Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2002-04-26
2004-04-20
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
Reexamination Certificate
active
06723941
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a wire electric-discharge machining apparatus.
BACKGROUND OF THE INVENTION
FIG. 8
is a block diagram that shows a wire electric-discharge machining apparatus based on a conventional art of this type. In
FIG. 8
, reference numeral
1
denotes a wire electrode,
2
a workpiece,
3
a wire supplying unit,
4
a dielectric fluid supplying unit,
5
a machining power supply,
6
an average voltage measurement unit,
7
a control parameter setting unit,
8
a relative feed rate determining unit,
9
a control unit, and
10
a driving unit.
The wire supplying unit
3
feeds out the wire electrode
1
at an appropriate speed, and allows this to travel while giving an appropriate tension to the wire electrode
1
. The dielectric fluid supplying unit
4
supplies a dielectric fluid in a minute gap between the wire electrode land the workpiece
2
. The machining power supply
5
applies a pulse-like voltage between the wire electrode
1
and the workpiece
2
so that a discharge is generated between the wire electrode
1
and the workpiece
2
. The average voltage measurement unit
6
measures an average voltage between the wire electrode
1
and the workpiece
2
. The control parameter setting unit
7
sets a reference voltage and a target feed rate based on machining conditions set by the user. The relative feed rate determining unit
8
calculates a relative feed rate between the wire electrode
1
and the workpiece
2
by using the average voltage measured by the average voltage measurement unit
6
and the reference voltage and the target feed rate set by the control parameter setting unit
7
to supply the relative feed rate to the control unit
9
. The control unit
9
relatively moves the wire electrode
1
and the workpiece
2
at the relative feed rate thus calculated through the driving unit
10
.
The sequence of calculating the relative feed rate between the wire electrode
1
and the workpiece
2
in the relative feed rate determining unit
8
will be explained below.
First, the relative feed rate determining unit
8
compares the measured average voltage obtained by the average voltage measurement unit
6
with a preset short circuit reference voltage. The short circuit reference voltage is a voltage used as a reference when a case in which the measured average voltage is below this voltage is determined that the generation of a discharge is impossible because the wire electrode
1
and the workpiece
2
are in contact with each other. For example, when the wire electrode
1
is made of brass with the workpiece
2
being made of steel, the above-mentioned relative feed rate determining unit
8
generally sets the short circuit reference voltage to a value in the range of 10 to 15 V.
As a result of comparison between the measured average voltage and the short circuit reference voltage, when the measured average voltage goes below the short circuit reference voltage, the relative feed rate determining unit
8
sets the relative feed rate to a comparatively great negative value. Consequently, the wire electrode
1
and the workpiece
2
are separated from each other at a high speed, thereby making it possible to eliminate a short circuit state between them.
On the other hand, when the measured average voltage is not less than the short circuit reference voltage, the relative feed rate determining unit
8
sets the relative feed rate by executing the following calculations. In other words, the relative feed rate determining unit
8
divides the target feed rate given from the control parameter setting unit
7
by a difference between the reference voltage and the short circuit reference voltage to obtain a proportional constant. Next, the relative feed rate determining unit
8
calculates a difference (hereinafter, simply referred to as error voltage) between the measured average voltage and the reference voltage, multiplies this error voltage by the proportional constant, and obtains a value as a compensation feed rate. Lastly, the relative feed rate determining unit
8
adds the target feed rate to this compensation feed rate to decide a relative feed rate.
The relationship between the relative feed rate and the measured average voltage, obtained through the above-mentioned calculations, is collectively shown in the graph of FIG.
9
. In other words, the relative feed rate is made to be proportional to the difference between the measured average voltage and the short circuit reference voltage, and the relative feed rate is set to be equal to the target feed rate when the measured average voltage is equal to the reference voltage.
However, the above-mentioned conventional art has a problem such that the value of the proportional constant is decided uniquely by the values of the target feed rate and the reference voltage. In other words, the proportional constant is a constant to determine the degree in which the relative feed rate is changed in response to variations in the measured average voltage, that is, the constant by which control gain is decided, so that this is the most basic and essential constant to define the machining control characteristic. Nevertheless, the constant is uniquely decided by the target feed rate and the reference voltage that are decided according to the settings of a machining plate thickness and the machining power supply
5
, and therefore there is a problem that desired control characteristics cannot be set freely. Moreover, when the measured average voltage is not less than the short circuit reference voltage, the control gain becomes constant in all the areas. Therefore, if the response property is intended to be improved by increasing the proportional constant in the vicinity of the reference voltage in order to increase the machining speed, the amount of overshoot is inevitably increased as well. Thereby, the measured average voltage tends to easily go below the short circuit reference voltage in the vicinity of the short circuit reference voltage. As a result, the relative feed rate becomes frequently negative to repeat short circuit and opening, thus the machining state may become quite unstable.
In order to solve these problems, a modified technique has been proposed in which a voltage that makes the relative feed rate zero is set to be higher than the short circuit reference voltage. In this modified technique, the proportional constant is obtained not through calculation of the target feed rate and the reference voltage, but through multiplication of the error voltage by this proportional constant using the preset value to set a value as a compensation feed rate. Further, the target feed rate is added to this compensation feed rate to decide the relative feed rate. However, when the compensation feed rate is negative with its absolute value being greater than the target feed rate, the relative feed rate obtained through the calculations becomes negative, and in this case, zero is set as the relative feed rate. The reason why the relative feed rate is set to zero when the relative feed rate obtained through the calculations is negative, is explained as follows. When the relative feed rate is set negative, the wire electrode
1
backs up on the path. The wire electrode
1
, which has once backed, again advances the same path, i.e., the path that has been once machined when a positive and relative feed rate is subsequently set. In this case, the side face of the workpiece
2
is machined, resulting in an excessively machined state. Therefore, the wire electrode
1
is allowed to back only when the backing is inevitably required, such as a case in which short circuit is occurring. In another cases, the wire electrode
1
is stopped to wait for the recovery of the state, which makes it possible to achieve far better machining quality even if the measured average voltage becomes low.
The relationship between the relative feed rate obtained through the above-mentioned modified technique and the measured average voltage is collectively shown in the graph of FIG.
10
. In other words, i
Sato Tatsushi
Shibata Jun'ichi
Evans Geoffrey S.
Mitsubishi Denki & Kabushiki Kaisha
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