THESIS
2002
xvii, 148 leaves : ill. ; 30 cm
Abstract
Assembly precision is very important as it can greatly affect the quality, yield and cost of many products. Thus far, the research on assembly precision has more focused on the analysis of the precision for a given and completed assembly process. However, selection of appropriate assembly processes with appropriate parameters can be of great significance in determining precision of the final assembled product. It appears that this important area is under-researched and has not yet been fully explored. In this thesis, a comparative study of different assembly processes and different process parameters is conducted to analyse their impact on assembly precision and dimensional variation of the final products. In particular, we are investigating how different mechanical alignment methodolo...[
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Assembly precision is very important as it can greatly affect the quality, yield and cost of many products. Thus far, the research on assembly precision has more focused on the analysis of the precision for a given and completed assembly process. However, selection of appropriate assembly processes with appropriate parameters can be of great significance in determining precision of the final assembled product. It appears that this important area is under-researched and has not yet been fully explored. In this thesis, a comparative study of different assembly processes and different process parameters is conducted to analyse their impact on assembly precision and dimensional variation of the final products. In particular, we are investigating how different mechanical alignment methodologies, part fixture locating layouts and part-to-part joining geometries affect the assembly precision in the presence of form and surface errors of the workpieces. One of the benefits of this work is to provide some guidance for improving the assembly precision by selecting better approaches and better parameters.
Due to the complexity and difficulty of building a multiple-station monitoring system for assembly processes, theoretical modelling is used for the study. First, a state space model is developed to provide a mathematical representation of variation propagation in multistage assembly processes. Secondly, simulation method is used to study the effect of part imperfection such as part dimensional error, surface waviness and roughness. A methodology is developed to generate surface spanning from short-range to long-range based on various surface characterizations. Autoregressive integrated moving average (ARIMA) time series modelling is utilized to simulate the machined surface short-range and medium-range profiles, which appear to be non-Gaussian and non-stationary. A Bezier curve is applied to fit the surface long-range profile that appears to have a certain repeatable pattern depending on material and process condition. Finally, the superimposition method is used to generate the complete surface spanning from short to long range.
To date, relatively little attention has been paid to the design of alignment systems and how the parts should be located with respect to the mechanical alignment system in the assembly process. The effect of using various part surfaces as datums for making the parts and assembling the parts is investigated in this thesis. Our analysis shows that if the MAS is not properly selected, the form errors as well as surface waviness and roughness of the workpieces to be assembled can badly limit the level of accuracy achievable. Such cases are common, where fabrication operations are done on parts before they are assembled. The study shows that if the workpieces are aligned with the same orientation, using similar or identical MAS for the fabrication processes and assembly processes, then the effect of the form errors as well as surface waviness and roughness of the workpieces can be greatly suppressed. The precision level can be readily improved from hundreds of microns to several microns and there is almost a two-orders-of-magnitude difference in the assembly precision between the good assembly case and the bad assembly case.
Different types of part fixtures locating layouts and part-to-part joint geometries can significantly affect the assembly precision. In this thesis, we investigate the effects of three types of part-to-part joint geometries on assembly precision in the presence of part dimensional error, surface roughness and waviness. We confirm that fixture error, workpiece dimensional error, surface roughness and waviness can have significant effects on assembly precision. In addition, our analysis shows that the selection of joint and joining process can fairly affect the dimensional accuracy of part-against-part joints.
Industrial case studies of front rail and rear gate opening assemblies during the automotive body manufacturing process have been investigated. Some of the potential impacts of the typical assembly approach and error sources have been identified.
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