# Process-Oriented Tolerance and Variation Management: Review and Classification
## 3 Classification Results and Main Approaches
### Analysis of Single Processes  
The \"analysis of single processes\" category is the second largest, with 47 publications. The approaches and models used in this category are more diverse than those in the previous category. Therefore, only a selection of possible approaches for analyzing single processes is presented below. All assigned publications and approaches can be found in Table 1 in Appendix A. The approaches developed by [PERSON] et al. and [PERSON] et al. for tolerance analysis or variation modeling are largely assigned to the single process class. This is particularly applicable to the use case of a hollow box and a couple of circular profiles [24; 25; 26; 27; 28; 29; 30], as it theoretically models a general, abstract assembly process without considering further interactions between different processes in a process chain. Further considerations focus on glue modeling [55], deviations in drilling [56], and milling [56; 57] or on the manufacture of lightweight products [58; 59] and thus correspond to a consideration of single processes. Generally, the approaches by [PERSON] et al. and [PERSON] et al. consider manufacturing signatures [24; 25; 26; 27; 28; 29; 30] and operating conditions [24; 25; 27; 28; 30]. Various procedures including Jacobian, torsor, variational, and vector loop are utilized and compared with each other [25; 26]. Moreover, they develop and employ a new variational model [28; 29]. The approaches and methods of [PERSON] and [PERSON] for variation simulation are predominantly categorized as \"analysis of single processes\". The majority of the publications concentrate on the particular modeling and simulation of specific influences and properties inherent to a given process. Examples of this include the development of a method for transforming the variation in the test data into variations in the contacts between the workpiece and the locators [49], the use of critical material and process parameters and the resulting geometric deviations in a meta-model [60], and the development of a method for analyzing the variation in surface-to-surface contact of a cutting tool [61]. In particular, the working group employs a detailed analysis and simulation of welding processes. Furthermore, the class \"analysis of single processes\" is also the subject of their detailed consideration. Various factors are used to simulate variations or optimize the joining and spot welding sequence. These include heating and cooling processes together with the geometric tolerances of the components [62], the spot welding sequence [5] including the springback calculation [63], and the joining sequences [6; 64], joining points [64], and joining forces [64]. Tools like the Method of Influence Coefficients (MIC) [63] are utilized for this purpose. The MIC is also employed in other publications, for example, by [PERSON] et al. [65] to assess the assembly configuration when analyzing flexible sheet metal structures or by [PERSON] et al. [66] in the variation analysis for compliant aerostructure assembly. [PERSON] et al. 's \"rigid-compliant hybrid variation analysis method\" also falls under this category. It is used to analyze ladder structures, primarily for the aircraft structure assembly. The analysis is centered on variations resulting from rigid body locating errors and part geometric errors. They are modeled using rigid-body kinematics and mechanistic methods, and subsequently integrated into a rigid-compliant hybrid variation model. Due to the low production volume in the aircraft industry and the often-unknown probability distribution, [PERSON] et al. propose a novel variation analysis method. A Monte Carlo interval approach is used for this [67]. As previously outlined, the approach adopted by [PERSON]'s research group is primarily oriented towards practical applications and a systematic methodology. In this context, particular emphasis is placed on the utilization of key characteristics, key characteristic flowdown, and tolerance chains.