# Process-Oriented Tolerance and Variation Management: Review and Classification
###### Abstract  
In the context of tolerance management, the consideration of manufacturing and assembly processes is becoming increasingly important. The main drivers for this are, above all, short development times and high-quality requirements, leading to tight tolerances. To overcome the resulting challenges, many publications address the process-oriented tolerance management. However, since multiple terms and definitions for describing activities that link tolerance management with the production process exist, it is hard to obtain a comprehensive overview on the topic. Therefore, this paper presents a review of existing approaches. The aim is to identify similarities and differences of existing approaches and present them with the help of a classification. For this purpose, among others, work from the areas of process-oriented tolerance management, stream of variation, state space modeling, and variation propagation in multistation manufacturing and assembly systems is considered. Based on the definition of the summarizing term \"process-oriented tolerance and variation management\", a classification of this thematic area will be introduced. process-oriented tolerance; stream of variation; state space modeling; process-oriented tolerance and variation management  
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Management and Automation Management Review and Classification  
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on manufacturing, assembly, and supplier management are elevating the importance of including process variables as a significant consideration. This is particularly the case with regard to the interchangeability of processes, such as when different suppliers manufacture the same component using different processes [2]. The process information is also essential for determining the optimal tolerances [1]. Variations in process parameters have a significant impact on product deviations, not only in manufacturing but also in assembly processes [1]. Therefore, it is essential to give greater consideration to process parameters and information [2]. This includes process parameters and information from the manufacturing, assembly, and measurement process, such as locator dimensions and variations [2], fixture layouts [4], but also joining and spot welding sequences [5; 6], spindle thermal variations and cutting tool wear variations [7], measurement uncertainties [1], or interactions along the process chain [2]. This results in the aforementioned process-oriented tolerance management, in which process information from manufacturing, measurement, and assembly is taken into account in order to determine optimal tolerances (Figure 1) [1]. Therefore, the literature provides numerous concepts regarding process-oriented tolerancing [8]. The most prominent approaches in this field are, of course, those that describe themselves as process-oriented tolerancing or process-oriented tolerance management. Examples include [PERSON] et al. [2], [PERSON] et al. [9], [PERSON] et al. [10], and [PERSON] et al. [11]. However, there are significant differences in their specific implementations and scopes. [PERSON] et al. and [PERSON] et al. both use the stream of variation (SoV) with a state space model to represent the variation propagation as the basis for their process-oriented tolerancing [2; 9; 12]. [PERSON] et al. specifically focus on multistage assembly systems, modeling not only variation propagation but also the tolerance-variation relation and process degradation. In the context of tolerance synthesis, this approach is intended to optimally allocate tolerances to the fixture in the assembly process [2]. [PERSON] et al. analyze multistage manufacturing processes using an extended SoV approach. The approach includes critical process variables and cost functions [9; 12]. [PERSON] et al. adopt a comprehensive perspective and attempt to predict deviations based on process variables while optimizing tolerance allocation.