Rikard Söderberg

Computer Aided Assembly Robustness Evaluation

This paper presents a methodology and a software that allows assemblies to be evaluated with respect to robustness and geometrical stability. The assembly robustness evaluation aims at detecting design and assembly solutions that are sensitive to variation and may cause problems later during production. It is based on Suhs independence axiom, stating that in a good, uncoupled design, each functional requirement is satisfied by one and only one design parameter. The methodology enables the designer to evaluate the geometrical sensitivity of the assembly, what the sources of variation are, their importance for the overall robustness and in what order to improve the design. A possible result from the analysis may be that the concept must be changed in some way, either by changing the way parts are located with respect to each other or by using assembly fixtures for positioning. The software is implemented in a Microsoft Windows environment and has an IGES interface that enables the designer to import comput...

Accuracy of CAD/CAM-guided surgical template implant surgery on human cadavers: Part I

STATEMENT OF PROBLEM An optimal method for approaching the clinical surgical situation, when using preoperatively, virtually planned implant positioning, is to transfer data to a CAD/CAM-guided surgical template with the definitive position of the implant placed after surgery. The accuracy of CAD/CAM-guided surgeries must be determined to provide safe treatment. PURPOSE The purpose of this study was to compare the deviation between the position of virtually planned implants and the position of implants placed with a CAD/CAM-guided surgical template in the mandible and the maxilla in human cadavers. MATERIAL AND METHODS Ten maxillae and 7 mandibles, from completely edentulous cadavers, were scanned with CT, and 145 implants (Brånemark RP Groovy) were planned with software and placed with the aid of a CAD/CAM-guided surgical template. The preoperative CT scan was matched with the postoperative CT scan using voxel-based registration. The positions of the virtually planned implants were compared with the actual positions of the implants. Data were analyzed with a t test (alpha=.05). RESULTS The mean measurement differences between the computer-planned implants and implants placed after surgery for all implants placed were 1.25 mm (95% CI: 1.13-1.36) for the apex, 1.06 mm (95% CI: 0.97-1.16) for the hex, 0.28 mm (95% CI: 0.18-0.38) for the depth deviation, 2.64 degrees (95% CI: 2.41-2.87) for the angular deviation, and 0.71 mm (95% CI: 0.61-0.81 mm) for the translation deviation. CONCLUSIONS The results demonstrated a statistically significant difference between mandibles and maxillae for the hex, apex, and depth measurements in the variation between the virtually planned implant positions and the positions of the implants placed after surgery with a CAD/CAM-guided surgical template.

Computer-aided robustness analysis for compliant assemblies

This paper presents an analysis tool that allows non-rigid parts and assemblies to be evaluated with respect to geometrical robustness. The robustness evaluation aims at detecting design and assembly solutions that are sensitive to variation and may cause problems later on during production. The analysis tool enables the designer to evaluate the geometrical sensitivity of a part or assembly, what the sources of variation are, their importance for the overall robustness and in what order to improve the design. It allows designers and production engineers to evaluate and improve different types of locating, clamping and joining schemes with respect to non-rigid deformation of parts due to part variation and fixture variation. The analysis tool is implemented in a Windows environment and has interfaces to IGES, VRML, STL and ABAQUS, which enables the user to import computer-aided design geometry, finite element meshes and stiffness matrices, and to perform robustness and tolerance analysis for different locating, clamping and joining layouts. The results are presented in stability matrices, as sensitivity factors and with colour-coding of the sensitivity of the geometry.

Managing Physical Dependencies through Location System Design

Many geometrical quality problems that arise during production and assembly can be traced back to the way parts are designed and located to each other; that is, how the interface geometry and locating schemes are designed and selected. Today, locating schemes are often generated after the geometry has been set, or as a consequence of the part geometries being designed. In many situations, locating schemes are not deliberately designed or defined at all. This often results in assembly and positioning situations that are not clearly defined, analysed or understood by the designer. Since the way parts are located to each other is critical for how geometrical variation will propagate and cause variation in critical product dimensions, more emphasis should be put on this activity in early design phases in order to avoid assembly and production problems later on. This work proposes a structured top-down procedure for selecting locating systems for parts and subassemblies, and presents design guidelines to assist assembly modelling. The procedure can be seen as a first step in robust design and tolerance analysis in the area of geometry design and assurance. The proposed procedure utilizes an assembly dependency matrix, a locating scheme library, a number of different part-sensitivity analyses and a set of design guidelines.

An integrated approach to technology platform and product platform development

Platforms may enable offering a variety of products to the market while keeping the development cost down. Reusing design knowledge is a key concept, whether manifested as reusing parts, ideas, concepts, or technologies. This article describes processes and information technology solutions for holistically working with both technology platforms and product platforms. A platform framework was developed for managing information and to support the processes. The use of the framework is illustrated through a case study performed at a subsupplier in the aerospace industry focusing on technology development, platform-based product development, and platform configuration. A wiki system supports the technology platform, containing electronic guidelines, methods, and information about the technologies. To support the product platform, a product lifecycle management architecture is created. A turbine rear structure from a turbofan engine is used as an example, requiring several different analysis technologies to be used and coordinated when creating a variant. The solution is a product lifecycle management architecture created based on the technology platform. It integrates a product data management system, a computer-aided design tool, two computer-aided engineering tools, and a configurator.

Tolerance Chain Detection by Geometrical Constraint Based Coupling Analysis

Assembly tolerance chains are often the root cause for low geometrical robustness and high manufacturing costs. This paper presents a framework for function means modeling in configuration design that allows for modeling and analysis of geometrical couplings, and detection of potential tolerance chains. Requirement decomposition is described, and geometrical couplings on different hierarchical assembly levels are modeled, analyzed and explained. The treatment of sub-assemblies is especially described. The presented framework includes the use of locating schemes for positioning parts in assemblies. A general positioning system for modeling datum frames and locating schemes for parts and sub-assemblies are presented and used. An automotive example is used to show how an overall geometrical product constraint is decomposed into locating schemes on individual parts and how potential tolerance chains are detected. Finally, geometrical coupling quantification and the use of fixtures as a mean for breaking toler...

Increased Concurrency between Industrial and Engineering Design Using CAT Technology Combined with Virtual Reality

These days, when an industrial design concept is evaluated in an aesthetic manner, all models used are nominal. If the design is evaluated with nominal models, variation aspects and design solutions that would greatly influence the overall quality appearance might not be discovered until the first test series are made. By using nonnominal models during the design process, important geometric aspects can be issued, and the need for physical test series can be reduced. In the automotive industry, especially in body design, the relationships between doors, hoods, fenders and other panels are critical for quality appearance. This article suggests how combining traditional Computer Aided Tolerance (CAT) tools with modern Virtual Reality (VR) tools has the potential to enhance concurrency between industrial and engineering design and provide support for the geometry process in early phases. Traditional nonnominal verification can then already be conducted in the concept phase using digital models instead of physical. A VR-CAT tool supported geometry design process is proposed from a holistic point of view. Results and indications from a case study, where prescribed VR-CAT tool has been tried out in an on going project at Volvo Cars is presented.

Tolerance Simulation of Compliant Sheet Metal Assemblies Using Automatic Node-Based Contact Detection

Tolerance simulation is a crucial tool for predicting the outcome in critical dimensions, used during early phases of product development in automotive industry. In order to increase the accuracy and the agreement with reality of the predictions even further, variation simulation software offer in some cases the possibility to perform compliant analysis, i.e. the parts are not restricted to be rigid. In the compliant analysis, contact modeling is an important tool to avoid that parts penetrate each other in the simulations. In this paper a simplified method for automatic contact detection, well suited for tolerance simulations, is suggested. The method is based on node to node contacts instead of contacts between a node and a surface, which is a common procedure. Using automatic contact detection can in many cases give rise to an excessively large number of contact pairs. Therefore, an algorithm for attenuation of the contact pairs is also presented. Traditionally, non-rigid variation simulations with contact modeling are very time consuming, but by using this kind of simplified contact modeling in combination with the Method of Influence Coefficient in the Monte Carlo simulations, the simulation times can be kept down. The method is tested on an industrial case study, with respect to both standard deviation and position. The correlation between simulated data and industrial inspection data is high and there is a considerable difference between simulations with and without contact modeling, showing that this is an important feature in non-rigid simulations. Inspection data is also compared to rigid simulations and to simulations with different number of contact pairs. The computational effort for the suggested node-node based contact modeling algorithm seems to be considerable less than when using traditional finite element software, but still, the agreement with inspection data is very good.

Assembly Root Cause Analysis: A Way to Reduce Dimensional Variation in Assembled Products

The objective of root cause analysis (RCA) is to make the trouble shooting dimensional error efforts in an assembly plant more efficient and successful by pinpointing the underlying reasons for variation. The result of eliminating or limiting these sources of variation is a real and long term process improvement. Complex products are manufactured in multileveled hierarchical assembly processes using positioning fixtures. A general approach for diagnosing fixture related errors using routine measurement on products, rather than from special measurements on fixtures, is presented. The assembly variation is effectively tracked down into variation in the fixture tooling elements, referred to as locators. In this way, the process engineers can focus on adjusting the locators affected by most variation. However, depending on the assembly process configuration, inspection strategy, and the type of locator error, it can be impossible to completely sort out the variation caused by an individual locator. The reason for this is that faults in different locators can cause identical dimensional deviation in the inspection station. Conditions guaranteeing diagnosability are derived by considering multiple uncoupled locator faults, in contrast to previous research focusing on single or multiple coupled locator faults. Furthermore, even if an assembly is not diagnosable, it is still possible to gain information for diagnosis by using a novel approach to find an interval for each locator containing the true underlying locator variation. In this way, some locators can be excluded from further analysis, some can be picked out for adjustment, and others remain as potential reason for assembly variation. Another way around the problem of diagnosability is to make a higher level diagnosis by calculating the amount of variation originating from different assembly stations. Also, a design for diagnosis approach is discussed, where assembly and inspection concepts allowing for root cause analysis are the objective.

STRATEGIES FOR OPTIMIZATION OF SPOT WELDING SEQUENCE WITH RESPECT TO GEOMETRICAL VARIATION IN SHEET METAL ASSEMBLIES

During the assembly process of sheet metal parts, a lot of factors affect the final geometrical quality. It is important to have knowledge about the characteristics of as many as possible of those factors, not only to be able to reduce their effect, but also to be able to include those factors in variation simulations. Those tolerance simulations are crucial tools in early stages in automotive industry in order to predict the outcome in critical dimensions and it is of course important to have as good accuracy as possible in the simulations. One of the factors affecting the final geometry is the spot welding sequence. In this paper it is shown how the spot welding sequence affects the amount of geometrical variation in a sheet metal assembly. A method for including the welding sequence in tolerance simulations is described. Of course, it is desirable to find an optimal sequence, i.e. a sequence that minimizes the geometrical variation in the final assembly. Since this is a fast growing problem - the number of possible sequences for N welding points is N!, it is not practicable to test all possible sequences. In this work some different strategies for finding an optimal sequence are tested on several industrial case studies. The tested strategies are based on general guidelines, on minimizing variation in each welding step respectively calculations of the movements in unwelded points in each step. The strategies based on general guidelines was not successful, neither was the one based on minimization of the variation in each step. The strategy based on movements in the unwelded points seems however promising. It resulted in the best or one of the better sequences for all of the eight tested industrial case studies. ©2010 by ASME.