# Integrating Geometric Dimensioning and Tolerancing with Additive Manufacturing: A Perspective
## 4 Digital Thread and MBD Integration  
A recent improvement in Additive Manufacturing is the combination of Model-Based Definition (MBD) with GD&T within the digital thread ecosystem. By ensuring that tolerance criteria are efficiently transferred from design to both manufacture and inspection, this integration is expected to improve the AM workflow's accuracy and efficiency. It is expected in the near future that GD&T information, material specifications, and other annotations will be embedded directly into a 3D CAD model, eliminating the reliance on traditional 2D drawings. Even though we live in an increasingly digital world, this central step is not easy to implement immediately. Many companies, especially SMEs, still work using 2D drawings for manufacturing. This requires the designer to indicate the geometric and dimensional tolerances on this drawing, rendering the analysis of AM processes more complex. By empowering MBD, even for traditional subtractive processes, it will be easier to consider the precise definition of tolerances on simple geometries but also on freeform surfaces and lattice structures. ISO standards are aware of this opportunity, as demonstrated by ISO 16792:2021 [26]--Digital Product Definition Data Practices, which specifies requirements for the preparation, revision, and presentation of digital product definition data, and supports two methods of application: 3D model only and 3D model with 2D drawing in digital format. Under these premises, organizations are turning to Additive Manufacturing (AM) as a solution to supply chain and manufacturing restrictions in today's more complicated production and sustainability contexts. However, companies must oversee intricate, interconnected, and data-driven events to deploy AM on an industrial scale. The secret to enhancing AM capability is the digital thread, a single, seamless strand of data that runs from the original design to the completed part. Digital thread's primary benefit is that it allows designers to directly implement GD&T into 3D models, ensuring their tolerance objectives are maintained throughout the manufacturing and inspection processes. When specifying tolerances on complex surfaces and interior aspects that are difficult to accurately depict in 2D drawings, this is especially helpful. Additionally, automatic tolerance checks are made easier during the process simulation and build preparation phases by incorporating GD&T within the MBD framework. In fact, automatic GD&T assignment and validation for AM-specific geometries is now supported by software programs like Siemens NX version 2306 (Siemens AG, Munich, Germany), PTC Creo (PTC Inc., Boston, MA, USA) and Autodesk Fusion 360 (Autodesk Inc. San Francisco, CA, USA). Lastly, because inspection tools may extract GD&T information directly from CAD models, GD&T annotations in the digital thread allow automated inspection utilizing sophisticated measuring systems like 3D scanners and CT scans, minimizing errors in manual interpretation. However, even though MBD is supported by several GD&T standards (such as ISO 1101 [27]), further harmonization is needed to handle AM's particular requirements, like layer-specific annotations and freeform tolerancing. Furthermore, data loss or incorrect interpretation of GD&T annotations in AM processes may result from a lack of connectivity between design, simulation, and manufacturing software. Fortunately, this issue was addressed in the past by leveraging the concept of Constructive Technological Entity that aims to embed the same object pieces of geometrical and technological information, targeting a better data transfer between different types of software (e.g. CAD like Siemens NX, version 2306 and CAM like Siemens NX CAM version 2024). As a final remark, it is possible to state that Artificial Intelligence-based methods will also assume their role in this process by leveraging the integration of GD&T and MBD by automating tolerance assignment based on part functionality and AM process capabilities [28].