4M Knowledge base - papers

A Friction Model for Microforming

H.J. Jeon, A.N. Bramley
Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK

Abstract

For the simulation of metal forming processes, input data relating to the tool-workpiece interface is necessary. For microforming applications the tool/workpiece interface conditions tend to dominate the process and it has been found that traditional methods of modeling the interface are not realistic. This paper describes an approach that seeks to describe friction by modelling the geometric surface roughness of the tool as opposed to the use of the traditional empirical friction coefficient or factor. This finite element based model has been validated experimentally in terms of loads and metal flow using the ring test and actual surface measurements. It enables more accurate and also more flexible modeling of friction. As such it will be very suitable for microforming applications.

Submitted on May 19, 2008 - 15:23.

Flexible Tool System for Creation of Surface Micro-Geometries

M. Rosochowska (a), K. Chodnikiewicz (a), R. Balendra (a), R. Smith (b)
a Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow G1 1XJ, UK
b Pascoe Engineering Ltd. Glasgow G53 7TD, UK

Abstract

The performance of functional surfaces can be improved by incorporating a pattern of micro-geometrical features; research has shown that surface micro-geometries (smg’s) extend the range of the hydrodynamic lubrication regime, thus reducing friction. The incorporation of smg’s on functional surfaces in mechanical-engineering/bioengineering applications requires a fast, tolerance-insensitive technique; a novel technique, conforming to these requirements is the subject of this paper. The developed tool system enables the creation of smg’s of a range of depths of 0.5~25μm, up to a rate of 50Hz on undulating surfaces; this tool system works in conjunction with a 3-axis CNC machine. This is essentially a high-speed indentation process that uses piezoelectric actuation. Experimental results show that the tool system and associated controls enable the production of smg’s of high consistency of pattern, density and geometry. The use of diamond-tipped tools enables the creation of smg’s on hard (>60HRC) steel surfaces; current configurations permit the creation of smg’s on plain and cylindrical surfaces; more complex forms would require a 5-axis CNC machine. Experimental results show that the form-errors are of the order of 5% and the distribution error is 2%. However, to produce dimples of the required dimensions, it is essential to know the relationship between the dimple depth and forming force; these data were extracted using FE analysis.

Submitted on May 19, 2008 - 15:45.

Microforming at elevated temperature – forming and material behaviour

B. Eichenhueller (a), E. Egerer (b), U. Engel (a)
a Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, D-91058 Erlangen / Germany
b Siemens AG, D-91058 Erlangen / Germany

Abstract

Manufacturing of metallic parts by forming methods is industrially widespread due to several advantages like good surface quality, high accuracy and good efficiency at concurrent high quantity. As a result of the steady miniaturisation of products, large quantities of smallest metallic parts with the above mentioned attributes are needed. Despite the advantages of forming methods, microparts are mainly produced by machining, because of problems caused by so-called size-effects. These effects occur by scaling down geometry and process parameters, leading to the fact that the existing know-how for conventional processes cannot be transferred unrestrictedly to the microscale. One reason for the difference between macro- and microscale is the number of grains within the forming area. At microscale only a small number of grains is directly involved in the forming process, so that the single grain, characterised by its individual size, orientation and position, gains influence on the process. The stochastic distribution of the grain characteristics leads to an inhomogeneous material behaviour and causes an increased scatter of the process parameters. To minimise the effect of inhomogeneous material behaviour, microforming at elevated temperature is applied. Experiments with different materials at elevated temperature show a homogenising effect which leads to a reduced process scattering. This indicates that elevated temperatures are suitable to minimise and control the size-effects at micro-forming processes.

Submitted on May 19, 2008 - 15:08.

Spring-back behaviour of thin metal foils in free bending processes

A. Diehl, U. Engel, M. Geiger
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, D-91058 Erlangen / Germany

Abstract

Metal foils attract a large field of applications, e.g. in micro-technology they are being used for sensors, actors, micro-electro-mechanical systems and in medical devices. Conventional sheet metal forming processes are in principle applicable for metal foil forming. Reducing the sheet thickness to the order of micrometers, however, causes various scaling effects. Therefore, the know-how of conventional sheet metal forming cannot be transferred directly to metal foil forming. A known phenomenon during foil forming is the reduction of strength of the material with decreasing thickness due to the increasing share of surface grains with fewer constraints to plastic flow on the overall volume. The opposed phenomenon is the increase of material strength regarding foils with mean grain sizes in the range of the foil thickness or even higher.

In the present paper basic research via scaled free bending tests is performed to investigate size effects in order to provide basic knowledge for the design of the process and of the components, respectively. An important factor in production accuracy of bending processes is the spring-back. In the current research spring-back of aluminium foils (Al 99.5) in dependence of the foil thickness is investigated with foil thicknesses ranging from 25 to 200 microns. Variation of the mean grain size/foil thickness ratio is achieved by different heat treatments. The experimental results are being compared with FE-simulations.

Submitted on May 19, 2008 - 15:06.

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