Channel spreading during the spark discharge for edm

(1)
(2)
(3)
(4)பைடு நூலகம்
(3
mgure 1: Suucms of discharge channels and base pains for EDM
(Single Pulse Discharges)
Which channel propagations were assumed up to now:
P cylindrically without geometrical change (type (1)) P cylindrically with radius propagation (type (I)) P multiple cathodic base point
The discharge channel structures and their gap conditions
The different models to the spark erosion brought also different variants out of the formation of discharge channels. However, for the computations of the erosion rate a cylindrical discharge channel was always assumed. During the measurements of single discharges partly completely different crater textures was determined, which cannot be caused by a cylindrical heat source. This development was a reason for the fact that different varriants of the formation of the discharge channel were regarded and examined. The optical observation are difficult by the fact that the electrode gap approximately amounts to 2 pm to 100 pm, and in the "normal case" a plate to plate arrangement is present. For the optical single pulse investigations the electrode arrangements plate to apex has to be used. In addition, exposure times smaller 200 ns are necessary, in order to receive sufficient resolution of the localization of the channel 121.
0-7803-7910-1/03/$17.00 02003 IEEE
297
b cylindrically with moved respectively jumping cathodic base point (type (2-5)) b multiple cathodic moved base points b moved anodic and cathodic base points (type
H.-P. Schulze', M. Guter', W. Rehbein', K. Meckel, and G. Wollenherg' Institut for Fundamental Electrical Engineering and EMC, Otto-von-Guericke-University Magdeburg, Germany * oelheld GmbH, Mineraliilwerk Stuttgart, Germany
Abstract: The paper presents the forms of the channel formation during spark erosion. The deviations from the cylindrical discharging channels leads to completely other removal craters and therefore for changed surface roughness. The different channel types are dependent on gap conditions and pulse parameters in the f m t place but too dependent on the compounds in the dielectric work liquid. For the projection the technological parameter like in particular roughness, are knowledge uecessary via the channel spreading and channel form, for off-line process models. The used examination methods are High Speed Framing Camera (HSFC) for the optical observation and the Confocal Lasers Scanning Microscopy (CUM) for the determination of the craters topologies.
Introduction
The electrical discharge machining (EDM) a technique werehy will lo4 to lo6 discharges per second the material is removed, in order to fabricate a desired shape. Technological investigations have showed, that with "identical" pulses the demanded technological parameters can he produced. Unfortunalety, because of different ignition conditions from pulse to pulse, the current and voltage curves deviate from each other. Investigations at single pulses should give more conclusions on the shape of the crater and thus on the roughness of the machined surface. The differences in the gap conditions at a single discharge and at a sequence of discharges are substantial, so that not all conclusions can be taken over for a pulse sequence. The "cold not contaminated gap conditions of the single discharge lead to deeper textures of the crater with stronger ejection of melted particles. The real conditions correspond to a "hot" contaminated gap, which is given by the sequence of the pulse and the gap flushing [I]. The optical view of the discharges is possible only in the "clean" working medium, so that the conclusions may he transferred to the real conditions only with an appropriate correction.
2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena Channel spreading during the spark discharge for selected conditions and working fluids
(3)).
Intluence of the work liquids The investigations of the influence of additives on the efficiency of the work liquid with the basis medium ndodecane was accomplished. The fmt group of additivs, who were analyzed by single discharges, was alicyclic and aromatic additives. Starting point for the typical channel propagations are the conditions of the streamer entry with these short electrode gaps of the spark erosion. The additives change thereby the electron transfer processes whereby the probability of cert i ranges for a streamer entry can he increased. an The alkanes are thereby generally the substances, which are smallest reactive. The n-alkanes have very small electron affiities and very high ionization potentials, i.e. they are bad electron acceptors and electron donors, because they need high activation energies. Anions of n-alkanes are still less stable than those of the cycloalkanes. The impact processes of the electrons do not produce molecular ions separate but it dominates small fragment ions, like CH-, CHI-, CH; or CzH -. Cycloalkanes are comparatively little reactive, temporary anions with life duration from less than a picosecond were ObServed. The ionization potentials are somewhat smaller than those of the n-alkanes. The ionizableness can be increased by suitable substituents .