Wire-discharge machining apparatus

Details Wire-discharge machining apparatus Wire-discharge machining apparatus

2000-07-11

2002-05-28

Evans, Geoffrey S. (Department: 1725)

Electric heating

Metal heating

Cutting or disintegrating

C219S069130

Reexamination Certificate

active

06396021

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wire-discharge machining apparatus. In particular, the invention relates to a wire-discharge machining apparatus which performs necessary and sufficient wire-breakage prevention operations, thereby allowing machining performance to be significantly improved.
2. Description of the Related Art
In wire-discharge machining, machining speeds are improved proportionally to an increase in machining energy. However, application of the machining energy in excess of a limit causes breakage of a wire electrode, thereby seriously reducing the machining speeds. Therefore, it has been a common practice to proceed with the machining while preventing wire breakage by limiting the energy input to the machining apparatus to below a predetermined limit.
Nevertheless, the level of the energy which causes wire breakage varies according to conditions. For instance, energy below a standard energy level tends to cause wire breakage at certain locations along the machining path such as an end surface or a step of a workpeice and a corner of the electrode path. A countermeasure commonly adopted to obviate this problem is to set a limit level of the machining energy significantly lower than a standard energy level throughout the machining process, particularly when the machining includes a condition which tends to cause wire breakage. This causes a problem in that, for example, the low limit set for the machining energy is applied not only during machining of portions where wire breakage tends to occur but also during machining of portions that are to be machined with energy of a normal level, with the result that the machining speed is unnecessarily reduced. Furthermore, indefiniteness of the low limit level of the machining energy makes it difficult to completely avoid wire breakage.
To solve these problems, for example, Japanese Unexamined Patent Application Publication No. 4-30915 proposed a method in which machining energy is reduced upon detection of a corner in an electrode path which is performed based on a sharp decrease in the number of the discharge pulses followed by a gradual increase of the same.
FIG. 11
is a schematic view of one example of a discharge-machining-conditions adjustment circuit used in conventional wire-discharge machining, as proposed in the above-mentioned publication. In this figure,
10
a
denotes a counter,
10
b
denotes a frequency-to-voltage converting circuit,
10
c
denotes a threshold-setting circuit, and
10
d
denotes a comparator.
FIG. 12
shows variation in the number of pulses and threshold of the pulses before and after passing through a corner in an electrode path. Referring now to
FIG. 12
, the solid line represents thresholds set in the threshold-setting circuit
10
c
, and the dotted line represents an actual number of pulses (that is, an output of the frequency-to-voltage converting circuit
10
b
).
In operation, the counter
10
a
counts discharge pulses, and the count value is converted at an interval of several milliseconds into a voltage value corresponding to the current number of pulses. The voltage value corresponding to the current number of pulses is input to one side of the comparator
10
d
and as well as to the threshold-setting circuit
10
c.
In the threshold-setting circuit
10
c
, when the input voltage sharply decreases, the number of pulses in 0.5 second is averaged by a short-time constant low-pass filter of, whereas, when the input voltage gradually increases, the number of pulses in 10 seconds is averaged by a long-time constant low-pass filter. In either case, a threshold is obtained through multiplication of the averaged filter output with a gain of 1.1 to 1.2, and the threshold thus obtained is output to the comparator
10
d
. When the current number of pulses is larger than the threshold, the comparator
10
d
generates a command signal for increasing the off time of the discharge pulses, thereby suppressing the increase of the number of discharge pulses. As a result, as shown in
FIG. 12
, the number of pulses is controlled so as not to exceed the threshold obtained by averaging the number of pulses over a short period when the number of discharge pulses sharply decreases and over a long period when the pulse number increases gradually.
In the conventional wire-discharge machining apparatus, breakage of wire electrode is prevented by the above-described method. However, wire-breakage frequently occur not only at the corners but also at various other non-linear portions of the electrode path, such as stepped portions where the work thickness abruptly varies and end-surface portions. The conventional method as described above can effectively be used only for the corners, leaving the problem unsolved for other portions where wire breakage is likely to take place.
Another big problem with the described method is that a detection parameter is the same as a control parameter. Insofar as a sharp decrease and a gradual increase of the discharge pulses are referenced to detect corners in the electrode path, the number of the discharge pulses is regarded as being a detection parameter. Meanwhile, with a long off-time set for a corner, since the number of the discharge pulses is directly controlled, the number of the discharge pulses serves also as a control parameter. In this case, it is not clearly known whether a decrease or an increase in the number of discharge pulses is to be attributed to the presence of a corner in the electrode path or to be understood as being the control results. This makes it difficult to precisely determine the start and the end of a corner, thus hampering adequate control at a corner of the electrode path.
SUMMARY OF THE INVENTION
To solve the problems as described above, an object of the present invention is to provide a wire-discharge machining apparatus that can sufficiently detect not only corners but also all other portions such as steps and end-surface portions of a work where wire-breakage occurs more frequently than at normal portions, and that can carry out necessary and sufficient wire-breakage prevention operations by using a control method that can discriminate between the parameter which is to be used for detection and the parameter which indicates the control results.
To achieve the above object, according to the present invention, there is provided a wire-discharge machining apparatus for machining a work by generating pulse-state discharges between a wire electrode and the work, comprising: evaluating means for measuring one of the cycle time, the frequency, and the ignition delay time of the discharges, for evaluating dispersion of the measured values, and for outputting an evaluation value for the dispersion; and control means for controlling machining conditions based on the evaluation value for the dispersion.
The arrangement may be such that the evaluating means evaluates at least one of the sample variance of the measured values, unbiased variance of the measured values, the standard deviation of the measured values, the variation coefficient of the measured values, the squared mean of the measured values, the distortion of the measured values, the kurtosis of the measured values, the mean deviation of the measured values, and the absolute values of the differences between the measured values and the mean value.
The evaluating means may comprise: means for determining the square of the mean of the measured values; means for determining the mean of the squares of the measured values; and means for determining the difference between the square of the mean of the measured values and the mean of the squares of the measured values.
The evaluating means may comprises: a function generator for producing outputs variable over two or more kinds in accordance with occurrence of a discharge; an integrator for integrating the outputs of the function generator; and means for outputting either the absolute value of the difference between the output of the integrator and the product of the integration period

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