Wire cut electric discharge machining apparatus

The wire cut electric discharge machining apparatus includes a tension applying device for tensing a wire electrode, an instruction signal generating device for providing a movement instruction and a tension instruction for a wire electrode, the speed detecting device for detecting an actual speed of the wire electrode, comparison circuitry for comparing the actual speed detected by the speed detecting device with an instructed speed of the movement instruction provided by the instruction signal generating device and the instruction value correcting device for correcting the tension instruction value which is provided by the instruction signal generating device according to a result of comparison of the comparison device. Hence, in the wire cut electric discharge machining apparatus, the tension of the wire electrode can be controlled with high accuracy.

BACKGROUND OF THE INVENTION 
This invention relates to a wire cut electric discharge machining apparatus 
having means for controlling the tension of a wire electrode with high 
accuracy 
FIG. 6 outlines the arrangement of a conventional wire cut electric 
discharge machining apparatus disclosed in Examined Japanese Patent 
Publication No. Sho-58-52776/(1983). 
In FIG. 6, reference numeral 1 designates a workpiece to be machined; 2, a 
wire electrode for machining the workpiece 1; 3, a wire bobbin for 
supplying the wire electrode 2; 4 and 4a, a pair of wire electrode feeding 
rollers for feeding the wire electrode 2; 5, a constant speed motor 
coupled directly to the wire electrode feeding roller 4; 6 and 6a, a pair 
of constant torque friction rollers for giving a predetermined tension to 
the wire electrode 2; 7, a brake coupled directly to the friction roller 
6; and 8 and 9, guide rollers provided above and below the workpiece to 
guide the wire electrode 2. 
Further in FIG. 6, reference characters 10 and 10a designates a pair of 
rollers for detecting the speed of movement of the wire electrode being 
fed (hereinafter referred to as "a pair of first wire electrode speed 
detecting rollers 10 and 10a", when applicable); 11 and 11a, a pair of 
rollers for detecting the speed of movement of the wire electrode used 
(hereinafter referred to as "a pair of second wire electrode speed 
detecting rollers 11 and 11a", when applicable); 12, a speed detecting 
unit for detecting the speed of rotation of the first wire electrode speed 
detecting roller 10; 13, a speed detecting unit for detecting the speed of 
rotation of the second wire electrode speed detecting roller 11; 14, an 
amplifier for amplifying the output signal of the speed detecting unit 13; 
15, an amplifier for amplifying the output signal of the speed detecting 
unit 12; 16, comparison means for comparing the outputs of the amplifiers 
14 and 15; 17, machining condition control means for changing the setting 
of tension of the wire electrode 2 according to the output signal of the 
comparison means 16; 18, change over means for switching the setting of 
tension of the wire electrode 2 according to the output signal of the 
machining condition control means 17; 19, a set of resistors for switching 
the current flowing in the brake 7 to adjust the setting of tension of the 
wire electrode 2, the set of resistors 19 consisting of a resistor 19a for 
decreasing the tension of the wire electrode, a resistor 19b for setting a 
tension suitable for the diameter of the wire electrode 2, and a resistor 
19c for increasing the tension of the wire electrode 2; and 20, an 
electric source for the brake 7. 
The conventional wire cut electric discharge machining apparatus thus 
constructed, and operation thereof will be described hereafter: 
The wire electrode 2 pulled out of the wire bobbin 3 is tensioned being 
held between the constant torque friction rollers 6 and 6a, and is 
extended through the first wire electrode speed detecting rollers 10 and 
10a. The wire electrode 2 thus extended is laid over the upper guide 
roller 8, so that it is extended towards the workpiece 1. The wire 
electrode 2 thus extended, passing through the machining portion of the 
workpiece 1, is laid over the lower guide roller 9, so that the direction 
of advance of the wire electrode 2 is changed. Thereafter, the wire 
electrode 2 passes through the pair of second wire electrode speed 
detecting rollers 11 and 11a, and is then run at constant speed being held 
between the wire electrode pulling rollers 4 and 4a. 
The tension of the wire electrode 2 is determined by the current which is 
supplied from the electric source 20 to the brake 7 coupled directly to 
the constant torque friction roller 6. On the other hand, the speed of 
movement of the wire electrode 2 is controlled as follows: That is, an NC 
(numerical control) device (not shown) provides an instruction signal to 
control the speed of rotation of the constant speed motor 5 coupled 
directly to the wire electrode pulling roller 4, thereby to control the 
speed of movement of the wire electrode 2. 
In machining the workpiece 1, a machining power source (not shown) applies 
machining current to the machining gap between the workpiece 1 and the 
wire electrode 2, while a drive unit (not shown) moves the workpiece 1 
relative to the wire electrode 2 according to a program stored in the NC 
device, so that the workpiece is machined into a required form. 
The wire electrode feeding speed is detected by the speed detecting unit 12 
coupled to the first wire electrode speed detecting roller 10, and the 
wire electrode winding (take-up) speed is detected by the speed detecting 
unit 13 coupled to the second wire electrode speed detecting roller 11. 
The output signals of the two speed detecting units 12 and 13 are applied 
through the amplifiers 15 and 14 to the comparison means 16, the output 
signal of which is applied to the machining condition control means 17. 
Normally, the change-over means 18 selects the resistor 19b. When the wire 
electrode winding speed is higher than the wire electrode supplying speed, 
the tension of the wire electrode 2 is increased. In this case, the 
machining condition control means 17 applies an output signal to the 
change-over means 18, so that the latter 18 selects the resistor 19a, as a 
result of which the set tension of the wire electrode 2 is decreased. 
When, on the other hand, the wire electrode feeding speed is higher than 
the wire electrode winding speed, the tension of the wire electrode 2 is 
decreased. In this case, the machining condition control means 17 applies 
an instruction signal to the change-over means 18, so that the latter 18 
selects the resistor 19c to increase the set tension of the wire 
electrode. 
That is, the tension of the wire electrode is so controlled that the 
elongation rate of the wire electrode 2 between before and after the 
machining is maintained in a predetermined range. 
The conventional wire cut electric discharge machining apparatus thus 
constructed requires the speed detecting unit for detecting the wire 
electrode winding speed. And, since the set tension of the wire electrode 
is changed stepwise with the change-over means, the tension cannot be 
adjusted with high accuracy, and accordingly the workpiece is decreased in 
machining accuracy. 
Further, when the wire is passed through an initial hole by an automatic 
wire supplying apparatus, tho probability of success to pass the initial 
hole is decreased. 
SUMMARY OF THE INVENTION 
An object of this invention is to eliminate the above-described difficulty 
accompanying a conventional wire cut electric discharge machining 
apparatus. More specifically, an object of the invention is to provide a 
wire cut electric discharge machining apparatus in which the tension of 
the wire electrode can be controlled with high accuracy without a speed 
detecting unit for detecting a wire electrode winding speed. 
The foregoing object of the invention has been achieved by the provision of 
a wire cut electric discharge machining apparatus according to the 
invention comprises: tension applying means for tensing a wire electrode; 
instruction signal generating means for providing a movement instruction 
and a tension instruction for the wire electrode; speed detecting means 
for detecting the actual speed of the wire electrode; comparison means for 
comparing the actual speed detected by said detecting means with an 
instruction speed of the movement instruction provided by the instruction 
signal generating means; and instruction value correcting means for 
correcting the tension instruction value provided by the instruction 
signal generating means according to a result of comparison of the 
comparison means. 
In the wire cut electric discharge machining apparatus according to the 
present invention, the tension of the wire electrode can be controlled 
with high accuracy even if varied, as a result of which the machining 
accuracy is markedly improved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
A first embodiment of this invention, a wire cut electric discharge 
machining apparatus, will be described with reference to FIG. 1. 
In FIG. 1, reference numeral 1 designates a workpiece to be machined; 2, a 
wire electrode for machining the workpiece 1; 3, a wire bobbin from which 
the wire electrode 2 is supplied; 4 and 4a, a pair of wire electrode 
pulling rollers for pulling the wire electrode 2; 5, a constant speed 
motor coupled directly to the wire electrode pulling roller 4; 6 and 6a, a 
pair of constant torque friction rollers for giving a desired tension to 
the wire electrode 2; 8 and 9, guide rollers provided above and below the 
workpiece 1 to guide the wire electrode 2; 30, a DC motor coupled directly 
to the friction roller 6; 31, a tachometer generator which is connected 
directly to one shaft of the DC motor 30 which is opposite to the other 
shaft which is coupled to the friction roller 6; 32, instruction signal 
generating means including speed instruction generating means 26 for 
producing a movement instruction for the wire electrode 2, and tension 
instruction generating means 27 for providing a tension instruction for 
the wire electrode 2; 33, comparison means for comparing the instruction 
speed of the wire electrode 2 from the speed instruction signal generating 
means 26 and the output signal of the tachometer generator 31; 34, 
instruction value correcting means for correcting the tension instruction 
value provided by the tension instruction signal generating means 27 
according to the output signal of the comparison means 33; and 35, a 
machining power source. 
FIG. 2 is a circuit diagram showing an example of the instruction value 
correcting means. The instruction value correcting means 34 comprises 
resistors R and R, a variable resistor VR, and an amplifier AMP. In such 
circuit, an amount of correction is varied by a variable resistor VR 
according to the output from the comparison means 33 in order to correct 
the tension of the wire electrode. 2 
The instruction value correcting means 34 operates to feed-back a part of 
the difference between the instruction speed of the wire electrode 2 
provided by the speed instruction generating means 26 and the output of 
the tachometer generator 31 to the tension instruction value provided by 
the tension instruction generating means 27, thereby to adjust (increase 
or decrease) of the tension instruction value in association with the 
behavior of the wire electrode moving system. 
The wire cut electric discharge machining apparatus thus constructed 
operates as follows: 
The wire electrode 2 supplied from the wire bobbin 3 is tensioned being 
held between the pair of constant torque friction rollers 6 and 6a, and is 
laid over the guide roller 8 so that the direction of movement of the wire 
electrode 2 is changed; that is, the wire electrode 2 is fed to the 
machining part of the workpiece 1. After passing through the workpiece 1, 
the wire electrode 2 is laid over the lower guide roller 9 so that its 
direction of movement is changed again. The wire electrode 2 thus laid is 
held between the pair of wire electrode pulling rollers 4 and 4a so that 
it is moved at a constant speed. The tension and the speed of the wire 
electrode are determined by the instruction signals provided by the 
instruction signal generating means 32. 
In machining the workpiece 1, the machining power source 35 applies 
machining current to the machining gap between the workpiece 1 and the 
wire electrode 2, while a drive unit (not shown) moves the workpiece 1 and 
the wire electrode 2 relative to each other according to a program stored 
in the instruction signal generating means 32, so that the workpiece is 
machined into a required form. 
With the wire cut electric discharge machining apparatus, the tension of 
the wire electrode 2 is controlled as follows: 
the wire electrode 2 is moved at a constant speed by the constant speed 
motor 5 coupled directly to the wire electrode pulling roller 4. A certain 
tension is applied to the wire electrode 2 by controlling the torque of 
the DC motor 30 coupled directly to the constant torque friction roller 6. 
In the wire electrode moving system, a spring math system is formed by the 
inertial moments of the wire bobbin 3, the rollers and the motors with the 
wire electrode 2 as a spring. Hence, it is impossible to control the 
variation in tension of the wire electrode 2, which is due to the natural 
frequency of the wire electrode moving system, only with the 
above-described wire electrode tension controlling system. If, when the 
tension changes in the above-described manner, a tension greater than the 
tensile strength of the wire electrode 2 is applied to the latter 2, then 
the wire electrode 2 is broken so that the machining operation is 
suspended; and in addition the machined surface is scratched by the 
vibration of the wire electrode 2 which is caused by the variation in 
tension,. 
In order to eliminate this difficulty, fundamentally the torque of the DC 
motor 30 is controlled so that the tension of the wire electrode 2 is 
maintained constant. That is, the following method is employed when the 
tension is changed by the natural frequency of the wire electrode moving 
system: When the actual tension of the wire electrode 2 is increased, the 
set tension of the wire electrode 2 is decreased, whereas when the actual 
tension is decreased, the set tension is increased. 
In the wire cut electric discharge machining apparatus according to the 
invention, the speed of rotation of the DC motor 30 is detected by using 
the tachometer generator 31, and the speed of rotation of the DC motor 
thus detected is compared with the speed instruction value, as the wire 
electrode moving speed, provided by the speed instruction generating means 
26 in the comparison means 33. When the tension is maintained unchanged, 
the comparison means provides no output signal, so that the tension of the 
wire electrode 2 is not changed. If the tension of the wire electrode 2 is 
increased, the constant torque friction roller 6 is pulled, and the speed 
of the DC motor is increased accordingly. As a result, a positive signal 
is applied to the comparison means 33, so that the wire electrode tension 
instruction value outputted by the tension instruction generating means 27 
is decreased by the instruction value correcting means 34, and the 
instruction value thus decreased is applied to the DC motor 30. 
When the tension of the wire electrode 2 is decreased, the DC motor 30 is 
rotated slowly, as a result of which a negative signal is applied to the 
comparison means 33. Hence, the wire electrode tension instruction value 
provided by the tension instruction generating means 27 is increased by 
the instruction value correcting means 34, and the instruction value thus 
increased is applied to the DC motor 30. 
Thus, the wire electrode 2 is maintained constant in tension between the 
constant torque friction roller 6 coupled to the DC motor and the wire 
electrode pulling roller 4. 
In the above-described embodiment, the wire electrode moving speed 
instruction and the wire electrode tension instruction are provided by the 
instruction signal generating means 32; however, the embodiment may be so 
modified that these instructions are provided by separate units, 
respectively. Furthermore, in the above-described embodiment, the wire 
electrode is tensioned with the DC motor; however, it may be tensioned 
with an AC motor whose torque can be controlled. The same effect may be 
obtained by using a brake. 
In the above-described embodiment, the tachometer generator coupled to the 
DC motor is employed to detect the wire electrode supplying rate; however, 
the invention is not limited thereto or thereby. For instance, similarly 
as in the above-described conventional wire cut electric discharge 
machining apparatus, a wire electrode supplying speed detecting roller, 
and a speed detecting unit may be provided. That is, in order to detect 
the wire electrode supplying rate, the embodiment may be changed or 
modified in various manners. 
FIG. 3 shows a second embodiment of the invention. In the embodiment, the 
technical concept of the invention is applied to the automatic wire 
electrode supplying operation in a wire cut electric discharge machining 
apparatus with an automatic wire electrode supplying device. 
In FIG. 3, reference character la designates an initial hole about 0.5 mm 
in diameter which is formed in a workpiece 1; 2, a wire electrode about 
0.1 mm to 0.3 mm in diameter for machining the workpiece 1; 3, a wire 
bobbin on which the wire electrode 2 has been wound; 20, a pretension 
motor coupled to the shaft of the wire bobbin 3; 6 and 6a, a pair of brake 
rollers for tensing the wire electrode 2; 8, a guide roller provided above 
the workpiece 1 to change the direction of advance of the wire electrode 
2; 21 and 21a, a pair of feed rollers for sending the wire electrode 2 
into the initial hole 1a of the workpiece 1; 22, a guide pipe for guiding 
the wire electrode 2; 23, an upper wire guide for supporting the wire 
electrode 2 above the workpiece 1; 24, a feed motor coupled directly to 
the feed roller 21; 25, a brake coupled directly to the brake roller 6; 
26, speed instruction generating means for applying a rotation speed 
instruction value to the feed motor 24; 27, tension instruction generating 
means for applying a torque instruction value to the brake 25; 28, a 
tachometer generator connected directly to one end portion of the shaft of 
the brake 25 to the other end portion of which the brake roller 6 is 
connected; 29, an amplifier for amplifying the output signal of the 
tachometer generator 28; 33, comparison means for comparing the output of 
the amplifier 29 with the signal of the feed roller 21 which is controlled 
by the speed instruction generating means 26; and 34, instruction value 
correcting means for correcting the signal of the tension instruction 
generating means 27 according to the output signal of the comparison means 
33. The speed instruction generating means 26 and the tension instruction 
generating means 27 form instruction signal generating means 32. 
The operation of the second embodiment thus constructed will be described. 
The wire electrode 2 supplied from the wire bobbin 3 is tensioned being 
held between the brake rollers 6 and 6a, and laid over the guide roller 8 
and held between the feed rollers 21 and 21a. Thus, the wire electrode 2 
is run at a constant speed. The wire electrode 2 passes through the guide 
pipe 22, and is inserted into the initial hole la of the workpiece 1 while 
being positioned by the upper wire guide 23 relative to the workpiece 
piece 1. The pretension motor 20 is connected to the shaft of the wire 
bobbin 3 to give a light tension of the order of 200 g to the wire 
electrode so that the latter may not be slackened between the wire bobbin 
3 and the brake rollers 6 and 6a. The tension applied to the wire 
electrode 2 is determined by the current flowing in the brake 25 coupled 
directly to the brake roller 6, and the current is determined by the 
output signal of the tension instruction generating means 27. The speed of 
insertion of the wire electrode 2 into the initial hole 1a of the 
workpiece 1 is determined by the voltage which is applied to the feed 
motor 24, and the voltage is determined by the speed instruction 
generating means 26. On the other hand, the speed of rotation of the brake 
25 is detected by the tachometer generator 28 coupled directly to the 
brake 25. In the comparison means 33, the signal for the feed roller 21 
which is supplied from the speed instruction generating means 26 is 
compared with the wire electrode movement speed signal provided by the 
brake section. The amplifier 29 is to match the signal of the feed roller 
21 from the speed instruction generating means 26, and the wire electrode 
movement speed signal provided by the brake section with each other in the 
characteristic of output. The instruction value correcting means 34 
corrects the signal of the tension instruction generating means 27 
according to the output signal of the comparison means 33, thereby to 
adjust (increase or decrease) the current applied to the brake 25, to 
stabilize the tension of the wire electrode moving system. For instance 
when the speed instruction value is larger than the speed of movement of 
the wire electrode in the brake section, in the wire moving system the 
tension is decreased. Hence, a negative signal corresponding to the 
difference between the speed instruction value and the speed of movement 
of the wire electrode is outputted by the comparison means 33, so that the 
instruction value correcting means 34 corrects the output signal of the 
tension instruction generating means 27 so as to increase the tension. On 
the other hand, when the speed instruction value is smaller than the speed 
of movement of the wire electrode in the brake section, in the wire moving 
system the tension is increased. Hence, a positive signal corresponding to 
the difference between the speed instruction value and the speed of 
movement of the wire electrode is outputted by the comparison means 33, so 
that the instruction value correcting means 34 corrects the output signal 
of the tension instruction generating means 27 so as to decrease the 
tension. 
In the above-described embodiment, the speed of the wire electrode 2 in the 
brake section is detected with the tachometer generator 28 coupled 
directly to the brake 25. However, the same effect can be obtained by 
modifying the embodiment as shown in FIGS. 4 and 5. In FIG. 4, the 
tachometer generator 28 is connected directly to the shaft of the brake 
roller 6. In FIG. 5, a pair of speed detecting rollers 50 and 50a are 
additionally provided, and the tachometer generator 28 is coupled to the 
speed detecting roller 50. 
In the above-described embodiment, the speed of the wire electrode is 
detected by the tachometer generator; however the same effect can be 
obtained by a method in which an encoder is employed to detect the speed 
of movement of the wire electrode through frequency-to-voltage conversion. 
Furthermore in the above-described embodiment, the brake is employed as an 
actuator for tensing the wire electrode 2; however, the same effect can be 
obtained by an electric motor such as a DC motor or AC motor. 
Moreover in the above-described embodiment, the motor, the brake, the 
tachometer generator, and the encoder are coupled directly to the rollers; 
however, the same effect can be obtained by connecting them through gears 
or the like to the rollers so that their speeds are reduced.