Doctoral theses

Increasing quality requirements combined with high cost pressure make efficient quality management indispensable, especially for manufacturing companies. As part of quality management, complaints management ensures that complaints are received and processed and that the causes of defects are identified and eliminated. The 8D method is a tool for the profitable documentation and processing of complaints. Parallel to the execution of the 8D method, all steps performed are recorded in an 8D report. There are no objective internal inspection or control bodies that ensure the integrity and correctness of an 8D report.

Therefore, this thesis aims to support the verification of 8D-reports by an automated evaluation system. Thereby, 8D-reports should be evaluated both formally and in terms of content.

8D-Report, computational linguistics, quality assessment, complaints

As a result of the energy turnaround and the expansion of the grid, energy-intensive manufacturing companies are confronted with further increases in energy costs and are urged to reduce them through their own measures. In this context, energy costs can be positively influenced by organisational measures, whereby the actual power requirement remains unchanged. This can be done within the machine allocation planning.

However, logistical target values, such as a high delivery reliability, must not be neglected. Therefore, in this thesis a method for energy- and logistics cost oriented machine allocation planning was developed, which considers energetic and logistic target values simultaneously during planning. In the developed method, the planned peak load was used as an energetic target value and valued monetarily via the performance price. The planned schedule deviation in order issue was used as the logistical target value as the target value in the method and was valuated in monetary terms via the stock of manufacturing orders with early planned completion.

The developed method was converted into a mathematical model and a problem-specific algorithm was developed to solve the model. By applying the developed method it could be confirmed that with the additional consideration of the energetic target figure planned load peak in the machine allocation planning, the total costs resulting from the implementation of the planning can be significantly reduced compared to a sole consideration of the logistic target figure. Furthermore, the qualitative effects of the variation of the considered input variables on the total costs resulting from the implementation of the machine allocation plan could be shown.

Machine occupancy planning, energy costs, performance price, schedule deviation

To determine the influence of the process combination of cross wedge rolling with multidirectional forming on the burr formation in single-cylinder crankshaft preforms, cross wedge rolling and multidirectional forming geometries were determined. For this purpose, FEM simulations were first used to narrow down the value range of the five varied parameters for subsequent experimental investigations.

The parameters were the shoulder angle, the reduction in cross-sectional area, the bearing offset, the forming speed and the workpiece temperature. After the evaluation of both investigations, there was potential for adjustments to the FEM simulations. In the course of the search for suitable parameters for adaptation, friction was identified as significant in the FEM simulations.

On the one hand, the values were varied within the friction coefficient friction factor model used; on the other hand, the suitability of another friction model, the IFUM friction model, was examined.The latter influenced the results of the FEM simulations in such a way that a sufficient approximation to the experimental investigations was achieved and a mathematical model could be formed on this basis.

cross wedge rolling, multidirectional forming, crankshaft, friction

Ever-stricter requirements for the quality of the finished product and the energy consumption pose difficult challenges for industry, especially the automotive industry. The market demands high-quality components at a reasonable cost.

Modern technologies and innovative methods help to meet this challenge. Process monitoring has become a key technology and enables the improvement of safety, quality, and efficiency in various fields. Until recently, production ? from the design of the aluminum melting furnace to the daily process ? relied largely on tra-ditional methods and experience.

This thesis investigates a new method for monitoring a melt-ing process and shape changes in the furnace by means of optical sensors for the first time. To this end, this thesis deals with an innovative analysis algorithm for using an optical meas-uring system that is able to monitor the melting block despite the red-hot furnace walls.

melting process, analysis algorithm, light-field camera, process monitoring

In the burr-free forging of aluminium, material can unintentionally penetrate into the gap areas between the tool elements, thus creating so-called tinsel burr.

Burr makes handling and positioning of the components in subsequent forging or machining operations more difficult and thus leads to position and tolerance errors on the component. In this thesis, two different methods for the first numerical prediction of the formation of flake burr were developed.

For specific areas of finished parts, it was possible to qualitatively predict the formation of flake burr in all areas considered. Furthermore, a method for the reduction of flake burr by geometrical optimization of preforms using evolutionary algorithms was developed. The optimized preforms met or exceeded the established quality requirements compared to conventionally developed preforms with a significantly reduced optimization time of less than ten minutes.

Solid forming, burr-free forging, flash, forecast, optimization

When forging aluminum, the main cause of tool failure is adhesion-related wear. The high affinity for the adhesion of aluminum alloys to the steel materials of the forging dies results in so-called "stickers", which have a negative influence on the forging result. As a result, the engravings on the tools must be regularly overhauled. The overhaul interval depends largely on the workpiece geometry and temperature as well as on the tool temperature, material and surface.

This paper develops a model that predicts the adhesive wear and thus determines the tool life or the time of potential tool failure. This model is based on the correlation of simulative and experimental data. After evaluating the sensitivity of the varied parameters, influencing variables such as forming or die temperature are combined in a data mining model. With the help of various algorithms, the surface quality and therefore the tool life can be predicted after simulation of any aluminum forming process.

forging, adhesion, aluminum, wear

Automated guided vehicle systems (AGVS) have become indispensable in advanced production facilities. Due to significant progress in the field of AGVS and the increased automation within production plants, the potential applications for AGVS increase. So far the roadmaps for the vehicles are mostly generated manually, which leads to long and laborious planning phases. This thesis examines how system planners' knowledge can be integrated into a pathfinding algorithm in order to combine human logic with mathematical optimization to generate roadmaps for AGVS that are both efficient and applicable.

The combination of mathematical planning and human planning was achieved by combining a fuzzy inference system with a traditional pathfinding algorithm - the A* algorithm. The fuzzy inference system stores the knowledge of the system planners in the form of fuzzy rules and the output of the rules directly influence the path planning of the A* algorithm.

pathfinding, expert system, fuzzy logic, automated guided vehicle system, automated guided vehicle

Rising procurement costs for electrical energy and an increase in electricity price volatility due to the increased feed-in of renewable energies endanger the international competitiveness of manufacturing companies in Germany. To meet this challenge, the demand for energy must be adapted to the energy supply in the medium term. Demand-side energy management requires companies to consume more energy when energy is cheap. A starting point for this is the production control, since this has the task to implement the production plan created by production planning due to frequent unavoidable disturbances. It thus determines when which order of production is received and processed. In this thesis the influence of the production control on the energy costs is described and integrated into an existing active model of the production control. Based on the active model, a sequential rule is developed as a decision model, which takes into account not only the energy prices, but also the time-related urgency of the individual orders and thus enables energy cost savings while taking into account the logistical target values. Finally, the simulation-based validation of this sequence of rules proves part of the model of action. Based on the validated impact model, the application prerequisites and the influences on the logistic target values ??of an energy price-oriented order sequence rule are described.

Production control, sequencing, energy prices, energy costs, electricity price volatility

The introduction of continuous condition monitoring of machinery and equipment is associated with obstacles, especially for retrofits in the maritime sector. Sensor networks in conjunction with wireless, energy-autonomous sensor nodes completely avoid the cabling effort for energy supply and data transmission, so that the sensors for measuring value acquisition can be integrated with a single installation effort. Energy Harvesting systems allow the sensor nodes to be supplied with energy from the immediate environment.
The aim of the present work is therefore to develop a model that enables the energetic design of wireless, energy self-sufficient sensor nodes for condition monitoring of machines and plants.

wireless sensor network, energy harvesting, energy-independent sensors, condition monitoring

Automated guided vehicles (AGVs) are unsteady conveyors and suitable for logistics. To reduce installation costs and to make AGVs economically more feasible for small- and medium-sized enterprises (SMEs), autonomously acting AGVs are used. These AGVs can be ordered decentrally via natural communication channels of humans. Most frequently, people interact via speech. The aim of this thesis is to examine how well a decentralized speech control for autonomously operating AGVs works in intralogistics and which impact the operator's cognitive information processing has. For this purpose, a speech control system for AGVs and an electroencephalography (EEG)-based measurement system were developed to investigate cognitive information processing. Four independent studies were conducted to evaluate the interaction with the AGVs. The results confirm the assumption that a speech control for AGVs in intralogistics can be used speaker-independently. However, noise has a significant influence on the recognition error of the speech control. In addition, it was found that the interaction is realizable in real time and that inattention and relaxation of the operator have a significant effect on the work result.

cognition, speech control, automated guided vehicles (AGVs), human-machine-interaction (HMI)

The preform design of forging processes mainly influences the economics of a multistage forging process and is very time consuming. This thesis presents an automated approach for preform design, aiming to reduce the development effort for multistage forging processes. Therefore analytical equations are derived to describe the quality of preforms. The equations take into account all main quality (formfilling, folds) and economic parameters (amount of flash, forming force, manufacturing expenses) for preforms.

To solve the multicriteria optimization problem an evolutionary algorithm is used. The results show that it is possible to obtain a suitable preform by using the algorithm in less than 60 seconds. Furthermore a method to reduce the amount of flash for cross wedge rolled preforms is developed in this thesis. Based on an equation describing the minimum required amount of flash in each area of the final part an optimized mass pre-distribution is achieved. Compared to conventional preforms a flash reduction of 66 % is reached.

bulk forming, evolutionary algorithm, preform optimization, flash reduction

The most common method of bulk forming processes is closed die forging with flash. In these processes a surplus of material is used to avoid forging defects such as a missing filling of the cavities. The surplus material is driven through the flash gap and forms the flash. Therefore, the flash gap design has a distinct influence on the material flow within the die and thereby on the cavity filling, the material usage and other process parameters. In indus-try the flash gap is fixed once the dies are manufactured and cannot be changed without additional manufacturing.

A moveable flash gap, which can be changed vertically during the forging operation, was developed within this thesis. It can be used to influence the material flow within the forming process. Thus, the filling of the cavity can be improved. By the use of a variable flash gap in the forging process of a trial part, a maximum increase in the cavity filling of 4.6 mm (17.2 %) was achieved. Furthermore, the influences of different process properties, such as the billet temperature, the trigger force or the height of the moveable flash gap, were evaluated.

To allow the application of a variable flash gap to different forging parts, a guideline was designed. For that purpose, forging parts were classified into characteristic zones. Within the zones the influence of differently designed flash gaps were evaluated using FEA. The result of this evaluation is a recommendation on possible designs of a variable flash gap for differ-ent forging parts, to allow an improvement of the cavity filling.

bulk forming, finite element analysis (FEA), flash gap, material flow, die design

Industrial value creation is driven increasingly by the market dynamics. The control of the dynamics in the field of production and logistics is crucial for business success. The value creation within production networks is also gaining a stronger relevance. A higher process orientation leads to stronger dynamic interactions between the pro-duction stages of the network.

Manufactures need to know, how they can control the dynamics within production processes and the impacts to the supplying and receiving processes. A new model of explaining dynamic processes is developed in this work. Deterministic and stochastic dynamic effects are described based on that model. Furthermore their impact to the logistical objectives is calculated with known logistical models and actions for control-ling dynamics are recommended.

Based on these results a decision model for converging processes is introduced. The model enables to adjust the single reactions of the supplying processes to higher demand by synchronizing their logistical responsiveness.

dynamics, production planning and control, production networks

Warm forging is an attractive alternative to hot forging due to the preferable properties of the forged work pieces. However, the lower forging temperature results in higher yield stresses of the material to be formed. Therefore, the forging die within warm forging is rather loaded mechanically than thermally compared to hot forging. To protect forging dies against wear appropriate die materials and coatings are necessary. Based on their high strength and outstanding friction properties hydrogenated amorphous carbon films could be a suitable wear protection coating for warm forging. 

Different types of hydrogenated amorphous carbon films were deposit on forging dies and their wear behaviour was characterised following warm forging trials. The results prove the applicability of the coatings to protect warm forging dies against wear. A hydrogenated amorphous carbon film doped with 40 at-% of chromium demonstrated its efficacy as a wear protection coating up to billet temperatures of 950 °C. The resulting surface temperature of the forging die then is about 510 °C.

warm forging, wear, diamond-like carbon (DLC)

The development of forging sequences is important to producing flashless forging components. To achieve form filling in the final form an optimal forging sequence is required. To reach this aim the preforming steps are essential. An optimal volume distribution in the preform is the requirement for a complete form filling in the final form.

In this dissertation "Cross wedge rolling and bi-directional forging as a preforming operation for the production of crankshafts" the preforming processes cross wedge rolling and bi-directional forming are analysed. These preforming processes are ana-lysed for the applicability to forging crankshafts in a flashless forging process. The development of the preforming processes and the forging sequences will be done with the Finite-Element-Analysis (FEA). In experimental tests the preforming process cross wedge rolling will be analysed.

For reducing process steps in the forging sequence of crankshafts a direct combina-tion of cross wedge rolling and bi-directional forming will be analysed. For this, a development for a new bi-directional forming tool will be done.

cross-wedge-rolling, bi-directional forming, crankshaft, flashless forging, FEA

The companies in the tool making industry are exposed to a high intensity of competition. To get customer orders, they have to prepare reliable quotes in a short time. The available rudimental and partially incomplete information about the tool is of limited suitability for a reliable estimation of manufacturing costs. This thesis aims to support the tool making companies with tender preparation through a method of feature-based estimation of manufacturing costs. Essential basis forms a digital representation of the tool, which is generated by an automated CAD model building using technical guidelines. The method is implemented in the form of an expert system. The cost estimation is done, firstly, by determining the costs for machining by an analytical method based on the CAD model. Second, the costs for manufacturing processes are determined by means of a data mining based method by features derived from the CAD model and serve as inputs for prediction models. Using practical examples, the application of the expert system is described in practice and the method is evaluated. Thereby the practical applicability and limitations of the method are shown.

tool making, tool shop, quotation costing, data mining, expert system

The midterm utilization and resource planning process for a press shop network demonstrates a huge challenge. In order to produce highly economical, it is necessary to balance the poles between maximum utilizations, homogeneous distributions of utilizations between press lines and a minimum of manpower. Complex machine processes, fluctuating machine performance and fluctuating demands for stampings need a new simulation-based planning method to increase planning accuracy in the press shop. The objectives are the definition and choice of simulation-based production modes, and the installation of the necessary capacities and production resources. The development of a specific planning method for the press shop, including a following validation of an application example, will result in the midterm planning corridor in press shop after the analysis of all relevant parameters of the planning process. To ensure a holistic simulation-based utilization and resource planning maximum efficiently, the realization and implementation takes place with the help of a new software tool. The development of multiple simulated plan scenarios, and a subsequent holistic assessment of the simulatio ? processes after changed boundary conditions provide a quantitative analysis of process simulation. The result is an increase in planning accuracy between planned values and current values in the production. Consequently, further improvements of efficiencies of capacities and requirements of resources into the press shop network are required.

planning process, allocation, utilization, resources, simulation

Through the use of cooling systems in injection molding, the reduction of cycle time and increasing the quality of molded parts is desired. New manufacturing processes such as selective laser melting enable the layered structure of cooling systems and allow the free design of cooling systems which is close to the part geometry. With these form-fitting cooling systems, for example, the tempering cycle times can be reduced further. However, the degree of freedom in the design of cooling circuits increases with the new production method, which makes the manual design difficult and extends the simulation times. Automated design methods of conformal cooling systems based on geometric and process parameters are currently not available. An innovative software helps by constructing close-contoured channels and reducing the cycle time with minimized engineering effort.

injection molding, conformal cooling systems, design, nature-inspired algorithms

In comparison to hot forging, warm forged parts contain advantages e. g. improved surface qualities, lower surface decarburization and smaller geometrical tolerances. The geometrical spectrum of warm forged parts is limited. Cross wedge rolling allows an increase of this geometrical spectrum by cylindrical parts. Additionally, further potential e. g. material saving and short cycle times can be adapted. A method for process lay-out of cross wedge rolling processes is developed and a parameter field for warm cross wedge rolling is identified. This method contains FEAsimulations and experimental analysis with a geometrically downsized cross wedge rolling process. For the experimental trials a new apparatus is developed, which can be integrated in a hydraulic press and allows mounting of several tools in flat wedge configuration. The results of the FEA-simulations and downsized trials are adapted to a cross wedge rolling process for preforming of a steering link integrated into an industrial warm forging process chain.

forming, cross wedge rolling, warm forging, downsizing, process lay-out

Due to their diverse properties and versatile aesthetics, natural stones are used as construction materials, i.e. for floor coverings, stair steps and as facade cladding. The production of slabs and tiles made of natural stone is mainly performed with rotating diamond cutting discs. In the production of slabs and tiles the cutting rate is more than 60%, leading to a loss of raw material as well as high energy consumption, and tool costs. By using thin cutting discs the width of the cutting kerf can be reduced and the efficiency of the abrasive cutting process can be increased. However, the reduced stiffness of the thin cutting disc may lead to higher tool deflection and reduced cut quality. The objective of this work is to investigate a closed loop control for an abrasive cutting process using thin cutting discs. The used diamond cutting disc is 40% thinner than normally used tools in the industry. For the implementation of a closed loop control a monitoring system is being developed. It allows monitoring the axial tool vibrations throughout the grinding process. Integrating piezoelectric elements in the grinding tool, the vibrations can be detected directly at their source. This ensures a high accuracy of the detecting process. Based on studies, a closed loop cut-off control is being developed and studied. The closed loop control varies the feed rate depending on the axial tool vibrations. Through the developed closed loop control further information on the vibration characteristics of a cutting disc during a process are obtained. Furthermore, approaches to increase the cutting quality of thin cutting discs by a closed loop control are presented.

cut-off grinding, natural stone, process monitoring, process control

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