Electric heating – Metal heating – By arc
Patent
1992-10-19
1994-07-05
Paschall, Mark H.
Electric heating
Metal heating
By arc
21912403, 21912154, 21912139, 21912144, 21912157, B23K 1000, B23K 9073
Patent
active
053269551
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a standoff control method and an apparatus for controlling the standoff between a torch of a plasma cutting machine and a workpiece to be cut, the plasma cutting machine being arranged to cut the workpiece by generating a plasma arc between an electrode thereof and the workpiece.
BACKGROUND ART
A plasma cutting machine has a nozzle disposed to surround an electrode thereof so as to form a fluid passage for an operation gas and throttle a plasma generated in front of the leading portion of the electrode between the electrode and the workpiece to be cut so that the temperature of the plasma is raised and the size of the plasma arc is made to be uniform for the purpose of obtaining an excellent cut surface. However, if the distance between the torch composed of the electrode and the nozzle and the workpiece to be cut is changed, the plasma arc cannot be maintained or an excellent cut shape cannot be obtained because the size of the plasma arc is changed. Hence, the plasma cutting machine has been arranged in such a manner that the distance (hereinafter called a "standoff") between the torch and the workpiece to be cut is kept constant so that the cutting work is performed satisfactorily.
In a case where the standoff is controlled, it has been known that the arc voltage between the workpiece to be cut and the electrode or the nozzle has a proportional relationship with the standoff. The relationship can be utilized to detect the arc voltage, and the detected arc voltage is maintained at a constant value (refer to Japanese Patent Laid-Open No. 57-195582 for example).
However, the aforesaid conventional method is arranged in such a manner that the quantity of the displacement of the torch is simply changed in accordance with the level of the deviation between the detected arc voltage and a reference voltage. Therefore, if the deviation is excessively large, it takes excessive time to realize a predetermined standoff, causing a delay of response to take place. As a result, a satisfactory cutting accuracy cannot be obtained. The aforesaid method is arranged in such a manner that, if the deviation has become excessively large, an abnormal voltage detection circuit disposed individually from a circuit for controlling the standoff detects it so as to turn off the arc voltage. Therefore, the efficiency of the cutting operation deteriorates. What is worse, if the deviation becomes excessively toward negative values, the torch is moved downwardly and the arc is turned off by the abnormal voltage detection circuit so that the deviation is converted into positive values. Hence, the continuation of the downward movement of the torch will cause the torch to collide with the workpiece to be cut, and therefore there is a risk that the torch or the workpiece to be cut is damaged.
Furthermore, consideration has not been taken against the change in the arc voltage with respect to a rise in the arc voltage or a change in the cutting speed due to the consumption of the electrode. As a result, the quantity of the standoff is changed due to the consumption of the electrode or a change in the cutting speed. Therefore, an excellent cutting operation cannot be performed.
In another case in which rebounds of molten metal adhere to a portion adjacent to the nozzle after the torch has approached the workpiece to be cut, or in another case in which the gas flow is disordered due to deformation of the nozzle, a double-arc takes place, for example, between the workpiece to be cut and the nozzle. Hence, the nozzle can be damaged, causing the cutting work to be interrupted. Therefore, an apparatus has been suggested (refer to, for example, Japanese Patent Publication No. 60-43831) which is arranged in such a manner that attention has been paid to the fact that the output voltage from the power source for generating the plasma arc is reduced when the torch comes closer to the workpiece to be cut than a predetermined distance and when the impedance has been reduced, and the output from the pow
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Hasegawa Masahiko
Kurkawa Iwao
Nishi Yozo
Nozaki Eiichi
Wakui Atsushi
Kabushiki Kaisha Komatsu Seisakusho
Paschall Mark H.
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