JPH0328262B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a processing method and apparatus for performing processing such as cutting and welding using a plasma arc.

Prior Art A method for processing metals using a plasma arc is usually configured as shown in FIG. In the figure, the start switch S is closed and the output of the power source E is supplied to the torch 1 and the workpiece 4,
The tip of the electrode 101 of the torch 1 and the squeezing nozzle 10
2, a pilot arc is generated by the current limited by the limiting resistor R, and the solenoid valve 2 is opened to supply working gas from the gas cylinder 3, which has been adjusted to a pressure and flow rate suitable for plasma arc processing. Flow. This pilot arc turns the working gas into a hot ionized gas stream. When the nozzle 102 is brought close to the workpiece 4 in this state, the ionized gas flow reaches the workpiece 4, a plasma arc is generated from the electrode 101 toward the workpiece 4, and machining is started.

Here, a power source E with drooping characteristics or constant current characteristics is used, and the pilot arc is limited to an extremely small value by a resistor R in order to reduce wear and tear on the nozzle 102. The no-load voltage of the power supply required to maintain the pilot arc stably in a high-speed gas flow from a high-pressure source is extremely high, and small torches with small pilot arc currents use voltages exceeding 250V. ing. The reason for this can be roughly considered as follows. In conventional plasma arc processing, the gas supply source must be at a considerably high pressure in order to flow a large amount of gas at high speed even during the pilot arc, and this gas flow is strongly squeezed at the nozzle exit. Most of the pressure loss occurs in this part, and for this reason, the pilot arc generation part in the nozzle has a high pressure close to the pressure of the gas supply source, and therefore the arc voltage has a high value. Furthermore, gas molecules ionized by the pilot arc flow away at high speed, making it impossible to maintain a sufficient ionized state, necessitating an even higher voltage. In such a nozzle where gas flows out under high pressure and at high speed, the voltage-current characteristics of the arc exhibit extremely negative characteristics in the small current range, as shown by the solid line in FIG. FIG. 2 shows an example of the voltage-current characteristics of an arc in nitrogen gas, and the solid line shows the characteristics when the pressure is 2 kg/cm ² . As can be seen from the figure, when the voltage is several amperes, such as during a pilot arc, the arc voltage exceeds 200V. In order to stably maintain a high voltage pilot arc in such a negative characteristic region, a no-load voltage corresponding to the highest voltage in these voltage regions is required.

On the other hand, after switching to plasma arc,
The current is large and the arc voltage at that time is also relatively low. For this reason, even though a high no-load voltage is not required during actual machining, it is necessary to prepare a power supply with a high no-load voltage simply to maintain the pilot arc, and the capacity of the power supply becomes unnecessary. Not only does this result in a large amount of damage, but also the high pilot arc voltage applied to the nozzle section is extremely dangerous in semi-automatic processing machines where processing is carried out by hand. Furthermore, the electrode 101 and the nozzle 10
2 is short-circuited for some reason such as flying spatter, the full voltage of the power source E will be applied to the nozzle, which will not only prevent the pilot arc from occurring but also create an extremely dangerous situation.
As a way to eliminate this phenomenon, we use a gas such as argon, which has a low ionization voltage and good arc stability, as the gas flowed during the pilot arc, and use nitrogen gas, which has an excellent arc cooling effect and is inexpensive, during plasma processing. A method has also been proposed in which the gas is switched to use a gas containing a small amount of hydrogen gas, etc. However, the arc may become unstable before and after switching the gas, or a defective part may be formed at the start of machining. , it wasn't very practical.

Progress of the Invention The inventors conducted experiments based on the above considerations and found the following phenomenon. In other words, if the pressure of the gas supplied during the pilot arc is gradually lowered to reduce the flow rate, the arc voltage will drop significantly for the same pilot arc current, and the pilot arc will stably occur even with a low no-load voltage power supply. It turned out that it was possible to maintain it. The dashed line in Figure 2 above is an example of the experimental results, and the dashed line 1 in the same figure shows a gas supply pressure of 1 kg/kg using the same torch.
cm ² and the broken line 2 is the arc voltage and current characteristics when it is 0.5Kg/cm ² . Therefore, if the gas supplied during pilot arcing is supplied from a gas source with a lower pressure than during plasma processing and the flow rate is small, no-load voltage as low as 180V can be achieved even if the same type of gas as used during plasma processing is used. It can be seen that a sufficiently stable pilot arc can be maintained with the power supply. However, in this case, it is expected that it will be difficult to transition from the pilot arc to the main arc because the arc power will be low, so the present invention was developed as a result of various experimental studies to solve this problem.

Summary of the Invention The present invention provides (1) a gas supplied during a pilot arc that is the same type of gas as that used during a plasma arc;
By supplying the gas from a low-pressure gas source and reducing the flow rate, the no-load voltage of the power supply required to generate and maintain the pilot arc is reduced; (2) the torch nozzle is brought into almost contact with the workpiece; That is, by bringing them into contact or at close range, the transition from the pilot arc to the plasma arc during processing is facilitated even at low arc voltages, and (3) the torch nozzle and the workpiece are maintained in a substantially contact state during processing. In addition to shortening the arc length and lowering the arc voltage during machining to further reduce the power supply capacity, (4) maintaining the nozzle and workpiece in a nearly contact state reduces the transition to plasma arc and Because the processing is stable, it is possible to keep the torch nozzle and the workpiece connected in a completely short-circuited state at all times, which eliminates the risk of electric shock.
Together with the use of a low no-load voltage power supply, this is designed to ensure the safety of workers as much as possible.

Embodiment FIG. 3 is a connection diagram showing an example of an apparatus for carrying out the method of the present invention. 5 and 6 in the same figure
is a pressure reducing valve and a solenoid valve installed in the working gas passage.The pressure reducing valve 5 obtains a low-pressure, low-flow gas flow during the pilot arc, and when the solenoid valve 6 opens, a high-pressure, large-flow gas flow for plasma processing is obtained. I'm starting to get a flow. 7 is a first output control circuit that controls the power supplied during plasma processing, 8 is a second output control circuit that controls the small current output that is supplied during pilot arcing, and these solenoid valves 6,
The first output control circuit 7 and the second output control circuit 8 are each switched by a switching circuit 9 operated by the output of the machining current detector 10. In the embodiment shown in the figure, at the start of machining, a voltage is applied between the electrode 101 and the nozzle 102 through the second output control circuit in response to a signal from a starting circuit (not shown), and the solenoid valve 2 opens and the pressure reducing valve 5 A small amount of gas is supplied into the nozzle through. At this time, a high voltage from a separately provided high frequency generator or the like is applied between the electrode 101 and the nozzle 102 to break the insulation between the electrode 101 and the nozzle 102 and start the pilot arc. This pilot arc is a small current determined by the second output control circuit, but the amount of gas supplied is also small because the solenoid valve 6 is still closed, and the pilot arc voltage is stable at a relatively low voltage. maintained. Next, when the nozzle 102 is brought close to the workpiece 4 until it almost contacts it, the high temperature ionized gas generated by the pilot arc reaches the surface of the workpiece and transfers to the main arc. This current is detected by the machining current detector 10, and upon receiving this output, the switching circuit 9 supplies a switching signal to the solenoid valve 6, first and second output control circuits immediately or after a predetermined time delay. do. As a result, the solenoid valve 6 opens and supplies a large amount of high-pressure gas necessary for plasma arc processing to the nozzle 102. Further, the output control circuit is switched from the second control circuit to the first control circuit, and large power for plasma arc processing is supplied between the electrode 101 and the workpiece 4, and processing is started. At this time, the nozzle 102
is in almost contact with the workpiece 4, and the nozzle 102 is short-circuited to the workpiece with a cable, but the current during plasma processing does not flow through the nozzle 102, and almost all the current flows through the workpiece. Flows into things.
The reason for this is that during plasma arc processing, the outside of the arc column is strongly cooled by a high-pressure, high-velocity gas flow, and for this reason the arc is narrowed into an extremely thin line and has strong directivity in the direction of the gas flow. This is thought to be because when the nozzle and the workpiece are close together and the arc column is relatively short, there is almost no shunting to the nozzle on the side of the arc column. This was discovered by the present inventors who inserted an extremely thin insulator between the nozzle and the workpiece to separate the current flowing through the nozzle and the workpiece, and compared the current waveform of each part with the waveform of the entire machining current. It was also confirmed in the experiment that no current flows through the nozzle after that, except for a very short transition period until the arc is established. For this reason, it can be seen that even if the nozzle is brought into contact with the workpiece and the workpiece and nozzle are short-circuited by a cable, plasma arc processing is not affected at all.

In this way, during plasma arc processing, processing is continued by a powerful arc column directed from the electrode toward the workpiece, but when the end of the workpiece is reached and there is no longer anyone for the plasma arc to reach, the processing current detector 10 When the output disappears, the solenoid valve 6 closes and the output current becomes a small current and automatically returns to the original pilot arc, so even when machining a workpiece that is intermittent, the start and stop operations can be performed every time. This is convenient because there is no need to do this.

In order to restrict the gas flow during the pilot arc, instead of using a pressure reducing valve as shown in the example of FIG. 3, the gas flow rate may be directly adjusted using a flow rate regulating valve or a throttle valve. Further, the switching of the gas flow and the switching of the power supply output may be performed at the same time, but the switching of the power supply output may be delayed slightly from the switching of the gas flow. In any of the above cases, as shown in Figure 3, it is better to switch the gas flow by using a solenoid valve that is connected in parallel with a pressure reducing valve for flow restriction or a throttle valve to bypass it. This method is more convenient than the system in which piping is switched using a solenoid valve because there is no temporary stoppage of the gas flow or generation of pulsating flow during switching, and the arc can be kept more stable. In addition, separate power supplies may be prepared for the pilot arc and for machining. In this case, the equipment will be slightly more complicated, but it is possible to use the optimal no-load voltage and capacity for each. It's even more convenient because you can do it.

Effects of the Invention As described above, the present invention has the following advantages: (1) By supplying a small amount of working gas to the pilot arc at a low pressure, it is possible to stably operate the pilot arc even when power for the pilot arc is supplied from a power source with a low no-load voltage. (2) By bringing the nozzle into almost contact with the workpiece, even a low pilot arc voltage can easily transfer to the main arc. (3) By keeping the nozzle in almost contact with the workpiece during machining, the arc length during machining can be shortened, making it possible to use a low arc voltage for plasma machining. Coupled with the ability to lower the no-load voltage during pilot arcing, the power supply capacity can be greatly reduced, and electric shock accidents can be reduced.

(4) Furthermore, by electrically shorting the nozzle and the workpiece, no load voltage will be applied to the nozzle even if the electrode inside the torch and the nozzle come into contact due to some kind of accident. Even in semi-automatic processing where the torch is hand-held, the risk of electric shock can be completely eliminated.

It has excellent effects such as:

[Brief explanation of the drawing]

Figure 1 is a connection diagram showing a conventional example of plasma arc processing equipment, Figure 2 is a diagram showing the relationship between arc voltage and current in plasma arc processing, and Figure 3 is a diagram showing the plasma arc processing method of the present invention. FIG. 2 is a connection diagram showing an example of a device for 1... Torch, 2, 6... Solenoid valve, 3... Gas cylinder, 4... Workpiece, 5... Pressure reducing valve, 7...
1st output control circuit, 8...2nd output control circuit, 9...switching circuit, 10...machining current detector,
101... Electrode, 102... Nozzle.

Claims (1)

[Claims] 1. Gas is supplied to the torch, a pilot arc is generated between the electrode and the nozzle of the torch to form an ionized gas flow, and then the pilot arc is transferred to the workpiece and the electrode In the plasma arc machining method, the nozzle of the torch and the workpiece are connected by a cable, and the nozzle of the torch and the workpiece are connected by a cable, and the nozzle of the torch and the workpiece are A pilot arc is generated by supplying power from a low-voltage power supply to the workpiece and supplying gas at a low pressure or small amount at a value that allows the low-voltage arc to maintain stability, and by bringing the nozzle into approximately contact with the workpiece. A plasma arc machining method in which the supplied gas and arc power are each increased to values suitable for plasma machining after being transferred to an arc, and machining is performed with the nozzle substantially in contact with the workpiece. 2. In a plasma arc processing device that performs processing while bringing the torch nozzle into approximately contact with the workpiece,
A throttle valve provided in a working gas piping line to limit the pressure or flow rate, a solenoid valve connected in parallel to the throttle valve to bypass the throttle valve, and a solenoid valve having a relatively low no-load voltage during pilot arcing. A processing power supply that can switch between a small current output and a large current output suitable for plasma arc processing, and a processing power supply that opens the solenoid valve when processing current flows to the workpiece and starts the plasma arc. and a plasma arc start detection circuit that outputs a switching signal for switching the output of the processing power source from a small current to a large current. Plasma arc processing equipment supplied between the electrode and the workpiece.