# Integrating Geometric Dimensioning and Tolerancing with Additive Manufacturing: A Perspective
## 1 Introduction  
As widely recognized, Geometric Dimensioning and Tolerancing (GD&T) is a method for specifying and conveying engineering tolerances on engineering drawings and computer-generated 3D models, using a symbolic language that explains the nominal geometry of a physical object and the acceptable variations from it [1]. The nominal (theoretically perfect) geometry of parts and assemblies, the permitted variation in the size, form, orientation, and location of individual features, and how features may differ from one another so that a component is deemed adequate for its intended use are all defined by GD&T. This framework addresses traditional subtractive manufacturing techniques like machining, turning, and milling, and establishes geometric and dimensional criteria for designed components [2, 3]. These processes allow the production of either simple parts with prismatic or revolving features or more complex geometries (especially when CNC machine centers are adopted); in any case, such parts are characterized by a well-defined topology and, depending on the process itself, by specific and usually consistently annotatable tolerance values. Quite the reverse, Additive Manufacturing (AM) enables the creation of complex geometries and freeform shapes that challenge traditional tolerancing approaches [4]. Mainly due to the special features of additive processes, including layer-by-layer fabrication, the manufacturing of complex geometries, and material variability, the adoption of GD&T in AM is expected to be significantly developed in the next years. Standardized techniques to properly specify and communicate dimensions and geometrictolerances of additive manufactured parts will be required to deal with issues like the implementation of topology-optimized designs, surface roughness, and the presence of anisotropic features on a macroscopic scale (e.g., resulting from layer-by-layer manufacturing) and on a microscopic scale (e.g., due to the thermal history in the AM process). In particular, the main challenge is to ensure that AM components fulfill functional requirements while aiding efficient production and quality control, especially when integrated into machines composed of both subtractive manufactured and AM components. To achieve dimensional compliance with the original CAD model, some approaches are used to manage geometric, dimensional, or shape errors in AM. For instance, stair-stepping, a rough surface caused by AM's layer-by-layer bottom-up method, can potentially result in a geometrical error. Along with component shrinkage, these geometrical errors are also influenced by material qualities, process parameters, support structures, and surface approximation errors brought on by slicing techniques. As a consequence, the design phase needs to take into account the expected actual geometry of the manufactured parts, planning a set of actions to ensure its compliance with the originally designed digital part. Accordingly, the geometrical mismatch between the printed items and the CAD and toolpath models is a significant problem to be confronted with when AM is involved as a primary method for building a part [5]. Because of the alterations made during the slicing step to build a toolpath model that depends on different slicing settings, the printed part is not the same as the original CAD model. The goal is to reduce the deviations and/or compensate them based on the process conditions [6]. However, to function as a comprehensive geometric quality sensing and control strategy, the integration of GD&T and AM requires considerable development.