# Integrating Geometric Dimensioning and Tolerancing with Additive Manufacturing: A Perspective
## 9 Discussion and Conclusions  
The integration between Geometric Dimensioning and Tolerancing and Additive Manufacturing is a critical aspect of ensuring the reliability, consistency, and functionality of AM-produced components. Unlike subtractive manufacturing, where the manufacturing process is relatively simpler, AM throws many challenges, including complex freeform geometries, anisotropic material properties, and process-induced deviations that are not easily implemented in the GD&T standard. All these issues indicate a shift from traditional GD&T approaches toward more flexible and encompassing methodologies. The first important development is the adaptation of GD&T principles to AM-specific geometries, where functional datum systems and freeform surface tolerancing are gaining prominence. Because layer-by-layer fabrication creates distinct roughness patterns and dimensional irregularities, it is also important to incorporate tolerance stack-up models and surface texture analysis specifically designed for AM. Additionally, the integration of Model-Based Definition (MBD) into the digital thread seems to be a challenging yet effective strategy for ensuring continuity from design to manufacturing by reducing interpretation mistakes and enabling automated tolerance checking. Again, the key to guaranteeing AM's industrial scalability includes standardization and benchmarking. Not by chance, standardized procedures for the inspection, tolerancing, and quality control of AM components are being developed by organizations like ASME and ISO. Finally, a significant step forward is the increasing reliance on non-contact inspection techniques, which allows for the verification of complex internal and external geometries. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**ISO Rules** & **ASME Rules** \\ \hline Rule 1: Independence Principle & Rule 1: Envelope requirement \\ Each dimensional or geometric prescription must be respected in itself (independently) unless a particular relationship is specified & When specifying only dimensional tolerances, these always apply under maximum envelope conditions [equivalent to ISO Rule 2]. No modifier is used. \\ \hline Rule 2: Envelope requirement & Rule 2: Interdependence \\ The shape of an element must never exceed the envelope of the perfect shape corresponding to the maximum material condition. ISO adopts modifier E & To establish an interdependence between geometric and dimensional tolerances, a modifier has to be adopted according to the principle of Maximum Material or Minimum Material. ISO adopts modifiers M and L \\ \hline Rule 3: Interdependence & Rule 3: Regardless of Feature Size \\ To establish an interdependence between geometric and dimensional tolerances, a modifier has to be adopted according to the principle of Maximum Material or Minimum Material. ISO adopts modifiers M and L & Each dimensional or geometric prescription must be respected in itself (independently) unless a particular relationship is specified. ASME adopts modifier RFS \\ \hline \hline \end{tabular}
\end{table} 
Table 1: ISO [37] and ASME [38] rules for assemblies. However, challenges still exist. The development of interoperability among various AM procedures, software tools, and GD&T standards continues to evolve and calls for constant improvement, as well as industry cooperation. Furthermore, current GD&T frameworks do not adequately handle the additional tolerancing issues presented by multi-material and functionally graded AM components. It is also important to note that when a given AM process cannot ensure a set of tolerances that comply with the functional requirements of a part, AM either cannot be selected as an appropriate process, or it needs to be followed by another additional manufacturing process. This is particularly true for complex assemblies consisting of many mated parts, where strict control of the tolerance chain is required in addition to the tolerances of the specific parts.