Showing papers in "Production Engineering in 2008"

Categories of size effects

Abstract: Size effects play an important role in up-or downscaling of processes and the transfer of knowledge to the processing in new geometric dimensions like micro- and nanotechnology. A definition of size effects and a systematic order for size effects is proposed. The main categories are density, shape and microstructure size effects, which can be subdivided into further subcategories. Using the example of size effects in the strength of materials it is shown that the systematic can be used to get the different effects unambiguous into an order, which helps to avoid apparent contradictions in experimental findings.

Orbital drilling kinematics

Abstract: In orbital drilling the tool (special end mill) moves relative to the work piece on a helical course Because of the three-dimensional tool path and the superimposed rotary cutting motion a complex machining motion results which determines the contact conditions of the tool The objective of this study is to describe mathematically the occurring cutting conditions over the engagement angle as a function of the technology parameters: bore diameter, tool diameter and the gradient of the helical course On the basis of this mathematical description further fundamental and qualitative statements regarding the cutting process of orbital drilling can be determined The theoretical description confirmed a change of the cutting characteristic over the penetration engagement angle of the tool from drilling to milling

Determination of tribological conditions within hot stamping

Abstract: Innovative hot sheet metal forming technologies are gaining an increasing significance in the scope of application of more and more innovative high and ultra high strength steels in the automotive industry. With respect to a numerical process design beside the mechanical and the thermal material characteristics the friction coefficient represents an important input parameter for finite element (FE) simulation. Within the scope of this paper an evaluation method for the determination of the friction coefficient μ for the direct hot stamping process of boron-manganese steels will be presented. Therefore cup deep drawing tests at elevated temperatures following the time-temperature-characteristic of the hot stamping process are carried out. For the calculation of the friction coefficient the approach according to Siebel for the modeling of the maximum drawing force is used.

Machining induced residual stress in structural aluminum parts

Abstract: Machining operations of aluminum structural parts are typically carried out under high feeds and high cutting speeds. Under these conditions, high thermomechanical loads are exerted on the workpiece, which may result in changes in the subsurface material. Residual stresses can be one of the machining induced changes and can lead to considerable rejection rates caused by part distortion. Due to their significant economic importance, it is essential to understand the influence of the machining process on the residual stresses in aluminum. This paper presents the influence of the machining parameters as well as the cutting edge geometry on residual stress of workpieces made out of a forged aluminum alloy.

Cartesian compliance model for industrial robots using virtual joints

Abstract: Industrial robots represent a promising, cost-saving and flexible alternative for machining applications. Due to the kinematics of a vertical articulated robot the system behavior is quite different compared to a conventional machine tool. The robot’s stiffness is not only much smaller but also position dependent in a non-linear way. This article describes the modeling of the robot structure and the identification of its parameters with focus on the analysis of the system’s stiffness. Therefore a method for the calculation of the Cartesian stiffness based on the polar stiffness and the use of the Jacobian matrix is introduced. Furthermore, so called virtual joints are used. With this method it is possible to model each joint of the robot with three degrees of freedom. Beside the gear stiffness the method allows the consideration of the tilting rigidity of the bearing and the link deformations to improve the model accuracy. Based on the results of the parameter identification and the calculation of the Cartesian stiffness the experimental model validation is done.

Prediction and inline compensation of springback in roll forming of high and ultra-high strength steels

Abstract: With the rising interest in lightweight construction, the usage of high and ultra-high strength steels has increased remarkably during the last years. Unfortunately, these steel grades show more springback than mild steels do, which leads in consequence to lower dimensional accuracy. To improve the bended part’s geometry considerable trial-and-error work is necessary since the influence of different bending-parameters (e.g. the bending radius, sheet thickness, yield strength, Young’s modulus, the material’s strain hardening coefficient, …) on the amount of springback is still unknown. The aim of the paper at hand is therefore the investigation of springback for different parameter combinations. Furthermore, an online calibration system for occurring springback during roll forming is presented to compensate springback independent of material or process parameters.

Mechanisms in the generation of grinding wheel topography by dressing

Abstract: For the process of dressing vitrified bonded grinding wheels with diamond tools it has been unknown how the wheel topography is generated. Moreover, the influence of the kinematical dressing parameters on the wheel wear behavior has not been quantified. In the course of this article the grinding wheel was dealt with as a porous ceramic composite. In FEM simulations common dressing forces and usual dressing tool geometries were applied. The results were verified by dressing tests and grinding wheel scratch tests which show the wheel wear mechanisms. The common practice of decreasing the grinding wheel surface roughness by a finishing dressing stroke has to be reconsidered, because previous dressing strokes with higher depths of cut can weaken the grinding wheel structure and lead to an unsteady phase with high grinding wheel wear after dressing.

Electron beam sintering of metal powder

Abstract: The laser is widely used in additive layer manufacturing for producing parts on the basis of polymer and metal powder. In the metal area, the capacity of laser technologies is noticeably exhausted. The electron beam (eb) technology offers higher power density and beam velocity and therefore seems more suitable for sintering high-tensile steel powder in an economic way. In this paper, a comprehensive method is implemented to develop the eb sintering. In comparison to laser-based applications, different physical effects occur and have to be controlled. Therefore, the paper focusses on those physical effects and the measures, which have to be taken in order to achieve a reliable process. The result of this work is a prototype machine, in which the eb is used for sintering high-tensile steel directly from CAD files to three-dimensional, non-porous parts.

Determination of size-dependent friction functions in sheet metal forming with respect to the distribution of the contact pressure

Abstract: The aim of this paper is to optimize the analytical model developed in previous work (Hu et al. in Determination of the friction coefficient in deep drawing, process scaling. In: Vollertsen F, Hollmann F (eds) Proceeding of the 1st colloquium of DFG priority program process scaling. BIAS-Verlag, ISBN 3-933762-14-6, Bremen, pp 27–34, 2003; Hu and Vollertsen in J Technol Plast 29:1–9, 2004; Vollertsen and Hu in Annu CIRP 55(1):291–294, 2006) with respect to the distribution of the contact pressure at the drawing radius. A size-dependent friction function was acquired based on the experimentally measured punch force from strip drawing with deflection, which can identify the tribological size effects in sheet metal forming. This function was implemented in the FEM-simulation. The distribution of the contact pressure at the drawing radius was assumed to be uniform in the previous analytical model, which is not right, since the simulated punch force versus punch travel curve showed a difference of about 11% from the experimental curve (Vollertsen and Hu in Annu CIRP 55(1):291–294, 2006). In the new analytical model the non-uniform distribution of contact pressure between the work piece and the tools was taken into account. The simulated curve using the friction function from the new model shows a better agreement with the experimental curve.

Surface hardening by strain induced martensitic transformation

Abstract: The hardness and fatigue strength achieved by strain hardening are normally noticeable lower than those attained by thermal or thermochemical heat treatments. Strain or deformation induced martensitic transformation of residual austenite can increase the strength achieved by mechanical surface hardening processes considerably. In this paper, an approach is presented where workpieces with a high content of metastable austenite are used for hardening the surface layer. The microstructure has to be sufficiently stable, in order to ensure that the material can be machined without being changed by strain induced transformation of the residual austenite. After machining, high Hertzian contact stresses are introduced by deep rolling, so that a strain induced martensitic transformation of the residual austenite takes place. At the same time deep rolling produces the surface finish of the part. By this method, a surface hardening without a heat treatment process within the production line can be realized. A conceivable use of this method could be the production of bearings or guideways.

Development and design of an active work piece holder driven by piezo actuators

Abstract: The use of piezo actuators in machine tools is a current topic. There are many research works which show the advantages of this kind of actuators in various fields around machining. In this paper an active work piece holder is presented, which is designed for reducing the dynamic displacement between the tool and the work piece relatively. The paper shows the design process for the fixture, the finite element analysis of the mechanical parts and the actuators. Also a specially designed power amplifier is presented which fits best to the needs of voltage and frequency range of the active work piece holder. First experimental results are shown.

Manufacturing of molds for replication of micro cube corner retroreflectors

Abstract: Retroreflectors composed of micron-sized cube corner elements are widely used as reflectors on clothes for safety reasons or for measuring applications. Due to the small size of the individual reflector elements structured films can be flexed widely without affecting the geometry of each element too much so that it can still fulfill its function. However, the efficiency of triangle cube corner reflectors widely used today is limited. It could be improved by applying structures with a hexagonal shape resulting in a significantly better performance of the reflector. Up to now these structures are not machinable at micron size (50–500 μm) in sufficient quality. This paper introduces a new machining process, diamond micro chiselling, which is capable of generating these micro retroreflectors and other microoptic structure geometries not machinable so far.

Service provision as a sub-model of modern business models

Abstract: Today’s turbulent economic environment confronts Germany’s tooling industry with new challenges. To compete with China and with the new eastern members of the European Union, German companies must come up with innovative products and services to stand out from their competitors. New types of business models shall offer individual solutions to the customers, integrating product and accompanying product-services to a product-service-system.

Machining of micro rotational parts by wire electrical discharge grinding

Abstract: Micro rotational parts are used in several industrial sectors. Well-known applications are micro shafts of gears, ejector pins in forming tools, pin electrodes for micro electrical discharge drilling or micro stamping dies. Depending on the geometrical complexity of micro rotational parts different process variants of micro electrical discharge machining characterized by a rotating work piece can be used: wire electrical discharge grinding (WEDG) with fine wire electrodes, electrical discharge turning (EDT) with micro structured tool electrode, cylindrical electrical discharge grinding (CEDG) with micro profiled disk electrode. Characteristic to these process variants is the superimposed relative motion between the rotating electrodes and the feed. This relative motion can be varied in a wide circumferential velocity range to improve the material removal process. The paper gives an overview of kinematic and technological restrictions and requirements of the WEDG process influencing the process behavior with respect to the technological requirements of micromachining.

An inverse approach to the numerical design of the process sequence of tailored heat treated blanks

Abstract: While in industries lightweight construction gain an increasingly significant role and as weight reduction is often done with aluminum sheets, advanced production technologies have to be developed to be competitive for this evolution. Since steel sheet metal parts cannot be substituted directly with aluminum due to its minor formability, the usage of so called tailored heat treated blanks (THTB) is presented in this work. THTB are locally heat treated aluminum blanks from the 6,000-series alloy which exhibit a specific strength pattern optimized to the forming operation leading to a significantly improved formability for the manufacturing of complex aluminum car body parts. The enhancement of the formability is reached by a local heat treatment before the forming process. Due to the strong interdependency of heat treatment and forming operation, a numerical investigation of the process sequence is a prerequisite for a cost-effective usage of the THTB. An inverse approach on basis of a finite element simulation enables the determination of process parameters for an optimized THTB, thus having an effective and efficient engineering method for this technology.

Using PCD for machining CGI with a CO 2 coolant system

Abstract: Polycrystalline diamond is widely used as a economic cutting material for machining non-ferrous materials such as aluminum. It is perceived that diamond cannot be used for cutting ferrous materials due to the high affinity of carbon to iron. Nevertheless, under certain conditions it is possible to use diamond materials for cutting ferrous metals. In order to avoid graphitization of the diamond matrix, it is necessary to keep the cutting temperature below the critical level of diamond graphitization. This paper presents the influence of a cryogenic CO2 coolant strategy on the cutting process using PCD tools for cutting high strength compacted graphite iron (CGI). Investigations show, that tool wear behavior strongly correlates with the cutting speed, the cutting forces, cutting temperatures, and surface roughness of the workpiece. The test results show, that the tool life of PCD for cutting cast iron is dependent on the diamond grain size, the binder material, and the cutting parameters.

Design of dynamic multi sensor systems

Abstract: Recent increase in machine automation, machine capability, and the performance of machining processes lead to high demands on process monitoring systems. In the Center of Production Engineering in Hannover Gentelligent® manufacturing systems are researched, which will be able to meet those requirements. One enabling factor for these systems is the “feeling” capability, achieved by multi sensor systems. This paper copes with the design of these systems. This includes on the one hand the development of sensor configurations and on the other hand a concept for sensor fusion. For the search of optimum sensor configurations a previously developed algorithm is extended to consider the dynamic behavior of the sensor systems.

Tool path generation for ultra-precision machining of free-form surfaces

Abstract: The generation of tool paths for ultra-precision machining is still a limiting factor in the manufacturing of parts with complex optical surfaces. In conventional machining as well as in complex five axes machining the application of CAD- and CAM-software for the generation of tool paths is state of the art. But these software solutions are not able to generate tool paths according to the high requirements of ultra-precision machining. This paper describes possible ways to generate tool paths for ultra-precision machining when the optical surface can be analytically described or when the surface data is derived from optical design software. Ultra-precision milling experiments with different tool paths have been carried out and the quality of the machined geometry has been evaluated concerning the achievable form accuracy.

Modeling regenerative workpiece vibrations in five-axis milling

Abstract: During milling—especially of thin-walled components—the dynamic behavior of the workpiece-tool-machine-system influences the milling process and particularly the quality of the resulting workpiece surface. This article focuses on the presentation of a simulation concept for predicting regenerative workpiece vibrations, which combines a finite element model for analyzing the dynamic behavior of the workpiece with a time domain simulation for the five-axis milling process. Both concepts, their linking, and the experimental setup for verifying the simulation will be described. A comparison of the simulation results with the data measured in experiments with regard to the vibration frequencies as well as the surface quality will be given.

Reduction of wear induced surface zone effects during hard turning by means of new tool geometries

Abstract: Tool wear during hard turning influences the properties of the workpiece surface and subsurface layer significantly. Due to increasing flank face wear at the cutting edge, the contact conditions between tool and workpiece are changed. The mechanical and thermal load in the workpiece surface increases during the process. This favors the formation of white layers and of residual stress gradients in the subsurface zone of hardened workpieces whereby the components life time is reduced. The article presents novel modifications of the tool geometry, which leads to a considerable prolongation of the tool life time. This advanced tool design enables the production of constant material properties in the surface and subsurface zone during a broad time window.

Production engineering for self-organizing complex systems

Abstract: Today’s models in production engineering are mainly based on quasi-stationary design assumptions. With increasing dynamics, models reflecting the nonlinear characteristics of production are becoming increasingly relevant. Several approaches have been formulated to design self-organizing systems, leading to good results with regards to the efficiency in which dynamics are mastered by local interaction of a system’s entities. While in self-organization a system’s characteristics emerge bottom-up, effectiveness is designed into a system by an analytical, top-down design approach. The paper discusses how a target oriented analytical design approach can be combined with the concept of self-organization and shows its application and impact on practical examples.

Pre-treatment by dry ice blasting for adhesive bonding

Abstract: In recent years, the constant development of adhesives and coatings has contributed to the establishment of the adhesive and coating technology in several fields. Based on findings in macro-molecular chemistry, the cohesive strength, e.g. the internal strength of adhesives, has been enhanced continuously. In contrast to the adhesive strength, the mechanical, chemical and physical adhesion, the cohesive strength is relatively strong. As a consequence, adhesion is usually the weakest link of an adhesive bond, thus, most often it is adhesive fracture that can be observed. Pre-treatment and the use of so-called coupling reagents represent an approach to improve the adhesive strength. The paper presents dry ice blasting as a pre-treatment process and a process model with which the reasons for the increased adhesive strength can be clarified. With respect to the relevant blasting parameters, the effects of a pre-treatment with dry ice blasting on roughness, surface topography, specific surface, mechanical activation, surface energy and chemical composition depending on relevant parameters have been determined.

Simulation of the manufacturing process chain of welded frame structures

Abstract: Numerical solutions of manufacturing process chains strongly focus on supporting the production of frame structures in the vehicle industry. Frame structures play a central role in all means of transportation, dominating the total mass in automobiles, trains, airplanes, and even space ships. In the case of automobiles, the frame structure mass constitutes about 30% of the total mass. In the case of the automotive industry, the mass-production, the high-quality standards and the need for mass minimization all increase the necessity to apply virtual manufacturing methods in the production cycle of frame structures. In scope of this paper, a method of chaining the numerical analysis of the structural effect on frame components during successive manufacturing processes is introduced. The change of thermal, metallurgical, and mechanical properties in structures during manufacturing is analyzed by applying theoretical models depending on the identified problem.

Model-driven development of PLC software for machine tools

Abstract: Developing PLC software for modern machine tools is becoming more difficult because of the increasing functionality and resulting complexity. An approach for managing this is provided by the model-driven development of the control software. However, this innovative development method requires both a procedure that is adapted to the specific application domain and suitable modeling techniques. The article describes an approach appropriate for machine tools. The focus is on introducing the necessary description techniques and a methodology for their use in product development.

High-power laser welding of contaminated steel sheets

Abstract: A deep penetration laser welding process differs from a heat conduction laser welding process in the generation of a steam capillary through the material. The so called keyhole is formed if high energy densities of the laser beam vaporize the base material, leading to a vapor-filled channel which is surrounded by a liquid weld pool. The shape and the oscillations of the keyhole are affected by various physical parameters of the welding process. Under constant process conditions a balance of pressure keeps the capillary open. If the material is contaminated by a layer with physical properties differing from the bulk material (e.g. oil), the equilibrium is disturbed in a negative way. The coating evaporates during the melting of the base material, resulting in vapor locks at the capillary and the weld pool, leading to pores and failures along the weld seam. In order to understand and avoid this effect, it is the aim to analyze and describe the failure generating mechanisms. Additionally, a control method of the laser power is investigated which influences the keyhole’s oscillations and could suppress an unwanted collapse.

Development of ultra high performance concrete dies for sheet metal hydroforming

Abstract: The paper concentrates on investigation results dealing with ultra high performance concrete (UHPC) dies for the use in sheet metal hydroforming. A special mixture of UHPC for these tools with its mechanical properties and its manufacturing is presented. The developed UHPC features a compressive strength of approx. 250 MPa and a Young’s modulus of approx. 50 GPa. Applying internal pressure, the UHPC die (without confinement or reinforcement) failed between 80 and 96 MPa. The forming test proved a good shape accuracy of the sheet metal parts but revealed critical tribological conditions. Drawing foil and already formed sheet metal parts were tested successfully as interlayer and led to an appropriate material flow and surface quality of the sheet metal parts.

Valuation of changeable production systems

Abstract: The introduction of changeable production systems increases the adaptability of factories in an uncertain environment. This paper analyzes the valuation of the economic efficiency of such systems using the real options theory. The developed approach proposes a structured valuation process for selecting options related to production engineering and factory planning. Based on a classification of possible modifications of a production system, options profiles are derived. Considering the existing uncertainty of a company’s environment these profiles may be utilized to identify real options which ought to be included when valuating a technical production system design.

Microstructuring of functional surfaces by means of cutting processes

Abstract: Microstructuring of mechanically and thermally highly stressed surfaces offers an innovative alternative for adjusting tribological properties and thus reducing friction losses and wear. At the Institute of Production Engineering and Machine Tools (IFW) functional surfaces are structured via cutting. This paper introduces to the basic principles of manufacturing microstructures in ductile materials foccussing cutting tool- and process-dimensioning. Scratch tests are carried out at planar surfaces to describe the basic chacacteristics when producing structures in micron range. Due to the small structure depths, size effects have to be considered. On this account, influences and interrelations of cutting edge microgeometry, material properties, as well as cutting parameters are investigated. In order to avoid further finishing operations, sharp structures with tolerable burr formation have to be achieved.

Warm forging: new forming sequence for the manufacturing of long flat pieces

Abstract: Warm forming of steel is an economic and ecologic alternative to the conventional hot forging technology. It offers several advantages like decreased energy input, reduced scale formation, better surface quality and closer tolerances. Warm forming is common for rotation symmetric parts but has not been applied for long flat pieces yet. The main obstacles prohibiting the transfer are the missing of a suitable preforming technology and the problems to keep the narrow temperature tolerances despite the increased number of forming operations compared to rotation-symmetric process routes. In a cooperative effort within a European consortium of forges and scientific institutes, closed-die warm forging processes for long flat pieces have been developed. To cover a wide range of longitudinal geometries, two model products, a connecting rod and a steering link, with unequal requirements towards the production process have been selected. The process developed for the steering link is discussed in this article.

Improved NC path validation and manipulation with augmented reality methods

Abstract: Five-axis milling offers many advantages over the conventional three-axis milling process. However, because of the potentially complex motions, it is difficult for the machine tool operator to anticipate the actual movement based on the NC program. In this paper a software system for the NC path validation and manipulation during the milling process is introduced. This system is meant to expand the information given to the machine tool operator by enriching the view with data of a concurrently running milling simulation. The simulation is synchronized with the real-world machine tool movement by detecting the position of a marker that is mounted on the head stock. With this combination of real-world view and computer-generated data, which is called Augmented Reality, the machine tool operator is able to detect critical situations—like collisions between tool holder and workpiece or excessive forces—and may adjust the NC program accordingly.