Volker Wittstock

Thermal deformations of cutting tools: measurement and numerical simulation

Challenges for machining include greater and greater material removal rates coupled with an increase in the use of difficult to machine materials, as well as environmental-friendly dry or minimum quantity lubrication machining, small manufacturing batches and frequently changed manufacturing orders. These trends are accompanied by high temperatures in the machining process and large, variable heat flows causing thermo-elastic displacements of the tool, the workpiece and the clamping devices. Although the displacements are small, in the range of a few micrometers, they have assumed more and more importance because of growing requirements for manufacturing accuracy. Thermo-elastic displacements of the tool due to heat flow during machining are investigated and analysed in this paper. Temperatures and displacements are measured on a test bed equipped with measuring instruments. The identification of the thermal boundary and contact conditions is supported by finite element models. Knowledge of the heat flows resulting from the machining process is a prerequisite for control of and compensation for displacements. Since these heat flows either cannot be measured or can only be measured with enormous effort, heat flows are determined by means of numerical simulation of the machining process itself. This strategy has been previously used as a systematic approach for turning in orthogonal cutting conditions. However, further investigations are needed for oblique turning conditions, milling and drilling operations.

Virtual-Reality-Based Simulation of NC Programs for Milling Machines

This paper addresses Virtual Reality-assisted pre-testing of NC programs and describes the benefits of testing them with respect to processing on the NC control unit. Thereby collisions of machine axes can be recognized, downtimes can be minimized and costs arising from faulty NC programs can be cut. Newly-engineered visualization of material removal in real-time-capable VR systems facilitates an even more realistic simulation of the production process, and thereby enables even those users who are not specialists in the field to quickly comprehend the ever more complex processes involved, and thus to recognize potential errors more effectively. By using the hardware in loop-coupling between a real NC control unit and a virtual machine model, errors caused by the NC control unit itself can be recognized.

Joining by forming of piezoceramic macro-fiber arrays within micro-structured surfaces of aluminum sheets

Functional integration of smart materials in sheet metal enables lightweight composite parts which are enhanced by new functionalities. Locally integrated piezoceramic/metal composites consist of a prefabricated array of ten parallel piezoceramic macro-fibers with dimensions of 0.277 mm by 0.232 mm by 10 mm which are joined in micro-formed cavities within the surface of an aluminum sheet metal. By the use of joining by forming, the interference–fit, preload and form–fit of macro-fiber arrays are achieved in a single process step. The paper describes investigations of the joining by forming process in formal planned experiments using the design of experiments method. The influence of the dimensions and preparation of the joining partners, the maximum forming force and the velocity of the forming stamp are varied. The interference–fit and preload depend on the maximum forming force. In contrast, the quality of the form–fit is primarily related to the geometric dimensions and the forming force. Fiber fractures and incipient cracks are the major failure mechanisms during joining by forming of the macro-fibers. The number of cracks is significantly reduced by the use of lower die velocities, lower maximum joining forces and the introduction of additional geometric elements in the microstructure of the metal surface. Concluding, constraints with regard to the design of parts and the process are derived from the experiments.

A Framework for User Tests in a Virtual Environment

This paper describes the setup and development of an innovative framework for conducting user tests in a virtual environment. As it is the main purpose to evaluate the user’s reactions to user interfaces on mobile devices, an Android-based smartphone and a tablet computer were linked to a Virtual Reality (VR) system. The framework allows user interaction to trigger certain events in the immersive environment and vice versa interaction with the virtual environment can trigger interface behaviour. For hand-free navigation through the virtual scene a Wii Balance Board is used. The framework is the basis for user tests to be conducted within the uTRUSTit (Usable Trust in the Internet of Things) project supported by the EU under the Seventh Framework Programme.

Fields of Application of Coupling a Real NC Control Unit With Virtual Reality Technology

This paper presents an approach on how to combine the advantages of both, the integration of a real NC control unit in the NC simulation as well as the visualization using the Virtual Reality (VR) technology. As a first step it will be described how to establish an effective coupling between a real NC control unit and a virtual machine model. Therefore the setup of the VR simulation environment, like the preparation of the virtual machine model with its kinematic structure, will be presented. Based on this coupling, different fields of application such as detection and prevention of collisions between the machine axes as well as the material removal at the work piece will be described. Finally the use of this coupling for training purposes will be presented.Copyright

Control concept for piezo-based actuator-sensor-units for uniaxial vibration damping in machine tools

Additional piezo-based components in drive trains can significantly improve the dynamic behaviour of machine tools. In this article we present a piezo-based actuator-sensor-unit that is able to reduce uniaxial vibrations in ball screw driven feed axis of machine tools. A complex model of a feed axis including ASU was developed to design a controller. The control concept is based on the direct velocity feedback. A modular test bench was designed, assembled and investigated to verify the ASU’s suitability for different feed axis configurations. In conclusion, the results for the different configurations where evaluated regarding economical aspects.

Influence of Electrode Layers on the Fracture Strength of PZT Ceramics

A newly developed process chain enables the production of active structural components. Thereby lead zirconate titanate (PZT) fibres are integrated in metal sheets by joining by forming. For the further development of the process chain to a high volume production the fracture strength of PZT should be increased. This paper pursues the approach to increase the strength by selected electrode layers. For this purpose, various electrode layers are tested on PZT plates in strength tests with the ball on three balls test (B3B-test). The evaluation of the recorded values is carried out according to statistically methods. The results show that the strength of PZT can be increased by electrode layers up to 30%. In addition, it was established that a high tensile strength and a high Young’s modulus of the electrode layers do not have a positive effect on the strength of PZT.

Monitoring concept for structural integration of PZT-fiber arrays in metal sheets: a numerical and experimental study

ABSTRACT The technique joining by forming allows the structural integration of piezoceramic fibers into locally microstructured metal sheets without any elastic interlayers. A high-volume production of the joining partners causes in statistical deviations from the nominal dimensions. A numerical simulation on geometric process sensitivity shows that the deviations have a high significant influence on the resulting fiber stresses after the joining by forming operation and demonstrate the necessity of a monitoring concept. On this basis, the electromechanical behavior of piezoceramic array transducers is investigated experimentally before, during and after the joining process. The piezoceramic array transducer consists of an arrangement of five electrical interconnected piezoceramic fibers. The findings show that the impedance spectrum depends on the fiber stresses and can be used for in-process monitoring during the joining process. Based on the impedance values the preload state of the interconnected piezoceramic fibers can be specifically controlled and a fiber overload.

In-Process Monitoring of Joining Operations for Piezoceramic Elements

The direct integration of piezo elements into micro-structured aluminum sheets is a new approach for adaptronics and lightweight constructions. With the integration of the active piezoceramic elements the aluminum sheets gain sensor and actuator functionalities. The mechanical interconnections and the preload of the piezoceramic elements are an important issue for the sensor and actuator capability of the later smart material. Post-process inspection methods to characterize the mechanical interconnection of the joining partners and the performance of the transducer after the joining operation are state of the art. Scope of the paper is the development of a novel in-process monitoring method that utilizes the piezoceramic transducer as inherent sensor for failure mode detection and preload evaluation during the joining by forming operation. Within this study, results of forming experiments with array batches of interconnected piezoceramic elements are presented. The piezoceramic batches are electrically contacted inside the joining tool and utilized as material inherent sensor during joining by forming experiments. Test samples are characterized by impedance spectroscopy during the joining operation. Based on the experimental results, a novel in-process-monitoring method utilizing the piezoceramic joining partners as inherent sensor is outlined. It is shown, that with this method a sufficient preload can be adjust on the basis of the intensity of the resonance peak without an overload. Furthermore, error effects to the transducer can be detected at an early stage.

Influence of surface processing on the fracture strength of structurally integrated PZT fibers in shaped sheet metal parts

In the present state of the art, the function integration into lightweight metal structures is generally based upon adhesive bonding of sensors or actuators to the surface. A new technology enables a direct structural integration of lead-zirconatetitanate (PZT) fibers into local microstructures of metal sheets and subsequent joining by forming. This provides a complete functional integration of the piezoelectric ceramic in the metal for sensors and actuators purposes. In a further process step, the composite is shaped by deep drawing with a cup with double curvature radii of 100 mm into a complex 3D surface. During the shaping process it is expected that the PZT- fibers get damaged with the result of degradation of the piezoelectric function. This paper describes the application of various surface processing methods to improve the shaping behavior of the piezoceramic fibers. The production of interconnected parallel fibers is based on piezoceramic plates. The plates are treated by different surface processing. One experimental series is lapped and another series is extra polished by chemical mechanical polishing (CMP). The resulting plates were examined with regard to the fracture strength and the degradation of the piezoelectric properties during manufacturing and operation. It has been shown that the lapped and polished plates have a clearly better persistence with regard to the shaping processes compared to the unprocessed plates. The best results in this process were achieved by the polished plates, which is also transferable to the fibers. Furthermore, the piezoelectric characteristics were better preserved by the lapped and polished plates and fibers.