U.S. patent application number 11/157804 was filed with the patent office on 2005-12-29 for wire cutting device of wire electric discharge machine.
This patent application is currently assigned to FANUC LTD. Invention is credited to Kawahara, Akiyoshi, Nakashima, Yasuo.
Application Number | 20050284846 11/157804 |
Document ID | / |
Family ID | 35063315 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050284846 |
Kind Code |
A1 |
Nakashima, Yasuo ; et
al. |
December 29, 2005 |
WIRE CUTTING DEVICE OF WIRE ELECTRIC DISCHARGE MACHINE
Abstract
When starting a wire cutting operation, a first predetermined
current is supplied from a power unit to a wire, and at the same
time predetermined tension is applied from a tension applying
device to the wire. When a first predetermined time elapses since
the start of the wire cutting operation, the current supplied from
the power unit is switched to a second predetermined current which
is smaller than the first predetermined current, and also the
predetermined tension is continuously applied, whereby the wire is
cut.
Inventors: |
Nakashima, Yasuo;
(Minamitsuru-gun, JP) ; Kawahara, Akiyoshi;
(Minamitsuru-gun, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
FANUC LTD
Yamanashi
JP
|
Family ID: |
35063315 |
Appl. No.: |
11/157804 |
Filed: |
June 22, 2005 |
Current U.S.
Class: |
219/69.12 |
Current CPC
Class: |
B23H 7/04 20130101; B23H
7/102 20130101 |
Class at
Publication: |
219/069.12 |
International
Class: |
B23H 007/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2004 |
JP |
191994/2004 |
Claims
What is claimed is:
1. A wire cutting device of a wire electric discharge machine,
comprising: a power unit which supplies power for initiating fusion
of a wire; a tension applying device which applies tension to the
wire; a progress monitoring device which monitors progress of a
work operation for fusing the wire; and a control device which
instructs the power unit for a current value and the tension
applying device for a tension value, wherein the control device
instructs the power s unit to supply a first predetermined current
to the wire unit until the progress monitoring device judges that a
predetermined progress level for fusing the wire is reached, and to
supply a second predetermined current, which is smaller than the
first predetermined current, to the wire after the progress
monitoring device judges that the predetermined progress level for
fusing the wire is reached.
2. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the progress monitoring device is
timing means, which begins measuring the time when the power unit
starts to supply a current for fusion to the wire, and judges that
the predetermined progress level is reached after measuring a
predetermined time.
3. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the progress monitoring device is
temperature detecting means for detecting temperature of the wire,
which judges that the predetermined progress level is reached when
the wire temperature reaches a predetermined temperature.
4. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the progress monitoring device is
wire stretched amount detecting means which detects the stretched
amount of the wire and judges that the predetermined progress level
is reached when the stretched amount of the wire reaches a
predetermined amount.
5. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the control device instructs the
tension applying device to continue the application of fixed
tension to the wire before and after the progress monitoring device
judges that the predetermined progress level is reached.
6. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the control device instructs the
tension applying device to apply first predetermined tension to the
wire until the progress monitoring device judges that the
predetermined progress level is reached, and to apply second
predetermined tension, which is larger than the first predetermine
tension, to the wire after the progress monitoring device judges
that the predetermined progress level is reached.
7. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the control device instructs the
tension applying device to apply first predetermined tension to the
wire, and further instructs the power unit to supply a first
current until the progress monitoring device judges that the
predetermined progress level is reached, and, after the progress
monitoring device judges the predetermined progress level is
reached, instructs the tension applying device to apply second
predetermined tension, which is larger than the first predetermined
tension, to the wire, and further instructs the power unit to
supply a second current, which is smaller than the first
current.
8. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the power unit comprises a voltage
generator in which an output voltage is variable.
9. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the power unit comprises a voltage
generator having fixed output voltage, and a current limiting
circuit having variable impedance.
10. The wire cutting device of a wire electric discharge machine
according to claim 1, wherein the power unit comprises a voltage
generator having fixed output voltage, and a switching element
having a variable duty ratio.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wire cutting device of a
wire electric discharge machine, which, in the wire electric
discharge machine, applies a current through a wire and applies
tension to fuse the wire.
[0003] 2. Description of the Prior Art
[0004] A wire electric discharge machine provides a machining start
hole in a workpiece beforehand, inserts a wire into the machining
start hole, and thereafter machines the workpiece by electric
discharge. Particularly, when machining a plurality of workpieces,
or when taking out a plurality of machining objects from one
workpiece in continuous unattended operation, when one work is
finished, a wire is cut, then a wire is automatically inserted into
the next machining start hole for the next machining to be
automatically connected, and the next machining is performed.
Therefore, a highly reliable automatic wire cutting/connection
function is absolutely necessary. This function is for
automatically cutting a wire after machining which is started from
a certain machining start hole is finished, moving a wire guide to
the next machining start hole, then automatically connecting the
wire in that position to which the wire guide was moved, and then
starting the next machining. Speeding up automatic wire
cutting/connection operation is an important development subject
for reducing machining time.
[0005] As a conventionally proposed method for automatically
cutting a wire, there is a method in which a current is supplied to
a wire, the wire is then heated by means of its own resistance and
is at the same time tensioned, whereby the wire is fused by heat
(see, for example, Japanese Patent Application Laid-open No.
2-53528). According to this method, in comparison with a mechanical
cutting method where a cutter or the like is used, an end of the
wire can be formed into a pointy shape, so the wire can be smoothly
led through a wire guide and workpiece at the time of wire
connection. Therefore, speedy and secure wire connection is
possible.
[0006] In order to reduce time to cut a wire, the time until the
temperature at which a wire can be cut is reached may be reduced by
increasing a current to be supplied to the wire so as to increase
the heat value of the wire. However, if a current is increased too
much, an end of the wire turns into a spherical shape, so the wire
cannot be inserted smoothly, and it takes time for wire connection.
Moreover, wire connection may be failed. As described in Japanese
Patent Application Laid-open No. 2-237724, the shape of an end of a
wire changes according to values of a current and tension applied
to the wire, thus, in order to realize the shape of wire that is
suitable for automatic wire connection, it is necessary to select
an optimum value of a current and of tension in accordance with the
material and wire diameter.
[0007] Also, there is proposed a method in which a wire is held for
a predetermined amount of time while being applied with a
predetermined of tension and a fusing current, for the purpose of
forming an end portion of the wire into a pointy shape without
damaging an object to be machined or a part such as a wire guide
(see, for example, Japanese Patent Application Laid-open No.
5-305522). Here, "a predetermined time" indicates time until the
wire is fused under a condition where a fusing current is applied
and then the end portion of the wire is formed into a spherical
shape (an optimum value of tension and an optimum value of a fusing
current).
[0008] In this manner, a value of a current supplied to a wire is
determined itself, resulting in a problem where time for cutting
the wire cannot be reduced. It should be noted that a method for
changing a value of a current supplied to a wire during fusing
operation is described in Japanese Patent Application Laid-open No.
5-220624 and Japanese Patent Application Laid-open No. 4-331022.
Japanese Patent Application Laid-open No. 5-220624 describes
increases of the amount of current with the course of time during
fusing operation will be able to save determining, before starting
the fusing operation, a value of the supplied current in accordance
with the type or wire diameter of the wire. Further, Japanese
Patent Application Laid-open No. 4-331022 describes how a wire is
securely fused regardless of variation in values of resistance
within a disconnecting circuit, by increasing the amount of a
current after a predetermined amount of time since the cutting has
started.
SUMMARY OF THE INVENTION
[0009] A wire cutting device of a wire electric discharge machine
according to the present invention has a power unit which supplies
power for initiating fusion of a wire, a tension applying device
which applies tension to the wire, a progress monitoring device
which monitors progress of a work operation for fusing the wire,
and a control device which instructs the power unit for a current
value and the tension applying device for a tension value. The
control device supplies a first predetermined current to the wire
from the power unit until the progress monitoring device judges
that a predetermined progress level for fusing the wire is reached,
and then supplies a second predetermined current, which is smaller
than the first predetermined current, from the power unit to the
wire after the progress monitoring device judges that the
predetermined progress level for fusing the wire is reached.
[0010] A first embodiment of the wire cutting device of a wire
electric discharge machine according to the present invention has a
power unit which supplies power for initiating fusion of a wire, a
tension applying device which applies tension to the wire, and a
control device which instructs the power unit for a current value
and the tension applying device for a tension value. When applying
a current and tension to the wire to fuse the wire by heat, at the
time of starting a wire cutting operation the first predetermined
current is supplied from the power unit to the wire, and also
predetermined tension is applied to the wire by the tension
applying device. Moreover, when a first predetermined time elapses
since the time when the wire cutting operation is started, the
current supplied from the power unit is switched to a second
predetermined current, which is smaller than the first
predetermined current, and further the predetermined tension is
continuously applied to the wire, whereby the wire is cut.
[0011] Until when a predetermined time elapses since the start of
wire cutting, the current supplied to the wire is the first
predetermined current value corresponding to the material and wire
diameter of the wire, and increase in temperature of the wire is
accelerated. Thereafter, the current is reduced to the second
predetermined current value when cutting the wire, whereby the wire
end is formed to have a pointy shape, which is suitable for
automatic wire connection. It should be noted that tension is still
continuously applied to the wire even after reducing the current,
and the wire cutting is initiated during the period.
[0012] In the wire cutting device according to the first
embodiment, the progress monitoring device, which monitors progress
of a work operation for fusing the wire, is timing means which
begins measuring the time when supply of a current to the wire is
started, and switches the intensity of the current supplied to the
wire, after measuring a predetermined amount of time.
[0013] A second embodiment of the wire cutting device of a wire
electric discharge machine according to the present invention has a
power unit which supplies power for initiating fusion of a wire, a
tension applying device which applies tension to the wire, a
control device which instructs the power unit for a current value
and the tension applying device for a tension value, and a wire
temperature detection device. When applying a current and tension
to the wire to fuse the wire, at the time of starting a wire
cutting operation a first predetermined current is supplied from
the power unit to the wire, and also predetermined tension is
applied to the wire by the tension applying device. A wire
temperature which is detected by the wire temperature detection
device is monitored, and when the wire temperature reaches a first
predetermined temperature, the current supplied from the power unit
is switched to a second predetermined current, which is smaller
than the first predetermine current, and further the predetermined
tension is continuously applied to the wire, whereby the wire is
cut.
[0014] In the wire cutting device according to the second
embodiment, the progress monitoring device, which monitors progress
of a work operation for fusing the wire, is temperature detecting
means which is, or is not, brought into contact with the wire, and
switches the intensity of the current supplied to the wire, after
the wire temperature reaches a predetermined temperature.
[0015] A third embodiment of the wire cutting device of a wire
electric discharge machine according to the present invention has a
power unit which supplies power for initiating fusion of a wire, a
tension applying device which applies tension to the wire, a
control device which instructs the power unit for a current value
and the tension applying device for a tension value, and a wire
stretched amount detection device. When applying a current and
tension to the wire to fuse the wire, at the time of starting a
wire cutting operation a first predetermined current is supplied
from the power unit to the wire, and also predetermined tension is
applied to the wire by the tension applying device. A stretched
amount of the wire which is detected by the wire stretched amount
detection device is monitored, and when the stretched amount of
wire reaches a first predetermined amount, the current supplied
from the power unit is switched to a second predetermined current,
which is smaller than the first predetermine current, and further
the predetermined tension is continuously applied to the wire,
whereby the wire is cut.
[0016] In the wire cutting device according to the third
embodiment, the progress monitoring device, which monitors progress
of a work operation for fusing the wire, is detecting means which
detects a stretched amount of the wire, and switches the intensity
of the current supplied to the wire, after the stretched amount of
the wire reaches a predetermined value.
[0017] According to the wire cutting device of a wire electric
discharge machine of the present invention, when a work operation
for fusing the wire develops to a specific situation (when a
predetermined time elapses since the start of wire cutting, when
the wire temperature reaches a predetermined temperature, or when
the stretched amount of wire reaches a predetermined wire stretched
amount), the current supplied to the wire is the first
predetermined current value corresponding to the material and wire
diameter of the wire, and increase in temperature of the wire is
accelerated, whereby the time for cutting the wire can be reduced.
Thereafter, the current is reduced to the second predetermined
current value, tension is still continuously applied to the wire
even after reducing the current, and the wire cutting is initiated
during the period. Consequently, the wire end can be formed to have
a pointy shape, which is suitable for automatic wire connection.
Furthermore, by reducing the current applied to the wire and
increasing the tension, the wire end can be prevented from being
formed into a spherical shape, and the wire can be cut
securely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The abovementioned objects, other objects, and the
characteristics of the present invention may become clear from the
following descriptions of embodiments which are provided with
reference to the attached drawings, in which:
[0019] FIG. 1 is a configuration diagram showing the outline of a
wire cutting device of a wire electric discharge machine which is
related to an embodiment of the present invention;
[0020] FIG. 2 is a circuit diagram showing a first example of a
power unit which is used in the wire cutting device of FIG. 1;
[0021] FIG. 3 is a circuit diagram showing a second example of the
power unit;
[0022] FIG. 4 is a circuit diagram showing a third example of the
power unit;
[0023] FIG. 5 is a time chart showing the relationship between a
current supplied to a wire and tension applied to the wire in
accordance with the passage of time, the time chart being related
to a first example of a wire cutting process in which a fixed
amount of tension is applied during the time from when supply of a
current is started until when the wire is cut;
[0024] FIG. 6 is a time chart showing the relationship between a
current supplied to a wire and tension applied to the wire in
accordance with the passage of time, the time chart being related
to a second example of the wire cutting process in which a small
amount of tension is applied at the start of supplying a current,
and a tension value is changed to a larger value when the current
is reduced;
[0025] FIG. 7 is a flow chart showing a process which is related to
the first example and is executed by a control device; and
[0026] FIG. 8 is a flow chart showing a process which is related to
the second example and is executed by the control device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] FIG. 1 is a configuration diagram showing the outline of a
wire cutting device of a wire electric discharge machine which is
related to an embodiment of the present invention. In FIG. 1, a
wire 1 is fed by a take-up roller 6, which is rotary driven by a
wire drive motor (not shown), through a roller with brake 2, an
upper guide 3, a workpiece 4 of an object to be machined, and a
lower guide 5. Conducting elements 8a and 8b are connected to the
wire at the fusing positions, which are established between the
roller with brake 2 and the upper guide 3, and a current for
initiating fusion of the wire is supplied from a power unit 7
through the conducting elements 8a and 8b. Also, a wire temperature
detection device 9 for detecting a wire temperature in a noncontact
state with the wire is disposed so as to face the fusing positions
on the wire 1. The wire drive motor (not shown) for rotary driving
a brake function of the roller with brake 2 and the take-up roller
6 constitutes a tension applying device which applies tension to
the wire when cutting the wire. Further, the take-up roller 6 is
provided with a wire stretched amount detection device 10 which
detects a stretched amount of the wire. The wire stretched amount
detection device 10 is constituted by, for example, an encoder
which detects the amount of rotation of the roller corresponding to
the stretched amount of the wire, and a value detected by the wire
stretched amount detection device 10 is the stretched amount of the
wire.
[0028] A control device 11 is a control device of the wire electric
discharge machine, and, in this embodiment, serves also as a
control device of the wire cutting device. The control device 11 is
the same as conventional control devices, thus a detailed
explanation thereof is omitted. The control device 11 has a
processor (CPU), a memory constituted by a ROM and RAM, an
input-output interface, a display/manual input unit having a
display constituted by a liquid crystal, CRT or the like and an
input portion constituted by a keyboard, control panel or the like,
and an axis control circuit for feed axes X, Y and Z in the wire
electric discharge machine.
[0029] In a wire cutting process, the control device 11 instructs
the power unit 7 for a current value and the tension applying
device (wire drive motor) for a tension value. The control device
11 further performs input of a wire temperature detection value
obtained by the wire temperature detection device 9, input of a
detection value of a stretched amount of the wire, which is
obtained by the wire stretched amount detection device 10, and
control of a current value for initiating fusion, which is sent
from the power unit 7.
[0030] In order to fuse the wire 1 by heat by applying a current
and tension to the wire 1 using the wire cutting device according
to the present invention, first of all, a first predetermined
current is supplied from the power unit 7 while the wire 1 is
applied with a predetermined tension by the tension applying device
at the time of starting a wire cutting operation. Thereafter, when
a predetermined time elapses since the start of the wire cutting
operation, or when the wire temperature detected by the wire
temperature detection device 9 reaches a predetermined wire
temperature, or alternatively when the stretched amount of the wire
detected by the wire stretched amount detection device 10 reaches a
predetermined stretched amount of the wire, the current supplied
from the power unit 7 is reduced to a second predetermined current
which is smaller than a first predetermined current, and the
predetermined tension is continuously applied, to initiate fusion
of the wire. Therefore, the power unit 7 changes the current
supplied to the wire 1.
[0031] Several examples of the power unit 7 which is used in the
wire cutting device of the present invention are described
hereinafter. FIG. 2 is a circuit diagram showing a first example of
the power unit 7. A power unit 7a according to the first example is
constituted by a circuit in which a current-limiting resistance 22
is connected in series with a voltage generator 21 having variable
output voltage.
[0032] FIG. 3 is a circuit diagram showing a second example of the
power unit. A power unit 7b according to the second example is
constituted by a circuit in which a current limiting resistance 32
and a current limiting resistance 33 together with a relay contact
point 34 are connected in parallel to the output side of a voltage
generator 31 having fixed output voltage, and the combined
resistance of the current limiting resistances is changed by
switching the relay contact point 34 by relay.
[0033] FIG. 4 is a circuit diagram showing a third example of the
power unit. A power unit 7c according to the third example is
constituted by a circuit in which a current limiting resistance 43
is connected to the output side of a voltage generator 41 having
constant output voltage, via a switching element 42, wherein the
ratio between the time when the current is on and the time when the
current is off (for example, the ratio of the time when the
switching element 42 turns on the current every predetermined cycle
of time, in other words, the duty ratio) is changed.
[0034] FIG. 5 is a time chart showing the relationship between a
current supplied to the wire 1 and tension applied to the wire 1 in
accordance with the passage of time. In a first example of the wire
cutting process which is described below, a predetermined tension
is applied to the wire 1 during the time from when supply of a
current is started until when the wire is cut. The tension to be
applied is fixed and is not changed.
[0035] The first example of the wire cutting operation is described
hereinafter. FIG. 7 is a flow chart showing a process which is
executed by the processor (referred to as "CPU" hereinafter) of the
control device 11. Execution of this process is practically started
upon completion of machining of a part of the workpiece 4 by
electric discharge.
[0036] First, a first predetermined current, predetermined tension,
second predetermined current, and the time for supplying the first
predetermined current (first predetermined time) that correspond to
the material and diameter of the wire 1, which is an object of use,
and that are preset and stored in cutting condition storage means
of the memory are read out (step S01). Here, the first and second
predetermined currents and the predetermined tension are set in
advance as conditions for forming the end portion of the fused wire
into a pointy shape.
[0037] Next, the brake is turned on (step S02), a predetermined
torque is used to drive the wire drive motor in order to apply the
set predetermined tension to the wire 1 (step S03), an instruction
is outputted to the power unit 7 in order to supply the first
predetermined current from the power unit 7 (step S04), and a timer
for measuring the time elapsed since the start of wire cutting is
reset to start measuring time (step S05). Then, whether the time
elapsed since the start of wire cutting, the time being measured by
the timer, reaches the set time for supplying the first
predetermined current (first predetermined time) or not is
monitored (step S06).
[0038] If the time elapsed since the start of wire cutting, the
time being measured by the timer, reaches the time for supplying
the first predetermined current, the process proceeds from the step
S06 to a step S07 in which an instruction is outputted to the power
unit 7 in order to switch the current, which is supplied from the
power unit 7, to the second predetermined current which is smaller
than the first predetermined current in value (step S07). Next, the
value of the supply current supplied from the power unit 7 to the
wire 1 is read (step S08), and whether the wire is cut or not is
judged based on the supply current value (step S09). It should be
noted that the predetermined tension is continuously applied to the
wire 1. When the wire 1 is cut, conduction between the conducting
element 8a and the conducting element 8b is interrupted, whereby
the supply current value becomes 0 A. The CPU repeats the process
between the step S08 and the step S09 until the wire 1 is cut.
[0039] If the wire 1 is cut, the process proceeds from the step S09
to a step S10 in which the wire drive motor is stopped and the
brake is turned off in order to stop applying the tension to the
wire 1 (step S10), supply of the current by the power unit 7 is
stopped (step S11), and the wire cutting process is ended.
[0040] In the case of a zinc coated wire having a wire diameter of
0.3 mm, when current of 8 ampere is continuously supplied to the
wire during the time from when the cutting operation is started
until when the wire is cut, the cutting time is approximately ten
seconds. However, when current of 12 ampere is constantly supplied
as the first predetermined current for three seconds since the time
when the cutting operation is started, and thereafter the current
is switched to 8 ampere as the second predetermined current, the
cutting time is reduced to approximately seven seconds.
[0041] In the first example of the abovementioned wire cutting
process, the time when the current is switched from the large first
predetermined current to the small second predetermined current is
judged by the time elapsed since the start of wire cutting;
however, instead, the time when the wire temperature detected by
the wire temperature detection device 9 reaches a predetermined
wire temperature (first predetermined temperature, or the
temperature at which the wire may be fused) may be taken as the
time for switching the supply current, or alternatively, the time
when the stretched amount of the wire which is detected by the wire
stretched amount detection device 10 reaches a predetermined wire
stretched amount (when the wire temperature rises, whereby the wire
starts to stretch, and the wire stretched amount reaches a first
predetermined value) may be taken as the time for switching the
supply current. In this case, the predetermined wire temperature or
predetermined stretched amount of the wire corresponding to the
material or diameter of the wire may be stored in the cutting
condition storage means which is previously set in the memory, the
wire temperature or the stretched amount of the wire may be read
out in the step S01, and judgment may be made in the step S06
whether or not a current wire temperature or stretched amount of
the wire reaches the predetermined value.
[0042] In the first example of the abovementioned wire cutting
process, the predetermined tension is applied to the wire 1 during
the time from when supply of a current is started until when the
wire is cut, where the tension is fixed and is not changed.
Alternatively, the tension may be changed.
[0043] FIG. 6 is a time chart showing the relationship between a
current supplied to the wire 1 and tension applied to the wire 1 in
accordance with the passage of time. In a second example of the
wire cutting process which is described below, first predetermined
tension is applied to the wire when supply of a current is started,
and the tension applied to the wire is switched to second
predetermined tension which is larger than the first predetermined
tension upon switching the supply current from the first
predetermined current to the second predetermined current which is
smaller than the first predetermined current.
[0044] The second example of the wire cutting operation is
described hereinafter. FIG. 8 is a flow chart showing a process
which is executed by the processor (referred to as "CPU"
hereinafter) of the control device 11. First, a first predetermined
current, first predetermined tension, second predetermined current,
second predetermined tension, and the time for supplying the first
predetermined current (first predetermined time) that correspond to
the material and diameter of the wire 1, which is an object of use,
and that are stored in the cutting condition storage means which is
previously set in the memory are read out (step S21). Here, the
first predetermined tension is smaller than the second
predetermined tension.
[0045] Next, the brake is turned on (step S22), a predetermined
torque is used to drive the wire drive motor in order to apply the
first predetermined tension to the wire 1 (step S23), an
instruction is outputted to the power unit 7 in order to supply the
first predetermined current from the power unit 7 (step S24), and a
timer for measuring the time elapsed since the start of wire
cutting is reset to start measuring time (step S25). Then, the
process proceeds from the step S25 to a step S26 in which is
monitored whether or not the time elapsed since the start of wire
cutting, the time being measured by the timer, reaches the time for
supplying the first predetermined current (first predetermined
time).
[0046] Through the process as described above, the first
predetermined current is supplied from the power unit 7 to the wire
1, and at the same time the first predetermined tension is applied
to the wire 1 when the wire cutting operation is started.
Accordingly, the wire can be prevented from being cut, and at the
same time the wire end can be prevented from being formed into a
spherical shape when the first predetermined current is
supplied.
[0047] If the time elapsed since the start of wire cutting, the
time being measured by the timer, reaches the time for supplying
the first predetermined current (first predetermined time), the
process proceeds from the step S26 to a step S27 in which an
instruction is outputted to the power unit 7 in order to switch the
current, which is supplied from the power unit 7, to the second
predetermined current (step S27). Also, a predetermined torque is
used to drive the wire drive motor in order to apply the second
predetermined tension to the wire 1 (step S28). Consequently, the
wire temperature is considered that the wire temperature reaches
the temperature at which the wire may be fused, and the second
predetermined tension is applied to the wire 1 to monitor the wire
be fused. Specifically, the value of the supply current supplied
from the power unit 7 to the wire 1 is read (step S29), and whether
the wire is cut or not is judged based on the supply current value
(step S30). The CPU repeats the process between the step S29 and
the step S30 until the wire 1 is cut.
[0048] If the wire 1 is cut, the process proceeds from the step S30
to a step S31 in which the wire drive motor is stopped and the
brake is turned off in order to stop applying the tension to the
wire 1 (step S31), supply of the current by the power unit 7 is
stopped (step S32), and the wire cutting process is ended.
[0049] It should be noted that, in the second example of the wire
cutting process, the tension is increased simultaneously with the
decrease in the current; however, the same effect can be obtained
even when the tension is increased after the current is decreased.
For example, the time for increasing the tension can be judged by
the time when a second predetermined time (predetermined time which
is different from the first predetermined time for switching the
current from the first predetermined current to the second
predetermined current) elapses since the time when the current is
decreased, or, for example, by the time when the wire temperature
reaches a second predetermined temperature (predetermined
temperature which is different from the first predetermined
temperature for switching the current from the first predetermined
current to the second predetermined current) after the current is
decreased, or alternatively, for example, by the time when the
stretched amount of the wire reaches a second predetermined value
(a predetermined value which is different from the first
predetermined value for switching the current from the first
predetermined current to the second predetermined current) after
the current is decreased.
[0050] In the above-described first and second examples of the wire
cutting process, the time when the wire is cut off is detected by
monitoring the current which initiates fusion, to detect that the
current is no longer supplied. However, the cutting off of the wire
may be detected by monitoring a driving current of the wire drive
motor applying tension to the wire, to detect that a load torque
has been changed since the tension is no longer applied due to wire
cutting.
[0051] Moreover, in the first and second examples of the wire
cutting process, although the current is switched from the first
predetermined current to the second predetermined current, the
value of the current may be switched in multiple stages, such as
from the second predetermined current to a third predetermined
current, the third predetermined current to a fourth predetermined
current, or the like. Further, the tension of the wire may be
switched in multiple stages, such as from the second predetermined
tension to third predetermined tension, the third predetermined
tension to fourth predetermined tension, or the like. It should be
noted that, as with the first and second examples of the wire
cutting process, the time for the multi-stage switching can be
judged by the passage of predetermined time, the time when the wire
temperature reaches the predetermined temperature, the time when
the stretched amount of wire reaches the predetermined value, or
the like. Furthermore, the current value or the tension may be
switched in accordance with a predetermined switching pattern for
the current value or a predetermined switching pattern for the
tension.
* * * * *