Evaluating different strategies to achieve the highest geometric quality in self-adjusting smart assembly lines

Digital twin-driven productions have opened great opportunities to increase the efficiency and quality of production processes. Smart assembly lines are one of these opportunities in which the effects of geometric variations of the mating parts on the assemblies can be minimized. These assembly lines utilize different techniques, including selective assembly and locator adjustments, to improve the geometric quality. This paper signifies that the achievable improvements through these techniques are highly dependent on the utilized fixture layout for the assembly process. Hence, different design methods and productions that can be followed in a smart assembly line are discussed. Furthermore, different scenarios are applied to two industrial sample cases from the automotive industry. The aptest design strategy for each improvement technique is determined. Moreover, the strategy that can result in the highest geometric quality of assemblies through a smart assembly line is defined

Shape characterisation of sheet metal assembly variation with a view to quality assessment and dimensional control

Sheet metal assembly is a complex process involving component-to-component and component-to-tooling interactions. A key characteristic of sheet metal assemblies, the flexibility of components, means that variation does not stack-up according to the additive theorem of variance that applies to rigid bodies. Instead, components can be bent and distorted into conforming or non-conforming shapes by assembly interactions. This characteristic of flexibility also means that in comparison to rigid body assembly, additional aspects of the assembly process, such as clamp sequence and weld sequence,can influence the way in which variation propagates. Through a detailed understanding of the influence of assembly processes on variation propagation, manufacturers can adjust their processes to target particular quality assessment criteria: in this thesis, it is firstly demonstrated how assembly processes such as clamping sequence can be altered to control different variation patterns (and therefore quality) in sheet metal assemblies. However, in order to truly optimise a sheet metal assembly process for dimensional control, there must be a well defined quality assessment framework from which to select the best processes. The most commonly adopted measures of assembly quality are based on the mean and standard deviation of a set of assumedly statistically independent measurement points. Such approaches are perhaps not the best measure of assembly quality. This is primarily due to their inability to adequately capture a key characteristic of assemblies: correlated variation patterns. This thesis proposes that assembly quality cannot be simply assessed by the mean and variance of a set of assumedly statistically independent measurement points, and that correlated variation patterns in the form of bows, buckles, twists and ripples also form a large part of assembly quality perceptions. Two key methods were therefore developed to better characterise assembly variation: the multivariate statistical shape model, and the local shape descriptors. These shape charaterisation measures overcome key limitations of existing univariate quality measures including an inability to capture correlated variation patterns, monitor non-normally distributed data, interpret high dimensional data, and measure local variation patterns of different sizes or scales. Through addressing these limitations, the proposed shape characterisation methods provide significant advancements in the ability of manufacturers to accurately measure variation and discriminate between differing levels of assembly quality, and are particularly well suited for the interpretation of high dimensional measurement data made available by optical co-ordinate measuring machines. The new shape characterisation methods therefore provide a framework for achieving new levels of quality assessment, with a view to the ultimate goal of developing optimal dimensional control strategies for sheet metal assemblies

Individualizing assembly processes for geometric quality improvement

Dimensional deviations are a consequence of the mass production of parts. These deviations can be controlled by tightening production tolerances. However, this solution is not always desired because it usually increases production costs. The availability of massive amounts of data about products and automatized production has opened new opportunities to improve products\u27 geometrical quality by individualizing the assembly process. This individualization can be conducted through several techniques, including selective assembly, locator adjustments, weld sequence optimization, and clamping sequence optimization in a smart assembly line for spot-welded sheet metal assemblies. This study focuses on two techniques of individualizing the assembly process, selective assembly, and individualized locator adjustments in assembly fixtures. The existing studies and applications of these methods are reviewed, and the research gaps are defined. The previous applications of selective assembly are limited to linear and rigid assemblies. This study develops the application of selective assembly for sheet metal assemblies. This research addresses another research gap regarding the selective assembly of sheet metals by reducing the calculation cost associated with this technique. This study also develops a new locator adjustment method. This method utilizes scanned geometries of mating parts to predict the required adjustments. Afterward, a method for individualized adjustments is also developed. Considering applied and residual stresses during the assembly process as constraints is another contribution of this research to locator adjustments. These methods are applied to three industrial sample cases and the results evaluated. The results illustrate that individualization in locator adjustments can increase geometrical quality improvements three to four times.Accumulation of the potential improvements from both techniques in a smart assembly line is also evaluated in this study. The results indicate that combining the techniques may not increase the geometrical quality significantly relative to using only individualized locator adjustments.A crucial factor in the achievable improvements through individualization is the utilized assembly fixture layout. This study develops a method of designing the optimal fixture layout for sheet metal assemblies. Different design and production strategies are investigated to acquire the maximum potential for geometrical improvements through individualization in self-adjusting smart assembly lines

Robust dimensional variation control of compliant assemblies through the application of sheet metal joining process sequencing

Imperfections are inherent in every manufactured part - when the hundreds of sheet metal components that form the automotive Body In White (BIW) are assembled together, significant deformation and variability are possible. Although early work by Takazawa (1980) showed that compliant components can absorb individual component variability when assembled, interactions between the components and successive operations complicates analysis of the assembly process and prediction of the assembled output. Therefore, improving vehicle dimensional quality requires more detailed knowledge of the assembly process and control of features critical to functionality and aesthetic appeal. In the automotive industry, these features include: uneven gaps and flushness between panels, high closure forces, and incorrect seal gaps leading to leaks and excessive noise. Despite significant research in the field of compliant assembly, there have not been sufficiently detailed studies regarding the joining sequence process. Further, existing works are based on a number of assumptions that limits the applicability of their results. This thesis addresses this gap by utilising the joining process sequence to control deformations and minimise dimensional variation during the assembly of complex non-ideal compliant components. In this work, a geometry class to represent complex compliant assemblies is presented; the interactions of process sequences and variations examined; the criteria for robust sequence selection established; and a method for the rapid identification of robust sequences is developed. In addressing the aim of this research a number of key findings were developed. A broad method of classifying the input variation of the components is presented. Using this basis, identifying when the joining process has a significant influence on final assembly dimensionality can be established. The pre-existing guidelines of fixed-to-free end were then further generalised for complex geometries, resulting in the approach of most-to-least rigid configuration, noting the importance of prior joining operations and the fixture boundary conditions. In determining the potential impact of the joining sequence, the need to consider the build-up of internal stresses while modelling the assembly process is highlighted. A novel method, which analyses the natural frequency shift of the structure between successive joins, is presented as a technique of calculating a robust joining sequence. This technique requires no knowledge of the part deformations, only the component geometry, fixture configuration and weld locations, and hence is more practical to industry. Experimental studies to validate the simulation-based work were then performed. Although sequence-based trends are identifiable in some of the extracted data patterns, the twist induced in the experimental structure was less significant when compared to the simulated results. This difference is a result of a number of factors which are then postulated and analysed. Further investigation of this effect would be beneficial to further validate the approach. To build on the work in this thesis, two notable directions in addition to further industrial validation have been identified. By assessing the functional impact of variation patterns in measurement data, functional variation sequence synthesis could be investigated; where the goal is to select a sequence that best controls these critical functional variations. Evolvable assembly systems that utilise the input work and holding forces to optimise the sequence operations and minimise potential spring-back of the component can also be considered. This strategy would negate the need for the additional measurement step for process feedback which currently hampers existing adaptive techniques. With between four and six thousand joins performed per vehicle, an estimated 300 billion joining operations are performed annually worldwide. With minimal knowledge available to industry regarding the impact of the joining process sequence, the results from this work have the potential to significantly improve quality in the automotive market

Jig-Less Assembly for Aerospace Manufacture

Due to the high level of investment required to compete successively in the global aerospace and automotive markets, these industries are forced to form partnerships wherever possible and thereby share their resources appropriately. This in turn has brought about the requirement to provide a standardized flexible design and manufacturing capability in which interchangability and compatibility may take place. Current assembly practices and associated tooling can be traced back to the earliest days of aircraft production and have become relatively expensive and inflexible in today’s environment. The final assembly stage has been recognized to be a key area which has the potential to offer substantial returns as well as play a major role in any change management process within the organisation. Assembly tooling, jigs and fixtures, are required to support and maintain positional accuracy of components during assembly. Traditional jigs and fixtures make up for the short comings at the product design and manufacturing phases and add significantly to the final product costs and reduce flexibility in the production process. Jig-Less Assembly Concept (JAC) has been defined and researched with the aim to integrate and optimize various tools and techniques with which to reduce or eliminate the assembly tooling currently in use. The outcome of the research presents a comprehensive critique of the processes involved in and pertaining to the assembly of typical airframe assemblies. The thesis forms a platform from which to move forward towards the embodiment of the concept of jig-less assembly. Particular attention is drawn from the research to the need for appropriate organisational and management strategies as well as technical innovation in the adoption of a jig-less approach to airframe assembly. Together with BAe Airbus and Military this collaborative research seeks to define the scope of JAC by identifying and evaluating the issues and constraints, to enable the development of supportive techniques in unison with best practice engineering within a robust and sustainable manufacturing system. This commercially focused R & D required liaison and working at all levels within a variety of industrial sites using live case studies at Filton and Chester.MPhi

The 20th Aerospace Mechanisms Symposium

Numerous topics related to aerospace mechanisms were discussed. Deployable structures, electromagnetic devices, tribology, hydraulic actuators, positioning mechanisms, electric motors, communication satellite instruments, redundancy, lubricants, bearings, space stations, rotating joints, and teleoperators are among the topics covered

The non-destructive testing of adhesively bonded structures

Imperial Users onl

Sustainable design of hydrocarbon refrigerants applied to the hermetic compressor

International environmental concern led to the control and phase out of traditional chlorofluorocarbon refrigerants (CFCs) under the terms of the Montreal protocol. CFCs used in domestic applications were initially replaced with hydrofluorocarbons (HFCs) such as R134a which has a zero ozone depletion potential (ODP). The use of HFCs has also come under scrutiny as they have high global warming potential (GWP) and inferior thermodynamic and lubricating properties and have been replaced by hydrocarbon (HC) refrigerants such as R600a in much of the domestic European and Asian markets. Despite this, there has been little research into the long-term environmental consequences of their application. Domestic refrigeration compressors were analysed to ascertain the tribological contact conditions for both R600a and R134a systems. A novel pressurised micro-friction test machine was developed to simulate the tribological conditions of the critical components using aluminium on steel samples. Refrigerant charges of R600a with mineral oil (MO) and poly-ol-ester (POE) lubricant and R 134a with POE were tested for their tribological performance within the test rig. Experimental tribological information is presented from the physical test procedures to establish wear mechanisms and friction coefficients within the critical components. The tribological performance is used to predict deterioration in energy consumption and system durability. Results indicate that for higher contact stresses R600a MO charges provide a lower wear regime than R600a and R134a POE charges. At lower contact stresses the R600a and R134a POE charges provide a very low wear, very low friction regime. Despite contact conditions lead to a faster deterioration in durability, hence increase in energy consumption compared to the R600a system