Effects of Nozzle Diameter, Nozzle Length, Standoff Distance and Secondary Flow on Plasma Cutting Speed

Abstract

Summary form only given. The thermal cutting industry has seen significant changes in markets and processes since the 80's. The main cutting processes of this industry are oxy-fuel, plasma and laser cutting. Laser dominates the thin gauge materials market producing high quality cuts. This is followed by high definition plasma that appeared in the early 90's. Plasma cutting processes are able to cut up to 50.8 mm thick material at higher current settings, i.e. 200 A and above. Oxy-fuel cutting dominates the thicker gauge market. With the push of laser technology towards thicker materials at faster speeds, i.e., LASOX, plasma cutting has to adapt in order to differentiate itself by cutting at even higher speeds on medium gauge materials, i.e. in the range of 12.7 mm. Therefore, the focus of this talk will be on factors affecting cutting speed of the plasma cutting process. In order to achieve this goal a number of factors were investigated using design of experiments. Factors with higher responses were selected and further investigated. The orifice length, diameter and standoff distance are important factors affecting the speed (orifice diameter D and length L are often coupled in the literature as an L/D ratio), and to a lesser extent secondary gas shrouding. L/D ratio is also a function of current level related parameters (current, plasma gas flow, secondary gas flows, etc.). Generally, increasing the value of this ratio improves the cutting speeds; however, the survivability of the consumables often limits higher values. Proper standoff distance is critical in order to insure proper level of energy density of the jet at the work piece. The secondary shrouding, while often presented in the literature as a significant factor, has a small effect on the cut speeds but a major one in other aspects of the cutting process (parts life and piercing). It is important to note that the means by which the secondary shrouding is implemented is an important factor at various current levels.