Design and construction of a plasma cutting machine with an automatic system for "fish mouth" cuts in round tubes

Abstract

In the metalworking sector, the fishmouth cutting process for round tubes is a high-demand operation in the fabrication of metal structures. However, its manual execution using chip removal with punches and drills results in material loss, dimensional inaccuracies, and risks for the operator. This paper describes the design, construction, and validation of a plasma cutting machine with an automatic rotation system, designed to perform fishmouth cutting on round steel tubes with thicknesses from 2 mm to 6 mm and diameters from 2" to 5". The system integrates a 1/8 hp geared motor operating at 10 RPM, an aluminum pulley transmission mechanism with a 2:1 gear ratio, SKF double-row bearings, and a Porten PS-EC40 plasma cutter. Electrical control is implemented using an electromagnetic contactor with latching, start and stop pushbuttons, and pilot lights. Structural calculations confirmed that the 30×30×3 mm ASTM A-36 square tube withstands the applied loads, with a calculated stress of 16,165.80 Pa compared to an allowable stress of 165.47 MPa. Experimental validation demonstrated that the system performs continuous, uniform, and reproducible 360° cuts, reducing operating time and material losses compared to the manual method. The prototype was implemented in the welding workshop of the "Central Técnico" Higher Technological Institute in Quito, Ecuador, as a platform for practical learning in automated cutting processes.