Abstract
In a plasma arc cutting, the behaviour of anode or cathode spots in a cutting kerf affects not only the distribution of heat input in the work piece but also the quality of the kerf or cut surface. Using a stack plate of water-cooled copper, the current in each plate was measured with arc voltage in order to clarify the fundamental behaviour of such a spot. Moreover, the spot motion in actual cutting was observed by an optical method, and analyzed from the correlation of the fluctuations between radiation intensity from the spot and arc voltage.Essentially, the sopt is carried away from the upper to lower side on the front surface of the cutting zone or the edge surface of the work piece due to the momentum of plasma and cold surrounding gas streams and due to the magnetic force acting on the spot and arc column. When the voltage between the arc column and the front surface of the kerf reaches a breakdown voltage, a new spot is suddenly made on the upper side. Consequently, the spot periodically repeats such a motion, so that the arc voltage fluctuates as sawteeth waves in several kHz to several 10 kHz corresponding to the spot motion.The above consideration shows that the wandering region of the spot is governed by the breakdown voltage of the sheath and the potential gradient of the arc column. Especially, effect of the former is considerably large. At reverse polarity, the breakdown voltage is higher than at straitgh polarity, because the voltage to make a new cathode spot can not be neglected as well known from the restrike phenomena in A.C. arc. Thus, the spot wandering regon at reverse polarity is extended in comparison with that at straight polarity.It is a noticeable fact that the cathode spot motion is remarkably affected by the condition of the plate surface and surrounding gas. If air mixing into the plasma stream increases, the cathode sopt seeks out an oxide film, and behaves the same as in cleaning phenomena at reverse polarity of TIG arc welding. Arc voltage, therefore, shows the characteristic triangle waves in several 100 Hz.
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