Papers by Author: Berend Denkena

Abstract: The machining of difficult-to-cut materials such as titanium plays a key role in several industries such as aerospace or medical. Approaches to overcome many difficulties when machining these materials can be an appropriate coating system for cemented carbide cutting tools. However, the atmosphere under which machining takes place, influencing the chemical tool wear, has not been taken into consideration. This work examines the tribochemical wear resistance of TiN, TiAlN and CrAlN coated carbide tools under different atmospheric conditions when cutting Ti6Al-4V. Air, technically pure argon and silane-doped argon is used to determine the influence of different oxygen levels on the wear behaviour of the tools. It has been found that oxidation of tools and tool coatings plays a significant role in tool wear when dry cutting titanium. Best results were generated using CrAlN and uncoated inserts where an increase in tool life up 50 % can be achieved when cutting in oxygen levels corresponding to extreme high vacuum (XHV) adequate atmospheres by using silane-doped argon. The benefits of XHV adequate atmospheres also have an effect on TiAlN-and TiN based coatings, but the chemical interaction of Ti element in the coating with the workpiece material, which presumably reduces wear resistance of cutting tools, cannot be outweighted or equalised by applying oxygen free atmospheres.

Abstract: In this study a novel inverse hybrid experimental-simulative approach to the determination of the thermal tool load as a function of the coating properties during orthogonal turning of AISI4140 with Cr_1-xAl_xN-coated cemented carbide tools is presented. The approach consists of an experimental determination of the internal tool temperatures by means of fiber-optic pyrometry as input for an inverse FEM-based simulation algorithm to calculate the surface temperatures. Based on a parameter study, the coating thickness s and the thermal conductivity of the coating λ_c were identified as the main factors influencing the thermal tool load. The combined influence of these properties was described via the thermal resistance R. It could be shown that the average thermal load on the tool surface increases with increasing thermal resistance R.

Abstract: Wire cutting, originally developed for the processing of natural stone, is increasingly used for the machining of reinforced concrete and metallic structures. In this field of application the diamond wire sawing tools are subjected to a higher-than-average wear. Furthermore the metallic bonding of the cutting beads is not suitable for machining metallic structures, because of a missing self-sharpening effect. Within this paper, conventional wire sawing tools are investigated in pure steel structures. These tools are supposed to be suitable for the cutting process in accordance to their tool concept. The manufacturing process and the grain sizes have an influence to the tool performance. From these investigations requirements for a new tool concept will be derived.

Abstract: High-strength structure components can be built by the application of an advantageously designed geometry. These structures are challenging to manufacture due to their complex shape. This motivates the design of a new system for quality assurance of the geometric properties of these structures. Firstly, different measurement approaches and their usability for the measurement of high-strength structure components are discussed. Then an optimized solution is suggested. A pattern projection method is introduced, whereas the patterns adapt locally to the measured surface. The basic object of the research are methods of the optic three-dimensional measurement of structural elements with the help of adaptable pattern projection that allows to adjust the measuring process to the local geometry and reflecting characteristics of structural elements. This technique takes care of the special properties of high-strength structure components within the production process. It is described, how a measurement speed can be achieved that can catch up with a common production frequency without interrupting the flow of the parts. Furthermore, the measurement can take place in an industrial environment, because it is relatively tolerant to movements of the measured structure. Therefore, the measurement procedure is divided into two steps. Within the adaptation-step the luminescence and the resolution of the projected pattern is adapted to the measured object. The actual picture is taken and calculated in a second step in order to be robust against vibrations. The result of the measurement is an optimized point cloud of several million points. The paper concludes with a description of the data processing procedures necessary in order to evaluate the correctness of the measured structure.

Abstract: Novel manufacturing technologies for high-strength structural components of aluminium allow a local modification of material properties to respond to operational demands. Machining and finishing processes for changing material properties like deep rolling or rubbing are to be combined to a single process step. The intention is the controlled adjustment of the component’s properties by the modification of its subsurface. For that purpose the essential understanding of the interaction mechanisms of the basic processes turning, deep rolling and rubbing is necessary. Influences of the tool geometry as well as of the process parameters on the material properties are investigated. The results will be extended by parameter studies within numerical simulations. Thereafter, combinations of the basic processes in process sequences are analyzed to their ability to modify the subsurface properties. In consideration of these results, a prototypic combined turn-rolling tool is developed

Abstract: The wire sawing process is a highly flexible cut-off grinding process. Nowadays it is not only used in its traditional field of application (processing of natural stone) but also in the civil construction industry and for the destruction of metallic structures. In order to extend the knowledge about the process it is one goal of the Institute of Production Engineering and Machine Tools (IFW) of the Leibniz Universität Hannover, Germany, to investigate the cutting mechanisms. These are supposed to be the basis for a tool design taking into account the specific field of application. The results presented within this paper describe an approach to explain the interdependencies of the system and manipulable variables for the wire sawing process. From these interrelationships the cutting mechanisms will be derived.

Abstract: Five-axis-grinding is a process, which merges high surface quality and high shape accuracy for high precision machining of multiple curved surfaces. Until now, characteristic parameters, which describe the interaction of the grinding tool and the workpiece are not available. In the present paper models for the analytical determination of the contact condition for surface normal grinding of double curved surfaces with toric grinding wheels are introduced to evaluate and configure the machining parameters.

Abstract: Cohesive damage of PVD-coated cemented carbide cutting tools is ascribed to the residual stress state of the substrate subsurface. The present paper shows the formation of the substrate residual stress in the process chain as well as the stability of the single process steps referred to the scattering of the residual stress values. Depth resolved residual stress measurements across coating and substrate subsurface show a layer in the substrate, where possibly tensile stress occurs, from where cohesive damage may be initialized during tool use. Results of experiments are presented, where the influence of parameter variations in pre coating processes on the residual stress state is investigated. The characteristics of compressive residual substrate stress during the final PVD-process is presented as well as a correlation between coating and substrate stress.

Abstract: The need for new cutting tool technologies is driven by the constantly increasing performance of machine tools and the rising market competition. Current research results show that an improved combination of the cutting edge macro- and microgeometry, together with an appropriate substrate and coating, leads to a significant enhancement of cutting tool performance. Furthermore, inappropriate cutting edge microgeometries cause, in addition to the higher production costs, a reduction of the tool life. Hence, it is essential to produce tailored cutting edge microgeometries with high precision and process reliability. This paper presents the influence of brushing process parameters on the size and the form of produced cutting edges of indexable inserts. This leads to a better understanding and higher quality of the cutting edge preparation process by means of abrasive brushes. Furthermore, the process reliability of 5-axes brushing is analyzed. An example of a tool life map presents the significantly enhanced tool performance through cutting edge preparation and its sensitivity towards varying the cutting edge microgeometry.

Abstract: Premature collapse in terms of cohesive damage of PVD-coated carbide cutting tools often results in a time and cost consuming immediate interrupt of the cutting process. It is assumed that the residual stress state of the composite coating – substrate in combination with external loads during tool use is responsible for cohesive damage. The X-ray diffraction methods sin2 and scattering vector are applied for determination of the residual stress depth distribution in the coating and the substrate’s subsurface. Investigations of the residual stress state of commercial PVD-coated carbide cutting tools are presented. It is determined to what extent the single process steps during tool manufacturing are responsible for the final residual stress state of the PVD-coated tool. Furthermore the meaning of the PVD-coating process for the substrate’s residual stress state is investigated. Moreover, possibilities of controlling the residual stress state of the substrate by changing process variables of selected process steps are analyzed.