# Integrating Geometric Dimensioning and Tolerancing with Additive Manufacturing: A Perspective
## 3 Surface Texture and Tolerance Stack-Up  
Since AM is mostly layer-by-layer based, tolerance stack-up and surface texture requirements are different from those adopted in conventional manufacturing. Layer thickness, material deposition technique, powder size, and post-processing methods are some of the parameters that affect both the final shape of the part (which deviates from the designed part and the traditional machined surfaces) and the surface finish in AM parts. These variations provide difficulties for conventional GD&T procedures and ISO tolerancing standards.  
In terms of surface texture, it should be noticed that the layering process causes AM parts to generally have anisotropic surface roughness, with the vertical (build) direction frequently having rougher surfaces than the horizontal. As a consequence, standard roughness parameters such as Ra (Arithmetic Average Roughness), Rz (Maximum Height), or Sa (Surface Area Roughness) may not fully define the complexity of AM surface profiles [16]. Furthermore, AM may use support structures to stabilize overhanging features [17], leaving residual marks or artifacts that affect the surface finish. The immediate consequence is that designers should be aware of the difference between specific tolerances for post-processed areas and as-built surfaces. As seen by ISO/ASTM 52920:2023 standard [18], which establishes rules for test artifacts in AM to assess surface roughness, standards are progressively concentrating on specifying surface finish requirements, especially for AM procedures. Moreover, several authors [19, 20, 21] are proposing the use of area-based parameters (Sa, Sq)to better characterize roughness in AM parts compared to the traditional line-based parameters (e.g., Ra). In addition, inspection methods based on Reverse Engineering [22; 23] methods, such as optical 3D scanning, light interferometry, and CT, can be used for assessing complex AM surfaces, thus allowing GD&T standards to enable precise evaluation of surface profiles.  
Regarding tolerance stack-up, dimensional variability in individual parts brought on by AM process features (such as warping or shrinking) might result in unforeseen tolerance stack-up problems in assemblies, including AM components. For instance, when tolerances are not properly maintained across many AM components, parts with tight assembly clearances (e.g., turbine blades) cannot operate under design conditions. To deal with this issue, GD&T practices will increasingly include provisions for as-built tolerances, which reflect the raw AM surface finish directly from the 3D printer, and post-processed tolerances that will ensure tighter limits achieved through secondary processes such as machining, polishing, or chemical finishing [24]. Additionally, process-induced variations like warping and thermal distortions will be incorporated into tolerance stack-up models for AM parts, and sophisticated simulation techniques like digital twins and finite element analysis (FEA) will aid in the prediction of tolerance accumulations in assemblies.  
Unfortunately, since existing GD&T standards primarily focus on single-material components, it is still difficult to obtain consistent tolerance values for multi-material or graded-material AM parts. As a result, future studies are anticipated to focus on this area [25].