connect to any brand of measurement device enables you to select the right CMM or 3-D scanner.
  • A common software platform that supports all major CMM and 3-D scanner Model Based Enterprise brands can reduce training and support costs.

Design Phase: Defines GD&T to ensure product meets specifications. GD&T provides a consistent, sound method for communicating design intent.

Engineering Phase: Simulates GD&T of the product assembly variation and performance. Virtual simulation of the entire assembly of parts under max/ min variation conditions identifies assembly issues before manufacturing. Virtual simulation identifies key characteristics critical to assembly and operation. Reduce costs by identifying non-key characteristics and relaxing their tolerances.

Manufacturing Phase: Guides the selection and management of manufacturing processes.

Inspection Phase: Specify inspection process for effective assessment of manufacturing process and end product quality. Facilitate gaging requirements and automate coordinate metrology. Define, monitor and report on statistical process control (SPC) of key characteristics. Three-dimensional SPC findings can be communicated back to design phase for closed loop continuous improvement.

Maintenance Phase: Maintenance, repair and overhaul (MRO) processes reference GD&T while inspecting product for wear and reassembly. LOng Term ARchival (LOTAR) for sustainment of product until end of life requires three technology life cycles be considered and are product data for at least 50 years, the data storage for about 10 years and the software application for about three years.

Model-based GD&T Product Definition Data: Also known as Product Manufacturing Information (PMI). Denotes the totality of data elements required to completely define a product. Product definition data includes geometry, topology, relationships, tolerances, attributes and features necessary to completely define a component part or an assembly of parts for the purpose of design, analysis, manufacture, test and inspection. See ASME Y14.41-2012 Digital Product Definition Data Practices or The Boeing Corp. D6-51991 “Quality Assurance Standard for Digital Product Definition,” which specifies requirements for their global supply chain.

Geometric Dimensioning and Tolerancing: GD&T is a system for defining and communicating engineering tolerances. The primary purpose of GD&T is to ensure a product is manufacturable, assemble-able and functional. The primary benefits of GD&T are: it encourages robust design with guaranteed assemblability and functionality; reduces cost through unambiguous communication and looser tolerances; and establishes 3-D metrology as a scientific process. See ASME Y14.5-2009, Dimensioning and Tolerancing or ISO 1101 Geometrical Product Specifications.

Annotations: Dimensions, tolerances, notes, text or symbols visible without any manual or external manipulation.

Digital Element: Geometric element, feature, group of features, annotation, associated group, or attribute that exists in a data set.

Associativity: The established relationship between digital elements.

Model-based GD&T: The primary purpose of model-based GD&T is to clearly communicate GD&T across the manufacturing enterprise by associating GD&T to the 3-D CAD model and storing them and transporting them in the computer-aided design (CAD) file. The primary value of model-based GD&T is increased when time to market is critical; supply chains are dispersed; and establishing and monitoring quality is of high importance. Three-dimensional software applications that support model-based GD&T associativity can be identified by checking the user interface for three digital elements that are associated to each other. These are the GD&T annotation, GD&T annotation tree, and the GD&T associated model feature. Associativity between the elements is visually communicated by the highlighting of all three elements when selecting any one of the elements.

Associativity is the established relationship between digital elements.