Hans-Werner Hoffmeister

Micro abrasive pencils with CVD diamond coating

Abrasive pencils (burs) for grinding can be made by coating cemented carbide bodies with a rough chemical vapour deposition (CVD) diamond layer. These tools have advantages compared to conventional abrasive pencils made by electroplating or sintering of diamond grains. Different CVD diamond abrasive pencils were manufactured with tip diameters ranging from 0.06 to 2.0 mm. A hot-filament CVD reactor was used in combination with a substrate holder system having a capacity of up to 240 tools. The grain size is the most important property of grinding tools. It must therefore be adjusted carefully in the coating process. The basic influence of tool geometry and coating parameters on the crystal size is presented. By prolonging the coating time to 90 h, crystal sizes of up to 50 μm can be achieved. Manufacturing of abrasive pencils even with such long coating times can be economical, because thousands of tools could be coated in one process by industrial coating in bigger reactors. The relation between the crystal size and the roughness of ground workpieces is presented.

Closed-form inverse kinematics of 6R milling robot with singularity avoidance

For a 6R milling robot, it is necessary to convert the postprocessing cutter locations (CL) into the robot’s revolute joint variables. This paper introduces an algorithm for calculating the forward and inverse kinematics of a 6R robot according to the CL data generated by conventional CAD/CAM systems. A redundant mechanism is analyzed to avoid the singular configurations and joint limits. The Denavit–Hartenberg (D–H) convention is referred to for developing the forward kinematics, and a closed-form solution of the inverse kinematics is presented by means of kinematic decoupling. A fundamental approach with modifying factor for avoiding singularity are developed with regard to three-axis and five-axis CL data. A gap bridging strategy is applied to reduce the jerk motion caused by tool retraction and cut paths connection. Finally, the result is conducted to simulation and machining test to verify the algorithms.

Mechanical roughing of cylinder bores in light metal crankcases

The increasing demand on vehicles with reduced fuel consumption and emission is the reason why aluminum is frequently used as material for crankcases in the automotive industry. Eutectic aluminum alloys, however, have the disadvantage that they cannot be used for the cylinder bore surface due to their tribological qualities. A method to increase the wear resistance is to coat the surface by using a thermal spray process. In this case a coating material e.g. a steel wire is melted. The molten particles solidify at the boring surface so that a wear resistant coating of the cylinder bore is produced. In order to guarantee that the coating adheres to the aluminum, it is necessary to roughen the surface. This paper describes a method of roughing the surface of the cylinder bores of light metal crankcases by using a modified fine boring process.

Parallel kinematic concept for stationary high performance cutting in wood machining centers

In recent years parallel kinematic machines for wood machining have come into use more frequently. Despite first promising prototypes, these machines are single solutions for specific applications. To meet the requirements of shorter product life cycles and higher product diversity, high flexibility is demanded of the machining system. This paper presents a new wood machining center obtaining both, the reduction of the primary and secondary processing times. The machine concept, based on a parallel kinematic structure, allows high operating speeds and accelerations not only for workpiece machining but also for handling. Thus, the machine can be used without any external handling devices. The kinematic structure originates from a plane closed five-bar chain with two linear drives and additional drive axes for stroke and rotation. In order to increase the useable workspace a continuous motion between different assembly modes is realized. To guarantee a high feed rate and to minimize set-up times, an optimized dust exhaustion is included.

Development and design of micro machine tool components via numerical simulation

This paper deals with machine tool components for use in a small machine tool for micro machining. The goal is an automated manufacturing process and to reduce manual handling by the operator as far as possible. This includes a miniaturized clamping device for fixing a test workpiece via freezing of water. Using the Finite Element Method, the thermal deformation of the test workpiece made of 100Cr6 could be analyzed as well as the static stiffness and dynamic behavior of the clamping device. Additionally, an automated workpiece supply using electromagnets and a pneumatic cylinder is presented. For extraction of chips, an adapted extraction hood was developed. The paper presents first results of performed Computational Fluid Dynamic simulations regarding velocity and streamlines of particles. Furthermore, a demonstrator of the micro machine tool shows the current installation space, enabled with the developed machine tool components. Here, the machine tool frame is made of CFRP due to its thermal stability.

Process-integrated quality assurance in wood machining centers with the help of image processing

With rising feed rates of wood working machines, it is important to realize a process-integrated quality assurance. The quality control of produced workpieces is mainly carried out by the operator. As his capabilities are limited, an automated quality control system can help to assure a constant quality level. Due to the high production speeds, only an optical measurement can be used. While CCD-camera-based control systems are generally applied in wood working, no approach has been made so far to measure the surface roughness which is caused by the machining of the workpiece. This paper presents two different quality control systems for an assessment of the surface quality of a workpiece. In planing of solid wood, the size of the waves on the surface is most significant for workpiece quality. An image processing system has been developed which measures the width of the waves by means of a special illumination. In machining chipboards, the size and number of cracks in the edge are very important. Here, a camera-based system is presented which is able to measure the complete workpiece edge even at high feed rates with a sufficient resolution.

Punching in industrial wood machining: an alternative production process to drilling

Drilling of wood is besides milling, sawing and grinding one of the most important production processes in industrial furniture manufacturing. For the body assembly and the fixing of fittings, boreholes are essential. They have immediate effect on quality and production costs. The process step drilling limits the output of stationary and throughfeed machines, because the workpieces have to be stopped to create the drillings at a relatively low feed speed. An essentially increased performance cannot be reached by the improvement of conventional drilling devices. The aim of the work presented in this paper is the development of an alternative production process in which blind holes can be punched. Thereby thin-walled, cylinder-shaped tools are pressed into different solid and derived timber workpieces by a simple translative movement. For it special experimental setups have been designed. The new production process opens possibilities for an optimization and acceleration of process cycles in industrial wood machining.

Modularly Structured, (Re-)Configurable Machines for Micro Machining by Means of Cooperative Motion Generation

A concept is presented, which allows the individual combination of several machine modules into a flexible manufacturing unit by using a modular principle. Here, these are size-adjusted machine tools with a much better ratio of installation space to the workpiece size than that of conventional machines for micro manufacturing. This results not only in a smaller area in the shopfloor, but also in cost savings for acquisition and operation of the machines. A modular and (re)configurable machine concept also ensures a user-oriented machine design that can be easily and quickly adapted to special manufacturing tasks. The low dead load permits additionally a high mobility and quickly changeable installations of multiple machines within a process chain.

Stanznageln — Prozesssimulation mit Hilfe der Finiten Elemente Methode

Der zunehmende Einsatz hochstfester Werkstoffe in Fahrzeugkarosserien in Kombination mit werkstofflichen Mischbauweisen erfordert die Entwicklung neuer warmearmer mechanischer Fugeverfahren. Die Prozesssimulation mittels Finiter Elemente Methode stellt dabei eine Moglichkeit zur Verfahrensentwicklung dar. In diesem Beitrag werden die Anforderungen und Moglichkeiten an die Prozesssimulation stanzender mechanischer Fugeverfahren aufgezeigt.