Doctoral theses

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

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

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

Disruptions in productions with negative impact on the objectives of production logistics need to be indentified and resolved. Enterprises often lack of sufficient qualified employee who know how to control the controllable logistic relations in production and thus to realize a targeted improvement of a logistic process. Moreover, a profound knowledge about the compley relations between logistic objectives and measures against interferences in production is often missing. An approach for a model-based diagnose of disruptions in production logistics will be developed in this thesis. It will be exemplified how to implement the approach in an assistance system. Staff will get assistance in detection of deviations, finding causes and deriving reommendations for measures.

production logistics, diagnostics, assistance system, model based rules, web of causes and effects

The performance of humans is subject to fluctuations throughout the day. As criticism of a method developed by Graf power curve is an example the lack of differentiation of people according to biorhythms (morning and evening type) call. In addition, the study looked no spiritual component. An analysis of circadian fluctuations in performance of the people in industrial assembly has not yet explicitly examined. With the present study this gap is closed. On the basis of an appropriate industrial work system performance circadian variations in manual assembly activities depending on different factors (e.g. "Sex", "Age", "biorhythm") are initially recorded and analyzed. The relationship between power fluctuations and the factors determined is statistically backed up by data analytical methods. Finally, are areas of application (e.g. As shift work, timing adjustment) recognized for harnessing the proven power fluctuations and demonstrated potential to account.

performance efficiency fluctuation, assembly, industrial work system, data analysis method

Companies in which the typical tool cycle is used for their production tools and be found with reclamation and reuse are, seek to reduce the average delay deployment of ready tools. To improve the availability of production the number of tools in the circuit or the tool life of the tools can be increased. However, this in turn leads to higher cost. Thus, the achievement of an objective goes to the expense of another. The central part of the work presents the modeling of a typical tool cycle is in a simulation model. Thus, the basic relationship between the indicators and default provisioning tool inventory and their controllability by organizational and technological measures within the framework of simulation studies have shown.

tool allocation delay, tool logistics, tool stock, tool supply, material flow simulation

Sheet metal parts for fastening often require pin-shaped joining elements. In case of high requirements on the tightness or the electrical contact arc stud welding processes are favored. Arc stud welding processes up to now are executed on separate welding stations. For it handling operations and storage of parts are necessary. To shorten the process chain solutions for integrating the arc stud welding in sheet metal working tools are required. In this thesis a solution for capacitor discharge (CD) arc stud welding with tip ignition in sheet metal working tools is presented. The developed welding module is automated in a tool using a metal strip conveyor system. In experiments the attainable joining quality, the interaction between tool and welding process and the process stability by automatic line operation are analyzed. On the basis of practice-orientated disturbances the robustness of the integrated welding process is examined.

sheet metal, capacitor discharge arc stud welding, cd arc stud welding, tip ignition, follow-on tool

Producing companies have to face an increasing number of phase-outs due to shortening product life cycles. Phase-outs show a high potential of cost saving in terms of avoiding remaining inventory after the phase-out. Although this potential is well known an efficient management of phase-outs does not take place. Methods, models or key performance indicators for an efficient phase-out management do not exist. An additional challenge is the collaboration of various companies to produce a defined product. The coordination and collaboration of these companies additionally exacerbate the efficient conduction of a phase-out. This work presents a methodical support for a phase-out management in production networks.

production planning, supply chain management, phase-out management

The flexible customisation of production and logistic processes is an increasing challenge for companies in order to save their entrepreneurial success. This entails requirements concerning production control, which have to be contemplated and interpreted holistocally not at last due to an increasing importance of assembly for the overall manufacturing and assembling processes. The cost effective and timely supply of the assembly with in-house and an increasing amount of external material hasn´t been explored much to date from a scientific point of view. This logistic challenge can be supported by a concept of an assembly synchronous job control. The synchronisation approach focuses on the compensation of assembly disturbances as well as on the enhancement of flexibility in workflow changes. On the basis of an holistic approach of synchronisation between the manufacture of parts and assembly (horizontal synchronisation) the developed concept of a forward-looking prevention of missing parts and an holistic, assembly oriented synchronisation of material supply processes for assembly (vertical synchronisation) wll be expanded by aiming at on-time-delivery improvements of partner orders.

production logistics, production management, production control, assembly, synchronisation

In the dissertation the basics of capacity related external processing will be described. Different approaches to selecting the amount of production to be moved to external production will be analyzed. Additionally approaches to the evaluation of external processing scenarios are discussed. The different approaches will then be checked according to the requirements for viable solutions. The objective of this dissertation is the development of a methodology for triggering a capacity related external processing between press shops. The structure of the methodology will be explained by describing its single steps and the analyzed elements of the production process. Furthermore, the procedures and applications of the steps will be delineated. The functionality and the practicability of the methodology will be shown within a realistic cooperation scenario.

press shop, sheet metal part production, capacity related external processing, cooperation

This paper elaborates the open scientific question with practical relevance to producing enterprises at which service level an article of a production warehouse should be provided to production. The objective of this paper is to develop a decision supporting logistic model calculating the service level of an article which should be achieved when considering the follow-up costs of a stockout situation (stockout costs) on the one hand and the costs of the logistic organization on the other hand (i. e. warehousing). The new logistic model’s elaborative work is based on known stock out cost types (i. e. by REICHMANN) as well as on the characteristic curves for the service level and the delivery delay developed by WIENDAHL and NYHUIS.

characteristic curves, delivery delay, logistic costs, production, service level, stock, stockout co

Considerably, flashless precision forging offers the potential for material saving. To secure form filling, in conventional forging in open dies, e. g. of crankshafts, a surplus of mass of about 50 percent is used. Compared to conventional forging with flash, the economical potential of flashless precision forging is based on smaller tolerance classes and the shortening of the process chain. The newly developed forging sequence of a two cylinder crankshaft for flashless precision forging consists of four stages. The pre-forming takes place in two lateral extrusion processes, with an innovative multi-directional forging tool being used. The final flashless precision forging produces the final geometrie of the crankshaft. To achive the required tolerances of IT 7 to IT 9, a newly developed method for the compensation of the shrinkage behaviour is implemented. This method uses not only a homogeneous gauge but considers the inhomogeneous tension and temperature profiles in the forged parts.

flashless precision forging, crankshaft, shrinkage compensation, FEA

Defects like folds can arise using forging for the production of long flat pieces made of aluminium. A special defect is the formation of inner folds. These can be seen in the grain flow. Inner folds have a negative effect on the dynamic properties of the forged part. As a production process, forging can be divided into single-directional and multi-directional forging. The formation of inner folds was observed at the single-directional forging. By using the multi-directional forging, a forming operation working from different directions, the forming can be set variably. Thus the development of folds can be prevented. A newly developed method can help in the selection of the forming process and in determining an appropriate tool geometry. Here especially the area is adapted, where the development of inner folds occur. Therefore a calculation model was developed. It integrates a computer-aided identification of the inner folds. Using this model, a correction of the parametrically constructed forging tool is possible.

multidirectional-forging, long flat pieces, aluminium, fibre orientation

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