Observing Dimensional Variation in an Automotive Sheet-Metal Sub-Assembly

This paper presents a measurement case study of an automotive stamped sheet metal sub-assembly, in order to build a better understanding of flexible component tolerance stack-up in a physical production line assembly. Here, corresponding points are measured before and after assembly to capture changes in variation. Variation levels, variation stack-up, and positional shifts were investigated. It was found that that in terms of a Process Potential Index (Pp), the assembly was dimensionally better than its components. In some regions variation levels were found to be higher after assembly compared to before: in other regions, variation levels were found to be lower after assembly than before. Observation of positional shifts of features over the assembly process revealed consistent directional changes. Suspected causes are datum-shifts between assembly stations and measurement gauges, and assembly processes deforming/shifting the components in a consistent manner. The findings of this study begin to identify characteristics of variation and tolerance stack-up in flexible sheet-metal assemblies.