Electrical discharge machining (EDM)

A. Herrero (a), S. Azcarate (a), A. Rees (b), A. Gehringer (c), A. Schoth (c), J.A. Sanchez (d)

(a) Micro & Nano Technologies Dep., Fundacion Tekniker, Avda. Otaola 20, 20600 Eibar, Spain
(b) Manufacturing Engineering Centre, Cardiff University, Cardiff, CF24 3AA, UK
(c) IMTEK, University of Freiburg, Georges-Koehler Allee 103, EG-79110, Freiburg, Germany
(d) Dep. of Mechanical Engineering – Faculty of Engineering of Bilbao, Avda. Urquijo s/n, 48013 Bilbao, Spain

Abstract

Apart from the important role that Micromachining and Ultraprecision machining has provided to the development of improved or innovative miniaturised products, these techniques have also attracted the interest of the researchers to obtain the highest accuracy and a thorough analysis of the principles governing the material removing mechanisms. The present article exposes the theoretical analysis of some aspects of the thin WEDM that drop the process accuracy in terms of minimum machinable slot or corner over/undercutting. The scaled electrode dimensions and the reduced power supply with respect to the normal process causes a different influence of the process variables and contributes to obtain complementary information about the WEDM process. The different force components contributing to the wire deformation are discussed and some of them are analyzed from a theoretical point of view presenting analytical calculations to evaluate their expected magnitude and pointing out the difficulties to obtain an experimental characterisation of each phenomena.

J. Fleischer, M. Deuchert, C. Kühlewein, C. Ruhs
Institute of Production Science (wbk), Universität Karlsruhe (TH), Kaiserstrasse 12, 76131 Karlsruhe, Germany

Abstract

The geometry of micro milling tools currently in use have been adopted from macro tools, assuming that chip formation and process kinematics are analogical in both types of tools [1]. Experience has proved that micro tools respond to influences in a very different way than macro tools [2]. Oftentimes, structural details such as the rake angle and the twist angle impede further miniaturization and are impossible to achieve with conventional manufacturing techniques. Therefore it is necessary to get a comprehensive understanding of the entire process by taking a structure mechanical and cutting technological approach to micro milling tools in order to be able to optimize them. Another objective consists in the production of these miniaturized milling tools by means of force-free procedures such as laser ablation and electrical discharge machining.
The present state of research already puts the deficits of the currently available tools on display. Insufficient manufacturing tolerances of ±10 μm, constitute a substantial change of cutting conditions for the commonly used lateral infeed or feed per tooth of a few micrometers. Sometimes, only one cutting edge is engaged, which results in increased wear and, therefore, reduced durability, increased cutting forces, minor surface quality and a higher probability of milling cutter breakage. For that reason, a single-edged geometry has been proposed. It guarantees clear adjustment of the process parameters feed per edge and lateral infeed. For that purpose, stability analyses of simple stylus geometries have been conducted by means of FEM simulations. The resulting tool with a diameter down to 30 μm was machined on the EDM-machine at the wbk (Sarix SX 100). First tests have been carried out that prove the ability of these tools to cut steel.

A. Rees (a), E. Brousseau (a), S.S. Dimov (a), H. Gruber (b), I. Paganetti (b)

(a) Manufacturing Engineering Centre, Cardiff University, CF24 3AA, UK
(b) AGIE AG für Industrielle Elektronik, Losone, Switzerland

Abstract

This paper investigates the technological capabilities of a micro machining process for performing Wire Electro Discharge Grinding (WEDG). In particular, micro Wire Electrical Discharge Machining (μWEDM) is employed in combination with a rotating submergible spindle to perform WEDG. In this paper, the effects of different factors on the achievable surface finish after WEDG are investigated. In particular, an experimental study employing the Taguchi parameter design method is conducted to identify the most important main cut machining parameters that affect the surface quality of the machined parts. Then, the obtained results are used to analyse the effects of the investigated parameters on the achievable surface roughness, and ultimately to select the optimum technological parameters for performing WEDG. The process parameters that statistically have a significant influence on the surface finish are presented. The study shows that by optimising the main cut machining parameters of WEDG a level of surface finish comparable to that of μWEDM can be achieved.

K. Liu, J. Peirs, E. Ferraris, B. Lauwers, D. Reynaerts
Afd. PMA, Department of Mechanical Engineering, Katholieke Universiteit Leuven, Leuven, BE-3001, Belgium

Abstract

The Electrical Discharge Machining (EDM) behaviour and machining properties of advanced engineering Si3N4-based ceramic composites Si3N4-TiN are investigated and discussed in this paper. Two types of EDM machining configurations, micro-EDM milling and die-sinking EDM, are employed in the investigation. Relaxation type of pulse is used, and the performances of EDM process in the form of material removal rate, tool wear and surface quality are studied. These tests result in a performance comparison and a discussion on the ceramic composites material removal mechanism. The feature of material removal mechanism is characterised as chemical decomposition of Si3N4 and TiN at elevated temperature rather than melting/evaporation. The generation of nitrogen gas bubbles leads to a porous and foamy top surface structure. Due to the ideal mechanical and physical property of Si3N4-TiN ceramic composites, an application example - a turbine impeller -
as a crucial component in a micro power generation system is manufactured with obtained knowledge in both
machining configurations.

A. Herreroa, L. Uriartea, J. Esmorisa, J.A. Sánchezb, L.N. Lopez de Lacalleb
a Fundación Tekniker, Avda. Otaola 20, 20600, Eibar, Spain
b Dpto. Ing. Mecánica, ETSII, Alameda Urquijo s/n, 48013, Bilbao, Spain

Abstract

The analysis of WEDM is still nowadays an important field of research due to the difficulties to measure the process characteristics: narrow gap (~10 mm), dirty environment (oil or deionised water), high frequency (>100 kHz), etc. Nevertheless, the WEDM technology has been improved thanks to the theoretical and empirical results of different research groups that have made use of state of the art technologies to measure temperature distributions, displacements, frequencies or electrical signals for spark characterisation. The accurate measurement of machined parts has also brought light to the machining process, being this aspect critical for the improvement of the EDM technology.

In the last years, the growing tendency to miniaturisation has promoted the research of production techniques capable to produce small components with very high precision. EDM technology, due to the low processing forces, was immediately identified as one applicable technology for the production of moulds and dies. The technological research in the field has been very important, reducing the minimum wire diameter from Ø0.1 mm to Ø0.02 mm, the machine components have evolved to provide a finer control of all process parameters, specially the wire traction force, the machine feed and the spark energy. Thanks to the research in WEDM, nowadays it is known that, during the process, electrostatic, electrodynamic, electromagnetic, dielectric and wire traction forces act on the wire. Many of these forces push and pull the part from the workpiece. The result of all these forces acting on the wire is an error of the machined shape that, in normal WEDM, is of only a few microns (3~20 mm depending on part height). This error is specially important when machining flat walls and machining corners in which the feeding direction change.

Despite using lower energy values, due to the origin of the different forces acting on the wire and the low tensile strength of wires smaller than Ø0.1 mm (considered as thin wires), the errors that can be found in miniature parts and microparts are bigger than the corresponding values in conventional WEDM. The present paper analyses the errors that appear when applying thin wire EDM (Ø0.03 mm) to the machining of 3 mm height components made of tungsten carbide, it presents the difficulties that are found when trying to characterise the errors in small components. A possible error analysis approach is presented and then the errors are discussed.