In manufacturing, especially in assembly systems, every operation plays a role in shaping the quality of the final product. The influence of these operations can carry through each stage, ultimately affecting the quality of the finished products delivered to customers. Understanding how these subassemblies and operations contribute to the overall product quality is essential for identifying root causes of issues, resolving quality problems, and driving continuous improvement.  

A manufacturing system is complex, with numerous processes and variables affecting the overall quality of products, as illustrated below in Figure 1. This dynamic applies whether you’re looking at a single operation or the entire system. Given this complexity, it’s important to ask: How does each process step or factor relate to the product’s final quality? For instance, how does a specific process in producing a subassembly affect the quality of the completed product? This insight is key to effective problem-solving and quality control.   

Understanding quality transmissibility 

The effect of manufacturing operations on product quality depends on several factors, including the specific quality attributes of a subassembly and how the final product is assembled.

For instance, take the dimensional quality of a door assembly: It directly affects the door gap in the vehicle body assembled, as shown in Figure 2. It’s about quality transmissibility, referring to how we understand the connection between a subassembly and the overall final assembly. 

Often, the relationship between the subassemblies and the final assembly is qualitative, like noticing that a smaller door size results in a larger door gap. However, a quantitative relationship is more useful when it comes to problem-solving and continuous improvement. For example, if the variation of door dimensions accounts for 40% of the variation of the door gap, assuming the door dimensions are centered on the nominal, then we have a clear understanding of its contribution. To build this kind of qualitative understanding, you need data of the quality attributes from both the subassembly and final product for modeling the quality transmission.  

Modeling quality transmissibility 

Once you have the necessary data and the concept of quality transmissibility, you can model the relationship between the subassembly and the final product’s quality attributes. The relationship may follow a linear trend, as shown in Figure 3. A smaller slope in this model indicates a weaker relationship, suggesting that in this case the door assembly has less of an effect on the car-door gap in the final vehicles. From a product engineering perspective, the goal is often to design systems where the slope approaches zero for key quality attributes, meaning the quality of the subassembly has minimal influence on the final product’s quality.  

For complex products, the relationship between subassembly and final assembly quality may not be linear. For example, the door gap on a car can also be affected by the door opening in the body structure. In these cases, the relationship might look more like a curve, as shown by the dashed line in Figure 3. Understanding these relationships can significantly speed up root cause analysis and support ongoing improvements on the production floor.  

Another challenge arises from interactions between different subassemblies. A small quality issue on a subassembly might be acceptable on its own but could become problematic if combined with quality issues from neighboring subassemblies. For example, if both the door and the door opening have borderline dimensional quality, the combined variation might make the door gap unacceptable. Unknown factors, like residual stress in subassemblies, can also worsen the dimensional quality of a door and door opening.  

With a solid understanding of the quality reflection concept, along with its benefits and challenges, we begin investigating specific quality issues and their relationships. By collecting the necessary data, we can build transmission models tailored to specific cases, providing accurate insights that drive quality improvement projects. 

Conclusion

This article introduces the concept of how quality attributes are transmitted from subassemblies to the final assembly, and using reflection modeling to quantify their relationships. With actual data, this approach can establish a clear, quantitative link between subassembly and final assembly quality, which is invaluable for problem-solving and driving improvement. This approach can be applied in real-world case studies of quality improvement on the production floor, offering further insights for product design. By applying this method, organizations can refine their processes and make more informed decisions. Further studies, especially focusing on major subassemblies, are needed to fully explore quality transmission and achieve tangible improvements in managing the quality of final products.  

Reference 

Tang, H. Quality Planning and Assurance—Principles, Approaches, and Methods for Product and Service Development. Wiley & Sons, 2021. ISBN-13: 978-1119819271.