Fundamental Studies on Cutting Phenomena in Plasma Arc Cutting (2nd Report)

Distribution of Anode of Cathode Spot within Cutting Croove

Abstract

In a plasma arc cutting, the wandering region of anode or cathode spot within a cutting groove was measured by the optical method, and the correlation between the spot wandering region and the cutting front shape was investigated.
The cutting front shape is characterized as shown in Fig. 3, and the anode or cathode spot mainly wanders in the BC region within the cutting groove. Then, the cutting front can be divided into three parts inspite of sorts of working gases and electrode polarities;
Region I; which takes upper part of the cutting front, and in which the cutting front is mainly melted by the heat flux from arc column.
Region II: which takes middle part of the front, and in which the most of anode or cathode spot exists and the cutting front melted by the anode or cathode energy in addition to the arc column energy.
Region III; which takes lower part of the front, and in which the cutting front is chiefly melted by the heat transfer from high temperature and high velocity plasma stream.
The spot wandering region II is essentially governed by the distance between torch axis and the center of cutting front, so that the region II shifts upwards and narrows according as cutting speed and/or working gas flow rate generally increase. At the reverse polarity in high flow rate of working gas (Ar), the region II, however, slightly expands toward downward according to increasing the gas flow rate, since the voltage to make a new cathode spot becomes to be neglected as mentioned in Report 1.
In nitrogen plasma arc cutting, peculiar anode spot motion is observed when the lower part of region III exists within the cutting groove. The peculiar anode spot slowly repeats the motion from lower part to upper part within the groove in period of about one second, and makes rough cut surface. Such a inconvenient spot motion is caused by the bigness of the ratio of distance between torch axis and center of cutting front against the cutting depth. So, the peculiar anode spot motion is resolved by using a working gas in which high percentage argon or oxygen are mixed into nitrogen, since the ratio is small in such conditions.