Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1997-11-03
2001-02-06
Easthom, Karl D. (Department: 2832)
Electric heating
Metal heating
Cutting or disintegrating
C219S069160, C219S069170, C219S069130
Reexamination Certificate
active
06184486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention deals with electrode tools for electroerosion (EDM) where a cavity or a relief is machined by hollowing out the part at great speed and in successive layers or slices (i.e. if “peelpocketting”) by means of a rotating electrode tool of a simple shape which is independent of the cavity to be eroded, called “EDM slices milling”. The fastidious designing and manufacturing of electrodes of complex shapes is thus avoided, thanks to an automatic breakdown of the geometry and to a machining simulation.
2. Background Information
This EDM slice milling method with wear compensation has already been described in the European patent application EP 555818 filed by the applicant. It concerns machining at a high wear rate where the length of a simply shaped electrode tool decreases rapidly, but without any apparent lateral wear. The longitudinal wear of the electrode tool is compensated by a regular, coaxial additional advance, directed of course towards the workpiece and corresponding to a theoretical value of that wear. The value of this advance is deduced by calculation based on technological preexisting data and on the geometry of the electrode tool, in such a way so as to keep the active end of the electrode tool parallel to the plane of the slice to be hollowed out. Moreover, the geometry of the three-dimensional volume to be hollowed out has been memorized as a superposition of virtually parallel slices (or layers). The active end of the electrode tool makes to and fro movements, or snaking movements or other sweeping movements, in the plane of each of these slices so as to erode them successively down to the bottom of the cavity to be machined.
Since EDM slices milling is a machining method with a high wear rate, the applicant has chosen, at a first stage, to use solid electrodes, in particular cylindrical ones, rather than hollow tubes. This choice corresponds to the most recent state of the art, that is to say the work done by M. M. Tsuchiya, T. Kaneko and S.Shoda (in particular “Three-dimensionally controlled EDM Using Column-Shaped Electrodes; J. Japan soc. Electrical Machining Eng. 1983, 17 (34), 30 42”) .
Tubular electrodes, at first sight, have as many drawbacks as advantages. As a matter of fact, it did not really seem appropriate to use a tubular tool which gets worn out much faster than a solid one in spite of its known advantage offering the possibility of tracing grooves with a flat bottom. Indeed, as stated in Applicant's EP 555818, after a short transitional phase, the active end of the electrode tool takes an invariable profile called an “asymptotic or stationary shape”. As illustrated in
FIG. 1
, the stationary shape of the tool tip
100
in the case of a rotating hollow tube
120
is a truncated cone
130
. Using the tool tip
100
, it is possible to trace adjacent grooves
140
with a flat bottoms
150
. This facilitates erosion of successive flat slices, each being of a constant thickness, E wherein the thickness of the wall of the electrode is &tgr;. The other known advantage of using tubular electrode tools in EDM machining is when a good irrigation of the machining gap is necessary by injecting or aspiring the dielectric through the hollow tube. In the case of EDM slices, such milling is only of little use, since the cavity to be machined is much larger than the tool.
The applicant has discovered quite unexpectedly that, thanks to the use of tubular electrodes, computer aided preparation of machining of EDM slices milling, as described in applicant's parallel application filed concurrently herewith (Attorney Docket No. 032530-007 (U.S. Ser. No. 08/780,127), having priority on Application No. 02 484/93-9 filed Aug. 20, 1993 in Switzerland), becomes possible. Such preparation, (e.g., of the programming of the plane paths at zero wear of the tool for each slice, the wear compensation, or the simulation of machining by calculating the material removal and by programming the machining speeds along these plane paths) seemed utopian and hardly implementable in view of the complexity subsequent to the accumulation of all of the differences between reality and the ideal universe of simulation. The tubular electrodes thus do not only possess as the sole advantage that of machining flat bottoms. Tubular electrodes have also made it possible to formulate, without any risk for the geometry of the finished part, simplified hypotheses to program computer aided preparation of EDM slices milling, and to envisage the calculation of theoretical material removal and machining speed at each point of the plane paths at zero wear.
SUMMARY OF THE INVENTION
The objective of the present invention is therefore an electrode tool for EDM slices milling made up of an electricity conducting tube that is hollow or at least partly filled with a material which wears faster than the tube itself. In the latter case of the composite electrodes, the stationary shape is the same as that of the hollow tube, for example the trunk of a cone for cylindrical tubes. Being used by rotating, instead of being cylindrical, the tube may have a square cross section or any other polygonal section.
Another objective of the present intervention is the utilization of such an electrode tool on an EDM slices milling machine. It is moreover advantageous to use an electrode tool the geometry of which is adapted to its utilization for EDM slices milling, because said geometry is suited to the computer aided preparation method of this kind of EDM machining.
A tube with a any sized inner or outer diameter, or a wall with any thickness, will not necessarily be adequate for EDM slices milling. The choice of the correct inner or outer diameter and of the correct thickness is moreover a matter of compromise. For a given outer diameter it is advisable to choose the least possible thickness to obtain as large a groove as possible, provided the mechanical stability of the tool is preserved. But the thinner the wall, the faster the tool wears out. In general, it is preferable that the ratio between the thickness of the wall and the inner diameter is situated between {fraction (1/10)} and ⅓, but this depends on the material of which the tube is made.
It is furthermore advantageous to standardize the sizes of the tubes envisaged to serve as electrodes for EDM slices milling, in order to avoid the directional changes of its path. Since the tool follows a sweep path, these directional changes will occur rather frequently.
Lastly, the known wear compensator does not permit to really “simulate” the machining, because it does not provide any means to evaluate the variation of the material cut and to deduce therefrom the machining speed according to the curved abscissa.
Accordingly, the objective of the present invention is therefore to elaborate a method to prepare machining by EDM milling which would effectively simulate the machining by taking into account the theoretical material removal and the machining speed corresponding to each point of the path and which would take into account the various factors affecting the actual wear of the electrode tool, but without modifying the principle of the existing wear compensation module., There is, therefore, no need to have recourse to a continuous adaptation of the additional advance in response to a continuous control of longitudinal wear of the tip.
Another additional objective was to introduce into this machining preparation, a strategy anticipating the direction changes of the path so as to prevent the tool from overshooting caused by inertias and the response time of the EDM system.
REFERENCES:
patent: 2818491 (1957-12-01), Matulaitis
patent: 3591761 (1971-07-01), Bederman et al.
patent: 4168426 (1979-09-01), Furakawa
patent: 4608476 (1986-08-01), Shimizu
patent: 5354961 (1994-10-01), Diot et al.
patent: 555 818 B1 (1995-12-01), None
patent: 2 211 775 (1989-07-01), None
patent: 62-297019 (1987-12-01), None
patent: 1228986 A1 (1986-05-01), None
patent: 1662781 A1 (1991-07-01), None
patent: 170
Briffod Jean-Paul
Diot Jean-Claude
Naville Gerald
Zaugg Willi
Burns Doane Swecker & Mathis L.L.P.
Charmilles Technologies S.A.
Easthom Karl D.
LandOfFree
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