Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2000-10-16
2003-07-29
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069130, C219S069180
Reexamination Certificate
active
06600125
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method and apparatus for electrical discharge or electrochemical machining of workpieces in which the machining electrode is moved relative to a workpiece, the machining comprises cycles and appropriate process parameters are adjusted for machining of the current cycle, and especially with consideration of process parameters of the current cycle.
BACKGROUND OF THE INVENTION
Numerous cyclic machining methods are described in the prior art, especially planetary EDM methods.
Such a generic planetary EDM method is known, for example, from “Industrie-Anzeiger”, Special Issue, April 1981, page 109ff. This method involves planetary EDM of cavity sinkings, in which a cyclic translatory movement is conducted for machining between the machining electrode and workpiece laterally in planes arranged transversely to the direction of advance. Each plane is then machined cyclically by the machining electrode, the cyclic translatory movements being used in the form of orbital movements to widen a machining or for layered machining.
During electrical discharge machining, material removal occurs on the electrically conducting workpiece, owing to the fact that the machining electrode is brought to the workpiece and electrical discharge occurs. Machining then occurs by the translatory movement between the electrode and the workpiece in an appropriate working liquid. A gap width control superimposed on the translatory movement ensures maintenance of the appropriate spacing between the machining electrode and the workpiece.
It is known from the generic document that the speed of the translatory movement can be controlled as a function of the current process state, in order to avoid losses of efficiency, for example, to carry out the translatory movement with increased speed in the no-load section (sections of the cyclic translatory movement, in which material removal no longer occurs during machining). A shortcoming in this method, however, is that strong fluctuations of the true path can occur within one cycle as a function of the geometry of the reference path in the course of machining. Moreover, braking or delay at tricky sites is problematical at increased speeds. Specifically, it can happen during abrupt changes in direction, for example, that braking is not carried out in a timely fashion and, in the extreme case, the machining electrode can even collide with the workpiece. Because of this, the gap width regulation system can be overtaxed, so that incorrect discharges increasingly occur, which result in increased wear of the machining electrode.
A further developed planetary EDM method is known from EP 0 340 569, which seeks to solve the above problems. Each planetary revolution (cycle)is subdivided into a specific number of angle positions of the planetary angle, and the true path, i.e., the effective deflection amplitude of the machining electrode, is stored for each angle position. The reference path for the pending planetary revolution is then determined in advance from the true values from at least one of the past planetary revolutions and the expected volume of removal. The process runs at almost constant planetary speed. The most erosion-intense discharges possible and therefore a shorter machining time are also sought. The true movement, as mentioned, can be determined from one or more preceding planetary revolutions or, alternatively, from a trial run without erosion. The emphasis in this method therefore lies in accelerating the machining process.
To summarize, it can be stated that this document considers the geometric data of previous planetary revolutions, in order to adjust the geometric data of the pending planetary revolution. Process parameters of previous planetary revolutions are not considered.
In principle, in all known devices, the corresponding CNC control views machining as completed as soon as the undersize is reached. The undersize to be reached, however, refers to a theoretical gap width stored in process parameter data sets. A change in effective gap width, as is usually produced by the gap width control, remains unrecognized, which can have extremely harmful effects on contour accuracy.
OVERVIEW OF THE DISCLOSED DEVICE
The underlying task of the disclosed device is to improve the generic method to the extent that an improved geometric machining accuracy is made possible without time losses, especially to increase process safety and reproducibility.
To solve the task in a generic method, stored process parameters of at least one previous cycle are also used to adjust the process parameters of the current cycle.
The invention proceeds from the observation that, in cyclic machining, a location-bound development of certain process parameters can be established. If a disturbance, for example, occurs at one site of a cycle, the development of this disturbance is also generally apparent in the subsequent cycle at the same site.
Based on this observation, the illustrated device stores the process behavior for already performed cycles and evaluates the stored data to establish certain process parameters of the pending cycle, so that sections of the pending cycle to be machined can be arrived at in advance. For example, depending on whether a disturbance-free section, a no-load section or a critical section is expected, the process parameters to be used are adjusted accordingly. Process disturbances and time losses can therefore largely be avoided to advantage, good shape trueness can be achieved and the process optimized overall.
Preferably, at least one process parameter is measured during at least one preceding cycle. At least one process parameter to be adjusted in the current cycle is derived from this measured parameter. The at least one derived process parameter is stored during the preceding cycle and the at least one stored process parameter is adjusted during the current cycle.
The cycles, including machining (which is also referred to as cyclic machining below), preferably involve planetary erosion during cavity sinking or pocketing during wire erosion. However, any wire erosion machining, in which a full cut and a certain number of aftercuts are carried out, can, in principle, be referred to as cyclic.
Process parameters permanently stored in a table, alternatively or additionally, are preferably used for adjustment of the process parameters, which, in particular, are established with reference to optimization of erosion, with reference to performance of no-load and/or with reference to performance of the guaranteed disturbance-free machining. Stipulated process parameters can therefore also be permanently stored for specific standard situations, like no-load machining, or also for specific compositions of the workpiece, machining electrode, geometry being machined, etc., which can then be called up in specific situations for instantaneous machining. These can be called up, in particular, for those situations when instabilities in process parameter control occur.
The preceding cycles arc preferably investigated by a reference/actual value comparison of the measured process parameter on sections in which a machining disturbance, a no-load condition and/or another condition deviating from the normal case occurs, and adjusted process parameters are set, at least in the corresponding sections of the current cycle. Sections of the cycle are advantageously characterized, in which specific situations occur, and for which optimized process parameters can be worked out in the subsequent cycle with respect to the situation.
Adjusted process parameters are preferably already set in one region before the corresponding sections of the current cycle. For example, abrupt transitions in process parameters that otherwise could lead to machining inaccuracies can therefore be advantageously avoided.
The length of the region is preferably chosen as a function of planetary speed. For the case of wire erosion, the planetary speed corresponds to the wire advance speed.
Machining is preferably divided into a number of layers and
Boccadoro Marco
Bonini Stefano
Buzzini Andrea
Agie SA
Evans Geoffrey S.
Marshall Gerstein & Borun
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