Effects of Laval Nozzle on Precision Micro Cutting of Thin Metal Plate by Pulsed YAG Laser

Abstract

The stagnation pressure, which is the sum of static and dynamic pressures, has a great influence on machining performance in laser cutting. Therefore, increasing the velocity of assist gas is effective in reducing the height of dross. In this paper, Laval nozzle was newly designed, because it can increase the velocity of assist gas flow spouted from nozzle tip. Effects of Laval nozzle on assist gas flow and the machining results in precision cutting of thin metal plate by a pulsed YAG laser were experimentally investigated. Assist gas flow from Laval nozzle does not have Mach Shock Disk and goes more straight than a traditional convergent nozzle. Therefore, the pressure on a workpiece increases, since assist gas flow from Laval nozzle can utilize energy more efficiently. However, caution is necessary in setting the cylinder gas pressure, since Laval nozzle has many unstable regions, where the pressure on the workpiece changes periodically. Using Laval nozzle makes it possible to reduce the height of dross even under low cylinder gas pressure condition compared to convergent nozzle. Thus the consumption of flow quantity of assist gas can be reduced, when the same height of dross is desired. Moreover, it was pointed out that the convexed inner wall of nozzle leads to the reduction of dross generation and unstable region.